arXiv subjects

Anish Ghoshal

Publications and source records attributed to Anish Ghoshal.

At least 19 recordsLinked to original sources

arXiv2026

Primordial Black Hole Assisted Dirac Leptogenesis

Dirac leptogenesis offers a qualitatively different route to the baryon asymmetry: total lepton number is conserved, but equal and opposite asymmetries are stored in the Standard Model and right handed neutrino sectors, with only the visible component processed by electroweak sphalerons. We present the first dedicated study of how evaporating primordial black holes (PBHs) modify this mechanism. In a minimal charged-scalar mediator setup, we solve the coupled Boltzmann system including thermal production, Hawking emission of the mediator $X$ and of right handed neutrinos, the PBH contribution to the expansion rate, and entropy injection. We find that Hawking emission can populate $X$ in regions where thermal production is inefficient, allowing its subsequent CP-violating decays to enhance the baryon asymmetry. For representative benchmarks, PBHs extend the successful region from mediator masses around $10^9\,{\rm GeV}$ down to about $10^6\,{\rm GeV}$ and can compensate for smaller effective CP asymmetries. This effect is not universal: where thermal Dirac leptogenesis is already efficient, PBHs can instead reduce the final asymmetry. Their evaporation also deposits energy in the right handed neutrino sector and contributes to $\Delta N_{\rm eff}$. In the PBH parameter scan, part of the region that reproduces the observed baryon asymmetry lies below the Planck reference level $\Delta N_{\rm eff}=0.284$ and above the projected CMB-S4 sensitivity $\Delta N_{\rm eff}=0.06$.

hep-ph↗

arXiv2026

Impact of Scale-dependent Primordial Non-Gaussianity on Scalar-induced Gravitational Waves

Scalar-induced gravitational waves (SIGWs) probe primordial curvature perturbations beyond cosmic microwave background scales, while primordial black holes (PBHs) probe the rare tail of the same statistics. We ask whether SIGW morphology can reveal scale-dependent primordial non-Gaussianity. We adopt the internal-leg separable kernel, $\mathcal F_{\rm NL}(\mathbf{k};\mathbf q,\mathbf{k}-\mathbf q)=\bar f_{\rm NL}f(q)f(\lVert\mathbf{k}-\mathbf q\rVert)$, and compute Gaussian, reducible, and connected contributions through $\mathcal O(\bar f_{\rm NL}^{4})$ in the second-order tensor-source approximation. We compare persistent power-law running with a UV-matched tanh weight that saturates at high scalar momentum. Across narrow, finite-width, asymmetric, and multi-slope scalar spectra, running produces more than amplitude renormalization. Power-law running shifts the scalar support sampled by non-Gaussian convolutions, generating peak displacement, asymmetric shoulders, and persistent ultraviolet deformations. The tanh template instead produces a transition-localized modification and approaches a momentum-independent ultraviolet plateau. Analytical estimates of the perturbative hierarchy, weighted slopes, log-normal saddle displacement, peak shifts, and infrared scaling explain the trends and the enhanced sensitivity of the $\mathcal O(\bar f_{\rm NL}^{4})$ sector. For localized sources with finite weighted moments, smooth running preserves the leading $k^3$ infrared behavior up to logarithmic corrections. Since the PBH mass scales as $M\propto k^{-2}$, preferential high-wavenumber weighting conditionally favors lower PBH masses in a narrow-support interpretation. Peak position, spectral curvature, shoulders, and ultraviolet slopes are diagnostics for PTAs, LISA, and third-generation observations, providing finite-order templates for primordial interactions on otherwise inaccessible scales.

astro-ph.CO↗

arXiv2026

Exact Lattice Identities and Continuum-Limit Dyson--Schwinger Equations for Yang-Mills Theory

