4,250 research outputs found

### Measuring Space-Time Geometry over the Ages

Theorists are often told to express things in the "observational plane". One
can do this for space-time geometry, considering "visual" observations of
matter in our universe by a single observer over time, with no assumptions
about isometries, initial conditions, nor any particular relation between
matter and geometry, such as Einstein's equations. Using observables as
coordinates naturally leads to a parametrization of space-time geometry in
terms of other observables, which in turn prescribes an observational program
to measure the geometry. Under the assumption of vorticity-free matter flow we
describe this observational program, which includes measurements of
gravitational lensing, proper motion, and redshift drift. Only 15% of the
curvature information can be extracted without long time baseline observations,
and this increases to 35% with observations that will take decades. The rest
would likely require centuries of observations. The formalism developed is
exact, non-perturbative, and more general than the usual cosmological analysis.Comment: Originally written for the Gravity Research Foundation 2012 Awards
for Essays on Gravitation and received Honorable Mentio

### The Effect of a Non-Thermal Tail on the Sunyaev-Zeldovich Effect in clusters of galaxies

We study the spectral distortions of the cosmic microwave background
radiation induced by the Sunyaev-Zel'dovich (SZ) effect in clusters of galaxies
when the target electrons have a modified Maxwell-Boltzmann distribution with a
high-energy non-thermal tail. Bremsstrahlung radiation from this type of \\
electron distribution may explain the supra-thermal X-ray emission observed in
some clusters such as the Coma cluster and A2199 and serve as an alternative to
the classical but problematic inverse Compton scattering interpretation. We
show that the SZ effect can be used as a powerful tool to probe the electron
distribution in clusters of galaxies and discriminate among these different
interpretations of the X-ray excess. The existence of a non-thermal tail can
have important consequences for cluster based estimators of cosmological
parameters.Comment: 14 pages, 3 figures, version to be published in ApJ. Let

### Cosmic Microwave Background Radiation Anisotropy Induced by Cosmic Strings

We report on a current investigation of the anisotropy pattern induced by
cosmic strings on the cosmic microwave background radiation (MBR). We have
numerically evolved a network of cosmic strings from a redshift of $Z = 100$ to
the present and calculated the anisotropies which they induce. Based on a
limited number of realizations, we have compared the results of our simulations
with the observations of the COBE-DMR experiment. We have obtained a
preliminary estimate of the string mass-per-unit-length $\mu$ in the cosmic
string scenario.Comment: 8 pages of TeX - [Color] Postscript available by anonymous ftp at
ftp://fnas08.fnal.gov:/pub/Publications/Conf-94-197-A, FERMILAB-Conf-94/197-

### Amorphous silica between confining walls and under shear: a computer simulation study

Molecular dynamics computer simulations are used to investigate a silica melt
confined between walls at equilibrium and in a steady-state Poisseuille flow.
The walls consist of point particles forming a rigid face-centered cubic
lattice and the interaction of the walls with the melt atoms is modelled such
that the wall particles have only a weak bonding to those in the melt, i.e.
much weaker than the covalent bonding of a Si-O unit. We observe a pronounced
layering of the melt near the walls. This layering, as seen in the total
density profile, has a very irregular character which can be attributed to a
preferred orientational ordering of SiO4 tetrahedra near the wall. On
intermediate length scales, the structure of the melt at the walls can be well
distinguished from that of the bulk by means of the ring size distribution.
Whereas essentially no structural changes occur in the bulk under the influence
of the shear fields considered, strong structural rearrangements in the ring
size distribution are present at the walls as far as there is a slip motion.
For the sheared system, parabolic velocity profiles are found in the bulk
region as expected from hydrodynamics and the values for the shear viscosity as
extracted from those profiles are in good agreement with those obtained in pure
bulk simulations from the appropriate Green-Kubo formula.Comment: 23 pages of Late

### Advanced techniques for determining long term compatibility of materials with propellants

A method for the prediction of propellant-material compatibility for periods of time up to ten years is presented. Advanced sensitive measurement techniques used in the prediction method are described. These include: neutron activation analysis, radioactive tracer technique, and atomic absorption spectroscopy with a graphite tube furnace sampler. The results of laboratory tests performed to verify the prediction method are presented

### Primordial Non-Gaussianity: Baryon Bias and Gravitational Collapse of Cosmic String Wakes

I compute the 3-D non-linear evolution of gas and dark matter fluids in the
neighbourhood of cosmic string wakes which are formed at high redshift
($z\simeq 2240$) for a ``realistic'' scenario of wake formation. These wakes
are the ones which stand out most prominently as cosmological sheets and are
expected to play a dominant r\^ole in the cosmic string model of structure
formation. Employing a high-resolution 3-D hydrodynamics code to evolve these
wakes until the present day yields results for the baryon bias generated in the
inner wake region. I find that today, wakes would be $1.5 h^{-1}$ Mpc thick and
contain a 70% excess in the density of baryons over the dark matter density in
their centre. However, high density peaks in the wake region do not inherit a
baryon enhancement. I propose a mechanism for this erasure of the baryon excess
in spherically collapsed objects based on the geometry change around the
collapsing region. Further, I present heuristic arguments for the consequences
of this work for large scale structure in the cosmic string model and conclude
that the peculiarities of wake formation are unlikely to have significant
import on the discrepancy between power spectrum predictions and observations
in this model. If one invokes the nucleosynthesis bound on $\Omega_b$ this
could be seen as strengthening the case against $\Omega_m=1$ or for low Hubble
constants.Comment: 21 pages, 7 figures, 2 tables, prepared with the AASTeX package.
Minor modifications, results unchanged. ApJ in press, scheduled for Vol. 50

### A Geometrical Test of the Cosmological Energy Contents Using the Lyman-alpha Forest

In this Letter we explore a version of the test of cosmological geometry
proposed by Alcock and Paczynski (1979), using observations of the Lyman-alpha
forest in the spectra of close quasar pairs. By comparing the correlations in
absorption in one quasar spectrum with correlations between the spectra of
neighboring quasars one can determine the relation of the redshift distance
scale to the angle distance scale at the redshift of the absorbers, $z \sim 2 -
4$. Since this relationship depends on the parameters of the cosmological
model, these parameters may be determined using the Lyman-alpha forest. While
this test is relatively insensitive to the density parameter $\Omega_m$ in a
dust-dominated universe, it is more sensitive to the presence of a matter
component with large negative pressure (such as a cosmological constant
$\Lambda$) and its equation of state. With only 25 pairs of quasar spectra at
angular separations $0.5' - 2'$, one can discriminate between an $\Omega_m =
0.3$ open universe ($\Lambda=0$) and an $\Omega_m = 0.3$ flat
($\Lambda$-dominated) universe at the $4-\sigma$ level. The S/N can be enhanced
by considering quasar pairs at smaller angular separations, but requires proper
modeling of nonlinear redshift space distortions. Here the correlations and
redshift space distortions are modeled using linear theory.Comment: 13 pages, 2 ps figures, submitted to ApJ

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