Anish Ghoshal

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 Γ and the abundance-weighted mass Y i M , independently of the microphysical origin of the gravitational-wave source. Using numerical scans, we derive semianalytic relations between these characteristic frequencies and the beyond the standard model 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 Γ , although the second feature, and hence the inference of Y i M , 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.

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 3.8σ evidence for a non-zero intergalactic magnetic field from anisotropic pair-halo searches using 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 ∼ 10 -9 G at coherence length λ ₀ ∼ 10 ^-3 -10 ^-1 Mpc, consistent with lower bounds on the IGMF inferred from blazar observations by MAGIC, H.E.S.S. and Fermi -LAT. We show that the ALP parameter region consistent with γ-ray blazar data (assuming maximal helicity) simultaneously produces stochastic GW signals detectable at future space-based interferometers, such as LISA, over the ALP decay constant range 10 ³ GeV≲ f _a ≲ 10 ⁵ 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 ≳ 0.1 GeV, accessible to next-generation intensity and energy-frontier experiments.

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 backgrounds-ΛCDM, CPL, CPL+Λ, 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 diagnostics such as H ( z ), q ( z ) and 𝒪 _m ( 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 ≃ 1.6 - 1.8 for ET and z ≃ 2.6 - 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.

A systematic approach is presented for using CMB observables and reheating temperature for discriminating between various models of inflation and certain freeze-in dark matter scenarios. It is applied to several classes of α -attractor models as an illustrative example. In the first step, all independent parameters of the inflationary potential are expressed in terms of the CMB observables (the three parameters — by the scalar spectral index n _s , scalar amplitude A _s and the tensor-to-scalar amplitude ratio r ). For a standard reheating mechanism characterized by the inflaton equation of state parameter w and its effective dissipation rate Γ the reheating temperature is uniquely fixed in terms of the CMB observables measured for some pivot scale k _* . There are striking consequences of this fact. The model independent bounds on the reheating temperature, the BBN lower bound and the upper bound of the order of the GUT/Planck scale, translate themselves for each class of models into very narrow ranges of the allowed values of the spectral index n _s ,( k _* ), providing their strong tests by the present and future CMB data. The recent tension between Planck and DESI-ACT results has strong impact on our conclusions. Furthermore, given a class of inflaton models satisfying those tests, the reheating temperature is an interesting portal to link the CMB observables to the particle physics scenarios that are sensitive to it. As an example, non-thermal dark matter (DM) production mechanisms are discussed. One obtains then a consistency check between theories of inflation and DM production. If the future precision of the CMB data will constrain the reheating temperature beyond the model independent bounds, further constraints on the DM production will follow.

A bstract: 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 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.

We investigate the B − L gauge extension of the Standard Model that the Dirac seesaw mechanism with thermal leptogenesis leaves imprints in the early Universe via the stochastic gravitational background emanating from a network of cosmic strings when B − L symmetry is broken. With right-handed neutrino mass lighter than the typical scale of grand unification, the B − L symmetry protecting the right-handed neutrinos leads to constraints on the Yukawa couplings for both Dirac and Majorana scenarios. Estimating the predicted gravitational wave background we find that future space-borne missions could probe the range concerning thermal Dirac leptogenesis. In a comparative analysis between such probes of gravitational waves sourced from cosmic strings in Dirac and Majorana leptogenesis in the B − L extension, based on the energy scales of the leptogenesis, for instance, gravitational waves detectors will be able to probe the scale of Dirac leptogenesis up to 10 9 GeV , while for Majorana leptogenesis it would be upto 10 12 GeV .