Primordial Black Holes as a dark matter candidate
Pith reviewed 2026-05-14 23:24 UTC · model grok-4.3
The pith
The detection of gravitational waves has renewed interest in primordial black holes as a dark matter candidate.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim of the paper is that the detection of gravitational waves from mergers of tens of solar mass black hole binaries has led to a surge in interest in primordial black holes as a dark matter candidate, and it provides an overview of their formation in the early universe via collapse of large density perturbations generated by inflation along with the various current and future constraints on their present day abundance.
What carries the argument
Primordial black holes formed via the collapse of large density perturbations generated by inflation, which sets their mass spectrum and determines their viability as dark matter.
If this is right
- If primordial black holes comprise a substantial fraction of dark matter, their binary mergers could account for observed gravitational wave events.
- Improved future observations in microlensing, gravitational waves, and CMB could further restrict or eliminate allowed mass ranges for primordial black holes.
- Key open questions include additional formation mechanisms and effects like evaporation for low-mass black holes.
- Constraints from different methods can be combined to map out the allowed parameter space for their abundance.
Where Pith is reading between the lines
- Confirmation of primordial black holes as dark matter would imply specific features in the early universe inflation that produced the required density perturbations.
- This could connect to tests of inflation models using future cosmic microwave background polarization data.
- Non-detection in upcoming surveys would strengthen the case for particle dark matter candidates instead.
Load-bearing premise
The review assumes that existing constraints from lensing, CMB, and GW data can be combined without major systematic tensions or unaccounted formation channels.
What would settle it
A precise measurement showing that the black hole merger rate or mass distribution matches only stellar formation channels with no room for a primordial component, or conversely a discovery of a unique signature like a specific mass window populated only by primordial objects, would settle the question.
read the original abstract
The detection of gravitational waves from mergers of tens of Solar mass black hole binaries has led to a surge in interest in Primordial Black Holes (PBHs) as a dark matter candidate. We aim to provide a (relatively) concise overview of the status of PBHs as a dark matter candidate, circa Summer 2020. First we review the formation of PBHs in the early Universe, focusing mainly on PBHs formed via the collapse of large density perturbations generated by inflation. Then we review the various current and future constraints on the present day abundance of PBHs. We conclude with a discussion of the key open questions in this field.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript provides a concise overview of the status of Primordial Black Holes (PBHs) as a dark matter candidate circa summer 2020. Motivated by gravitational-wave detections of tens-of-solar-mass black-hole mergers, it reviews PBH formation in the early Universe (primarily via collapse of large inflationary density perturbations), surveys current and future observational constraints on the present-day PBH abundance, and identifies key open questions.
Significance. If the synthesis holds, the review is a timely snapshot of a rapidly evolving subfield, useful for consolidating literature on inflation-generated PBHs and the interplay of lensing, CMB, and GW constraints. No new derivations or machine-checked results are claimed, but the structured aggregation of external constraints and explicit discussion of open questions constitute a service to the community.
major comments (1)
- Abstract: the claim that constraints from lensing, CMB, and GW data can be reviewed together rests on the unstated assumption that systematic tensions or unaccounted formation channels do not invalidate a simple combination; without the full text this central organizing premise cannot be verified and is load-bearing for the overview's utility.
Simulated Author's Rebuttal
We thank the referee for their comments on our manuscript. We address the major comment below and are happy to clarify the scope of the review.
read point-by-point responses
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Referee: Abstract: the claim that constraints from lensing, CMB, and GW data can be reviewed together rests on the unstated assumption that systematic tensions or unaccounted formation channels do not invalidate a simple combination; without the full text this central organizing premise cannot be verified and is load-bearing for the overview's utility.
