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.

http://sci.esa.int/science-e-media/img/62/Compo_CMB_Planck_WMAP_v1_3k.jpg

Date: 20 Mar 2013
Satellite: Planck
Depicts: Cosmic Microwave Background
Copyright: ESA and the Planck Collaboration; NASA / WMAP Science Team: "This image shows temperature fluctuations in the Cosmic Microwave Background as seen by ESA's Planck satellite (upper right half) and by its predecessor, NASA's Wilkinson Microwave Anisotropy Probe (WMAP; lower left half) A smaller portion of the sky is highlighted in the all-sky map and shown in detail below. With greater resolution and sensitivity over nine frequency channels, Planck has delivered the most precise image so far of the Cosmic Microwave Background, allowing cosmologists to scrutinise a huge variety of models for the origin and evolution of the cosmos. The Planck image is based on data collected over the first 15.5 months of the mission; the WMAP image is based on nine years of data."

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:

Behnam Javanmardi, Bonn, 19.04.2013

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.

http://sci.esa.int/science-e-media/img/67/Planck_anomalies_Bianchi_on_CMB_orig.jpg

Date: 21 Mar 2013
Satellite: Planck
Copyright: ESA and the Planck Collaboration: "Two Cosmic Microwave Background anomalous features hinted at by Planck's predecessor, NASA's Wilkinson Microwave Anisotropy Probe (WMAP), are confirmed in the new high precision data from Planck. One is an asymmetry in the average temperatures on opposite hemispheres of the sky (indicated by the curved line), with slightly higher average temperatures in the southern ecliptic hemisphere and slightly lower average temperatures in the northern ecliptic hemisphere. This runs counter to the prediction made by the standard model that the Universe should be broadly similar in any direction we look. There is also a cold spot that extends over a patch of sky that is much larger than expected (circled). In this image the anomalous regions have been enhanced with red and blue shading to make them more clearly visible".

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

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.

By Behnam Javanmardi and Pavel Kroupa  (22.04.2013): "The Planck results on the cosmic Microwave background" on SciLogs. See the overview of topics in The Dark Matter Crisis.

7 Responses to “The Planck Results on the Cosmic Microwave Background”

  1. Hossein Reply | Permalink

    Does Planck say anything about Modified gravity models or MOND hypothesis?
    Can we rule out MOND or any alternative to CDM by using planck data?

  2. Benoit Famaey Reply | Permalink

    @Hossein, the CMB is a hard nut to crack in mond, definitely... with or without Planck. One anyway needs a relativistic theory for it to be computed, and one also needs to know whether one allows for an additional hot DM component (such as sterile neutrinos) or not.

    Leaving aside that possibility of additional hot DM, I'd say that TeVeS and generalized Einstein-aether theories, based on vector fields, probably cant produce the right CMB (the best reference for this probably being http://arxiv.org/abs/1002.0849). One could not say they are 100% excluded, but I would not bet money on those. But there are of course many other mond relativistic theories around. It is actually one of the great misunderstandings of our time that most fellow astrophysicists tend to conflate TeVeS with mond.

    For instance, there is Milgrom's bimetric mond with twin matter fields (http://arxiv.org/abs/1006.3809), for which I am not aware of a CMB prediction, or Deffayet's non-local theory (http://arxiv.org/abs/1106.4984) for which the prediction is probably too hard to compute, or Blanchet's dipolar dark fluid (http://arxiv.org/abs/0807.1200 a dipolar fluid which is also called "dipolar dark matter", but which is nothing like CDM in galaxies, since the density of the fluid is negligible in galaxies, yet its dipole actually creates mond there), which definitely fits the CMB. Some non-gaussianities were predicted, but these are slightly unconstrained, so at the moment even the strong planck constraints on non-gaussianities still fit the bill. So, there is at least one mond theory around that perfectly fits planck's CMB.

