arXiv subjects

Vivekanand Mohapatra

Publications and source records attributed to Vivekanand Mohapatra.

7 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↗

arXiv2025

Cosmological bounds on dark matter annihilation using dark ages 21-cm signal

We investigate the impact of dark matter (DM) annihilation on the global 21-cm signal during the dark ages and cosmic dawn eras. The 21-cm line provides a complementary probe for studying the nature of dark matter beyond standard cosmological observables. In the standard $Λ$CDM framework, the expected absorption amplitude of the dark ages global 21-cm signal is approximately $-42\, \rm mK$. However, energy injection from DM annihilation can significantly heat and ionize the intergalactic medium, potentially altering or even erasing this absorption feature. We evaluate the thermal and ionization history of the gas to derive an upper bound on $f_χ^2 \langle σv \rangle / M_χ$ using the dark ages signal, which is free from astrophysical uncertainties. After incorporating observational and theoretical uncertainties arising from future lunar-based experiments and variations in cosmological parameters, respectively -- we obtain a conservative upper limit of $f_χ^2\langleσv\rangle/M_χ\lesssim 10^{-27}~\rm cm^3\,s^{-1}\,\rm GeV^{-1}$. This constraint is stronger than the bounds derived from Planck (2018) data for mass $\lesssim 10~\rm GeV$.

astro-ph.CO↗

arXiv2025

Revisiting constraints on superconducting cosmic strings in light of Dark Ages global 21-cm signal

The Superconducting Cosmic Strings (SCS) are a special case of cosmic strings that have a core carrying a charged field. When SCS passes through magnetized regions, the charged particles in the string experience a Lorentz force, which can produce radiation on the entire electromagnetic spectrum. This radiation can inject energy into the surrounding plasma, resulting in a modification of the thermal and ionization evolution of the intergalactic medium (IGM) and, subsequently, the global 21-cm signal. The signatures of SCS in the post-recombination era have been primarily studied in the low-frequency (radio) regime, which does not impact the state of the IGM. In this work, we study the effect of decaying SCS on the dark ages global 21-cm signal $(δT_b)$, considering both the ionizing and radio radiation. The dark ages signal can provide pristine cosmological information free from astrophysical uncertainties, as the universe was primarily homogeneous during this era in the absence of baryonic structure formation. Considering a change in the $δT_b$ at redshift $z\sim 89$ from the $Λ\rm CDM$ framework, we derive an upper bound on the decay efficiency parameter, $g\equiv g(I,~Gμ_s)$, to be $\lesssim 5.1\times10^{14}\, \rm GeV^2$, where, $I$ and $Gμ_s$ represent the loop current and string tension of SCS, respectively.

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

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↗

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↗