arXiv subjects

J. S. Bagla

Publications and source records attributed to J. S. Bagla.

At least 55 records · Page 3Linked to original sources

arXiv2005

Observational constraints on low redshift evolution of dark energy: How consistent are different observations?

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/rho 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 WMAP observations of CMB anisotropies and abundance of rich clusters of galaxies. For models with a constant w, the LCDM 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 LCDM 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 Omega_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 three. Allowing for variation in w has an impact on the values for other cosmological parameters in that the allowed range often becomes larger. (Abridged)

astro-ph↗

arXiv2005

Gravitational Collapse in an Expanding Universe: Scaling Relations for Two-Dimensional Collapse Revisited

We investigate non-linear scaling relations for two-dimensional gravitational collapse in an expanding background using a 2D TreePM code and study the strongly non-linear regime ($\barξ\leq 200$) for power law models. Evolution of these models is found to be scale-invariant in all our simulations. We find that the stable clustering limit is not reached, but there is a model independent non-linear scaling relation in the asymptotic regime. This confirms results from an earlier study which only probed the mildly non-linear regime ($\barξ\leq 40$). The correlation function in the extremely non-linear regime is a less steep function of scale than reported in earlier studies. We show that this is due to coherent transverse motions in massive haloes. We also study density profiles and find that the scatter in the inner and outer slopes is large and that there is no single universal profile that fits all cases. We find that the difference in typical density profiles for different models is smaller than expected from similarity solutions for halo profiles and transverse motions induced by substructure are a likely reason for this difference being small.

astro-ph↗

arXiv2005

Gravitational collapse in an expanding background and the role of substructure I: Planar collapse

We study the interplay of clumping at small scales with the collapse and relaxation of perturbations at much larger scales. We present results of our analysis when the large scale perturbation is modelled as a plane wave. We find that in absence of substructure, collapse leads to formation of a pancake with multi-stream regions. Dynamical relaxation of plane wave is faster in presence of substructure. Scattering of substructures and the resulting enhancement of transverse motions of haloes in the multi-stream region lead to a thinner pancake. In turn, collapse of the plane wave leads to formation of more massive collapsed haloes as compared to the collapse of substructure in absence of the plane wave. The formation of more massive haloes happens without any increase in the total mass in collapsed haloes. A comparison with the Burgers' equation approach in absence of any substructure suggests that the preferred value of effective viscosity depends primarily on the number of streams in a region.

astro-ph↗

arXiv2005

Comments on the size of the simulation box in cosmological N-Body simulations

N-Body simulations are a very important tool in the study of formation of large scale structures. Much of the progress in understanding the physics of high redshift universe and comparison with observations would not have been possible without N-Body simulations. Given the importance of this tool, it is essential to understand its limitations as ignoring the limitations can easily lead to interesting but unreliable results. In this paper we study the limitations arising out of the finite size of simulation volume. This finite size implies that modes larger than the size of the simulation volume are ignored and a truncated power spectrum is simulated. If the simulation volume is large enough then the mass in collapsed haloes expected from the full power spectrum and from the truncated power spectrum should match. We propose a quantitative measure based on this approach that allows us to compute the minimum box size for an N-Body simulation. We find that the required box size for simulations of LCDM model at high redshifts is much larger than is typically used. We can also use this approach to quantify the effect of perturbations at large scales for power law models and we find that if we fix the scale of non-linearity, the required box size becomes very large as the index becomes small. The appropriate box size computed using this approach is also an appropriate choice for the transition scale when tools like MAP (Tormen and Bertschinger, 1996) that add the contribution of the missing power are used.

astro-ph↗

arXiv2004

Cosmological N-Body Simulations

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.

astro-ph↗

arXiv2004

WMAP constraints on low redshift evolution of dark energy

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 {\it with the constraints from WMAP observations} restricts large variation of $ρ(z)$ at low 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.

astro-ph↗

arXiv2004

Cosmological N-Body simulation: Techniques, Scope and Status

Cosmological N-Body simulations have become an essential tool for studying formation of large scale structure. These simulations are computationally challenging even though the available computing power gets better every year. A number of efficient algorithms have been developed to run large simulations with better dynamic range and resolution. We discuss key algorithms in this review, focusing on techniques used and their efficacy. N-Body simulations solve a model that is an approximation of the physical model to be simulated, we discuss limitations arising from this approximation and techniques employed for solving equations. Apart from simulating models of structure formation, N-Body simulations have also been used to study aspects of gravitational clustering. Simulating formation of galaxies requires us to take many physical process into account; we review numerical implementations of key processes.

astro-ph↗

arXiv2004

A Parallel TreePM Code

We present an algorithm for parallelising the TreePM code. We use both functional and domain decompositions. Functional decomposition is used to separate the computation of long range and short range forces, as well as the task of coordinating communications between different components. Short range force calculation is time consuming and benefits from the use of domain decomposition. We have tested the code on a Linux cluster. We get a speedup of 31.4 for 128^3 particle simulation on 33 processors; speedup being better for larger simulations. The time taken for one time step per particle is 6.5 micro seconds for a 256^3 particle simulation on 65 processors, thus a simulation that runs for 4000 time steps takes 5 days on this cluster.

astro-ph↗

arXiv2003

Cosmology with tachyon field as dark energy

We present a detailed study of cosmological effects of homogeneous tachyon matter coexisting with non-relativistic 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) ~ t^{2/3} as t goes to infinity. This eliminates the future event horizon present in LCDM models and is an attractive feature from the string theory perspective. A comparison with supernova 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 and are also consistent with requirements of structure formation. They do require fine tuning of parameters but not any more than in the case of LCDM or quintessence models.

