arXiv subjects

Pravin Kumar Natwariya

Publications and source records attributed to Pravin Kumar Natwariya.

At least 19 recordsLinked to original sources

arXiv2026

Overview of 21cm Experiments at high redshift with SKAO

We provide an overview of the eight SKAO Science Book chapters that motivate the Epoch of Reionisation and Cosmic Dawn experiments with SKA-Low. We describe the individual SKA-Low experiments and expected sensitivity - power spectrum, tomography, 21-cm forest, cross-correlations, building on the broad observational plan laid out in the 2015 SKA Science Book. Finally, we outline features of the telescope that will be critical for the success of EoR/CD science, e.g., beam apodization, substations, and multi-beaming.

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

Sensitivity toward dark matter annihilation imprints on 21-cm signal with SKA-Low: A convolutional neural network approach

This study investigates the sensitivity of the radio interferometers to identify imprints of spatially inhomogeneous dark matter annihilation signatures in the 21-cm signal during the pre-reionization era. We focus on the upcoming low-mode survey of the Square Kilometre Array (SKA-Low) telescope. Using CNNs, we analyze simulated 3D 21-cm differential brightness temperature maps generated via the DM21cm code, which is based on 21cmFAST and DarkHistory, to distinguish between spatially homogeneous and inhomogeneous energy injection/deposition scenarios arising from dark matter annihilation. The inhomogeneous case accounts for local dark matter density contrasts and gas properties, such as thermal and ionization states, while the homogeneous model assumes uniform energy deposition. Our study focuses on two primary annihilation channels to electron-positron pairs ($e^+e^-$) and photons ($γγ$), exploring dark matter masses from 1 MeV to 100 MeV and a range of annihilation cross-sections. For $γγ$ channel, the distinction across dark matter models is less pronounced due to the larger mean free path of the emitted photons, resulting in a more uniform energy deposition. For $e^+e^-$ channel, the results indicate that the CNNs can effectively differentiate between the inhomogeneous and homogeneous cases. Despite observational challenges, the results demonstrate that these effects remain detectable even after incorporating noise from next-generation radio interferometers, such as the SKA. We find that the inhomogeneous dark matter annihilation models can leave measurable imprints on the 21-cm signal maps distinguishable from the homogeneous scenarios for the dark matter masses $m_{\rm DM}=1$ MeV and the annihilation cross-sections of $\geq 5 \times 10^{-30}~{\rm cm^3/sec}$ ($\geq 5 \times 10^{-29}~{\rm cm^3/sec}$ for $m_{\rm DM}=100$ MeV) for moderate SKA-Low noise.

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

Axion-Photon Conversion In Magnetized Universe: Impact On The Global 21-cm Signal

The reported anomalous global 21-cm signal $(T_{21})$ from the cosmic dawn era by Experiment to Detect the Global Epoch of Reionisation Signature (EDGES) could hint towards new physics beyond the standard model. The resonant conversion of the axion-like particles (ALPs) into photons in the presence of primordial magnetic fields (PMFs) could be a viable solution. However, the strength of the PMFs can change over the time as they can decay by ambipolar diffusion and turbulent decay. Consequently, PMFs can dissipate their energy into the intergalactic medium (IGM), which can alter the global 21-cm signal. We simultaneously consider both magnetic heating of IGM and resonant conversion of ALPs to derive physically motivated upper bounds on the coupling strength $(g_{aγ})$ and magnetic field strength $(B_n)$. Our findings report that, for $B_n= 0.1\,\rm nG$, $g_{aγ}B_n\lesssim (3.6\times 10^{-4}-3\times 10^{-3})$ is required to recover standard $T_{21}=-156\,\rm mK$, while a deeper absorption of $-500$ mK pushes the upper bound to $g_{aγ}B_n\lesssim (6.5\times 10^{-4}-5.7\times 10^{-3})$.

