Research Papers (TP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/3
Browse
228 results
Search Results
Item Crisis in cosmology: Observational constraints on Omega and H(Overseas Publishers Association, 1996-03-18) Bagla, J. S.; Padmanabhan, T.; Narlikar, J. V.This review of recent observations of cosmological interest seeks to take stock of how they constrain the standard hot big bang models with or without inflation. We look at two specific series indicative of this class of models. In one series the flatness condition of inflation requires that the density parameter shall be unity. Of late this statement has been relaxed somewhat to include the cosmological constant also as a contributor to the density parameter. Hence we ha»e used this "generalised" flatness condition. The other series of models does not need (be cosmological constant but assumes that the curvature parameter k = -1. Both these models are currently being pushed as "the" models of the universe. The observational constraints used by us are the measurements of the Hubble constant and the deceleration parameter, the ages of globular clusters, the abundance of primordial deuterium, the abundance of rich clusters, the baryon content of galaxy clusters and the abundance of high rsdshift objects. These constraints essentially limit the allowed values of the cosmological parameters. Our findings are that with measurements within their quoted error bars, the available parameter space has shrunk to negligible proportions. For survival of the standard models, therefore, one needs to take recourse to two normally unpalatable steps: (i) to doubt the existing error bars and hope to expand them and (ii) to fine-tone the theoretical parameters so that they fall within the available space. This is the essence of our perception of the crisis in cosmology.Item Cosmology with tachyon field as dark energy(American Physical Society, 2003-03-14) Bagla, J. S.; Jassal, H. K.; Padmanabhan, T.We present a detailed study of cosmological effects of homogeneous tachyon matter coexisting with nonrelativistic matter and radiation, concentrating on the inverse square potential and the exponential potential for the tachyonic scalar field. A distinguishing feature of these models ~compared to other cosmological models! is that the matter density parameter and the density parameter for tachyons remain comparable even in the matter dominated phase. For the exponential potential, the solutions have an accelerating phase, followed by a phase with a(t)}t2/3 as t!`. This eliminates the future event horizon present in cold dark matter models with a cosmological constant (LCDM) and is an attractive feature from the string theory perspective. A comparison with supernova type Ia data shows that for both the potentials there exists a range of models in which the universe undergoes an accelerated expansion at low redshifts which are also consistent with the requirements of structure formation. They do require fine-tuning of parameters but not any more than in the case of L CDM models or quintessence models.Item Cosmology Today: Models and constraints(Indian Academy of Sciences, 1995-03-12) Padmanabhan, T.Cosmological models for structure formation are severely constrained by several of the recent observational results. we now have observations which probe the power spectrum of fluctuations from about 0.5h-1 Mpc. these probes and the constraints they imply on models for structure formation are reviewed.Item 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.Item Cosmological N-Body Simulations(Indian Academy of Sciences, 1997-08-12) Bagla, J. S.; Padmanabhan, T.In this review we discuss Cosmological N-Body codes with a special emphasis on Particle Mesh codes. We present the mathematical model for each component of N-Body codes. We compare alternative methods for computing each quantity by calculating errors for each of the components. We suggest an optimum set of components that can be combined reduce overall errors in N-Body codes.Item Cosmological constant—the weight of the vacuum(Elsevier Science Publishers, 2003-03-01) Padmanabhan, T.Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(Gℏ/c3)≈10−123. This review discusses several aspects of the cosmological constant both from the cosmological (Sections 1–6) and field theoretical (Sections 7–11) perspectives. After a brief introduction to the key issues related to cosmological constant and a historical overview, a summary of the kinematics and dynamics of the standard Friedmann model of the universe is provided. The observational evidence for cosmological constant, especially from the supernova results, and the constraints from the age of the universe, structure formation, Cosmic Microwave Background Radiation (CMBR) anisotropies and a few others are described in detail, followed by a discussion of the theoretical models (quintessence, tachyonic scalar field, …) from different perspectives. The latter part of the review (Sections 7–11) concentrates on more conceptual and fundamental aspects of the cosmological constant like some alternative interpretations of the cosmological constant, relaxation mechanisms to reduce the cosmological constant to the currently observed value, the geometrical structure of the de Sitter spacetime, thermodynamics of the de Sitter universe and the role of string theory in the cosmological constant problem.Item Constraints on unstable heavy neutrinos from cosmology(Indian Academy of Sciences, 1985-12-11) Vasanthi, M. M.; Padmanabhan, T.Cosmological scenario with massive unstable neutrinos are discussed. Restrictions on the mass and lifetime of the unstable neutrino are derived from (a) age and mass density of the universe and (b) the growth of primordial fluctuations. It will not be possible to accommodate unstable neutrinos with masses above ~ `1 keV in standard cosmology unless they have exceedingly small lifetimeItem Constraints on the shape of the density spectrum from COBE and galaxy surveys(Wiley-Blackwell, 1992-10-28) Padmanabhan, T.; Narasimha, D.Item Constraints on ΩB, Ωm, and h from MAXIMA and BOOMERANG(American Astronomical Society, 2001-07-01) Padmanabhan, T.; Sethi, Shiv K.We analyze the BOOMERANG and MAXIMA results in the context of simplest inflationary universes: ΩTotal = 1, ns~ 1. We attempt to constrain three other parameters-h, ΩB, and Ωm from these observations. We show that (1) the data are consistent with the values of Ωm and h inferred from other observations and (2) the value of ΩBh² is too high to be compatible with big bang nucleosynthesis observations at the 2 σ level for ns = 1. We also include two cosmic background imager(CBI) band powers in our analysis. However, the inclusion of CBI band powers doesn't affect our conclusions.Item Conserved quantities from piecewise Killing vectors(IOP Publishing, 1988-11-10) Dray, Tevian; Padmanabhan, T.In the presence of symmetries, conserved quantities can be obtained by contracting the stress-energy tensor with a killing vector. We generalize this result to piecewise Killing vectors by giving sufficient conditions for the construction of an associated conserved quantity. A typical example namely, namely two stationary space-times joined together in such a way that the resulting space-time is not stationary, is treated in detail.