arXiv subjects

Debaprasad Maity

Publications and source records attributed to Debaprasad Maity.

At least 19 recordsLinked to original sources

arXiv2026

Inflaton accretion onto primordial black holes during reheating

Primordial Black Holes (PBHs) forming prior to Big Bang Nucleosynthesis evolve during the reheating epoch, an environment dominated by an oscillating inflaton field decaying into a relativistic thermal bath. In this work, we track the complete lifecycle of PBHs within this coupled inflaton-radiation background. Utilizing $\alpha$-attractor E-models, we analytically anchor the reheating initial conditions directly to Cosmic Microwave Background observations. By matching exact scalar field solutions in a Schwarzschild spacetime to the cosmological far-zone, we derive the cycle-averaged mass accretion rate and couple it to the growing radiation bath. We find that this combined accretion induces a highly non-linear enhancement of the final PBH mass. Because the Hawking evaporation timescale scales cubically with mass, PBHs forming near their critical runaway limits experience a massive extension of their lifespans. Surviving deeper into the radiation-dominated era triggers a multi-order-of-magnitude amplification in their emitted Stochastic Gravitational Wave Background (SGWB).

astro-ph.CO↗

arXiv2026

The Magnetic Origin of Primordial Black Holes: Ultralight PBHs and Secondary GWs

Ultralight primordial black holes (PBHs) provide a compelling window into early-Universe cosmology. Following our earlier work, we explore a mechanism for the formation of ultralight PBHs sourced by primordial inflationary magnetic fields, without invoking an ultra-slow-roll phase of inflation. We propose a magnetogenesis model in which large curvature perturbations are induced at small scales, leading to the efficient production of ultralight PBHs across a broad mass spectrum. We analyze the phenomenological implications of these ultralight PBHs for early-Universe cosmology, particularly during reheating. We compute the resulting stochastic gravitational wave (GW) background generated by both the electromagnetic spectrum and evaporating PBHs, which exhibits distinctive features tied to the underlying magnetogenesis model parameters. Our results demonstrate that inflationary magnetic fields can serve as a viable and testable origin for ultralight PBHs, opening new avenues for probing the interplay between inflation, magnetogenesis, PBHs, and primordial gravitational waves.

astro-ph.CO↗

arXiv2026

Cosmological correlators from the Inflation end to CMB sky via reheating

We investigate the imprint of post-inflationary evolution on primordial cosmological correlators by relaxing the standard assumption of instantaneous reheating. We compute the power spectrum and bispectrum for a conformally coupled and non-minimally coupled ($\xi \neq \frac{1}{6},0$) scalar field with cubic self-interaction, across a finite reheating epoch characterized by an effective equation of state $w$, and reheating temperature $T_{\rm reh}$. We find that for a conformally coupled scalar field, the signatures of reheating, such as the modification to oscillatory features, are strictly confined to the subhorizon regime. On superhorizon scales, the correlators receive negligible corrections at leading order. In sharp contrast, for a non-minimally coupled field, we identify a distinct phenomenology where non-trivial values of $\xi$ and reheating equation of state $w$, can induce a tachyonic enhancement of the field modes on superhorizon scales. This enhancement modifies both the power spectrum and bispectrum, marking a sharp deviation from the instantaneous transition limit. Our results demonstrate that to the leading order while conformal coupling shields large-scale correlations from the expansion history, non-minimal coupling breaks this degeneracy and opens an observable window into the physics of reheating.

hep-th↗

arXiv2026

Dark matters are Inert, or FIMPy, or WIMPy or UFOy: An inflationary gravitational particle production

