arXiv subjects

Subinoy Das

Publications and source records attributed to Subinoy Das.

At least 37 records · Page 2Linked to original sources

arXiv2020

Non-thermal Hot Dark Matter from Inflaton/Moduli Decay: The Momentum Distribution and Relaxing the Cosmological Mass Bound

Decay of the inflaton or moduli which dominated the energy density of the universe at early times leads to a matter to radiation transition epoch. We consider non-thermal sterile dark matter particles produced as decay product during such transitions. The particles have a characteristic energy distribution - that associated with decays taking place in a matter dominated universe evolving to radiation domination. We primarily focus on the case when the particles are hot dark matter, and study their effects on the Cosmic Microwave Background (CMB) and Large Scale Structure (LSS), explicitly taking into account their non-thermal momentum distribution. Our results for CMB angular power and linear matter power spectra reveal interesting features - such as an order of magnitude higher values of hot dark matter mass in comparison to the thermal case being consistent with the present data. We observe that this is related to the fact that $ΔN_{\rm eff}$ and the hot DM energy density can be independent of each other unlike the case of thermal or non-resonantly produced sterile hot DM. We also find features in the CMB at low $\ell$ angular power potentially related to supersonic transmission of hot dark matter through the photon-baryon plasma.

astro-ph.CO↗

arXiv2019

Alleviating the $H_0$ and $σ_8$ anomalies with a decaying dark matter model

The Hubble tension between the $Λ$CDM-model-dependent prediction of the current expansion rate $H_0$ using Planck data and direct, model-independent measurements in the local universe from the SH0ES collaboration disagree at $>3.5σ$. Moreover, there exists a milder $\sim 2σ$ tension between similar predictions for the amplitude $S_8$ of matter fluctuations and its measurement in the local universe. As explanations relying on unresolved systematics have not been found, theorists have been exploring explanations for these anomalies that modify the cosmological model, altering early-universe-based predictions for these parameters. However, new cosmological models that attempt to resolve one tension often worsen the other. In this paper, we investigate a decaying dark matter (DDM) model as a solution to both tensions simultaneously. Here, a fraction of dark matter density decays into dark radiation. The decay rate $Γ$ is proportional to the Hubble rate $H$ through the constant $α_{\rm dr}$, the only additional parameter of this model. Then, this model deviates most from $Λ$CDM in the early universe, with $α_{\rm dr}$ being positively correlated with $H_0$ and negatively with $S_8$. Hence, increasing $α_{\rm dr}$ (and allowing dark matter to decay in this way) can then diminish both tensions simultaneously. When only considering Planck CMB data and the local SH0ES prior on $H_0$, $\sim 1$\% dark matter decays, decreasing the $S_8$ tension to $0.3σ$ and increasing the best-fit $H_0$ by $1.6$ km/s/Mpc. However, the addition of intermediate-redshift data (the JLA supernova dataset and baryon acoustic oscillation data) weakens the effectiveness of this model. Only $\sim 0.5$\% of the dark matter decays bringing the $S_8$ tension back up to $\sim 1.5 σ$ and the increase in the best-fit $H_0$ down to $0.4$ km/s/Mpc.

astro-ph.CO↗

arXiv2019

Are $H_0$ and $σ_8$ tensions generic to present cosmological data?

Yes, for a wide range of cosmological models ($Λ$CDM, non-interacting $w_z$CDM or models with possible interactions between dark energy and dark matter, in either phantom or non-phantom regimes). In the recent past there have been many attempts to solve the tension between direct measurements of $H_0$ and $σ_8 \sqrt{Ω_{0 {\rm m}}}$ from the respective low redshift observables and indirect measurements of these quantities from the cosmic microwave background (CMB). In this work we reconstruct a model independent approach that boils down to different classes of cosmological models under suitable parameters choices. We test this parameterization against the latest Planck CMB data combined with recent BAO, SNeIa datasets and the R16 direct $H_0$ measurements, and compare among different cosmological models. Our analysis reveals that a strong positive correlation between $H_0$ and $σ_8$ is more or less generic, irrespective of the choice of cosmological models. We also find that present data slightly prefers a phantom equation of state for dark energy and a slight negative value for effective equation of state for dark matter (which is a direct signature of interacting models) with a relatively high value for $H_0$ consistent with R16 and simultaneously, a consistent value for $Ω_{0 {\rm m}}$. Thus, even though the tensions cannot be fully resolved, interacting models with phantom equation of state get a slight edge over the others for currently available data. We also see that allowing interaction between dark energy and dark matter may resolve the tension between the high redshift CMB data and individual low redshift datasets, but the low redshift datasets have inconsistencies between them (e.g. between BAO and $H_0$, SNeIa and BAO, and cluster counts and $H_0$) that are practically independent of the cosmological model.