Starting from SU(N) on the lattice, we give a rigorous derivation of the Dyson--Schwinger equations in the continuum limit. We formulate the Dyson--Schwinger identities for the lattice Yang--Mills theory directly in terms of the link variables $U_\mu(m)\in SU(N)$, exploiting the invariance of the Haar measure under left group translations. This provides an exact lattice derivation of the corresponding master equation for the Wilson action, expressed through left-invariant Lie derivatives acting on individual links. Because the construction is carried out directly on the compact gauge group, it avoids the ambiguities associated with introducing Lie-algebra valued gauge potentials as primary integration variables at finite lattice spacing. For practical applications, in a second part we then break down the gauge degree of freedom by choosing Feynman gauge. We analyze the continuum-limit form of the resulting lattice identities and derive equations for the one- and two-point connected functions. Under a further simplifying reduction, these equations close to a tractable scalar system. Our results establish a direct bridge between exact lattice identities and the functional equations commonly used in continuum nonperturbative studies of Yang--Mills theory.

hep-th↗

arXiv2026

Magnetically assisted primordial scalar perturbations: Scalar-Induced Gravitational Waves

Primordial magnetic fields (PMFs) provide a well-motivated source of cosmological perturbations through their anisotropic stress and may leave observable imprints in both the scalar and tensor sectors. In this work, we study scalar metric perturbations sourced by a PMF evolving through a finite post-inflationary epoch characterized by a constant equation-of-state (EoS) parameter $w$. Working in a gauge-invariant framework, we derive the sourced evolution equation for the Bardeen gravitational potential in a general constant-$w$ background. Our analysis indicates faster (slower) growth of PMF-sourced scalar perturbations on superhorizon scales for a stiffer $w>1/3$ (softer $w<1/3$) background compared to the marginal radiation-dominated scenario with $w=1/3$. We then derive the scalar-induced gravitational wave (SIGW) background induced at second order by the magnetically generated scalar perturbations. Our analysis indicates a narrow viable parametric region for a kination-like post-inflationary era and a nearly scale-invariant PMF that may give rise to a PMF-sourced SIGW signal dominating over the direct PMF-generated tensor background. Simultaneously, the induced tensor modes are found to be perturbatively small compared to the magnetic sector, which, in turn, exerts negligible backreaction on the background. Interestingly enough, for an inflationary scale $H_{\rm inf}\sim10^3-10^9$ GeV and duration of reheating in $e$-folds $N_{\rm reh}\sim8-10$, the dominant PMF-sourced SIGW signal may be detectable by next-generation terrestrial and space-based interferometric GW detectors spanning the mHz$-$kHz frequency range.

astro-ph.CO↗

arXiv2026

Particle production from bubble collisions

Collisions of ultra-relativistic bubbles during cosmological phase transitions can produce particles much heavier than the transition scale. Previous analyses modelled this process as the off-shell decay of the scalar background. We show that its results parametrically overestimate hard particle production and depend on the gauge choice and the coordinate choice in field space. We propose an alternative formalism, analogous to the partonic description of high-energy collisions. In the ultra-relativistic limit, the colliding bubbles undergo nearly free passage and hard production arises from on-shell scatterings among the quanta constituting the Lorentz-contracted walls. We apply this approach to heavy scalar, fermion, and vector particle production, and study the implications for dark matter, leptogenesis, graviton production and primordial gravitational waves.

hep-ph↗

arXiv2026

Gravitational Waves from hybrid defects as probe of Flavor symmetry breaking: Machine-Learning Approach

