Showing papers on "Deceleration parameter published in 1996"

Measurements of the Cosmological Parameters Omega and Lambda from the First 7 Supernovae at z >= 0.35

Abstract: We have developed a technique to systematically discover and study high-redshift supernovae that can be used to measure the cosmological parameters. We report here results based on the initial seven of >28 supernovae discovered to date in the high-redshift supernova search of the Supernova Cosmology Project. We find a dispersion in peak magnitudes of sigma_{M_B} = 0.27 this dispersion narrows to sigma_{M_B,corr} = 0.19 after "correcting" the magnitudes using the light-curve "width-luminosity" relation found for nearby (z <= 0.1) type Ia supernovae from the Calan/Tololo survey (Hamuy et al. 1996). Comparing lightcurve-width-corrected magnitudes as a function of redshift of our distant (z = 0.35-0.46) supernovae to those of nearby type Ia supernovae yields a global measurement of the mass density, Omega_M = 0.88^{+0.69}_{-0.60} for a Lambda = 0 cosmology. For a spatially flat universe (i.e., Omega_M +Omega_Lambda = 1), we find Omega_M = 0.94 ^{+0.34}_{-0.28} or, equivalently, a measurement of the cosmological constant, Omega_Lambda = 0.06 ^{+0.28}_{-0.34} (<0.51 at the 95% confidence level). For the more general Friedmann-Lemaitre cosmologies with independent Omega_M and Omega_Lambda, the results are presented as a confidence region on the Omega_M-Omega_Lambda plane. This region does not correspond to a unique value of the deceleration parameter q_0. We present analyses and checks for statistical and systematic errors, and also show that our results do not depend on the specifics of the width-luminosity correction. The results for Omega_Lambda-versus-Omega_M are inconsistent with Lambda-dominated, low density, flat cosmologies that have been proposed to reconcile the ages of globular cluster stars with higher Hubble constant values.

The redshift of the gravitational lens of PKS1830-211 determined from molecular absorption lines

Abstract: GRAVITATIONAL lensing can be used to derive fundamental cosmo-logical parameters, provided that the redshift of the lens and the structure of its potential well are known1. The radio source PKS1830–211—a lensed quasar2–5 whose components are separated by about 1 arcsecond—is ideal for such an examination, but until recently the redshift of the lens has been unknown. Here we report the detection and identification of 12 molecular absorption lines in the spectrum of PKS1830–211; the lines originate in the lensing source, which is probably a spiral galaxy, at a redshift of ~0.89. Only one of the lensed components is covered by the intervening molecular gas; when combined with the strong variability in the quasar, this creates a unique opportunity to measure the difference in the light travel time for the two components, which depends in part on the Hubble constant and the cosmic deceleration parameter.

Binary inspiral, gravitational radiation, and cosmology.

Abstract: Observations of binary inspiral in a single interferometric gravitational wave detector can be cataloged according to signal-to-noise ratio $\ensuremath{\rho}$ and chirp mass $\mathcal{M}$. The distribution of events in a catalog composed of observations with $\ensuremath{\rho}$ greater than a threshold ${\ensuremath{\rho}}_{0}$ depends on the Hubble expansion, deceleration parameter, and cosmological constant, as well as the distribution of component masses in binary systems and evolutionary effects. In this paper I find general expressions, valid in any homogeneous and isotropic cosmological model, for the distribution with $\ensuremath{\rho}$ and $\mathcal{M}$ of cataloged events; I also evaluate these distributions explicitly for relevant matter-dominated Friedmann-Robertson-Walker models and simple models of the neutron star mass distribution. In matter-dominated Friedmann-Robertson-Walker cosmological models advanced LIGO detectors will observe binary neutron star inspiral events with $\ensuremath{\rho}g8$ from distances not exceeding approximately 2 Gpc, corresponding to redshifts of 0.48 (0.26) for $h=0.8(0.5)$, at an estimated rate of 1 per week. As the binary system mass increases so does the distance it can be seen, up to a limit: in a matter-dominated Einstein-de Sitter cosmological model with $h=0.8(0.5)$ that limit is approximately $z=2.7(1.7)$ for binaries consisting of two $10{M}_{\ensuremath{\bigodot}}$ black holes. Cosmological tests based on catalogs of the kind discussed here depend on the distribution of cataloged events with $\ensuremath{\rho}$ and $\mathcal{M}$. The distributions found here will play a pivotal role in testing cosmological models against our own universe and in constructing templates for the detection of cosmological inspiraling binary neutron stars and black holes.

