Monday, February 27, 2012

The ISW mystery II: Trying to see the invisible

[Note: I'm travelling at the moment and haven't had time to write as substantial a post this time around as I'd hoped. We're all new to this blogging business, so next time I'm travelling for this long I'll plan further ahead and have the post ready in advance, or something. Anyway, enough excuses from me... I'll be back in Helsinki in a few weeks and will hopefully then be able to write something more substantial...]

In my last post I introduced something known as the integrated Sachs-Wolfe (ISW) effect. You'll probably get more out of today's post if you've read that one. However, I've tried to make today's post as self-contained as possible, so don't fret if you're new to the blog or have forgotten things over the last six weeks.

Put most simply, the ISW effect is the very subtle heating and cooling of light as it travels through structures in the universe. In the standard model for the universe's history this ISW effect grows with time and is most significant when dark energy starts to dominate the universe late in its history. The effect occurs because the energy gained or lost by light as it climbs into or falls out of structures becomes smaller with time. Therefore light receives an overall change in energy when it travels through these structures.

Unfortunately, the ISW effect is tiny. It will happen to any light travelling anywhere through the universe, but it is really, really tiny. This means that, for almost every light source in the universe (galaxies, stars, supernovae, etc), we just don't know the initial light source well enough to be able to tell if it has changed by the tiny amount we expect from the ISW effect. But, there is one source for which we have a very clear, very precise prediction. This is the cosmic microwave background, or CMB (note: I introduced the CMB in this post). As regular readers of the blog might be starting to appreciate, the CMB is more or less every cosmologist's favourite data source.

Unfortunately even the CMB has tiny fluctuations in it. These arise because the source of the CMB, a plasma of hydrogen that once permeated the entire universe, was not uniform (I explained the shape of the fluctuations in the CMB in an earlier post). And, most unfortunately, even these tiny fluctuations, fluctuations so small that Nobel prizes were awarded for their detection, are bigger approximately the same size as [Edit: 16/3/2012] the predicted size of the ISW effect. I have to admit that I find this irony amusing. The ISW effect is so small that one of the most significant measurements humanity has ever made is just annoying noise in the quest to detect it.

Alas! So it seems that we can't even see the ISW effect in the CMB?

Not quite...

The thing is, we know the source of the ISW effect. It is caused by structures in the universe. If we could see those structures then we wouldn't just know that there is an ISW effect, we would also know on which points of the sky it should be bigger and on which points of the sky it should be smaller. This is crucial. At any point of the sky where we expect the ISW effect to make the CMB hotter/smaller there is no reason we should also expect the CMB itself to start off hotter/smaller. So, while we can't measure the ISW effect directly we can ask whether on average the CMB is hotter where we expect a hot ISW effect and whether on average the CMB is colder where we expect a cold ISW effect.

But can we see structures in the universe? Well, yes and no. We can of course see some things in the universe (or the sky would be dark at night!). But, unfortunately, most of the stuff in the universe can't be seen at all. The common name for this unseeable stuff is dark matter, but a much more appropriate name would be transparent matter. This is because dark matter isn't really “dark” at all, instead it's completely and utterly invisible. It doesn't emit light and any light emitted by something else goes right through it. Thankfully, dark matter does interact gravitationally (in fact, that's how we know it's there!). Now, gravity causes matter to attract matter. As a result, all the visible things in the universe tend to follow the dark matter around. If there is more dark matter somewhere, there is a greater probability that we will see stuff there, if there is less dark matter, there's a smaller probability we will see stuff. So that's the 'yes' and the 'no' of whether we can see the structures of the universe. No, we can't see every structure in the universe, but yes, we can see something else that on average follows these structures around.

And that's it. Attempting to detect the ISW effect is as simple as looking at the CMB and checking whether on average it is hotter in patches of the sky that have more luminous matter behind them and colder on patches where there is less luminous matter.

The results of such measurements comes next... (Now continued here)

Twitter: @just_shaun


  1. I've been thinking a lot recently about the hinge between 'visibility' and 'invisibility' and this post seems to suggest that the method of physics is in line with the conclusion I reached, too: that any attempt to track the invisible actually focuses with a real intensity on the VISIBLE. It's an interesting contradiction. Look forward to the next installment..

    1. The only other option in tracking the invisible is to listen to it, which is also quite common, though less common. Technically, I suppose you could smell, feel or taste it too, but it is difficult to do any of that quantitatively, or at all for things not on Earth. Sound and light is much easier to quantify.

      Future gravitational wave detectors will be recording the invisible with something that is also invisible. It is often written that a GW detector will be our way of listening to the universe, something I've always found quite eloquent and elegant.

