If anybody is interested, I'm currently drip-tweeting some of the constraints one can obtain from considering Planck and BICEP2 data together. BICEP2 did do a bit of this in their paper, but they only considered specific scenarios. They were also often a bit coy about the implications of the combined analysis. I'll try not to be ;-).
The results should only be seen as indicative, these aren't published, and never will be in this form (maybe they could be cited if used in a paper though!). They were provided to me by Sussex Uni's resident obtaining-cosmology-from-the-CMB expert Antony Lewis, after a hurried Tuesday adding the BICEP2 data to the Planck cosmology pipeline (i.e. CosmoMC) and may contain mistakes.
Antony has himself also made some of these results public at the Cosmo Coffee website.
Questions here, or on Twitter are most welcome. If you want to see specific cosmologies, I'll do my best to show them (if I have them), or ask Antony very nicely to provide them (no guarantees, of course).
You can find my Twitter account here: @just_shaun. Feel free to share!
Showing posts with label measure everything. Show all posts
Showing posts with label measure everything. Show all posts
Wednesday, March 19, 2014
Friday, March 14, 2014
"A major discovery", BICEP2 and B-modes
[Added note (on Monday): Well, wow, the rumours were, if anything, understated. I'm happy to go on record that, unless a mistake has been made, this is the greatest scientific discovery of the 21st century, and may remain so even once the century is over. I (and others) will write many more detailed summaries of what was observed over time, but BICEP2 have announced a discovery of primordial B-modes, which is extremely strong evidence of cosmological inflation (if it turns out to be scale invariant, inflation is as true as most accepted science). Matt Strassler has a good hastily written summary here. As does Liam McAllister at Lubos Motl's blog, here. Of course, this is just one experiment and maybe they've made a mistake, but the results look very robust at the moment.
Congratulations on being alive today readers! We just learned about how particles work at energies \(10^{13}\) times greater than even the LHC can probe, and about what was happening at a time much, much less than a nanosecond after the beginning of the Big Bang.]
............................................
[Added note (on Sunday): It seems highly probable that these rumours are essentially true. Although the precise details of the results aren't yet public, the BICEP2 PI, John Kovac, has sent a widely distributed email with the following information: Data and scientific papers with results from the BICEP2 experiment will go public and be viewable here at 2:45pm GMT on Monday. At the same time a technical webcast will begin at this address.
It's going to be an exciting day!]
.......................................................
The cosmology rumour mill exploded today. Harvard Astrophysics have issued a press release stating that, on Monday, they will announce a "major discovery".
This is the only hard-evidence of anything interesting on the way and it could be an announcement of anything that fits under the label of "astrophysics". This is important to keep in mind. However, for one reason or another (that is hard to nail down), cosmologists are suggesting that it is going to be about cosmology. The speculation is that it will be about the BICEP2 experiment, which has been measuring the polarisation in the CMB. The speculation is that BICEP2 have seen primordial "B-mode" polarisation.
If this speculation is true, this would be a result immense in its significance.
Primordial B-modes would be a smoking gun signal of primordial gravitational waves. This, alone, makes such a discovery important. Gravitational waves have not yet been observed, but are a prediction from general relativity. Therefore, such a discovery would be on the same level of significance as the discovery of the Higgs particle. We were almost certain it would be there, but it is good to finally see it.
However, the potential significance of such a result goes further because these primordial gravitational waves would need a source. The theory of cosmological inflation would/could be such a source. Inflation is a compelling theory, not without some problems, for how the universe evolved in its very earliest stages. If it occurred when the universe had a large enough temperature, it would generate primordial gravitational waves large enough to tickle the CMB enough to make these B-modes visible in the polarisation. As of yet, inflation has passed quite a few observational tests, but nothing has been seen that could be described as smoking gun evidence. A spectrum of primordial gravitational waves would very nearly be such a smoking gun. If the spectrum was scale invariant (i.e. if the gravitational waves have the same amplitude on all distance scales) that would be a smoking gun for inflation and accolades, Nobel Prizes, etc, etc, would flow accordingly.
All of this is just speculation, but some of it does seem to be coming from reputable sources. And some of my colleagues have been talking about tip-offs from people who wish to remain anonymous, so I figured I'd collect all the speculation I know of here in a post (let me know if I've missed anything):
The PI of BICEP2, John Kovac, gave a talk at the annual COSMO conference last year that had some pretty ambitious claims for how sensitive BICEP2 and similar experiments were going to be, so... well... we'll know on Monday. It should also be noted that, although the existence of these gravitational waves is a prediction of inflation, their amplitude is a free parameter and an amplitude this big is potentially a little surprising (for me, lower temperature inflation models just seem more compelling, others might disagree).
Twitter: @just_shaun
[Edit: The video of John Kovac's talk can be found here]
Congratulations on being alive today readers! We just learned about how particles work at energies \(10^{13}\) times greater than even the LHC can probe, and about what was happening at a time much, much less than a nanosecond after the beginning of the Big Bang.]
............................................
[Added note (on Sunday): It seems highly probable that these rumours are essentially true. Although the precise details of the results aren't yet public, the BICEP2 PI, John Kovac, has sent a widely distributed email with the following information: Data and scientific papers with results from the BICEP2 experiment will go public and be viewable here at 2:45pm GMT on Monday. At the same time a technical webcast will begin at this address.
It's going to be an exciting day!]
.......................................................
The cosmology rumour mill exploded today. Harvard Astrophysics have issued a press release stating that, on Monday, they will announce a "major discovery".
This is the only hard-evidence of anything interesting on the way and it could be an announcement of anything that fits under the label of "astrophysics". This is important to keep in mind. However, for one reason or another (that is hard to nail down), cosmologists are suggesting that it is going to be about cosmology. The speculation is that it will be about the BICEP2 experiment, which has been measuring the polarisation in the CMB. The speculation is that BICEP2 have seen primordial "B-mode" polarisation.
If this speculation is true, this would be a result immense in its significance.
Primordial B-modes would be a smoking gun signal of primordial gravitational waves. This, alone, makes such a discovery important. Gravitational waves have not yet been observed, but are a prediction from general relativity. Therefore, such a discovery would be on the same level of significance as the discovery of the Higgs particle. We were almost certain it would be there, but it is good to finally see it.
However, the potential significance of such a result goes further because these primordial gravitational waves would need a source. The theory of cosmological inflation would/could be such a source. Inflation is a compelling theory, not without some problems, for how the universe evolved in its very earliest stages. If it occurred when the universe had a large enough temperature, it would generate primordial gravitational waves large enough to tickle the CMB enough to make these B-modes visible in the polarisation. As of yet, inflation has passed quite a few observational tests, but nothing has been seen that could be described as smoking gun evidence. A spectrum of primordial gravitational waves would very nearly be such a smoking gun. If the spectrum was scale invariant (i.e. if the gravitational waves have the same amplitude on all distance scales) that would be a smoking gun for inflation and accolades, Nobel Prizes, etc, etc, would flow accordingly.
All of this is just speculation, but some of it does seem to be coming from reputable sources. And some of my colleagues have been talking about tip-offs from people who wish to remain anonymous, so I figured I'd collect all the speculation I know of here in a post (let me know if I've missed anything):
- Richard Easther, on the rumour and its implications
- Bruce Bassett, on the probability that the rumours are true
- The Guardian (the first major news source to pick up on this) with comments from various prominent cosmologists
- Jester/Resonaances on the context (i.e. earlier constraints on primordial B modes)
- Lubos Motl, amongst other things, explains what B mode polarisation actually is
- Philip Gibbs, at viXra log
- Peter Coles, amongst other things, on why gravitational waves mean there should be B-modes in the CMB polarisation
- Sesh Nadathur on why, amongst other things, the rumoured measurement would appear to be in tension with results from Planck and WMAP
The PI of BICEP2, John Kovac, gave a talk at the annual COSMO conference last year that had some pretty ambitious claims for how sensitive BICEP2 and similar experiments were going to be, so... well... we'll know on Monday. It should also be noted that, although the existence of these gravitational waves is a prediction of inflation, their amplitude is a free parameter and an amplitude this big is potentially a little surprising (for me, lower temperature inflation models just seem more compelling, others might disagree).
Twitter: @just_shaun
[Edit: The video of John Kovac's talk can be found here]
Monday, November 25, 2013
How does one measure the mass of a neutrino, using cosmology?
I'm going to tell you how, soon, humanity might measure the masses of neutrinos just by observing past events in the universe. I like this topic because it is one of the few situations in fundamental physics where a measurement of the greater universe might detect something about fundamental particles and/or their interactions, before we manage to measure it in a lab. Another example is the existence of dark matter; however the mass of dark matter will almost certainly be first measured in a lab. Perhaps with neutrinos it will go in the other direction?
