Primordial Lights; modelling luminous primordial black holes and their consequences for dark matter
Primordial Lights; research modelling luminous primordial black holes and their consequences for dark matter continues amidst the COVID-19 pandemic
For many researchers in the NYU physics department, collaboration is an essential tool in conceptualising and elaborating new areas of investigation. Plainly, the ongoing COVID pandemic has curtailed many opportunities for spontaneous discussion and debate. Students and faculty are accustomed to being surrounded by laboratories, blackboards and other physicists to test their ideas, but remote working and learning limits many of these resources. Despite these challenges, research methods have been adjusted and new objectives have been developed such that some researchers have found themselves thriving in the new work environment.
Assistant physics professor Yacine Ali-Haïmoud explains that he now meets more frequently with his students to discuss their research and has also found the time to finish off some long-term projects which had previously been lower priority. Normally, he operates by an “open office” policy, whereby his students can come by to get help with any issues they are facing or explain any recent discoveries. Since this is infeasible in a remote work setting, his research group has turned to arranging more regularly scheduled meetings to discuss progress.
As Ali-Haïmoud’s research group primarily focuses on theoretical cosmology and astrophysics, the lack of access to laboratory space has affected their work much less than it has for more experimentally driven teams. Due to closer collaboration within the group and a smooth transition to working remotely, they have achieved rapid progress on several ongoing projects. Trey Jensen, a PhD candidate within the department, explains that his project investigating black holes as a candidate for dark matter remains on-track for publication later this year.
Jensen was initially drawn to Ali-Haïmoud’s group due to his background working with quasars: black holes actively “consuming” gas and dust, producing immense amounts of light and heat in the process. This expertise in black holes has been invaluable in addressing another major topic in cosmology – the origin and make-up of dark matter. Although researchers have developed a strong quantitative understand of dark matter’s abundance throughout the universe, its material properties and composition remain unknown. At present, astrophysicists are investigating a number of candidates for what may constitute dark matter; one promising option are black holes formed in the early universe, so-called primordial black holes (PBHs). Jensen’s project aims to establish constraints on the fraction of dark matter which PBHs may constitute, and to identify an observable signal which may be detected by observational astronomers to verify this constraint.
Applying a model designed by Ali-Haïmoud and associates, Jensen is investigating the luminosity which would be created by PBHs in the early universe when it was more densely packed with gas and dust. In this matter-filled environment, matter spirals into the black hole at incredible speeds and rubs against other nearby particles, which forms vast amounts of heat and light through friction. The efficacy of this process is affected by the relative velocity of the black hole and the dust clouds it is travelling through; at higher relative speeds, the black hole produces less light. A simulation of this negative relationship between velocity and luminosity can be seen in Figure 2.
A model which considers the effect of relative velocity between PBHs and the dense dust clouds of the early universe allows for a more precise prediction of a “spatial statistic” pattern observable in the sky. By comparing this prediction to real-world data, Jensen can adjust the parameters of the model to find what abundance and size of PBHs most accurately reflects the light signals we observe from this time in the universe’s history. These parameters will contribute to the growing body of knowledge about the properties of dark matter, hopefully informing future research to better identify what its composition and how it is dispersed around the universe.
More about Ali-Haïmoud’s group’s work can be found on his website, including a recently completed paper articulating the “tension” which exists between two different experimentally determined values for Hubble’s Constant H0 (a value which describes the rate of expansion of the universe) which may be resolved if reframed from a new perspective. This insight into a decades-old problem in observational astronomy came from a fortuitous conversation over lunch with NYU post-doc Mikhail Ivanov; together, they claim that the tension between the two values of H0can be reframed as a tension instead between different values for T0, the mean temperature of recombination. At about -270 degrees Celsius, this value represents the average background temperature of the universe today as left behind by the afterglow of the Big Bang. Like H0, this temperature is found to have different values depending on the method of observation; Ali-Haïmoud and Ivanov explain how this tension is substitutable for the similar discrepancy in the recorded values of H0.
Should the shutdown continue for an extended time, Professor Ali-Haïmoud points out that the spontaneous interactions like the one motivated this research are unlikely to happen, and that this may challenge researchers who thrive on collaboration to develop new projects. Thankfully, some restrictions have recently been relaxed which will allow more researchers (including undergraduates) to return to in-person work. Hopefully the rich, collaborative environment which inspires and motivates students and faculty to pursue new ideas will steadily return as the new academic year draws near.