Small Effect Sheds Light on Dark Energy

Small Effect Sheds Light on Dark Energy

A new tool for examining the nature of the universe’s enigmatic dark energy may yield clues to the future of the cosmos.

In the late 1960’s two Russian scientists, Rashid Sunyaev and Yakov Zel’dovich, predicted that if radiation passes through an ionized cloud, the brightness of that radiation will change due to a physical process called inverse Compton scattering, where an electron in the cloud gives up some of its energy to an incoming photon.

Researchers have employed several millimeter-wavelength astronomical telescopes, including the South Pole Telescope (SPT) in Antarctica,1 to search for this Sunyaev-Zel’dovich (SZ) effect.2 Their focus is the cosmic microwave background (CMB) radiation as it passes through the hot intergalactic medium present in clusters of galaxies. By observing the SZ effect in several thousand clusters, researchers hope to place constraints on the physics of dark energy.

The South Pole is a forbidding place to work, but it is also the most ideal site on the planet for making millimeter-wavelength observations. Its high altitude (approx. 9,000 feet above sea level) provides a thin atmosphere, its cold temperatures permit a low amount of water vapor in the air, and its darkness over half the year ensures an extremely low turbulence in the atmosphere above the telescope. The telescope’s Gregorian design, where the secondary reflector is off-axis (see photo), allows an unobstructed field of view, minimizing any scattering of light from its structure. The telescope can observe in three frequency bands—90, 150, and 220 GHz—that are ideal for detecting the SZ effect on CMB radiation. Eventually, new sensors will be added that can also detect the polarization of the radiation.

What can the SPT tell us about the past and future of dark energy? John E. Carlstrom, one of the leaders in the SPT project and the S. Chandrasekhar Distinguished Professor in Astronomy and Astrophysics at the University of Chicago, says the telescope will examine clusters of galaxies to learn what role dark energy played in their evolution. “One of the important things we need to learn about dark energy is what influence it has had on structure,” Carlstrom says. If scientists can learn how the density of clusters changed over time, he says they can determine “constraints on the equation of state of dark energy.”

That is, they can get a more precise idea of whether dark energy is taking us toward a big rip, a big crunch, or something in between. Analyzing follow-up data from optical telescopes, the scientists will determine the mass, distance, and age of the clusters. They will then map the clusters in space and time to see how their density and structure evolved over billions of years under the competing pulls of gravity and dark energy. They hope to learn how much power dark energy exerted in the early universe, how it evolved to dominate the universe now and, by extension, how much power it may wield in the future.3 Such knowledge will present a means to test RTB’s biblical creation model.