The «Planck» was launched by the European Space Agency aiming at a detailed record of the microwave background radiation, a relic of the primordial plasma filling the universe during the first hundred thousand years after the Big Bang, that is still observable today. ESA's Planck satellite has been surveying the microwave and submillimetre sky since August 2009. After 15.5 months of continuous observation, the collaboration recently published the first cosmological results of this mission. These results can be read in two different ways. The first is the excellent agreement between Planck data and the standard model of cosmology, that can be described by just six parameters. Planck satellite allows for a much improved measure of these parameters. The second is that the results confirm an “almost perfect Universe”, revealing some peculiar unexplained features that might require new physics. What is more interesting is that both sentences are correct.

A map of the sky at optical wavelengths shows a prominent horizontal band which is the light shining from our own Milky Way. The superimposed strip shows the area of the sky mapped by Planck during the First Light Survey.The colour scale indicates the magnitude of the deviations of the temperature of the Cosmic Microwave Background from its average value, as measured by Planck at a frequency close to the peak of the CMB spectrum (red is hotter and blue is colder).The large red strips trace radio emission from the Milky Way, whereas the small bright spots high above the galactic plane correspond to emission from the Cosmic Microwave Background itself. (Image Credit @ESA)

In the standard model, the Universe is described as homogeneous and isotropic on very large scales. It is also spatially flat, from which it follows that the mean energy density is equal to a critical value (about 6 protons per cubic meter). Of this total energy density, it is remarkable that more than 95% is in a form never directly detected in the laboratory. Planck determined with a percent precision that 26% of energy is in form of Cold Dark Matter (that may consist of sub-atomic particles interacting very weakly with ordinary matter) and 69% of Dark Energy (some energy similar to vacuum energy, that does not dilute with the cosmological expansion like ordinary matter, needed to explain that the Universe is now observed to be in an accelerated expansion).

Cosmic structure is the result of the slow growth of tiny density fluctuations that arose immediately after the Big Bang. The photons that make up the CMB show these density fluctuations at the epoch they are emitted, i.e. only 380,000 years after the Big Bang. The intensity of the CMB radiation is in fact not exactly the same in all directions, but displays tiny changes which are translated into small differences in temperature. For example, areas in the CMB map with the lowest apparent temperatures correspond to the highest density regions, which developed to what we observe today as galaxies, galaxy clusters or even larger cosmological structures. The statistical analysis of these temperature fluctuations provides information useful to cosmologists.

Moreover, the CMB photons have travelled have travelled for over 13 billion years across the Universe, witnessing the dramatic changes that took place during the various cosmic epochs such as the formation of stars, galaxies and galaxy clusters.

Before Planck, NASA designed two space missions to map the sky at the wavelengths of the microwave background radiation. The first, COBE, was launched in 1989 and the second, called WMAP, in 2001. The results of the two gave answer to many critical questions about the age of the universe and the structures that can be found in it.

The WMAP mission had 20 times better resolution than COBE, showing details 20 times smaller. This helped to give accurate answers to the basic questions of cosmology, and to establish the standard cosmological model on firm ground. A spectacular outcome of WMAP was the confirmation of coherent acoustic oscillations propagating in the early universe, a feature predicted in the simplest cosmologival model since the seventies, that was not testable by COBE by lack of resolution. The coherent oscillations strongly support the idea that the first inhomogeneities in the universe were seeded during the accelerated expansion stage called inflation. The results of WMAP also challenged two of the basic assumptions of the standard cosmological model, isotropy and homogeneity. These assumptions appeared to be satisfaied very well on scales smaller than the size of our observable universe, while no clear conclusion could be inferred for the largest observable scales.

Planck has a 3 times better resolution than WMAP, and its best detector are 25 times more sensitive. It delivered the “ultimate map of temperature anisotropies”,  since instrumental noise is now below the theoretical error called “cosmic variance”. It confirms the standard cosmological model with a much better precision than WMAP, and the search  for deviations for this model returned a null result. Planck could also test with much better sensitivity than WMAP an important prediction of the simplest cosmological scenarios: the  Gaussian distribution of CMB fluctuations. It is however intriguing that the indications collected by WMAP for  deviations from isotropy and homogeneity on very large scale have been confirmed with Planck. Two of the main discrepancies are: an asymmetry of the northern to the southern hemisphere of the sky, as one of the two hemispheres appears to have a significantly stronger signal on average; and a lack of signal on large angular scales. An additional peculiar element in the data is the presence of a so-called 'cold spot': one of the low-temperature spots in the CMB extends over a patch of the sky that is much larger than expected. The question was born whether these discrepancies were real or an effect of experimental uncertainties or of foreground emissions? Planck data confirmed the presence of these discrepancies, ruling out any instrumental or astrophysical foreground origin. However, their detection are not strong enough to exclude anomalies to be simply due to statistical fluctuations or “cosmic variance”.

The anisotropies of the Cosmic microwave background (CMB) as observed by Planck. The CMB is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380 000 years old. It shows tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structure: the stars and galaxies of today. (Image Credit @ ESA and the Planck Collaboration)

One of the possible ways to explain the anomalies present in the large-scale pattern of the CMB invites cosmologists to re-examine one of the pillar assumptions of the standard model – isotropy. Cosmologists are now facing an interesting dilemma: on the one hand, the standard model of cosmology is still the best way to describe the CMB data, although it includes elements that still lack solid theoretical understanding such as dark matter, dark energy, and inflation. On the other hand, the anomalies seen by Planck highlight that the model should be at the very least extended, if not radically modified.

For the Planck mission another important measurement is related to the gravitational lensing, caused by the interaction between CMB photons and the large-scale structures that they encountered during their journey (i.e. galaxies and galaxy clusters). Since gravitational lensing arises later in cosmic history, it provides cosmologists with an extra set of information about the late Universe. This enables them to better constrain several additional parameters, such as the properties of dark energy, and the acceleration rate of cosmic expansion. Planck also answered questions related to the mass of neutrinos and the number of massless or very light relic particles in the universe (for instance, sterile neutrinos). Earlier cosmology data hinted that there could be more light relics than just photons and ordinary neutrinos, but Planck results bring no evidence for extra species.

It is clear that Planck shed light to the 13-billion-year history of cosmic structure formation. Perhaps the true tests of inflation—and of Planck's full prowess—will be revealed with the release of the polarization data, currently slated for early 2014. In fact, analysis of statistical properties of CMB polarization could provide direct information on the inflationary epoch, at the very early moments of the Universe.

The PH newsletter asked from Julien Lesgourgues and Leonardo Senatore, two of the staff members of the TH unit and Jan Haman, a post-doctoral fellow in the group who are actively engaged in theoretical cosmology to discuss and share their views on the recent Planck Results. Read more about ….title of the article+link.