Astrophysicists fill gaps in the history of the Universe

July 20202- EPFL. An international consortium of scientists has analyzed, as part of a vast program of cosmological surveys, several million galaxies and quasars, thus retracing a more continuous history of the Universe and offering a better understanding of the mechanisms of its expansion. The latest 6 year-long survey called eBOSS was initiated, and led in part, by EPFL astrophysicist Jean-Paul Kneib.

Thanks to the extensive theoretical models describing the Universe after the Big Bang, as well as observation of the Cosmic Microwave Backgound Radiation (CMBR), the infant Universe is relatively well known. Scientists have also explored its expansion history over the most recent few billion years from Supernovae distance measurements and galaxy maps, including those from previous phases of the SDSS. “We know both the ancient history of the Universe and its recent expansion history fairly well, but there’s a troublesome gap in the middle 11 billion years,” says cosmologist Kyle Dawson of the University of Utah, who leads the team announcing today’s results. “Thanks to five years of continuous observations, we have worked to fill in that gap, and we are using that information to provide some of the most substantial advances in cosmology in the last decade.”

“Taken together, detailed analyses of the eBOSS map and the earlier SDSS experiments, we have now provided the most accurate expansion history measurements over the widest-ever range of cosmic time,” says Will Percival of the University of Waterloo, eBOSS’s Survey Scientist. “These studies allow us to connect all these measurements into a complete story of the expansion of the Universe.”

The finalized map shows filaments of matter and voids that more precisely define the structure of the Universe since its beginnings, when it was only 380,000 years old. From there, the researchers measured the recurring patterns in the distribution of galaxies, thus identifying several key cosmological parameters, including the density of hypothetical dark matter and energy in the Universe, with a high degree of precision.

To carry out this survey, the teams involved in the eBOSS project looked at different galactic tracers that reveal the mass distribution in the Universe. For the part of the map relating to the Universe six billion years ago, researchers observed the oldest and reddest galaxies. For more distant eras, they concentrated on the youngest galaxies, the blue ones. To go back further, that is to say up to eleven billion years, they used quasars, galaxies whose super-massive black hole is extremely luminous.

Slower expansion?

This map reveals the history of the Universe, and in particular, that the expansion of the Universe began to accelerate at some point and has since continued to do so. This seems to be due to the presence of dark energy, an invisible element that fits naturally into Einstein's general theory of relativity but whose origin is not yet understood.

When eBOSS observations are compared with studies of the Universe’s early days, discrepancies appear in estimates of the Universe’s expansion rate. The currently accepted expansion rate, called the "Hubble constant", is 10% slower than the value calculated from the distances between the galaxies closest to us. It is unlikely that this 10% difference is random due to the high precision and wide variety of data in the eBOSS database.

To date, there is no commonly accepted explanation for these disagreements between the different estimations of the speed of expansion, but the fact that a still unknown form of matter or energy from the early Universe could have left traces in our history is an interesting possibility.

EPFL astrophysicist Jean-Paul Kneib

References

SDSS website: https://www.sdss.org/

The Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Cosmological Implications from two Decades of Spectroscopic Surveys at the Apache Point observatory

The Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: one thousand multi-tracer mock catalogues with redshift evolution and systematics for galaxies and quasars of the final data release

The completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Large-scale Structure Catalogues and Measurement of the isotropic BAO between redshift 0.6 and 1.1 for the Emission Line Galaxy Sample

The Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Growth rate of structure measurement from anisotropic clustering analysis in configuration space between redshift 0.6 and 1.1 for the Emission Line Galaxy sample