Group led by Brandeis professor makes new claims about nature of black holes and gravitational waves
A recently published study led by Prof. David Roberts (PHYS), the William R. Kenan, Jr. Professor of Astrophysics at Brandeis, as well as researchers at the Raman Research Institute in India, offers new insights into the implications of gravitational waves predicted in Albert Einstein’s general theory of relativity, concluding that the prevalence of these waves is much smaller than Einstein had originally predicted.
According to Roberts, the single largest source of gravitational waves in the universe is understood to be pairs of supermassive black holes spiraling toward each other until they eventually coalesce.
Roberts described these gravitational waves as the “holy grails of astrophysics at the moment,” partly because it has been difficult to study them and partly because understanding how they work would be a great scientific achievement.
These types of waves have never been directly detected, Roberts said in an interview with the Justice, but the team’s research found only 20 percent as many binary black holes as had previously been estimated using the same technique, meaning that the strength of the gravitational waves emitted would be weaker.
Roberts said that the beginnings of the project came during a sabbatical he took to the National Radio Astronomy Observatory in New Mexico in 2012, where he met two astronomers from India — Lakshmi Saripalli and Ravi Subrahmanyan — who would eventually become his collaborators.
The idea for the recently completed study was originally his Raman collaborators’, but a previous request for the necessary time with the type of telescope they needed was rejected.
Roberts said, “We decided we would do the next best thing which was take data that had already existed and work with those,” a practice he said is common in his field.
With the help of undergraduate students Jake Cohen ’15 and Jing Lu — who has since transferred to a different university — Roberts processed 52 of the 100 objects from the large sample of data derived from the Karl G. Jansky VLA radio telescope, which is housed in the National Science Foundation in New Mexico. From this fragmentary data, Roberts said, the group was able to generate more detailed images.
Roberts emphasized the role of undergraduate work in completing the project. “To me, the most significant part of this is that the bulk of the work was done by Brandeis undergraduates,” he said.
“They received training from me on how to do the technical part of the experiments, and they became quite experts on it.” Although Lu transferred after one year on the project, Cohen stayed on for a second year, and the research eventually became his honor’s thesis, Roberts noted.
On Nov. 1, Roberts will begin taking a more in-depth look at the more likely of the 52 objects to contain binary black holes, observing them in high-resolution using a technique know as VLVI, which stands for very long baseline interferometry.
“There we’re looking for actual binary evidence of black holes when the black holes are very close together, and there’s no direct observational evidence of that yet. So if that succeeds and we’re lucky — that would require us to be very lucky — that could even be more exciting than the other work,” said Roberts.
A proposal to observe all 100 of the objects with the VLA telescope in New Mexico is currently being processed by the committee tasked with deciding who can use the telescope and when.
“The way you get time to use the instruments is that you write a proposal that is refereed competitively with the other proposals. Our proposal would be one of hundreds to use the telescope.”
Roberts and his colleagues will be notified whether their latest proposal has been accepted in approximately six weeks.
The committee accepts proposals on a biannual basis and does not discriminate against projects like Roberts’ that would require longer use of the telescope.
—Abby Patkin contributed reporting.