    Clearly, the problem here is that MOND is not a real theory, it is a general paradigm: it is a paradigm on galaxy scales, and so only galaxy scales can falsify the paradigm (by either showing that the paradigm quantitatively fails for the weak-field limit in some galaxies, or even more straightforwadly by directly detecting CDM particles). But then, there should of course also be precise theories behind the paradigm, and these theories should be falsifiable on *all* scales, including the CMB. Among these theories, I'd say that TeVeS and generalized Einstein-aether are probably close to being falsified there, but not the other ones I know of. This is also a problem of manpower: to falsify theories, you need people making the actual predictions.

  3. Caroline Copley Reply | Permalink

    Non-astrophysicist comment from me I'm afraid....
    The universe is oh so brilliantly flawed.
    An explosion causes things to fly off in all directions and a huge expansive wave.
    An explosion happens not because things are perfect but because they are uneven.
    Why does our window of the universe look like a perfect egg- do we really know if the boundary is not in fact a crenulated edge, perhaps with acceleration in some parts and not in others?
    Why do humans assume regularity when Nature teaches us through our genes that it is the error that gets us here. It has taken biology quite some time to realise that deterministic equations were never going to describe reality. Now it is more like fractal complexity. Perhaps astrophysics is in a similar boat?
    Of course nonlinearity makes sense so thanks to MOND for pointing that out.
    Could this explain the pendulum problem, and also the accelerating satellites problem where things just on the edge of our very own planet don't seem to add up?
    There is Nothing if things are all right.

  4. Dr. Volt Reply | Permalink

    I keep checking your website but hardly ever see an update. How can you call it a "crisis" if there is not a continuing flow of information and opinion? I encourage you to post more often...or is this a black hole with just an occasional and random seepage of matter that matters?

  5. Pavel Reply | Permalink

    Dear Dr. Volt,

    yours is a good point. Apologies - we are extremely busy with day to day issues on research and administration (I am also currently the director of the institute), so unfortunately very little time is available for writing on this blog. But we do aim to add posts in the future.

    To keep you a little busy, consider this part of the crisis:

    "Evidence for a ~300 Mpc Scale Under-density in the Local Galaxy Distribution" by Keenan et al., 2013, ApJ, in press
    ( http://adsabs.harvard.edu/abs/2013arXiv1304.2884K ).
    Compare with "Missing dark matter in the local universe" by Karachentsev, 2012
    ( http://adsabs.harvard.edu/abs/2012AstBu..67..123K ).

    Thus, while the standard cosmological model does not allow such large variations in matter density on the scales reported, it seems the Milky Way is in a major underdensity.

  6. Curtis Manning Reply | Permalink

    Perhaps velocity fields within a redshift of about z=1 are responsible? In my model of the universe, based on the observed concentration of hydrogen clouds toward the centers of galaxy voids, it is matter and not dark energy that is being formed (see Manning, 2002 ApJ 574, 19), and recent, upublished work based on the same HST data shows that void centers have cloud concentrations ten times that near void edges. This is suggestive of matter creation instead of dark energy since big bang models predict void centers have the lowest concentrations of clouds. It is possible that the microwave background is a product of these events (i.e., mini-bangs), rather than a remnant of an ancient recombination event. In that case, the inertial frames of creation events could be affected by the large mass flows of the low redshift universe, which could produce red- or blue-shifted patches on the sky. Therefore, some modeling of velocity flows from known superclusters should be considered as sources of the large red and blue patches on the PLANCK map - the source of the large scale anomalies. My calculations suggest variations on order of 10^-5 are possible. In my model, the creation rate is declining at a moderate pace so that it is still an accelerating expansion. The deceleration parameter is q=-2/5. Higher redshift has a greater creation rate, but lower mass flows, and low redshift has higher mass flows but a lower creation rate, so there will be an optimal redshift, probably between about 0.2 and 0.5, for modeling the large-scale anomalies.

Leave a Reply


3 × one =