CMB Bounds on Primordial Black Holes via Radiation Capture

summary

The gist

The gist This work explores how neutrino capture and photon accretion by primordial black holes, if existing, would impact the redshift evolution of radiation and dark matter densities, and therefore

In short

The study models how primordial black holes (PBHs) capture neutrinos and photons from the cosmic background radiation. This interaction modifies cosmic density evolution and affects CMB anisotropies. Findings show that only extremely massive PBHs leave observable traces on the CMB, setting stringent upper limits on their abundance depending on the mass range studied.

Key concepts

PBH-Radiation Interaction Mechanism: This models PBHs gravitationally capturing background neutrinos and photons. This capture transfers energy from the radiation sector into the black hole's mass, effectively changing how cosmic densities evolve over time.
Capture Cross Sections: The probability of a neutrino or photon being captured depends on the black hole's mass, quantified by specific cross-section formulas. These cross-sections are crucial because they determine the rate at which PBHs interact with the surrounding radiation bath.
CMB Observability Constraints: The effect of this interaction on CMB temperature and polarization is highly sensitive to the PBH mass. Current Planck data constrain the fraction of PBHs ($f_{ m pbh}$) to be very small, especially for heavier masses like $10^{15} M_ ext{odot}$ and above.

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This episode discusses

The paper

CMB Bounds on Primordial Black Holes via Radiation Capture · Read on arXiv

Department of Physics, Shahid Beheshti University · School of Astronomy, Institute for Research in Fundamental Sciences (IPM) · Department of Mathematics and Statistics, The University of Lahore

We explore the capture of neutrinos and photons in the cosmic neutrino and photon background by primordial black holes (PBHs). We model this phenomenon as a gravitational interaction that effectively modifies the continuity equations for radiation and PBH densities and the cosmic expansion history. We find that the observability of this modified cosmic history is highly sensitive to PBH mass, and only extraordinarily massive PBHs would leave observable trace on the temperature and E-mode polarization of the cosmic microwave background (CMB). Specifically, Planck data restrict PBH abundance to f pbh 10-1 for PBH masses above 10 15 M, getting considerably tighter for higher masses. We expect substantial improvement as high-resolution measurements of larger CMB multipoles become available. A future cosmic-variance-limited experiment, with max=7000, would set f pbh 10-1-8 times 10-5 (for the fiducial Λ CDM cosmology) across 10 13-10 18M. These constraints would be comparable to the current limits at the high-mass end of the spectrum [Carr et al, 2026]. The gravitational interaction of PBHS with the cosmic background radiation and its imprints on CMB would thus provide an independent complementary probe of extraordinarily massive PBH abundance.

DOI: 10.1103/tl5k-67r8

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "CMB Bounds on Primordial Black Holes via Radiation Capture".

Jocelyn: Here's the gist of the paper. The gist This work explores how neutrino capture and photon accretion by primordial black holes, if existing, would impact the redshift evolution of radiation and dark matter densities, and therefore the anisotropies of CMB.

Vera: PBH-Radiation Interaction Mechanism The paper models this phenomenon as a gravitational interaction that effectively modifies the continuity equations for radiation and PBH densities and the cosmic expansion history The capture of neutrinos and photons in the cosmic neutrino and photon background by primordial black holes is modeled as a gravitational interaction that effectively modifies the continuity equations for radiation and PBH densities and the cosmic expansion history Consider a population of uniformly distributed PBHs in a radiation bath deep in the radiation-dominated era, consisting of photons and neutrinos Neutrinos, on the other hand, are free streaming at nearly the speed of light Neutrinos and photons passing within the corresponding critical impact parameter of a PBH are irreversibly captured and increase the black hole mass This would effectively transfer energy from the radiation budget of the cosmos to its matter sector.

Jocelyn: Capture Cross Sections and Rates The capture cross section for neutrinos is given by σν = 27π(GM)2, and for photons, it is found to be σγ ≈ 45π(GM)2.

Vera: The rate of neutrino or photon capture by a single PBH would be Rx = σxnx.

Paper summary: Vera: So, we're looking at "CMB Bounds on Primordial Black Holes via Radiation Capture." The authors are essentially using this novel interaction to set bounds on primordial black holes based on CMB observations.

Jocelyn: They explore this as a gravitational interaction that effectively modifies the continuity equations for radiation and PBH densities and the cosmic expansion history, which is how they model the phenomenon <ref:2606.05133#pg1>.

Subrahmanyan: The core idea is that capturing neutrinos and photons in the cosmic neutrino and photon background increases the black hole mass by effectively transferring energy from the radiation budget of the cosmos to its matter sector <ref:2606.05133#pg2>.

Vera: And this is modeled as a fluid approximation analogous to phenomenological treatments of interacting dark matter–dark energy models in the literature <ref:2606.05133#pg1>.

Jocelyn: They give us the continuity equations for component x and PBHs with this energy exchange taken into account, which involves rho dot pbh plus three H rho pbh equals Q nu plus Q gamma <ref:2606.05133#pg3>.

Subrahmanyan: The expansion history is described by H z equals H zero h omega b zero (one + z) cubed plus (one minus fpbh) omega dm zero (one + z) cubed plus omega pbh(z) plus omega gamma(z) plus omega nu(z) plus Omega lambda i over two <ref:2606.05133#pg3>.

Vera: The interaction vanishes in the limiting case of tiny PBHs, ignoring evaporation for a moment, because the capture cross section depends quadratically on the Schwarzschild radius and thus on black hole mass <ref:2606.05133#pg3>.

Jocelyn: This means that as long as this PBH-radiation interaction matters, with negligible right hand sides for equations three, four and seven, these PBHs would not be distinguishable from particle dark matter <ref:2606.05133#pg3>.

Subrahmanyan: There is also a point they make that there is no interaction before PBH formation at zpbh <ref:2606.05133#pg3>.

Conclusion: Vera: So, to wrap up "CMB Bounds on Primordial Black Holes via Radiation Capture," the authors are using this new idea of black hole capture to put constraints on how many primordial black holes there are based on what we see in the cosmic microwave background.

Jocelyn: They did it by modeling a gravitational interaction between these black holes and the neutrino and photon backgrounds that changes how the universe expands over time <ref:2606.05133#pg1>.

Subrahmanyan: This is interesting because it’s not about standard dynamical effects; they are looking at a purely gravitational coupling affecting the energy budget of those components, which connects to structure formation in a different way <ref:2606.05133#pg3>.

Vera: What this means for us is that we get a new way to probe these extraordinarily massive black holes if they exist, using CMB data as a filter <ref:2606.05133#pg1>.

Subrahmanyan: The paper shows that Planck data already restricts the fraction of PBHs to be less than one for masses starting above ten fifteen solar masses, and they get even tighter as those masses go up <ref:2606.05133#pg2>.

Vera: And they point to future observations, suggesting we expect substantial improvement as high-resolution measurements of larger CMB multipoles become available <ref:2606.05133#pg1>.

Vera: And they point to future observations like the gradient of epsilon A Morgulis and K Malal on page eight of thirty <ref:2606.05133#pg1>.

Jocelyn: The final conclusion is that the predators respond to the external short wave signal by a short wave pattern, p zero equals (grad plus grad epsilon) <ref:2606.05133#pg1>.

Subrahmanyan: And the amplitude equals x t is nothing else than the mean predators density <ref:2606.05133#pg1>.

Vera: So, to wrap up this discussion on "CMB Bounds on Primordial Black Holes via Radiation Capture." The authors are using this new idea of black hole capture to put constraints on how many primordial black holes there are based on what we see in the cosmic microwave background.

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