Wednesday, June 12, 2013

Cosmological perturbations post-Planck - wrap up




David Lyth, the curvaton and the power asymmetry






Wednesday, June 5, 2013


This is a continuation of posts about the Cosmological Perturbations post-Planck conference being held in Helsinki this week. You can see my introduction post here and my first post from the conference here. If you're a member of the general public and want to understand more, please ask. If you're a cosmologist and want to add anything to what I've written, please add a comment.

The Curvaton

Yesterday I tried to introduce what the curvaton is. We had a few talks yesterday that related to this particular framework for generating the initial perturbations in the density of the universe (the curvaton is named the "curvaton" because, in this framework it is responsible for perturbations in the curvature of space-time). Prior to the Planck release, the curvaton had become quite a popular model because it would be capable of producing a distribution of density perturbations that was almost, but not quite Gaussian. This is quite a technical sounding term, non-Gaussian. To hopefully simplify it a little, I'll say that a Gaussian distribution is the familiar bell-shaped curve of a normal distribution. There is no a priori reason to expect that the distribution of the primordial density perturbations has to be Gaussian, but in many aspects of physics (and statistics in general) a Gaussian distribution does turn out to be the default.

However, even before Planck, we knew that the distribution of primordial density perturbations was close to Gaussian. Planck was going to be capable of measuring this distribution even more accurately and thus would be sensitive to even more subtle deviations from a Gaussian distribution. Ordinary inflation predicts a deviation from a perfect Gaussian distribution that would have been too small to detect with Planck. And, the WMAP satellite's measurements of the CMB provided a small amount of evidence that the perturbations did deviate from being Gaussian. This would have been fascinating to discover, and if WMAP's best-fit distribution had been true, Planck would have detected it beyond "all reasonable doubt".

Unfortunately, as we all know now, Planck found that WMAP's evidence was (probably) a statistical fluke (they do happen). So where, does that leave the curvaton?

David Wands gave a talk addressing precisely this question. I suppose the spirit of this talk (and a few others so far) could be summed up by one sentence on one of David's slides: "absence of evidence is not evidence of absence". This sounds like a cop-out, and of course to a certain degree it is. I'm certain David would have preferred to have been giving his talk in the context of a definitive detection of a slightly non-Gaussian distribution of density perturbations. He could tell us which specific curvaton models are favoured, which are ruled out, what needs done to tell the curvaton apart from other mechanisms that can generate non-Gaussianity, etc. On the other hand, the quoted sentence is also true. While the curvaton could generate detectably non-Gaussian perturbations, it could also generate perturbations that Planck wouldn't have distinguished from Gaussian ones.

However, the situation now is that there is no (strong) observational evidence that prefers a curvaton type mechanism to simple inflation. It is customary in this sort of situation to appeal to Occam's Razor and say that, in the absence of evidence that distinguishes between them, the simpler model should be preferred. In the case of the curvaton, I think this is probably going to be the community's consensus, for now (though if you're in the community and you disagree, speak up!).

The hemispherical power asymmetry

Having said that, David Lyth spoke yesterday about one of the infamous WMAP anomalies that Planck confirmed. David's choice of anomaly was the "hemispherical power asymmetry". This anomaly comes from the fact that the amplitude of the fluctuations in the temperature of the CMB along one particular line of sight seems to be systematically slightly larger than the amplitude in the opposite direction. I say "systematically" because this larger amplitude seems to persist when you average the CMB over a range of angular scales. Obviously, for any single angular scale there will be a direction of maximum asymmetry, but it wouldn't be expected that this direction of maximum asymmetry would be the same for other angular scales. The anomaly is then a combination of the fact the magnitude of this asymmetry is unlikely in the standard cosmology and the fact that all angular scales seem to have the same maximal direction of asymmetry.

I want to pause for just a moment to stress something for the people outside of the cosmology community who like to dwell on anomalies like this to claim that cosmology needs to be over-turned. This asymmetry is small (of the order of a few percent). The thing is though that Planck (and WMAP before it) has measured the CMB so incredibly accurately that even very small effects can now be noticed with quite strong statistical significance. Therefore, even if it turns out that this hemispherical asymmetry is more than a statistical fluke, this doesn't mean that the universe is very asymmetric. The universe would be almost symmetric, with a small perturbation away from perfect symmetry. It is certainly conceivably possible that some other, very different, model, will replace the current model (many cosmologists desperately hope for this); however whatever this model is it will still describe an almost perfectly symmetric universe, because that's not a theoretical prejudice, that's observed fact!

