Will Percival, the department’s Distinguished Chair in Astrophysics, is a relatively new arrival at the University, where he is building a team to study one of the most challenging problems in contemporary physics —understanding why Universe’s expansion is currently accelerating.
Specifically, Percival studies Baryon Acoustic Oscillations (BAO), a relic pattern created by acoustic waves in the early universe, which are visible across galaxies and can be employed as a way to measure the expansion of the cosmos. Percival describes BAOs as “a cosmic ruler” that can measure the rate of cosmological expansion. Galaxy surveys — both planned and ongoing — will use the power of BAOs to actually measure cosmological expansion and further an understanding of dark energy, the name given to the unknown physics driving the cosmological acceleration.
“We’re tackling some pretty big questions, and see the search for dark energy as an incredible opportunity”
said Percival. “There’s three quarters of the content in the universe that we know very little about. All we know are its basic properties; we know what it has to do to give the observations that we see — basically accelerate the expansion of the universe — but it’s not something we can isolate in the lab or manipulate at the moment. So we’re in this discovery phase where we want to get more data and more information.”
The engineering is in place to undertake new experiments to aid in the study of dark energy. Groundbreaking instruments such as DESI — the Dark Energy Spectroscopic Instrument — will soon be placed on large telescopes. They will measure the effect of dark energy on the expansion of the universe by obtaining optical spectra for tens of millions of galaxies and quasars, constructing a 3D map spanning the nearby universe out to distances 11 billion light years away. DESI, which is in the final stages of completion, will be able to obtain 5000 galaxy spectra at once, an order of magnitude beyond current capabilities. Scientists hope to get the first usable scientific data from the project early in 2020.
“The key thing we want from these experiments is lots and lots of galaxy spectra”
said Percival. “25 years ago, people would point their telescope at one galaxy, get the spectra of that galaxy, and then they wouldpoint the telescope at another galaxy. The big experimental apparatus that has changed this process is the multi-object spectrograph, with which we can obtain multiple spectra simultaneously.”
To obtain its data, the DESI spectrograph robotically places the ends of fiber optic cables on the known locations of galaxy targets, feeding the light to spectrographs, which then disperse and measure the light flux as a function of frequency.
While technologies for galaxy surveys may have advanced considerably in recent years, physicists still don’t have much to go on when it comes to understanding dark energy. “We have so little data,” said Percival. “Given limited observational constraints, theorists have gone a bit wild in their imaginations.”
Some scientists theorize that dark energy results from a previously unknown fundamental force called quintessence, said to fill the universe like a fluid. Another class of models theorizes dark energy through modifications to gravity. Said Percival, “You can make the expansion of the universe appear to accelerate by making gravity weaker on very large scales or at very low densities.” Scientists have also pointed out that dark energy might operate like Einstein’s discarded idea of a cosmological constant; while mathematically simple, this idea is conceptually difficult to understand.
What we do know is that over the next decade our understanding of the Universe will fundamentally change and that Percival and his group will be at the forefront of the forthcoming revolution in our understanding.
