216 research outputs found
Limits on Cosmic Chiral Vortons
We study chiral vorton production for Witten-type superconducting string
models in the context of a recently developed analytic formalism. We delineate
three distinct scenarios: First, a low energy regime (including the electroweak
scale) where vortons can be a source of dark matter. Secondly, an intermediate
energy regime where the vorton density is too high to be compatible with the
standard cosmology (thereby excluding these models). Finally, a high energy
regime (including the GUT scale) in which no vortons are expected to form. The
vorton density is most sensitive to the order of the string-forming phase
transition and relatively insensitive to the current-forming transition. For a
second-order string transition, vorton production is cosmologically disastrous
for the range 10^{-28}\lsim G\mu \lsim 10^{-10} (10^{5} GeV \lsim T_{c}
\lsim 10^{14} GeV), while for the first-order case we can only exclude
10^{-20}\lsim G\mu \lsim 10^{-14} (10^{9} GeV \lsim T_{c} \lsim 10^{12}
GeV). We provide a fitting formula which summarises our results.Comment: 9 LaTeX pages, 5 .eps files; submitted to Phys.Lett.
Cosmological Consequences of String-forming Open Inflation Models
We present a study of open inflation cosmological scenarios in which cosmic
strings form betwen the two inflationary epochs. It is shown that in these
models strings are stretched outside the horizon due to the inflationary
expansion but must necessarily re-enter the horizon before the epoch of equal
matter and radiation densities. We determine the power spectrum of cold dark
matter perturbations in these hybrid models, finding good agreement with
observations for values of and comparable
contributions from the active and passive sources to the CMB. Finally, we
briefly discuss other cosmological consequences of these models.Comment: 11 LaTeX pages with 3 eps figure
Constraining spatial variations of the fine-structure constant in symmetron models
Article / Letter to editorLeids Instituut Onderzoek Natuurkund
Topological defects: A problem for cyclic universes?
We study the behaviour of cosmic string networks in contracting universes,
and discuss some of their possible consequences. We note that there is a
fundamental time asymmetry between defect network evolution for an expanding
universe and a contracting universe. A string network with negligible loop
production and small-scale structure will asymptotically behave during the
collapse phase as a radiation fluid. In realistic networks these two effects
are important, making this solution only approximate. We derive new scaling
solutions describing this effect, and test them against high-resolution
numerical simulations. A string network in a contracting universe, together
with the gravitational radiation background it has generated, can significantly
affect the dynamics of the universe both locally and globally. The network can
be an important source of radiation, entropy and inhomogeneity. We discuss the
possible implications of these findings for bouncing and cyclic cosmological
models.Comment: 11 RevTeX 4 pages, 6 figures; version to appear in Phys. Rev.
Does a varying speed of light solve the cosmological problems?
We propose a new generalisation of general relativity which incorporates a
variation in both the speed of light in vacuum (c) and the gravitational
constant (G) and which is both covariant and Lorentz invariant. We solve the
generalised Einstein equations for Friedmann universes and show that arbitrary
time-variations of c and G never lead to a solution to the flatness, horizon or
problems for a theory satisfying the strong energy condition. In
order to do so, one needs to construct a theory which does not reduce to the
standard one for any choice of time, length and energy units. This can be
achieved by breaking a number of invariance principles such as covariance and
Lorentz invariance.Comment: 4 LaTeX pages, no figures. To appear in Phys. Lett.
Accurate Calibration of the Velocity-dependent One-scale Model for Domain Walls
We study the asymptotic scaling properties of standard domain wall networks
in several cosmological epochs. We carry out the largest field theory
simulations achieved to date, with simulation boxes of size 20483, and confirm
that a scale-invariant evolution of the network is indeed the attractor
solution. The simulations are also used to obtain an accurate calibration for
the velocity-dependent one-scale model for domain walls: we numerically
determine the two free model parameters to have the values
and , which are higher precision than (but in agreement
with) earlier estimates.Comment: 8 pages, version to appear in Phys. Lett. B. arXiv admin note:
substantial text overlap with arXiv:1110.348
Alternative Data Reduction Procedures for UVES: Wavelength Calibration and Spectrum Addition
This paper addresses alternative procedures to the ESO supplied pipeline
procedures for the reduction of UVES spectra of two quasar spectra to determine
the value of the fundamental constant mu = Mp/Me at early times in the
universe. The procedures utilize intermediate product images and spectra
produced by the pipeline with alternative wavelength calibration and spectrum
addition methods. Spectroscopic studies that require extreme wavelength
precision need customized wavelength calibration procedures beyond that usually
supplied by the standard data reduction pipelines. An example of such studies
is the measurement of the values of the fundamental constants at early times in
the universe. This article describes a wavelength calibration procedure for the
UV-Visual Echelle Spectrometer on the Very Large Telescope, however, it can be
extended to other spectrometers as well. The procedure described here provides
relative wavelength precision of better than 3E-7 for the long-slit
Thorium-Argon calibration lamp exposures. The gain in precision over the
pipeline wavelength calibration is almost entirely due to a more exclusive
selection of Th/Ar calibration lines.Comment: Accepted for publication in New Astronom
Cosmological effects of scalar-photon couplings: dark energy and varying-α models
We study cosmological models involving scalar fields coupled to radiation and discuss their effect on the redshift evolution of the cosmic microwave background temperature, focusing on links with varying fundamental constants and dynamical dark energy. We quantify how allowing for the coupling of scalar fields to photons, and its important effect on luminosity distances, weakens current and future constraints on cosmological parameters. In particular, for evolving dark energy models, joint constraints on the dark energy equation of state combining BAO radial distance and SN luminosity distance determinations, will be strongly dominated by BAO. Thus, to fully exploit future SN data one must also independently constrain photon number non-conservation arising from the possible coupling of SN photons to the dark energy scalar field. We discuss how observational determinations of the background temper- ature at different redshifts can, in combination with distance measures data, set tight constraints on interactions between scalar fields and photons, thus breaking this degeneracy. We also discuss prospects for future improvements, particularly in the context of Euclid and the E-ELT and show that Euclid can, even on its own, provide useful dark energy constraints while allowing for photon number non-conservation
Extending the velocity-dependent one-scale model for domain walls
We report on an extensive study of the evolution of domain wall networks in Friedmann-Lemaˆıtre- Robertson-Walker universes by means of the largest currently available field-theory simulations. These simulations were done in 40963 boxes and for a range of different fixed expansion rates, as well as for the transition between the radiation and matter eras. A detailed comparison with the velocity-dependent one-scale (VOS) model shows that this cannot accurately reproduce the results of the entire range of simulated regimes if one assumes that the phenomenological energy loss and momentum parameters are constants. We therefore discuss how a more accurate modeling of these parameters can be done, specifically by introducing an additional mechanism of energy loss (scalar radiation, which is particularly relevant for regimes with relatively little damping) and a modified momentum parameter which is a function of velocity (in analogy to what was previously done for cosmic strings). We finally show that this extended model, appropriately calibrated, provides an accurate fit to our simulations
CMB constraints on spatial variations of the vacuum energy density
In a recent article, a simple `spherical bubble' toy model for a spatially
varying vacuum energy density was introduced, and type Ia supernovae data was
used to constrain it. Here we generalize the model to allow for the fact that
we may not necessarily be at the centre of a region with a given set of
cosmological parameters, and discuss the constraints on these models coming
from Cosmic Microwave Background Radiation data. We find tight constraints on
possible spatial variations of the vacuum energy density for any significant
deviations from the centre of the bubble and we comment on the relevance of our
results.Comment: Minor changes; to appear in Astroparticle Physic
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