IUCAA Preprints
Permanent URI for this communityhttp://localhost:4000/handle/11007/149
Browse
6 results
Search Results
Item Semi analytic approach to understanding the distribution of neutral hydrogen in the universe : Comparison of simulations with observations(2000-08-25) Choudhury, T. Roy; Srianand, R.; Padmanabhan, T.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 (Ωm = 0.4, ΩΛ = 0.6 and h = 0.65), we obtain, using statistics obtained from the transmitted flux, constraints on (i) the combination f = (ΩBh2 )2 /J−12, where ΩB is the baryonic density parameter and J−12 is the total photoionisation rate in units of 10−12 s−1 , (ii) temperature T0 corresponding to the mean density and (iii) the slope γ of the effective equation of state of the IGM at a mean redshift z ≃ 2.5. We find that 0.8 < (T0/104 K) < 2.5 and 1.3 < γ < 2.3. while the constraint obtained on f is 0.0202 < f < 0.0322 . A reliable lower bound on J−12 can be used to put a lower bound on ΩBh2 , which can be compared with similar constraints obtained from Big Bang Nucleosynthesis (BBN) and CMBR studies. We find that if J−12 > 1.2, the lower bound on ΩBh2 is in violation of the BBN value.Item Semi analytic approach to understanding the distribution of neutral hydrogen in the universe(2000-10-28) Choudhury, T. Roy; Padmanabhan, T.; Srianand, R.Analytic derivations of the correlation function and the column density distribution for neutral hydrogen in the intergalactic medium (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. We have taken a completely ana- lytic approach, which allows us to explore a wide region of the parameter space for our model. The analytic results have been compared with observations to constrain var- ious cosmological and IGM parameters, whenever possible. Two kinds of correlation functions are defined : (i) along the line-of-sight (LOS) and (ii) 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, γ are of the same order, which means that we cannot constrain both the parameters simultaneously. However, it is possible to constrain γ and its evolution using the observed LOS correlation func- tion at different epochs provided one knows the background cosmology. We suggest that the constraints on the evolution of γ obtained 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 obtain the excess probability, over random, of finding two neutral hydrogen overdense regions separated by an angle θ. We find that this excess probability 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 ≃ 2.31 using the column density distribution. However, one cannot rule higher values of γ at higher redshifts.Item Issue of choosing nothing : What determines the low energy vacuum state of nature?(2000-09-25) Padmanabhan, T.; Choudhury, T. RoyStarting from an (unknown) quantum gravitational model, one can invoke a sequence of approx- imations 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.Item Can the clustered dark matter and the smooth dark energy arise from the same scalar field?(2011-07-06) Padmanabhan, T.; Choudhury, T. RoyCosmological observations suggest the existence of two different kinds of energy densities domi- nating at small (< ∼ 500 Mpc) and large (> ∼ 1000 Mpc) scales. The dark matter component, which dominates at small scales, contributes Ωm ≈ 0.35 and has an equation of state p = 0, while the dark energy component, which dominates at large scales, contributes ΩV ≈ 0.65 and has an equation of state p ≃ −ρ. It is usual to postulate weakly interacting massive particles (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) p1 − ∂iφ∂iφ acting as 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 = ρV /ρDM of the energy densities of the two dark components and expansion rate n of the universe [with a(t) ∝ t n] in the form n = (2/3)(1 +r). For r ≈ 2, we get n ≈ 2 which is consistent with observations.Item Theoretician's analysis of the supernova data and the limitations in determining the nature of dark energy(2011-07-05) Padmanabhan, T.; Choudhury, T. RoyCurrent cosmological observations show a strong signature of the existence of a dark en-ergy component with negative pressure. The most obvious candidate for this dark energy is the cosmological constant (with the equation of state wX = p/ρ = −1), which, however,raises several theoretical ifficulties. This has led to models for dark energy componentwhich 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 wX 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 wX. (ii) Although the full data set of supernova observa- tions (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 in- troduced two parameters, which can be obtained entirely from theory, to study the sensitivity of the luminosity distance on wX. Using these two parameters, we have argued that although one can determine the present value of wX accurately from the data, one cannot constrain the evolution of wX.Item Quasi normal modes in Schawarzschild-DeSitter spacetime: A simple derivation of the level spacing of the frequencies(2011-07-05) Choudhury, T. Roy; Padmanabhan, T.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 calcu- lating 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.