2000 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/629
Browse
8 results
Search Results
Item Vanishing of cosmological constant in nonfactorizable geometry(2000-07-20) Padmanabhan, T.; Shankaranarayanan, S.We generalize the results of Randall and Sundrum to a wider class of four-dimensional space- times including the four-dimensional Schwarzschild background and de Sitter universe. We solve the equation for graviton propagation in a general four dimensional background and find an explicit solution for a zero mass bound state of the graviton. We find that this zero mass bound state is normalizable only if the cosmological constant is strictly zero, thereby providing a dynamical reason for the vanishing of cosmological constant within the context of this model. We also show that the results of Randall and Sundrum can be generalized without any modification to the Schwarzschild background.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 Probes of the vacuum structure of quantum fields in classical backgrounds(2000-11-12) Sriramkumar, L.; Padmanabhan, T.We compare the different approaches presently available in literature to probe the vacuum structure of quantum fields in classical electromagnetic and gravitational backgrounds. We compare the results from the Bogolubov transformations and the effective Lagrangian approach with the response of monopole detectors (of the Unruh-DeWitt type) in non-inertial frames in flat spacetime and in inertial frames in different types of classical electromagnetic backgrounds. We also carry out such a comparison in inertial and rotating frames when boundaries are present in flat spacetime. We find that the results from these different approaches do not, in general, agree with each other. We attempt to identify the origin of these differences and then go on to discuss its implications for classical gravitational backgrounds.Item Method of complex paths and general covariance of Hawking radiation(2000-12-15) Shankaranarayanan, S.; Srinivasan, K.; Padmanabhan, T.We apply the technique of complex paths to obtain Hawking radiation in different coordi- nate representations of the Schwarzschild space-time. The coordinate representations we consider do not possess a singularity at the horizon unlike the standard Schwarzschild co- ordinate. However, the event horizon manifests itself as a singularity in the expression for the semi-classical action. This singularity is regularized by using the method of complex paths and we find that Hawking radiation is recovered in these coordinates indicating the covariance of Hawking radiation. This also shows that there is no correspondence between the particles detected by the model detector and the particle spectrum obtained by the quantum field theoretic analysis – a result known in other contexts as well.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 Hypothesis of path integral duality. III: Applications to QED(2000-11-14) Shankaranarayanan, S.; Padmanabhan, T.We use the modified propagator for quantum field based on a “principle of path integral duality” proposed earlier in a paper by Padmanabhan to investigate several results in QED. This procedure modifies the Feynman propagator by the introduction of a funda- mental length scale. We use this modified propagator for the Dirac particles to evaluate the first order radiative corrections in QED. We find that the extra factor of the modified propagator acts like a regulator at the Planck scales thereby removing the divergences that otherwise appear in the conventional radiative correction calculations of QED. We find that:(i) all the three renormalization factors Z1, Z2, and Z3 pick up finite correc- tions and (ii) the modified propagator breaks the gauge invariance at a very small level of O(10−45). The implications of this result to generation of the primordial seed magnetic fields are discussed.Item Constraints on omegaB, omega M, and h from MAXIMA and BOOMERANG(2000-10-24) Padmanabhan, T.; Sethi, Shiv K.We analyse the BOOMERANG and MAXIMA results in the context of models with ΩTotal = 1 and ns = 1. We attempt to constrain three other parameters—h, ΩB, and Ωm—from these observations. We show that: (a) the value of ΩBh2 is too high to be compatible with primordial nucleosynthesis observations at 95% confidence level (b) universe with age greater than 12Gyr is ruled out at 95% confidence level and (c) the value of Ωmh is too high to be compatible with the shape of the power spectrum of gravitational clustering. In effect, our analysis shows that models with ΩTotal = 1 and n = 1 are ruled out by BOOMERANG and MAXIMA observations.