Professor David Roberts (PHYS) and two undergraduate students found that pairs of supermassive black holes at the centers of X-shaped galaxies are significantly rarer than previously estimated—a discovery that has large implications for further research of the universe.

The concept for the project was introduced to Roberts by colleagues and collaborators Lakshmi Saripalli and Ravi Subrahmanyan from the Raman Research Institute in Bangalore, India, while Roberts was at the National Radio Astronomy Observatory (NRAO) in New Mexico in 2012. Roberts’ two Indian colleagues had a similar project rejected because they hadn’t consulted existing data first. Roberts offered to do their project using existing data and brought this project to Brandeis in January 2013.

Here at Brandeis, Roberts enlisted the help of two undergraduate students: Jake Cohen ’15 and Jing Liu. Roberts taught these students how to take previously collected data out of public data archives. In the archives, the students found data on 52 of the X-shaped galaxies they wished to study and then constructed images of the galaxies.

Then Roberts, Saripalli and Subrahmanyan looked at the images that the Brandeis undergraduates had created and used their expertise to determine the most likely explanation for the X-shape. Only in approximately 20 percent of cases did Roberts and his colleagues decide that binary black holes at the center of the galaxy were the most plausible explanation for the galaxies’ X-shape.

Roberts said that the most significant result of these findings has to do with gravitational radiation, which is a disturbance in space and time that moves at the speed of light and was predicted by Einstein’s general theory of relativity in 1915. While there is indirect evidence of gravitational radiation, no direct evidence of exists. However, it is known that binary black holes are one of the most significant sources of gravitational radiation.

Prior to this study, X-shaped galaxies were thought to be “the signposts” of binary black holes. The results of this project, however, suggest that because there are fewer binary black holes than originally thought, there are probably also significantly lower levels of gravitational radiation than originally thought.

Roberts says that finding and directly observing gravitational radiation is “sort of the last great piece that is missing from our way of understanding the universe.” In his opinion, finding gravitational radiation is important because it would allow scientists to test the general theory of relativity. Because binary black holes are the most significant source of gravitational radiation, knowing how many binary black holes there are would give us an indication of how much gravitational radiation there should be. This, Roberts stressed, is the importance of the project.

Roberts plans to continue his investigation into binary black holes in two ways. First, on Nov. 1 most of the galaxies that were determined to be possible candidates for binary black holes are going to be observed using Very Long Baseline Interferometry (VLBI), is a technique that gives angular resolution that is 300 times larger than with the Very Large Array (VLA).

“If there are any binary black holes in there, we might see two different black holes in the same object,” Roberts said. This would be very exciting as no current examples exist of “galaxies with two black holes very close together.” This new project will use computerized telescopes located in Europe to conduct the VLBI according to Roberts’ instructions.

The second continuation of this original project remains in the proposal phase. Roberts has proposed to use VLA at the NRAO in New Mexico in order to do the original experiment “right.” The original project that took place at Brandeis relied heavily on fragmented and pre-existing data. He has proposed looking at not just the 52 objects about which they could find existing data, but all 100 objects that were in the original sample proposed. This research would take place over two years and include many researchers from India, New Mexico and Brandeis.

For Roberts, “The most important thing about the whole project was that all the substantial work was done by … two Brandeis undergraduates.”