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The cosmic microwave background anisotropies, or CMB, observed by the Planck mission of ESA. The CMB is a snapshot of the oldest light of our cosmos, engraved in the sky while the Universe was only 380,000 years old. It presents tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structures: the stars and galaxies of today. The first view of this sequence shows anisotropies of the CMB temperature at the full resolution obtained by Planck. In the second view, the temperature anisotropies were filtered to show mainly the detected signal on scales of about 5º in the sky. The third view shows the filtered temperature anisotropies with additional indication of the direction of the polarized CMB fraction. A small part of the CMB is polarized – it vibrates in a favored direction. This is the result of the last encounter of this light with electrons, just before starting his cosmic journey. For this reason, the polarization of the CMB retains information on the distribution of matter in the primitive Universe, and its configuration in the sky follows that of the minute fluctuations observed in the temperature of the CMB. These images are based on data from the Planck Legacy press release, the latest mission data release, published in July 2018. Credit: ESA / Planck Collaboration
ESA's Planck satellite has found no new evidence of the mysterious cosmic anomalies that appeared on its temperature map of the Universe. The latest study does not rule out the potential relevance of the anomalies, but it does mean that astronomers have to work even harder to understand the origin of these confusing features.
The latest results from Planck come from an analysis of the polarization of cosmic microwave background radiation (CMB), the oldest light in cosmic history, issued when the Universe only had 380,000 years.
The initial analysis of the satellite, made public in 2013, is focused on the temperature of this radiation in the sky. This allows astronomers to investigate the origin and evolution of the cosmos. While it largely confirmed the standard picture of the evolution of our universe, Planck's first map also revealed a number of anomalies difficult to explain in the standard cosmological model.
Anomalies are weak features on the sky that appear at large angular scales. These are certainly not artifacts produced by the behavior of the satellite or the processing of the data, but they are sufficiently weak to constitute statistics, fluctuations extremely rare but not totally excluded by the standard model.
Alternatively, the anomalies could be a sign of "new physics", a term used to refer to natural processes still unrecognized that would extend the known laws of physics.
To further investigate the nature of the anomalies, the Planck team examined CMB polarization, which was revealed after careful analysis of multi-frequency data designed to eliminate microwave sources in the foreground, including gases and the dust in our own galaxy way.
A summary of the history of the Universe for nearly 14 billion years, showing in particular the events that contributed to the cosmic microwave background. The timeline at the top of the illustration shows an artistic view of the cosmos's evolution on a large scale. The processes described range from inflation, the brief period of accelerated expansion that the Universe experienced when it was only a fraction of a second, to the exit of the CMB, the oldest light in our universe, printed in the sky when the cosmos only 380,000 years ago; and "dark ages" at the birth of the first stars and galaxies, which reionized the Universe a few hundred million years ago to the present day. The tiny quantum fluctuations generated during the inflationary period are the seeds of the future structure: the stars and galaxies of today. After the end of inflation, dark matter particles began to congregate around these cosmic seeds, slowly building a cosmic network of structures. Later, after the publication of the CMB, normal matter began to fall into these structures, eventually giving rise to stars and galaxies. The inserts below show an enlarged view of some microscopic processes unfolding during cosmic history: tiny fluctuations generated during inflation to the dense soup of light and particles that filled the air. 39, primitive universe; from the last scattering of the electron light, to the origin of the CMB and its polarization, to the reionization of the Universe, caused by the first stars and galaxies, which induced an additional polarization on the CMB. Credit: ESA
This signal is the best measure to date of the CMB polarization E modes, and dates back to the time when the first atoms formed in the Universe and the CMB were released. It is produced in the same way as the light scattered by the electronic particles just before the electrons are collected into hydrogen atoms.
Polarization provides an almost independent view of CMB. Therefore, if the anomalies should also appear, astronomers would be more likely to believe that they could be caused by new physical data rather than by statistics.
Although Planck was not originally designed to focus on polarization, his observations were used to create the most accurate sky charts on CMB polarization to date. These were published in 2018, greatly improving the quality of Planck's first polarization maps, released in 2015.
When Planck's team examined this data, it found no obvious signs of anomalies. At best, the analysis, published today in Astronomy and astrophysics, revealed some weak clues indicating that some of the abnormalities may be present.
"Planck's polarization measurements are fantastic," says Jan Tauber, scientific director of ESA's Planck project.
"Despite the excellent data we have, we do not see any trace of a significant anomaly."
