ICTS

I will talk about the `observed' phase-space of dark matter in Milky Way, from the very centre to outer edge, and also from nearby galaxies. I will also discuss the implications for dark matter searches.

The inflationary paradigm of the early Universe has been extraordinarily consistent with the observations of the cosmic microwave background radiation, however, the microphysics governing it is not well understood. In this talk, I will focus on the warm inflation scenario and investigate its imprint on the observations. Warm inflation is a well-motivated and general description of inflation, in which the dissipative effects in a coupled inflaton-radiation system govern the dynamics of the Universe. Here, the inflaton interactions with radiation fields are not neglected; thus, there is a non-zero temperature during the inflationary phase. As a result, the dynamics of the Universe is modified, and the primordial power spectrum in warm inflation has distinct signatures compared to the standard cold inflation. I will discuss my study on some models of warm inflation and generation of primordial black holes (PBH) from them. PBH refers to the black holes that could have been produced in the very early Universe. Unlike the astrophysical black holes, which are the end products of stellar evolution, PBH spans a wide mass range. In particular, we focus on generating PBHs with mass in the range ($10^{17}-10^{23}$ ) g, which can explain the full dark matter abundance. Further, we calculate the scalar-induced secondary gravitational waves spectrum associated with the PBH formation and explore their signatures in the gravitational wave detectors. This is crucial for understanding the physics of inflation, dark matter and gravitational waves.

In this paper we have considered an interacting model of dark energy where the dark sectors are allowed to interact among themselves through an assumed relation. By solving the perturbation equations numerically, we have studied the imprints on the growth of matter as well as dark energy fluctuations. It has been found that for higher rate of interaction strength for the coupling term, visible imprints on the dark energy density fluctuations are observed at the early epochs of evolution. In order to check the viability of the model, we have tested the interacting model with different observational datasets and have looked into the evolution of the density contrast, and the effects on the CMB temperature fluctuation as well as matter power spectrum.

Moduli potential loses its minima due to external energy sources of inflaton energy density or radiation produced at the end of inflation. But, the non-existence of minima does not necessarily mean destabilization of moduli. In fact, the destabilization of moduli is always dependent on the initial field values of the fields. In this work, we study carefully how the effects of reheating ease the problem of moduli destabilization. The associated time scale to produce the thermal bath allows a larger initial field range to stabilize the field. Contrary to the usual notion, the allowed initial field range is larger for higher temperatures when the effective potential is of a run-away nature. This eases the moduli destabilization problem for heavy mass moduli. For low mass moduli (≲ 30 TeV), the allowed field range still causes the cosmological moduli problem by violating the BBN constraints unless its initial abundance is suppressed.

Conditions for strong first-order phase transition and generation of observable gravitational wave (GW) signals are very restrictive to the profile of the Higgs potential. Working in the minimal extension of the SM with a new gauge singlet real scalar, I’ll show that the production of signals relevant for future GW experiments, such as LISA, can favor depleted resonant and non-resonant di-Higgs rates at colliders for phenomenologically relevant regimes of scalar mixing angles and masses for the heavy scalar. I’ll discuss the emergence of these di-Higgs blind spot configurations in GWs and also show that di-boson channels, ZZ and WW , can restore the phenomenological complementarities between GW and collider experiments in these parameter space regimes.

Type 1a supernovae have been used as distance rulers to study the kinematics of the universe and serve as the primary evidence of dark energy and the recently growing Hubble tension. The latest type 1a compilation released is Pantheon+, but there is some debate within the literature about the many corrections employed in the dataset and its treatment of uncertainties. We examine the peculiar velocity corrections in the Pantheon+ dataset and estimate the cosmological parameters using the Maximum Likelihood technique.

