arXiv subjects

Mehrdad Mirbabayi

Publications and source records attributed to Mehrdad Mirbabayi.

At least 19 recordsLinked to original sources

arXiv2026

Primordial black hole formation in matter domination

We study Primordial Black Holes (PBHs) formed by the collapse of rare primordial fluctuations during an early period of Matter Domination. The collapse threshold strongly depends on the shape of the peaks, decreasing as they become flatter and hence rarer. In the extreme limit of a top-hat perturbation, Harada, Kohri, Sasaki, Terada, and Yoo have argued that the growth of velocity dispersion prevents the formation of black holes unless the initial peak is larger than $ζ_{\rm th} \sim ζ_{\rm rms}^{2/5}$. Including the shape distribution of the peaks, we find that for a realistic cosmic abundance of PBHs, the effective threshold is larger, $ζ_{\rm th} \sim ζ_{\rm rms }^{1/10}$. And this model requires $ζ_{\rm rms}\sim 10^{-1}$, which is much larger than the observed value at the CMB scales. Hence, PBH formation during Matter Domination is barely more efficient than Radiation Domination. We estimate the dimensionless spin parameter to be $a_{\rm rms} \sim ζ_{\rm rms}^{7/4}\ll 1$, slightly larger than PBHs formed in Radiation Domination.

gr-qc↗

arXiv2026

How Heavy Can Moduli Be?

In Kaluza-Klein (KK) compactification of gravitational theories, moduli fields, which are scalar fields associated to the deformations of the compact manifold, are typically lighter than the KK gravitons. However, a universal limit on their mass does not seem to exist. We provide numerical evidence that a light scalar particle, with mass ratio to the first KK graviton $(m_{\rm sc}/m_{1KK})^2 \leq {4/3}$, is necessary for the consistency of the $4d$ effective theory of KK gravitons. This can be interpreted as a limit on how rigidly the compact manifold can be stabilized.

hep-th↗

arXiv2025

Structure Formation with Warm White Noise: Effects of Finite Number Density and Velocity Dispersion in Particle and Wave Dark Matter

We investigate the evolution of density perturbations in dark matter, including the new combined effects of finite number density and non-zero velocity dispersion. Using a truncated BBGKY hierarchy, we derive analytical expressions for the dark matter power spectrum during radiation and matter domination. A component of warm white noise emerges in our analysis, which arises due to the finite number density and undergoes scale-dependent evolution because of the velocity dispersion. Although free streaming erases adiabatic initial perturbations on small scales, warm white noise persists below the free-streaming length and grows during matter domination, with growth suppressed below the dark matter Jeans length. Our calculated power spectra agree with $N$-body simulations in the linear regime and accurately predict halo mass functions in the nonlinear regime. Effects of warm white noise can emerge on observable quasi-linear scales for ultralight dark matter produced after inflation with a subhorizon correlation length. Our formalism is applicable to these scenarios (with de Broglie-scale quasi-particles), to cases in which dark matter includes macroscopic structures (such as primordial black holes), and to traditional warm and cold dark matter scenarios.

astro-ph.CO↗

arXiv2025

Early Growth of Structure in Warm Wave Dark Matter

We explore the growth of structure in wave-like dark matter models, where the field and density spectra are peaked at sub-horizon wavenumbers. Starting with the Schrödinger-Poisson system, we derive the scale-dependent evolution of the matter power spectrum during radiation and matter domination. We find a suppression of adiabatic perturbations during radiation domination, controlled by a free-streaming length, and scale-dependent growth of the initially white-noise isocurvature power, controlled by a Jeans scale during matter domination. The results are in qualitative, and in some regimes quantitative, agreement with the quasi-particle picture. We verify the analytic results of the power spectrum with 3+1-dimensional cosmological Schrödinger-Poisson simulations. We propose an analytic formula for the halo mass function, which is in rough agreement with the simulation results at early times after matter-radiation equality. Our simulations show that early halos typically host a soliton.

astro-ph.CO↗

arXiv2025

An infrared bound on the ultraviolet bounce

Sometimes a local minimum is known to be a metastable vacuum inside the low-energy EFT, but the true vacuum lies outside, and the bounce solution mediating the decay cannot be found. For single-field decay, Espinosa has proposed a family of configurations called ``pseudo-bounces'' as a way to constrain the decay rate. They are parametrized by the central field value $Φ_0$ and constructed without the knowledge of the true vacuum. We prove that the pseudo-bounce family has a monotonically decreasing decay exponent whose end point and minimum is the bounce action, and this continues to hold when gravitational effects are non-negligible. We then use this to estimate the decay rate (in the radion direction) of the promised AdS$_3$ vacuum of the Standard Model.

