A new probe of the small-scale primordial power spectrum: astrometric microlensing by ultracompact minihalos

TL;DR

This paper proposes using astrometric microlensing by ultracompact minihalos (UCMHs) to probe the small-scale primordial power spectrum, with Gaia potentially setting new constraints if no lensing is detected.

Contribution

It introduces a novel method of constraining the primordial power spectrum through astrometric microlensing observations of UCMHs by Gaia.

Findings

Gaia can detect UCMHs with masses larger than a few solar masses.

Non-detection by Gaia can set upper limits on UCMH abundance.

Constraints could improve bounds on the primordial power spectrum by three orders of magnitude.

Abstract

The dark matter enclosed in a density perturbation with a large initial amplitude (delta-rho/rho > 1e-3) collapses shortly after recombination and forms an ultracompact minihalo (UCMH). Their high central densities make UCMHs especially suitable for detection via astrometric microlensing: as the UCMH moves, it changes the apparent position of background stars. A UCMH with a mass larger than a few solar masses can produce a distinctive astrometric microlensing signal that is detectable by the space astrometry mission Gaia. If Gaia does not detect gravitational lensing by any UCMHs, then it establishes an upper limit on their abundance and constrains the amplitude of the primordial power spectrum for k~2700 Mpc^{-1}. These constraints complement the upper bound on the amplitude of the primordial power spectrum derived from limits on gamma-ray emission from UCMHs because the astrometric microlensing signal produced by an UCMH is maximized if the dark-matter annihilation rate is too low to affect the UCMH's density profile. If dark matter annihilation within UCMHs is not detectable, a search for UCMHs by Gaia could constrain the amplitude of the primordial power spectrum to be less than 1e-5; this bound is three orders of magnitude stronger than the bound derived from the absence of primordial black holes.