astro-ph.CO↗

arXiv2018

On dark matter - dark radiation interaction and cosmic reionization

An intriguing possibility for the dark sector of our universe is that the dark matter particle could interact with a dark radiation component. If the non-gravitational interactions of the dark matter and dark radiation species with Standard Model particles are highly suppressed, then astrophysics and cosmology could be our only windows into probing the dynamics of such a dark sector. It is well known that such dark sectors would lead to suppression of small scale structure, which would be constrained by measurements of the Lyman-$α$ forest. In this work we consider the cosmological signatures of such dark sectors on the reionization history of our universe. Working within the "ETHOS" (effective theory of structure formation) framework, we show that if such a dark sector exists in our universe, the suppression of low mass dark matter halos would also reduce the total number of ionizing photons, thus affecting the reionization history of our universe. We place constraints on the interaction strengths within such dark sectors by using the measured value of the optical depth from the Planck satellite, as well as from demanding a successful reionization history. We compare and contrast such scenarios with warm dark matter scenarios which also suppress structure formation on small scales. In a model where dark matter interacts with a sterile neutrino, we find a bound on the ETHOS parameter $a_4\lesssim 1.2\times10^6\textrm{ Mpc}^{-1}$. For warm dark matter models, we constrain the mass $m_{\textrm{WDM}}\gtrsim 0.7\textrm{ keV}$, which is comparable to bounds obtained from Lyman-$α$ measurements. Future 21-cm experiments will measure the global history of reionization and the neutral hydrogen power spectrum, which could either lead to stronger constraints or discovery of secret dark sector interactions.

astro-ph.CO↗

arXiv2018

Constraints on dark matter models from the observation of Triangulum-II with the Fermi Large Area Telescope

Triangulum-II, a newly discovered dwarf spheroidal galaxy, is a strong candidate for the indirect search of dark matter through the detection of $γ$-ray emission that could originate from the pair- annihilation of weakly interacting massive particles (WIMPs). We here report on the analysis of almost seven years Fermi Gamma-Ray Space Telescope data of Triangulum-II which was taken during its all sky survey operation mode. No excess $γ$-ray emission has been detected above 100 MeV from Triangulum-II. We derive the upper limits on $γ$-ray flux assuming both the power-law spectra and the spectra related to WIMP annihilation. In this work, we have considered several theoretical WIMP (neutralinos here) models envisioning both thermal and non-thermal production of WIMPs, and put limits on pair-annihilation cross-section of WIMPs to constrain the parameter space related to those theoretical models.

astro-ph.HE↗

arXiv2017

The effects of the small-scale behaviour of dark matter power spectrum on CMB spectral distortion

After numerous astronomical and experimental searches, the precise particle nature of dark matter is still unknown. The standard Weakly Interacting Massive Particle(WIMP) dark matter, despite successfully explaining the large-scale features of the universe, has long-standing small-scale issues. The spectral distortion in the Cosmic Microwave Background(CMB) caused by Silk damping in the pre-recombination era allows one to access information on a range of small scales $0.3 \, {\rm Mpc} < k < 10^4 \, \rm Mpc^{-1}$, whose dynamics can be precisely described using linear theory. In this paper, we investigate the possibility of using the Silk damping induced CMB spectral distortion as a probe of the small-scale power. We consider four suggested alternative dark matter candidates---Warm Dark Matter (WDM), Late Forming Dark Matter, Ultra Light Axion dark matter and Charged Decaying Dark Matter; the matter power in all these models deviate significantly from the $Λ$CDM model at small scales. We compute the spectral distortion of CMB for these alternative models and compare our results with the $Λ$CDM model. We show that the main impact of alternative models is to alter the sub-horizon evolution of the Newtonian potential which affects the late-time behaviour of spectral distortion of CMB. The $y$-parameter diminishes by a few percent as compared to the $Λ$CDM model for a range of parameters of these models: LFDM for formation redshift $z_f = 10^5$ (7\%); WDM for mass $m_{\rm wdm} = 1 \, \rm keV$ (2\%); CHDM for decay redshift $z_{\rm decay} = 10^5$ (5\%); ULA for mass $m_a = 10^{-24} \, \rm eV$ (3\%). We also briefly discuss the detectability of this deviation in light of the upcoming CMB experiment PIXIE, which might have the sensitivity to detect this signal from the pre-recombination phase.

