I still remember that day over twenty years ago when I was just starting out as a graduate student in theoretical physics, and hearing the faculty and students buzzing about this shocking new discovery from the astrophysics and cosmology community. Not only was the Universe expanding, but it seemed to be accelerating.

Subsequent studies using supernovae and the cosmic microwave background confirmed this discovery, leading to the dark energy problem in cosmology. Approximately three quarters of the energy content of the Universe is not in the form of matter, but appears as some form of substance that is invisible to light, does not coalesce into any form of structure, and has the effect of making spacetime expand faster than previously believed.

And no one knows what dark energy is.

Today saw the start of a new five year mission to map out dark energy, with the astronomers at Kitt Peak National Observatory (located near Tucson, Arizona) using the new Dark Energy Spectroscopic Instrument (DESI) to measure and study tens of millions of galaxies and other distant objects in the universe.

In the end they expect to carefully map the position and velocity of thirty million distant galaxies and other objects, and create a model of how space is expanding. Unlike previous galactic surveys, DESI will measure approximately ten times as many distant objects, and will also carefully measure the spectra of light that they are emitting to deduce their velocities. By averaging over several objects in a region, scientists can calculate how much space itself is expanding (rather than the movement of galaxies within space). By analyzing this model, physicists connected with the project expect to create a map of the density of dark energy at each point. 

And it isn't just dark energy that the new device will be able to study. By mapping the expansion rate of spacetime and the movement of millions of distant, massive objects, physicists and astronomers will also be able to test the predictions of the general theory of relativity and the properties of gravitational forces on large scales. At this point there is no reason to doubt the validity of Einstein's theory, but it also wouldn't be the first time that we have been surprised at the complexity of nature.

So if everything goes well, in five years we will have a much more precise model of the Universe and a better understanding of the nature of dark energy. It is truly an amazing time to be a scientist.