Microwave cosmic background (CMB) polarization amplitude map, observed by the ESA Planck satellite. Although Planck observed fluctuations in the CMB up to very small angular scales, these images were filtered to show mainly the detected signal at fairly large scales in the sky, of the order of 5 degrees – for comparison, the full moon extends about half a degree. On these large scales, a number of anomalies are observed in the temperature of the CMB. These are features difficult to explain in the standard model of cosmology, which is based on the assumption that the Universe, on a large scale, has the same properties when it is observed. in all directions. The most serious anomaly is a signal deficit observed on scales of about 5 degrees, which is about 10% lower than expected. Other abnormal features are a significant divergence of the signal observed in the two opposite hemispheres of the sky (the two hemispheres are delimited by the large U-shaped curve of the image, that of the north being in the center) and – called "cold spot" – a large low temperature spot with an unusually high temperature profile (the location of this spot is also shown at the bottom right). Such anomalies have not been detected, at least at a significant level, in Planck's observations on the polarization of CMB. A comparison between the top map, showing the total Planck measurement – including the signal and the noise – and the bottom map, showing only the noise – indicates that some abnormal features may be present, such as for example an asymmetry of power between the two hemispheres, but they are statistically unconvincing. The absence of statistically significant anomalies on the polarization maps does not exclude the potential relevance of those observed in the temperature, but makes it even more difficult to understand the origin of these puzzling features. . The shaded areas on the maps were masked during the analysis to avoid residual emissions in the foreground of our Milky Way or extragalactic sources that affect the cosmological results. Credit: ESA / Planck Collaboration
At first glance, this would seem to make the anomalies more likely than statistics, but in reality, this does not rule out a new physics because nature might be more delicate than we do it. think.
For the moment, there is no convincing assumption about the type of new physics that could be the source of the anomalies. Thus, it is possible that the responsible phenomenon only affects the temperature of the CMB, but not the polarization.
From this point of view, although the new analysis does not confirm the existence of a new physics, it imposes significant constraints on it.
The most serious anomaly appearing on the CMB temperature map is a signal deficit observed at large angular scales of the sky, around five degrees – for comparison, the full moon covers about half a degree. At these large scales, Planck's measurements are about ten percent lower than the standard model of cosmology predicts.
Planck also confirmed, with high statistical confidence, other abnormal features to which reference was made in previous CMB temperature observations, such as a significant divergence of the signal observed in the two opposite hemispheres of the sky, and a so-called effect. cold point – a large low temperature spot with an unusually high temperature profile.
"At the time of the first publication, we announced that Planck would test the anomalies using its polarization data.The first series of polarization cards sufficiently clear for this purpose was published in 2018, we now have the results, "says Krzysztof M Górski, one of the authors of the new paper, Jet Propulsion Laboratory (JPL), Caltech, US.
Cosmic Microwave Background Temperature (CMB) map observed by the ESA Planck satellite. Although Planck observed fluctuations in CMB up to very small angular scales, these images were filtered to show mainly the detected signal at fairly large scales in the sky, around 5 degrees and more. The moon extends over about a half degree. On these large scales, a number of anomalies are observed in the temperature of the CMB. These are features difficult to explain in the standard model of cosmology, which is based on the assumption that the Universe, on a large scale, has the same properties when it is observed. in all directions. The most serious anomaly is a signal deficit observed on scales of about 5 degrees, which is about 10% lower than expected. Other abnormal features are a significant divergence of the signal observed in the two opposite hemispheres of the sky (the two hemispheres are delimited by the large U-shaped curve of the image, that of the north being in the center) and – called "Cold spot" – a large low temperature spot with an unusually high temperature profile (also shown at bottom right). A comparison between the top map, showing the total Planck measurement – including the signal and the noise – and the bottom map, showing only the noise, indicates that the abnormal features are clearly not artifacts, as they are effectively present in the signal and not in the source. noise. Such anomalies have not been detected, at least at a significant level, in Planck's observations on the polarization of CMB. The absence of statistically significant anomalies on the polarization maps does not exclude the potential relevance of those observed in the temperature, but makes it even more difficult to understand the origin of these puzzling features. . The shaded areas on the maps were masked during the analysis to avoid residual emissions in the foreground of our Milky Way or extragalactic sources that affect the cosmological results. Credit: ESA / Planck Collaboration
Unfortunately, the new data did not push the debate further, as the latest results do not confirm or deny the nature of the anomalies.
"We have evidence that in the polarization maps there might be a power asymmetry similar to that observed in the temperature maps, although it remains statistically unconvincing," adds Enrique Martínez González, who is also co-leader. author of the article. , from the Instituto de Física de Cantabria in Santander, Spain.
Although Planck's results are analyzed in greater depth, it is unlikely that they will lead to any new significant results on this subject. The obvious way forward is a dedicated mission specifically designed and optimized to study the polarization of CMB, but this is in at least 10 to 15 years.
"Planck has provided us with the best data we will have for at least a decade," said Anthony Banday, co-author of the Institute for Research in Astrophysics and Planetology, Toulouse, France.
Meanwhile, the mystery of the anomalies persists.
From an almost perfect universe to the best of both worlds
Indefinite indefinite. Planck 2018 results. VII. Isotropy and statistics of the CMB, Astronomy and astrophysics (2019). DOI: 10.1051 / 0004-6361 / 201935201
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