hep-th↗

arXiv2025

An Observer's Measure of De Sitter Entropy

The two-point correlation function of a massive field $\langleχ(τ)χ(0)\rangle$, measured along an observer's worldline in de Sitter (dS), decays exponentially as $τ\to \infty$. Meanwhile, every dS observer is surrounded by a horizon and the holographic interpretation of the horizon entropy $S_{\rm dS}$ suggests that the correlation function should stop decaying, and start behaving erratically at late times. We find evidence for this expectation in Jackiw-Teitelboim gravity by finding a topologically nontrivial saddle, which is suppressed by $e^{-S_{\rm dS}}$, and which gives a constant contribution to $|\langle χ(τ)χ(0)\rangle|^2$. This constant might have the interpretation of the late-time average of $|\langle χ(τ)χ(0)\rangle|^2$ over all microscopic theories that have the same low-energy effective description.

hep-th↗

arXiv2024

Loosely coupled particles in warm inflation

Cosmological perturbations in warm inflation are conventionally described by an EFT. It consists of the inflaton field coupled to a radiation fluid and applies to energy scales of order Hubble $H$ and inflaton decay width $γ$, but well below temperature $T$. This EFT lacks a proper treatment of inflatons produced with energy $\sim T$. While the direct contribution of these ``UV inflatons'' to the super-horizon curvature perturbations is subdominant because of the cosmological redshift, for $γ\sim H$, their indirect contribution is potentially important -- by their effect on the equation of state, by being exchanged as long-lived intermediate states, and by sourcing gravitational waves with their sizable anisotropic stress. To include these effects, we add to the EFT the Boltzmann distribution of the gas of UV inflatons and compute some of the corrections.

astro-ph.CO↗

arXiv2024

A lower bound on dark matter mass

We argue that there is a lower bound of order $10^{-19}$ eV on dark matter mass if it is produced after inflation via a process with finite correlation length. We rely on non-detection of free-streaming suppression and white-noise enhancement of density perturbations as the observational inputs.

hep-ph↗

arXiv2024

A single-bubble source for gravitational waves in a cosmological phase transition

We show that quantum fluctuations of an expanding phase transition bubble give rise to gravitational wave (GW) emission, even when considering a single bubble, without bubble collisions or plasma effects. The ratio of GW energy to the total bubble energy reservoir increases with time as $\propto t$. If the bubble expands for long enough before percolation destroys it, back-reaction due to the GW emission becomes important after $t_{\rm br}\sim (16π^5) m_{\rm pl}^2R_0^3$, where $R_0$ is the bubble nucleation radius and $m_{\rm pl}$ is the reduced Planck mass. As seen by experiments today, the GW energy spectrum would appear blue. However, simple estimates suggest that the signal falls short of detection by even ambitious future experiments.

gr-qc↗

arXiv2024

The Density of Relic Neutrinos Near the Surface of Earth

It has been claimed that matter effects cause an asymmetry in the density of relic neutrinos versus antineutrinos near the surface of Earth, of order $O(G_F^{1/2})\sim 10^{-4}$, with the vertical extent $\sim 10$m. We argue that the effect is of order $O(G_F)\sim 10^{-8}$, with the vertical extent $\sim 1$mm.

hep-ph↗

arXiv2023

Probing de Sitter from the horizon

In a QFT on de Sitter background, one can study correlators between fields pushed to the future and past horizons of a comoving observer. This is a neat probe of the physics in the observer's causal diamond (known as the static patch). We use this observable to give a generalization of the quasinormal spectrum in interacting theories, and to connect it to the spectral density that appears in the Källén-Lehmann expansion of dS correlators. We also introduce a finite-temperature effective field theory consisting of free bulk fields coupled to a boundary. In matching it to the low frequency expansion of correlators, we find positivity constraints on the EFT parameters following from unitarity.

hep-th↗

arXiv2023

Shapes of non-Gaussianity in warm inflation

Sphaleron heating has been recently proposed as a mechanism to realize warm inflation when inflaton is an axion coupled to pure Yang-Mills. As a result of heating, there is a friction coefficient $γ\propto T^3$ in the equation of motion for the inflaton, and a thermal contribution to cosmological fluctuations. Without the knowledge of the inflaton potential, non-Gaussianity is the most promising way of searching for the signatures of this model. Building on an earlier work by Bastero-Gil, Berera, Moss and Ramos, we compute the scalar three-point correlation function and point out some distinct features in the squeezed and folded limits. As a detection strategy, we show that the combination of the equilateral template and one new template has a large overlap with the shape of non-Gaussianity over the range $0.01 \leqγ/H\leq 1000$, and in this range $0.7 <|f_{\rm NL}| < 50$.