The Universal Deceleration and Angular Diameter Distances to Clusters of Galaxies

Abstract: We show how the virial theorem can be applied to the hot gas in clusters of galaxies to obtain a yardstick, which could then be used to determine cosmological parameters. This yardstick relies on the assumptions of hydrostatic equilibrium and that the gas fraction is approximately constant. The constancy is checked empirically from a local population of clusters. By using the observed parameters consisting of temperature, surface brightness and radial profile $\beta$, one can calculate the expected core radius. Comparing it to the observed angular size, one can in principle calibrate the cosmological deceleration parameter $q_0$. We test this method on a small sample of 6 clusters, and show its promise and accuracy. The preliminary implications would be to suggest $q_0 \approx 0.85\pm 0.29$ with $1-\sigma$ statistical error bars, with several systematic uncertainties remaining. Taken at face value, this would argue against a cosmological constant. The method is robust to errors in the measurement of the core radius as long as the product of the central density and the core radius squared $\rho_0 r_c^2$ are well determined. New lensing and X-ray data can dramatically improve on the statistics.

Weak Lensing and the Measurement of q0 from Type Ia Supernovae

Abstract: On-going projects to discover Type Ia supernovae at redshifts z = 0.3 - 1, coupled with improved techniques to narrow the dispersion in SN Ia peak magnitudes, have renewed the prospects for determining the cosmic deceleration parameter q_0. We estimate the expected uncertainty in the Hubble diagram determination of q_0 due to weak lensing by structure in the universe, which stochastically shifts the apparent brightness of distant standard candles. Although the results are sensitive to the density power spectrum on small scales, the induced flux dispersion sigma_m 1.

Indeterministic Quantum Gravity IV. The Cosmic-length Universe and the Problem of the Missing Dark Matter

Abstract: This paper is a sequel of the series of papers [gr-qc/9409010, gr-qc/9505034, gr-qc/9603022], being an immediate continuation and development of the latter of them. In the Friedmann universe, the equation $\Omega_0=2q_0$ holds ($\Omega$ is density parameter, $q$ is deceleration parameter, and subscript 0 indicates present-day values), which gives rise to the problem of the missing matter as observational data give $\Omega_0<2q_0$. In the cosmic-length universe, $\Omega_0=2q_0- L/R_0^3H_0^2$ ($R_0$ is the radius of the universe, $H_0$ is Hubble constant), which lifts the problem. The cosmic length, $L=const \approx 1/H_0$, is the infimum of the set of maximal radii of a closed universe.

Dark Matter and Cosmology: CDM with a Cosmological Constant ($\Lambda$CDM) vs. CDM with Hot Dark Matter (CHDM)

Abstract: Here we discuss what are perhaps the two most popular variants of CDM that might agree with the data: \lcdm\ and CHDM. While the predictions of COBE-normalized \lcdm\ and CHDM both agree well with the available data on scales of $\sim 10$ to $100 \hMpc$, each has potential virtues and defects. \lcdm\ with $\Omega_0 \sim 0.3$ has the possible virtue of allowing a higher expansion rate $H_0$ for a given cosmic age $t_0$, but the defect of predicting too much fluctuation power on small scales. CHDM has less power on small scales, so its predictions appear to be in good agreement with data on the galaxy distribution, but it remains to be seen whether it predicts early enough galaxy formation to be compatible with the latest high-redshift data. Also, two very recent observational results favor high cosmic density, and thus favor $\Omega=1$ models such as CHDM over \lcdm\ --- (1) the positive deceleration parameter $q_0>0$ measured using high-redshift Type Ia supernovae, and (2) the low primordial deuterium/hydrogen ratio measured in two different quasar absorption spectra. We try to identify ``best'' variants of both \lcdm\ and CHDM, and discuss critical observational tests for both models.