We present a novel possibility that a network of domain walls bounded by cosmic strings generates a stochastic gravitational wave background (SWGB) signal originating from the spontaneous breaking of a gauged $U(1)_F$ flavor symmetry and the subsequent breaking of discrete $Z_2$ symmetry that accommodates dark matter. The gravitational wave (GW) spectrum produced by the string-bounded-wall network can be detected for high $U(1)_F$ breaking scales in forthcoming GW detectors including LISA, ET and SKA. The GW signal exhibits a distinctive frequency slope, in the infrared, compared to the standard cosmic-string case, in the frequency range between micro-hertz and hertz. We develop a possible strategy to distinguish and characterize GW spectrum of the hybrid defect from from other defects, such as stable cosmic strings, via employing the exact calculation with a machine-learning surrogate, based on a multilayer perceptron (MLP), trained on spectra obtained from the full numerical treatment. This is then used for rapid inference in the detector-specific signal-to-noise ratio (SNR) computation which also makes the process fast and efficient. We also discuss some possible complementarity between GW searches and Flavor observables in the laboratory.

astro-ph.CO↗

arXiv2026

Gaussian Process Reconstruction of Cosmological Parameters with Gravitational Wave Sirens using Machine Learning

Future gravitational wave (GW) standard siren catalogues will probe the late-time expansion history of the Universe across redshift ranges largely inaccessible to traditional electromagnetic observations. To determine how effectively this background distance information can distinguish between viable cosmological models, we introduce a model-independent reconstruction framework utilizing Gaussian Process Regression (GPR). Analyzing mock LISA and Einstein Telescope (ET) catalogues across six fiducial cosmological backgrounds-$\Lambda$CDM, CPL, CPL+$\Lambda$, interacting dark matter, interacting dark energy and axion inspired early dark energy. We reconstruct the comoving distance and its derivatives. Crucially, we propagated the full GP covariance, including derivative cross-covariances, to robustly evaluate the Hubble parameter $H(z)$ and other diagnostics such as $q(z)$, $\mathcal{O}_{m}(z)$ $w_{\rm total}(z)$ and $\kappa(z)$. While our analysis demonstrates that GW bright standard sirens faithfully recover fiducial expansion histories, applying pointwise marginal Hellinger distance reveals that background measurements alone do not provide decisive statistical separation among models. Instead, derivative sensitive diagnostics pinpoint specific redshift windows (e.g., $z\simeq1.6-1.8$ for ET and $z\simeq2.6-2.9$ for LISA) where future catalogues will maximize their discriminatory power. As machine learning methodologies become increasingly integral to astrophysics and cosmology, this Bayesian GPR pipeline offers a principled, nonparametric approach to precisely identifying where the most valuable cosmological information lies.

astro-ph.CO↗

arXiv2026

Primordial Black Hole from Tensor-induced Density Fluctuation: First-order Phase Transitions and Domain Walls

Gravitational waves generated by violent processes in the early Universe necessarily couple to scalar perturbations beyond linear order. We show that tensor perturbations produced by first-order phase transitions and by annihilating domain-wall networks source density fluctuations at second order, thereby opening a distinct channel for primordial-black-hole (PBH) formation. We compute the tensor-induced density spectrum for both sources and map the resulting PBH abundance onto the macroscopic parameters $(\alpha,\beta/H,T_\star)$ of a first-order phase transition and $(\alpha_{\rm ann},V_{\rm bias},\sigma)$ of a domain-wall network. Existing PBH limits therefore impose complementary constraints on early-Universe sources of stochastic gravitational waves. We find viable regions where one can have an observable gravitational-wave background and an appreciable abundance of asteroid-mass PBHs, including benchmark points that saturate the dark-matter abundance. Our results establish a direct, testable correlation among the source scale, the gravitational-wave spectrum, and the PBH mass function, distinguishing this tensor-induced channel from PBHs formed through delayed vacuum decay or direct defect collapse. We also discuss viable particle physics origin of such FOPT and DW, and therefore, constraints on such microphysics, either in the visible, or in dark sector models.

astro-ph.CO↗

arXiv2026

Primordial Magnetogenesis and Gravitational Waves from ALP-assisted Phase Transition