What is a neutrino?
I guess that before telling you how to measure a neutrino's mass, it might be pertinent to tell you what a neutrino is and how we can know it has mass before we've measured that mass. Well...
When an atomic nucleus decays, the decay products we see are other nuclei, electrons and/or positrons. These visible products always carry less energy and momentum than the amount that the initial nucleus had. This suggests strongly that some unknown other particle is also being created in the decay and that we just can't see it. This hypothetical particle was dubbed the neutrino and when theories were developed for the force responsible for nuclear decays, the neutrino became an important part of them. And, eventually, neutrinos were detected directly. It took a while because neutrinos interact incredibly weakly, which means you need either a lot of neutrinos or a lot of transparent stuff for the neutrino to interact with (or both) before you will see them.
Initially, it was assumed that neutrinos are massless. They don't need to be massless, but for a long time there was no evidence that they did have mass, so the simplest assumption was that they didn't. There are three types of neutrinos: those emitted in interactions with electrons, those emitted in interactions with muons and those emitted in interactions with tau particles. If neutrinos were massless, then a neutrino emitted as an electron neutrino would always remain an electron neutrino. Similarly, a muon neutrino would always remain a muon neutrino. However, if neutrinos do have mass, then a neutrino emitted in an interaction with an electron will actually travel as a superposition of an electron neutrino, muon neutrino and tau neutrino. The net result being that this neutrino could be detected as a different type of neutrino. Therefore, a smoking gun thing to look for when determining whether neutrinos have mass is this characteristic signal whereby one type of neutrino appears to oscillate into another type of neutrino.
This effect was then seen and seen and seen again. Neutrinos appear to have mass. From the perspective of particle physics this is a bit weird. Neutrinos must have really small masses and it is unclear why these masses are so small. Unfortunately, this mechanism of neutrino oscillations doesn't directly give the masses of the neutrinos. Although, it can be used to measure the differences of the masses of the neutrinos, thus setting lower bounds on the possible masses of the neutrinos.
What has this got to do with cosmology?
What is a neutrino?
I guess that before telling you how to measure a neutrino's mass, it might be pertinent to tell you what a neutrino is and how we can know it has mass before we've measured that mass. Well...
When an atomic nucleus decays, the decay products we see are other nuclei, electrons and/or positrons. These visible products always carry less energy and momentum than the amount that the initial nucleus had. This suggests strongly that some unknown other particle is also being created in the decay and that we just can't see it. This hypothetical particle was dubbed the neutrino and when theories were developed for the force responsible for nuclear decays, the neutrino became an important part of them. And, eventually, neutrinos were detected directly. It took a while because neutrinos interact incredibly weakly, which means you need either a lot of neutrinos or a lot of transparent stuff for the neutrino to interact with (or both) before you will see them.
Initially, it was assumed that neutrinos are massless. They don't need to be massless, but for a long time there was no evidence that they did have mass, so the simplest assumption was that they didn't. There are three types of neutrinos: those emitted in interactions with electrons, those emitted in interactions with muons and those emitted in interactions with tau particles. If neutrinos were massless, then a neutrino emitted as an electron neutrino would always remain an electron neutrino. Similarly, a muon neutrino would always remain a muon neutrino. However, if neutrinos do have mass, then a neutrino emitted in an interaction with an electron will actually travel as a superposition of an electron neutrino, muon neutrino and tau neutrino. The net result being that this neutrino could be detected as a different type of neutrino. Therefore, a smoking gun thing to look for when determining whether neutrinos have mass is this characteristic signal whereby one type of neutrino appears to oscillate into another type of neutrino.
This effect was then seen and seen and seen again. Neutrinos appear to have mass. From the perspective of particle physics this is a bit weird. Neutrinos must have really small masses and it is unclear why these masses are so small. Unfortunately, this mechanism of neutrino oscillations doesn't directly give the masses of the neutrinos. Although, it can be used to measure the differences of the masses of the neutrinos, thus setting lower bounds on the possible masses of the neutrinos.
What has this got to do with cosmology?
Wednesday, April 3, 2013
The universe as seen by Planck - Day Two
The cosmic microwave background (CMB) is the best probe we've yet found to study the early universe. The CMB's temperature is very nearly uniform. However this temperature does have very small anisotropies that can be used to study sound waves that existed in the primordial universe. The Planck satellite (an ESA funded experiment) has mapped these temperature anisotropies over the entire sky with the best resolution to date. Last month, Planck released its data and it immediately became the new benchmark for the testing of cosmological models and the measurement of cosmological parameters.
This week ESA is hosting the first conference since Planck released its data. The conference is at ESTEC in the Dutch town of Noordwijk. I am attending this conference and will be doing my best to write updates about what was discussed during the week.You can read my introductory post where I give my motivation for doing this, here.
The CMB is not just useful for studying the primordial universe. As soon as the CMB forms, everywhere in the universe, it travels freely, in every direction, at the speed of light. This means that, in every direction, the CMB we measure here on Earth today has travelled to us from a point billions of light years away. In principle, this makes the CMB not just a really good probe of the state of the universe where and when it was emitted, but also of everything it passed on its way to us.
This secondary use for the CMB turns out to be very useful and many of the highlights from Planck relate to the way in which the CMB interacts on its way to us. The existence of matter in the universe affects the CMB gravitationally. This causes the CMB to bend towards regions of over-density and away from regions of under-density. It also causes the CMB's temperature to shift as it falls into and out of over and under-dense regions. This first effect is known as lensing and one of Planck's most impressive results is a map of the locations of matter in the universe through this lensing effect. The second effect is known as the Sachs-Wolfe effect, something I've written about in some detail.
There is a third way that the CMB is significantly affected by the intervening universe. Within clusters of galaxies there is a lot of hot gas. If the CMB passes through a cluster it can scatter off electrons in this hot gas. The effect of this scattering on the CMB is known as the Sunyaev-Zeldovich (SZ) effect. Therefore, we should be able to use the CMB to detect the lines of sight along which the most massive clusters lie.
We can. And Planck has.
This week ESA is hosting the first conference since Planck released its data. The conference is at ESTEC in the Dutch town of Noordwijk. I am attending this conference and will be doing my best to write updates about what was discussed during the week.You can read my introductory post where I give my motivation for doing this, here.
The CMB is not just useful for studying the primordial universe. As soon as the CMB forms, everywhere in the universe, it travels freely, in every direction, at the speed of light. This means that, in every direction, the CMB we measure here on Earth today has travelled to us from a point billions of light years away. In principle, this makes the CMB not just a really good probe of the state of the universe where and when it was emitted, but also of everything it passed on its way to us.
This secondary use for the CMB turns out to be very useful and many of the highlights from Planck relate to the way in which the CMB interacts on its way to us. The existence of matter in the universe affects the CMB gravitationally. This causes the CMB to bend towards regions of over-density and away from regions of under-density. It also causes the CMB's temperature to shift as it falls into and out of over and under-dense regions. This first effect is known as lensing and one of Planck's most impressive results is a map of the locations of matter in the universe through this lensing effect. The second effect is known as the Sachs-Wolfe effect, something I've written about in some detail.
There is a third way that the CMB is significantly affected by the intervening universe. Within clusters of galaxies there is a lot of hot gas. If the CMB passes through a cluster it can scatter off electrons in this hot gas. The effect of this scattering on the CMB is known as the Sunyaev-Zeldovich (SZ) effect. Therefore, we should be able to use the CMB to detect the lines of sight along which the most massive clusters lie.
We can. And Planck has.
Monday, April 1, 2013
The universe as seen by Planck (conference)
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| The 47th ESLAB symposium. All the cool kids will either be there, or watching it live on the webcast. Are you one of the cool kids? |
This week I will be at a scientific conference, organised by ESA. In ESA's words, this conference is "An international conference dedicated to an in-depth look at the initial scientific results from the Planck mission". The conference is taking place in the small Dutch down of Noordwijk. At this conference there will be many people from within the Planck collaboration, who I'm sure will be delighted to finally be able to talk about their work and many people like myself who have spent the last few years eagerly anticipating the Planck collaboration's results.
The conference will have a live webcast here, you should watch some of it.
I will also be blogging during the conference. My goal is to try to write a new post here each day summarising the most interesting talks and discussions from the conference that day.
Why am I doing this?
