Universe to WMAP: ΛCDM Rules, OK?

Article written: 8 Feb , 2010
Updated: 24 Dec , 2015

The Wilkinson Microwave Anisotropy Probe (WMAP) science team has finished analyzing seven full years’ of data from the little probe that could, and once again it seems we can sum up the universe in six parameters and a model.

Using the seven-year WMAP data, together with recent results on the large-scale distribution of galaxies, and an updated estimate of the Hubble constant, the present-day age of the universe is 13.75 (plus-or-minus 0.11) billion years, dark energy comprises 72.8% (+/- 1.5%) of the universe’s mass-energy, baryons 4.56% (+/- 0.16%), non-baryonic matter (CDM) 22.7% (+/- 1.4%), and the redshift of reionization is 10.4 (+/- 1.2).

In addition, the team report several new cosmological constraints – primordial abundance of helium (this rules out various alternative, ‘cold big bang’ models), and an estimate of a parameter which describes a feature of density fluctuations in the very early universe sufficiently precisely to rule out a whole class of inflation models (the Harrison-Zel’dovich-Peebles spectrum), to take just two – as well as tighter limits on many others (number of neutrino species, mass of the neutrino, parity violations, axion dark matter, …).

The best eye-candy from the team’s six papers are the stacked temperature and polarization maps for hot and cold spots; if these spots are due to sound waves in matter frozen in when radiation (photons) and baryons parted company – the cosmic microwave background (CMB) encodes all the details of this separation – then there should be nicely circular rings, of rather exact sizes, around the spots. Further, the polarization directions should switch from radial to tangential, from the center out (for cold spots; vice versa for hot spots).

And that’s just what the team found!

Concerning Dark Energy. Since the Five-Year WMAP results were published, several independent studies with direct relevance to cosmology have been published. The WMAP team took those from observations of the baryon acoustic oscillations (BAO) in the distribution of galaxies; of Cepheids, supernovae, and a water maser in local galaxies; of time-delay in a lensed quasar system; and of high redshift supernovae, and combined them to reduce the nooks and crannies in parameter space in which non-cosmological constant varieties of dark energy could be hiding. At least some alternative kinds of dark energy may still be possible, but for now Λ, the cosmological constant, rules.

Concerning Inflation. Very, very, very early in the life of the universe – so the theory of cosmic inflation goes – there was a period of dramatic expansion, and the tiny quantum fluctuations before inflation became the giant cosmic structures we see today. “Inflation predicts that the statistical distribution of primordial fluctuations is nearly a Gaussian distribution with random phases. Measuring deviations from a Gaussian distribution,” the team reports, “is a powerful test of inflation, as how precisely the distribution is (non-) Gaussian depends on the detailed physics of inflation.” While the limits on non-Gaussianity (as it is called), from analysis of the WMAP data, only weakly constrain various models of inflation, they do leave almost nowhere for cosmological models without inflation to hide.

Concerning ‘cosmic shadows’ (the Sunyaev-Zel’dovich (SZ) effect). While many researchers have looked for cosmic shadows in WMAP data before – perhaps the best known to the general public is the 2006 Lieu, Mittaz, and Zhang paper (the SZ effect: hot electrons in the plasma which pervades rich clusters of galaxies interact with CMB photons, via inverse Compton scattering) – the WMAP team’s recent analysis is their first to investigate this effect. They detect the SZ effect directly in the nearest rich cluster (Coma; Virgo is behind the Milky Way foreground), and also statistically by correlation with the location of some 700 relatively nearby rich clusters. While the WMAP team’s finding is consistent with data from x-ray observations, it is inconsistent with theoretical models. Back to the drawing board for astrophysicists studying galaxy clusters.

Seven Year Microwave Sky (Credit: NASA/WMAP Science Team)

I’ll wrap up by quoting Komatsu et al. “The standard ΛCDM cosmological model continues to be an exquisite fit to the existing data.”

Primary source: Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation (arXiv:1001.4738). The five other Seven-Year WMAP papers are: Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies? (arXiv:1001.4758), Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Planets and Celestial Calibration Sources (arXiv:1001.4731), Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results (arXiv:1001.4744), Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Power Spectra and WMAP-Derived Parameters (arXiv:1001.4635), and Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Galactic Foreground Emission (arXiv:1001.4555). Also check out the official WMAP website.

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7 Responses

  1. Lawrence B. Crowell says

    This is great. The polarization of anisotropies and inverse Compton scattering signatures in the Sunyaev-Zel’dovich effect indicate stochastic fluctuations associated with inflation. This gives some weight to /\CDM which has the universe emerging according to a quantum fluctuation of a scalar field. This scalar field reached a minimum across the cosmological horizon length with slightly different coupling strengths to the local Ricci curvature. These anisotropies with the SZ shadows and polarizations are, as the UT post here by Jean Tate indicates, weak correlations to these much earlier processes in the universe.