Sufficiently strong first-order phase transitions (FOPTs) in the early Universe can simultaneously produce an observable stochastic gravitational wave background (SGWB) and a large-scale primordial magnetic field (PMF). The recent $3.8\sigma$ evidence for a non-zero intergalactic magnetic field from anisotropic pair-halo searches using \textit{Fermi}-LAT data further motivates a cosmological origin. We investigate an FOPT-origin of both cosmic signatures, namely, PMF and SGWB, and their correlation, within a minimal axion-like particle (ALP) framework in which a global $U(1)$ symmetry is spontaneously broken through radiative corrections, with the ALP sector coupled to the Standard Model (SM) via a Higgs-portal. We compute the present-day PMF amplitude and coherence length for both maximally helical and non-helical configurations, accounting for inverse cascade effects. For maximally helical configurations, we find peak field strengths up to $B_0 \sim 10^{-9}$ G at coherence length $\lambda_0 \sim 10^{-3}-10^{-1}$ Mpc, consistent with lower bounds on the IGMF inferred from blazar observations by MAGIC, H.E.S.S. and {\it Fermi}-LAT. We show that the ALP parameter region consistent with $\gamma$-ray blazar data (assuming maximally helical fields) simultaneously produces stochastic GW signals detectable at future space-based interferometers, such as LISA, etc., over the ALP decay constant range $10^3~\text{GeV} \lesssim f_a \lesssim 10^5~\text{GeV}$. We demonstrate that these correlated constraints can be directly mapped onto effective ALP couplings to SM particles, e.g., photons, gluons, and fermions. This establishes a multi-messenger complementarity between cosmological observables and laboratory/astrophysical ALP searches, with combined constraints preferring relatively heavy ALPs, $m_a \gtrsim 0.1~\text{GeV}$, accessible to next-generation intensity and energy-frontier experiments.

hep-ph↗

arXiv2026

Irreducible Gravitational Wave Background as a Particle Detector

Beyond the standard model particles can imprint a temporary early matter-dominated epoch on a stochastic gravitational-wave background. For a freely propagating background whose production has ceased, we show that the resulting feature contains two characteristic frequencies that determine the particle decay rate $\Gamma$ and the abundance-weighted mass $Y_iM$, independently of the microphysical origin of the gravitational-wave source. Using numerical scans, we derive semi-analytic relations between these characteristic frequencies and the BSM particle parameters, and quantify the parameter ranges over which these relations remain valid. For backgrounds that remain actively sourced, the end of early matter domination continues to define a source-independent cosmological frequency scale controlled by $\Gamma$, although the second feature, and hence the inference of $Y_iM$, is source dependent. Once a particle model and production mechanism are specified, the measured combinations can be translated into masses and microscopic couplings. Remarkably, decay lengths targeted by upcoming long-lived-particle searches map directly onto the nanohertz range in which NANOGrav has reported evidence for a stochastic gravitational-wave background.

hep-ph↗

arXiv2026

CMB signatures of gravity-mediated dark radiation in $\mathbf{\Delta N_{\rm eff}}$

Measurement of $N_{\rm eff}$ in the CMB (Cosmic Microwave Background) observations, like Planck 2018 and BBN (Big Bang Nucleosynthesis) has already set stringent constraints on the interaction strength of light particles beyond the Standard Model (BSM). Despite such negligible couplings of such BSM particles to the visible sector, they are inevitably produced in the early universe through gravity-mediated processes. If a sizable density of light particles survives around CMB formation, they may act as dark radiation (DR) contributing to $N_{\rm eff}$ at CMB epoch. In this work, we study the production of such light BSM particles through the gravity-mediated scatterings in an effective field theory (EFT) setup assuming that all non-gravitational couplings of the BSM particle are negligible. Since the production is sensitive to the spin of the produced particle, we perform a concrete analysis for two representative cases: scalar dark Higgs DR and vector dark photons DR.Using the Planck 2018 observations, we find constraints on the reheating temperature ($T_{\rm RH}$) and background equation of state ($w_\Phi$) during reheating in such scenarios featuring dark Higgs and dark photon. A comparative discussion involving gravity-mediated production of Dirac right-handed neutrinos ($\nu_R$) and light axion-like particles (ALP) is also presented. Finally, for completeness, we also analyze the scenario where the production occurs through a generic spin-2 mediator characterized by an effective scale $\Lambda$ delineating the parameter space that is currently ruled out from Planck-2018 and can be probed by the future CMB experiments like LiteBird, Simon Observatory, CMB-S4, CMB-HD.