Cosmology: going beyond the big bang

Abstract: Forty years ago cosmology was the realm of a handful of astronomers and two numbers were the holy grail: the Hubble constant, which describes the expansion of the universe, and the deceleration parameter, which describes how this expansion is slowing down due to gravity. Today, cosmology is an exciting area of research that attracts scientists ranging from astronomers and astrophysicists to experimental particle physicists and string theorists. The Hubble constant and the deceleration parameter have still not been measured, although they may be soon. The new holy grail, however, is to show that all structure in the universe evolved from quantum fluctuations.

Effects of Weak Gravitational Lensing from Large-Scale Structure on the Determination of $q_0$

Abstract: Weak gravitational lensing by large-scale structure affects the determination of the cosmological deceleration parameter $q_0$. We find that the lensing induced dispersions on truly standard candles are $0.04$ and $0.02$ mag at redshift $z=1$ and $z=0.5$, respectively, in a COBE-normalized cold dark matter universe with $\Omega_0=0.40$, $\Lambda_0=0.6$, $H=65$km/s/Mpc and $\sigma_8=0.79$. It is shown that one would observe $q_0=-0.44^{+0.17}_{-0.05}$ and $q_0=-0.45^{+0.10}_{-0.03}$ (the errorbars are $2\sigma$ limits) with standard candles with zero intrinsic dispersion at redshift $z=1$ and $z=0.5$, respectively, compared to the truth of $q_0=-0.40$ in this case, i.e., a 10\% error in $q_0$ will be made. A standard COBE normalized $\Omega_0=1$ CDM model would produce three times as much variance and a mixed (hot and cold) dark matter model would lead to an intermediate result. One unique signature of this dispersion effect is its non Gaussianity. Although the lensing induced dispersion at lower redshift is still significantly smaller than the currently best observed (total) dispersion of $0.12$ mag in a sample of type Ia supernovae, selected with the multicolor light curve shape method, it becomes significant at higher redshift. We show that there is an optimal redshift, in the range $z\sim 0.5-2.0$ depending on the amplitude of the intrinsic dispersion of the standard candles, at which $q_0$ can be most accurately determined.

On the Possible Variations of the Hubble Constant with Distance

Abstract: Current measurements of the Hubble constant H0 on scale less than ∼ 100 Mpc appear to be controversial, while the observations made at high redshift seem to provide a relatively low value. On the other hand, the Hubble expansion is driven by the matter content of the universe. The dynamical analysis on scale of a few ∼ 10 Mpc indicates that the matter density Ω0 is only ∼ 0.2–0.3, which is significantly smaller than Ω0 = 1 predicted in the standard inflation model. This might support the tendency of a decreasing Hubble constant towards distance. In this paper, we discuss the influence of a possible variant Hubble constant on two fundamental relations in astronomy: the magnitude-redshift (m–z) and the number-magnitude relations. Using a distant type Ia supernova at z = 0.458, we show that the deceleration parameter q0 or Ω0 cannot be determined from the m–z relation at moderate/high redshift unless the variation of the Hubble constant is a priori measured. It is further demonstrated that the number density of distant sources would be underestimated when their local calibration is employed, which may partially account for the number excess of the faint blue galaxies observed at moderate/high redshift. Subject headings: cosmology: distance scale — large-scale structure of universe

