Cosmology with DESI Emission-Line Galaxies
The Growth of Structure Tension
The growth of structure, or $\sigma_8$, refers to how much matter clumps in a sphere of $8$ megaparsec at redshift $0$. This is an important cosmological parameter because its value signifies how the dark sector affects the large-scale structure. For example, the more dark energy density there is in the Universe, the lower is the value of $\sigma_8$ because matter will have less ability to clump or cluster together over time.
What’s interesting is that when we measure $\sigma_8$ using data from recent Universe and compare that value with the value derived from data based on early Universe, they persistently disagree with each other at $2 - 3 \sigma$ level. While this level of disagreement does not necessarily mean the discovery of ``new physics”, it is nevertheless puzzling as to what may be going on.
Combined with the more well-known Hubble tension, the $\sigma_8$ tension could potentially point to the existence of not-yet-understood galaxy physics, or perhaps more excitingly, a hint of “new fundamental physics”.
Dark Energy Spectroscopic Instrument
The Dark Energy Spectroscopic Instrument (DESI) is an ongoing spectroscopic galaxy survey that aims to observe $\sim 50$ million objects to generate the most precise $3$-D map of the Universe to-date. In this effort, DESI will be looking back as far as $~10$ billion years ago, or when the Universe was just $\sim 3$ billion years old.