As galaxies began to form in the early universe, the universe continued to expand. Gravitational attraction between galaxies worked to bring galaxies together in superclusters, while dark energy and the resulting cosmic expansion worked to separate these clusters. As a result, the universe is filled with narrow clusters of galaxies separated by huge cavities of mostly empty space.
The scale of these clusters and cavities is based on the speed at which the universe has expanded over time. The effect is similar to the way air molecules are grouped together by the variable pressure of sound waves, so the effect is known as baryon acoustic oscillation (BAO). Through this effect, astronomers can study dark energy by measuring the position and redshift of more than one million galaxies. The collection and analysis of galaxies was performed for the first time by the Baryon Oscillation (BOSS) spectroscopic study. It was then expanded to eBOSS, which published its first results.
This new study analyzes galaxies at a distance of 0.7 to 1.8 billion light-years, studying the BAO effect just like earlier BOSS studies. But eBOSS also looked at an effect known as red-shift space distortion (RSD). This allowed the team to take into account the movement of the galaxy in space, as well as space expansion.
Within the standard model of cosmology, the distance of a galaxy can be determined by its redshift. As the universe expands everywhere, the farther away a galaxy is, the greater the expansion of space between us and the greater the redshift. But galaxies also move in space, and their relative motion can also contribute to red or bluish. As a result, the total redshift may be distorted, making our BAO measurements less accurate. With RSD, the team can report this statistically, making their overall results much more accurate.
Combining BAO and RSD, the team confirmed the existence of dark energy to a stunning 11-Sigma confidence level. A scientific result for 5-Sigma is usually accepted as confirmation. The result of 11-Sigma is so strong that it is almost so certain that we can achieve it. The dark energy and the accelerating expansion that it drives are definitely real.
Of course, we still don’t know what dark energy really is. One idea is that dark energy is an inherent property of space and time. A cosmological constant that causes the universe to expand. Another is that dark energy is an energy field that fills the universe as the fifth major force. To distinguish between these contradictory patterns, we must not only confirm the existence of dark energy, but also whether it changes over time or varies depending on the direction we look. Research such as eBOSS will give us the data we need to understand the cosmic secret of dark energy.
Reference: Gong-Bo Zhao, et al. “The completed SDSS-IV extended spectroscopic study of the Baryon oscillation: a multitracer analysis in Fourier space to measure the growth and rate of expansion of the cosmic structure.” Monthly notices of the Royal Astronomical Society 504.1 (2021): 33-52.