Browsing by Author "Choudhury, T. Roy"

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Can the clustered dark matter and the smooth dark energy arise from the same scalar field?
(2011-07-06) Padmanabhan, T.; Choudhury, T. Roy
Cosmological 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.
Can the clustered dark matter and the smooth dark energy arise from the same scalar ﬁeld?
(American Physical Society, 2002-10-29) Padmanabhan, T.; Choudhury, T. Roy
Cosmological observations suggest the existence of two different kinds of energy densities dominating 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 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) √1 - deli φ deli φ acting as ıt 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) ∝ tn) in the form n = (2/3) (1+r) . For r ≈ 2, we get n ≈ 2 which is consistent with observations.
Concept of temperature in multi-horizon spacetimes: analysis of Schwarzschild–De Sitter metric
(Springer, 2007-07-26) Choudhury, T. Roy; Padmanabhan, T.
In case of spacetimes with single horizon, there exist several well-established procedures for relating the surface gravity of the horizon to a thermodyna-mic temperature. Such procedures, however, cannot be extended in a straightforward manner when a spacetime has multiple horizons. In particular, it is not clear whether there exists a notion of global temperature characterizing the multi-horizon space-times. We examine the conditions under which a global temperature can exist for a spacetime with two horizons using the example of Schwarzschild–De Sitter (SDS) spacetime. We systematically extend different procedures (like the expectation value of stress tensor, response of particle detectors, periodicity in the Euclidean time etc.) for identifying a temperature in the case of spacetimes with single horizon to the SDS spacetime. This analysis is facilitated by using a global coordinate chart which covers the entire SDS manifold. We ﬁnd that all the procedures lead to a consistent picture characterized by the following features: (a) In general, SDS spacetime behaves like a non-equilibrium system characterized by two temperatures. (b) It is not possible to associate a global temperature with SDS spacetime except when the ratio of the two surface gravities is rational. (c) Even when the ratio of the two surface gravities is rational, the thermal nature depends on the coordinate chart used. There exists a global coordinate chart in which there is global equilibrium temperature while there exist other charts in which SDS behaves as though it has two different temperatures. The coordinate dependence of the thermal nature is reminiscent of the ﬂat spacetime in Minkowski and Rindler coordinate charts. The implications are discussed.
Constraining reionization and first
(2011-08-12) Choudhury, T. Roy
Cosmological parameters from supernova observations: A critical comparison of three data sets
(EDP Sciences, 2004-09-15) Choudhury, T. Roy; Padmanabhan, T.
We extend our previous analysis of cosmological supernova type Ia data (Padmanabhan & Choudhury 2003) to include three recent compilation of data sets. Our analysis ignores the possible correlations and systematic effects present in the data and concentrates mostly on some key theoretical issues. Among the three data sets, the first set consists of 194 points obtained from various observations while the second discards some of the points from the first one because of large uncertainties and thus consists of 142 points. The third data set is obtained from the second by adding the latest 14 points observed through HST. A careful comparison of these different data sets help us to draw the following conclusions: (i) All the three data sets strongly rule out non-accelerating models. Interestingly, the first and the second data sets favour a closed universe; if Ωtot ≡ Ωm + ΩΛ, then the probability of obtaining models with Ωtot > 1 is >∼0.97. Hence these data sets are in mild disagreement with the “concordance” flat model. However, this disagreement is reduced (the probability of obtaining models with Ωtot > 1 being ≈0.9) for the third data set, which includes the most recent points observed by HST around 1 < z < 1.6. (ii) When the first data set is divided into two separate subsets consisting of low (z < 0.34) and high (z > 0.34) redshift supernova, it turns out that these two subsets, individually, admit non-accelerating models with zero dark energy because of different magnitude zero-point values for the different subsets. This can also be seen when the data is analysed while allowing for possibly different magnitude zero-points for the two redshift subsets. However, the non-accelerating models seem to be ruled out using only the low redshift data for the other two data sets, which have less uncertainties. (iii) We have also found that it is quite difficult to measure the evolution of the dark energy equation of state wX(z) though its present value can be constrained quite well. The best-fit value seems to mildly favour a dark energy component with current equation of state wX < −1, thus opening the possibility of existence of more exotic forms of matter. However, the data is still consistent with the the standard cosmological constant at 99 per cent confidence level for Ωm >∼ 0.2.
Issue of choosing nothing : What determines the low energy vacuum state of nature?
(2000-09-25) Padmanabhan, T.; Choudhury, T. Roy
Starting 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.
Issue of choosing nothing: What Determines the low energy Vacuum state of nature?
(World Scientiﬁc Publishing Company, 2000-09-28) Padmanabhan, T.; Choudhury, T. Roy
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.
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.
Quasinormal modes in schwarzschild–de sitter spacetime: A simple derivation of the level spacing of the frequencies
(American Physical Society, 2004-03-25) Choudhury, T. Roy; Padmanabhan, T.
It is known that the imaginary parts of the quasinormal 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 de Sitter spacetime. It is not clear what the structure of the QNMs would be for the Schwarzschild–de Sitter (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 an 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 a comparison with previous work is presented.
Reply to "Comment on 'Quasinormal modes in schwarzschild–de sitter spacetime: A simple derivation of the level spacing of the frequencies"
(American Physical Society, 2011-05-23) Choudhury, T. Roy; Padmanabhan, T.
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.
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.
Semi analytic approach to understanding the distribution of neutral hydrogen in the universe: Comparison of simulations with observations
(American Astronomical Society, 2001-05-23) Srianand, R.; Padmanabhan, T.; Choudhury, T. Roy
Semi-analytic approach to understanding the distribution of neutral hydrogen in the Universe
(Wiley-Blackwell, 2000-05-15) 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 to perform one-dimensional simulations of lines of sight and analyse the properties of absorption systems. We have taken a completely analytic 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 various 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 owing to changes 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 function at different epochs provided that 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 reionization 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 out higher values of γ at higher redshifts.
Simple analytic model for the abundance of damped lyman alpha absorbers
(American Astronomical Society, 2002-03-28) Choudhury, T. Roy; Padmanabhan, T.
Theoretician's analysis of the supernova data and the limitations in determining the nature of dark energy
(2011-07-05) Padmanabhan, T.; Choudhury, T. Roy
Current 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.
Theoretician’s analysis of the supernova data and the limitations in determining the nature of dark energy
(Wiley-Blackwell, 2003-06-02) Padmanabhan, T.; Choudhury, T. Roy