arXiv subjects

Alekha C. Nayak

Publications and source records attributed to Alekha C. Nayak.

At least 19 recordsLinked to original sources

arXiv2026

Dark ages bounds on nonaccreting massive compact halo objects

We derive a complementary cosmological upper bound on the fraction of dark matter residing inside non-accreting massive compact halo objects (MACHOs) using the cosmic dawn and dark ages global 21-cm signal $(T_{21})$. MACHOs of mass $M\gtrsim 10^3~M_\odot$ moving through the baryonic fluid during post-recombination transfer kinetic energy to the intergalactic medium via dynamical friction, thereby raising the gas temperature and distorting the 21-cm signal predicted in the $Λ$CDM framework. We consider both a monochromatic distribution of MACHOs and two extended mass distributions: log-normal and critical collapse. Imposing the conditions that the deviation in the global 21-cm signal $ΔT_{21}$ does not exceed $50~\rm mK$ at $z\sim 17$ or $15~\rm mK$ at $z\sim 89$, and that no emission signal appears at $z \gtrsim 300$, we derive upper bounds on the MACHO fraction $f_M$ across the mass range $10^3 \lesssim M_c/M_\odot \lesssim 10^7$. The dark ages and $z \gtrsim 300$ criterion yield constraints that are both tighter and free from astrophysical uncertainties associated with star formation, particularly $f_M = 0.03$ for $M_c = 10^7~M_{\odot}$, providing a complementary cosmological window. The extended mass distributions yield bounds that are less stringent than those of their monochromatic counterparts.

astro-ph.CO↗

arXiv2026

Infrared Lines from Sterile-Neutrino Transition Magnetic Moments at JWST

We investigate infrared line signatures from radiatively decaying sterile-neutrino dark matter using publicly available JWST/NIRSpec IFU blank-sky observations. The main signal considered is the sterile-to-sterile transition $N_1\to N_2γ$, induced by the transition magnetic dipole coefficient $d_{NNγ}$, with $m_1>m_2$. In contrast to ordinary two-photon decays of axion-like or Majoron-like particles, the observed photon energy is not fixed by the full dark matter mass, but by the small mass splitting $Δm=m_1-m_2$. Thus, a keV-scale sterile-neutrino dark matter state can generate an eV-scale infrared photon line in the JWST band. We construct a $χ^2$-based line-search analysis using the NIRSpec IFU $\rm F170LP$-$\rm G235M$ blank-sky data toward $\rm GN\text{-}z11$, modelling the smooth continuum with a cubic spline and including the Milky Way halo decay flux. In the absence of a significant excess, we derive projected limits on $d_{NNγ}$ and on the decay width $Γ_{N_1\to N_2γ}$ for $0.1~{\rm eV}\lesssimΔm\lesssim1~{\rm eV}$. For a sterile component saturating the dark matter abundance, the strongest sensitivity reaches $d_{NNγ}\lesssim7\times10^{-14}~{\rm GeV}^{-1}$ and $Γ_{N_1\to N_2γ}\lesssim10^{-25}~{\rm s}^{-1}$. We also include a minimal anomalous-Majoron benchmark, $ω\toγγ$, obtaining JWST sensitivity to $λ_{ωγγ}\sim10^{-11}$-$10^{-9}~{\rm GeV}^{-1}$ for $m_ω\sim0.6$-$1~{\rm eV}$.

astro-ph.CO↗

arXiv2026

Constraining self-interacting ultrahigh-energy muon neutrinos by cosmic microwave background spectral distortion