In this letter, we explore the phenomenological impact of inflationary gravitational particle production in the physics of Dark Matter (DM). Large-scale DM fluctuations generated during inflation behave as gravitational particles upon their post-inflationary horizon reentry and alter the conventional Boltzmann dynamics of DM with a non-conserving source term, thereby producing significant phenomenological consequences. Within this framework, we analyze four distinct types of DM classified according to their production mechanisms. Dark matter may be completely non-interacting with the thermal bath, behaving as Inert Dark Matter. Alternatively, depending on the strength of its interactions with bath particles, DM may exhibit WIMPy, UFOy, or FIMPy behavior, sharing characteristics with their conventional counterparts. The late-time enhancement of the DM number density, driven by the successive horizon reentry of gravitationally produced low-momentum modes, enlarges the viable parameter space for both thermal and non-thermal DM scenarios. Remarkably, this expanded parameter space remains consistent with current constraints from $\Delta N_{\rm eff}$ and Lyman-$\alpha$ bound.

hep-ph↗

arXiv2025

Evaporation of Primordial Black Holes in a Thermal Universe: A Thermofield Dynamics Approach

We investigate the impact of a finite temperature environment on the Hawking radiation from black holes (BHs), with particular focus on Kerr BHs immersed in a cosmological thermal bath. The emitted particles from BHs interact with the thermal background and thermalize, leading to a modification in the Hawking radiation spectrum. By employing the methods of Thermofield Dynamics (TFD), a real time formalism of thermal quantum field theory, we derive the modified occupation numbers of the Hawking spectrum for asymptotically flat spacetimes like the Schwarzschild and the Kerr geometries. These corrections depend on the interplay between the BH temperature and the ambient bath temperature. We apply this formalism in the early universe reheating background scenario arising after inflation and demonstrate that the thermal correction to Hawking spectrum enhances the evaporation rate of primordial black holes (PBHs). As a result, the lifetime of PBH shortens compared to the zero temperature vacuum and leads to interesting cosmological consequences.

hep-th↗

arXiv2025

Revisiting PBH accretion, evaporation and their cosmological consequences

Primordial black holes (PBHs) provide a unique probe of the early Universe. Their cosmological evolution is governed by the competition between mass accretion and Hawking evaporation. In this paper we look into the details impact of accretion. Most of the earlier analysis relied on non-relativistic accretion models. In this work, we reinvestigate this in a fully relativistic framework for Kerr PBHs in the radiation-dominated era. We derive relativistic accretion rate and compute spin-dependent efficiency $\lambda_{\text{Kerr}}(a_*)$. Using this result, we construct coupled evolution equations for the PBH mass and spin that include both relativistic accretion and spin-dependent evaporation. Our analysis shows that relativistic accretion significantly increases PBH masses and consequently suppresses their spins, causing all PBHs to become effectively Schwarzschild well before evaporation. These effects strengthen the Big Bang Nucleosynthesis (BBN) bound on the initial PBH mass by a factor of $\sim 4$--$5$, reduce the mass required for survival to the present epoch to $\sim 2.7\times 10^{14}\,\mathrm{g}$, and shift the viable particle like DM parameter space. Notably the early accretion induced spin-down effect further washes out the well known high-frequency, spin-induced feature in the high frequency stochastic gravitational-wave background, modifying predictions for future detectors.

astro-ph.HE↗

arXiv2025

Extreme mass ratio inspirals in the cold vector dark matter environment

With regard to the observed dark matter density profile in galaxies and clusters, the scalar dark matter scenario has been previously studied for potential detectability through gravitational wave observations at measurable signal-to-noise ratios. In the present study, we consider the case of dark matter described by a massive vector field, also referred to as the Proca field. The density profile in the vicinity of the black hole is explicitly constructed for a broad range of dark matter mass, $\mu\sim 10^{-10}-10^{-15}{\mathrm eV}$, which allows it to exhibit both particle and wave-like characteristics. While in the particle regime, the computation of the DM density distribution is analytically tractable, we find it convenient to compute the same numerically in the wave regime. Nevertheless, in the outer region, the surrounding dark matter is assumed to follow a broken power-law distribution, represented by a Navarro-Frenk-White (NFW) profile with a central spike. For the purpose of investigating the detectability of the vector dark matter in the gravitational wave spectrum, we have modelled a stellar-mass black hole ($1M_{\odot}$) inspiralling into a Schwarzschild black hole of mass $10^4M_{\odot}$ within such a vector dark matter environment. With this setup, we analyzed the dephasing in the gravitational wave strain induced by vector dark matter and performed a Fisher forecast for upcoming LISA observations, with particular emphasis on the distinctive features in both the particle and wave regimes of the dark matter. Additionally, most of the important results have been compared with the scalar dark matter case.