Back to David Lyth's talk. David started by making a somewhat over the top proclamation (mentioned in a comment in an earlier post about the conference) that the detection of this asymmetry was as important as the detection of the fluctuations in the CMB themselves (by COBE). I would probably back David up that if the asymmetry is not a statistical fluke and is primordial in origin, that it does rank as highly in importance; however, it is not unlikely enough to rule out the possibility that it is a fluke, yet. However, that wasn't the main point of David's talk. He's a theorist so he wanted to explain where the asymmetry might have come from (and in the process try to make a prediction for how to check whether this explanation is true).

Here, fans of the curvaton might have had their interest piqued, because David's explanation needs the curvaton to work. The method he described was originally proposed by Adrienne Erickcek, Mark Kamionkowski and Sean Carroll (EKC). Thankfully, Sean is also a blogger and has written a blogpost about this method. You should check it out.

I will try to give my own description later, but David did have a clear consistency relationship that would be satisfied if the curvaton and EKC method was responsible for the asymmetry...

The final post of the conference now appears here...

Twitter: @just_shaun

Tuesday, June 4, 2013

Cosmological perturbations post Planck (CPPP) I

This week Helsinki is hosting a conference on the theoretical implications of the recent results from the Planck satellite. The official theme of the conference is cosmological perturbations post Planck. This is alluding to the fact that on large distance scales and at early times, the universe is very homogeneous (it is almost the same everywhere) but has small perturbations in things like its temperature and its density. Planck measures the temperature of the cosmic microwave background (CMB) today, which is an almost direct measurement of the density of the universe soon after the big-bang. This is because the CMB that came from the more dense bits of the universe lost a bit of energy climbing away from that little bit of extra matter and vice versa, the CMB gained energy falling out of less dense regions. Therefore, Planck has made an accurate measurement of the perturbations of the density of the early universe.

But what, are (some of) the implications of this measurement..? Hopefully this conference will elucidate that a little.

I'm going to do my best to describe what is said as the conference proceeds...

Planck's results

The conference started this morning with Helsinki's Mr Planck, Hannu Kurki-Suonio giving an overview of specifically what Planck found in its measurements. If you want a more detailed summary of this you can read some of my posts from when the data was released. The essence is, however, that the standard cosmological model, which had been settled on by most of the community as the simplest model that fits all the pre-Planck data works very, very well in a post-Planck world. There are a number of things about this model that are uncomfortable from a theoretical perspective, but it fits the data we measure extremely well.

But, there are some anomalies (which Hannu ran out of time to cover), which means there are some aspects of the data that aren't predicted by this simplest cosmological model. The anomalies are anomalies because they aren't overwhelmingly statistically significant. This simplest cosmological model only predicts the statistical properties of the perturbations in the universe and all of these anomalies are technically possible, they're just somewhat unlikely. They also don't have obvious explanations from well-motivated new physical effects. They could be statistical flukes. If you have a big enough set of data and look at it in enough different ways you will find anomalies, that's just what noise is. There are two questions that need asked when considering these anomalies:

  • Are there actually more anomalies than we would expect?
  • For each anomaly, is there a well-motivated model that can generate the effect seen without changing all of the many other things that aren't anomalous (either by completely replacing this simplest cosmological model, or by tweaking it in some way)?

The first question is almost impossible to answer. There are too many ways of looking at a data set this big and it's just too hard to quantify all the ways in which is isn't anomalous. This leaves us with just the second question. The reason why these anomalies are called anomalies is that we weren't expecting things like this and the reason for that is that none of the things we thought were well-motivated deviations from the simplest cosmological model predicted these things.

That's the playing field at the beginning of the conference.

The Curvaton

Many of today's talks were on the topic of the curvaton.

