378 research outputs found
Cosmic string induced CMB maps
We compute maps of CMB temperature fluctuations seeded by cosmic strings
using high resolution simulations of cosmic strings in a
Friedmann-Robertson-Walker universe. We create full-sky, 18-degree and 3-degree
CMB maps, including the relevant string contribution at each resolution from
before recombination to today. We extract the angular power spectrum from these
maps, demonstrating the importance of recombination effects. We briefly discuss
the probability density function of the pixel temperatures, their skewness and
kurtosis.Comment: 5 pages, 4 figures, submitted to PRD; v2: 6 pages, 5 figures, matches
published versio
Computing Non- Gaussian Maps of the CMB
We discuss methods to compute maps of the CMB in models featuring active
causal sources and in non-Gaussian models ofinflation. We show our large angle
results as well as some preliminary results on small angles. We conclude by
discussing on-going work.Comment: To appear in the proceedings of the XVth Rencontres de Blois. Added 2
reference
Re-growth of stellar disks in mature galaxies: The two component nature of NGC 7217 revisited with VIRUS-W
Previous studies have reported the existence of two counter-rotating stellar
disks in the early-type spiral galaxy NGC7217. We have obtained high-resolution
optical spectroscopic data (R ~ 9000) with the new fiber-based Integral Field
Unit instrument VIRUS-W at the 2.7m telescope of the McDonald Observatory in
Texas. Our analysis confirms the existence of two components. However, we find
them to be co-rotating. The first component is the more luminous (~ 77% of the
total light), has the higher velocity dispersion (~ 170 km/s) and rotates
relatively slowly (projected = 50 km/s). The lower luminosity second
component, (~ 23% of the total light), has a low velocity dispersion (~ 20
km/s) and rotates quickly (projected = 150 km/s). The difference in
the kinematics of the two stellar components allows us to perform a kinematic
decomposition and to measure the strengths of their Mg and Fe Lick indices
separately. The rotational velocities and dispersions of the less luminous and
faster component are very similar to those of the interstellar gas as measured
from the [OIII] emission. Morphological evidence of active star formation in
this component further suggests that NGC7217 may be in the process of
(re)growing a disk inside a more massive and higher dispersion stellar halo.
The kinematically cold and regular structure of the gas disk in combination
with the central almost dust-free morphology allows us to compare the dynamical
mass inside of the central 500pc with predictions from a stellar population
analysis. We find agreement between the two if a Kroupa stellar initial mass
function is assumed.Comment: accepted for publication by MNRA
Scattering of a Long Cosmic String by a Rotating Black Hole
The scattering of a straight, infinitely long string by a rotating black hole
is considered. We assume that a string is moving with velocity v and that
initially the string is parallel to the axis of rotation of the black hole. We
demonstrate that as a result of scattering, the string is displaced in the
direction perpendicular to the velocity by an amount kappa(v,b), where b is the
impact parameter. The late-time solution is represented by a kink and
anti-kink, propagating in opposite directions at the speed of light, and
leaving behind them the string in a new ``phase''. We present the results of
the numerical study of the string scattering and their comparison with the
weak-field approximation, valid where the impact parameter is large, b/M >> 1,
and also with the scattering by a non-rotating black hole which was studied in
earlier works.Comment: 27 pages, 14 figures, to be published in Classical and Quantum
Gravit
Cosmic string loop distribution on all length scales and at any redshift
We analytically derive the expected number density distribution of Nambu-Goto
cosmic string loops at any redshift soon after the time of string formation to
today. Our approach is based on the Polchinski-Rocha model of loop formation
from long strings which we adjust to fit numerical simulations and complement
by a phenomenological modelling of gravitational backreaction. Cosmological
evolution drives the loop distribution towards scaling on all length scales in
both the radiation and matter era. Memory of any reasonable initial loop
distribution in the radiation era is shown to be erased well before Big Bang
Nucleosynthesis. In the matter era, the loop distribution reaches full scaling,
up to some residual loops from the radiation era which may be present for
extremely low string tension. Finally, the number density of loops below the
gravitational cutoff is shown to be scale independent, proportional to a
negative power of the string tension and insensitive to the details of the
backreaction modelling. As an application, we show that the energy density
parameter of loops today cannot exceed 10^(-5) for currently allowed string
tension values, while the loop number density cannot be less than 10^(-6) per
Mpc^3. Our result should provide a more robust basis for studying the
cosmological consequences of cosmic string loops.Comment: 24 pages, 4 figures, uses iopart. References added, matches published
versio
Cosmic strings in a space-time with positive cosmological constant
We study Abelian strings in a fixed de Sitter background. We find that the
gauge and Higgs fields extend smoothly across the cosmological horizon and that
the string solutions have oscillating scalar fields outside the cosmological
horizon for all currently accepted values of the cosmological constant. If the
gauge to Higgs boson mass ratio is small enough, the gauge field function has a
power-like behaviour, while it is oscillating outside the cosmological horizon
if Higgs and gauge boson mass are comparable. Moreover, we observe that Abelian
strings exist only up to a maximal value of the cosmological constant and that
two branches of solutions exist that meet at this maximal value. We also
construct radially excited solutions that only exist for non-vanishing values
of the cosmological constant and are thus a novel feature as compared to flat
space-time. Considering the effect of the de Sitter string on the space-time,
we observe that the deficit angle increases with increasing cosmological
constant. Lensed objects would thus be separated by a larger angle as compared
to asymptotically flat space-time.Comment: 19 pages including 10 figures; discussion on deficit angle added;
Final version to appear in Phys. Lett.
Large Angle CMB Fluctuations from Cosmic Strings with a Comological Constant
In this paper, we present results for large-angle CMB anisotropies generated
from high resolution simulations of cosmic string networks in a range of flat
FRW universes with a cosmological constant. Using an ensemble of all-sky maps,
we compare with the COBE data to infer a normalization (or upper bound) on the
string linear energy density . For a flat matter-dominated model
() we find , which is lower
than previous constraints probably because of the more accurate inclusion of
string small-scale structure. For a cosmological constant within an
observationally acceptable range, we find a relatively weak dependence with
less than 10% higher.Comment: 5 pages, 5 figures; replaced with version to appear in Physical
Review
- âŠ