gr-qc↗

arXiv2025

GRMHD modelling of accretion flow around Sagittarius A$^*$ constrained by EHT measurements

We study low angular momentum, advective accretion flows around a Kerr black hole within the framework of general relativistic magnetohydrodynamics (GRMHD) in the steady state. By solving the full set of GRMHD equations, we aim to provide a comprehensive understanding of the behavior of magnetized plasma in the strong gravity regime near a rotating black hole. The accretion solutions are obtained for a set of input parameters, namely energy (${\cal E}$), angular momentum (${\cal L}$), magnetic flux ($\Phi$), and isorotation parameter ($I$). By systematically varying these parameters, we generate a family of global GRMHD accretion solutions that characterize the physical environment around the black hole. Using this approach, we investigate whether the inferred magnetic field strengths reported by the Event Horizon Telescope (EHT) for Sagittarius A$^*$ at various radii can be reproduced. We find that, for a broad range of parameter values, our model successfully recovers the EHT inferred magnetic field strengths with an accuracy of approximately $10\%$, offering a self-consistent framework for interpreting near-horizon accretion physics.

astro-ph.HE↗

arXiv2025

The Magnetic Origin of Primordial Black Holes: A Viable Dark Matter Scenario

Primordial Black Holes (PBHs) are compelling candidates for explaining the present-day relic abundance of cold dark matter (CDM), yet their formation typically requires finely tuned early-universe dynamics. In this work, we propose a novel PBH formation mechanism within a well-established magnetogenesis framework. This scenario simultaneously accounts for the large-scale magnetic fields observed today and generates an enhanced curvature power spectrum at intermediate scales, leading to PBH formation with masses that can survive until the present epoch. We identify a narrow reheating temperature range, $10^5\,\mathrm{GeV} \leq T_{re} \leq 3\times 10^5\,\mathrm{GeV}$, within which the resulting PBHs can constitute the entirety of the observed CDM abundance. Furthermore, our model predicts a stochastic gravitational wave (GW) background as a byproduct of the PBH formation process. Remarkably, the predicted GW signal lies within the sensitivity reach of upcoming space-based interferometers, such as the LISA, DECIGO, or SKA mission, offering a direct observational probe of this PBH generation mechanism.

astro-ph.CO↗

arXiv2025

Nonminimal infrared gravitational reheating in light of ACT observation

Inflation is known to produce large infrared scalar fluctuations. Further, if a scalar field $(\chi)$ is non-minimally coupled with gravity through $\xi \chi^2 R$, those infrared modes experience \textit{tachyonic instability} during and after inflation. Those large non-perturbative infrared modes can collectively produce hot Big Bang universe upon their horizon entry during the post-inflationary period. We indeed find that for reheating equation of state (EoS), $w_{\phi} > 1/3$, and coupling strength, $\xi>1/6$, large infrared fluctuations lead to successful reheating. We further analyze perturbative reheating by solving the standard Boltzmann equation in both Jordan and Einstein frames, and compare the results with the non-perturbative ones. Finally, embedding this infrared reheating scenario into the well-known $\alpha-$attractor inflationary model, we examine possible constraints on the model parameters in light of the latest ACT, DESI results. To arrive at the constraints, we take into account the latest bounds on tensor-to-scalar ratio, $r_{0.05}\leq 0.038$, isocurvature power spectrum, $\mathcal{P}_{\mathcal{S}} \lesssim 8.3\times 10^{-11}$, and effective number of relativistic degrees of freedom, $\Delta N_{\rm eff} \lesssim 0.17 $. Subject to these constraints, we find successful reheating to occur only for EoS $w_{\phi}\gtrsim 0.6$, which translates to a sub-class of $\alpha-$attractor models being favored and placing them within the 2$\sigma$ region in the $ n_s-r$ plane of the latest ACT, DESI data. In this range of EoS, we find that the coupling strength should lie within $2.11\lesssim\xi\lesssim 2.95$ for $w_{\phi}=0.6$. Finally, we compute secondary gravitational wave signals induced by the scalar infrared modes, which are found to be strong enough to be detected by future GW observatories, namely BBO, DECIGO, LISA, and ET.

