We determine the distances to 18 galaxy clusters with redshifts ranging from z ~ 0.14 to 0.78 from a maximum likelihood joint analysis of 30 GHz interferometric Sunyaev-Zeldovich effect (SZE) and X-ray observations. We model the intracluster medium (ICM) using a spherical isothermal β model. We quantify the statistical and systematic uncertainties inherent to these direct distance measurements, and we determine constraints on the Hubble parameter for three different cosmologies. These distances imply a Hubble constant of 60 km s-1 Mpc-1 for an ΩM = 0.3, ΩΛ = 0.7 cosmology, where the uncertainties correspond to statistical followed by systematic at 68% confidence. With a sample of 18 clusters, systematic uncertainties clearly dominate. The systematics are observationally approachable and will be addressed in the coming years through the current generation of X-ray satellites (Chandra and XMM-Newton) and radio observatories (Owens Valley Radio Observatory, Berkeley-Illinois-Maryland Association, and Very Large Array). Analysis of high-redshift clusters detected in future SZE and X-ray surveys will allow a determination of the geometry of the universe from SZE-determined distances.

Determining the cosmic distance scale from interferometric measurements of the Sunyaev-Zel'dovich effect

How to test stringy dark energy

There is now an enormously rich variety of experimental techniques being brought to bear on experimental searches for dark matter, covering a wide range of suggested forms for it. The existence of “dark matter”, in some form or other, is inferred from a number of relatively simple observations and the problem has been known for over half a century. To explain “dark matter” is one of the foremost challenges today — the answer will be of fundamental importance to cosmologists, astrophysicists, particle physicists, and general relativists. In this article, I will give a brief review of the observational evidence (concentrating on areas of current significant activity), followed by anequally brief summary of candidate solutions for the ‘dark matter’. I will then discuss experimental searches, both direct and indirect. Finally, I will offer prospects for the future.

/pdf/experimental-searches-for-dark-matter-3ojoczrzqc.pdf

Experimental Searches for Dark Matter.

It is suggested, by using a covariant lagrangian formalism to estimate the equation of state $w = p/\rho$, that stringy dark energy predicts $w < -1$, a negative pressure larger in magnitude than that for a cosmological constant or quintessence. This would lead to a later transition from decelerating to accelerating cosmological expansion; $w = - 4/3$ is briefly considered as one illustrative example.

/pdf/how-to-test-stringy-dark-energy-4syb7s1c3t.pdf

How to Test Stringy Dark Energy

Gravitational lensing directly measures mass density fluctuations along the lines of sight to very distant objects. No assumptions need to be made concerning bias, the ratio of fluctuations in galaxy density to mass density. Hence, lensing is a very useful tool to study the universe at low to moderate redshifts. 
We describe in detail a new method to trace light rays from redshift zero through a three dimensional mass distribution to high redshift. As an example, this method is applied here to a standard cold dark matter universe. We obtain a variety of results, some of them statistical in nature, others from rather detailed case studies of individual ``lines of sight". Among the former are the frequency of multiply imaged quasars, the distribution of separation of the multiple quasars, and the redshift distribution of lenses: all that as a function of quasar redshift. We find effects from very weak lensing to highly magnified multiple images of high redshift objects, which, for extended background sources, (i.e. galaxies), range from slight deformations of the shapes through tangentially aligned arclets up to giant luminous arcs. 
Different cosmological models differ, increasingly with redshift, in their predictions for the mass (thus gravitational potential) distributions. Our ultimate goal is to apply this method to a number of cosmogonic models and to eliminate some models whose gravitational lensing properties are inconsistent with those observed.

http://export.arxiv.org/pdf/astro-ph/9610096

Testing Cosmological Models by Gravitational Lensing: I. Method and First Applications

We describe and model high velocity ( ≈ 15 , 000 km s − 1 ) Ly α and H α emission from supernova remnant 1987A seen in September and October 1997 with the Space Telescope Imaging Spectrograph. Part of this emission comes from a reverse shock located at ≈ 75% of the radius of the inner boundary of the inner circumstellar ring and conﬁned within ± 30 ◦ of the equatorial plane. Departure from axisymmetry in the Ly α and H α emission correlates with that seen in nonthermal radio emission and reveals an asymmetry in the circumstellar gas distribution. We also see diﬀuse high velocity Ly α emission from supernova debris inside the reverse shock that may be due to excitation by nonthermal particles accelerated by the shock.

1998 New HST Observations of High Velocity Ly α and H α in SNR 1987A

The fate of the Universe, infinite expansion or a ldquo;big crunchrdquo;, can be determined by measuring the redshifts and brightness of very distant supernovae. These provide a record of changes in the expansion rate of the Universe over the past several billion years. The mass density, ?M, and cosmological-constant energy density ??, are measured from a data-set consisting of 42 high-redshift Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these supernovae, at redshifts between 0.18 and 0.83, are fit jointly with a set of supernovae from the Cal?n/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. We find ?Mflat = 0.28+0.09-0.08 (1? statistical)+0.05-0.04 (identified systematics). The data are strongly inconsistent with a ? = 0 flat cosmology, the simplest inflationary universe model. An open, ? = 0 cosmology also does not fit the data well: the data indicate that the cosmological constant is non-zero and positive, with a confidence of P(?>0) = 99%, including the identified systematic uncertainties. Thus, the Universe is found to be accelerating, i.e., q0 = ?M/2 - ?? < 0. The best-fit age of the universe relative to the Hubble time is t0flat = 14.9+1.4-1.1 (0.63/h) Gyr for a flat cosmology.

The Acceleration of the Universe:. Measurements of Cosmological Parameters from Type Ia Supernovae

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1998 New HST Observations of High Velocity Ly α and H α in SNR 1987A

The Acceleration of the Universe:. Measurements of Cosmological Parameters from Type Ia Supernovae