arXiv subjects

T. Roy Choudhury

Publications and source records attributed to T. Roy Choudhury.

63 records · Page 4Linked to original sources

arXiv2004

Quasi normal modes in Schwarzschild-DeSitter spacetime: A simple derivation of the level spacing of the frequencies

It is known that the imaginary parts of the quasi normal mode (QNM) frequencies for the Schwarzschild black hole are evenly spaced with a spacing that depends only on the surface gravity. On the other hand, for massless minimally coupled scalar fields, there exist no QNMs in the pure DeSitter spacetime. It is not clear what the structure of the QNMs would be for the Schwarzschild-DeSitter (SDS) spacetime, which is characterized by two different surface gravities. We provide a simple derivation of the imaginary parts of the QNM frequencies for the SDS spacetime by calculating the scattering amplitude in the first Born approximation and determining its poles. We find that, for the usual set of boundary conditions in which the incident wave is scattered off the black hole horizon, the imaginary parts of the QNM frequencies have a equally spaced structure with the level spacing depending on the surface gravity of the black hole. Several conceptual issues related to the QNM are discussed in the light of this result and comparison with previous work is presented.

gr-qc↗

arXiv2003

A theoretician's analysis of the supernova data and the limitations in determining the nature of dark energy

Current cosmological observations show a strong signature of the existence of a dark energy component with negative pressure. The most obvious candidate for this dark energy is the cosmological constant (with the equation of state w_X=p/ρ=-1), which, however, raises several theoretical difficulties. This has led to models for dark energy component which evolves with time. We discuss certain questions related to the determination of the nature of dark energy component from observations of high redshift supernova. The main results of our analysis are: (i) Even if the precise value of w_X is known from observations, it is not possible to determine the nature of the unknown dark energy source using only kinematical and geometrical measurements. We have given explicit examples to show that different types of sources can give rise to a given w_X. (ii) Although the full data set of supernova observations (which are currently available) strongly rule out models without dark energy, the high (z>0.25) and low (z<0.25) redshift data sets, individually, admit decelerating models with zero dark energy. Any possible evolution in the absolute magnitude of the supernovae, if detected, might allow the decelerating models to be consistent with the data. (iii) We have introduced two parameters, which can be obtained entirely from theory, to study the sensitivity of the luminosity distance on w_X. Using these two parameters, we have argued that although one can determine the present value of w_X accurately from the data, one cannot constrain the evolution of w_X.

astro-ph↗

arXiv2003

Physics of Structure Formation in the Universe

In recent years, unprecedented progress in observational cosmology has revealed a great deal of information about the formation and evolution of structures in the universe. This, in turn, has raised many challenging issues for the theorists. In the thesis, we have addressed two such key issues, namely, (a) the formation of baryonic structures and (b) the nature of dark matter and dark energy and the limitations in determining their nature from observations. The main results from the thesis are: (i) The baryons in the intergalactic medium at redshifts z \sim 2.5 can be modelled (both analytically and semi-analytically) by accounting for the non-linearities in the density field through lognormal approximation. Our results agree with observations, and can be used for constraining parameters related to the baryons. (ii) A simple model based on baryon conservation, along with observational estimates of cosmic star formation rate, correctly predicts the abundance of damped Lyman-alpha systems in the universe. (iii) The redshift distribution of gamma ray bursts can, in principle, be used for studying the physical conditions of the universe before reionization epoch (which is otherwise a difficult task). (iv) It might be possible that the dark matter and dark energy arise from the same scalar field, provided the equation of state has a dependence on the length scale. The possibility of using the kinematical and geometrical measurements (such as supernova observations) for determining the nature and evolution of the dark energy is discussed.

astro-ph↗

arXiv2002

Can the clustered dark matter and the smooth dark energy arise from the same scalar field ?

Cosmological observations suggest the existence of two different kinds of energy densities dominating at small ($ \lesssim 500$ Mpc) and large ($\gtrsim 1000 $ Mpc) scales. The dark matter component, which dominates at small scales, contributes $Ω_m \approx 0.35$ and has an equation of state $p=0$ while the dark energy component, which dominates at large scales, contributes $Ω_V \approx 0.65$ and has an equation of state $p\simeq -ρ$. It is usual to postulate wimps for the first component and some form of scalar field or cosmological constant for the second component. We explore the possibility of a scalar field with a Lagrangian $L =- V(ϕ) \sqrt{1 - \del^i ϕ\del_i ϕ}$ acting as {\it both} clustered dark matter and smoother dark energy and having a scale dependent equation of state. This model predicts a relation between the ratio $ r = ρ_V/ρ_{\rm DM}$ of the energy densities of the two dark components and expansion rate $n$ of the universe (with $a(t) \propto t^n$) in the form $n = (2/3) (1+r) $. For $r \approx 2$, we get $n \approx 2$ which is consistent with observations.