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| An absolutely wonderful image showing how the various all sky images of the CMB anisotropies have improved each decade. |
This won't be an easy task. The conference goes quite late each day and many topics will be covered, but I want to do this anyway. To understand why, first go watch my new favourite video on the internet. Brady Haran makes science videos and if you've never seen them, you should go check them out. I felt like Brady was taking the words out of my mind when I saw that video. One day the utopia that Brady and I envisage will exist and a Planck conference like this will be besieged by legions of fans. One auditorium will be fill of fans of non-Gaussianity and fans of Gaussianity, on opposite side, cheering their preference on. Another auditorium will be filled with fans of dark radiation, cheering their team on. Yet another will be filled with fans of the cosmological constant shouting their favourite chants at their mortal enemies, the quintessence crowd. But that day is not today.
Thursday, March 21, 2013
Following Planck's results today
For the people (new and old) who follow this blog and are interested in the Planck satellite's results, which are being announced today, here is a run-down of important things to know:
Although I won't be live-blogging the results, I will write a post later today summarising what we've learned and discussing the fall-out arising from all the new information.
- Richard Easther will be live-blogging the data release at this location. If you can't watch the release yourself you should follow Richard's post.
- The first ESA event is a general-audience press conference, very soon, at 10:00 CET, which you can watch here.
- The second ESA event is a press conference aimed at scientists and science journalists, which will stream at the same location, i.e. here. You should watch that even if you aren't a scientist because it will be when all the interesting bits are revealed. If you're confused, you can follow Richard's live-blogging and/or ask questions of scientists on Twitter with the hashtag #askplanck.
- The release of the scientific papers is scheduled for 12:00 CET at the ESA website (I'm not sure of the precise url, maybe here?).
Although I won't be live-blogging the results, I will write a post later today summarising what we've learned and discussing the fall-out arising from all the new information.
Enjoy the day!
Edit: The papers will appear here at 12:00 CET: http://www.sciops.esa.int/index.php?project=planck&page=Planck_Legacy_Archive
Edit: The papers will appear here at 12:00 CET: http://www.sciops.esa.int/index.php?project=planck&page=Planck_Legacy_Archive
Twitter: @just_shaun
Tuesday, March 19, 2013
Planck rumours will soon become Planck results
On Thursday, the Planck satellite will be revealing its first cosmological results. In terms of fundamental physics, this will be the biggest event since the Higgs discovery last year. In the cosmology community it is the biggest event for the best part of a decade (possibly in both directions of time). If you don't follow cosmology too closely, you might wonder why this particular experiment might generate so much excitement. After all, aren't there all sorts of experiments, all of the time?
If so, I hope you've come to the right place.
If you're unaware, Planck is a satellite put in space by the European Space Agency to measure the cosmic microwave background (CMB). The CMB is an incredibly useful source of cosmological information. The impending release of Planck's results on Thursday is big news because Planck has measured the CMB with better resolution than any other experiment that can see the whole sky. Planck might have discovered evidence of interesting new physics, such as extra neutrinos or additional types of dark matter. It might even reveal some effects relating to how physics works at energies we could never probe on Earth. But even if it hasn't discovered anything dramatically new, the precision with which Planck has measured the parameters of the standard cosmological model will immediately make it the new benchmark.
There have been surprisingly few rumours leaked to the rest of the cosmology community about what to expect on Thursday. This has resulted in the most pervasive rumour being that they have simply not found anything worth leaking. Whatever the reality, on Thursday rumours will become results.
What has Planck actually done that is so interesting?
If so, I hope you've come to the right place.
 |
| The sky as seen by Planck in 2010. Only, they hadn't removed the foregrounds yet. There's a whole Milky Way galaxy in the way. Why must they make us wait so long? |
If you're unaware, Planck is a satellite put in space by the European Space Agency to measure the cosmic microwave background (CMB). The CMB is an incredibly useful source of cosmological information. The impending release of Planck's results on Thursday is big news because Planck has measured the CMB with better resolution than any other experiment that can see the whole sky. Planck might have discovered evidence of interesting new physics, such as extra neutrinos or additional types of dark matter. It might even reveal some effects relating to how physics works at energies we could never probe on Earth. But even if it hasn't discovered anything dramatically new, the precision with which Planck has measured the parameters of the standard cosmological model will immediately make it the new benchmark.
There have been surprisingly few rumours leaked to the rest of the cosmology community about what to expect on Thursday. This has resulted in the most pervasive rumour being that they have simply not found anything worth leaking. Whatever the reality, on Thursday rumours will become results.
What has Planck actually done that is so interesting?
Tuesday, February 5, 2013
The “ISW mystery” deepens considerably (II)
[... continued from yesterday]
[Note added April 29: Through correspondence with some of the authors from what I label as the "French group" below I have learned that the density threshold was actually applied in their work as well. This makes things rather confusing as it means that their methods and the methods of "the DHB" are much more similar. However, they have also updated their paper to reflect new knowledge about the void catalogues and see a slightly more significant signal, similar to what Planck find (see note below). Everything is rather confusing right now. Again, once the dust has settled, I will write a post clearing everything up.]
[Note added March 21: Wow, sometimes science moves quickly. Today Planck released its data. They appear to confirm the anomalous spots in the original "Granett" (Hawaiian) result. They also appear to confirm the new anomalous result that was present in the paper that is now retracted (see the note below from March 19), albeit with a slightly reduced significance. It is unclear exactly what is going on, but it is clear that it is something interesting. I will keep you informed as things progress.]
[Noted added March 19: The paper described in the second half of this post (I called its authors the DHB) has been withdrawn from the journal it was submitted to (see the new abstract at this link: http://arxiv.org/abs/1301.6136). It is unclear whether the problems that the authors found in their analysis will affect their conclusions. However, I suggest you are cautious regarding how you interpret the conclusions I have drawn below based on this paper. I will keep you informed as/when things progress.]
Isn't this just "a posteriori" statistics?
There is another possible explanation for the mystery. The probability of ZOBOV picking out these lines of sight at random is exceedingly small (less than 0.003), but it isn't zero. Might this have just been a crazy fluke?
Suppose 100 different groups of physicists look for unexpected, but interesting, signals in cosmological data. Then, even if each group is very careful you still expect one of them to find something that would seem to them to be unlikely. Unfortunately, they would be the only ones to publish their results. So we wouldn't see one “detection” paper and ninety-nine papers consistent with no detection. We would just see the one “detection” paper.
The best way to determine whether this is what happened is to look for the signal in other surveys. If the original measurement was a fluke, it won't show up anywhere else. But, if it does show up again, then the chances that it was a fluke will significantly diminish.
The Friday before last a paper appeared that did exactly this. A French group took two catalogues of voids (so no over-densities), which have been produced by applying ZOBOV to a new catalogue of galaxies (these ones are closer to us). The French group then did more or less the same thing as the Hawaiians did. They examined images of the CMB along the lines of sight of these voids, averaged the temperature in all the images and checked whether the resulting signal could have happened at random.
They found no significant result.
This was quite sobering to read on the day. The paper did verify the significance of the original measurement, but not finding it in the new catalogues was highly suggestive that the story I painted above of a sort of community wide “look elsewhere effect” was true.
Hold on though!
Things at this date in time did look bad for the anomaly, but there was one important piece missing from the French group's analysis. The Hawaiians only used the most extreme over and under-dense regions in their analysis. ZOBOV found many more than 50 regions for them and if they had used all of them, they also wouldn't have obtained a statistically significant signal. This was always a crucial part of their analysis because we already knew from other observations that the observed ISW effect from most of the universe is as small as the predicted signal.
What would the French group have seen if they had only examined the most extreme voids?
A new observation
Three days later (last Monday) a mixture of physicists from Durham, Hawaii and Baltimore (the DHB) released a paper. It answered the question posed above. For anybody interested in finding new physics, the answer is very exciting.
[Note added April 29: Through correspondence with some of the authors from what I label as the "French group" below I have learned that the density threshold was actually applied in their work as well. This makes things rather confusing as it means that their methods and the methods of "the DHB" are much more similar. However, they have also updated their paper to reflect new knowledge about the void catalogues and see a slightly more significant signal, similar to what Planck find (see note below). Everything is rather confusing right now. Again, once the dust has settled, I will write a post clearing everything up.]
[Note added March 21: Wow, sometimes science moves quickly. Today Planck released its data. They appear to confirm the anomalous spots in the original "Granett" (Hawaiian) result. They also appear to confirm the new anomalous result that was present in the paper that is now retracted (see the note below from March 19), albeit with a slightly reduced significance. It is unclear exactly what is going on, but it is clear that it is something interesting. I will keep you informed as things progress.]