hep-ph↗

arXiv2026

Induced Multi-phase Inflation with Reheating: Leptogenesis and Dark Matter Production in Metric versus Palatini

We study non-minimally coupled scalar-induced multi-phase inflation in metric and Palatini gravity, considering linear, Brans-Dicke-like, and Higgs-like sectors. The scalar spectral index lies in the range $ n_s \simeq 0.93 \ \text{--} \ 0.98 $, consistent with \textit{Planck} and combined \textit{Planck}+ACT data. The tensor-to-scalar ratio can reach $ r \sim 0.03 $ in metric, whereas Palatini models generically predict $ r \lesssim 10^{-5} $. In the Palatini case, field excursions remain sub-Planckian, and the perturbative unitarity cutoff is raised. Reheating proceeds via perturbative inflaton decays into Higgs bosons and fermionic dark matter (DM) through the portal coupling $ \lambda_{12} $ and Yukawa coupling $ y_\chi $. Radiative stability of the inflationary plateau constrains the couplings to $ y_\chi, \lambda_{12} \sim 10^{-7} \ \text{--} \ 10^{-3} $, implying $ 4\,\mathrm{MeV} \lesssim T_{\rm rh} \lesssim 10^{15}\,\mathrm{GeV} $. Palatini realizations require smaller couplings and thus a narrower reheating window. Non-thermal DM production $\chi$ from inflaton decays is viable for DM mass $ m_\chi \sim \mathrm{keV} \ \text{--} \ \mathrm{PeV} $ with $ y_\chi \lesssim 10^{-6} $ over large parameter regions. We estimate the inflaton-right-handed neutrino (RHN) Yukawa coupling $ y_N $ required for successful baryogenesis via non-thermal leptogenesis within a Type-I seesaw framework, for the lightest RHN mass $ M_{N_1} \sim 10^{9} \ \text{--} \ 10^{14}\,\mathrm{GeV} $, provided $ M_{N_1} > T_{\rm max} $, where $ T_{\rm max} $ follows from radiatively consistent reheating. In Palatini scenarios, the lower maximal temperature and tighter stability bounds further restrict the leptogenesis parameter space.

hep-ph↗

arXiv2026

Review of strongly coupled regimes in gravity with Dyson-Schwinger approach

We analyze various gravity theories involving de-Sitter, quadratic $\mathcal{R}^2$ and non-minimally coupled scalar in the light of application of the Dyson-Schwinger technique involving exact background solution of the Green's function. We denote specific set of solutions for the metric to move towards a quantum analysis of the theory. This kind of solutions is identified as conformally flat metric. Such a conclusion naturally arises in the use of the Dyson-Schwinger equations in the study of the Yang-Mills theory through the mapping theorem. We show a sequence of cosmological phase transitions starting from the breaking of such conformal invariance that can be hindered by the presence of the non-minimal coupling.

