The DESI Dark Energy Survey - An Intriguing Controversy
So there’s been a bit of a kerfuffle over dark energy ever since the results of the Dark Energy Spectroscopic Instrument (DESI) survey dropped in 2023.
If you’re not familiar with dark energy, it’s a phenomenon where the universe’s expansion is accelerating over time, causing distant galaxies to move away from us faster and faster1. While we know very little about what causes this, we do know that if it continues then eventually all distant galaxies will be moving away faster than light in ~100 billion years2. When that happens, they will be physically impossible to see and so our galaxy will be alone in the observable universe, a lonely speck of light in an infinite abyss. What’s particularly interesting is that dark energy only became relevant about 5 billion years1 - not too long before Earth formed!
Now, we would very much like to know what dark energy is precisely because it is so important for the evolution of the universe. Thus we built DESI, a telescope out in the middle of the Arizona desert which is taking pictures of all galaxies in the local universe. The hope is that with a good idea of how a whole lot of galaxies are moving we’ll get a better idea of if and how dark energy varies over time. And the DESI data, plus things like cosmic microwave background studies and type Ia supernova surveys, indicate that dark energy does indeed vary, and that it is in fact weakening3!
The older survey SDSS on the left and the DESI survey on the right. Every dot is a galaxy.
David J. Schlegel & M. Zamani (CC BY-SA 4.0)
Now this result is not firmly established3 and new data from surveys like the one the Rubin Observatory is doing always have a chance of overturning it. But I find it really cool and would love if it were true - it’d be the key to a lot of new physics, and the universe might not get all dark and boring at the end of time! How nice!
References
1 🔒Frieman, J. A., Turner, M. S., & Huterer, D. (2008). Dark energy and the accelerating universe. Annual Review of Astronomy and Astrophysics, 46, 385-432. https://doi.org/10.1146/annurev.astro.46.060407.145243
2 Krauss, L. M., & Scherrer, R. J. (2007). The return of a static universe and the end of cosmology. General Relativity and Gravitation, 39, 1545–1550. https://doi.org/10.1007/s10714-007-0472-9
3 Giarè, W., Najafi, M., Pan, S., Di Valentino, E., & Firouzjaee, J. T. (2024). Robust preference for Dynamical Dark Energy in DESI BAO and SN measurements. Journal of Cosmology and Astroparticle Physics, 10, 35. https://doi.org/10.1088/1475-7516/2024/10/035
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