The neutrino telescopes have firmly established the existence of ultrahigh-energy neutrinos. Observations of these neutrinos offer a unique probe of neutrino self-interactions. This work investigates how the self-interacting neutrinos, mediated by scalar bosons, inject energy into the medium through radiative scattering with the cosmic neutrino background, leaving an imprint on the cosmic microwave background (CMB) spectrum. The energy injection into plasma in redshift ranges, $5\times10^4\lesssim z\lesssim2\times10^6$ and $ z\lesssim5\times10^4$, leads to $μ$-type and $y$-type CMB spectral distortions, respectively. Using observational constraints from Cosmic Background Explorer/Far Infrared Absolute Spectrophotometer (COBE/FIRAS) and projected sensitivities from Primordial Inflation Explorer (PIXIE) experiments for $μ$-type and $y$-type CMB distortions, we derive the stringent upper bounds on the self-interaction coupling strength as a function of mediator mass for neutrino interactions. We focus on flavor-specific self-interaction related to muon neutrinos and sub-GeV mass mediators ($m_ϕ$). We find the upper bound on the self-interaction coupling strength to be $\sim 2.8\times 10^{-4}$ for the muon neutrino, considering ultrahigh-energy muon neutrino energy to be 1 PeV and PIXIE projected upper bounds on $y$-type CMB spectral distortion. The bound remains constant till the mediator mass reaches the center-of-mass energy, and after that, it gets relaxed and becomes proportional to the mediator mass. We have also compared our results with existing bounds in the literature. Our findings indicate that CMB spectral distortion could play a decisive role in exploring neutrino physics beyond the standard model of particle physics, and future missions like PIXIE can provide valuable insights.

astro-ph.CO↗

arXiv2026

Constraining Primordial Black Holes via p-wave annihilation in light of CMB Spectral Distortion and 21-cm global signal

Primordial black holes (PBHs) can form spike density halos through the accretion of weakly interacting massive particles (WIMPs). In these halos, the enhanced density significantly boosts the annihilation rate of WIMPs. For Majorana dark matter annihilation into light fermions, the s-wave part of the annihilation cross section is helicity-suppressed, making the p-wave contribution dominant. We study the velocity-dependent p-wave annihilation case, whose resulting energy injection can modify the thermal and ionization history of the Universe, leaving observable imprints on the cosmic microwave background (CMB) spectrum and the global 21-cm signal. From the predicted energy injection into the plasma, we derive stringent upper limits on the fraction of dark matter in form of PBHs for p-wave annihilation models, based on the observational constraints of the CMB spectral distortions ($y$-type), and from the measurement of the 21-cm absorption signal at cosmic dawn. Our results highlight that accounting for the p-wave nature of annihilation is crucial for deriving robust constraints on the PBH abundance.

astro-ph.CO↗

arXiv2025

Bounds on decaying sterile neutrinos via magnetic dipole moment from COB intensity

A recent observation by Long Range Reconnaissance Imager (LORRI) mounted on NASA's New Horizons yielded the most accurate measurement of the cosmic optical background (COB). The reported COB intensity is $11.16\pm 1.35$ $\mathrm{nW/m^2/sr}$ at a pivot wavelength $ λ_{piv} = 0.608 \, μ\mathrm{m}$ observed in the range $ 0.4 \, μ\mathrm{m} \lesssim λ\lesssim 0.9 \, μ\mathrm{m} $. After subtracting the measured intensity from the deep Hubble Space Telescope count, diffused galactic light, and scattered light from bright star foregrounds, an anomalous intensity of $2.99 \pm 2.03~\mathrm{nW/m^2/sr}$ has been found. We considered radiatively decaying sterile neutrinos of keV mass scale, as dark matter candidate, that could contribute to this anomalous reported intensity. Using this, we derive upper bounds on the sterile-to-sterile transition magnetic moment. We find that sterile neutrinos with mass of $\mathcal{O}(\rm keV)$ scale take values of the transition magnetic moment in the range $ 3\times 10^{-13}\,\rm eV^{-1} - 10^{-9}\,\rm eV^{-1}$ to explain the anomalous intensity of $2.99\pm 2.03\,\rm nW/m^2/sr$. % Future experiments such as, Cosmological Advanced Survey Telescope for Optical-UV Research (CASTOR), James Web Space Telescope (JWST), and Spectro-Photometer for the history of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx) might help us derive a better bound on the sterile neutrinos.

astro-ph.CO↗

arXiv2025

In Search of Global 21-cm Signal using Artificial Neural Network in light of ARCADE 2

