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

Abstract

The dark matter enclosed in a density perturbation with a large initial amplitude (δρ/ρ 10 −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 10 −5 ; this bound is three orders of magnitude stronger than the bound derived from the absence of primordial black holes.

Neutrino signals from ultracompact minihalos and constraints on the primordial curvature perturbation

“…The methods used by us are mainly from Refs. [9,19,20] and here we only show the main description of the calculations.…”

“…The cosmological mass fraction at horizon entry, β(M H ), which then forms the UCMHs, is related to the present fraction of UCMHs [19,20],…”

“…[9,19], the authors first used the γ-ray observations to obtain the constraints on the abundance of UCMHs for different masses and then obtained the limit on the primordial curvature perturbations for the corresponding scales, P R (k) ∼ 10 −7 − 10 −6 for k ∼ 5 − 10 8 Mpc −1 . The complementary constraints can also be obtained from the effect of astrometric microlensing produced by UCMHs [20]. In this work, we will use the neutrino observations to obtain the constraints on the primordial power spectrum of curvature perturbations for the small scales.…”

Constraining the primordial power spectrum from SNIa lensing dispersion

“…Other methods of probing the primordial spectrum include methods such as Ly α [17], the absence of primordial black holes and Ultracompact Minihalos [19][20][21], measuring spectral distortions of the CMB blackbody spectrum [22][23][24], galaxy weak lensing [25], galaxy correlation functions [26] and cluster number counts [27]. All of which have either different systematics, different k range sensitivity, based on future data or some combination of the above.…”

Isocurvature Perturbation of Weakly Interacting Massive Particles and Small Scale Structure

“…They can be also the sources of neutrinos [24], detectable by IceCube [25]. Furthermore they can leave an imprint in the CMB by changing the reionization history of the Universe [26], produce a microlensing light curve [27,28], or change the direct detection rate [29].…”