      And technically speaking a lot of the stuff we already measure is "invisible", that is to say unable to be seen by a human eye (either because it is too faint or it is outside the visible spectrum). But, in reality it is still light that the detector is detecting, so it isn't really invisible.

      The dark matter I mention in the post is, however, most definitely invisible - though not silent.

    2. Actually, I suppose cosmic rays, which are particles traversing space and occasionally colliding with Earth, are our way of feeling space. Some of the rays *are* just very energetic light (as light gets more energetic it becomes more like a particle and less like a wave), but others are more genuine particles like protons, electrons, positrons and anti-protons (and even neutrinos).

      When cosmic rays hit a detector one isn't really "seeing" a distant source so much as being hit by something it threw at us.

      Taste and smell, though, I think will always be out of the question for measuring astrophysical sources.

    3. But isn't smell is the result of small particles of something hitting our senses? So physics probably has a version of smell. I'm glad to know there is a way of "feeling space" - what could be more appropriate, indeed necessary!, than for physics to have a haptic method.

      When art and artists try to 'see' or sense the invisible, it's precisely the full range of sensory that is activated. Interesting way of putting it - maybe this is a direction to explore as I continue the posts about scale and abstraction.

    4. I feel I should refer you to this:

      I would certainly be possible to recreate the smell or feel of another part of the Universe by recreating the small molecules or composite structures that are found there. The smells would be a bit boring since most of our olfactory receptors are specific to large organic molecules that are Earth-specific, but the feel aspect could be quite fun. I've heard, for example, that walking on the Moon is a bit like walking on cake because of the low gravity and dryness of the dust.

      Nonetheless, this is more a way of engaging interest in cosmology/astrobiology etc, rather than actually learning anything new.

    5. First, thank you for that.

      Second, I have to quibble with "... rather than actually learning anything new." Au contraire! Are you trying to tell me that the Smell-O-Scope has no aesthetic and epistemic value?! After all, and this is a quote, "there is also experimentation in philosophy" (and art). Smelling Mars - we'd learn a lot!

    6. I tried to write a comment disagreeing, but I talked myself out of it. There is indeed epistemic and aesthetic value in the Smell-O-Scope, but only in the form of the human experience of smelling (I'm pretty sure that's all you mean though anyway). It wouldn't be much value under either label because, as James points out, the smells would be very simple smells, but it certainly raises a tiny amount of wonder inside me when I imagine knowing what areas of the universe "smell like".

      I wonder what the aesthetic implications are of the discovery of giant clouds of ethanol in our own galaxy.

      I suppose in many ways neutrino detectors are kind of like smelling or tasting. They are built in such a way (i.e. underground) that no other particles could get there. Therefore, like taste or smell, where only particular chemicals will stimulate a response, (almost) only neutrinos will be able to stimulate a neutrino detector (though only because nothing else could get that far).

      It's a stretched analogy, but it doesn't hold zero aesthetic worth. There may be better examples, but none of them are coming to me at the moment.

    7. The significance of the human experience is, well, significant! I don't think it would matter how simple the smell was - it would give us some kind of conceptual access to the previously unimaginable.

      Can you post a picture of the giant clouds of ethanol? And speaking of neutrino detectors, did you see this:

    8. Surely you would agree though that there are more significant human experiences and less significant human experiences. My going to sleep tonight for something like the 10,000th time in my life will feel much less significant, to me at least, than when I finish my current research project. Experiencing the smell of space does pale in significance, even for the human experience of space, when compared to knowing what the matter in space is made of and where it came from and where it is going, etc, etc. There will be no *new* smell waiting for us in space, we evolved to smell earth things - space will smell like Earth.

      And I would have to quibble with the idea that smelling space takes it from unimagineable to imagineable. I can imagine things about space pretty well right now... and I'm not certain of this, but my guess would be that methane would be the most common thing in space that has an odour. I know what methane smells like. And claiming that is smells is also a bit far-fetched given that it won't be ion any atmosphere, so inhaling it would be impossible.

      Ethanol "picture"

      It isn't really an image of the ethanol, because the ethanol isn't visible. But the bright stuff is microwave emissions from the ethanol, I think.

    9. Regarding the neutrinos, the fact that a message has been sent using them is definitely cute, but it wasn't really all that difficult (once you have the neutrino beam and detector set up of course!).

      The experiment OPERA did (the one that thought the neutrinos were travelling faster than light) was both more interesting and more difficult. They have basically been sending a message encoded in neutrinos for years. That message was just the shape of the neutrino beam, but they measured that beam incredibly well and if that shape was changed in anyway they would have been able to notice it too.

    10. No edit... grrr..... The sentence in the comment above the one above this one should read...

      "And claiming that it** smells is also a bit far-fetched given that it won't be in** any atmosphere, so inhaling it would be impossible."