03 Jan Entering a New Era of Dark Energy Cosmology
There are several ways to probe the Universe’s expansion. The most established method is the one famously used by Perlmutter and others in the 1990s. It relies on type Ia supernovae, thermonuclear explosions of white dwarfs that are visible even in galaxies ten billion light years away. Since the intrinsic brightness of this type of supernova is known, these events serve as “standard candles”: the brightness observed on Earth can be used to calculate their distances. Astronomers can also determine the speed with which a supernova is moving away from us by measuring its redshift, the change in wavelength of spectral features caused by the Doppler effect. The redshift provides the cosmic time at which we are seeing the explosion, since objects that are receding faster are farther away in both space and time than ones receding more slowly. By measuring the distances of many supernovae as a function of their redshift, researchers can put together the expansion history of the Universe.
DESI will use a different method, based on measuring the large-scale distribution of galaxies. This distribution retains the signatures of a phenomenon known as baryon acoustic oscillations (BAOs). BAOs were produced by acoustic waves in the hot, primordial plasma making up the early Universe. These waves froze into place about 380,000 years after the big bang, when the plasma’s electrons and protons became bound into neutral atoms. This freezing ultimately resulted in subtle, periodic oscillations in the density of galaxies—pick a galaxy, and you are slightly more likely to find other galaxies at a specific distance away from it, a distance called the acoustic scale.
Since the acoustic scale at the freeze-out time can be calculated from first principles, “It’s like having a reference yardstick imprinted in the Universe,” says DESI director Michael Levi. The acoustic scale increased as the Universe expanded, so that it is now almost half a billion light years. Researchers can follow how this yardstick changed with time by studying galaxies at a range of redshifts.
Levi says that using BAOs to determine the Universe’s expansion history is less prone to systematic errors than interpreting supernova measurements, which requires detailed modeling of stellar explosions. Measuring the acoustic scale, however, is anything but straightforward. To measure it precisely, one must observe large numbers of galaxies across billions of light years. Previous BAO surveys mapped about 2.5 million galaxies. DESI will surpass the precision of those surveys by mapping over 35 million galaxies and 2.4 million quasars—extremely bright and distant galactic nuclei. From such maps, DESI will reconstruct the last 11 billion years of the Universe’s expansion. “It’s like taking a 3D MRI scan of the Universe,” says Levi.
DESI will be able to measure such a large number of objects thanks to robotic technology. On the focal plane of the telescope, 5000 optical fibers will each gather light from one galaxy and send the light to a spectrometer, which will provide the galaxy’s redshift. Every 20 minutes, the 5000 robotic positioners will realign the fiber ends, pointing them at a new set of galaxies and cycling through over 100,000 galaxies every night. To prepare the ground for DESI, three previous surveys have observed one third of the sky, collecting images of over one billion galaxies, from which the most promising targets for DESI were selected. These data are collected in a publicly available online Sky Viewer.
DESI opened its eyes to the night sky in October 2019, and its scientists and engineers have been testing and calibrating the instrument. “We are still wrapping our heads around the complexity of this machine,” says Levi. “It’s like using robots to control the most complex Alvin Ailey choreography tens of times every night.”