Understanding the astrophysical nature of the first stars remains an unsolved problem in cosmology. The redshifted global 21-cm signal $({T}_{21})$ acts as a treasure trove to probe the cosmic dawn era -- when the intergalactic medium was mostly neutral. Many experiments, like SARAS 3, EDGES, and DARE, have been proposed to probe the cosmic dawn era. However, extracting the faint cosmological signal buried inside a brighter foreground, $\mathcal{O}(10^4)$, remains challenging. Additionally, an accurate modelling of foreground and ${T}_{21}$ signal remains the heart of any extraction technique. In this work, we constructed the foreground signal $(T_{FG})$ from the global sky model and star formation history using Press-Schechter formalism to determine the $T_{21}$ signal with excess radio background following ARCADE 2 detection. Further, we incorporated static ionospheric distortion into the total signal and calculated the signal measured by an ideal antenna. We then trained an artificial neural network (ANN) for the extraction of a $T_{21}$ signal parameters signal measured by antenna with an R-square score $(0.5523 - 0.9901)$. Lastly, we used a Bayesian technique to extract $T_{21}$ signal and compared the finding with ANN's extraction.

astro-ph.CO↗

arXiv2025

Geometric optics analysis in Lorentz violating Chern-Simons electrodynamics

We study the geometric optics limit of the electrodynamics in the presence of Lorentz violating Chern-Simons term in (3+1) dimensions. The Chern-Simons term couples the dual electromagnetic tensor to an external four-vector and the electromagnetic gauge field. For a fixed external four-vector, such a Chern-Simons term violates Lorentz invariance while maintaining the gauge invariance of the theory. In this analysis, we look into the consequences of Lorentz symmetry violating Chern-Simons term within the geometric optics limit of light rays propagating from a source to an observation point. We argue that the Ricci tensor and the Lorentz-violating term modify the dynamical equation for the gauge vector field. However, in the geometric optics limit, neither the space-time curvature nor the Chern-Simons term influences the intensity of light. Unlike the intensity, the polarization of light, on the other hand, can be influenced by the Lorentz-violating Chern-Simons term. Due to such an effect in the presence of Chern-Simons term, the photon emitting from astrophysical objects can undergo a change in polarization as it propagates in space.

hep-ph↗

arXiv2024

Primordial Magnetic Fields in Light of Dark Ages Global 21-cm Signal

We study the constraints on primordial magnetic fields (PMFs) in light of the global 21-cm signal observed during the dark ages. Primordial magnetic fields can heat the intergalactic medium (IGM) via magnetohydrodynamic effects. We investigate the impact of magnetic heating on the dark ages global 21-cm signal and constrain the present-day strength of primordial magnetic fields and their spectral indices. Since there were no stars during the Dark Ages, measuring the global 21-cm signal can provide pristine cosmological information. However, detecting this signal using ground-based telescopes is challenging. Several lunar and space-based experiments, such as FARSIDE, DAPPER, and FarView, have been proposed to detect the signal in future. Our findings indicate that measuring the 21-cm global signal during the Dark Ages can provide stronger bounds compared to the existing constraints from Planck 2016. Specifically, the bounds are independent of astrophysical uncertainties and stronger for spectral indices $-2.84 \leq n_B \leq -1.58$. Additionally, we explore the dark-ages consistency ratio, which can identify any non-standard heating of the IGM by measuring the 21-cm signal at only three different redshifts. This approach could complement future experiments aimed at detecting the Dark Ages global 21-cm signal.

astro-ph.CO↗

arXiv2024

Constraining Viscous Dark Matter in light of CMB Spectral Distortion

We calculate the $μ$- and \textit{y}-type spectral distortions of Cosmic Microwave Background (CMB), taking a non-standard interaction between baryons and viscous dark matter. Using the CMB spectral distortion observations, we can constrain any exotic mechanism that may change the energy of the CMB photon, leading to a CMB spectrum distortion. Depending on the viscosity of dark matter, the energy transfer between dark matter and baryons may modify, leading to a modification in CMB distortion. The existing Cosmic Background Explorer (COBE)/FIRAS and the Primordial Inflation Explorer (PIXIE) set limits on \textit{y} and $μ$ types of distortions to $y = 1.5\times10^{-5}$, $μ= 9.0\times10^{-5}$ and $y = 10^{-8}$, $μ= 5.0\times10^{-8}$, respectively. In this paper, we discuss the pre-recombination contributions to $μ$ and \textit{y}-type distortions by viscous dark matter and constrain the parameter space using PIXIE bounds on spectral distortion.

astro-ph.CO↗

arXiv2022

Bounds on sterile neutrino lifetime and mixing angle with active neutrinos by global 21 cm signal

