A new experiment may help us figure out what ‘dark energy’ actually is

As an astronomer, there is no better feeling than achieving “first light” with a new instrument or telescope. It is the culmination of years of preparations and construction of new hardware, which for the first time collects light particles from an astronomical object. This is usually followed by a sigh of relief and then the excitement of all the new science that is now possible.

On October 22, the Dark Energy Spectroscopic Instrument (DESI) on the Mayall Telescope in Arizona, US, achieved first light. This is a huge leap in our ability to measure galaxy distances – enabling a new era of mapping the structures in the universe. As its name indicates, it may also be key to solving one of the biggest questions in physics: what is the mysterious force dubbed “dark energy” that makes up the 70 percent of the universe?

The cosmos is clumpy. Galaxies live together in groups of a few to tens of galaxies. There are also clusters of a few hundreds to thousands of galaxies and superclusters that contain many such clusters.

This hierarchy of the universe has been known from the first maps of the universe, which looked like a “stickman” in graphs by the pioneering Centre for Astrophysics (CfA) Redshift Survey. These striking images were the first glimpse of large-scale structures in the universe, some spanning hundreds of millions of light years across.

The CfA survey was laboriously constructed one galaxy at a time. This involved measuring the spectrum of the galaxy light – a splitting of the light by wavelength, or color – and identifying the fingerprints of certain chemical elements (mostly hydrogen, nitrogen, and oxygen).

These chemical signatures are systematically shifted to longer redder wavelengths due to the expansion of the universe. This “red shift” was first detected by the astronomer Vesto Slipher and gave rise to the now famous Hubble’s Law – the observation that more distant galaxies appear to be moving away at a faster rate. This means that galaxies that are close by appear to be moving away relatively slowly by comparison – they are less redshifted than galaxies far away. Therefore, measuring the redshift of a galaxy is a way to measure its distance.

Crucially, the exact relationship between redshift and distance depends on the expansion history of the Universe which can be calculated theoretically using our theory of gravity and our assumptions of the matter and energy density of the universe.