astro-ph.CO↗

arXiv2017

Static structure of chameleon dark Matter as an explanation of dwarf spheroidal galactic core

We propose a novel mechanism which explains cored dark matter density profile in recently observed dark matter rich dwarf spheroidal galaxies. In our scenario, dark matter particle mass decreases gradually as function of distance towards the center of a dwarf galaxy due to its interaction with a chameleon scalar. At closer distance towards galactic center the strength of attractive scalar fifth force becomes much stronger than gravity and is balanced by the Fermi pressure of dark matter cloud, thus an equilibrium static configuration of dark matter halo is obtained. Like the case of soliton star or fermion Q-star, the stability of the dark matter halo is obtained as the scalar achieves a static profile and reaches an asymptotic value away from the galactic center. For simple scalar-dark matter interaction and quadratic scalar self interaction potential, we show that dark matter behaves exactly like cold dark matter (CDM) beyond few $\rm{kpc}$ away from galactic center but at closer distance it becomes lighter and fermi pressure cannot be ignored anymore. Using Thomas-Fermi approximation, we numerically solve the radial static profile of the scalar field, fermion mass and dark matter energy density as a function of distance. We find that for fifth force mediated by an ultra light scalar, it is possible to obtain a flattened dark matter density profile towards galactic center. In our scenario, the fifth force can be neglected at distance $ r \geq 1\, \rm{kpc}$ from galactic center and dark matter can be simply treated as heavy non-relativistic particles beyond this distance, thus reproducing the success of CDM at large scales.

gr-qc↗

arXiv2016

The effects of the small-scale DM power on the cosmological neutral hydrogen (\HI) distribution at high redshifts

The particle nature of dark matter remains a mystery. In this paper, we consider two dark matter models---Late Forming Dark Matter (LFDM) and Ultra-Light Axion (ULA) models---where the matter power spectra show novel effects on small scales. The high redshift universe offers a powerful probe of their parameters. In particular, we study two cosmological observables: the neutral hydrogen (HI) redshifted 21-cm signal from the epoch of reionization, and the evolution of the collapsed fraction of HI in the redshift range $2 < z < 5$. We model the theoretical predictions of the models using CDM-like N-body simulations with modified initial conditions, and generate reionization fields using an excursion-set model. The N-body approximation is valid on the length and halo mass scales studied. We show that LFDM and ULA models predict an increase in the HI power spectrum from the epoch of reionization by a factor between 2--10 for a range of scales $0.1 4 \times 10^5$ (for LFDM) and the axion mass $m_a > 2.6 \times 10^{-23} \, \rm eV$ (for ULA). The comparison of the collapsed mass fraction inferred from damped Lyman-$α$ observations to the theoretical predictions of our models lead to the weaker bounds: $z_f > 2 \times 10^5$ and $m_a > 10^{-23} \, \rm eV$. These bounds are consistent with other constraints in the literature using different observables; we briefly discuss how these bounds compare with possible constraints from the observation of luminosity function of galaxies at high redshifts. In the case of ULAs, these constraints are also consistent with a solution to the cusp-core problem of CDM.

astro-ph.CO↗

arXiv2015

Small scale clustering of late forming dark matter

We perform a study of the nonlinear clustering of matter in the late-forming dark matter (LFDM) scenario in which dark matter results from the transition of a nonminimally coupled scalar field from radiation to collisionless matter. A distinct feature of this model is the presence of a damped oscillatory cutoff in the linear matter power spectrum at small scales. We use a suite of high-resolution N-body simulations to study the imprints of LFDM on the nonlinear matter power spectrum, the halo mass and velocity functions and the halo density profiles. The model largely satisfies high-redshift matter power spectrum constraints from Lyman-$α$ forest measurements, while it predicts suppressed abundance of low-mass halos ($\sim 10^{9}-10^{10}$ h$^{-1}$ M$_\odot$) at all redshifts compared to a vanilla $Λ$CDM model. The analysis of the LFDM halo velocity function shows a better agreement than the $Λ$CDM prediction with the observed abundance of low-velocity galaxies in the local volume. Halos with mass $M\gtrsim 10^{11}$ h$^{-1}$ M$_\odot$ show minor departures of the density profiles from $Λ$CDM expectations, while smaller-mass halos are less dense, consistent with the fact that they form later than their $Λ$CDM counterparts.

