1,409 research outputs found
Dark Energy and the Hubble Age
I point out that an effective upper limit of approximately 20 Gyr (for a
Hubble constant of 72 km/s/Mpc) or alternatively on the -independent
quantity , exists on the age of the Universe, essentially
independent of the unknown equation of state of the dominant dark energy
component in the Universe. Unless astrophysical constraints on the age of the
Universe can convincingly reduce the upper limit to below this value no useful
lower limit on the equation of state parameter for this component can be
obtained. Direct dating by stars does not provide a useful constraint, but
model-dependent cosmological limits from supernovae and the CMB observations
may. For a constant value of , a bound Comment: 4 pages, submitted to Ap. J. Lett (analytic asymptotic upper bound
now added
The X-ray Size-Temperature Relation for Intermediate Redshift Galaxy Clusters
We present the first measurements of the X-ray size-temperature (ST) relation
in intermediate redshift (z~0.30) galaxy clusters. We interpret the local ST
relation (z~0.06) in terms of underlying scaling relations in the cluster dark
matter properties, and then we use standard models for the redshift evolution
of those dark matter properties to show that the ST relation does not evolve
with redshift. We then use ROSAT HRI observations of 11 clusters to examine the
intermediate redshift ST relation; for currently favored cosmological
parameters, the intermediate redshift ST relation is consistent with that of
local clusters. Finally, we use the ST relation and our evolution model to
measure angular diameter distances; with these 11 distances we evaluate
constraints on Omega_M and Omega_L which are consistent with those derived from
studies of Type Ia supernovae. The data rule out a model with Omega_M=1 and
Omega_L=0 with 2.5 sigma confidence. When limited to models where
Omega_M+Omega_L=1, these data are inconsistent with Omega_M=1 with 3 sigma
confidence.Comment: ApJ: submitted April 7, accepted June 28, to appear Dec 1 (vol 544
Optimizing Observational Strategy for Future Fgas Constraints
The Planck cluster catalog is expected to contain of order a thousand galaxy
clusters, both newly discovered and previously known, detected through the
Sunyaev-Zeldovich effect over the redshift range 0 < z < 1. Follow-up X-ray
observations of a dynamically relaxed sub-sample of newly discovered Planck
clusters will improve constraints on the dark energy equation-of-state found
through measurement of the cluster gas mass fraction fgas. In view of follow-up
campaigns with XMM-Newton and Chandra, we determine the optimal redshift
distribution of a cluster sample to most tightly constrain the dark energy
equation of state. The distribution is non-trivial even for the standard w0-wa
parameterization. We then determine how much the combination of expected data
from the Planck satellite and fgas data will be able to constrain the dark
energy equation-of-state. Our analysis employs a Markov Chain Monte Carlo
method as well as a Fisher Matrix analysis. We find that these upcoming data
will be able to improve the figure-of-merit by at least a factor two.Comment: 11 pages, 8 figure
Accuracy of Mesh Based Cosmological Hydrocodes: Tests and Corrections
We perform a variety of tests to determine the numerical resolution of the
cosmological TVD eulerian code developed by Ryu et al (1993). Tests include
512^3 and 256^3 simulations of a Pk=k^{-1} spectrum to check for
self-similarity and comparison of results with those from higher resolution SPH
and grid-based calculations (Frenk et al 1998). We conclude that in regions
where density gradients are not produced by shocks the code degrades resolution
with a Gaussian smoothing (radius) length of 1.7 cells. At shock caused
gradients (for which the code was designed) the smoothing length is 1.1 cells.
Finally, for \beta model fit clusters, we can approximately correct numerical
resolution by the transformation R^2_{core}\to R^2_{core}-(C\Delta l)^2, where
\Delta l is the cell size and C=1.1-1.7. When we use these corrections on our
previously published computations for the SCDM and \Lambda CDM models we find
luminosity weighted, zero redshift, X-ray cluster core radii of (210\pm 86,
280\pm 67)h^{-1}kpc, respectively, which are marginally consistent with
observed (Jones & Forman 1992) values of 50-200h^{-1}kpc. Using the corrected
core radii, the COBE normalized SCDM model predicts the number of bright
L_x>10^{43}erg/s clusters too high by a factor of \sim 20 and the \Lambda CDM
model is consistent with observations.Comment: ApJ in press (1999
Fluoropyrimidine sensitivity of human MCF-7 breast cancer cells stably transfected with human uridinehosphorylase
The relationship between uridine phosphorylase (UP) expression level in cancer cells and the tumour sensitivity to fluoropyrimidines is unclear. In this study, we found that UP overexpression by gene transfer, and the subsequent efficient metabolic activation of 5-fluorouracil (5-FU) by the ribonucleotide pathway, does not increase the fluoropyrimidine sensitivity of MCF-7 human cancer cells. © 2001 Cancer Research Campaign http://www.bjcancer.co
Optical conductivity of the Frohlich polaron
We present accurate results for optical conductivity of the three dimensional
Frohlich polaron in all coupling regimes. The systematic-error free
diagrammatic quantum Monte Carlo method is employed where the Feynman graphs
for the momentum-momentum correlation function in imaginary time are summed up.
The real-frequency optical conductivity is obtained by the analytic
continuation with stochastic optimization. We compare numerical data with
available perturbative and non-perturbative approaches to the optical
conductivity and show that the picture of sharp resonances due to relaxed
excited states in the strong coupling regime is ``washed out''by large
broadening of these states. As a result, the spectrum contains only a
single-maximum broad peak with peculiar shape and a shoulder.Comment: 4 pages, 6 ps-figure
The Asymptotic Form of Cosmic Structure: Small Scale Power and Accretion History
We explore the effects of small scale structure on the formation and
equilibrium of dark matter halos in a universe dominated by vacuum energy. We
present the results of a suite of four N-body simulations, two with a LCDM
initial power spectrum and two with WDM-like spectra that suppress the early
formation of small structures. All simulations are run into to far future when
the universe is 64Gyr/h old, long enough for halos to essentially reach
dynamical equilibrium. We quantify the importance of hierarchical merging on
the halo mass accretion history, the substructure population, and the
equilibrium density profile. We modify the mass accretion history function of
Wechsler et al. (2002) by introducing a parameter, \gamma, that controls the
rate of mass accretion, dln(M) / dln(a) ~ a^(-\gamma), and find that this form
characterizes both hierarchical and monolithic formation. Subhalo decay rates
are exponential in time with a much shorter time scale for WDM halos. At the
end of the simulations, we find truncated Hernquist density profiles for halos
in both the CDM and WDM cosmologies. There is a systematic shift to lower
concentration for WDM halos, but both cosmologies lie on the same locus
relating concentration and formation epoch. Because the form of the density
profile remains unchanged, our results indicate that the equilibrium halo
density profile is set independently of the halo formation process.Comment: 17 pages, submitted to ApJ. Full resolution version avaliable at
http://www-personal.umich.edu/~mbusha/Papers/AccretionHistory.pd
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