Browsing by Author "Saini, Tarun Deep"

Now showing 1 - 8 of 8

Anisotropy dissipation in brane-world inflation
(2001-02-14) Maartens, Roy; Sahni, Varun; Saini, Tarun Deep
We examine the behavior of an anisotropic brane-world in the presence of inﬂationary scalar ﬁelds. We show that, contrary to naive expectations, a large anisotropy does not adversely aﬀect inﬂation. On the contrary, a large initial anisotropy introduces more damping into the scalar ﬁeld equation of motion, resulting in greater inﬂation. The rapid decay of anisotropy in the brane-world signiﬁcantly increases the class of initial conditions from which the observed universe could have originated. This generalizes a similar result in general relativity. A unique feature of Bianchi I brane-world cosmology appears to be that for scalar ﬁelds with a large kinetic term the initial expansion of the universe is quasi-isotropic. The universe grows more anisotropic during an intermediate transient regime until anisotropy ﬁnally disappears during inﬂationary expansion.
Exploring the expanding universe and dark energy using the statefinder diagnostic
(2011-07-05) Ujjaini, Alam; Sahni, Varun; Saini, Tarun Deep; et al.
The coming few years are likely to witness a dramatic increase in high quality Sn data as current surveys add more high redshift supernovae to their inventory and as newer and deeper supernova experiments become operational. Given the current variety in dark energy models and the expected improvement in observational data, an accurate and versatile diagnostic of dark energy is the need of the hour. This paper examines the Stateﬁnder diagnostic in the light of the proposed SNAP satellite which is expected to observe about 2000 supernovae per year. We show that the Stateﬁnder is versatile enough to diﬀerentiate between dark energy models as varied as the cosmological constant on the one hand, and quintessence, the Chaplygin gas and braneworld models, on the other. Using SNAP data, the Stateﬁnder can distinguish a cosmological constant (w = −1) from quintessence models with w > −0.9 and Chaplygin gas models with κ 6 15 at the 3σ level if the value of Ωm is known exactly. The Stateﬁnder gives reasonable results even when the value of Ωm is known to only ∼ 20% accuracy. In this case, marginalizing over Ωm and assuming a ﬁducial LCDM model allows us to rule out quintessence with w > −0.85 and the Chaplygin gas with κ 6 7 (both at 3σ). These constraints can be made even tighter if we use the Stateﬁnders in conjunction with the deceleration parameter. The Stateﬁnder is very sensitive to the total pressure exerted by all forms of matter and radiation in the universe. It can therefore diﬀerentiate between dark energy models at moderately high redshifts of z < 10.
Lens mapping algorithm for weak lensing
(2000-09-24) Saini, Tarun Deep; Raychaudhury, Somak
We develop an algorithm for the reconstruction of the two-dimensional mass distribution of a gravitational lens from the observable distortion of background galaxies. From the measured reduced shear γi/(1 − κ) the lens mapping is obtained, from which a mass distribution is derived. This is unlike other methods where the convergence κ is directly obtained. We show that this method works best for sub-critical lenses, but can be applied to a critical lens away from the critical lines. For ﬁnite ﬁelds the usual mass-sheet degeneracy is shown to exist in this method as well. We show that the algorithm reproduces the mass distribution within acceptable limits when applied to simulated noisy data.
Observing high-redshift Supernovae in lensed galaxies
(2000-01-28) Saini, Tarun Deep; Raychaudhury, Somak; Shchekinov, Yuri A.
Supernovae in distant galaxies that are grav- tationally lensed by foreground galaxy clusters make ex- cellent cosmological candles for measuring quantities like the density of the Universe in its various components and the Hubble constant. Distant supernovae will be more eas- ly detectable since foreground cluster lenses would mag- nify such supernovae by up to 3–4 magnitudes. We show that in the case of the lens cluster Abell 2218, the de- tectability of high-redshift supernovae is signiﬁcantly en- hanced due to the lensing eﬀects of the cluster. Since ensed supernovae will remain point images even when their host galaxies are stretched into arcs, the signal-to- noise ratio for their observation will be further enhanced, typically by an order of magnitude. We recommend moni- toring well-modelled clusters with several known arclets for the detection of cosmologically useful SNe around z = 1 and beyond.