astro-ph.CO↗

arXiv2015

How CMB and large-scale structure constrain chameleon interacting dark energy

We explore a chameleon type of interacting dark matter-dark energy scenario in which a scalar field adiabatically traces the minimum of an effective potential sourced by the dark matter density. We discuss extensively the effect of this coupling on cosmological observables, especially the parameter degeneracies expected to arise between the model parameters and other cosmological parameters, and then test the model against observations of the cosmic microwave background (CMB) anisotropies and other cosmological probes. We find that the chameleon parameters $α$ and $β$, which determine respectively the slope of the scalar field potential and the dark matter-dark energy coupling strength, can be constrained to $α< 0.17$ and $β< 0.19$ using CMB data alone. The latter parameter in particular is constrained only by the late Integrated Sachs-Wolfe effect. Adding measurements of the local Hubble expansion rate $H_0$ tightens the bound on $α$ by a factor of two, although this apparent improvement is arguably an artefact of the tension between the local measurement and the $H_0$ value inferred from Planck data in the minimal $Λ$CDM model. The same argument also precludes chameleon models from mimicking a dark radiation component, despite a passing similarity between the two scenarios in that they both delay the epoch of matter-radiation equality. Based on the derived parameter constraints, we discuss possible signatures of the model for ongoing and future large-scale structure surveys.

astro-ph.CO↗

arXiv2015

Constraint on noncommutative spacetime from PLANCK data

We constrain the energy scale of noncommutativity of spacetime using CMB data from PLANCK. We find that PLANCK data puts the lower bound on the noncommutativity energy scale to about 20 TeV, which is about a factor of two larger than a previous constraint that was obtained using data from WMAP, ACBAR and CBI. We further show that inclusion of data of $E$ mode of CMB polarization will not significantly change the constraint.

astro-ph.CO↗

arXiv2015

How Late can the Dark Matter form in our universe?

We put constraints on the epoch of dark matter formation for a class of non-WIMP (Weakly Interacting Massive Particle) dark matter candidates. These models allow a fraction of Cold Dark Matter (CDM) to be formed between the epoch of Big Bang Nucleosynthesis (BBN) and the matter radiation equality. We show that for such models the matter power spectra might get strong suppression even on scales that could be probed by linear perturbation theory at low redshifts. Unlike the case of Warm Dark Matter (WDM), where the mass of the dark matter particle controls the suppression scale, in Late Forming Dark Matter (LFDM) scenario, it is the redshift of the dark matter formation which determines the form of the matter power spectra. We use the Sloan Digital Sky Survey (SDSS) galaxy clustering data and the linear matter power spectrum reconstructed from the Lyman-$α$ data to find the latest epoch of the dark matter formation in our universe. If all the observed dark matter is late forming, we find lower bounds on the redshift of dark matter formation $z_f > 1.08 \times 10^5 $ at 99.73 $\%$ C.L from the SDSS data and $z_f > 9 \times 10^5$, at the same C.L, from the Lyman-$α$ data. If onlya fraction of the dark matter is late forming then we find tentative evidence of the presence of LFDM from the Lyman-$α$ data. Upcoming data from SDSS-III/BOSS (Baryon Oscillation Spectroscopic Survey) will allow us to explore this issue in more detail.

astro-ph.CO↗

arXiv2012

Origin of Delta N_eff as a Result of an Interaction between Dark Radiation and Dark Matter

Results from the Wilkinson Microwave Anisotropy Probe (WMAP), Atacama Cosmology Telescope (ACT) and recently from the South Pole Telescope (SPT) have indicated the possible existence of an extra radiation component in addition to the well known three neutrino species predicted by the Standard Model of particle physics. In this paper, we explore the possibility of the apparent extra dark radiation being linked directly to the physics of cold dark matter (CDM). In particular, we consider a generic scenario where dark radiation, as a result of an interaction, is produced directly by a fraction of the dark matter density effectively decaying into dark radiation. At an early epoch when the dark matter density is negligible, as an obvious consequence, the density of dark radiation is also very small. As the Universe approaches matter radiation equality, the dark matter density starts to dominate thereby increasing the content of dark radiation and changing the expansion rate of the Universe. As this increase in dark radiation content happens naturally after Big Bang Nucleosynthesis (BBN), it can relax the possible tension with lower values of radiation degrees of freedom measured from light element abundances compared to that of the CMB. We numerically confront this scenario with WMAP+ACT and WMAP+SPT data and derive an upper limit on the allowed fraction of dark matter decaying into dark radiation.