[Noted added March 19: The paper described in the second half of this post (I called its authors the DHB) has been withdrawn from the journal it was submitted to (see the new abstract at this link: http://arxiv.org/abs/1301.6136). It is unclear whether the problems that the authors found in their analysis will affect their conclusions. However, I suggest you are cautious regarding how you interpret the conclusions I have drawn below based on this paper. I will keep you informed as/when things progress.]
 |
| A really neat figure from arXiv:1301.5849 showing the locations and sizes of the various catalogues of voids being examined. A larger redshift means the void is further away from us and one Megaparsec (Mpc) corresponds to three million light years. The purple "Granett et al." box is the original catalogue used by the Hawaiian group back in 2008. |
Isn't this just "a posteriori" statistics?
There is another possible explanation for the mystery. The probability of ZOBOV picking out these lines of sight at random is exceedingly small (less than 0.003), but it isn't zero. Might this have just been a crazy fluke?
Suppose 100 different groups of physicists look for unexpected, but interesting, signals in cosmological data. Then, even if each group is very careful you still expect one of them to find something that would seem to them to be unlikely. Unfortunately, they would be the only ones to publish their results. So we wouldn't see one “detection” paper and ninety-nine papers consistent with no detection. We would just see the one “detection” paper.
The best way to determine whether this is what happened is to look for the signal in other surveys. If the original measurement was a fluke, it won't show up anywhere else. But, if it does show up again, then the chances that it was a fluke will significantly diminish.
The Friday before last a paper appeared that did exactly this. A French group took two catalogues of voids (so no over-densities), which have been produced by applying ZOBOV to a new catalogue of galaxies (these ones are closer to us). The French group then did more or less the same thing as the Hawaiians did. They examined images of the CMB along the lines of sight of these voids, averaged the temperature in all the images and checked whether the resulting signal could have happened at random.
They found no significant result.
This was quite sobering to read on the day. The paper did verify the significance of the original measurement, but not finding it in the new catalogues was highly suggestive that the story I painted above of a sort of community wide “look elsewhere effect” was true.
Hold on though!
Things at this date in time did look bad for the anomaly, but there was one important piece missing from the French group's analysis. The Hawaiians only used the most extreme over and under-dense regions in their analysis. ZOBOV found many more than 50 regions for them and if they had used all of them, they also wouldn't have obtained a statistically significant signal. This was always a crucial part of their analysis because we already knew from other observations that the observed ISW effect from most of the universe is as small as the predicted signal.
What would the French group have seen if they had only examined the most extreme voids?
A new observation
 | |
|
Three days later (last Monday) a mixture of physicists from Durham, Hawaii and Baltimore (the DHB) released a paper. It answered the question posed above. For anybody interested in finding new physics, the answer is very exciting.
Monday, February 4, 2013
The “ISW mystery” deepens considerably
 |
| Other than my initials, what secrets does the CMB hide that are waiting to be seen only when the CMB is examined in just the right way? |
This time last year I wrote a few posts describing what I called the “ISW mystery” (Part I, II, III and IV). A year has passed, it is time for an update on the mystery.
The very short summary is that things are starting to get more than a little bit exciting. All of the plausible ways in which the calculation of the expected ISW signal could have been wrong have been checked and eliminated as possibilities; if the measured signal is real, it is too large for the standard cosmological model. Much, much more excitingly, the observation that generated the mystery has now been repeated in another region of the universe and a very similar and equally anomalous signal was found; the apparent anomaly was not a statistical fluke.
The preprint of the paper describing this new observation was released just a week ago.
What is the “ISW mystery”?
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| The image that began the mystery. Why is that spot so hot, and how did it get that cold ring around it? |
A quick recap will probably be useful. The integrated Sachs-Wolfe (ISW) effect describes the heating and cooling of light as it passes through gravitational peaks and valleys late in the evolution of the universe. In the standard cosmological model, these peaks and valleys decay with time, so a light ray gains (or loses) more energy entering an over-dense (or under-dense) region of the universe than it loses (or gains) leaving it. The effect is very, very small. Almost every source of light in the universe is not known well enough to be used to detect it. Only the cosmic microwave background (CMB) is uniform enough that these tiny fluctuations could ever be detected.
However, even then, the primary fluctuations in the the temperature of the CMB are bigger than the secondary ones created by the ISW effect. We can measure these fluctuations but we could never know how much is due to the ISW effect and how much is primordial. The only thing we can do is look at the structures in the universe nearby and see if on average the CMB is slightly hotter (colder) along lines of sight where the nearby universe is over-dense (under-dense). The bigger, primordial fluctuations in the CMB should have nothing to do with local structures (the CMB has come from much further away). Therefore, if this signal were to be found in the CMB, the most plausible explanation would be an ISW effect.
A group in Hawaii decided to look for this signal in a slightly unusual way. Firstly, they made a catalogue of significant over and under-dense regions in a particular survey of galaxies. Then, they only examined patches of the CMB that existed along the line of sight of each of these regions. They then found that the patches aligned with over-densities were hotter on average than a randomly selected patch and those aligned with under-densities were colder (with more than “\(4\sigma\)” significance). This is what one would expect from an ISW effect. The “ISW mystery” is that these patches were too hot and too cold. The ISW effect simply shouldn't be that big.
The importance of checking the anomaly from every angle
Tuesday, September 25, 2012
Highlights from Beijing: COSMO 2012
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| The obligatory conference photo. The photographer spoke to us in Mandarin. I think what he was trying to say was "more intensity". |
Just over a week ago I was at the annual COSMO conference. This year's host was Beijing. I had originally intended to live blog this event, but the Great Wall of China (alternative link) managed to prevent that entirely.
What follows are some reflections on the scientific bits and pieces people presented at the conference that I happened to find interesting. It might be a bit technical, but please ask questions if I use jargon you don't understand. Also, if you're an expert and I write something you want to comment on, please do (especially if something I write is misleading or just plain wrong).
The topics I've chosen below just happen to be what I found memorable. I made no attempt to choose these topics by any sort of theme. I apologise if I've missed anything particularly interesting. Perhaps if you were there and think I missed out something interesting you can either mention it in the comments or write a guest post for us.
Neutrinos and precision cosmology
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| One of the first images captured by the Dark Energy Survey. The more interesting images it will take will be of very distant galaxies and won't look anywhere near as nice. This one is just for people to put in their blogs. |
Jan Hamann gave a talk on the future constraints that cosmology will provide for neutrino physics. I was pleasantly surprised by the power of large scale structure probes, such as Euclid.
We know from particle physics experiments that the difference between the masses of two of the neutrinos is more than 0.06 electron volts. This means that the heaviest neutrino must be heavier than 0.06 electron volts.
Monday, September 10, 2012
The human machine: coding and decoding
The previous post in this series can be found here.
It's been an exciting week for molecular biologists, and should have been for everyone else too! This week, the Encyclopaedia of DNA Elements programme has revealed its first results about the role of the 99% of the human genome that has, until now, represented a fairly sizeable gap in our understanding of how DNA works. This has made big waves in biological circles and has to some extent penetrated the mainstream media, on the BBC for example, but I thought I'd herald this great work by giving you a brief explanation of what DNA is, how it works, and why so much of it was a bit of mystery until now.
From humble beginnings
DNA is unbelievably complex yet unbelievably simple at the same time. The principles upon which it is based are extremely simple: a string of code made up of four chemical units (called nucleotides: G, C, A and T) on two intertwined strands where units on opposite strands are paired either G:C or A:T. The complexity that arises from such a basic principle emerges much in the same way that vastly complicated computer programs can emerge from the binary 1 and 0 system of computer code; principally, that it is a code there to store information that, when read correctly, is vast. And when you have around 3 billion units of this code in every cell in your body, that vastness can quickly become unfathomable!
Nonetheless, we've come a hell of a long way in the last 60 years. It was only in 1952 that the Hershey-Chase experiment conclusively demonstrated that DNA and not protein, as had also been suggested, was the information-carrier of the cell. Just a year later Watson, Crick, and Franklin discovered the now famous 'double-helix' structure of DNA, and the race was well and truly under way to decipher this mysterious molecule.
Wednesday, July 4, 2012
A Higgs Hunter's story...
[Note from Shaun: Here is Higgs hunter Mikko Voutilainen's account of the recent search for the Higgs. You can find the teaser to this post here. And my own, partially cynical, but ultimately upbeat, account of Higgs-things, here.]
