WMAP results — cosmology makes sense!

I’ll follow Mark’s suggestion and fill in a bit about the new WMAP results. The WMAP satellite has been measuring temperature anisotropies and polarization signals from the cosmic microwave background, and has finally finished analyzing the data collected in their second and third years of running. (For a brief explanation of what the microwave background is, see the cosmology primer.) I just got back from a nice discussion led by Hiranya Peiris, who is a member of the WMAP team, and I can quickly summarize the major points as I see them.

WMAP spectrum

  • Here is the power spectrum: amount of anisotropy as a function of angular scale (really multipole moment l), with large scales on the left and smaller scales on the right. The major difference between this and the first-year release is that several points that used to not really fit the theoretical curve are now, with more data and better analysis, in excellent agreement with the predictions of the conventional LambdaCDM model. That’s a universe that is spatially flat and made of baryons, cold dark matter, and dark energy.
  • In particular, the octupole moment (l=3) is now in much better agreement than it used to be. The quadrupole moment (l=2), which is the largest scale on which you can make an observation (since a dipole anisotropy is inextricably mixed up with the Doppler effect from our motion through space), is still anomalously low.
  • The best-fit universe has approximately 4% baryons, 22% dark matter, and 74% dark energy, once you combine WMAP with data from other sources. The matter density is a tiny bit low, although including other data from weak lensing surveys brings it up closer to 30% total. All in all, nice consistency with what we already thought.
  • Perhaps the most intriguing result is that the scalar spectral index n is 0.95 +- 0.02. This tells you the amplitude of fluctuations as a function of scale; if n=1, the amplitude is the same on all scales. Slightly less than one means that there is slightly less power on smaller scales. The reason why this is intriguing is that, according to inflation, it’s quite likely that n is not exactly 1. Although we don’t have any strong competitors to inflation as a theory of initial conditions, the successful predictions of inflation have to date been somewhat “vanilla” — a flat universe, a flat perturbation spectrum. This expected deviation from perfect scale-free behavior is exactly what you would expect if inflation were true. The statistical significance isn’t what it could be quite yet, but it’s an encouraging sign.
  • A bonus, as explained to me by Risa: lower power on small scales (as implied by n<1) helps explain some of the problems with galaxies on small scales. If the primordial power is less, you expect fewer satellites and lower concentrations, which is what we actually observe.
  • You need some dark energy to fit the data, unless you think that the Hubble constant is 30 km/sec/Mpc (it’s really 72 +- 4) and the matter density parameter is 1.3 (it’s really 0.3). Yet more proof that dark energy is really there.
  • The dark energy equation-of-state parameter w is a tiny bit greater than -1 with WMAP alone, but almost exactly -1 when other data are included. Still, the error bars are something like 0.1 at one sigma, so there is room for improvement there.
  • One interesting result from the 1st-year data is that reionization — in which hydrogen becomes ionized when the first stars in the universe light up — was early, and the corresponding optical depth was large. It looks like this effect has lessened in the new data, but I’m not really an expert.
  • A lot of work went into understanding the polarization signals, which are dominated by stuff in our galaxy. WMAP detects polarization from the CMB itself, but so far it’s the kind you would expect to see being induced by the perturbations in density. There is another kind of polarization (“B-mode” rather than “E-mode”) which would be induced by gravitational waves produced by inflation. This signal is not yet seen, but it’s not really a suprise; the B-mode polarization is expected to be very small, and a lot of effort is going into designing clever new experiments that may someday detect it. In the meantime, WMAP puts some limits on how big the B-modes can possibly be, which do provide some constraints on inflationary models.

Overall — our picture of the universe is hanging together. In 1998, when supernova studies first found evidence for the dark energy and the LambdaCDM model became the concordance cosmology, Science magazine declared it the “Breakthrough of the Year.” In 2003, when the first-year WMAP results verified that this model was on the right track, it was declared the breakthrough of the year again! Just because we hadn’t made a mistake the first time. I doubt that the third-year results will get this honor yet another time. But it’s nice to know that the overall paradigm is a comfortable fit to the universe we observe.

