2003 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/626
Browse
4 results
Search Results
Item viable cosmology with a scalar field coupled to the trace of the stress-tensor(2011-07-05) Sami, S.; Padmanabhan, T.We study the cosmological evolution of a scalar field that couples to the trace T = Ta a of energy momentum tensor of all the fields (including itself). In the case of a shallow exponential potential, the presence of coupling to the trace T in the field equation makes the energy density of the scalar field decrease faster thereby hastening the commencement of radiation domination. This effect gradually diminishes at later epochs allowing the scalar field to dominate the energy density again. We interpret this phase as the current epoch of cosmic acceleration with Ωφ = 0.7. A variant of this model can lead to accelerated expansion at the present epoch followed by a a(t) ∝ t 2/3 behaviour as t → ∞, making the model free from future event horizon. The main features of the model are independent of initial conditions. However, fine tuning of parameters is necessary for viable evolution.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.Item Gravity and the thermodynamics of horizons(2011-07-05) Padmanabhan, T.Abstract Spacetimes with horizons show a resemblance to thermodynamic systems and it is possible to associate the notions of temperature and entropy with them. Several as- pects of this connection are reviewed in a manner appropriate for broad readership. The approach uses two essential principles: (a) the physical theories must be formu- lated for each observer entirely in terms of variables any given observer can access and (b) consistent formulation of quantum field theory requires analytic continua- tion to the complex plane. These two principles, when used together in spacetimes with horizons, are powerful enough to provide several results in a unified manner. Since spacetimes with horizons have a generic behaviour under analytic continua- tion, standard results of quantum field theory in curved spacetimes with horizons can be obtained directly (Sections III to VII). The requirements (a) and (b) also put strong constraints on the action principle describing the gravity and, in fact, one can obtain the Einstein-Hilbert action from the thermodynamic considerations (Section VIII). The review emphasises the thermodynamic aspects of horizons, which could be obtained from general principles and is expected to remain valid, independent of the microscopic description (‘statistical mechanics’) of horizons.