hep-th↗

arXiv2002

Probing the dark ages with redshift distribution of GRBs

In this article, we explore the possibility of using the properties of gamma ray bursts (GRBs) to probe the physical conditions in the epochs prior to reionization. The redshift distribution of GRBs is modelled using the Press-Schechter formalism with an assumption that they follow the cosmic star formation history. We reproduce the observed star formation rate obtained from galaxies in the redshift range 0 < z < 5, as well as the redshift distribution of the GRBs inferred from the luminosity-variability correlation of the burst light curve. We show that the fraction of GRBs at high redshifts, whose afterglows cannot be observed in R and I band due to HI Gunn Peterson optical depth can, at the most, account for one third of the dark GRBs. The observed redshift distribution of GRBs, with much less scatter than the one available today, can put stringent constraints on the epoch of reionization and the nature of gas cooling in the epochs prior to reionization.

astro-ph↗

arXiv2002

A simple analytical model for the abundance of damped Ly-αabsorbers

A simple analytical model for estimating the fraction (Ω_{gas}) of matter in gaseous form within the collapsed dark matter (DM) haloes is presented. The model is developed using (i) the Press-Schechter formalism to estimate the fraction of baryons in DM haloes, and (ii) the observational estimates of the star formation rate at different redshifts. The prediction for Ω_{gas} from the model is in broad agreement with the observed abundance of the damped Ly-αsystems. Furthermore, it can be used for estimating the circular velocities of the collapsed haloes at different redshifts, which could be compared with future observations.

astro-ph↗

arXiv2001

Semi analytic approach to understanding the distribution of neutral hydrogen in the universe: Comparison of simulations with observations

Following Bi & Davidsen (1997), we perform one dimensional semi analytic simulations along the lines of sight to model the intergalactic medium (IGM). Since this procedure is computationally efficient in probing the parameter space -- and reasonably accurate -- we use it to recover the values of various parameters related to the IGM (for a fixed background cosmology) by comparing the model predictions with different observations. For the currently favoured LCDM model (Ω_m=0.4, Ω_Λ=0.6 and h=0.65), we obtain, using statistics obtained from the transmitted flux, constraints on (i) the combination f=(Ω_B h^2)^2/J_{-12}, where Ω_B is the baryonic density parameter and J_{-12} is the total photoionisation rate in units of 10^{-12} s^{-1}, (ii) temperature T_0 corresponding to the mean density and (iii) the slope γof the effective equation of state of the IGM at a mean redshift z \simeq 2.5. We find that 0.8 <(T_0/10^4 K)< 2.5 and 1.3<γ<2.3. while the constraint obtained on f is 0.020^2 1.2, the lower bound on Ω_B h^2 is in violation of the BBN value.

astro-ph↗

arXiv2000

Semi analytic approach to understanding the distribution of neutral hydrogen in the universe

Analytic derivations of the correlation function and the column density distribution for neutral hydrogen in the IGM are presented, assuming that the non-linear baryonic mass density distribution in the IGM is lognormal. This ansatz was used earlier by Bi & Davidsen (1997) to perform 1D simulations of lines-of-sight and analyse the properties of absorption systems. Our approach is completely analytic, which allows us to explore a wide region of the parameter space for our model. The analytic results have been compared with observations, whenever possible. Two kinds of correlation functions are defined: along the line-of-sight (LOS) and across the transverse direction. We find that the effects on the LOS correlation due to change in cosmology and the slope of the equation of state of the IGM, γare of the same order, which means that we cannot constrain both the parameters simultaneously. However, it is possible to constrain γand its evolution using the observed LOS correlation function at different epochs, provided one knows the background cosmology. We suggest that the constraints on the evolution of γobtained using the LOS correlation can be used as an independent tool to probe the reionisation history of the universe. From the transverse correlation function, we find that the excess probability, over random, of finding two neutral hydrogen overdense regions separated by an angle θ, is always less than 1 per cent for redshifts greater than 2. Our models also reproduce the observed column density distribution for neutral hydrogen and the shape of the distribution depends on γ. Our calculations suggest that one can rule out γ> 1.6 for z \simeq 2.31 using the column density distribution. However, one cannot rule higher values of γat higher redshifts.

astro-ph↗

arXiv2000

The Issue of Choosing Nothing: What Determines the Low Energy Vacuum State of Nature?

Starting from an (unknown) quantum gravitational model, one can invoke a sequence of approximations to progressively arrive at quantum field theory (QFT) in curved spacetime, QFT in flat spacetime, nonrelativistic quantum mechanics and newtonian mechanics. The more exact theory can put restrictions on the range of possibilities allowed for the approximate theory which are not derivable from the latter - an example being the symmetry restrictions on the wave function for a pair of electrons. We argue that the choice of vacuum state at low energies could be such a `relic' arising from combining the principles of quantum theory and general relativity, and demonstrate this result in a simple toy model. Our analysis suggests that the wave function of the universe, when it describes the large volume limit of the universe, dynamically selects a vacuum state for matter fields - which in turn defines the concept of particle in the low energy limit. The result also has the potential for providing a concrete quantum mechanical version of Mach's principle.

gr-qc↗