My talk will mainly focus on the reheating dynamics after inflation and its possible implication on dark matter (DM) and inflaton phenomenology. We consider reheating through various possible channels of inflaton going into massless scalars (bosonic reheating) and fermions (fermionic reheating) via non-gravitational and gravity-mediated decay processes. We further include the finite temperature effect on the decay process. Along with their precise roles in governing the dynamics, we compared the relative importance of different temperature-corrected decay channels in the gradual process of reheating depending on the reheating equation of state (EoS), which is directly related to inflaton potential. Particularly, the universal gravitational decay of inflaton is observed to play a crucial role in the reheating process for a large range of inflaton decay parameters. For our study, we consider typical α-attractor inflationary models. We further establish the intriguing connection among those different inflaton decay channels and the CMB power spectrum that can have profound implications in building up a unified model of inflation, reheating, and DM. We analyze both fermion and scalar DM with different physical processes being involved, such as gravitational scattering, thermal bath scattering, and direct inflaton decay. Gravitational decay can again be observed to play a crucial role in setting the maximum limit on DM mass, especially in the FIMP scenario. Depending on the coupling strength, we have analyzed in detail the production of both FIMP and WIMP-like DM during reheating, and their detailed phenomenological implications from the perspective of various cosmological and laboratory experiments. References: 1. Phys.Rev.D 106 (2022) 2, 023506, 2.e-Print: 2301.01641 3. e-Print: 2201.02348 (Accepted in Phys.Rev.D )

Topics: Production of ultralight dark matter from vacuum realignment. Linear theory structure formation. Bounds from the CMB and large scale structure. Schrodinger-Poisson equation inside galaxies. Looking to the future with intensity mapping. Formation and structure of axion stars. Superradiance.

Recent literature is replete with models and mechanisms for the production of primordial black holes (PBHs) that could constitute significant fraction of dark matter in the current universe. Among the various models, a class of inflationary models known as ultra slow roll (USR) inflation stands as a promising candidate. These models generate significant population of PBHs along with enhanced strengths of secondary gravitational waves (GWs). Given the plethora of models in the literature, it is imperative to look for unique features of USR, to distinguish it from other contenders. USR models, besides enhancing the primordial scalar power over small scales, lead to nulling of scalar power at a specific scale close to the location of the peak in the spectrum. Further, the scalar bispectra arising from USR models have non-trivial amplitudes and shapes, such that they may alter the observational predictions that are computed using Gaussian assumptions. For instance, they lead to significant corrections to the spectral density of secondary GWs around the peak amplitude. In this talk, I shall highlight these characteristic signatures of USR and discuss the prospect of observing them through different cosmological probes such as the upcoming GW and 21-cm missions. Reference: [1] H. V. Ragavendra and L. Sriramkumar, an invited review accepted in Galaxies [arXiv:2301.08887 [astro-ph.CO]]

We propose a new method to hunt for dark matter using dark forest/absorption features across the full electromagnetic spectrum, especially in the bands where there is a desert i.e. regions where no strong lines from baryons are expected. Such unique signatures can arise for dark matter models with a composite nature and internal electromagnetic transitions. In the presence of a background source, such as a quasar, such interactions in the dark matter halos can produce a series of closely spaced absorption lines, which we call the dark forest. The dark forest feature is a sensitive probe of the dark matter self-interactions and the halo mass function, especially at the low mass end. Moreover, the absorption of CMB photons by dark matter gives rise to a global absorption signal in the CMB spectrum which can explain the anomalous absorption feature detected by the EDGES collaboration.

Topics: Focus on the QCD axion. Basics of particle physics and direct detection. Symmetry breaking after inflation and relic density from topological defects. Formation of miniclusters. Phenomenology of miniclusters.

In the paradigm of hierarchical structure formation, the galaxies are thought to form and evolve inside a potential well environment (halos) of 'collisionless' and 'only gravitationally interacting' form of matter. These dark halos have formed at the peaks of initial density fluctuations due to gravitational instability and as observations have revealed, are the sites of most of the galaxy formation and evolution. Estimating the presence of these dark structures of halos by using available galaxy surveys itself is an important and challenging task. This information can then be used to find out the connection between the galaxy and halo properties. Our current understanding of the structure formation and evolution is driven by simulations. At large scales the full hydrodynamic simulations are not feasible due to computational cost. However using 'the connection' (scaling relations) between galaxy properties like for example, 'stellar mass' versus the 'halo mass', semi-analytical models of structure formation can constrain the effectiveness of physical processes as a function of redshift, thus bypassing the need of full simulation from scratch. In our work, to estimate the masses of dark matter halos which host - (the lens galaxies) the photometric galaxies from HSC survey, we employ the technique of measurement of 'weak gravitational lensing' signals. Weak lensing being purely gravitational phenomena, directly and fully probes the total matter content responsible for lensing of the background source galaxies. Thus we are able to probe this matter content associated with lens galaxies responsible for lensing. We show the halo-mass vs stellar mass relation as our concluding scaling relation from low to high redshift bins of lens galaxies further split into bins of stellar masses, and in doing so we represent the power of wide layer of HSC survey for statistical studies like weak lensing.