astro-ph.CO↗

arXiv2025

Impact of general relativistic accretion on primordial black holes

We demonstrate that general relativistic corrections to the accretion of relativistic matter onto primordial black holes (PBHs) can significantly enhance their mass growth during the early Universe. Contrary to previous Newtonian treatments, our analysis reveals that PBH masses can increase by an order of magnitude before evaporation, leading to substantial modifications of their lifetime and cosmological imprints. We quantify the resulting shifts in the minimum PBH mass constrained by Big Bang Nucleosynthesis (BBN), the revised lower bound for PBHs surviving today, and the dark matter parameter space allowed by PBH evaporation. Furthermore, we show that the enhanced accretion alters the high-frequency gravitational wave spectrum from PBH evaporation, potentially within the reach of future detectors. Our results provide a comprehensive, relativistically consistent framework to delineate the role of PBHs in early-universe cosmology and dark matter phenomenology.

astro-ph.CO↗

arXiv2025

Probing Reheating Phase via Non-Helical Magnetogenesis and Secondary Gravitational Waves

In the past two decades, significant advancements have been made in observational techniques to enhance our understanding of the universe and its evolutionary processes. However, our knowledge of the post-inflation reheating phase remains limited due to its small-scale dynamics. Traditional observations, such as those of the Cosmic Microwave Background (CMB), primarily provide insights into large-scale dynamics, making it challenging to glean information about the reheating era. In this paper, our primary aim is to explore how the generation of Gravitational Waves (GWs) spectra, resulting from electromagnetic fields in the early universe, can offer valuable insights into the Reheating dynamics. We investigate how the spectral shape of GWs varies across different frequency ranges, depending on the initial magnetic profile and reheating dynamics. For this, we consider a well-known non-helical magnetogenesis model, where the usual electromagnetic kinetic term is coupled with a background scalar. Notably, for such a scenario, we observe distinct spectral shapes with sufficiently high amplitudes for different reheating histories with the equation of state parametrized by ($w_{\rm re}$). We identify spectral breaks in the GW spectra for both $w_{\rm re}<1/3$ and $w_{\rm re}>1/3$ scenarios. We find that future GW experiments such as BBO, LISA, SKA, and DECIGO are well within the reach of observing those distinct spectral shapes and can potentially shed light on the underlying mechanism of the reheating phase.

astro-ph.CO↗

arXiv2025

Minimal Magnetogenesis: The Role of Inflationary Perturbations and ALPs, and Its Gravitational Wave Signatures

Any attempt to understand the ubiquitous nature of the magnetic field in the present universe seems to lead us towards its primordial origin. For large-scale magnetic fields, however, their strength and length scale may not necessarily originate from a singular primordial mechanism, namely inflationary magnetogenesis, which has been a popular consideration in the literature. In this paper, we propose a minimal scenario wherein a large-scale magnetic field is generated from the inflationary perturbation without any non-conformal coupling. Due to their origin in the inflationary scalar spectrum, these primordial fields are inherently weak, with their strength suppressed by the small amplitude of scalar fluctuations. We then consider the coupling between this large-scale weak primordial magnetic field and a light axion of mass $<10^{-28}$ eV, which is assumed to be frozen in a misaligned state until the photon decoupling. After the decoupling, when the universe enters into a dark age, the light axion coherently oscillates. By appropriately tuning the axion-photon coupling parameter $\alpha$, we demonstrate that a large-scale magnetic field of sufficient strength can indeed be generated through tachyonic resonance. We further show that the produced magnetic field induces a unique spectrum with multiple peaks of secondary gravitational waves, which the upcoming CMB-S4 can probe through B-mode polarization. The strength can be sufficient enough to violate the PLANCK bound on tensor-to-scalar ratio $r \lesssim 0.036$. Such a violation leads to a constraint on $\alpha \lesssim 80$. With this limiting value of the coupling, we find that present-day magnetic field strength could be as high as $10^{-10}$ Gauss at Mpc scale, consistent with observation.

