Lensing constraints on ultradense dark matter halos

TL;DR

This paper explores how microlensing surveys can detect ultradense dark matter halos formed from primordial density perturbations, potentially constraining small-scale primordial fluctuations and linking to black hole and gravitational wave phenomena.

Contribution

It demonstrates that current and future microlensing data can probe primordial curvature perturbations at small scales, highlighting the importance of halo internal structures for detection.

Findings

Current data may already constrain primordial perturbations at small scales.

Future HSC data could detect enhancements in the power spectrum at $k \, \sim 10^7/\mathrm{Mpc}$.

Detection prospects are sensitive to the internal structure assumptions of ultradense halos.

Abstract

Cosmological observations precisely measure primordial variations in the density of the Universe at megaparsec and larger scales, but much smaller scales remain poorly constrained. However, sufficiently large initial perturbations at small scales can lead to an abundance of ultradense dark matter minihalos that form during the radiation epoch and survive into the late-time Universe. Because of their early formation, these objects can be compact enough to produce detectable microlensing signatures. We investigate whether the EROS, OGLE, and HSC surveys can probe these halos by fully accounting for finite source size and extended lens effects. We find that current data may already constrain the amplitudes of primordial curvature perturbations in a new region of parameter space, but this conclusion is strongly sensitive to yet undetermined details about the internal structures of these ultradense halos. Under optimistic assumptions, current and future HSC data would constrain a power spectrum that features an enhancement at scales $k \sim 10^7/{\rm Mpc}$, and an amplitude as low as $\mathcal{P}_\zeta\simeq 10^{-4}$ may be accessible. This is a particularly interesting regime because it connects to primordial black hole formation in a portion of the LIGO/Virgo/Kagra mass range and the production of scalar-induced gravitational waves in the nanohertz frequency range reachable by pulsar timing arrays. These prospects motivate further study of the ultradense halo formation scenario to clarify their internal structures.