Authors: The abstract does not assert that the constraints can be combined in a simple or assumption-free manner. The manuscript reviews each observational probe (microlensing, CMB spectral distortions and anisotropies, gravitational-wave searches) in dedicated sections, explicitly stating the assumptions, mass ranges, and systematic uncertainties that apply to each. Where relevant, we note possible tensions between probes and the impact of non-standard formation channels. The full text therefore supplies the details needed to assess the validity of presenting the constraints side-by-side. We can add a sentence to the abstract clarifying that each constraint is discussed with its own caveats if the referee considers this helpful. revision: partial
Circularity Check
No significant circularity; review aggregates external literature
full rationale
This is a review paper that summarizes PBH formation and constraints from prior literature without deriving new equations, predictions, or fitted parameters. The available abstract contains only a historical statement about gravitational wave detections and an outline of topics covered; no self-definitional steps, fitted inputs presented as predictions, or load-bearing self-citations appear. The paper is self-contained as an overview relying on external sources rather than internal reductions.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Standard inflationary cosmology generates density perturbations that can collapse into PBHs
Forward citations
Cited by 60 Pith papers
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The swallowed spike: the formation of light primordial black hole structures around heavy seeds
Light PBHs around heavy primordial seeds form significantly less dense inner cores than particle DM because no studied torque mechanism prevents capture.
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Are Primordial Black Holes a Natural Dark Matter Candidate?
PBH dark matter spans all naturalness tiers, with some mechanisms as natural as WIMPs or freeze-in particles, determined by abundance map structure rather than candidate type.
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Accretion of Primordial Black Holes in Stellar Interiors
Self-consistent spherical accretion simulations show cooling-enhanced growth of PBHs with radiative efficiency ~10^{-2} in the bremsstrahlung regime, yielding a critical seed mass of ~10^{-16} M_sun to consume a solar...
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Carr criterion and mass gaps in non-singular primordial black hole formation
Non-singular gravity with regulator ℓ induces a PBH mass gap ~c²ℓ/G and a stronger Carr criterion δ_H > 2GM_gap/R_H - 1 when R_H ~ ℓ.
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Gravitational Waves from Black Hole Reheating: The Scalar-Induced Component
Accounting for the minimal mass spread of primordial black holes from gravitational collapse suppresses the Poltergeist GW background to the level of generic scalar-induced signals and reopens ultra-light PBH parameter space.
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Tunneling and tidal stripping in multifield ultralight dark matter halos
A semiclassical tunneling model shows that two-field ultralight DM halos have stability bounds that can be relaxed for some density-mass ratios but become more stringent across much of the parameter space compared to ...
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Probing the small-scale primordial power spectrum via relic neutrinos and acoustic reheating
Dissipation of small-scale primordial perturbations after neutrino decoupling cools relic neutrinos and reduces their abundance, enabling PTOLEMY to constrain the primordial curvature power spectrum to O(0.1) on scale...
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Gravitational wave emission from nonspherical collapse in an early matter-dominated era using N-body simulations
Full numerical N-body treatment is required for reliable gravitational wave predictions from nonspherical collapse in early matter-dominated eras, with resulting spectra mappable to detector sensitivities via horizon ...
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Microlensing of fast and slow compact objects
Microlensing surveys constrain fast and slow compact objects at masses and densities differing by orders of magnitude from dark matter limits due to speed-mass degeneracy in Einstein crossing times.
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Microscopic primordial black holes as macroscopic dark matter from large extra dimensions
In the ADD extra-dimension model, microscopic primordial black holes undergo runaway accretion and grow to macroscopic scales, allowing them to comprise all dark matter with initial abundances as low as 10^{-44}.
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Primordial black holes as cosmic accelerators of light dark matter: Novel direct detection constraints
Primordial black hole evaporation generates light fermionic dark matter capable of producing electron recoils in XENONnT, LZ, and PandaX-4T, enabling new constraints on DM-electron interactions after including Earth a...
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PBHs and GWs from Scaling Monopoles
Scaling monopoles generate PBHs via stochastic overdensities and GWs with correlated spectra, potentially with magnetically charged PBHs as a signature if the scaling ends via gauge boson mass.
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One Feature, Three Clocks: Phase-Locked Gravitational Waves, Primordial Black Holes, and Non-Gaussianity from Periodic Warm Inflation
Periodic warm inflation imprints one log-periodic feature on the curvature spectrum that saturates asteroid-mass PBHs, generates dual-band GW backgrounds, and offsets the bispectrum phase by a quarter cycle fixed by s...