The reason why verifying a successful model is such a big deal is that the model itself — LambdaCDM with inflationary perturbations — is such an incredible extrapolation from everyday experience into the far reaches of space and time. When we’re talking about inflation, we’re dealing with the first 10-35 seconds in the history of the universe. When we speak about dark matter and dark energy, we’re dealing with substances that are completely outside the very successful Standard Model of particle physics. These are dramatic ideas that need to be tested over and over again, and we’re going to keep looking for chinks in their armor until we’re satisfied beyond any reasonable doubt that we’re on the right track.

The next steps will involve both observations and better theories. Is n really less than 1? Is there any variation of n as a function of scale? Are there non-Gaussian features in the CMB? Is the dark energy varying? Are there tensor perturbations from gravitational waves produced during inflation? What caused inflation, and what are the dark matter and dark energy?

Stay tuned!

More discussion by Steinn Sigurðsson (and here), Phil Plait, Jacques Distler, CosmoCoffee. In the New York Times, Dennis Overbye invokes the name of my previous blog. More pithy quotes at Nature online and Sky & Telescope.

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62 Responses to WMAP results — cosmology makes sense!

  1. Pingback: Not Even Wrong » Blog Archive » Three-year WMAP Data Now Out

  2. ghazal says:

    Well as people had guessed, it seems the new result for tau is much lower than what was proposed before!

  3. Plato says:

    Okay, like I did in Mark’s other post. Maybe a trackback would be appropriate, I dunno?

    Dreamer, who sits at desk, looking out to window on universe, while a teacher gives critical evidence amazed.:) Sorry.

    As I was daydreaming….

    As a layman, such visualization, given the evidence of this map, is there not a way of seeing, that brings more perspective to all that data, or should we just stop and accept the picture as a 2d model of a 5d space? 🙂

    I am thinking of the “polarization points,” to the beginning times(red), and seeing in this way, tunnels going all over the place, in a “boundary” conditon(edge of the unniverse). I might have used the term wrong? To see, the overall dynamics of the universe “itself” doing a complete rotation?

  4. Robert says:

    There seems to be something wrong with your description of the large scale structure: l=1 is the dipole, 2 the quadrupole and 3 the octopole. 4 does not have a name that I know of and I just read the statement in one of the new papers that there has not much changed in l=2 and 3. So is it really l=4 that moved significantly?

  5. Aaron Bergman says:

    Were there any comments on the reason for the delay?

  6. Adam says:

    ‘It was really hard to get the polarisation data out’ is the gist of it.

  7. Sean says:

    Robert, sorry, I had just shifted by one. Fixed now.

  8. Risa says:

    As Adam said, the basic comment on the reason for the delay is that understanding the polarization results is just hard. As Hiranya pointed out today, they were looking for signal at 50 times the sensitivity that the instrument was designed for. I, for one, am glad they took their time instead of risking spontaneous emission of hundreds of papers exploring the unusual models predicted by an incorrect analysis of the data.

  9. Kea says:

    I feel immense relief. A few more things can be relegated to the dust bin, saving me from many nightmares in the future.

  10. Dumb Biologist says:

    One thing I’ve been kind of interested in were the anomalous octupole and quadrupole results, which various people appear to claim is due (pessimistically) to some foreground contamination of the data, or maybe even (optimistically) due to the universe being finite with some interesting topology. I’ve read (to the extent that I can comprehend the papers) criticisms of the analyses claiming some alignment with the eclipitic, implying, I think, that such a posteriori analysis finds trouble precisely where they’re looking for it, and is hence potentially spurious.

    So…I guess the octupole data looks better, and the quadrupole is still anomalous. However, other features of the data argue against a finite universe (if I read things at all correctly).

    Any thoughts on what’s going on?

  11. Kieran says:

    Were there any comments on the reason for the delay?

    They had to clear it with some 22 year old at the White House.

