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The largest maps of the cosmic distribution of matter

The Dark Energy Survey (DES) has made it possible to obtain the largest maps of the spatial distribution of matter in history, locating both ordinary matter and dark matter in the universe up to a distance of 7 billion light-years.

The international initiative has had significant participation from the Center for Energy, Environmental and Technological Research (CIEMAT), the Institut de Ciències de l’Espai (IEEC, CSIC), the Institut de Física d’Altes Energies (IFAE) and the Institute of Theoretical Physics (UAM-CSIC), in Spain.

The new results from the Dark Energy Survey use the largest sample of galaxies ever analyzed in cosmology, covering a huge region of the sky, to produce the most accurate measurements of the composition and growth of the universe in history. Scientists have determined that the way matter is distributed in space is consistent with the predictions of the standard cosmological model.

Over six years, DES observed 5,000 square degrees – almost an eighth of the celestial sphere – in 758 nights, cataloging hundreds of millions of objects. The results that are now being made public have been obtained from data taken during the first three years of the project – 226 million galaxies observed in 345 nights, of which 100 million are used in cosmology studies – to create the largest and most Accurate maps ever made of the distribution of matter in the recent universe.

Since DES studies both nearby galaxies and those billions of light-years away, its maps provide a large-scale panoramic picture of the universe as well as a movie of how that structure has evolved over time. of the last 7 billion years.

The Blanco telescope, from which the DES project has been carried out. The black cylinder contains DECam, the powerful camera with which the images of galaxies have been taken, and in whose construction Spanish groups played an important role. (Image: Reidar Hahn / Fermilab)

To test the current model of the universe, DES scientists have compared their results with measurements made by the Planck space observatory of the European Space Agency (ESA). Planck used the light signals known as microwave background radiation to observe the early universe, only about 380,000 years after the Big Bang. The Planck data offers a very accurate view of what the universe looked like 13 billion years ago, and the standard cosmological model predicts how the distribution of dark matter (and ordinary matter) should have evolved up to the present day. If DES observations do not fit this prediction, it is very possible that there are aspects of the universe that have not yet been discovered. Although the published results are consistent with the prediction, there are still indications, both in DES and in previous experiments, that matter in today’s universe is distributed, by a small percentage, more evenly than predicted, an intriguing finding. it deserves further investigation.

Ordinary matter makes up only 5% of the universe. Dark energy, which according to cosmologists produces the accelerated expansion of the universe counteracting the force of gravity, accounts for almost 70%. The remaining 25% is dark matter, the gravitational influence of which holds galaxies together. Both dark matter and dark energy remain invisible and mysterious, but DES tries to reveal their nature by studying how the competition between the two shapes the large-scale structure of the universe throughout cosmic history.

“DES has managed to restrict the properties of dark energy to a level of precision that rivals that obtained by studying the microwave background radiation and, moreover, complements it”, says Ignacio Sevilla, CIEMAT senior scientist. “It is exciting to have achieved one of the most accurate measurements ever obtained of the fundamental properties of the universe.”

DES has photographed the night sky using the 570-megapixel Dark Energy Camera (DECam) installed on the 4-meter diameter Víctor Manuel Blanco telescope, located at the Cerro Tololo Inter-American Observatory in Chile. DECam, one of the most powerful digital cameras in the world, was designed specifically for DES and was assembled and verified at Fermilab, USA. In the process of design, construction, verification and installation of DECam there was an important Spanish contribution.

“The challenge was of unprecedented complexity, it involved a multidisciplinary team of hundreds of people, an investment in millions of hours in supercomputers and required the development of techniques that will mark the future of the field in almost all aspects of analysis”, he says Martín Crocce, ICE researcher who co-chairs the large-scale structure group of the DES international collaboration. “We are entering a new era of our global understanding of the universe, with direct observations, ranging from the early universe, 380,000 years, to the recent universe, 13,000 million years later.”

To quantify the distribution of dark matter and the effect of dark energy, DES relies primarily on two physical phenomena.

First, that at very large scales galaxies are not distributed randomly through space, but rather form a spiderweb-like structure as a consequence of the gravitational pull of dark matter. DES has measured how this cosmic web has evolved throughout the history of the universe. The clustering of galaxies that make up the cosmic web, in turn, reveals the regions that contain a higher density of dark matter.