It's going to be hard for me to describe to you what the curvaton is given that I haven't ever properly told you what the inflaton is, but I'll give you a quick whirlwind introduction of both. The inflaton is the field that drove something called inflation. Inflation is a (hypothetical) period early in the history of the observable universe when the universe's expansion accelerated. This period is thought to have happened because it would have smoothed out any pre-exisiting inhomogeneities in the universe (in its density, in the curvature of space-time, in the number density of exotic types of matter, etc). There are issues with this because inflation needs the universe to be somewhat homogeneous even to get started, but despite that inflation still definitely leaves the universe more homogeneous than it found it, so at the very least it helps.

But, the thing that is most interesting about this potential inflationary period is that it would also seed very small perturbations in the otherwise homogeneous universe it left behind. This doesn't sound like much of a gain. Without inflation the problem was that there might be too much inhomogeneity, why should we celebrate this small amount of inhomogeneity inflation leaves behind? The answer to that is that, for a given inflationary model we can actually predict the statistical distributions of these post-inflation perturbations. This gives us something to measure and then compare to theory. In other words, we can gain evidence for or against inflation through observation. There may have been inhomogeneities around in the universe before inflation but we have (almost) no way of predicting them. Inflation lets us make predictions and test them.

So, what is the curvaton in all of this? Well, in the simplest models of inflation there is only one thing other than space-time that is around during inflation. This is the inflaton, the field driving this accelerated expansion. In a curvaton model, there is still an inflaton driving this expansion, but there is also as least one other thing around, the curvaton. And, in these models, it is the curvaton that produces the perturbations that we observe today. The inflaton still produces perturbations, but they decay over-time and the curvaton's don't (as quickly).

Why is this interesting? Why should one study a curvaton model?

That's a very good question. The first, not-quite-completely-joking answer I can give is because you can. It is a possible reality for the universe. This is what theoretical physics is about, thinking about what is possible and exploring the observational consequences if the possible were real. So, from that perspective, why just assume that, if inflation occurred, that a curvaton field wouldn't be present? The counter to this perspective is that it adds complexity to inflationary models.

 Unfortunately, it is now past midnight. More to come tomorrow...

Saturday, June 1, 2013

Cosmological perturbations post-Planck (conference)

Helsinki, as it looked when the Planck data was released, less than three months ago. (Image credit: Samuel Flender's facbook photos)
Hello people reading this (present and distant future). Next week, we're hosting a conference here in Helsinki. On balance, I enjoyed covering the last conference I attended (it was demanding, but rewarding). So, I'll cover this one too. This means, each day I'll try to write a summary of what I found interesting during the day's talks.

The last conference was very observationally based. It was hosted by ESA and was the first scientific conference after ESA released data (measured by the Planck satellite) on the temperature fluctuations in the cosmic microwave background. The conference next week will be quite different. Next week, we'll mostly be theorists. Of course, there really isn't a cold, hard, dividing line between a "theorist" and an "observer", but nonetheless, this conference will be much more focussed on what the measurements from Planck (and other past and future experiments) mean for the universe and its laws. Whereas that last conference also focussed on what it is Planck actually measured (and how they measured it).

This is quite exciting. Planck's release was something of a bombshell, even if this was just because it seemed to strongly confirm the simplest cosmological model that was designed to fit all the previous data. People weren't (aren't?) so content with that model, and were hoping/expecting for something new that might show us where to look to replace it. However, even if theorists aren't, it seems that Planck is content with the model.

The theoretical cosmology community has now had three months, a quarter of a year, to digest these results. So this conference will be interesting, even just at the very least to see how the community is dealing with the shell-shock from March. However, it will be more interesting to see what models look good, which don't, and where people have adjusted their attentions from and to in this three month period. Should we still be interested in exotic particles potentially being present in the early universe? What about "monopoles" and "domain walls"? What inflationary models are still appealing and which are on their way out? Why is everyone suddenly talking about primordial magnetic fields? If the universe is a little asymmetric, what caused the asymmetry?

That's the sort of thing to be looking out for next week!

Take a look at the programme for the conference. Whether you are a member of the general public or another cosmologist, if there is anything you see that you are interested in let me know and I'll make sure to pay particular attention to that talk and summarise it here afterwards. Absent from reader's suggestions I will write about the things I generally find interesting, anything that might have some sort of human interest value and things that get a lot of discussion (either during the talk, or afterwards). The more you interact (whether you are an expert or a member of the public), the closer to what you find interesting my blogging will be.