In a mean-field approximation within Kinetic Field Theory, it is possible to derive an accurate analytic expression for the power spectrum of resent-day, non-linear, cosmic density fluctuations. It depends on the gravity theory and the cosmological model via the expansion function of the background space-time, on the growth factor derived from it, and on the gravitational coupling strength, which may deviate from Newton’s constant in a manner depending on time and scale. In earlier work, we introduced a functional Taylor expansion around general relativity and the cosmological standard model to derive the effects of a wide class of modified-gravity theories on the non-linear power spectrum, assuming that such effects need to be small given the general success of the standard model. Here, we extend this class towards theories with small-scale screening, modelling screening effects by a suitably flexible interpolating function. We compare the Taylor expansion with exact solutions and find good agreement where expected. We find typical relative enhancements of the non-linear power spectrum between a few and a few ten per cent in a broad range of wave numbers between k ≳ 0.1 − 10 h Mpc, in good qualitative agreement with results obtained from numerical simulations.

In this talk, I discuss the evolution of various measures of quantumness of the curvature perturbation by integrating out the inaccessible entropic fluctuations in the multi-field models of inflation. In particular, I discuss the following measures of quantumness, namely purity, entanglement entropy and quantum discord. The models being considered in this talk are ones that produce large scale curvature power spectra similar to those produced by single-field models of inflation. More specifically, I consider different multi-field models which generate nearly scale invariant and oscillatory curvature power spectrum and compare their quantum signatures in the perturbations with the corresponding single-field models. I will show that, even though different models of inflation may produce the same observable power spectrum on large scales, they have distinct quantum signatures arising from the perturbation modes. This may allow for a way to distinguish between different models of inflation based on their quantum signatures.

During reheating coherent oscillations of a field dissipate its energy density and form a dominating thermal bath. In this talk, I will discuss the thermalization of the decay products produced via increasing dissipation rate and its implication on the production of dark matter in the reheating epoch.

Dedicated metre-sized detectors on Earth looking for dark matter particles via "direct detection" have caught none so far. This is perhaps because these particles are at least kilometres apart, thus coming with scant flux. I will discuss new strategies to catch them, and a search I performed in collaboration with the DEAP-3600 experiment, placing the first laboratory limits on particle dark matter at the Planck mass (1.2 x 10^19 GeV). I will also describe how neutrino experiments, with much larger detectors, can be repurposed to look for dark matter orders of magnitude beyond the Planck mass.

We study numerical evolution 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

Based on: JCAP 10 (2022) 018 (arXiv: 2207.07142). Certain inflationary models like Natural inflation (NI) and Coleman-Weinberg inflation (CWI) are disfavoured by cosmological data in the standard ΛCDM+r model (where r is the scalar-to-tensor ratio), as these inflationary models predict the regions in the ns-r parameter space that are excluded by the cosmological data at more than 2σ (here ns is the scalar spectral index). Cosmological models incorporating strongly self-interacting neutrinos (with a heavy mediator) are, however, known to prefer lower ns values compared to the ΛCDM model. Considering such neutrino self-interactions can, thus, open up the parameter space to accommodate the above inflationary models. In this work, we implement the massive neutrino self-interactions with a heavy mediator in two different ways: flavour-universal (among all three neutrinos), and flavour-specific (involving only one neutrino species). We implement the new interaction in both scalar and tensor perturbation equations of neutrinos. Interestingly, we find that the current cosmological data can support the aforementioned inflationary models at 2σ in the presence of such neutrino self-interactions.

After overviewing some of the short scale puzzles within the standard cold dark matter paradigm, I will discuss the idea of dark matter superfluidity as a tantalizing phenomenon in this context. One of the motivations is to provide a novel mechanism behind empirical correlations between dark and baryonic sectors within galaxies, by having an emergent long-range interaction between baryons mediated by superfluid phonons. I will discuss the challenges needed to be overcome for a successful realization of this paradigm. Another motivation is the superfluidity itself, without phonon-mediated forces. I will discuss the recent progress in understanding the structure of dark matter haloes in this scenario and the implications for the galactic dynamics.

