From Cosmic Bounces to Primordial Black Holes: The Role of Primordial Magnetic Fields

summary

The gist

The gist: This work investigates how primordial magnetic fields generated during a contracting phase in a non-singular ekpyrotic bounce source curvature perturbations and influence subsequent

In short

This work investigates how primordial magnetic fields generated during a contracting phase in a non-singular ekpyrotic bounce source curvature perturbations and influence subsequent primordial black hole formation. The study shows that these magnetic fields, amplified by time-dependent couplings, create an anisotropic stress that sources curvature perturbations. Using both monochromatic and critical collapse formalisms, the authors constrain the background cosmology to limit PBH dark matter fraction to $10^{-5}$ for benchmark parameters.

Key concepts

Ekpyrotic Bounce: This is a model of early universe cosmology where the universe contracts to a minimum size before 'bouncing' into an expanding phase. The paper uses this scenario because the contraction naturally maintains weak gauge coupling, unlike inflation, which requires fine-tuning for magnetic field generation.
Magnetogenesis in Bouncing Cosmology: This describes how primordial magnetic fields are generated during the contraction phase. A time-dependent coupling between the electromagnetic field and a scalar background breaks conformal invariance, causing gauge field fluctuations to amplify significantly while the universe is contracting.
Primordial Black Holes (PBH) Formation: PBHs are hypothetical black holes formed from large density perturbations in the early universe. The magnetic fields generated during the bounce act as a source for these curvature perturbations, which then collapse into PBHs during the subsequent radiation-dominated era.
Critical Collapse Prescriptions: This is a mathematical method used to calculate how many black holes form from a given initial perturbation scale. It accounts for the near-threshold scaling of density perturbations, allowing researchers to determine the mass distribution and total dark matter fraction of PBHs.

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This episode discusses

The paper

From Cosmic Bounces to Primordial Black Holes: The Role of Primordial Magnetic Fields · Read on arXiv

Ajeet Singhal, Shreya Banerjeea

Indian Institute of Technology (Indian School of Mines)

We investigate primordial black hole (PBH) formation from primordial magnetic fields generated during a non-singular ekpyrotic bounce. A time-dependent coupling between the electromagnetic field and a scalar background breaks conformal invariance and amplifies gauge field fluctuations during contraction, generating magnetic fields whose anisotropic stress dominates during radiation domination. By retaining finite-wavenumber cutoffs, we compute the ultraviolet-dominated curvature spectrum, evaluate density-perturbation variance, and determine PBH abundances using both monochromatic and critical-collapse prescriptions. For benchmark parameters, the critical collapse mass spectrum peaks at M 2.0 times10-3M, with a total PBH dark matter fraction of f PBH about 10-5, forming a subdominant subsolar PBH population. Enforcing the cosmological overclosure bound (f PBH 1) constrains the primary background parameters, namely the end-of-contraction Hubble scale (H end 1.6 times 10 3 GeV) and the contraction exponent (q 0.247), establishing a physical boundary consistent with the electromagnetic backreaction limit. Shifting to asteroid-mass PBHs or a larger dark matter fraction requires modifying the ultraviolet cutoff and contraction duration. This analysis highlights the essential role of finite-wavenumber cutoffs and assumed-collapse prescriptions in linking bounce magnetogenesis to the predicted PBH abundance.

Transcript

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

Vera: We're looking at the paper: "From Cosmic Bounces to Primordial Black Holes".

Jocelyn: The gist: This work investigates how primordial magnetic fields generated during a contracting phase in a non-singular ekpyrotic bounce source curvature perturbations and influence subsequent primordial black hole formation.

Vera: First, who's behind it and why it matters.

Paper summary: Vera: We're looking at how these magnetic fields are made during that contracting phase of a bounce, and now we’re looking at how those fields actually lead to primordial black holes in the later radiation era.

Jocelyn: This paper is called "From Cosmic Bounces to Primordial Black Holes: The Role of Primordial Magnetic Fields," and it's about connecting the dynamics of a bouncing universe directly to the formation of these black hole seeds.

Subrahmanyan: Exactly. The authors take this non-inflationary mechanism and see how it generates density perturbations that, when they evolve, can become black holes after the bounce.

Vera: It means we don't need inflation to get those initial seeds for black holes; a bounce with certain physics can create them naturally.

Jocelyn: But the paper really hammers home that this isn't a free ride; there are very specific conditions they have to meet regarding the Hubble scale at the end of contraction, H end, and the contraction exponent q.

Subrahmanyan: Those conditions are pretty strict because if you want to keep that fraction of black holes as dark matter under one, you’re limited to H end being less than or equal to one point six times ten cubed giga-electron volts and q being less than or equal to zero point two four seven.

Vera: And that puts a real constraint on the physics of the bounce itself, showing what kind of universe allows for this black hole production.

Jocelyn: It’s a testable link because if those constraints aren't met, then we can rule out this particular bouncing model for generating PBHs.

Subrahmanyan: The implications are that finding these black holes would actually tell us something specific about the magnetic fields during the contracting phase.

Vera: And while they give us a pathway to get PBHs from bounces, you still have to look at those statistical assumptions they made about how the curvature perturbations behave.

Jocelyn: That's where things get interesting because quadratic vector sources usually create non-Gaussianities, which means we need to check those effects in future observations.

Subrahmanyan: So this work sets up a pathway for looking at these specific parameters and connecting them to observable phenomena like gravitational waves.

Conclusion: Vera: We've looked at how magnetic fields get made during that contracting phase of a bounce, and now we’re looking at how those fields actually lead to primordial black holes in the later radiation era.

Jocelyn: That paper is titled "From Cosmic Bounces to Primordial Black Holes: The Role of Primordial Magnetic Fields." It's about connecting the dynamics of a bouncing universe directly to the formation of these black hole seeds.

Subrahmanyan: So, this work sets up a pathway for looking at these specific parameters and connecting them to observable phenomena like gravitational waves.

Vera: It shows that even starting with a bounce model, the way the magnetic field behaves statistically dictates how many black holes we actually expect to see.

Jocelyn: This paper provides the framework for those predictions, but it leaves us needing those future tests to see if they match what instruments like LISA or the Einstein Telescope might find.

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