Here it is, finally
[I assume the readers of this blog are somewhat familiar with the Higgs boson; if not, there's a nice summary on the CMS pages here]
So, this is the follow-up to the teaser I wrote a week ago. Now that everybody knows we found a Higgs boson at \(125.3\pm 0.6\) GeV, I'm free to talk about our finding, what it means and how we got there. Note the intentional use of 'a' Higgs there: although we, beyond reasonable doubt (less than one in a million chance of an error, to be precise), found a new particle, it's not 100% sure yet if it's *the* Higgs boson predicted by the standard model, or one of its many twins predicted by the hundreds of theories out there. There's even a tiny chance of it being an altogether different particle yet.
We actually already know a fair deal about this new particle besides the rather impressively precise estimate of its mass: it seems to be produced at a rate that matches the standard model prediction within about 20% uncertainty, it decays into bosons (W, Z and photon) and fermions (b-quarks and tau-leptons) roughly in the ratios predicted by the standard model, and in particular it decays into W and Z bosons in the ratio predicted by the standard model. The last point is rather important, because the Higgs mechanism, and the Higgs boson along with it, was invented to give mass to the W and Z bosons, and leave the photon massless. This also fixes the ratio of the decay rates to W and Z. If the new particle didn't decay into Z's and W's in just the right ratio, it couldn't be the Higgs boson we predicted.
We've also had a stab at determining the more abstract properties of the particle such as a quantum number called parity, but the statistics are low and the results still inconclusive. Predictions say we should be able to tell by the end of the current run, when we've collected 2--3 times the amount of data we have now. At this point we should also have more precise determination of the particle's decay rates in all the different channels, in order to gain more confidence in calling the particle a Higgs boson or something else.
So, is this the end, or the beginning of something new? I'm really hoping for the latter. If the new particle turns out to be 'just' the standard model Higgs boson and there's nothing new to be found, that would be fairly boring. If instead it's a Higgs twin, we may have just opened a window into a new landscape of particles.
At the moment it's too early to tell for sure, but there are a few interesting features to the way the new particle decays. It seems to decay into photons more often than expected, and to tau-leptons less often than expected. Taking all the decays to fermions together, they only seem to add up to about half of the rate predicted by the standard model, albeit with an error of about 50% as well. That coincidence is causing a bit of excitement nevertheless.
It might not be too bad for the standard model, though, it could just indicate that it's 'non-minimal'. While the Higgs coupling to W and Z is pretty tightly constrained, all the other particle masses are more of an ad-hoc addition to the theory, and there's some freedom to adjust how these particles couple to the Higgs boson without breaking everything else. Another good example of something that would require a 'non-minimal' standard model are the neutrino masses, which in the simplest expectation are exactly zero. We now know they are not zero, although we've still to nail down exactly how much they weigh (it's very very little in any case).
What for me was most interesting in this was to see first-hand how things have evolved towards a big discovery. Things started rolling about six months ago, when the first results from LHC Higgs boson searches were presented last December. Back then both ATLAS and CMS saw a hint of a Higgs at 125 GeV, with about 2-2.5 sigma statistical confidence. If you were a Higgs-believer, you could have given the signal more than 95% chance of being true.
After December it was decided that we wouldn't look at the 2012 data in the signal region before we had enough to confirm or refute the hint seen in 2011. This process is called blinding, and its important for making sure the analyzers are not unconciously affected by their prior expectations. Blinding is also one of the reasons we've tried to keep a lid on the results until today's seminar so that the experiments would not affect each other's findings between opening their signal box opening and presenting the final results. I think we were fairly successful in the end, although rumors started circulating on the blogs within days, and by yesterday almost every major newspaper (including Nature) had run a story on Higgs.
Between opening the signal box and seeing the first evidence of a new particle there was a whole lot of work going on for 2--3 weeks to prepare for ICHEP. The analyses added around 50% more data, the particle properties were studied in more detail, the CMS management had regular meetings with both ATLAS and CERN directors, people were working day and night to scrutinize the results, prepare documentation, etc. The final days were spent polishing plots, rehearsing presentations and fine-tuning press releases. Although I didn't happen to be at CERN during that period (I did attend the signal box opening in the beginning, though), I could at least participate through the almost daily video meetings and by keeping my own small piece of CMS running (I'm responsible for a team calibrating jets).
Just two days prior to the seminar there was also a presentation of the Tevatron Higgs results at Fermilab. The Tevatron people had done a superb job in squeezing every last bit of sensitivity out of their data and fell just a hair's width short of claiming evidence for the Higgs (they got 2.94 sigma by the most optimistic count, and needed 3.0). The Tevatron experiments collected data for ten years before shutting down last summer, and have the same amount of data (10 fb-1) available for analysis as the LHC experiments now. The lower collision energy of the Tevatron, 2 TeV versus 8 TeV at LHC, means roughly ten times less Higgs bosons are produced, but they still have better sensitivity in one single channel, the Higgs decaying into two b-quarks. I was watching that live on video, too, cheering for my old colleagues (I did my PhD on D0, one of the two experiments at the Tevatron).
And then, finally, today we had a chance to see how our colleagues and rivals at ATLAS were doing with their Higgs search. According to blog rumors, newspaper leaks and sensitivity estimate just a tad behind CMS, but never far. As it turned out, both CMS and ATLAS came up with the same significance in the end, within 0.1 sigma precision. Both experiments have now just made it to the 5-sigma milestone, and it's pretty clear that the signal has been effectively confirmed by at least three experiments (counting D0 and CDF together as a single Tevatron experiment).
P.S. I wrote a lengthy story about the box opening the same evening when I was at CERN, and stored it on a time capsule on my e-mail account. I'm not sure if it's interesting anymore, but at least I shouldn't be breaking any confidentiality rules by releasing it. [Shaun speaking: I now have this item in my possession, so if anyone wants to see it please let me know and I will upload it in a few days.]
Here it is, finally
[I assume the readers of this blog are somewhat familiar with the Higgs boson; if not, there's a nice summary on the CMS pages here]
So, this is the follow-up to the teaser I wrote a week ago. Now that everybody knows we found a Higgs boson at \(125.3\pm 0.6\) GeV, I'm free to talk about our finding, what it means and how we got there. Note the intentional use of 'a' Higgs there: although we, beyond reasonable doubt (less than one in a million chance of an error, to be precise), found a new particle, it's not 100% sure yet if it's *the* Higgs boson predicted by the standard model, or one of its many twins predicted by the hundreds of theories out there. There's even a tiny chance of it being an altogether different particle yet.
We actually already know a fair deal about this new particle besides the rather impressively precise estimate of its mass: it seems to be produced at a rate that matches the standard model prediction within about 20% uncertainty, it decays into bosons (W, Z and photon) and fermions (b-quarks and tau-leptons) roughly in the ratios predicted by the standard model, and in particular it decays into W and Z bosons in the ratio predicted by the standard model. The last point is rather important, because the Higgs mechanism, and the Higgs boson along with it, was invented to give mass to the W and Z bosons, and leave the photon massless. This also fixes the ratio of the decay rates to W and Z. If the new particle didn't decay into Z's and W's in just the right ratio, it couldn't be the Higgs boson we predicted.
We've also had a stab at determining the more abstract properties of the particle such as a quantum number called parity, but the statistics are low and the results still inconclusive. Predictions say we should be able to tell by the end of the current run, when we've collected 2--3 times the amount of data we have now. At this point we should also have more precise determination of the particle's decay rates in all the different channels, in order to gain more confidence in calling the particle a Higgs boson or something else.
So, is this the end, or the beginning of something new? I'm really hoping for the latter. If the new particle turns out to be 'just' the standard model Higgs boson and there's nothing new to be found, that would be fairly boring. If instead it's a Higgs twin, we may have just opened a window into a new landscape of particles.
At the moment it's too early to tell for sure, but there are a few interesting features to the way the new particle decays. It seems to decay into photons more often than expected, and to tau-leptons less often than expected. Taking all the decays to fermions together, they only seem to add up to about half of the rate predicted by the standard model, albeit with an error of about 50% as well. That coincidence is causing a bit of excitement nevertheless.
It might not be too bad for the standard model, though, it could just indicate that it's 'non-minimal'. While the Higgs coupling to W and Z is pretty tightly constrained, all the other particle masses are more of an ad-hoc addition to the theory, and there's some freedom to adjust how these particles couple to the Higgs boson without breaking everything else. Another good example of something that would require a 'non-minimal' standard model are the neutrino masses, which in the simplest expectation are exactly zero. We now know they are not zero, although we've still to nail down exactly how much they weigh (it's very very little in any case).