astro-ph.CO↗

arXiv2022

A comment about the cosmology on a bubble wall

The interface between a big bubble of true AdS$_D$ vacuum expanding inside a false AdS$_D$ vacuum is a model of an inflating $D-1$ dimensional universe. It looks like an interesting setup to study fundamentals of inflation. A recent computation shows that the prediction of this model for the wavefunction of the universe disagrees with that of Hartle and Hawking. We show that this discrepancy is because the effective $D-1$ dimensional description of the model is spin-$0$ (Nordström) gravity rather than spin-2 (Einstein) gravity.

hep-th↗

arXiv2022

Snowmass Theory Frontier: Effective Field Theory

We summarize recent progress in the development, application, and understanding of effective field theories and highlight promising directions for future research. This Report is prepared as the TF02 "Effective Field Theory" topical group summary for the Theory Frontier as part of the Snowmass 2021 process.

hep-ph↗

arXiv2022

Snowmass White Paper: Effective Field Theories in Cosmology

Small fluctuations around homogeneous and isotropic expanding backgrounds are the main object of study in cosmology. Their origin and evolution is sensitive to the physical processes that happen during inflation and in the late Universe. As such, they hold the key to answering many of the major open questions in cosmology. Given a large separation of relevant scales in many examples of interest, the most natural description of these fluctuations is formulated in terms of effective field theories. This was the main avenue for many of the important modern developments in theoretical cosmology, which provided a unifying framework for a plethora of cosmological models and made a clear connection between the fundamental cosmological parameters and observables. In this review we summarize these results in the context of effective field theories of inflation, large-scale structure, and dark energy.

astro-ph.CO↗

arXiv2021

Asymptotic Fragility, Near $AdS_2$ Holography and $T\bar{T}$

We present the exact solution for the scattering problem in the flat space Jackiw-Teitelboim (JT) gravity coupled to an arbitrary quantum field theory. JT gravity results in a gravitational dressing of field theoretical scattering amplitudes. The exact expression for the dressed $S$-matrix was previously known as a solvable example of a novel UV asymptotic behavior, dubbed asymptotic fragility. This dressing is equivalent to the $T\bar{T}$ deformation of the initial quantum field theory. JT gravity coupled to a single massless boson provides a promising action formulation for an integrable approximation to the worldsheet theory of confining strings in 3D gluodynamics. We also derive the dressed $S$-matrix as a flat space limit of the near $AdS_2$ holography. We show that in order to preserve the flat space unitarity the conventional Schwarzian dressing of boundary correlators needs to be slightly extended. Finally, we propose a new simple expression for flat space amplitudes of massive particles in terms of correlators of holographic CFT's.

hep-th↗

arXiv2021

Gravity as an ensemble and the moment problem

If a bulk gravitational path integral can be identified with an average of partition functions over an ensemble of boundary quantum theories, then a corresponding moment problem can be solved. We review existence and uniqueness criteria for the Stieltjes moment problem, which include an infinite set of positivity conditions. The existence criteria are useful to rule out an ensemble interpretation of a theory of gravity, or to indicate incompleteness of the gravitational data. We illustrate this in a particular class of 2D gravities including variants of the CGHS model and JT supergravity. The uniqueness criterium is relevant for an unambiguous determination of quantities such as $\overline{\log Z(β)}$ or the quenched free energy. We prove in JT gravity that perturbation theory, both in the coupling which suppresses higher-genus surfaces and in the temperature, fails when the number of boundaries is taken to infinity. Since this asymptotic data is necessary for the uniqueness problem, the question cannot be settled without a nonperturbative completion of the theory.

hep-th↗

arXiv2021

Low-temperature entropy in JT gravity

For ensembles of Hamiltonians that fall under the Dyson classification of random matrices with $β\in \{1,2,4\}$, the low-temperature mean entropy can be shown to vanish as $\langle S(T)\rangle\sim κT^{β+1}$. A similar relation holds for Altland-Zirnbauer ensembles. JT gravity has been shown to be dual to the double-scaling limit of a $β=2$ ensemble, with a classical eigenvalue density $\propto e^{S_0}\sqrt{E}$ when $0 < E \ll 1$. We use universal results about the distribution of the smallest eigenvalues in such ensembles to calculate $κ$ up to corrections that we argue are doubly exponentially small in $S_0$.

hep-th↗