astro-ph.CO↗

arXiv2025

Generalizing the Bogoliubov vs Boltzmann approaches in gravitational production

We investigate the spectral behavior of scalar fluctuations generated by gravity during inflation and the subsequent reheating phase. We consider a non-perturbative Bogoliubov treatment within the context of pure gravitational reheating. We compute both long and short-wavelength spectra, first for a massless scalar field, revealing that the spectral index in part of the infrared (IR) regime varies between $-6$ and $-3$, depending on the post-inflationary equation of state (EoS), $0\leq w_\phi\leq1$. Furthermore, we study the mass-breaking effect of the IR spectrum by including finite mass, $m_{\chi}$, of the daughter scalar field. We show that for $m_{\chi}/H_{\rm e} \gtrsim 3/2$, where $H_{\rm e}$ is the Hubble parameter during inflation, the IR spectrum of scalar fluctuations experiences exponential mass suppression, while for smaller masses, $m_{\chi}/H_{\rm e}<3/2$, the spectrum remains flat in the IR regime regardless of the post-inflationary EoS. For any general EoS, we also compute a specific IR scale, $k_m$, of fluctuations below which the IR spectrum will suffer from this finite mass effect. In the UV regime, oscillations of the inflaton background lead to interference terms that explain the high-frequency oscillations in the spectrum. Interestingly, we find that for any EoS, $1/9 \lesssim w_\phi \lesssim 1$, the spectral behavior turns out to be independent of the EoS, with a spectral index $ -6$. We have compared this Bogoliubov treatment for the UV regime to perturbative computations with solutions to the Boltzmann equation and found an agreement between the two approaches for any EoS, $0 \lesssim w_\phi \lesssim 1$. We also explore the relationship between the gravitational reheating temperature and the reheating EoS employing the non-perturbative analytic approach, finding that reheating can occur for $w_\phi \gtrsim 0.6$.

gr-qc↗

arXiv2025

Black holes in thermal bath live shorter: implications for primordial black holes

Hawking radiation from a non-extremal black hole is known to be approximately Planckian. The thermal spectrum receives multiple corrections including greybody factors and due to kinematical restrictions on the infrared and ultraviolet frequencies. We show that another significant correction to the spectrum arises if the black hole is assumed to live in a thermal bath and the emitted radiation gets thermalised at the bath temperature. This modification reshapes the thermal spectrum, and leads to appreciable deviation from standard results including modification in the decay rate of black holes. We argue that this altered decay rate has significance for cosmology and, in a realistic setting, show that it alters the life time of primordial black holes (PBHs) in the early universe. In particular, the very light PBHs formed right after the end of inflation decay faster which may have interesting phenomenological implications.