Second, DES detects the imprint of dark matter using the weak gravitational lensing effect. When a distant galaxy emits light, the trajectory of the photons that compose it is disturbed by the gravitational effect exerted by the distribution of masses that are along its path. As a consequence, when we observe this galaxy, its shape is very slightly different from the original one, and the pattern of these distortions depends on the quantity and distribution of matter along the path of the light.

“By analyzing the subtle distortions of our 100 million galaxies, DES has been able to trace the distribution of matter that produces them,” explains Marco Gatti, a predoctoral researcher at IFAE (now at the University of Pennsylvania) and who has co-led the group. who makes the matter maps. “These are the largest matter maps ever created, covering an eighth of the sky and showing mostly dark matter, which does not emit light and cannot be detected by traditional methods.” This analysis has been partly possible thanks to new techniques for modeling large-field maps and large simulations carried out by Spanish groups and distributed on a new Big Data platform (CosmoHub), housed in the Port d’Informació Científica (PIC), a CIEMAT and IFAE data center.

Analyzing the huge amount of data collected by DES has been a formidable task. The team began by analyzing the first year of data, and the results were released in 2017. This process prepared the researchers to use more sophisticated techniques on larger data sets, including the largest sample of galaxies ever used to study. the weak gravitational lensing effect.

For example, calculating the redshift of a galaxy – the change in the wavelength of its light due to the expansion of the universe – is an important step in measuring the change, both in the spatial distribution of the galaxies and in the effect weak gravitational lensing, throughout cosmic history. “A key point has been the development of new methodologies to measure the redshift of the 100 million galaxies, directly related to their distances, which makes it possible to produce a 3D map of the universe,” says Giulia Giannini, predoctoral researcher at IFAE and one of those responsible for these measures. “Several independent methods have been combined applying advanced statistical techniques, more sophisticated and precise, to characterize the relationship between colors and positions of galaxies and their redshifts with the highest possible accuracy, something essential to obtain unbiased results.”

This and other advances, both in the measurements and in the theoretical description of the observations, were coupled with an increase in the amount of data of a factor three with respect to the first year, to allow the team to determine the density and uniformity of the universe with unprecedented precision.

“These precise measurements are the result of an analysis that is carried out with extreme care at all points, from the data collection at the telescope to the calculation of the final results. Among many other factors, we have corrected for the impact of external elements, such as stars or atmospheric effects, on our data. ” says Martín Rodríguez Monroy, a predoctoral researcher at CIEMAT, and one of those responsible for measuring the spatial distribution of nearby galaxies. “It is a great satisfaction to see how all the effort is translated into such precise and robust results.”

Along with analyzing the signals from the weak gravitational lensing effect, DES also measures other indicators that constrain the cosmological model in independent ways: the distribution of galaxies at very large scales (the acoustic oscillations of baryons), the number of clusters of massive galaxies and high-precision measurements of the brightness and redshift of Type Ia supernovae. These additional measurements are combined with weak gravitational lensing analysis to provide even more stringent constraints for the standard model.

“The DES data are unique because they allow us to test the cosmological model by studying very different phenomena,” says Santiago Ávila, IFT postdoctoral researcher in charge of analyzing the relationship between the initial conditions of the Universe and the observed distribution of galaxies. “The larger scales reveal us some sound waves generated in the early universe (the acoustic oscillations of baryons) and also how the first structures were formed from quantum fluctuations generated during cosmological inflation” – he adds.

DES completed its observations of the night sky in 2019. With the experience gained from analyzing the data presented today, the team is now ready to tackle the full set, which will double the number of galaxies used in the results. that today are made public. The final DES analysis is expected to extract an even more accurate view of the dark matter and dark energy in the universe. In addition, the methods developed by the DES science team have paved the way for future maps that delve even more deeply into the mysteries of the cosmos.

The Dark Energy Survey is a collaboration of more than 400 scientists from 25 institutions in seven countries.

Spain was the first international group to join the United States to found, in 2005, the DES project and participates through three institutions, two of them in Barcelona (the Institut de Ciències de l’Espai (IEEC, CSIC), and the Institut de Física d’Altes Energies, IFAE) and one in Madrid (the Center for Energy, Environmental and Technological Research, CIEMAT), as well as researchers from the Institute of Theoretical Physics, IFT (CSIC-UAM). (Source: CSIC)

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