gr-qc↗

arXiv2026

INFLAVON: CMB as cosmic tracer of Flavor physics

We unify one of the most widely studied frameworks to explain the hierarchical structure of the flavor sector in the Standard Model, the Froggatt-Nielsen mechanism, with cosmic inflation. We propose that the complex scalar field, the so-called flavon, which breaks the Froggatt-Nielsen $U(1)$ symmetry and generates the Yukawa couplings of the Standard Model, to also drive inflation, which we dub as Inflavon. After inflation ends, the decay of the inflavon reheats the Universe, establishing a novel link between early Universe cosmology and flavor physics. As concrete examples, we present realizations where the inflavon potential is described by an $\alpha$-attractor potential. We then compute the resulting CMB observables, specifically the spectral index ($n_s$), the tensor-to-scalar ratio ($r$), and the amplitude of scalar perturbations ($A_s$) as functions of the underlying Froggatt-Nielsen model parameters. We identify the parameter space in Froggatt-Nielsen models involving the scale of Flavor symmetry breaking $\Lambda_{\rm FN}$ and FN charges which is ruled out by Planck and ACT data, as well as the region that could be probed by next-generation CMB experiments like CMB-S4, SO and LiteBIRD. We also discuss inflavon as dark matter and its isocurvature constraints.

hep-ph↗

arXiv2026

Cosmic Collider Gravitational Waves sourced by Right-handed Neutrino production from Bubbles: Testing Seesaw, Leptogenesis and Dark Matter

We study a minimal type-I seesaw framework in which a first-order phase transition (FOPT), driven by a singlet scalar, produces right-handed neutrinos (RHNs) through bubble collisions, realizing a cosmic-scale collider that probes ultra-high energy scales. The resulting RHN distribution sources novel low-frequency gravitational-waves (GWs) in addition to the standard bubble-collision contribution. A stable lightest RHN can account for the observed dark matter (DM) relic abundance for masses as low as $M_{1} \equiv m_{\rm DM} \gtrsim 10^{6}\,\mathrm{GeV}$, with the associated novel GW signal accessible in LISA, ET and upcoming LVK detectors. If the RHNs are unstable, their CP-violating decays generate the observed baryon asymmetry via leptogenesis for $M_{1} \gtrsim 10^{11}\,\mathrm{GeV}$ and phase transition temperatures $T_* \gtrsim 10^{6}\,\mathrm{GeV}$, for which the novel GW spectrum is detectable in ET, BBO and upcoming LVK. If RHN decays also populate a dark-sector fermion with mass $m_{\chi} \in [10^{-4},10^{4}],\mathrm{GeV}$, successful co-genesis of baryons and asymmetric dark matter occurs for $T_* \gtrsim 10^{7}\,\mathrm{GeV}$ and $M_{1} \gtrsim 10^{9}\,\mathrm{GeV}$, naturally explaining $\Omega_{\rm DM} \simeq 5\Omega_{\rm B}$. The corresponding GW signals are testable with LISA, ET, and BBO. Finally, we analyze a UV-complete multi-Majoron model, based on a global $U(1)_N \times U(1)_{\rm B-L}$ extension, motivated from the hierarchy of lepton masses, which we dub as Mojaron collider. The corresponding FOPT in this model leaves a distinctive GW signature arising from RHN production during $U(1)_N$ symmetry breaking detectable by BBO, ET and upcoming LVK. Successful leptogenesis is realized for heaviest RHN mass $M_3 \sim 10^{10}\,\mathrm{GeV}$ and a $U(1)_N$ breaking vev $v_2 \sim \mathcal{O}(\mathrm{TeV})$, which sets the seesaw scale.

astro-ph.CO↗

arXiv2025

Cosmic Strings Gravitational Wave Probe of Leptogenesis: Thermal, Non-thermal, Near-resonant and Flavourful

Breaking a global or local $U(1)_{\rm B-L}$ symmetry at high scales simultaneously generates Majorana masses for heavy right-handed neutrinos and produces a network of cosmic strings. The evolution and decay of these strings source a stochastic gravitational-wave background that may be probed by current and future gravitational-wave experiments, while the decays of the resulting massive right-handed neutrinos can generate the baryon asymmetry of the Universe via leptogenesis. We derive analytical bounds for successful leptogenesis with a global and a local $U(1)_{B-L}$ symmetry, separately finding an absolute lower bound on the lightest right-handed neutrino mass $M_1 > 1.74 \times 10^{8}\,\mathrm{GeV}$ for thermal initial conditions and $M_1 > \mathcal{O}(10^{6})\,\mathrm{GeV}$ for non-thermal initial conditions. Allowing for near-resonant leptogenesis relaxes these bounds to TeV scale in both cases making it a viable target at collider searches complementing the GW signals. Full flavour effects are included, and crucially, we determine the region where successful leptogenesis can be probed through gravitational-wave observations in upcoming experiments such as LISA and Einstein Telescope. Importantly, we find that flavour effects rescue regions of the parameter space that are ruled out due to current CMB or gravitational wave measurements.