Reconstructing the cosmic equation of state from supernova distances
(2015-03-01) Saini, Tarun Deep; Raychaudhuri, Somak; Sahni, Varun; Strarobinsky, A.A.
Response of distance measures to the equation of state
(Wiley-Blackwell, 2003-04-05) Saini, Tarun Deep; Padmanabhan, T.; Bridle, Sarah
We show that the distance measures (such as the luminosity and angular diameter distances) are linear functionals of the equation of state function w(z) of the dark energy to a fair degree of accuracy in the regimes of interest. That is, the distance measures can be expressed as a sum of (i) a constant and (ii) an integral of a weighting function multiplied by the equation of state parameter w(z). The existence of such an accurate linear response approximation has several important implications. (1) Fitting a weighted average of w(z). (2) Any polynomial (or other expansion coefficients) can also be expressed as weighted sums of the true w. (3) A replacement for the commonly used heuristic equation for the effective w, as determined by the cosmic microwave background, can be derived and the result is found to be quite close to the heuristic expression commonly used. (4) The reconstruction of w()z by Huterer & Starkman can be expressed as a matrix inversion. In each case the limitations of the linear response approximation are explored and found to be surprisingly small.
Statefinder - anew geometrical diagnostic of dark energy
(2002-02-21) Sahni, Varun; Saini, Tarun Deep; Starobinsky, A. A.
We introduce a new cosmological diagnostic pair {r, s} called Stateﬁnder. The Stateﬁnder is a geometrical diagnostic and allows us to characterize the properties of dark energy in a model independent manner. The Stateﬁnder is dimensionless and is constructed from the scale factor of the Universe and its time derivatives only. The parameter r forms the next step in the hierarchy of geometrical cosmological parameters after the Hubble parameter H and the deceleration parameter q, while s is a linear combination of q and r chosen in such a way that it does not depend upon the dark energy density. The Stateﬁnder pair {r, s} is algebraically related to the equation of state of dark energy and its ﬁrst time derivative. The Stateﬁnder pair is calculated for a number of existing models of dark energy having both constant and variable w. For the case of a cosmological constant the Stateﬁnder acquires a particularly simple form. We demonstrate that the Stateﬁnder diagnostic can eﬀectively diﬀerentiate between diﬀerent forms of dark energy. We also show that the mean Stateﬁnder pair can be determined to very high accuracy from a SNAP-type experiment.
Using Gravitational Lensing to study damped Lyman - alpha clouds
(2000-03-29) Saini, Tarun Deep; Bharadwaj, Somnath; Sethi, Shiv K.
We investigate the possibility of detecting HI emission from gravitationally lensed HI clouds (akin to damped Lyman-α clouds) at high redshift by carrying out deep radio observations in the ﬁelds of known cluster lenses. Such observations will be possible with present radio telescopes only if the lens substantially magniﬁes the ﬂux of the HI emission. While at present this holds the only possibility of detecting the HI emission from such clouds, it has the disadvantage of being restricted to clouds that lie very close to the caustics of the lens. We ﬁnd that observations at a detection threshold of 50 Jy at 320 MHz (possible with the GMRT) have a greater than 20% probability of detecting an HI cloud in the ﬁeld of a cluster, provided the clouds have HI masses in the range 5 × 108 M⊙ ≤ MHI ≤ 2.5 × 1010 M⊙. The probability of detecting a cloud in- creases if they have larger HI masses, except in the cases where the number of HI clouds in the cluster ﬁeld becomes very small. The probability of a detection at 610 MHz and 233 MHz is comparable to that at 320 MHz, though a deﬁnitive statement is diﬃcult owing to uncertainties in the HI content at the redshifts corresponding to these fre- quencies. Observations at a detection threshold of 2 Jy (possible in the future with the SKA) are expected to detect a few HI clouds in the ﬁeld of every cluster provided the clouds have HI masses in the range 2 × 107 M⊙ ≤ MHI ≤ 109 M⊙. Even if such observations do not result in the detection of HI clouds, they will be able to put useful constraints on the HI content of the clouds.