arXiv subjects

Shikhar Mittal

Publications and source records attributed to Shikhar Mittal.

14 recordsLinked to original sources

arXiv2026

High-Redshift Signatures from the Cosmic Dawn and the Epoch of Reionization

In this chapter, we provide a comprehensive overview of the astrophysical and cosmological processes that shape the 21-cm signal during Cosmic Dawn and the Epoch of Reionization. We investigate both standard and exotic signatures potentially observable with SKA-Low. Standard signatures are those expected within the $\Lambda$CDM framework, including contributions from the first stars, galaxies, and black holes. Exotic signatures are more speculative indicating new physics, such as primordial black holes, modifications to the dark matter sector, non-standard primordial fluctuations, or strongly emitting radio galaxies. The effects of these different sources or scenarios are evaluated in the context of the expected sensitivity of SKA-Low, considering the AA* and AA4 configurations. The chapter aims to provide an overview of the theoretical landscape of 21-cm signatures and to highlight how the forthcoming SKA-Low observations will improve our understanding of astrophysical processes at early times and may open the door towards new physics beyond the $\Lambda$CDM framework.

astro-ph.CO↗

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

Implications of SARAS3 data for Coulomb-like interacting dark matter

The 21-cm signal from cosmic dawn is a potentially sensitive probe of interactions between dark matter (DM) and baryons. We investigate the implications of the SARAS3 non-detection in the 55.5-84.4 MHz band for Coulomb-like interacting DM (IDM). In contrast to earlier constraint analyses that focused primarily on baryon cooling, we model the interaction self-consistently by including both excess cooling of the gas and the suppression of structure formation, which delays the onset of star formation and hence suppresses the Ly$\alpha$, X-ray, and ionizing backgrounds at early times. We perform a joint Bayesian fit of a global 21-cm signal model and a flexible foreground model to the SARAS3 antenna temperature, and find that the signal parameters remain weakly constrained after marginalizing over the foregrounds. The null result is nonetheless informative: the data disfavour deep absorption features within the observed band, with the strongest bound at $z = 23.6$ ($\nu \approx 57.7$ MHz), where $T_{21} \gtrsim -277.6$ mK at $3\sigma$. Comparing the IDM and standard cold dark matter scenarios, we find no statistically significant preference for IDM (Bayes factor $B \approx 1.7$). While we do not constrain the strength of baryon-DM interactions, the SARAS3 non-detection places a meaningful upper bound on the amplitude of the global 21-cm signal in this class of models.

astro-ph.CO↗

arXiv2026

Optimisation of calibration sources for global 21-cm experiments: the REACH case

The spin-flip 21-cm signal from the Cosmic Dawn and the Epoch of Reionization is an essential probe of the conditions that led to the formation of the first luminous objects in the early Universe. However, its detection remains a major challenge owing to its low strength compared to the bright foregrounds and the requirement of precise calibration of the instrument to prevent systematics that could hinder a detection or lead to false inferences. REACH (Radio Experiment for the Analysis of Cosmic Hydrogen) is a radiometer experiment designed to detect this sky-averaged signal in the frequency range of 50--130~MHz. Using a wide-beam antenna, REACH calibration relies on internal reference sources, covering a broad range of temperatures and reflection coefficients. The choice of type and number of calibrators used significantly influences the quality of the calibration. This work investigates these effects and introduces a novel method for selecting an optimal set of calibration sources. With an optimised set, we aim to reduce calibration time, thereby increasing sky integration time while preserving calibration accuracy. We explore two optimisation strategies: one applied across the full receiver band and another performed on a frequency-by-frequency basis. Finally, we demonstrate that, with a total calibration time comparable to the conventional full-calibrator set, an optimised set with fewer calibrators achieves approximately a $15~\%$ reduction in calibrated temperature noise and improved absolute calibration of the instrument. This has implications for better calibration strategies in similar radiometer experiments.

