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?

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.

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?

Highlights from Beijing: COSMO 2012

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

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.

Why do galaxies rotate?

[Note from Shaun: The following is a guest post from cosmologist Bjoern Malte Schaefer, who works at the University of Heidelberg. He also writes at the blog, Cosmology Question of the Week, which, although aimed at undergraduate and postgraduate students of cosmology, is worth a look for everyone.]

Image illustrating why there is a relationship between angular momentum direction and inclination angle (and hence  the apparent shape of a galaxy)

Why do galaxies rotate?

Galaxies rotate, every child knows that. Looking at the images of grand spiral galaxies it is quite suggestive to think how all the stars and gas that make up a galaxy all move in a more or less orderly fashion about the galaxy's centre. However, when we think about mechanisms through which galaxies can acquire angular momentum the matter seems very obscure: how do they start rotating in the first place? 

The formation of cosmic structure, including galaxies and the larger clusters and superclusters in which they are embedded, is a fluid mechanical phenomenon, where gravity is the only force acting on the  distribution of matter on large scales. It is in fact gravity that amplified the tiny fluctuations present in the primordial distribution of matter that filled the early Universe (matter here means mostly cold dark matter) and caused them to evolve into the large-scale structure that we observe in the present Universe. These tiny fluctuations grew by self-gravity: a region in the matter distribution that is slightly denser than its surroundings generates a gravitational pull and accumulates more matter, hence its density increases with time.

At first sight it is very difficult to imagine how gravity could introduce rotation. After all, on the scales of cosmic structures, gravity is well approximated by the scalar Newtonian potential, which is parity invariant and does not possess any chirality: At which point would a galaxy in the forming decide whether it should rotate clockwise or counterclockwise? The answer to this lies in a process called tidal shearing, which consists in a misalignment between the tidal forces (the second derivatives of the gravitational field) and the moment of inertia of the protogalaxy (the second moments of the matter distribution). The meaning of these technical terms can be explained quite easily: Imagine a curling stone sliding along the sheet, where the ice surface on its left side is a bit smoother compared to the right side. The stone will be set into clockwise rotation by this difference in force. For a protogalaxy, it is the variation in gravitational force moving the galaxy along that introduces rotation, and is responsible for the galaxy's angular momentum.

The ISW mystery III: How did the CMB get so hot?

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...

Congratulations Cosmology

The news today had an almost poetic structure to it, as if it was part of a well crafted story.

Firstly, for all those rock dwellers out there, the Nobel Prize in physics, announced today, went to two groups who, in 1998, turned the world of cosmology upside down. Their discovery subsequently planted a great big question mark in the rest of the world of fundamental physics, for it was the observations made by today's Nobel prize winners that caused the entry into the standard cosmological model of that most mysterious of concepts:

Dark Energy

The observation these groups made is all the more beautiful for how mundane it actually is. They simply made the observation that a bunch of distant supernovae were dimmer than had previously been expected. But, still today, nobody has any good idea what causes this dimming. Subsequent cosmological observations have made it clear that the dimness of these supernovae probably means the expansion of the universe is accelerating, but why? And if it isn't accelerating, what profound property of our universe is so cleverly mimicking this acceleration.

This was great news for cosmology and if the fact that observations of exploding stars can completely change our understanding of the make up of the universe wasn't poetic enough, today also happened to be the day that ESA announced its next wave of big experiments. One of these was Euclid (artist's impression below). The confirmation of Euclid's eventual launch is also great news for cosmology. On the same day that one beautiful cosmological experiment wins the most prestigious prize available to a scientist, another beautiful cosmological experiment is announced.

What is ESA's stated goal for Euclid? Nothing else but:

To understand the nature of dark energy and dark matter by accurate measurement of the accelerated expansion of the Universe through different independent methods.

And so, while today the big news headlines were of the Nobel Prize being awarded to the two groups who pointed out an enormous cosmological mystery, it is not outside the realm of possibility that hiding subtly in the news background the experiment that will solve this mystery was also today finally made a certainty.

You couldn't write a better script if you tried.

An artist's impression of the Euclid satellite

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