The emergence of the first stars and galaxies, often termed as cosmic dawn, and subsequent ionization of the Universe constitute crucial epochs in the cosmic timeline. Studying the nature of the first sources of radiation is essential to understanding their evolution. The spin-flip transition from neutral hydrogen, 21-cm signal, constitutes a powerful probe of the early Universe. Tracing its brightness at different cosmological redshifts and spatial scales can unravel several poorly constrained astrophysical properties of the first stars and galaxies and their interaction with the intergalactic medium. However, observing the elusive 21-cm signal from high redshifts is challenging due to multiple factors, which include exceptionally bright foregrounds and instrumental systematics. In this talk, I will discuss some of these challenges and how they are being addressed by experiments worldwide. I will draw examples from SARAS and HERA, 21-cm experiments aiming to detect the global and spatial fluctuations of the 21-cm signal, respectively. I will discuss their calibration techniques, unique systematic and foreground handling approaches, and how proposed space-based observations from PRATUSH can overcome several challenges. I will conclude with the latest results from these experiments, which include inference on the anomalous detection from the EDGES experiment, constraints on the properties of the galaxies in the early Universe, and the improved limits by a joint model for the interferometric and global signal experiments.

The expansion history between the end of inflation and big bang nucleosynthesis, is not accessible from any direct observations at our disposal. The advent of gravitational wave astronomy and recent developements involving primordial black holes (PBH) as a dark matter candidate has offered us a novel way to probe this intermediate phase. We find different reheating histories to leave distinct effects on both the PBH formation and associated second order stochastic gravitational wave background (SGWB). Particularly a matter dominated reheating causes an aditional amplification in SGWB spectrum. We further explore the possibility of ultra-low mass PBHs to dominate the universe for brief duration before evaporation. Taking into account both the inflationary and PBH density fluctuations, we show that such a scenario leads to an uniquely shaped doubly peaked SGWB spectrum, detectable in future GW observatories. Detection or non-detection of such a signal will confirm or constrain the PBH parameters, as well as PBH evaporation induced baryogenesis, dark matter and dark radiation.

Recently, ultra-light Scalar Field Dark Matter (SFDM) has emerged as a promising alternative to the standard Cold Dark Matter (CDM) model. However, viability of models with no self-interactions (Fuzzy Dark Matter), in solving the small scale issues of CDM is under question while relevant masses are increasingly constrained using various astrophysical and cosmological observations. It is then interesting to consider the effects of self-interactions on these constraints. First we shall look at the impact of self-interactions on mass and radius of stable structures (solitons). Then we shall look at two astrophysical observations that can potentially constrain self-interactions: amount of mass contained within some region around the galactic center and galactic rotation curves of dwarf galaxies. We find that for the former, smaller values of attractive self-interactions are allowed compared to repulsive self-interactions. For the latter, repulsive self-interactions are preferred as compared to attractive or no self-interactions. For repulsive self-interactions, probed values of self-interaction strength is $\sim \mathcal{O}(10^{-90})$.

Recent sky surveys have discovered a large number of stellar substructures. It is highly likely that there are dark matter (DM) counterparts to these stellar substructures. We examine the implications of DM substructures for electron recoil (ER) direct detection (DD) rates in dual phase xenon experiments. We have utilized the results of the LAMOST survey and considered a few benchmark substructures in our analysis. Assuming that these substructures constitute ∼10% of the local DM density, we study the discovery limits of DM-electron scattering cross sections considering one kg-year exposure and 1, 2, and 3 electron thresholds. With this exposure and threshold, it is possible to observe the effect of the considered DM substructure for the currently allowed parameter space. We also explore the sensitivity of these experiments in resolving the DM substructure fraction. For all the considered cases, we observe that DM having mass O(10)MeV has a better prospect in resolving substructure fraction as compared to O(100)MeV scale DM. We also find that within the currently allowed DM-electron scattering cross-section; these experiments can resolve the substructure fraction (provided it has a non-negligible contribution to the local DM density) with good accuracy for O(10)MeV DM mass with one electron threshold.