When the largest map of the universe ever created begins to shake the decades long model upon which cosmologists have relied, what does that say? The three dimensional picture constructed by the Dark Energy Spectroscopic Instrument, or DESI, brings that query to life. On the surface, the visual is stunning enough on its own, with each dot of light marking a galaxy, and our planet located right in the middle of the frame. But it is more than just the visual: DESI has plotted more than 47 million galaxies and quasars, as well as more than 20 million stars the most detailed 3D map of our universe to date.
That is important due to dark energy, which refers to the phenomenon behind the accelerating expansion of the universe, constituting around 70% of everything that exists according to our calculations, but which we have been unable to identify since first theorizing about it nearly two decades ago. The idea behind DESI is to finally shed some light on this mystery by examining the distribution of galaxies through cosmic history. Located atop the Nicholas U. Mayall 4 meter Telescope in Arizona, DESI is a system of 5,000 fiber optic ‘eyes’ capable of collecting light from far off points and analyzing their spectra. Thus, instead of just providing us with another pretty map of stars, DESI gives us a precise way to study how cosmic structure developed over the past 11 billion years, which is where the traces of dark energy lie.
But even more important than the images themselves is the information behind them. Observations of phenomena like dark energy rely on so called ‘baryon acoustic oscillations’, which are essentially a ‘scale’, imprinted on matter during the early history of the universe thanks to sound waves propagating throughout it. Using that as a baseline, scientists can examine its changing size and thus learn how quickly the universe was expanding at any point in time. And in contrast to earlier surveys of this kind such as BOSS, DESI’s larger dataset provides a sharper look at the phenomenon, and a better test of Lambda Cold Dark Matter.
That is why the map in and of itself is a very exciting piece of news. Using DESI data, researchers found that dark energy might not be constant after all. Nathalie Palanque Delabrouille, a scientist at Berkeley Labs involved in the DESI collaboration, summarized the potential implications of this discovery quite succinctly: “This is a major paradigm shift. ” she said, adding, “The weakening acceleration observed by DESI can no longer be explained with a cosmological constant.” Indeed, should the trend hold throughout the whole five year span of observations, the current paradigm regarding the history of the universe would require significant revision. Even DESI’s technical performance has played a role in this.
According to Berkeley Labs, DESI has finished collecting the entire five year dataset before the allotted time, and exceeded the original goal of 34 million galaxies and quasars, which means it will continue its work until 2028 and increase the coverage by around 20%, also returning to certain areas in order to make the sample denser. Consequently, while the currently available image is certainly spectacular on its own, it is only a step towards answering the key questions related to dark energy and potentially rewriting the rules of cosmology altogether. For now, one could say that the map of the universe serves double duty by offering us both a breathtaking image of the night sky, and the data to prove that it behaves in unexpected ways.