Sterile neutrinos can be a possible candidate for dark matter. Sterile neutrinos are radiatively unstable and can inject photon energy into the intergalactic medium (IGM). The injection of photon energy into IGM can modify the temperature and ionization history of IGM gas during cosmic dawn. Theoretical models based on the ΛCDM framework predict an absorption profile in the 21 cm line during the cosmic dawn era. Recently, the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) collaboration confirmed such an adsorption signal. Injection of energy into IGM can modify the absorption amplitude in the 21 cm signal. Considering the 21 cm absorption signal at cosmic dawn, we constrain the lifetime of sterile neutrinos and the mixing angle of sterile neutrinos with active neutrinos. We also compare these bounds with other astrophysical observational bounds.

hep-ph↗

arXiv2022

Constraining spinning primordial black holes with global 21-cm signal

We study the upper projected bounds on the dark matter fraction in the form of the primordial black holes (PBHs) with a non-zero spin by using the absorption feature in the global 21-cm signal at redshift z ~ 17. The mass and spin are fundamental properties of a black hole, and they can substantially affect the evaporation rate of the black hole. The evaporating black hole can inject energy into the intergalactic medium and heat the gas. Subsequently, it can modify the absorption amplitude in the global 21-cm signal. Therefore, the absorption feature in the 21-cm signal can provide a robust bound on PBHs. We analyse the projected constraints on the dark matter fraction in the form of both spinning and non-spinning PBHs. The constraints are more stringent for spinning PBHs than non-spinning ones. We also compare these bounds with other observations and find the most stringent lower constraint on PBHs mass, which is allowed to constitute the entire dark matter to 6.7 x 10^17 g for extremal spinning PBHs.

astro-ph.CO↗

arXiv2020

Dark matter admixed neutron star properties in the light of gravitational wave observations: a two fluid approach

We consider the effect of density dependent dark matter on the neutron star mass, radius, and tidal deformability. Nuclear matter (normal matter) as well as the fermionic dark matter sector is considered in a mean field model. We adopt the two fluid formalism to investigate the effect of dark matter on the neutron star properties. In the two fluid picture, there is no direct interaction between the dark matter and the nuclear matter. Rather these two sectors interact only through gravitational interaction. The nuclear matter sector is described by the $σ-ω-ρ$ meson interaction in the "FSU2R" parameterization. In the dark matter sector, we use the Bayesian parameter optimization technique to fix the unknown parameters in the dark matter equation of state. In the two fluid picture, we solve the coupled Tolman-Oppenheimer-Volkoff (TOV) equations to obtain the mass and radius of dark matter admixed neutron stars (DANSs). We also estimate the effect of the density dependent dark matter sector on the tidal deformability of dark matter admixed neutron stars (DANSs).

nucl-th↗

arXiv2020

Baryon-Dark matter interaction in presence of magnetic fields in light of EDGES signal

We have shown that in presence of a cosmic magnetic field the bounds on baryon dark matter cross-section ($\hat σ$), dark-matter mass ($m_d$) and values of the magnetic field ($B_0$) can strongly influence each other. This requires to rework the bounds on $\hat σ\,$, $m_d$ and $B_0$ which can explain the observed absorption signal by EDGES collaboration. The upper limit on the magnetic field strength can modify in presence of baryon-dark matter interaction cross-section. In the presence of a strong magnetic field, a large baryon-dark matter interaction cross-section is required to balance magnetic heating of gas to explain the EDGES signal as compared to a weak magnetic field. Subsequently, the strong magnetic-fields can even erase the 21 cm signal--this gives an upper bound on the strength of magnetic-fields, dark-matter mass and baryon-dark matter cross-section. In the special case when $\hat σ$=0, one can recover the bound on magnetic field strength calculated in [1]. In this work we find that the allowed range of the primordial magnetic field can increase by three orders of magnitude in comparison with [1]. We get upper bound on the magnetic field strength: $3.48\times10^{-6}$ G for the dark matter mass $\lesssim 10^{-2}$ GeV.