astro-ph.IM↗

arXiv2026

Circuit Modeling for In Situ 21 cm Radiometer Calibration

Recent experiments in cosmology, particularly those aimed at detecting the faint, redshifted, global 21 cm hydrogen line (depth < ~200 mK, z > 7.5), have imposed stringent new requirements on radiometer calibration. In this work, we present a framework for circuit modeling and parameter inference to strengthen these calibration pipelines. This new approach enables in situ characterization of otherwise immeasurable systematics using physically motivated models. A combination of frequentist and Bayesian techniques are employed in a pipeline that supports iterative modeling, robust parameter estimation, and detailed uncertainty quantification. The framework is applied to the REACH telescope, where the precise correction of variations in the radio signal paths arising from component aging or environmental effects is critical. Circuit models of REACH's calibration sources are developed, with the goal of predicting source temperature corrections that are conventionally obtained from laboratory measurements. By fitting the models to measured data using a convolutional cost function, a strong agreement with RMS residuals no worse than -37 dB is obtained. However, Bayesian inference reveals that the resulting temperature corrections can have uncertainties on the order of 1 to 2 K, caused by reflection coefficient degeneracies, measurement noise, and errors in the models. To combat this, posteriors obtained from laboratory measurements are employed as updated priors, reducing correction uncertainties down to 75 mK. Ultimately, the framework provides a means of dynamically accounting for drift in system non-idealities over time, addressing the increasing precision demands of global 21 cm radio astronomy.

astro-ph.IM↗

arXiv2025

ECHO21: a tool for modelling global 21-cm signal from dark ages to reionization

We introduce a Python package called ECHO21 for modelling the global 21-cm signal from the dark ages through cosmic dawn to the end of reionization. Leveraging its analytical framework, ECHO21 generates a single model in $\mathcal{O}(1)\,$s, allowing a large number of signals to be generated efficiently by distributing models across multiple cores. Thus, it is ideal for performing astrophysical or cosmological inference from a given 21-cm dataset. We offer six astrophysical parameters that control the Lyman-$\alpha$ (Ly$\alpha$) emissivity, X-ray emissivity, emissivity of ionizing photons, and star formation rate. Beyond its efficiency some of the attractive and novel features in ECHO21 relative to previously published codes are inclusion of Ly$\alpha$ heating, ability to vary the standard cosmological parameters as easily as the astrophysical parameters, and different models of star formation rate density (physically-motivated, a semi-empirical, and an empirically-motivated). With a number of 21-cm experiments soon to provide cosmic dawn 21-cm data, ECHO21 is a flexible and extensible new open-source package for making quick but sufficiently realistic astrophysical inferences. We make our code publicly available.

astro-ph.CO↗

arXiv2024

Impact of extragalactic point sources on the low-frequency sky spectrum and cosmic dawn global 21-cm measurements

Contribution of resolved and unresolved extragalactic point sources to the low-frequency sky spectrum is a potentially non-negligible part of the astrophysical foregrounds for cosmic dawn 21-cm experiments. The clustering of such point sources on the sky, combined with the frequency-dependence of the antenna beam, can also make this contribution chromatic. By combining low-frequency measurements of the luminosity function and the angular correlation function of extragalactic point sources, we develop a model for the contribution of these sources to the low-frequency sky spectrum. Using this model, we find that the contribution of sources with flux density $>10^{-6}\,$Jy to the sky-averaged spectrum is smooth and of the order of a few kelvins at 50--$200\,$MHz. We combine this model with measurements of the galactic foreground spectrum and weigh the resultant sky by the beam directivity of the conical log-spiral antenna planned as part of the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) project. We find that the contribution of point sources to the resultant spectrum is $\sim0.4\%$ of the total foregrounds, but still larger by at least an order of magnitude than the standard predictions for the cosmological 21-cm signal. As a result, not accounting for the point-source contribution leads to a systematic bias in 21-cm signal recovery. We show, however, that in the REACH case, this reconstruction bias can be removed by modelling the point-source contribution as a power law with a running spectral index. We make our code publicly available as a Python package labelled epspy.

astro-ph.CO↗

arXiv2023

Radiative transfer of Lyman-$\alpha$ photons at cosmic dawn with realistic gas physics

