Primordial Black Holes

The nature of dark matter remains one of the biggest puzzles in cosmology today. Primordial black holes (PBHs) that formed very early in the history of the Universe have been suggested as possible candidates to make up a large fraction of dark matter. Cosmological observations including the cosmic microwave background, as well as various microlensing surveys carried out from both ground and space have ruled out a vast parameter range for the possible masses of such primordial black holes. However, there exists a window in the mass range 10-11 – 10-14 M where such PBHs remain unexplored thus far. Such PBHs could even make up the entirety of the dark matter, thus solving the dark matter puzzle without recourse to any new particle physics. Such mass scales cannot be explored with light in the optical wavelength range, as the Schwarzschild radii of such black holes turns out to be smaller than the wavelength of light. \daksha~with its wide wavelength coverage the high energy band will probe this mass range. The two \daksha~satellites will enable a unique parallax microlensing experiment for the first time, where the same GRB can be observed with two different lines-of-sight. For a GRB at z=2, the PBH microlensing optical depth, i.e., the probability that a primordial black hole will be closely aligned to its line-of-sight to cause microlensing of the GRB, is ~0.1. The PBH will be at a different impact parameter compared to the GRB for the two different satellites, thus causing a difference in the flux of the measured GRB as seen from the two satellites. A null detection of a difference in flux can constrain the PBH abundance in the unexplored mass range. Alternatively a detection of a difference in the detected fluxes of the same GRB from the two satellites will open an exciting possibility of finding such tiny primordial black holes for the very first time.

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