hep-th↗

arXiv2024

Thermal and nonthermal dark matters with gravitational neutrino reheating

We have discussed in detail how neutrinos produced from inflaton solely through gravitational interaction can successfully reheat the universe. For this, we have introduced the well-known Type-I seesaw neutrino model. Depending on seesaw model parameters, two distinct reheating histories have been realized and dubbed as i) Neutrino dominating: Following the inflaton domination, the universe becomes neutrino dominated, and their subsequent decay concludes the reheating process, and ii) Neutrino heating: Despite being sub-dominant compared to inflaton energy, neutrinos efficiently heat the thermal bath and produce the radiation dominated universe. Imposing baryon asymmetric yield, the $\Delta N_{\rm eff}$ constraint at Big Bang Nucleosynthesis (BBN) considering primordial gravitational waves (PGW), we have arrived at the following constraints on reheating equation of state to lie within $0.5\lesssim w_\phi\lesssim1.0$. In these neutrino-driven reheating backgrounds, we further performed a detailed analysis of both thermal and non-thermal production of dark matter (DM), invoking two minimal models, namely the Higgs portal DM and classical QCD pseudo scalar axion. An interesting correlation between seemingly uncorrelated DM and Type-I seesaw parameters has emerged when confronting various direct and indirect observations. When DMs are set to freeze-in, freeze-out, or oscillate during reheating, new parameter spaces open, which could be potentially detectable in future experiments, paving an indirect way to look into the early universe in the laboratory.

hep-ph↗

arXiv2024

Probing a nonminimal coupling through superhorizon instability and secondary gravitational waves

In this paper, we investigate the impact of scalar fluctuations ($\chi$) non-minimally coupled to gravity, $\xi\chi^2 R$, as a potential source of secondary gravitational waves (SGWs). Our study reveals that when reheating EoS $\wre < 1/3$ and $\xi \lesssim 1/6$ or $\wre > 1/3$ and $\xi \gtrsim 1/6$, the super-horizon modes of scalar field experience a \textit{Tachyonic instability} during the reheating phase. Such instability causes a substantial growth in the scalar field amplitude leading to pronounced production of SGWs in the low and intermediate-frequency ranges that are strong enough to be detected by Planck and future gravitational wave detectors. Such growth in super-horizon modes of the scalar field and associated GW production may have a significant effect on the strength of the tensor fluctuation at the Cosmic Microwave Background (CMB) scales (parametrized by $r$) and the number of relativistic degrees of freedom (parametrized by $\dneff$) at the time of CMB decoupling. To prevent such overproduction, the PLANCK constraints on tensor-to-scalar ratio $r \leq 0.036$ and $\dneff \leq 0.284$ yield a strong lower bound on $\xi$ for $\wre < 1/3$, and upper bound on the value of $\xi$ for $\wre > 1/3$. Taking into account all the observational constraints we found the value of $\xi$ should be $ \gtrsim 0.02$ for $\wre =0$, and $\lesssim 4.0$ for $\wre \geq 1/2$ for a wide range of reheating temperature within $10^{-2} \lesssim \Tre \lesssim 10^{14}$ GeV, and for a wide range of inflationary energy scales. Further, as one approaches $\wre$ towards $1/3$, the value of $\xi$ remains unconstrained. Finally, we identify the parameter regions in $(\Tre,\xi)$ plane which can be probed by the upcoming GW experiments namely BBO, DECIGO, LISA, and ET.

astro-ph.CO↗

arXiv2024

Study of a Tilted Thin Accretion Disk around a Kerr-Taub-NUT black hole

The accreting collapsed object GRO J1655-40 could contain the gravitomagnetic monopole (GMM), and it was shown to be better described by the Kerr-Taub-NUT (KTN) spacetime instead of the Kerr spacetime. The warped accretion disk has also been observed for the same collapsed object. Motivated by these, we study a tilted thin inner accretion disk around a KTN black hole. Such a tilting could have a significant effect on the X-ray spectral and timing features via the Lense-Thirring effect. Taking into account the contribution from the inner accretion disk for the KTN black hole, here we calculate the radial profile of a tilt angle. Depending on the numerical values of the viscosity of the accreting material and Kerr parameter, GMM tends the angular momentum of the disk to align along the black hole's spin axis, or to make it more tilted. Our solution for the radial profile of the tilted disk around a KTN black hole could be useful to probe the strong gravity regime, and could also give indirect evidence for the existence of GMM in nature.

astro-ph.HE↗