Lyman-$\alpha$ photons enable the cosmic dawn 21-cm signal through a process called the Wouthuysen-Field effect. An accurate model of the signal in this epoch hinges on the accuracy of the computation of the Ly$\alpha$ coupling, which requires one to calculate the specific intensity of Ly$\alpha$ photons emitted from the first stars. Most traditional calculations of the Ly$\alpha$ coupling assume a delta-function scattering cross-section, as the resonant nature of the Ly$\alpha$ scattering makes an accurate radiative transfer (RT) solution computationally expensive. Attempts to improve upon this traditional approach using numerical RT have recently emerged. However, some of these treatments suffer from assumptions such as a uniform gas distribution, coherent scattering in the gas frame and isotropic scattering. While others which do not account for these only do so through certain schemes along with core-skipping algorithms. We present results from a self-consistent Monte Carlo RT simulations devoid of any of the assumptions in the previous work for the first time. We find that gas bulk motion is the most important effect to account for in RT resulting in an RMS difference of 38% in the 21-cm signal and anisotropic scattering being the least important effect contributing to less than 3% RMS difference in 21-cm signal. We also evaluate the 21-cm power spectrum and compare that with the traditional results at cosmic dawn. This work points the way towards higher-accuracy models to enable better inferences from future measurements.

astro-ph.CO↗

arXiv2022

Implications of the cosmological 21-cm absorption profile for high-redshift star formation and deep JWST surveys

Apart from its anomalously large depth, the cosmological 21-cm absorption signal measured by the EDGES collaboration also has a shape that is distinctly different from theoretical predictions. Models with non-traditional components such as super-adiabatic baryonic cooling or an excess radio background explain the depth of the observed profile, but still conspicuously fail to explain its shape. In this paper, we quantify the requirements imposed by the EDGES measurement on sources of Ly $α$ and X-ray photons in the presence of excess radio background at cosmic dawn. In extreme cases, the Ly $α$ and X-ray emissivities require to be enhanced by up to an order of magnitude relative to traditional models. Furthermore, this enhancement needs to be active only for a short duration. We find that under conventional assumptions for the cosmic star formation rate density, standard stellar populations are incapable of meeting these conditions. Only highly unusual models of massive metal-free stars seem to provide a possible mechanism. Conversely, if the sources of Ly $α$ and X-ray photons are compelled to have standard properties, the EDGES measurement puts strong demands on the cosmic star formation rate density. This provides interesting falsifiable predictions for high-redshift galaxy surveys enabled by \textit{James Webb Space Telescope} (\textit{JWST}). We derive predictions for galaxy UV luminosity functions and number densities, and show that a deep \textit{JWST} survey with a limiting UV magnitude of $m_\mathrm{UV,lim}=32$ would potentially be able to rule out the predictions enforced by the EDGES measurement.

astro-ph.CO↗

arXiv2022

Constraining primordial black holes as dark matter using the global 21-cm signal with X-ray heating and excess radio background

Using the global 21-cm signal measurement by the EDGES collaboration, we derive constraints on the fraction of the dark matter that is in the form of primordial black holes (PBHs) with masses in the range $10^{15}$-$10^{17}\,$g. Improving upon previous analyses, we consider the effect of the X-ray heating of the intergalactic medium on these constraints, and also use the full shape of the 21-cm absorption feature in our inference. In order to account for the anomalously deep absorption amplitude, we also consider an excess radio background motivated by LWA1 and ARCADE2 observations. Because the heating rate induced by PBH evaporation evolves slowly, the data favour a scenario in which PBH-induced heating is accompanied by X-ray heating. Also, for the same reason, using the full measurement across the EDGES observation band yields much stronger constraints on PBHs than just the redshift of absorption. We find that 21-cm observations exclude $f_{\mathrm{PBH}} \gtrsim 10^{-9.7}$ at 95% CL for $M_{\mathrm{PBH}}=10^{15}\,$g. This limit weakens approximately as $M_{\mathrm{PBH}}^4$ towards higher masses, thus providing the strongest constraints on ultralight evaporating PBHs as dark matter over the entire mass range $10^{15}$-$10^{17}\,$g. Under the assumption of a simple spherical gravitational collapse based on the Press-Schechter formalism, we also derive bounds on the curvature power spectrum at extremely small scales ($k\sim 10^{15}\,$Mpc$^{-1}$). This highlights the usefulness of global 21-cm measurements, including non-detections, across wide frequency bands for probing exotic physical processes.