astro-ph↗

arXiv2003

TreePM: A code for Cosmological N-Body Simulations

We describe the TreePM method for carrying out large N-Body simulations to study formation and evolution of the large scale structure in the Universe. This method is a combination of Barnes and Hut tree code and Particle-Mesh code. It combines the automatic inclusion of periodic boundary conditions of PM simulations with the high resolution of tree codes. This is done by splitting the gravitational force into a short range and a long range component. We describe the splitting of force between these two parts. We outline the key differences between TreePM and some other N-Body methods.

astro-ph↗

arXiv2002

Prospects for detecting neutral hydrogen using 21cm radiation from large scale structure at high redshifts

We estimate the signal from large scale structure at high redshifts in redshifted 21cm line. We focus on z ~ 3 and the LCDM cosmology. We assume that neutral hydrogen is to be found only in galaxies, and normalise the total content to the density parameter of neutral hydrogen in damped Lyman alpha absorption systems (DLAS). We find that the rms fluctuations due to the large scale distribution of galaxies is very small and cannot be observed at angular scales probed by present day telescopes. We have used the sensitivity of the Giant meter-wave Radio Telescope (GMRT) for comparison. We find that observations stretching over 1000hours will be required for a 3 sigma detection of such objects.

astro-ph↗

arXiv2002

Performance Characteristics of TreePM codes

We present a detailed analysis of the error budget for the TreePM method for doing cosmological N-Body simulations. It is shown that the choice of filter for splitting the inverse square force into short and long range components suggested in Bagla (2002) is close to optimum. We show that the error in the long range component of the force contributes very little to the total error in force. Errors introduced by the tree approximation for the short range force are different from those for the inverse square force, and these errors dominate the total error in force. We calculate the distribution function for error in force for clustered and unclustered particle distributions. This gives an idea of the error in realistic situations for different choices of parameters of the TreePM algorithm. We test the code by simulating a few power law models and checking for scale invariance.

astro-ph↗

arXiv2001

A Comparison of Simulated and Analytic Major Merger Counts

We use large volume, high resolution, N-body simulations of 3 different $Λ$CDM models, with different clustering strengths, to generate dark matter halo merging histories. Over the reliable range of halo masses, roughly galaxy groups to rich clusters of galaxies, we quantify the number density of major mergers for two different time intervals and compare with analytic predictions based on the extended Press-Schechter (1974) theory.

astro-ph↗

arXiv1999

The Observed Evolution of Galaxy Clustering Vs. Epoch-Dependent Biasing Models

We study the observed evolution of galaxy clustering as a function of redshift. We find that the clustering of galaxies decreases as we go from observations of the local Universe to $z \sim 2$. On the other hand, clustering of the Lyman break galaxies at $z \sim 3$ is very strong, comparable to the clustering of present day galaxies. However there are three major factors to take into account while comparing clustering measurements coming from various surveys: the so-called ``scale-dependence'' effect, due to measurements being made at different scales; the ``type-selection'' effect introduced by the fact that different galaxy surveys select different populations which do not have the same clustering amplitudes; and the Malmquist bias. We correct for the first two effects and discuss the implications of Malmquist bias on the interpretation of the data at different $z$. Then we compare the observed galaxy clustering with models for the evolution of clustering in some fixed cosmologies. Correcting for the scale-dependence effect significantly reduces the discrepancies amongst different measurements. We interpret the observed clustering signal at high redshift as coming from objects which are highly biased with respect to the underlying distribution of mass; this is not the case for $z\simlt 2$ where measurements are compatible with the assumption of a much lower biasing level which only shows a weak dependence on $z$. Finally we propose a model-independent test that can be used to place a lower limit on the density parameter $Ω_0$.

astro-ph↗

arXiv1998

Evolution of Galaxy and Quasar Clustering

We study the evolution of correlation function of dark matter halos in the CDM class of models. We show that the halo correlation function does not evolve in proportion with the correlation function of the underlying mass distribution. Earliest halos to collapse, which correspond to rare peaks in the density field, cluster very strongly. The amplitude of halo correlation function decreases from its initial, large, value. This decrease continues till the average peaks have collapsed, after which, the amplitude grows at a slow rate. This behaviour is shown to be generic and the epoch of minimum amplitude depends only on the rms fluctuations in mass at the relevant scale and, to a much smaller extent, on the slope of the power spectrum at that scale. We discuss the relevance of this result for interpretation of observations of galaxy and quasar clustering.

astro-ph↗

arXiv1998

Clustering in Redshift Space at High Redshifts

Recent observations show a large concentration of galaxies at high redshift. At first sight strong clustering of galaxies at high redshifts seems to be in contradiction with the models of structure formation. In this paper we show that such structures are a manifestation of the strong clustering of rare peaks in the density field. We compute the frequency of occurrence of such large concentrations of galaxies in some models of structure formation.

astro-ph↗

arXiv1998

Neutral hydrogen at high redshift: probing structure formation

Large inhomogeneities in neutral hydrogen in the universe can be detected at redshifts $z \leq 10$ using the redshifted 21cm line emission from atomic hydrogen. This paper reviews the expected evolution of neutral hydrogen and presents estimates for future surveys of HI at $z \approx 3$. We also discuss the possibility of detecting neutral hydrogen at higher redshifts.

astro-ph↗

arXiv1997

Evolution of Galaxy Clustering

We show that the galaxy correlation function does not evolve in proportion with the correlation function of the underlying mass distribution. Earliest galaxies cluster very strongly and the amplitude of the galaxy correlation function decreases from this large value. This continues till the average peaks have collapsed, after which, the galaxy correlation function does not evolve very strongly.

astro-ph↗