On the Possible Variations of the Hubble Constant with Distance

Abstract: Current measurements of the Hubble constant $H_0$ on scale less than $\sim100$ Mpc appear to be controversial, while the observations made at high redshift seem to provide a relatively low value. On the other hand, the Hubble expansion is driven by the matter content of the universe. The dynamical analysis on scale of a few $\sim10$ Mpc indicates that the matter density $\Omega_0$ is only $\sim0.2$--$0.3$, which is significantly smaller than $\Omega_0=1$ predicted in the standard inflation model. This might support the tendency of a decreasing Hubble constant towards distance. In this paper, we discuss the influence of a possible variant Hubble constant on two fundamental relations in astronomy: the magnitude-redshift ($m$--$z$) and the number-magnitude relations. Using a distant type Ia supernova at $z=0.458$, we show that the deceleration parameter $q_0$ or $\Omega_0$ cannot be determined from the $m$--$z$ relation at moderate/high redshift unless the variation of the Hubble constant is {\it a priori} measured. It is further demonstrated that the number density of distant sources would be underestimated when their local calibration is employed, which may partially account for the number excess of the faint blue galaxies observed at moderate/high redshift.

Cosmological tests for determining real spatial and time dimensionalities of our Universe

Abstract: In the framework of the homogeneous and isotropic cosmological model with arbitrary (noninteger) space and time dimensionalities, we derive the three classical cosmological tests: visual bolometric magnitude, angular distance and the number of sources versus redshift. We also obtain the deceleration parameter and the age and modern radius of the Universe.

Status of Cosmological Parameters: $\Omega_0\approx 0.3$ vs. $\Omega=1$

Abstract: The cosmological parameters that I discuss are the Hubble parameter $H_0 \equiv 100 h$ km s$^{-1}$ Mpc$^{-1}$, the age of the universe $t_0$, the average density $\Omega_0$, and the cosmological constant $\Lambda$. To focus the discussion, I concentrate on the the value of $\Omega_0$ in currently popular models in which most of the dark matter is cold, especially Cold + Hot Dark Matter (CHDM) and flat ($\Omega_0 + \Omega_\Lambda=1$) low-$\Omega$ CDM with a Cosmological Constant ($\Lambda$CDM). The evidence would favor small $\Omega_0 \approx 0.3$ if (1) the Hubble parameter actually has the high value $h \approx 0.75$ favored by many observers, and $t_0 \geq 13$ Gy; or (2) the baryonic/total mass ratio in clusters of galaxies is actually $\sim 15$\%, about 3 times larger than expected for standard BBN in an $\Omega=1$ universe, $\Omega_b \approx 0.0125 h^{-2}$, despite the recent measurement by Tytler of $D/H=2.4\times 10^{-5}$ in two high-redshift Lyman limit systems, implying $\Omega_b\approx 0.024 h^{-2}$. The evidence would favor $\Omega=1$ if (1) the POTENT analysis of galaxy peculiar velocity data is right, in particular regarding outflows from voids or the inability to obtain the present-epoch non- Gaussian density distribution from Gaussian initial fluctuations in a low- $\Omega$ universe; or (2) the preliminary LSND report indicating neutrino mass $\gsim 2.4$ eV is right, since that would be too much hot dark matter to allow significant structure formation in a low-$\Omega_0$ $\Lambda$CDM model. Statistics on gravitational lensing of quasars provide an upper limit on $\Lambda$, and the preliminary results on the deceleration parameter $q_0=\Omega_0/2-\Omega_\Lambda$ on very large scales from high-redshift Type Ia supernovae suggest that $\Omega_0 \sim 1$ and $\Omega_\Lambda$ is small.

Regarding the Deceleration of the Universe

Abstract: A proposed strategy for determining the deceleration parameter entails measuring the deviation from a linear (Hubble) distance-red shift relation. However, even at moderate red shifts, z > 0.2, the deviation does not depend on q_0 alone, but also on the equation-of-state of the universe.