Penn researchers help map universe’s dark matter
Members of the Dark Energy Survey (DES) have released the first in a series of maps that show the concentration of dark matter in the cosmos. Analysis of the distribution of the dark matter in the maps will improve the understanding of how and where galaxies form, as well as help scientists to probe the nature of dark energy, the mysterious force believed to be causing the accelerating expansion of the universe.
Penn researchers played a leading role in this international effort, which was centered in the research group of Bhuvnesh Jain, a professor and the Edmund J. and Louise W. Kahn Term Chair in the Natural Sciences in the Department of Physics & Astronomy in the School of Arts & Sciences. As the co-coordinator of the DES’s gravitational lensing group, Jain and his colleagues study how otherwise-invisible dark matter bends the light emitted by distant galaxies, giving clues to its location and concentration.
Vinu Vikram, co-lead author of the study that produced the maps, conducted the work while a postdoctoral researcher at Penn. The survey’s project scientist, Gary Bernstein, the Reese W. Flower Professor of Astronomy and Astrophysics at Penn, designed key elements of the survey and its algorithms. Research scientist Mike Jarvis led the first stage of the galaxy image analysis; the mass maps and other lensing results rely on the elegant methods developed by his team. Graduate student Dillon Brout and undergraduates Andrew Neil and Charles Davis also contributed to the Penn team’s work.
“We measured the barely perceptible distortions in the shapes of about 2 million galaxies to construct these new maps,” Vikram says.
These dark matter maps make use of early DES observations and cover only about 3 percent of the area of sky DES will document over its five-year mission. The survey has just completed its second year. As scientists expand their search, they’ll be able to better test current cosmological theories by comparing the amounts of dark and visible matter.
Those theories suggest that galaxies will tend to form where there are large concentrations of dark matter, and thus stronger gravity, present. The maps show large filaments of matter along which visible galaxies and galaxy clusters lie and cosmic voids where very few galaxies reside.
“The cosmic background radiation left over from the Big Bang has ‘hot’ and ‘cold’ spots over the whole sky,” Jain says. “Starting from these primordial ripples in the early universe, gravity led to the growth of filaments and the emptying out of the voids we see in the maps.”
“As the universe ages,” Bernstein says, “these structures get more prominent. Mass attracts mass, and so the rich gets richer. And because most of this mass is dark matter, its concentration tells you where galaxies are more likely to form. That we see galaxy clusters in the same places where the gravitational lensing analysis tells us the dark matter is most concentrated demonstrates that this story makes sense.”
Follow-up studies of some of the enormous filaments and voids, and the enormous volume of data collected throughout the survey will reveal more about this interplay of mass and light.