The largest galaxy survey yet conducted with the Hubble Space Telescope confirms Einstein’s theory of General Relativity.
A group of astronomers lead by Tim Schrabback from the Leiden University and including Assistant Professor of Physics Marusa Bradac and Chris Fassnacht, associate professor of physics at UC Davis, studied over 446 000 galaxies in 575 slightly overlapping views of the same part of the Universe using the Advanced Camera for Surveys (ACS) onboard Hubble. It took nearly 1000 hours of observations, during which Hubble circled the Earth nearly 600 times.
“The sheer number of galaxies used is unprecedented, but more important is the precise information we could obtain about the invisible inhabitants of our Universe. And along the way, we were able to say, that Einstein seems to be right after all,” Schrabback said.
Since the 1930’s astronomers have found evidence that only a small fraction of the mass in our Universe is made of directly visible matter. Most mass consists of dark matter, which neither emits nor absorbs any light, but which shows its presence only through the gravitational force it generates. In comparison, visible galaxies are like the tip of the iceberg, where the bulk of the matter remains hidden.
To learn about the distribution of the dark matter, the team used a phenomenon called gravitational lensing, a direct prediction of Einstein’s General Relativity: as light travels through space, its path is slightly bent by the gravity of the matter it passes, distorting the images of distant galaxies.
In addition to the Hubble data, the researchers used ground-based telescopes to assign distances to 194 000 of the galaxies. This helped the team to create a three-dimensional picture for this part of the Universe, giving much more information — just as a three-dimensional CAT scan of a patient gives a doctor a lot more information than an X-ray.
With the three-dimensional picture of this region of the Universe, the scientists were able to test General Relativity and the cosmological constant, which Einstein once nicknamed his “biggest blunder.”
The researchers found that their measurements are in excellent agreement with these two concepts. Furthermore, their results confirm that the cosmological constant, or its generalization, dark energy, currently dominates the energy budget of the Universe and speeds up the cosmic expansion.
Using gravitational lensing this way is a hot technique right now, Bradac said, because it is the only way to measure the distribution of dark matter in the Universe. For example, Fassnacht was co-author on another recent paper that used a single gravitational lens to measure the Hubble Constant. The new study uses many more galaxies and measures dark matter over a much larger scale, Bradac said.
Future surveys will test these theories even more precisely, and maybe someday show whether General Relativity needs to be modified on cosmic scales.
The results will appear in an upcoming issue of Astronomy & Astrophysics.