The scientific results of the first three years of observing the cosmos with Des, the Dark Energy Survey, were presented on May 27th. Results in agreement with the predictions of the standard model of cosmology, albeit with an indication of possible discrepancy regarding the “graininess” of the universe
Over four hundred female scientists from seven countries. 758 nights of telescope observations, distributed between 2013 and 2019. Hundreds of millions of objects cataloged. Objective: To produce the most accurate description ever of seven billion years of universe history. An impressive effort, that of the Dark Energy Survey (Des) observing campaign , which culminated last May 27 in the presentation of 29 scientific articles with the results of the first three years of observations. Results that substantially confirm the validity of the standard cosmological model (the so – called Lambda-Cdm model , the one with dark energy and dark matter, so to speak), albeit showing a graininess ( clumpiness, in English) slightly lower than expected, as if today’s universe were a few percentage points more uniform – therefore with fewer “lumps” – than predicted based on observations of the early universe.
And it is precisely from the early universe – and in particular from the “photograph” of the cosmos 380 thousand years after the Big Bang, immortalized with great precision in the maps of the Planck space telescope – that the Des survey started . If we take the universe back then (therefore over 13 billion years ago) as a starting point and apply the cosmological model to predict its evolution to the present day, the scientists of Des said to themselves, then comparing the result with the observations of the survey we will be able to evaluate the goodness or otherwise of the cosmological model itself. Observations that were conducted with the Dark Energy Camera – a 570 megapixel digital camera mounted on the Blanco Telescope– with the aim of producing the most extensive and precise maps ever obtained of the distribution of galaxies in the universe in relatively recent times. We are talking about 226 million galaxies observed in 345 nights, one hundred million of which were then used for cosmological analysis.
«The published results show compatibility between Des’s predictions and measures. However, some elements of Des’s analysis and other competing experiments continue to suggest a slight tension between the observations of the nearby universe and those of the primitive universe “, explains referring to the excess of uniformity from which we mentioned earlier Marco Raveri , postdoc researcherto UPenn – the University of Pennsylvania at Philadelphia – who led the statistical analysis on this discrepancy. A discrepancy, the authors of the study underline, is not large enough to exclude that it is only a statistical fluctuation: the analysis of the remaining half of Des data, expected for the next few years, will be important to try to arrive at an answer definitive.
The methods used by Des’s team to trace the distribution of dark matter and quantify the effect of dark energy are mainly based on the observation of two effects due to the gravitational attraction of dark matter on normal matter and on light. The first is the large-scale distribution of galaxy clusters – a cosmic web that traces the density of the underlying dark matter. The second is weak gravitational lensing , or the slight but statistically significant distortion of the shape of objects in the background introduced by the presence of large masses – mainly dark matter – between them and us who observe them.
“By analyzing the subtle distortions on our one hundred million galaxies, Des was able to trace the distribution of matter that produces them”, says Marco Gatti , also a postdoc researcher at UPenn, at the head of the group that elaborates the maps. of matter. “These are the largest matter maps ever created: they cover an eighth of the sky and show, above all, dark matter, which does not emit light and cannot be detected with traditional methods.”
During the survey, the Dark Energy Camera of Des has repeatedly focused on ten regions of the sky selected to obtain as many “deep fields”, so as to be able to glimpse even the most distant galaxies. The measurement of the redshift – therefore of the distance – of these remote galaxies was then used to precisely calibrate the rest of the survey .
“A key point was the development of new methodologies to measure the redshift of one hundred million galaxies, which makes it possible to produce a 3D map of the universe”, observes one of those responsible for these measurements, Giulia Giannini , PhD researcher at IFAE, the Barcelona Institute of High Energy Physics. “Various independent methods have been combined, applying advanced statistical techniques, more sophisticated and precise than those adopted so far, to characterize the relationship between color and position of galaxies and their redshift with the greatest possible accuracy, which is fundamental to avoid obtaining false results “.
The next step will come with the results of the data analysis of the remaining three years of survey, and should lead to an even more accurate picture of dark matter and dark energy in the universe.
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