The most precise 3D map of galaxies supports the standard cosmological model
A team of researchers from the collaboration Sloan Digital Sky Survey III (SDSS-III) have presented this week the latest results obtained from the map obtained by the consortium with data collected over the past ten years. The analysis of the data has been carried out by researchers of this collaboration, including a team from the Institute of Cosmos Sciences at the University of Barcelona (ICCUB, IEEC-UB).

The results presented this week are the work of the galaxy group of the Baryon Oscillation Spectroscopic the Survey (BOSS), one of the programs of SDSS-III. They contain the measurements of 1.2 million galaxies over a quarter part of the sky to create a 3D map of the structure of the Universe.


Section map of the large-scale structure of the Universe obtained by the BOSS program of the Sloan Digital Sky Survey collaboration. Each point of the image indicates the position of a galaxy whose light was emitted 6,000 million years ago. Each color indicates their distance from Earth, in a range from yellow for the closest ones, to violet for the most distant ones. The distribution of galaxies forms clusters, filaments and voids, constituting what is known as the cosmic web. Image Credit: Daniel Eisenstein / SDSS-III.

The challenge of BOSS has been to combine precise measurements of how galaxies group in the large-scale structure of the Universe, which is known as the cosmic web. These galaxies span a volume of the observable Universe equivalent to a cube of 8,500 million light years on a side, combined with a detailed modeling using cosmological simulations.

The results are presented in a main article and twelve supporting articles published in the digital repository ArXiv and submitted to the scientific journal Monthly Notices of the Royal Astronomical Society. Among the seventy authors who have signed the main article are Licia Verde, ICREA researcher of the ICCUB, and Antonio Cuesta of the same institution. Both are also members of the Institute for Space Studies of Catalonia (IEEC). Other coauthors include researchers from the Institute of Astrophysics of the Canary Islands (IAC) and the Institute for Theoretical Physics (UAM-CSIC).

One of the main consequences of the results from BOSS is that they constrain very precisely the expansion history of the Universe, which places very restrictive limits to theoretical models of dark energy alternative to the cosmological constant introduced by Einstein.

“In fact, it looks like BOSS results are consistent with a cosmological model of a flat Universe dominated by a cosmological constant, and with a cold dark matter component, which corresponds to the standard cosmological model developed in the last twenty years”, says Licia Verde, ICCUB researcher.

The scale of the Universe

In order to get this map of the cosmic web, BOSS has been able to establish a measurement of the distance to galaxies and quasars at cosmological scales, specifying the relationship between the distance to these objects and the expansion of the Universe. The light of these observed galaxies was emitted between 2,000 and 7,000 million years ago, covering approximately half of the expansion history of the Universe, whose age is estimated at about 13,800 million years. The data obtained trace the tug-of-war between gravity and expansion of the Universe, during its phase of accelerated expansion. Thus, the map presented by BOSS allows astronomers to measure the rate of expansion of the Universe and thus determine the amounts of dark matter and dark energy that make up the Universe today.


Section of the 3D map built by BOSS. The rectangle on the left contains 120,000 galaxies which accounts for 10% of all measured by BOSS. Each point of the rectangle corresponds to the spectroscopic measurement of a galaxy, which is what makes the two-dimensional image a three-dimensional map, extending our vision of the Universe to 7,000 million years in the past, about half of the estimated age of the Universe. Image Credit: Jeremy Tinker / SDSS-III.

To carry out this map BOSS has used a technique based on the measurement of the so-called baryon acoustic oscillations (BAO), which are acoustic waves, also called pressure waves, that spread through matter in the early Universe, leaving their footprint on the small density fluctuations that existed at its beginning. These waves have a known length, allowing scientists to measure distances and thus deduce the expansion rate of the Universe in the past.

“The baryon acoustic oscillations method used by BOSS, has become one of the essential pillars of modern cosmology to understand the expansion history of the Universe and hence dark energy”, says Licia Verde.

The main fact in which this technique is based is that galaxies tend to be separated by a typical distance, which astronomers call the BAO scale. The primordial measurement of the BAO scale has been perfectly determined by observations of the cosmic microwave background made by the Planck satellite, which estimates the length of this BAO scale as 481 million light-years.

BOSS: Building the ‘cosmic web’

Another major feature in this article has been the study of the implications that BOSS data have when combined with the measurements of the cosmic microwave background by the Planck satellite. Researchers have analyzed from this data combination any possible deviations from the standard cosmological model regarding the curvature of the universe, dark energy, or the theory of gravity, and in all cases the result has been negative, a fact that reinforces the standard cosmological model.

Galaxies analyzed by BOSS reach to a distance of about 20 BAO scales, whereas the observable Universe has its horizon in about 100 BAO scales, so future missions, currently under construction, will continue to seek these modifications at distances greater than those reached by BOSS.

“But what is impressive about the BOSS experiment is that we have been able to measure cosmological distances with an precision of 1%. That is, if all galaxies we have observed were placed in a cube whose length is 20 meters on a side, just by looking to pairs of galaxies that are separated about 1 meter from each other, we have managed to measure the distances to all of them with a precision of centimeters”, said Antonio Cuesta del ICCUB. Furthermore, “thanks to this data combination we have experienced a leap in the quality of our measurements of the cosmological parameters, and we have established a firm foundation on the future search for modifications of the standard cosmological model”, concludes the ICCUB (IEEC-UB) researcher.

Researchers at the University of Barcelona have led the computation of the correlation functions of BOSS galaxies and of artificial catalogs that simulate the observed data. This correlation function is precisely what determines the number of pairs of galaxies separated by a given distance.

Reference of the main article:

Alam, S. et al. (BOSS Collaboration). “The clustering of galaxies in the SDSS-III Baryon completed Oscillation Spectroscopic Survey: Cosmological analysis of the DR12 galaxy sample.” ArXiv, July 2016.

More information at the SDSS Collaboration press release
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