astro-ph.CO↗

arXiv2019

Cosmological Dark Matter in a Conformal Model

We study the collider, astrophysical and cosmological constraints on the dark matter sector of a conformal model within the framework of both the freeze out as well as the freeze in mechanism. The model has a dark sector with strong self interactions. This sector couples weakly with the Standard Model (SM) particles via a scalar messenger. The lightest dark sector particle is a pion-like fermion anti-fermion bound state. We find that the model successfully satisfies the constraints coming from the Higgs decay to the visible as well as the invisible sector. We have used the results of the dark matter direct detection experiments, such as, XENON1T in order to impose bounds on the parameters of the model. The model satisfies the indirect detection constraints of gamma ray from the galactic center and Dwarf spheroidal galaxies. We also determine the parameter range for which it satisfies the astrophysical constraints on the dark matter self coupling.

hep-ph↗

arXiv2019

Confronting nuclear equation of state in the presence of dark matter using GW170817 observation in relativistic mean field theory approach

We confront admixture of dark matter inside neutron star using gravitational wave constraints coming from binary neutron star merger. We consider a relativistic mean field model including $σ-ω-ρ$ meson interaction with NL3 parameterization. We study fermionic dark matter interacting with nucleonic matter via Higgs portal mechanism. We show that admixture of dark matter inside the neutron star soften the equation state and lower the value of tidal deformability. Gravitational wave GW170817 observation puts an upper bound on tidal deformability of a binary neutron star with low spin prior at 90\% confidence level, which disfavors stiff equation of state such as Walecka model with NL3 parameterization. However, we show that Walecka model with NL3 parameterization with a fermionic dark matter component satisfy the tidal deformability bound coming from the GW170817 observation.

hep-ph↗

arXiv2019

Viscous dark matter and 21 cm cosmology

The EDGES experiment has detected the global absorption signal of 21 cm line at $z\sim17$ in cosmic dawn era and reported its amplitude larger than the standard cosmological prediction. One of the possible explanation requires that the baryons were much cooler than the standard scenario. This requires an interaction between the dark and baryonic sectors with some appropriate cross-section, $ \hatσ $. In this work, we examine the role that dissipative effects of cosmic fluid might play in influencing the 21 cm signal. We show that the presence of viscous dissipation of dark matter can significantly affect the energy transfer between the baryonic and dark matter fluids. It is demonstrated that the inclusion of the dissipative mechanism in the dark sector, strongly modify the earlier constraints on dark matter mass and $ \hatσ $ obtain from EDGES observation. Further, we argue that EDGES absorption signal can put an independent bound on dark matter viscosity which is many order of magnitude larger than the maximum viscosity allowed by the structure formation.

astro-ph.CO↗

arXiv2019

Very special relativity induced phase in neutrino oscillation

In Very Special Relativity (VSR), the neutrino mass term is coupled with the VSR preferred axis, and hence Lorentz violating in nature. Beyond standard model physics predicts neutrino magnetic moment which is linearly proportional to the mass eigenstates of the neutrinos. We report an additive kinematic phase in the neutrino flavor oscillation due to the neutrino magnetic moment in the VSR framework. This phase is proportional to the coupling between the VSR preferred axis with the external magnetic field as well as the spin of the neutrino. Furthermore, we predict time variation in the neutrino oscillation with a period of one sidereal day.

hep-ph↗

arXiv2017

Phenomenological Implications of Very Special Relativity

We discuss several phenomenological implications of Very Special Relativity (VSR). It is assumed that there is a small violation of Lorentz invariance and the true symmetry group of nature is a subgroup called SIM(2). This symmetry group postulates the existence of a fundamental or preferred direction in space-time. We study its implications by using an effective action which violates Lorentz invariance but respects VSR. We find that the problem of finding the masses of fundamental fermions is in general intractable in the presence of VSR term. The problem can be solved only in special cases which we pursue in this paper. We next determine the signal of VSR in torsion pendulum experiment as well as clock comparison experiment. We find that VSR predicts a signal which is different from other Lorentz violating theories and hence a dedicated data analysis is needed in order to impose reliable limits. Assuming that signal is absent in data we determine the limits that can be imposed on the VSR parameters. We also study the implications of VSR in particle decay experiments taking the charged pion and kaon decay as an example. The effective interaction between the charged pion and the final state leptons is related to the fundamental VSR mass terms through a loop calculation. We also predict a shift in the angular dependence of the decay products due to VSR. In particular we find that these no longer display azimuthal symmetry with respect to the momentum of the pion. Furthermore the azimuthal and polar angle distributions show time dependence with a period of a sidereal day. This time dependence provides us with a novel method to test VSR in future experiments.

hep-ph↗