The Other Ninety-Five Percent
How do you study what you can’t see? When it comes to dark matter and dark energy, which we now know make up about 95 percent of our universe, astronomers have started to use an old technique: gravitational lensing. Thanks to Albert Einstein’s theory of general relativity, we know that matter curves space, and light will follow those curves. This allows observers on Earth to “see” dark objects by measuring how light from a distant source bends around it.
In 2010, Eric Jullo, now at the Aix-Marseille University in France, and his team at NASA’s Jet Propulsion Laboratory used Hubble to demonstrate the use of gravitational lensing to study dark matter and dark energy. They looked at Abell 1689, a massive galaxy cluster about 2.2 billion light-years away, “for the stunning number of lensed images it contains,” explains Jullo. By measuring how much each galaxy’s light is distorted, the astronomers used the relativity theory to calculate the mass in the cluster they couldn’t see.
The team eventually published this image, taken with Hubble’s Advanced Camera for Surveys, with the galaxies in false color, while “in purple we overlaid a reconstruction of the matter distribution.” With the information from Hubble, Jullo says, “we showed that the two groups of galaxies visible in the image are actually associated with two distinct halos of dark matter” that affect their orientation, shape, and density. Scientists still know very little about the nature of dark matter and dark energy, but reconstructions like the one Jullo’s team made “can tell us about the growth of the universe.”
Hubble’s ability to take detailed lensing images is just one contribution to the study of this exotic matter. After ground observations of distant supernovas in 1998 revealed that the universe’s expansion was speeding up—thus revealing the existence of dark energy, a discovery that in 2011 garnered a Nobel Prize in physics—astronomers turned to Hubble to confirm these astounding findings. The space telescope’s images were able to eliminate other explanations: This mysterious repulsive energy is causing the universe to expand at ever-faster rates. The continuing search for dark matter and dark energy is now one of the biggest inquiries in science, and Hubble will surely play a part in answering it.