Research Papers (TP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/3

Browse

Now showing 1 - 4 of 4

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.
Observational constraints on low redshift evolution of dark energy: How consistent are different observations?
(American Physical Society, 2005-11-04) Jassal, H. K.; Bagla, J. S.; Padmanabhan, T.
The dark energy component of the Universe is often interpreted either in terms of a cosmological constant or as a scalar ﬁeld. A generic feature of the scalar ﬁeld models is that the equation of state parameter w P= for the dark energy need not satisfy w 1 and, in general, it can be a function of time. Using the Markov chain Monte Carlo method we perform a critical analysis of the cosmological parameter space, allowing for a varying w. We use constraints on w z from the observations of high redshift supernovae (SN), the Wilkinson Microwave Anisotropy Probe (WMAP) observations of cosmic microwave background (CMB) anisotropies, and abundance of rich clusters of galaxies. For models with a constant w, the CDM(cold dark matter) model is allowed with a probability of about 6% by the SN observations while it is allowed with a probability of 98.9% by WMAP observations. The CDM model is allowed even within the context of models with variable w: WMAP observations allow it with a probability of 99.1% whereas SN data allows it with 23% probability. The SN data, on its own, favors phantom-like equation of state (w< 1) and high values for NR. It does not distinguish between constant w (with w< 1) models and those with varying w z in a statistically signiﬁcant manner. The SN data allows a very wide range for variation of dark energy density, e.g., a variation by factor ten in the dark energy density between z 0 and z 1 is allowed at 95% conﬁdence level. WMAP observations provide a better constraint and the corresponding allowed variation is less than a factor of 3. Allowing for variation in w has an impact on the values for other cosmological parameters in that the allowed range often becomes larger. There is signiﬁcant tension between SN and WMAP observations; the best ﬁt model for one is often ruled out by the other at a very high conﬁdence limit. Hence results based on only one of these can lead to unreliable conclusions. Given the divergence in models favored by individual observations, and the fact that the best ﬁt models are ruled out in the combined analysis, there is a distinct possibility of the existence of systematic errors which are not understood.
Understanding the origin of CMB constraints on dark energy
(Wiley-Blackwell, 2010-03-08) Jassal, H. K.; Bagla, J. S.; Padmanabhan, T.
We study the observational constraints of cosmic microwave background (CMB) temperature and polarization anisotropies on models of dark energy, with special focus on models with variation in properties of dark energy with time. We demonstrate that the key constraint from CMB observations arises from the location of acoustic peaks. An additional constraint arises from the limits on ΩNR from the relative amplitudes of acoustic peaks. Further, we show that the distance to the last scattering surface is not how the CMB observations constrain the combination of parameters for models of dark energy. We also use constraints from supernova observations and show that unlike the gold and silver samples, the Supernova Legacy Survey (SNLS) sample prefers a region of parameter space that has a significant overlap with the region preferred by the CMB observations. This is a verification of a conjecture made by us in an earlier work. We discuss combined constraints from Wilkinson Microwave Anisotropy Probe 5-yr and SNLS observations. We find that models with w ~−1 are preferred for models with a constant equation-of-state parameters. In case of models with a time-varying dark energy, we show that constraints on evolution of dark energy density are almost independent of the type of variation assumed for the equation-of-state parameter. This makes it easy to get approximate constraints from CMB observations on arbitrary models of dark energy. Constraints on models with a time-varying dark energy are predominantly due to CMB observations, with supernova constraints playing only a marginal role.
WMAP constraints on low redshift evolution of dark energy
(Wiley-Blackwell, 2004-10-29) Jassal, H. K.; Bagla, J. S.; Padmanabhan, T.
The conceptual difﬁculties associated with a cosmological constant have led to the investigation of alternative models in which the equation of state parameter, w = p/ρ,of the dark energy evolves with time. We show that combining the supernova type Ia observations with the constraints from WMAP observations restricts large variation of ρ(z)atlow redshifts. The combination of these two observational constraints is stronger than either one. The results are completely consistent with the cosmological constant as the source of dark energy.