What for me was most interesting in this was to see first-hand how things have evolved towards a big discovery. Things started rolling about six months ago, when the first results from LHC Higgs boson searches were presented last December. Back then both ATLAS and CMS saw a hint of a Higgs at 125 GeV, with about 2-2.5 sigma statistical confidence. If you were a Higgs-believer, you could have given the signal more than 95% chance of being true.
After December it was decided that we wouldn't look at the 2012 data in the signal region before we had enough to confirm or refute the hint seen in 2011. This process is called blinding, and its important for making sure the analyzers are not unconciously affected by their prior expectations. Blinding is also one of the reasons we've tried to keep a lid on the results until today's seminar so that the experiments would not affect each other's findings between opening their signal box opening and presenting the final results. I think we were fairly successful in the end, although rumors started circulating on the blogs within days, and by yesterday almost every major newspaper (including Nature) had run a story on Higgs.
Between opening the signal box and seeing the first evidence of a new particle there was a whole lot of work going on for 2--3 weeks to prepare for ICHEP. The analyses added around 50% more data, the particle properties were studied in more detail, the CMS management had regular meetings with both ATLAS and CERN directors, people were working day and night to scrutinize the results, prepare documentation, etc. The final days were spent polishing plots, rehearsing presentations and fine-tuning press releases. Although I didn't happen to be at CERN during that period (I did attend the signal box opening in the beginning, though), I could at least participate through the almost daily video meetings and by keeping my own small piece of CMS running (I'm responsible for a team calibrating jets).
Just two days prior to the seminar there was also a presentation of the Tevatron Higgs results at Fermilab. The Tevatron people had done a superb job in squeezing every last bit of sensitivity out of their data and fell just a hair's width short of claiming evidence for the Higgs (they got 2.94 sigma by the most optimistic count, and needed 3.0). The Tevatron experiments collected data for ten years before shutting down last summer, and have the same amount of data (10 fb-1) available for analysis as the LHC experiments now. The lower collision energy of the Tevatron, 2 TeV versus 8 TeV at LHC, means roughly ten times less Higgs bosons are produced, but they still have better sensitivity in one single channel, the Higgs decaying into two b-quarks. I was watching that live on video, too, cheering for my old colleagues (I did my PhD on D0, one of the two experiments at the Tevatron).
And then, finally, today we had a chance to see how our colleagues and rivals at ATLAS were doing with their Higgs search. According to blog rumors, newspaper leaks and sensitivity estimate just a tad behind CMS, but never far. As it turned out, both CMS and ATLAS came up with the same significance in the end, within 0.1 sigma precision. Both experiments have now just made it to the 5-sigma milestone, and it's pretty clear that the signal has been effectively confirmed by at least three experiments (counting D0 and CDF together as a single Tevatron experiment).
P.S. I wrote a lengthy story about the box opening the same evening when I was at CERN, and stored it on a time capsule on my e-mail account. I'm not sure if it's interesting anymore, but at least I shouldn't be breaking any confidentiality rules by releasing it. [Shaun speaking: I now have this item in my possession, so if anyone wants to see it please let me know and I will upload it in a few days.]
Tuesday, July 3, 2012
On its own, a Higgs discovery would be grim (II)
[...continued from yesterday]
This is like, instead of mapping the entire globe, the ocean explorers found that they had simply reached the edge of all navigable land, and, as far as any vessel could see, beyond that, there was just apparently endless ocean. The explorers would know, from measuring the curvature of the Earth, that Earth was a globe and had a finite extent, but the radius of the Earth would be so enormous that they would never be able to come close to traversing it by boat. It would also be as if, on the last few islands this civilisation discovered, there were all sorts of indications that there must be new land out there somewhere. Only there was no way of knowing where, or how far away, it was. The Higgs, for these explorers, would be one, last, island, discovered far into the wilderness of this ocean, farther from the mainland than anything else except the top quark (another island, alone in the wilderness). To reach either island would require the finest ship imaginable and would require a journey of decades.
Such a civilisation would be left to wonder, 'what is it that is out there in that wilderness?' But, they would be unable to answer their question until the invention of the aeroplane hundreds of years later. The next land might be just over the horizon, or it could be on the other side of the globe. This world, is where particle physics will find itself if the LHC finds the Higgs and nothing else.
The LHC's great, great grandparent in this journey of exploration was Ernest Rutherford who fired alpha particles at gold and discovered the atomic nucleus. Where Rutherford was the first of this kind, the LHC (or ILC) might be the last. For just over 100 years, collision experiments have been one of the driving forces of fundamental physics. The photos interspersed throughout this post show a collection of some of the more famous colliders during this period. But, just as the days of the ocean explorer had to eventually come to an end and the romantic tales of discovery that came with them ceased to be written, so might we have to fare colliders well and accept that the Higgs is the last of its kind.
If such an event occurs, a thought should be spared for all the map-makers of this oceanic world (the theoretical physicsists of the last thirty years), who, for decades, have built ever more complicated maps showing that Higgs island would not be alone. They had fascinating and compelling arguments for why Higgs island should be surrounded by exotic new islands, completely different to anything we've encountered before, many maps even showed new continents. The map-makers will have built entire careers making those maps, but if the islands and continents turn out not to be next to Higgs island, they're simply not there; however much we thought they should be. Of course, these continents may very well still exist, somewhere out of HMS Large Hadron Collider's range, but the map makers themselves would never get the chance to know.
This is like, instead of mapping the entire globe, the ocean explorers found that they had simply reached the edge of all navigable land, and, as far as any vessel could see, beyond that, there was just apparently endless ocean. The explorers would know, from measuring the curvature of the Earth, that Earth was a globe and had a finite extent, but the radius of the Earth would be so enormous that they would never be able to come close to traversing it by boat. It would also be as if, on the last few islands this civilisation discovered, there were all sorts of indications that there must be new land out there somewhere. Only there was no way of knowing where, or how far away, it was. The Higgs, for these explorers, would be one, last, island, discovered far into the wilderness of this ocean, farther from the mainland than anything else except the top quark (another island, alone in the wilderness). To reach either island would require the finest ship imaginable and would require a journey of decades.
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| The Large Electron Positron Colllider (most accurate measurer of the precision electroweak parameters) |
Such a civilisation would be left to wonder, 'what is it that is out there in that wilderness?' But, they would be unable to answer their question until the invention of the aeroplane hundreds of years later. The next land might be just over the horizon, or it could be on the other side of the globe. This world, is where particle physics will find itself if the LHC finds the Higgs and nothing else.
The LHC's great, great grandparent in this journey of exploration was Ernest Rutherford who fired alpha particles at gold and discovered the atomic nucleus. Where Rutherford was the first of this kind, the LHC (or ILC) might be the last. For just over 100 years, collision experiments have been one of the driving forces of fundamental physics. The photos interspersed throughout this post show a collection of some of the more famous colliders during this period. But, just as the days of the ocean explorer had to eventually come to an end and the romantic tales of discovery that came with them ceased to be written, so might we have to fare colliders well and accept that the Higgs is the last of its kind.
If such an event occurs, a thought should be spared for all the map-makers of this oceanic world (the theoretical physicsists of the last thirty years), who, for decades, have built ever more complicated maps showing that Higgs island would not be alone. They had fascinating and compelling arguments for why Higgs island should be surrounded by exotic new islands, completely different to anything we've encountered before, many maps even showed new continents. The map-makers will have built entire careers making those maps, but if the islands and continents turn out not to be next to Higgs island, they're simply not there; however much we thought they should be. Of course, these continents may very well still exist, somewhere out of HMS Large Hadron Collider's range, but the map makers themselves would never get the chance to know.
Monday, July 2, 2012
On its own, a Higgs discovery would be grim
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| Rutherford, with the first ever particle collider |
Why the Higgs is cool
If rumours are to be believed, then, in two days time, CERN will announce the discovery of a new particle and it will be called Higgs. To the degree that the discovery of any new particle is a pretty big deal, this will be a pretty big deal.
To put things into perspective, not only will this be the discovery of an entirely new particle, if the standard model of particle physics is correct, this will also be the discovery of an entirely new fundamental particle. That is, it won't be made up of any constituent pieces. Also, the field that it will be excited from will not have been directly detected ever before. And that's not even it. Other aspects of the Higgs are also completely new. For example, the way it behaves when you rotate it will be unique amongst all the fundamental particles we've discovered so far, which is quite curious because its rotational properties will be the simplest (i.e. it has no spin at all).
So, irrespective of everything I'm about to write I want to first stress the following: the discovery of a Higgs-like particle is pretty damn cool and a great achievement of exploration for humanity.