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Memoirs of the curvaton: non-perturbative non-Gaussianity and supermassive primordial black holes
Curvaton self-interactions in non-quadratic potentials produce a local non-Gaussian map that enables supermassive primordial black hole formation at peak amplitudes of order 10^{-5} while remaining consistent with μ-d...
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Stochastic constant-roll inflation beyond the hilltop with the spectral method
Spectral solution of the Fokker-Planck operator for hilltop constant-roll inflation shows rare crossing trajectories dominate the mean, so the median yields a coarse-grained ΔN distribution whose exponential tail flat...
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Eigenvalue formulation of Stochastic Inflation and application to large perturbation generating inflationary features
A new eigenvalue method is introduced to compute the PDF of stochastic e-folds in inflation, recovering a known flat-potential result and analyzing constant-drift cases in narrow and broad well limits.
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Opening the Window of Ultra-Light PBHs by Exorcising the Poltergeist
Incorporating the general-relativity mass tail df_PBH/d ln M ∝ M^3.78 smooths PBH evaporation, suppresses the scalar-induced GW signal by orders of magnitude, and reopens the ultra-light PBH window for the hot Big Bang.
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Primordial Black Hole from Tensor-induced Density Fluctuation: First-order Phase Transitions and Domain Walls
Tensor perturbations from first-order phase transitions and domain wall annihilation induce curvature fluctuations at second order that form primordial black holes, allowing asteroid-mass PBHs to comprise all dark mat...
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The Gravitational Spectral Radio Forest: A Signature of Primordial Black Holes
Asteroid-mass primordial black holes induce a Riemann tidal splitting of the 2P_{3/2} hydrogen state, turning the 9.9 GHz line into a ~2 GHz bandwidth gravitational spectral radio forest in H II regions with accretion...
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Axial Quasi-normal Modes of Admixed Neutron Stars
Increasing the bosonic dark matter fraction in admixed neutron stars shifts axial quasi-normal mode frequencies and damping times, can reorder mode hierarchy, and drives a transition from neutron-star-like to boson-st...
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Gravitational Waves from a Black Hole Falling Radially into a Thin-Shell Traversable Wormhole
Analytic gravitational waveforms from radial test-particle infall into a thin-shell traversable wormhole exhibit a characteristic pulse-gap structure from repeated throat crossings and lie within reach of ground-based...
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Gravitational Properties of the Monopole Bag
Monopole bags in axionic backgrounds gravitationally collapse into horizonless states or dyonic regular black holes that evade singularities while retaining axionic hair.
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Asteroid-mass Primordial Black Holes as Dark Matter from Supersymmetry
Supersymmetry with heavy particles above ~10^5 GeV enhances asteroid-mass PBH production via transient equation-of-state softening, allowing them to comprise all dark matter unlike in the Standard Model.
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Boson star-black hole binaries: initial data and head-on collisions
A one-body conformal-factor correction stabilizes boson star-black hole initial data, enabling gravitational-wave analysis that shows higher multipoles can discriminate mixed mergers from pure black-hole binaries.
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High-energy neutrino constraints on primordial black holes as dark matter
High-energy neutrino telescopes constrain sub-asteroid mass primordial black holes with extended mass functions as dark matter, yielding limits slightly weaker than but independent of gamma-ray bounds.
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Primordial Neutron Stars
Large initial baryon asymmetry allows Hubble patches to collapse into primordial neutron stars arrested by nuclear pressure, requiring later entropy dilution to match observed Y_B and BBN.
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Primordial black holes and the velocity acoustic oscillations features in 21 cm signals from the cosmic Dark Ages
Primordial black holes generate up to 30% amplitude VAO wiggles in 21 cm signals from the Dark Ages at redshifts 20-40 even at dark matter fractions as low as 10^-13.
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Constraining memory-burdened primordial black holes with graviton-photon conversion and binary mergers
Derives gamma-ray upper limits on memory-burdened PBH dark matter abundance using graviton-photon conversion during recombination and merger-induced semiclassical evaporation, excluding mass windows 7.5e5-4.4e7 g and ...