astro-ph.CO↗

arXiv2022

Background of radio photons from primordial black holes

We compute the isotropic radiation background due to Hawking emission from primordial black holes (PBHs), and examine if this background is a viable option in explaining the excess radiowave background observed by the ARCADE2 and LWA1 experiments at $\lesssim 1\,$GHz. We find that even under the extreme assumption that all of the dark matter is in the form of PBHs, the radio brightness temperature induced by Hawking evaporation of PBHs is $\mathcal{O}(10^{-46})\,$K, highly subdominant compared to the cosmic microwave background. The main reason for this is that for PBHs in the mass range $\sim10^{12}$-$10^{14}\,$kg, which can be constrained by Hawking emission, the spectrum peaks at $10^7$ to $10^5\,$eV. As the Hawking spectrum is power law suppressed towards lower energies, negligible flux of $μ$eV photons is obtained. The peak of the Hawking spectrum shifts to lower energies for higher masses, but the number density is low and so is the specific intensity. Because Hawking emission from PBHs is thus unable to explain the observed excess radio background, we also consider the alternative possibility of radio emission from gas accretion onto supermassive PBHs. These PBHs can readily produce strong radio emission that could easily explain the ARCADE2/LWA1 excess.

astro-ph.CO↗

arXiv2021

Lyman-$α$ coupling and heating at Cosmic Dawn

The global 21-cm signal from the cosmic dawn is affected by a variety of heating and cooling processes. We investigate the impact of heating due to Lyman-$α$ (Ly~$α$) photons on the global 21-cm signal at cosmic dawn using an analytical expression of the spectrum around the Ly~$α$ resonance based on the so-called `wing approximation'. We derive a new expression for the scattering correction and for the first time give a simple close-form expression for the cooling due to injected Ly~$α$ photons. We perform a short parameter study by varying the Ly~$α$ background intensity by four orders of magnitude and establish that a strong Ly~$α$ background is necessary, although not sufficient, in order to reproduce the recently detected stronger-than-expected 21-cm signal by the EDGES Collaboration. We show that the magnitude of this Ly~$α$ heating is smaller than previously estimated in the literature by two orders of magnitude or more. As a result, even a strong Ly~$α$ background is consistent with the EDGES measurement. We also provide a detailed discussion on different expressions of the Ly~$α$ heating rate used in the literature.

astro-ph.CO↗

arXiv2020

Path integral Monte Carlo method for the quantum anharmonic oscillator

The Markov chain Monte Carlo (MCMC) method is used to evaluate the imaginary-time path integral of a quantum oscillator with a potential that includes both a quadratic term and a quartic term whose coupling is varied by several orders of magnitude. This path integral is discretized on a time lattice and calculations for the energy and probability density of the ground state and energies of the first few excited states are carried out on lattices with decreasing spacing to estimate these quantities in the continuum limit. The variation of the quartic coupling constant produces corresponding variations in the optimum simulation parameters for the MCMC method and in the statistical uncertainty for a fixed number of paths used for measurement. The energies and probability densities are in excellent agreement with those obtained from numerical solutions of Schrödinger's equation.

physics.comp-ph↗

arXiv2019

Reflection of a Point Object in an Arbitrary Curved Mirror

In this work, I have derived the equation of the curve obtained on reflection of a point object in an arbitrary curved mirror if the object and the mirror are placed on the 2D Cartesian plane. I have used only the basic laws of reflection of classical geometric optics and elementary coordinate geometry. Several examples are provided and compared with Gaussian optics. We also see how the equations reduce to the standard mirror formula under the paraxial approximation.

physics.optics↗