Beyond the hype
However, the Higgs is no God particle and it is not the origin of all the mass in the universe (or even a significant proportion of it). No great mysteries of the universe are about to be solved on Wednesday. The Higg's significance in our understanding of the universe is similar to the understanding gained when the last piece of a jigsaw is finally placed in a puzzle. Placing that last piece produces an enormous amount of cathartic pleasure (more so than any other individual piece). But, the image in the puzzle has become clear long before that final piece is placed. The role the Higgs plays in the standard model of particle physics is to break a certain symmetry in nature, the electroweak symmetry. All the other pieces of this broken symmetry have been found, some quite a long time ago.
Monday, June 25, 2012
The Higgs: To be, or not to be?
[Note from Shaun: The following is a guest post from Higgs Hunter, Mikko Voutilainen. Mikko is a colleague of mine here in Helsinki. He is a postdoc working on the CMS experiment at the LHC in CERN. Below, he rhetorically asks the Higgs boson whether it exists or not. The irony of this is that Mikko asks this question, non-rhetorically, for a living and it is quite possible that he has already received an answer. He cannot (unfortunately) tell us this answer, yet. You should consider the following a teaser for what will follow next Wednesday when CERN unveils its latest results to the world. On that date, Mikko has promised to give us another guest post where he will reveal everything he knows about, The Higgs... (I've even heard rumours that this follow-up post has already been written)]
To be, or not to be?
That's a question for the Higgs boson to answer, and we might know soon enough. CERN just (well, three days ago really, but everybody here was out in the countryside celebrating Midsummer) published a press release about having a seminar on the new results on Wednesday 4th of July.
Coincidence that it's also Independence Day for the folks in the US? Probably yes, although my collaboration, the Compact Muon Solenoid (CMS) experiment at CERN, does have a strong representation from the States, including our spokesperson Joe Incandela.
The real reason, though, is that the 4th of July is also the eve of a major particle physics conference, ICHEP, starting in Melbourne. The ATLAS and CMS experiments will deliver the preliminary results of their 2012 data analysis there, and the seminar will be a kickoff for these presentations (you can see the live broadcast at webcast.cern.ch).
The experiments at the Large Hadron Collider stopped collecting data only on the 18th of June, and everybody is now busily analysing this dataset. We actually collected quite a nice bunch of data, just over 6/fb, which is a bit better than last year. The collision energy was also raised from 7 TeV to 8 TeV, which should increase the production rate of possible Higgs bosons by 20--30%.
People are really eager to see the new results, and for a reason. The data collected in 2011 showed some hints of a Higgs boson in the 124-126 GeV range. The amount of data collected this year is nearly equal to that collected last year so the results are directly comparable. We should be able to see whether the earlier trends are still there, or whether they've gone away. Either way, it should be pretty exciting.
The predictions made earlier indicate that a combination of the 2011 and 2012 datasets should get pretty close to five sigma, the traditional standard for a discovery in the field. Or, we should be able to rule the existence of the Higgs boson out at a 95% confidence level from the whole remaining mass window.
What happens in a week depends both on the hard work of the physicists, who are improving the sensitivity of their analysis, and, due to statistical fluctuations, pure luck. If we're unlucky, the existence of the Higgs boson may still remain a mystery, but if we're lucky, we might end the quest earlier than expected.
So, what if we find the Higgs or not? Is it the answer to Life, the Universe, and Everything? Or a piece in the puzzle of the origin of mass for the elementary particles? The latter, more likely.
If we find that the Higgs boson lacks existence, much of the theoretical work done in particle physics for the past few decades will end up in the dustbin. It's not all that bad, really, because it will allow the theorists to start from a clean slate, and that's often been a very fruitful thing. The experimentalists will continue to hunt for other particles that could replace the Higgs boson.
If the Higgs boson is found, it's properties will have to be scrutinized carefully. There are many theories out there besides the Standard Model of particle physics that predict the Higgs boson (or bosons) so determining it's precise identity might take a while. Many of the alternative theories also predict other particles, leaving plenty of work to be done for the experimentalists.
[Note: Mikko writes for a Finnish language blog, Higgs Hunters. This post is an English translation of his latest post at Higgs Hunters.]
To be, or not to be?
That's a question for the Higgs boson to answer, and we might know soon enough. CERN just (well, three days ago really, but everybody here was out in the countryside celebrating Midsummer) published a press release about having a seminar on the new results on Wednesday 4th of July.
Coincidence that it's also Independence Day for the folks in the US? Probably yes, although my collaboration, the Compact Muon Solenoid (CMS) experiment at CERN, does have a strong representation from the States, including our spokesperson Joe Incandela.
The real reason, though, is that the 4th of July is also the eve of a major particle physics conference, ICHEP, starting in Melbourne. The ATLAS and CMS experiments will deliver the preliminary results of their 2012 data analysis there, and the seminar will be a kickoff for these presentations (you can see the live broadcast at webcast.cern.ch).
The experiments at the Large Hadron Collider stopped collecting data only on the 18th of June, and everybody is now busily analysing this dataset. We actually collected quite a nice bunch of data, just over 6/fb, which is a bit better than last year. The collision energy was also raised from 7 TeV to 8 TeV, which should increase the production rate of possible Higgs bosons by 20--30%.
 |
| The amount of data collected in 2010, 2011 and 2012. One fb-1 amounts to almost 100 trillion proton-proton collisions. |
People are really eager to see the new results, and for a reason. The data collected in 2011 showed some hints of a Higgs boson in the 124-126 GeV range. The amount of data collected this year is nearly equal to that collected last year so the results are directly comparable. We should be able to see whether the earlier trends are still there, or whether they've gone away. Either way, it should be pretty exciting.
The predictions made earlier indicate that a combination of the 2011 and 2012 datasets should get pretty close to five sigma, the traditional standard for a discovery in the field. Or, we should be able to rule the existence of the Higgs boson out at a 95% confidence level from the whole remaining mass window.
 |
| Predictions for the significance of a Higgs signal as a function of the boson mass. The combination of 2011 (5 fb-1, 7 TeV) and 2012 (5 fb-1, 8 TeV) data will correspond to roughly the average of the two red lines. |
What happens in a week depends both on the hard work of the physicists, who are improving the sensitivity of their analysis, and, due to statistical fluctuations, pure luck. If we're unlucky, the existence of the Higgs boson may still remain a mystery, but if we're lucky, we might end the quest earlier than expected.
So, what if we find the Higgs or not? Is it the answer to Life, the Universe, and Everything? Or a piece in the puzzle of the origin of mass for the elementary particles? The latter, more likely.
If we find that the Higgs boson lacks existence, much of the theoretical work done in particle physics for the past few decades will end up in the dustbin. It's not all that bad, really, because it will allow the theorists to start from a clean slate, and that's often been a very fruitful thing. The experimentalists will continue to hunt for other particles that could replace the Higgs boson.
If the Higgs boson is found, it's properties will have to be scrutinized carefully. There are many theories out there besides the Standard Model of particle physics that predict the Higgs boson (or bosons) so determining it's precise identity might take a while. Many of the alternative theories also predict other particles, leaving plenty of work to be done for the experimentalists.
[Note: Mikko writes for a Finnish language blog, Higgs Hunters. This post is an English translation of his latest post at Higgs Hunters.]
Friday, April 13, 2012
Are you racist (even if you don't think you are)?
The video above describes something known in psychology as the Implicit Association Test (IAT). Keon, the dude in the video, does a good job of explaining what the test is and why it is interesting so watch and gain in understanding.
The reason why the IAT is interesting (for me at least) is that it can have quite startling results. The example Keon has chosen for his video is a racism test. Effectively, the test will determine whether the person taking the test has an implicit racism. This is not a conscious, directed, racism that might motivate someone to join a hate-group, or take some deliberate racist action, but instead a sub-conscious, implicit, racism that might direct behaviour that is more instinctive or reactive. In other words, the IAT detects racism that the person being tested won't necessarily even know they have.
The point is, you could be as tolerant and open minded as you want to be, but you live in a society that has racist undertones in it and this will have influenced you. Watch the video, take the test and see how much.
Be prepared to be surprised by the results though (whatever your own race or view on racism happens to be)...
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I happen to be friends with Keon and he posted this on Facebook a few weeks ago. I took the test in the video and had a couple of immediate objections. Firstly I found both of the final stages of the test difficult and, while I suspected I found the second one harder, I wasn't sure. But secondly, and what I thought was far more importantly I suspected strongly that the ordering of the tests conditioned me. In the process of the test I got used to associating good with white and left and bad with black and right. So I suspected that my difficulty in the final stage had more to do with the difficulty in changing than a genuine difficulty in associating good with black faces.