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Isotropy, anisotropies and non-Gaussianity in the scalar-induced gravitational-wave background: diagrammatic approach for primordial non-Gaussianity up to arbitrary order
Extends diagrammatic approach for scalar-induced gravitational waves to arbitrary-order local PNG, deriving semi-analytic spectra for energy density, anisotropies, bispectrum and trispectrum up to quartic terms.
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Running into tension: primordial black holes from ultra-slow-roll inflation, spectral running, and the Hubble tension
EDE models increase inferred α_s from CMB data, strengthening tension with USR PBH models that predict negative running.
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Cancellation of one-loop time dependence in superhorizon curvature perturbations from all scales
One-loop time dependence in superhorizon curvature perturbations cancels when all-scale contributions and boundary terms are included consistently via the δN formalism.
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A Solar-System Window for Hidden Stellar Companions
The paper builds an illustrative mass-distance window for hidden-brane companions and finds overlap with the minimum mass of a QCD-scaled hidden star at the upper end of the allowed range.
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A New Origin of the Big Bang from Dark-Sector-Induced Vacuum Decay and Its Gravitational-Wave Signal
A model in which inflaton energy goes exclusively to a dark sector, delaying SM thermalization until a false-vacuum decay produces a GW background with present-day Omega_GW up to 3e-8.
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$\tt BlackHawk$ $\tt v3.0$: Hawking Radiation from Regular Black Holes
BlackHawk v3.0 adds Hawking temperatures and greybody factors for multiple regular black hole metrics to an existing public code via numerical routines.
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Electromagnetic Signatures From Primordial Black Holes in the Solar System
Calculations indicate AMEGO-X could detect PBH transits within 0.1 AU while HAWC and LHAASO could observe explosions out to 0.1-0.5 pc, with future events at ~1000 AU potentially producing measurable electromagnetic s...
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Electromagnetic Signatures From Primordial Black Holes in the Solar System
Calculations show AMEGO-X could detect PBH transits within 0.1 AU of Earth while HAWC and LHAASO could see explosions out to 0.1-0.5 pc, with future 1000 AU bursts potentially yielding measurable EM signals unlike the...
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Inflaton Accretion onto Primordial Black Holes During Reheating
Inflaton accretion during reheating drives non-linear PBH mass growth that extends lifetimes and amplifies emitted SGWB by multiple orders of magnitude.
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Fixing the Renormalization of Inflationary Loops via Ward Identities
Ward identities from large gauge symmetry impose model-independent constraints on renormalizing inflationary loops and non-perturbatively govern the infrared power spectrum evolution.
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Gravitational Properties of the Monopole Bag
Monopole bags in axion models can collapse into horizonless objects or dyonic regular black holes that evade singularities and retain axionic structure through Chern-Simons effects.
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Purely Quadratic Non-Gaussianity from Tachyonic Instability: Primordial Black Holes and Scalar-Induced Gravitational Waves
Purely quadratic non-Gaussianity from tachyonic instability allows narrow curvature spectra to exponentially suppress primordial black hole overproduction via correlation coefficient ρ approaching -1 while retaining s...
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Dark ages bounds on non-accreting massive compact halo objects
Upper bounds on the dark matter fraction in MACHOs of 10^3 to 10^7 solar masses are derived from limits on distortions to the global 21-cm signal at z~17, z~89, and z>300.
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Memory-Burden Suppression of Hawking Radiation and Neutrino Constraints on Primordial Black Holes
Memory-burden backreaction deforms the Hawking spectrum to suppress its high-energy tail, lowering total luminosity and neutrino flux by a factor set by a single suppression parameter and thereby relaxing IceCube boun...
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Self-resonance preheating in deformed attractor models: oscillon formation and evolution
Deformed alpha-attractor T-models with a Gaussian feature near the minimum yield more smaller shorter-lived oscillons during self-resonance preheating, suppressing energy in oscillons and altering the high-frequency g...
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Evaporation of Primordial Black Holes in a Thermal Universe: A Thermofield Dynamics Approach
Thermal bath corrections derived via thermofield dynamics enhance the evaporation rate of primordial black holes, shortening their lifetimes relative to zero-temperature calculations.
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Primordial black holes formation in inflationary $F(R)$ models with scalar fields
Authors add induced gravity and a polynomial potential to an F(R) model, transform to a two-field chiral cosmology, and find parameter choices that match ACT inflation data while yielding PBH masses compatible with da...
