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A new probe of the small-scale primordial power spectrum: astrometric microlensing by ultracompact minihalos
Cosmology and Nongalactic Astrophysics · Submitted 6 Feb 2012
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim 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.
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The abstract, sentence by sentence
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.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | 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). | background 1.00 | 0.07 | 0.03 | asserted 1.00 |
| s1 | 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. | background 0.50 | 0.10 | 0.04 | asserted 0.97 |
| s2 | 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. | result 0.43 | 0.29 | 0.18 | asserted 0.60 |
| s3 | 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}. | interpretation 0.72 | 0.18 | 0.45 | asserted 0.86 |
| s4 | 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. | interpretation 0.88 | 0.22 | 0.07 | asserted 0.97 |
| s5 | 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. | result 0.65 | 0.44 | 0.60 | hedged possibility 0.99 |
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Original 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.
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References
How the literature uses it
38 arXiv papers cite this in 56 sentences
Papers citing it · 37 sentences without a confident label
Contests it 1 paper
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This finding contradicts the assumption made by previous UCMH work [ref] , throwing into question power spectrum constraints that have been derived from this theory.
Density profiles of ultracompact minihalos: Implications for constraining the primordial power spectrum · Conclusion · contest 0.91
Uses its method or tool 2 papers
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The cosmological mass fraction at horizon entry, $β(M_H)$, which then forms the UCMHs, is related to the present fraction of UCMHs [ref] ,
Neutrino signals from ultracompact minihalos and constraints on the primordial curvature perturbation · Constraints on the primordial curvature perturbation · 0.81
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For example, for local and equilateral bispectral templates, the dimensionless skewness in the dark matter density perturbations at horizon crossing for modes that enter the horizon during radiation domination is [equation] We have integrated over the bispectra, using a tophat window function and the transfer function described in the Appendix of [ref] .
Number Counts and Non-Gaussianity · The approximate PDF and inflation models · 0.96
Compares with it 1 paper
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which is in close agreement with [ref] .
PBH assisted search for QCD axion dark matter · More accurate UCMH profile · 1.00
Cites it as background 14 papers
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For more detailed discussions one can see the appendixes in Refs. [ref] .
Neutrino signals from ultracompact minihalos and constraints on the primordial curvature perturbation · Constraints on the primordial curvature perturbation · 0.93
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The primordial power spectrum can also be constrained by searching for UCMHs using astrometric microlensing [ref] or macrolens distortions [ref] or by constraining the UCMH abundance using pulsar timing arrays [ref] .
Density profiles of ultracompact minihalos: Implications for constraining the primordial power spectrum · Constraining the power spectrum · 0.85
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In addition, UCMHs have been investigated through their gravitational lensing effect [ref] and their contribution to the cosmic reionization [ref] .
Cosmological contribution from population III stars in ultracompact minihalos · Introduction · 0.93
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