astro-ph.CO↗

arXiv2011

Cosmological Limits on Hidden Sector Dark Matter

We explore the model-independent constraints from cosmology on a dark-matter particle with no prominent standard model interactions that interacts and thermalizes with other particles in a hidden sector. Without specifying detailed hidden-sector particle physics, we characterize the relevant physics by the annihilation cross section, mass, and temperature ratio of the hidden to visible sectors. While encompassing the standard cold WIMP scenario, we do not require the freeze-out process to be nonrelativistic. Rather, freeze-out may also occur when dark matter particles are semirelativistic or relativistic. We solve the Boltzmann equation to find the conditions that hidden-sector dark matter accounts for the observed dark-matter density, satisfies the Tremaine-Gunn bound on dark-matter phase space density, and has a free-streaming length consistent with cosmological constraints on the matter power spectrum. We show that for masses <1.5 keV no region of parameter space satisfies all these constraints. This is a gravitationally-mediated lower bound on the dark-matter mass for any model in which the primary component of dark matter once had efficient interactions -- even if it has never been in equilibrium with the standard model.

astro-ph.CO↗

arXiv2010

Dark Matter Decaying into a Fermi Sea of Neutrinos

We study the possible decay of a coherently oscillating scalar field, interpreted as dark matter, into light fermions. Specifically, we consider a scalar field with sub-eV mass decaying into a Fermi sea of neutrinos. We recognize the similarity between our scenario and inflationary preheating where a coherently oscillating scalar field decays into standard model particles. Like the case of fermionic preheating, we find that Pauli blocking controls the dark matter decay into the neutrino sea. The radius of the Fermi sphere depends on the expansion of the universe leading to a time varying equation of state of dark matter. This makes the scenario very rich and we show that the decay rate might be different at different cosmological epochs. We categorize this in two interesting regimes and then study the cosmological perturbations to find the impact on structure formation. We find that the decay may help alleviating some of the standard problems related to cold dark matter.

astro-ph.CO↗

arXiv2009

The Dark Side of the Electroweak Phase Transition

Recent data from cosmic ray experiments may be explained by a new GeV scale of physics. In addition the fine-tuning of supersymmetric models may be alleviated by new O(GeV) states into which the Higgs boson could decay. The presence of these new, light states can affect early universe cosmology. We explore the consequences of a light (~ GeV) scalar on the electroweak phase transition. We find that trilinear interactions between the light state and the Higgs can allow a first order electroweak phase transition and a Higgs mass consistent with experimental bounds, which may allow electroweak baryogenesis to explain the cosmological baryon asymmetry. We show, within the context of a specific supersymmetric model, how the physics responsible for the first order phase transition may also be responsible for the recent cosmic ray excesses of PAMELA, FERMI etc. We consider the production of gravity waves from this transition and the possible detectability at LISA and BBO.

hep-ph↗

arXiv2008

Phenomenology of Hybrid Scenarios of Neutrino Dark Energy

We study the phenomenology of hybrid scenarios of neutrino dark energy, where in addition to a so-called Mass Varying Neutrino (MaVaN) sector a cosmological constant (from a false vacuum) is driving the accelerated expansion of the universe today. For general power law potentials we calculate the effective equation of state parameter w_{eff}(z) in terms of the neutrino mass scale. Due to the interaction of the dark energy field (acceleron) with the neutrino sector, w_{eff}(z) is predicted to become smaller than -1 for z>0, which could be tested in future cosmological observations. For the considered scenarios, the neutrino mass scale additionally determines which fraction of the dark energy is dynamical, and which originates from the cosmological constant like vacuum energy of the false vacuum. On the other hand, the field value of the acceleron field today as well as the masses of the right-handed neutrinos, which appear in the seesaw-type mechanism for small neutrino masses, are not fixed. This, in principle, allows to realise hybrid scenarios of neutrino dark energy with a high-scale seesaw where the right-handed neutrino masses are close to the GUT scale. We also comment on how MaVaN Hybrid Scenarios with high-scale seesaw might help to resolve stability problems of dark energy models with non-relativistic neutrinos.

astro-ph↗

arXiv2006

Late Forming Dark Matter in Theories of Neutrino Dark Energy

We study the possibility of Late Forming Dark Matter (LFDM), where a scalar field, previously trapped in a metastable state by thermal or finite density effects, begins to oscillate near the era matter-radiation equality about its true minimum. Such a theory is motivated generally if the dark energy is of a similar form, but has not yet made the transition to dark matter, and, in particular, arises automatically in recently considered theories of neutrino dark energy. If such a field comprises the present dark matter, the matter power spectrum typically shows a sharp break at small, presently nonlinear scales, below which power is highly suppressed and previously contained acoustic oscillations. If, instead, such a field forms a subdominant component of the total dark matter, such acoustic oscillations may imprint themselves in the linear regime.

astro-ph↗