Josh Frieman illuminates dark energy with a telescope and a camera.
When Josh Frieman, PhD'89, was an undergrad at Stanford, famed British cosmologist Dennis Sciama gave a physics colloquium there that inspired Frieman to view cosmology as archaeology on the grand scale. A scientist can study distant objects in the universe—how they are distributed and how they are moving—and fit them together like pottery shards to discover what the early universe looked like.
As a UChicago astronomy and astrophysics professor and Fermilab staff scientist since 1988, Frieman builds on this idea of piecing together observations of the universe to understand how it evolved and what it's made of. He leads the Dark Energy Survey (DES), a project launched last August that takes photographs of distant galaxies using a 570-megapixel camera mounted on the Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in the Andes mountains of Chile. The survey, which brings together more than 300 scientists from 25 organizations around the world, takes about 200 pictures every night, each logging about 80,000 galaxies, and over five years the survey will document more than 300 million of them. Frieman hopes to better understand dark energy, contributing to his already considerable work, for which the Royal Astronomicl Society named him a 2014 honorary fellow.
Ninety-five percent of the universe is thought to be dark—25 percent dark matter, which holds galaxies together, and 70 percent dark energy, which pushes galaxies apart. Matter and dark matter both obey laws of gravity, scientists believe, so if the universe were composed of only matter, "we would expect the expansion of the universe to gradually slow down," says Frieman. Instead, the expansion is accelerating. "There must be something pushing matter away from other matter. Dark energy is something we invented that has that property, that it's gravitationally repulsive. But we don't know what it is."
One possible explanation is the energy of empty space. According to quantum mechanics, energy can be "borrowed' from a vacuum by way of "virtual particles." Energy can convert into a particle and an anti-particle—like an electron and its oppositely charged counterpart, the positron—and those particles immediately annihilate each other and disappear back into energy. "Empty space isn't completely empty; we rather think of it as a kind of roiling sea of virtual pairs of particles continuously popping out of and back into the vacuum." That energy of the vacuum would act as a gravitationally repulsive force that could accelerate the expansion.
Empty space isn't completely empty.
– Josh Frieman
The Dark Enegry Survey is actually two surveys in one: observing the history of the universe's expansion and investigating the growth of large-scale structures, which are increasingly larger organizations of matter, such as galaxy clusters.
Frieman and his team analyze cosmic expansion by taking pictures of the same spots every few nights, looking for supernovae. Frieman previously led the Supernova Survey that was part of the second Sloan Digital Sky Survey; from 2005 through 2007, it discovered and measured hundreds of type Ia supernovae, exploding stars that shine as bright as a whole galaxy for a couple of weeks and then fade over a few months. "They can be used as 'standard candles' to measure cosmic distances," Frieman explains, and were instrumental in discovering cosmic acceleration in 1998. Frieman and his colleagues will use the thousands of supernovae found in this new, deeper survey to compare how fast the universe is expanding today versus billions of years ago.
The survey of large-scale structure growth, on the other hand, will examine the tug-of-war between the gravitational pull from dark matter and gravitational repulsion from dark energy. Photographing one-eighth of the sky, survey researchers will apply three techniques to evaluate the data. One is to map galaxies and compare how tightly they are clustered compared to billions of years ago. The next technique is to "pan out" and take a census of galaxy clusters compared to billions of years ago.
The third technique considers the appearance of the individual galaxies themselves. When light leaves a distant galaxy, it travels in a straight line toward us, but gravity can bend the path of light. Because large-scale structures sit between us and the light source, and those structures exert a gravitational pull, the light we observe follows a curved path that makes the galaxy appear distorted, a tiny effect called "weak gravitational lensing." Frieman and his colleagues look at thousands of galaxies near each other to see if they are all distorted in the same way, and use the effect to map the large-scale dark matter structures.
The Dark Energy Survey could provide more precise measurements supporting the case that dark energy is the energy of a vacuum. Prior measurements are consistent with this explanation but have a great deal of uncertainty. Yet Frieman thinks it would be more interesting if the DES does not support vacuum energy, that it might be something else or something strange going on with gravity. "That would be tremendously exciting because it would mean that there's some new fundamental physics that we don't understand," he says. "We'll have to wait and see." —M.S.