Initial Perturbations on Ultra-Small Scales and Primordial Black Holes in the Era of Gravitational-Wave Astronomy

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This award funds the research activities of Professor Yacine Ali-Haimoud at New York University.After decades of increasingly detailed observations, it is well established that the Universe contains a large amount of unknown matter that interacts little, if at all, with known forms of matter. Understanding the nature of this "dark matter" is one of the major unsolved problems in physics. Primordial black holes (PBH's) are fascinating dark-matter candidates. PBH's could have formed in the infant Universe, before the first stars even existed. Professor Ali-Haimoud aims to develop tests of PBH's by predicting their effect on maps of the early Universe and examining the gravitational waves they would emit, and by comparing these predictions with observations. In addition, he will develop a novel way to study the initial conditions of the Universe by quantifying their impact on certain cosmological phase transitions. As such, this research serves the national interest by promoting the progress of fundamental science in the United States. This research will also have significant broader impacts. Professor Ali-Haimoud will include the results of this research in his graduate and undergraduate physics courses and will also directly engage students in these investigations. In addition, he will develop a course for underserved New York City high-school students on the physics of sailing in which he hopes to foster enthusiasm for physics by drawing parallels between sailing and gravitational waves and black holes. More specifically, the first and main focus of this research is to establish robust limits on the abundance of PBH's of about one to one thousand solar masses through their impact on cosmic microwave background (CMB) anisotropies, and especially their gravitational-wave signals for the Laser Interferometer Gravitational-Wave Observatory (LIGO). Professor Ali-Haimoud will extend his analytic calculations of the merger rate of PBH binaries to realistic mass distributions, estimate the resulting stochastic gravitational-wave background accounting for high eccentricities, and establish through numerical simulation whether PBH binaries formed in the early Universe are significantly affected by non-linear structures. He will also extend his work on CMB limits by estimating the self-consistent luminosity of accreting PBH's, and by studying the nature of accretion (spherical vs. disk-like). The second project involves exploring an entirely new idea: the impact of small-scale baryon perturbations on the global recombination history. This effect can probe the currently unconstrained small-scale baryon isocurvature perturbation. Professor Ali-Haimoud will explore this idea quantitatively by studying perturbed radiative transfer in the Lyman-alpha line during recombination.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.