I mentioned this to Keon and he suggested I try either taking the test again, reversing the steps in the video, or even better, going here and taking a randomised version of the test. I chose the second option and took that test multiple times, with different orderings. I also took different versions of the test. Unfortunately, I repeatedly came out of the test having shown either a slight or a moderate preference for white faces, rather than as I would have hoped, no preference for either. This was entirely independent of the ordering of the test. If you have doubts as well then I strongly recommend that you check out the link above.
I don't rationally associate white with good and black with bad. But sub-consciously, I clearly find it easier to think white=good than I do black=good. I'm glad this test made me aware of this. Some people (especially around my realm of natural sciences) like to view the likes of psychology somewhat disparagingly because of its supposed lack of testability. Well, the people who came up with the IAT have done a stellar job of overcome this supposed boundary and have revealed very interesting things about human psychology.
Your thoughts, as always, are welcomed... (are you sub-consciously racist? were you surprised with the result? Do you doubt the effectiveness of the test?)
Keon has made lots of other videos on psychology. His YouTube channel can be found here.
[Edit: Sesh points out in this comment that even if the test does prove the existence of implicit preferences, it is not accurate to use the word "racism" to describe these associations. I have to admit that, despite my use of the word in this post, I do agree. Is this a far call?]
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Monday, April 9, 2012
The ISW mystery III: How did the CMB get so hot?
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| An example map of what the ISW effect would look like on the sky if we could observe it directly (arxiv:1003.0974) |
This is probably my most technical post to date. I don't want my contributions to this blog to be (all the time) popularised, ready to consume, pieces of quirky science. I want you to know what we, the cosmologists, are thinking and wondering about each day, beyond just vague explanations about accelerated expansion, dark energy and dark matter. I want you to understand how we're trying to explore the mysteries of cosmology. What are we measuring, how are we measuring it and what are we hoping it will tell us? Doing this has to be a two-way process. I invest time writing, doing my best to make things like the Integrated Sachs-Wolfe effect understandable and you invest time and concentration trying to understand.
The benefit for you of doing this is great. You will become engaged in the science in real time. When the mystery I'm about to reveal finally gets solved you will already be there waiting, expecting. You will be able to bask in the wonder of the discovery during the moments of discovery. Hence, it will also be your discovery.
You will only get this though, if you concentrate and think and read this post through. So, if you don't get it the first time, think about it and read it again. And, if you are left confused by anything, ask!
Recap of earlier posts
I'm going to describe in this post why there is an “ISW mystery”. I described in this post that the integrated Sachs-Wolfe (ISW) effect is the subtle heating and cooling of light as it passes through over and under-dense regions/structures in the universe. For most of the universe's history this effect was effectively non-existent. Any energy gained by light falling into a structure in the universe was perfectly balanced by the energy lost by the light climbing out of the structure. However, late in the universe's history something starts pushing the universe apart and as a result there is a net energy change. The gravitational well is smaller when the light leaves the structure than when it goes in.
Then, in this post, I explained that the ISW effect is incredibly small. This makes observing it very difficult. We can't observe it directly by looking at light from galaxies, quasars, supernovae, stars, etc. because we don't know the temperature of the light's source well enough. In fact, there is only one source of light that we do know well enough to use it to detect the ISW effect. This is the cosmic microwave background (CMB), which I introduce here. Unfortunately, even the tiny fluctuations in the temperature of the CMB are of the same size as the expected ISW temperature shifts. So, we still can't observe the ISW effect directly. What we can do though, is observe it on average. We know that the ISW effect occurs as light travels through over and under-dense regions in space. So what we can do is look for over and under-densities and ask whether the CMB is hotter on average when it has passed through an over-density and colder on average when it has passed through an under-density. How to find structures in space and what I mean by on average is covered in the post you are about to read...
Friday, March 16, 2012
At the neutrino OPERA, the fat lady is warming up.
If you weren't living under a rock last year, you will remember the neutrinos that seemed to be going faster than the speed of light. Everybody and their cat wrote about it, including me (though my cat only had editorial supervision of the final manuscript).
Earlier this year, OPERA (the experiment that apparently saw faster than light neutrinos) updated their press release informing us that they had spotted a few mistakes in their experiment. The mistake was an unfortunate one, but wasn't in OPERA's actual analysis or method itself. The mistake was basically a piece of faulty hardware. Normally you don't need to know things at a neutrino detector to within nanoseconds, so this fault hadn't been noticed before.
I don't hold OPERA to be at fault for going public with their initial findings. They had checked everything they could think of and still had this seemingly crazy result. If the result stood up to scrutiny and was repeated by other experiments, then they'd made one of the biggest discoveries for a long time, if not ever. Once they had checked everything they could think of, any lingering mistake is far more likely to be found by a group trying to repeat their experiment, than by someone in their collaboration.
I didn't write about this update, mostly because I was travelling, but also because nothing conclusive had yet occurred. OPERA made a mistake, fine, but it wasn't proven yet that this mistake was responsible (of course everyone, myself included, strongly suspected that it was – but we all strongly expected that some mistake had been made, and said so, from the start!).
Today, something genuinely new can be added to the story, due to ICARUS, another neutrino experiment. Ironically, both ICARUS and OPERA are at the same laboratory, at Gran Sasso, and in fact measure exactly the same neutrino beam from CERN. The new piece of the story is that ICARUS has measured the speed of these neutrinos and found them to be consistent with the speed of light, and crucially, inconsistent with the results from OPERA.
Here is the crucial figure from their article:
As a final passing comment I want to add that I am still in awe of the fact that both ICARUS and OPERA could make such a precise measurement of the speed of neutrinos. OPERA's mistake was unfortunate, but it doesn't change how impressive the measurement itself was. It just changes how impressive the result is.
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Earlier this year, OPERA (the experiment that apparently saw faster than light neutrinos) updated their press release informing us that they had spotted a few mistakes in their experiment. The mistake was an unfortunate one, but wasn't in OPERA's actual analysis or method itself. The mistake was basically a piece of faulty hardware. Normally you don't need to know things at a neutrino detector to within nanoseconds, so this fault hadn't been noticed before.
I don't hold OPERA to be at fault for going public with their initial findings. They had checked everything they could think of and still had this seemingly crazy result. If the result stood up to scrutiny and was repeated by other experiments, then they'd made one of the biggest discoveries for a long time, if not ever. Once they had checked everything they could think of, any lingering mistake is far more likely to be found by a group trying to repeat their experiment, than by someone in their collaboration.
I didn't write about this update, mostly because I was travelling, but also because nothing conclusive had yet occurred. OPERA made a mistake, fine, but it wasn't proven yet that this mistake was responsible (of course everyone, myself included, strongly suspected that it was – but we all strongly expected that some mistake had been made, and said so, from the start!).
Today, something genuinely new can be added to the story, due to ICARUS, another neutrino experiment. Ironically, both ICARUS and OPERA are at the same laboratory, at Gran Sasso, and in fact measure exactly the same neutrino beam from CERN. The new piece of the story is that ICARUS has measured the speed of these neutrinos and found them to be consistent with the speed of light, and crucially, inconsistent with the results from OPERA.
Here is the crucial figure from their article:
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| The neutrinos' arrival at ICARUS and OPERA. δt=0 corresponds to when light would arrive. |
The purple bars show the scatter in time when neutrinos were measured to arrive at ICARUS. δt=0 corresponds to when light would arrive. Clearly the ICARUS neutrinos are arriving at the same time as light would. The bars on the right are what OPERA's previous measurements indicated.
So, given that (a) a lot of other (measured!) things in physics would be very difficult to reconcile with something going faster than light (b) We know OPERA have made a mistake that they need to fix and (c) ICARUS is measuring the exact same neutrino beam at the exactly same place and finding the neutrinos to be travelling at light speed, it seems that, in this opera, "the fat lady" might not just be warming up, but could very well be in the final stages of her aria.
But, as Matt Strassler pointed out in his own blog, there is a silver lining to all of this.
In fact, I'm sure this precision timing will be extremely useful in increasing the bit-rate of the neutrinos that have very recently been used to send digital messages through the ground, something that is impossible with any other known form of communication.The experimental particle physics community has learned how to make long-range distance and timing measurements that are more precise and more accurate than were ever possible before. Don’t be surprised if this knowledge turns out to be useful, in some unexpected way, in future experiments.
As a final passing comment I want to add that I am still in awe of the fact that both ICARUS and OPERA could make such a precise measurement of the speed of neutrinos. OPERA's mistake was unfortunate, but it doesn't change how impressive the measurement itself was. It just changes how impressive the result is.
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