  12. Dumb Biologist says:

    I should have read Dr. Carroll’s post more carefully. I think the statement about “non-Gaussian” fluctuations is meant to address finiteness, among perhaps other things…

  13. Haelfix says:

    A few Ekyprotic models and some of the original textbook inflation models are ruled out with greater confidence.

    Eternal inflation seems to still fit the bill perfectly in naturalness.

    The lack of B modes at the resolution is troubling for a few models as well.

    Either way I love this experiment, its beautiful and deep and im glad to be alive when it happened.

  14. PK says:

    Why is there a relatively large uncertainty around l = 200?

  15. Count Iblis says:

    Has anyone here already worked out the limits on the DM-baryon cross section implied by the new WMAP data?

  16. Scott O says:

    Very exciting results. But I can’t help but feel a little bit of unease when experimenters reanalyze their data and suddenly find better agreement with their standard model’s predictions. Does anyone else besides me worry that somehow the analyzers are subconsciously biasing the way they do the analysis in order to “improve” the results? You know—tweak a cut here, throw out a dubious data point there? It’s so easy to do. Maybe future experiments should be doing a blind analysis of some sort.

  17. Hiranya says:

    Sean, thanks a lot for this great post.

    #2: Yes and no. The new tau result is actually very close to the best fit model from the likelihood analysis of the first year data (0.1), but for somewhat different reasons. It is indeed smaller than the correlation function analysis form year 1. We have made great improvements in the way we analyze the polarization data, where as Risa notes we are digging deep to extract a tiny signal that the satellite was not actually designed to detect.

    #13: I am not aware of why Ekpyrotic models would be ruled out, per se. And lack of B modes at the level we are able to detect at the moment is not troubling for inflation models. Our sensitivity to B modes is very limited.

    #16: I am not sure why you say we are “suddenly” finding better agreement with the standard model. If you look at our first year papers, we had very good agreement with the standard LCDM model, which has been confirmed by the new analysis. I am not sure why you say we are biasing the analysis to improve the results – if you look at the paper we analyse more than a dozen models of varying degrees of baroqueness, and none of the improves the fit enough to justify adding extra degrees of freedom.

  18. Scott O says:

    Hiranya, if you read again what I wrote, I did NOT say that WMAP is biasing their analyses. I do not know if there is any bias or not. However, although the LambdaCDM model was clearly a good fit in 2003, the fit now seems to be improved in some respects. If I understand things, the l=3 mode has moved closer to agreement. According to Sean’s post, several other points have moved closer to agreement with the LCDM model. Do you have a chi^2 per degree of freedom to report for the overall LCDM fit, which would be one way to quantify how good the agreement is?

    There are numerous reasons why the fit may have improved. Perhaps increased data (better signal/noise) has decreased the experimental uncertainties. But any time an experiment is trying to do precision tests of a model, and the analysts know what result they expect to get, there is a potential for bias to be introduced into an analysis. ALL precision experiments face this problem (and the WMAP team is to be congratulated for turning cosmology into a precision experiment!)

    Let me be also clear that when bias does occur in an analysis, it is almost never deliberate. There are very many unconscious ways in which bias can happen—whenever the analyst has to make choices about how to do the analysis (eg. what data sets to include, what foreground model to subtract, etc.), then potentially one might wind up be influenced by the impact on the final results.

    The high energy physics field has embraced blind analyses in a major way over the last several years due to these issues. Examples of recent analyses which were done blindly or are being done blindly include the BaBar CP violation results, the SNO solar neutrino measurements, and the upcoming MINOS and MiniBooNE results. In each case, the analysis is done as much to protect the analysts from their conscious or unconscious biases as anything else.

    Are there particular steps that WMAP has taken to guard against the introduction of biases? Are there things that could be done in future experiments to prevent analyst bias from impacting the result? I think it would be naive to assume that bias just can’t happen in this kind of work. This kind of precision testing of a favoured model is EXACTLY where blind analyses are often needed.

    Lest I sound too critical, let me finally congratulate WMAP on what is an impressive and high quality set of work! It’s really beautiful, even if I do feel like asking some difficult questions about blind analyses.

