788 research outputs found
Interactions of cosmological gravitational waves and magnetic fields
The energy momentum tensor of a magnetic field always contains a spin-2
component in its anisotropic stress and therefore generates gravitational
waves. It has been argued in the literature (Caprini & Durrer \cite{CD}) that
this gravitational wave production can be very strong and that back-reaction
cannot be neglected. On the other hand, a gravitational wave background does
affect the evolution of magnetic fields. It has also been argued (Tsagas et al.
\cite{Tsagas:2001ak},\cite{Tsagas:2005ki}) that this can lead to very strong
amplification of a primordial magnetic field. In this paper we revisit these
claims and study back reaction to second order.Comment: Added references, accepted for publication in PR
Testing Superstring Theories with Gravitational Waves
We provide a simple transfer function that determines the effect of an early
matter dominated era on the gravitational wave background and show that a large
class of compactifications of superstring theory might be tested by
observations of the gravitational wave background from inflation. For large
enough reheating temperatures > 10^9 \GeV the test applies to all models
containing at least one scalar with mass < 10^{12}\GeV that acquires a large
initial oscillation amplitude after inflation and has only gravitational
interaction strength, i.e., a field with the typical properties of a modulus.Comment: 5 pages 2 figures, v2: changes in presentation, refs revised, matches
version in print in PR
Constraints on the neutrino mass and the cosmological constant from large scale structure observations
The observational data on the large scale structure (LSS) of the Universe are
used to establish the upper limit on the neutrino content marginalized over all
other cosmological parameters within the class of adiabatic inflationary
models. It is shown that the upper 2 limit on the neutrino content can
be expressed in the form or, via the
neutrino mass, eV.Comment: 5 pages, to appear in the proceedings of the CAPP2000 Conference,
Verbier, Switzerland, July, 200
CMB anisotropies in the presence of a stochastic magnetic field
Primordial magnetic fields present since before the epoch of matter-radiation
equality have an effect on the anisotropies of the cosmic microwave background.
The CMB anisotropies due to scalar perturbations are calculated in the gauge
invariant formalism for magnetized adiabatic initial conditions. Furthermore
the linear matter power spectrum is calculated. Numerical solutions are
complemented by a qualitative analysis.Comment: 26 pages, 21 figures; sections 2 and 4 expanded; matches version
published in PR
Acoustic peaks and dips in the CMB power spectrum: observational data and cosmological constraints
The locations and amplitudes of three acoustic peaks and two dips in the last
releases of the Boomerang, MAXIMA and DASI measurements of the cosmic microwave
background (CMB) anisotropy power spectra as well as their statistical
confidence levels are determined in a model-independent way. It is shown that
the Boomerang-2001 data (Netterfield et al. 2001) fixes the location and
amplitude of the first acoustic peak at more than 3\sigma confidence level. The
next two peaks and dips are determined at a confidence level above 1\sigma but
below 2\sigma. The locations and amplitudes of the first three peaks and two
dips are 212+/-17, 5426+/-1218\mu K^2, 544+/-56, 2266+/-607\mu K^2, 843+/-35,
2077+/-876\mu K^2, 413+/-50, 1960+/-503\mu K^2, 746+/-89, 1605+/-650\mu K^2
respectively (1\sigma errors include statistical and systematic errors). The
MAXIMA and DASI experiments give similar values for the extrema which they
determine. The determined cosmological parameters from the CMB acoustic extrema
data show good agreement with other determinations, especially with the baryon
content as deduced from standard nucleosynthesis constraints. These data
supplemented by the constraints from direct measurements of some cosmological
parameters and data on large scale structure lead to a best-fit model which
agrees with practically all the used experimental data within 1\sigma. The
best-fit parameters are: \Omega_{\Lambda}=0.64^{+0.14}_{-0.27}, \Omega_{m}=
0.36^{+0.21}_{-0.11}, \Omega_b=0.047^{+0.093}_{-0.024},
n_s=1.0^{+0.59}_{-0.17}, h=0.65^{+0.35}_{-0.27} and \tau_c=0.15^{+0.95}_{-0.15}
(plus/minus values show 1\sigma upper/lower limits obtained by marginalization
over all other model parameters). The best-fit values of \Omega_{\nu} and T/S
are close to zero, their 1\sigma upper limits are 0.17 and 1.7 respectively.Comment: 34 pages, 10 figures; accepted by ApJ; some corrections in the text
are made and a few references are adde
Tachyonic perturbations in AdS orbifolds
We show that scalar as well as vector and tensor metric perturbations in the
Randall-Sundrum II braneworld allow normalizable tachyonic modes, i.e.,
possible instabilities. These instabilities require nonvanishing initial
anisotropic stresses on the brane. We show with a specific example that within
the Randall-Sundrum II model, even though the tachyonic modes are excited, no
instability develops. We argue, however, that in the cosmological context
instabilities might in principle be present. We conjecture that the tachyonic
modes are due to the singularity of the orbifold construction. We illustrate
this with a simple but explicit toy model.Comment: 11 pages, matches published versio
Magnetogenesis in Higgs-Starobinsky inflation
In the framework of mixed Higgs-Starobinsky inflation, we consider the
generation of Abelian gauge fields due to their nonminimal coupling to gravity
(in two different formulations of gravity -- metric and Palatini). We couple
the gauge-field invariants and
to an integer power of the scalar curvature
in Jordan frame and, treating these interactions perturbatively, switch
to the Einstein frame where they lead to effective kinetic and axial couplings
between gauge fields and inflaton. We determine the power spectra, energy
densities, correlation length, and helicality of the generated gauge fields for
different values of the nonminimal coupling constants and parameter . We
analytically estimate the spectral index of the magnetic power spectrum
and show that for it is possible to get the scale-invariant or even
red-tilted spectrum for a wide range of modes that implies larger correlation
length of the generated fields. On the other hand, the magnitude of these
fields typically decreases in time becoming very small in the end of inflation.
Thus, it is difficult to obtain both large magnitude and correlation length of
the gauge field in the frame of this model.Comment: 20 pages, 7 figures, submitted to Phys. Rev.
Large Scale Structure Formation with Global Topological Defects. A new Formalism and its implementation by numerical simulations
We investigate cosmological structure formation seeded by topological defects
which may form during a phase transition in the early universe. First we derive
a partially new, local and gauge invariant system of perturbation equations to
treat microwave background and dark matter fluctuations induced by topological
defects or any other type of seeds. We then show that this system is well
suited for numerical analysis of structure formation by applying it to seeds
induced by fluctuations of a global scalar field. Our numerical results are
complementary to previous investigations since we use substantially different
methods. The resulting microwave background fluctuations are compatible with
older simulations. We also obtain a scale invariant spectrum of fluctuations
with about the same amplitude. However, our dark matter results yield a smaller
bias parameter compatible with on a scale of in contrast to
previous work which yielded to large bias factors. Our conclusions are thus
more positive. According to the aspects analyzed in this work, global
topological defect induced fluctuations yield viable scenarios of structure
formation and do better than standard CDM on large scales.Comment: uuencoded, compressed tar-file containing the text in LaTeX and 12
Postscript Figures, 41 page
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