arXiv subjects

Subhasis Maiti

Publications and source records attributed to Subhasis Maiti.

9 recordsLinked to original sources

arXiv2026

Spectral Fingerprints Beyond Degeneracies in Primordial Gravitational-Wave Sources

Stochastic gravitational-wave backgrounds from different primordial mechanisms can produce identical peak frequencies and amplitudes, obscuring their physical origin. We show that the broadband spectral shape breaks this degeneracy. Comparing scalar-induced and gauge-field-induced gravitational waves for identical primordial spectral profiles and cosmological evolution, we find that their spectra retain distinct source-dependent signatures even after matching both the peak frequency and amplitude. While the two mechanisms share a universal deep-infrared behavior, their tensor-source kernels generate different peak morphologies and ultraviolet asymptotic scalings. In particular, the ultraviolet spectrum provides a direct diagnostic of the underlying source mechanism that is insensitive to the overall normalization. Our analysis shows that broadband spectral information contains additional source-dependent signatures that are inaccessible from peak observables alone.

astro-ph.CO↗

arXiv2026

Gravitational Waves from Post-Inflationary Magnetism: Direct and Scalar-Induced Contributions

We study stochastic gravitational waves generated in a post-inflationary magnetogenesis scenario with time-dependent gauge couplings during inflation and reheating. In this setup, magnetic anisotropic stress directly sources gravitational waves, while the induced curvature perturbations generate an additional scalar-induced GW component. We compare the spectral behavior of the two contributions and find that the magnetic component dominates the peak amplitude, whereas the scalar-induced contribution becomes important on larger scales. For blue magnetic spectra with $n_{\rm b}\geq3/2$, both spectra follow the universal infrared scaling $\Omega_{\rm GW}(f\ll f_{\rm peak})\propto f^3$. However, their ultraviolet behaviors differ significantly for $f>f_{\rm peak}$, leading to distinct spectral features. For suitable reheating and magnetogenesis parameters, the resulting GW signal naturally extends into the nano-Hz range relevant for pulsar timing array observations, while remaining consistent with current bounds. The distinct spectral features of the two components may provide a useful probe of reheating dynamics and primordial magnetogenesis.

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↗

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

Magnetogenesis from Sawtooth Coupling: Gravitational Wave Probe of Reheating

The detection of gravitational waves (GWs) by LIGO-Virgo and pulsar timing arrays (PTAs) has opened a new window into early universe cosmology. Yet, the origin of large-scale magnetic fields and the dynamics of the reheating epoch remain poorly understood. In this work, we study the generation of secondary GWs (SGWs) sourced by primordial magnetic fields produced via a Sawtooth-type coupling during reheating with a general background evolution. We show that the reheating equation of state significantly influences the spectral shape and amplitude of the magnetic fields. While a scale-invariant spectrum is typically needed to match observational bounds, this coupling naturally produces a strongly blue-tilted spectrum that remains consistent with current constraints. Crucially, the magnetic field continues to grow during reheating, leading to a GW signal with a broken power-law spectrum and a distinctive blue tilt on super-horizon scales. This SGW signal can fall within the sensitivity of upcoming detectors such as LISA, DECIGO, and BBO. The unique spectral features make this scenario distinguishable from other sources, offering a viable mechanism for cosmic magnetogenesis and a novel probe of the reheating era through GW observations.

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↗

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

Constraining inflationary magnetogenesis and reheating via GWs in light of PTA data

By leveraging the limits on primordial magnetic fields (PMFs), their contributions to secondary gravitational waves (GWs), and the recent observations by the pulsar timing arrays (PTAs), we arrive at constraints on the epoch of reheating. We find that the combined spectral energy density of primary and secondary (generated by the PMFs) GWs can be described as a broken power law with different indices. We show that PMFs with blue spectra and appropriate reheating scenarios can successfully explain the PTA observations without invoking any new physics.

astro-ph.CO↗