  19. D R Lunsford says:

    Surprise! The Neocons have done it again!


  20. Hiranya says:

    Scott #18: It is true we do not carry out particle physics type blind analyses, though if you have suggestions of how to apply such techqniues to CMB analyses I would very interested to hear them, since we should adapt any techniques we can to make future analyses better!

    That said, we take many steps to make sure our results are robust. In terms of models, we do not merely test the predictions of one model, but analyse the data in terms of many models, with many data combinations. There is nothing to inherently bias us towards LCDM in such an approach. For the predictions of the LCDM model itself, we compare the predictions of our best fit LCDM model against numerous astrophysical data sets at many scales and redshifts and check for consistency.

    l=3 moving closer to LCDM has no connection to the LCDM model fit itself – we have reanalysed the low l TT data with an optimal estimator to derive the Cls rather than our previous suboptimal method. You won’t find anyone in the field criticizing this improvement in analysis technique as a bias. You can find the chi^2/l for the best fit LCDM model in Fig 17 of the Hinshaw et al. paper. l=3 has very little weight in the LCDM fit because of the large cosmic variance there, so it is difficult to claim the change has “improved” the LCDM fit.

    We point out in exhaustive detail the limitations of our analysis (especially in the case of polarization analysis) in terms of foregrounds and other systematics uncertainties. Furthermore, all of our data and statistical analyses are (or will be soon) publicly available online so that anyone can download it and test, improve, and extend our analyses. A large number of researchers did this with our first year data, and hopefully even more will use the new data!

  21. Tom Renbarger says:

    Tau, n_s, and sigma-8 were the hot discussion topics in our impromptu lunchtime journal club. Regarding tau, I found it interesting that even as the error bars tightened on it, it actually became it bit less inconsistent with zero than in the 1-year release. It was also offered that the new result significantly diminishes the required output from Pop III stars to explain reionization.

  22. Plato says:

    A franco-american team of cosmologists [1] led by J.-P. Luminet, of the Laboratoire Univers et Théories (LUTH) at the Paris Observatory, has proposed an explanation for a surprising detail observed in the Cosmic Microwave Background (CMB) recently mapped by the NASA satellite WMAP. According to the team, who published their study in the 9 October 2003 issue of Nature, an intriguing discrepancy in the temperature fluctuations in the afterglow of the big bang can be explained by a very specific global shape of space (a “topology”). The universe could be wrapped around, a little bit like a “soccer ball”, the volume of which would represent only 80% of the observable universe!


  23. Hiranya says:

    #14: That grey band of uncertainty is “cosmic variance”! the fact that we only have one sky to measure. The black error bars on the points are our actually instrumental noise errors. The red points are the red model curve binned the same way as the theory.

  24. Plato says:

    Layman wondering

    Sound Waves in the CMB

    With thought of the vacuum, it’s hard to consider sound, but if you look at the picture in a different way, it just seems to make sense? If some condition(harmonic osscillation evident in place of nothing) was realize in the state of the vacuum, could such analogies be far reaching, then first assumed in that recogition of WMAP? 🙂

    With the discovery of sound waves in the CMB, we have entered a new era of precision cosmology in which we can begin to talk with certainty about the origin of structure and the content of matter and energy in the universe.-Wayne Hu

  25. Haelfix says:

    Well lets see if I get this right, any expert feel free to chime in. Cyclic models are tunable so they still fit (the original version I have from lecture notes is ruled out already from WMAP1 but I see you can tune it to fit current lambda CDM bounds, I suspect you might argue the tuning becomes a little more unnatural).

    To distinguish Ekyprotic from inflation you really need to look at the gravitational wave background. Cylcic models tend to have departure from scale invariant gravitational spectra. In principle some gravitational experiments in the next ten years should be able to detect inflationary modes (which could rule out the cyclic universe)

    Also *any* departure from gaussian density perturbations would be fatal I think (even very small departures), at least for the vanilla models without adding extra contrived stuff.