The Planck Results on the Cosmic Microwave Background
Guest contribution by Behnam Javanmardi
Prologue by Pavel Kroupa:
The much awaited Planck results on the CMB have been published recently. The results are consistent with those arrived at by using Wilkinson Microwave Anisotropy Probe (WMAP) measurements.
This agreement is excellent news, because it means that the two missions are consistent and thus the Planck data enhance our confidence in what we know about the CMB.
But, what do the results mean in terms of our physical understanding of the universe?
In this guest contribution by PhD student Behnam Javanmardi, who is studying cosmological models in Bonn since the Fall of 2012, some of the problems raised by the Planck CMB map are discussed:
Contribution by Behnam Javanmardi:
The European Space Agency (ESA) launched the Planck satellite on 14 May 2009 to the second Lagrange point of the Sun-Earth system (L2), at a distance of 1.5 million kilometers from the Earth, for observing the Cosmic Microwave Background (CMB), the afterglow of the Big Bang. On 21 March 2013, the Planck collaboration released the data with a series of papers on their scientific findings. Planck observed the CMB sky in different frequency bands, some of which are sensitive to the foregrounds (anything between us and that cosmic radiation, e.g. the disk of the Milky Way). This allows to remove the foregrounds and reach to an image of the Universe when it was very young.
Statistical analysis of this image (which shows small temperature fluctuations corresponding to small density contrasts at that time) gives us valuable information about our Universe. In the following, some major Planck's results are reviewed with the main focus on the problems cosmologists now face, given these results. Technical details can be found in the Planck 2013 Results Papers.
The current Standard Cosmological Model (ΛCDM) has a set of parameters and the Planck collaboration reported the values for these parameters by fitting the model to the data. For example, the best fit ΛCDM parameters resulted in a 6% lower value for the density parameter of dark energy (Planck: ΩL=0.686±0.020 vs WMAP-9: ΩL=0.721±0.025) and an 18% higher value for the density parameter of dark matter (Planck: Ωm=0.314±0.020 vs WMAP-9: Ωm=0.279±0.025) than the results of the previous all sky CMB survey, i.e. WMAP. As can be seen from these numbers, the two parameters are consistent with each other within the measurment uncertainties. Thus, the Planck mission has nicely confirmed the WMAP fit to the standard model of cosmology.
The main interesting result from Planck was the confirmation of some features that have been revealed by WMAP data. Before Planck, there were some doubts about the cosmic origin of these features, but since the precision of Planck's map is much higher than that of WMAP and the Planck collaboration was working nearly 3 years to carefully extract any foreground emission and those features are still present, we have to accept with a much higher confidence that these may be real features of the CMB sky.
These features or anomalies, which the standard model of cosmology did not expect, are significant deviations from large scale isotropy. But large scale isotropy is one of the two fundamental assumptions that form the Cosmological Principle and simply states that the Universe we observe must not be direction-dependent. Among these features found in the CMB one can mention a “Cold Spot” which is a low-temperature region much larger than expected. And, a “Hemispherical Asymmetry” has been detected: the northern ecliptic hemisphere has on average a significantly lower signal than the southern one. The latter leads to this question: why is the orientation of this asymmetry more or less aligned with the orbital angular momentum of the Earth? Is it a not-yet understood measurement bias or a data reduction bias or a coincidence? As the Earth orbits the Sun, its orbital angular momentum remains pointing into the same direction in the Milky Way. Perhaps a remnant Milky Way foreground contamination may play a role here.
The other assumption of the cosmological principle, i.e. that the initial temperature (and density) fluctuations had Gaussian distribution, has also been tested by the Planck collaboration and no significant deviation from it was reported, except for a few signatures which were interpreted to be associated with the above-mentioned anomalies.
Furthermore, the power-spectrum calculated using the Planck data (which is one of the main statistical tools for analyzing the CMB map) has a ≈2.7σ deviation from the “best fit ΛCDM model” at low-ℓ (ℓ ≤ 30) multipoles or large angular scales.
Regarding the test of inflation (a hypothesis which says that the early Universe was inflated by a factor of at least 10^(78) in less than 10^(-36) seconds), the models with only one scalar field are preferred by the Planck results and more complex inflationary scenarios do not survive. However, a recent paper by Ijjas et al (2013) has gone through the problems of inflation considering the results from both the Planck satellite and the LHC,
“The odd situation after Planck2013 is that inflation is only favored for a special class of models that is exponentially unlikely according to the inner logic of the inflationary paradigm itself”
as they mention. The forthcoming results on polarization of the CMB from Planck will cast light on this issue.
As mentioned above, although the ΛCDM model is consistent with the overall picture as seen by Planck, it fails to account for these observed anomalies and the deviation of the power-spectrum at large scales. In addition, the three major elements of the ΛCDM model, i.e. dark matter, dark energy and inflation, still lack a firm theoretical understanding. Therefore, cosmologist should try to look for a model in which the recent observed features are no longer “anomalies” and are predicted by the model itself.
Epilogue by Pavel Kroupa:
The Planck data thus demonstrate that not all is well with our understanding of cosmology, that is, the CMB poses hitherto unanswered problems. But even if the CMB had been in perfect agreement with the expectations from the current standard model of cosmology, what would this have implied for our physical understanding of cosmology?
First of all, an elementary if not trivial truth is that consistency of a model with a set of data does not prove the model. Thus, claiming that Planck establishes the existence of (cold or warm) dark matter and dark energy would be an unscientific statement. For example, the cosmological model by Angus & Diaferio (2011, see their fig.1) shows that the CMB can be reproduced with a non-CDM/WDM model, therewith proving the non-uniqueness of the models.
Furthermore, irrespective of any success or failure of the standard (or any other) cosmological model in reproducing some large-scale data, the highly significant problems encountered on the local cosmological scale of 100Mpc and below remain hard facts to be solved: See
- the Nature review paper "Nearby galaxies as pointers to a better theory of cosmic evolution" by Peebles & Nusser 2010,
- "Missing dark matter in the local universe" by Karachentsev 2012,
- the Cambridge University Press review research paper "The dark matter crisis: falsification of the current standard model of cosmology" by Kroupa 2012, and
- "The failures of the standard model of cosmology require a new paradigm" by Kroupa, Pawlowski & Milgrom 2012.
Behnam Javanmardi's final statement above,
"Therefore, cosmologist should try to look for a model in which the recent observed features are no longer “anomalies” and are predicted by the model itself.",
emphasises that cosmology is one of the least understood of the physical sciences.