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Regular black holes with gravitational self-energy as dark matter
Incorporating non-local gravitational self-energy from a T-duality-inspired model yields a regular neutral black-hole metric with extremal Planck-mass particle-black-hole solutions that are thermodynamically stable an...
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The Intrinsic and Extrinsic Hierarchy Problems
The Hierarchy Problem splits into Intrinsic (RG-induced cutoff sensitivity) and Extrinsic (UV augmentation making IR theory appear finetuned) versions, with the latter formalized as a paradox whose solutions are class...
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Imprints of energy injection by compact dark stars in the 21-cm signal
Compact dark stars from asymmetric dark matter may inject energy that significantly deviates the 21-cm brightness temperature evolution from standard cosmology, offering a new probe for particle dark matter.
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Gravitational Waves from Primordial Black Holes: Connecting Low-Frequency Scalar-Induced Signatures to High-Frequency Binary Mergers
Establishes a model-independent link between scalar-induced GW backgrounds and PBH binary merger signals, including the mass-independent relation f_peak = 1.79 f_ISCO.
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Gravitational Waves from Multiple First-Order Phase Transitions in a Scenario with Early Matter Domination
Early matter domination with time-dependent decay rates produces multiple first-order phase transitions whose gravitational wave signatures encode the transition and reheating temperatures.
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Ultralight dark matter mixed with primordial black holes
In mixed ULDM-PBH halos the continuum PBH contribution alters soliton structure at the tens-of-percent level while discrete shot noise induces mode transitions too slow to matter on galactic timescales.
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Primordial black hole formation in bulk-viscous cosmology
Bulk viscosity raises the critical collapse threshold for primordial black holes by an amount comparable to the viscosity strength and increases the resulting black hole masses.
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Primordial Black Hole Triggered Type Ia Supernovae II: Comparison with Supernova Remnants and Galactic Chemical Evolution
PBH-triggered SN Ia models across metallicities match some observed light curves and remnants, constrain the explosion channel fraction via chemical evolution modeling, and indicate PBHs as a potentially major early-u...
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CMB Bounds on Primordial Black Holes via Radiation Capture
Radiation capture by primordial black holes heavier than 10^15 solar masses modifies cosmic expansion enough for Planck CMB data to bound their abundance below 10 percent.
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Effects of formation channels and gravitational lensing on stochastic gravitational wave background
Using HBI on GWTC-4 data the authors compute lensed SGWBs for ABHs and PBHs and conclude that LIGO and ET can distinguish the two formation channels in specific frequency ranges, with ET offering broader coverage.
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Precision Analysis for $\boldsymbol{H_0}$ Using Upcoming Multi-band Gravitational Wave Observations
Multi-band GW observations of PBHs can reduce H0 uncertainty to ≲2 km/s/Mpc (conservative) or O(0.1) km/s/Mpc (optimistic) via Fisher forecasts on M_PBH and f_PBH.
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In-depth analysis of the clustering of dark matter particles around primordial black holes. Part III: CMB constraints
CMB data limits the s-wave annihilation cross section of thermal dark matter particles to ≲ 10^{-30} cm³/s scaled by PBH fraction and mass for PBHs heavier than ~10^{-10} solar masses.
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Probing Primordial Black Holes with upcoming Radio Telescopes: a case study for LOFAR2.0, FAST Core Array and BINGO
LOFAR2.0, FAST Core Array and BINGO can constrain the PBH dark matter fraction f_PBH below 0.16-0.39 for masses above 10^{-2} to 10 solar masses via FRB lensing statistics.
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Primordial Black Hole signatures from femtolensing and spectral fringe of Gamma Ray Bursts
Spectral fringes in a few GRB observations indicate possible femtolensing by primordial black holes, providing upper limits on their fractional abundance as dark matter.
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Primordial Black Holes Evaporating before Big Bang Nucleosynthesis
PBHs must exceed 10^9 g to affect BBN observables, yielding beta upper limits from 10^{-17} to 10^{-19} for masses 10^9-10^10 g, with public code provided.
discussion (0)
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