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↗

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

Machine Learning Constraints on Dark Matter Annihilation during the Epoch of Reionization: A CNN Analysis of the 21-cm Signal

We explore the impact of dark matter annihilation on the 21-cm signal during the cosmic dawn and epoch of reionization (EoR). Using modified 21cmFAST simulations and convolutional neural networks (CNNs), we investigate how energy injected into the intergalactic medium (IGM) through dark matter annihilation affects the evolution of the 21-cm differential brightness temperature. Focusing on two annihilation channels, photon-photon ($γγ$) and electron-positron ($e^+e^-$), we examine a broad range of dark matter masses and annihilation cross-sections. Our results show that CNNs outperform traditional power spectrum analysis by effectively distinguishing between subtle differences in simulated 21-cm maps produced by annihilation and non-annihilation scenarios. We also demonstrate that the structure formation boost, driven by dark matter clumping into halos and subhalos, significantly enhances the annihilation signal and alters the thermal and ionization history of the IGM. This enhancement leads to a noticeable effect on the 21-cm signal, including a shift from absorption to emission as dark matter annihilation heats the IGM at lower redshifts. By incorporating observational noise from upcoming radio interferometers, particularly the Square Kilometer Array (SKA), we show that these effects remain detectable despite observational challenges. We find that the dark matter annihilation models can leave measurable imprints on the 21-cm signal distinguishable from the non-annihilation scenarios for the dark matter masses $m_{\rm DM}=100$ MeV and the annihilation cross-sections of $\langle σv\rangle \simeq 10^{-31}~{\rm cm}^3/{\rm s}$ ($\langle σv\rangle \simeq 10^{-32}~{\rm cm}^3/{\rm s}$ for $m_{\rm DM}=1$ MeV and $\langle σv\rangle \simeq 10^{-24}~{\rm cm}^3/{\rm s}$ for $m_{\rm DM}=1$ TeV).

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↗

arXiv2023

Formation of a Bose Star in a Rotating Cloud

In this paper, we study the evolutions of a self-gravitating cloud of bosonic dark matter with finite angular momentum and self-interaction. This is achieved by using the sixth-order pseudospectral operator splitting method to solve the system of nonlinear Schrödinger and Poisson equations. The initial cloud is assumed to have mass density randomly distributed throughout three-dimensional space. The dark matter particles in the initial cloud are in the kinetic regime, i.e., their de Broglie wavelength is much smaller than the halo size. It is shown that Bose stars are indeed formed in the numerical simulation presented here. The presence of angular momentum and self-interaction in the initial cloud can significantly influence the star formation time in a non-trivial fashion. Furthermore, the plots of the vorticity magnitude profile after the star formation time indicate that the formed star may not have any intrinsic angular momentum for the cases when the self-interaction among the particles is either negligible or attractive. These results are in agreement with the earlier analytical studies of an isolated rotating Bose star. However, for the case of repulsive self-interaction, the vorticity magnitude analysis shows a possibility that the star formed in the numerical simulations may possess intrinsic angular momentum. It is also shown that the average mass and radius diagrams of the star are strongly influenced by the presence of angular momentum in the initial cloud.

astro-ph.CO↗

arXiv2023

21 cm Line Astronomy and Constraining New Physics

The 21 cm signal appears to be a treasure trove to provide an insight into the period when the first generation of luminous objects formed in the Universe. Hydrogen is the predominating fraction of the total baryonic matter during cosmic dawn (CD). Therefore, it is convenient and advantageous to study physics during CD using the 21 cm signal. The presence of any exotic source of energy can inject energy into the intergalactic medium (IGM) and heat the gas. Subsequently, it can modify the absorption amplitude in the global 21 cm signal. This feature can provide a robust bound on such sources of energy injection into the IGM gas.

astro-ph.CO↗

arXiv2022

Rotating Scalar Field and Formation of Bose Stars

We study numerical evolutions of an initial cloud of self-gravitating bosonic dark matter with finite angular momentum and self-interaction in kinetic regime. It is demonstrated that such a system can undergo gravitational condensation and form a Bose star. The results show that the gravitational condensation time is strongly influenced by the presence of finite angular momentum or the strength of self-interaction. We find that in the cases related with attractive or no self-interaction, there is no significant transfer of angular momentum from the initial cloud to the formed star. However, for the case repulsive interaction our results indicate that such a angular-momentum transfer is possible. These results are consistent with the earlier analytical work where the stability of the rotating boson star was considered [Dmitriev et al. 2021].

