The 11-sigma detection of darkish power relies on the measurement of over 1,000,000 extraordinarily distant galaxies

After galaxies began to form in the early universe, the universe continued to expand. The gravitational pull between galaxies caused galaxies to contract into superclusters, while dark energy and the resulting cosmic expansion drove these clusters apart. As a result, the universe is filled with tight galaxy clusters separated by huge cavities of mostly empty space.

The extent of these clusters and voids is based on the rate at which the universe has expanded over time. The effect is similar to the way air molecules are clumped together by the varying pressure of sound waves. Therefore the effect is called Baryon Acoustic Oscillation (BAO). This effect allows astronomers to study dark energy by measuring the position and redshift of more than a million galaxies. The collection and analysis of galaxies was first done by the Baryon Oscillation Spectroscopic Survey (BOSS). It was then extended to eBOSS, which published its first results.

A visualization of the Laniakea supercluster, which our galaxy is part of. Photo credit: Tsaghkyan / Wikimedia Commons

This new survey analyzed galaxies 0.7 to 1.8 billion light years away and examined the BAO effect just like the early BOSS studies. However, EBOSS also investigated an effect known as redshift space distortions (RSD). This enabled the team to take into account the movement of a galaxy in space as well as its cosmic expansion.

Within the standard model of cosmology, the distance of a galaxy can be determined by its redshift. As the universe expands everywhere, the further away a galaxy, the greater the expansion of the space between us and the greater the redshift. However, galaxies also move through space, and their relative movement can also contribute to a redshift or blueshift. As a result, the redshift overall could be skewed, making our BAO measurements less accurate. RSD allowed the team to account for this statistically, which made the overall results much more accurate.

By combining the BAO and RSD, the team confirmed the existence of dark energy to an amazing confidence level of 11-sigma. Usually a scientific result for 5-sigma is used as confirmation. A result at 11-sigma is so strong that it is about as certain as we can get it. Dark energy and the accelerated expansion that powers it are definitely real.

Of course, we still don’t know what dark energy 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 reason is that dark energy is an energy field that fills the universe like a fifth fundamental force. To distinguish between these conflicting models, we need to confirm not only the existence of dark energy, but also whether it changes over time or varies depending on the direction you are looking at. Studies like eBOSS provide us with the data we need to understand the cosmic mystery of dark energy.

Reference: Gong-Bo Zhao, et al. “The Completed Extended SDSS-IV Baryon Oscillation Spectroscopy Study: A Multitracer Analysis in Fourier Space to Measure the Growth and Rate of Expansion of the Cosmic Structure.” Royal Astronomical Society monthly notices 504.1 (2021): 33-52.

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