astro-ph.CO↗

arXiv2025

Grand Unification Higgs-$\mathcal{R}^2$ Inflation: Complementarity between Proton Decay and CMB Observables

We propose a predictive $SO(10)$ Grand Unified Theory (GUT) framework for cosmic inflation in the Palatini $\mathcal{R}^2$ formulation of gravity. In this model, a GUT Higgs field both drives inflation and induces intermediate-scale symmetry breaking, thereby linking primordial cosmology, gauge unification, and topological defect formation. A partial inflationary phase of $N_I \sim 10$--$17$ $e$-folds following monopole formation can dilute magnetic monopoles to abundances $Y_M \sim 10^{-35}$--$10^{-27}$. The model yields Cosmic Microwave Background (CMB) predictions of $0.955 \lesssim n_s \lesssim 0.974$, accommodating the tensions between Planck-BICEP ($n_s \approx 0.965$) and Planck+ACT ($n_s \approx 0.971$) via $\phi < M$ and $\phi > M$ branches repectively. The predicted tensor-to-scalar ratio $r \lesssim 8\times10^{-4}$ lies within current observational constraints and is accessible to forthcoming experiments, including the Simons Observatory and LiteBIRD. The resulting correlations between the unification scale $M_U$, the inflationary observables $(n_s, r)$, and proton-decay lifetimes highlight a complementarity between CMB measurements and proton-decay searches, with regions of parameter space testable in forthcoming experiments such as Hyper-Kamiokande and DUNE.

astro-ph.CO↗

arXiv2025

Gravitational Wave Spectral Shapes as a probe of Long Lived Right-handed Neutrinos, Leptogenesis and Dark Matter: Global versus Local B-L Cosmic Strings

The scale of the seesaw mechanism is typically much larger than the electroweak scale. This hierarchy can be naturally explained by $U(1)_{B-L}$ symmetry, which after spontaneous symmetry breaking, simultaneously generates Majorana masses for neutrinos and produces a network of cosmic strings. Such strings generate a gravitational wave (GW) spectrum which is expected to be almost uniform in frequency unless there is a departure from the usual early radiation domination. We explore this possibility in Type I, II and III seesaw frameworks, finding that only for Type-I, long-lived right-handed neutrinos (RHN) may provide a period of early matter domination for parts of the parameter space, even if they are thermally produced. Such a period leaves distinctive imprints in the GW spectrum in the form of characteristic breaks and a knee feature, arising due to the end and start of the periods of RHN domination. These features, if detected, directly determine the mass $M$, and effective neutrino mass $\tilde m$ of the dominating RHN. We find that GW detectors like LISA and ET could probe RHN masses in the range $M\in[0.1,10^{9}]$ GeV and effective neutrino masses in the $\tilde m\in[10^{-10},10^{-8}]$ eV range. We investigate the phenomenological implications of long-lived right-handed neutrinos for both local and global $U(1)_{B-L}$ strings, focusing on dark matter production and leptogenesis. We map the viable and detectable parameter space for successful baryogenesis and asymmetric dark matter production from right-handed neutrino decays. We derive analytical and semi-analytical relations correlating the characteristic gravitational-wave frequencies to the neutrino parameters $\tilde m$ and $M$, as well as to the relic abundances of dark matter and baryons.

astro-ph.CO↗