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↗

arXiv2021

Constraint on Primordial Magnetic Fields In the Light of ARCADE 2 and EDGES Observations

We study the constraints on primordial magnetic fields (PMFs) in the light of the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) low-band observation and Absolute Radiometer for Cosmology, Astrophysics and Diffuse Emission (ARCADE 2). ARCADE 2 observation detected extra-galactic excess radio radiation in the frequency range 3-90 GHz. The enhancement in the radio radiation is also supported by the first station of the Long Wavelength Array (LWA1) in the frequency range 40-80 MHz. The presence of early radiation excess over the cosmic microwave background can not be completely ruled out, and it may explain the EDGES anomaly. In the presence of decaying PMFs, 21 cm differential brightness temperature can modify due to the heating of the gas by decaying magnetic fields, and we can constraint the magnetic fields. For excess radiation fraction ($A_r$) to be LWA1 limit, we show that the upper bound on the present-day magnetic field strength, $B_0$, on the scale of 1 Mpc is $\lesssim 3.7$ nG for spectral index $n_B=-2.99$. While for $n_B=-1$, we get $B_0\lesssim1.1\times10^{-3}$ nG. We also discuss the effects of first stars on IGM gas evolution and the allowed value of $B_0$. For $A_r$ to be LWA1 limit, we get the upper constraint on magnetic field to be $B_0(n_B=-2.99)\lesssim4.9\times10^{-1}$ nG and $B_0(n_B=-1)\lesssim3.7\times10^{-5}$ nG. By decreasing excess radiation fraction below the LWA1 limit, we get a more stringent bound on $B_0$.

astro-ph.CO↗

arXiv2020

Viscosity in cosmic fluids

The effective theory of large-scale structure formation based on $Λ$CDM paradigm predicts finite dissipative effects in the resulting fluid equations. In this work, we study how viscous effect that could arise if one includes self-interaction among the dark-matter particles combines with the effective theory. It is shown that these two possible sources of dissipation can operate together in a cosmic fluid and the interplay between them can play an important role in determining dynamics of the cosmic fluid. In particular, we demonstrate that the viscosity coefficient due to self-interaction is added inversely with the viscosity calculated using effective theory of $Λ$CDM model. Thus the larger viscosity has less significant contribution in the effective viscosity. Using the known bounds on $\,σ/m$ for self-interacting dark-matter, where $\,σ\,$ and $m$ are the cross-section and mass of the dark-matter particles respectively, we discuss role of the effective viscosity in various cosmological scenarios.

astro-ph.CO↗

arXiv2020

EDGES signal in presence of magnetic fields

We study the 21-cm differential brightness temperature in the presence of primordial helical magnetic fields for redshift $z=10-30$. We argue that the $α$-effect that sets in at earlier time can be helpful in lowering the gas temperature to 3.2 degrees Kelvin at $z=17$. This effect can arise in the early Universe due to some parity violating high energy processes. Using the EDGES (Experiment to Detect the Global Epoch of Reionization Signature) results, we find the upper and lower limits on the primordial magnetic field to be $6\times 10^{-3}~{\rm nG} $ & $5\times 10^{-4}~{\rm nG}$ respectively. We also discuss the effect of Ly$α$ background on the bounds. Our results do not require any new physics in terms of dark matter.

astro-ph.CO↗

arXiv2020

Magnetic fields in a hot dense neutrino plasma and the Gravitational Waves

In the present work, we have studied the spectrum of the primordial gravitational waves due to magnetic instability in the presence of neutrino asymmetry. The magnetic instability generates a helical magnetic field on a large scale. The anisotropic stress generated by the magnetic field shown to be a source of primordial gravitational waves (GWs) at the time of matter-neutrino decoupling. We expect that the theoretically predicted GWs by this mechanism may be detected by Square Kilometer Array (SKA) or pulsar time array (PTA) observations. We also compare our findings with the results obtained by the earlier work where the effect of magnetic instability was not considered.

astro-ph.CO↗