eBOSS

eBOSS. The Extended Baryon Oscillation Spectroscopic Survey was a major cosmological experiment conducted as part of the fourth phase of the Sloan Digital Sky Survey (SDSS-IV). Operating from 2014 to 2019, it precisely mapped the large-scale structure of the universe to investigate the nature of dark energy and the geometry of cosmic expansion. The survey targeted millions of galaxies and quasars to trace matter distribution across vast cosmic epochs, providing critical data for modern cosmology.

Overview

eBOSS was designed as the successor to earlier baryon acoustic oscillations (BAO) surveys within the Sloan Digital Sky Survey, such as the BOSS project. It constituted a core component of SDSS-IV, utilizing the dedicated 2.5-meter telescope at the Apache Point Observatory in New Mexico. The project involved hundreds of scientists from institutions worldwide, including key contributions from Lawrence Berkeley National Laboratory, the Max Planck Institute for Astrophysics, and the Institute for Advanced Study. Its primary legacy is a comprehensive three-dimensional map of the universe spanning the last several billion years.

Scientific goals

The central objective was to measure the expansion history of the universe with unprecedented precision using the baryon acoustic oscillations as a standard ruler. This allowed scientists to constrain the properties of dark energy, testing whether it remains consistent with the cosmological constant from Einstein's theory of general relativity. Additional goals included measuring the growth of cosmic structure through redshift-space distortions, studying the properties of quasars and the intergalactic medium via the Lyman-alpha forest, and investigating the connection between galaxy properties and their surrounding dark matter halos. These measurements provided tests for theories of gravity and models of the early universe like inflation.

Instrumentation and observations

Observations were conducted using the SDSS telescope equipped with twin multi-object spectrographs capable of taking 1,000 spectra simultaneously. Key targets included luminous red galaxies up to a redshift of 0.8, emission line galaxies up to redshift 1.0, and a vast sample of quasars both as direct tracers and as backlights for studying the Lyman-alpha forest at redshifts above 2.0. The survey also incorporated data from the Time Domain Spectroscopic Survey for variable sources. This multi-tracer strategy, covering different cosmic epochs, was crucial for minimizing systematic errors and breaking degeneracies in cosmological parameters.

Key findings and cosmological results

The final cosmological analysis, presented in a series of papers in 2020, produced the most precise BAO measurements at the time across the redshift range 0.6 to 2.2. These results confirmed the accelerating expansion driven by dark energy and found consistency with the standard Lambda-CDM model. The data revealed a slight tension with some measurements of the Hubble constant from the Planck satellite, contributing to ongoing debates in cosmology. Furthermore, eBOSS provided detailed maps of the Lyman-alpha forest, yielding insights into the temperature and density of the intergalactic medium and placing constraints on the mass of the neutrino.

Collaboration and data releases

The eBOSS collaboration comprised over 100 institutions worldwide, managed by the SDSS collaboration. The project made all data publicly available through regular data releases, with the final cosmological results encapsulated in Data Release 16. This vast public dataset, hosted by the Space Telescope Science Institute through the Mikulski Archive for Space Telescopes, continues to support wide-ranging astronomical research beyond cosmology, including studies of galaxy evolution, quasar physics, and the Milky Way. The success of eBOSS directly paved the way for its successor, the Dark Energy Spectroscopic Instrument (DESI) survey.

Category:Sloan Digital Sky Survey Category:Cosmological surveys Category:Astronomical surveys