    The universe is a spacetime which is asymptotically approaching a de Sitter vacuum. Inflation was a sort of phase transition in the earliest universe where the structure was nearly de Sitter for a very brief period of time. The scalar field, or inflaton, cascaded down its field potential and induced this configuration. Upon reaching the bottom of this potential the inflation was “turned off,” and there was the phase of “reheating.” This reheating is analogous to the production of latent heat in a phase transition. Quantum uncertainty fluctuations in the field coupled with local regions of curvature, where this reheating had some measure of inhomogeneity, or as observed from a central location an anisotropy, that is frozen out in what we observe now. These are weak signature of early processes in the universe.

    To take this to some greater extent, the de Sitter spacetime is a time dependent conformal equivalent of flat spacetime, or Minkowski spacetime. The conformal factor is what determines the cosmological constant or /\. Quantum fluctuations may determine the value of the cosmological constant, where this is a major problem in string theory and cosmology, so that the frozen out value we observe represents how the universe is a quantum fluctuation that has been frozen out by a quantum induced phase of fields. The quantum fluctuation of the field serves to change the phase of a system as do thermal fluctuation in 1st and 2nd order phase transitions (eg. gas to solid etc).

    This is only the beginning. To probe further, we need to map the cosmic gravitational radiation background (CGRB), for in the early inflationary period quantum gravity decoupled from other fields in a manner analogous to the decoupling of radiation from matter 380,000 years after the big bang. Gravitational radiation would then carry stronger signals of the earliest moments of the universe. The trio of spacecraft designated LISA are a spacebased gravity wave interferometric system which may detect this radiation. In time we may be able to map the CGRB just as we have now the CMB.


  2. brundall says

    Yeah – what he said ^

  3. Torbjorn Larsson OM says

    That we are entering a period of inflation physics is really apparent by now. Not only are other theories pretty much excluded, but there is exclusion among inflationary models. Apparently the simplest models are disfavored (albeit the simplest less so), but sundry models of “new inflation” are presently less likely to fall to the axe of falsification.

    The beauty of this is that inflationary physics is rich of predictivity just as LambdaCDM itself. Bousso et al recently successfully falsification tests 6 predictions over 183 orders of magnitude including of course the cosmological constant of old.

    Which of course means that the prediction that our universe emerged out of the inflationary process chaotically is successfully tested as well.

    This is currently our best theory AFAIU, but it implies larger questions. It is tempting to believe that the inflationary universe (or “multiverse”) is eternal backwards, which it can be by pushing the sup inf boundary towards full “eternal symmetry” in all directions à la Linde.

    But that means either that the process has lost its memory. (Well, why not, eternity is a long time, and deterministic chaos at least is exponentially divergent.) Or if not, it means that the process was unlikely to start out close to its stationary fix point.

    In the later case it would have to go back to an origin in a specific quantum fluctuation (since FLWR universes are zero energy) through a process of other quantum fluctuations that started inflation locally. Which begs the question, “Who ordered that?” … er, I mean, what was “the pre-universe” field which fluctuated into the intermediate pre-universe that inflation could fluctuate out of, and which process was it a result of?

    My layman money is on the more parsimonious and tested theory, it’s inflation all the way back.

  4. Astrofiend says

    Ahh – the sublime beauty of it all.

  5. Astrofiend says

    Can’t wait for the Plank results 🙂

  6. Jean Tate says

    I’ve a question for Lawrence B. Crowell and Torbjorn Larsson OM (and anyone else who might know the answer).

    Table 1 in Komatsu et al. contains the six primary parameters of the simplest LCDM model (flat, Gaussian, adiabatic, power-law), together with seven derived ones.

    While some of these are relatively straight-forward (the Hubble constant, for example), many are not.

    Do you know of an online resource which concisely explains what each of these 13 parameters are?

    And what about the parameters in Table 2?

    I’ve asked this on the BAUT forum, but no answers yet.

  7. Lawrence B. Crowell says

    The Omega’s are for dark energy and matter (which includes dark matter) and Omega_b is baryonic matter or ordinary matter. H_0 is the Hubble constant, which is related to the Omegas. The Delta symbol at the bottom is the curvature fluctuation. n_s is the spectral scalar index z_reion is the redshift at reionization — when stars and galaxies started to form, which is about 10. the sigma is an optical depth and t_0 is the age of the universe.


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