4 research outputs found
Structure formation in the presence of dark energy perturbations
We study non-linear structure formation in the presence of dark energy. The
influence of dark energy on the growth of large-scale cosmological structures
is exerted both through its background effect on the expansion rate, and
through its perturbations as well. In order to compute the rate of formation of
massive objects we employ the Spherical Collapse formalism, which we generalize
to include fluids with pressure. We show that the resulting non-linear
evolution equations are identical to the ones obtained in the Pseudo-Newtonian
approach to cosmological perturbations, in the regime where an equation of
state serves to describe both the background pressure relative to density, and
the pressure perturbations relative to the density perturbations as well. We
then consider a wide range of constant and time-dependent equations of state
(including phantom models) parametrized in a standard way, and study their
impact on the non-linear growth of structure. The main effect is the formation
of dark energy structure associated with the dark matter halo: non-phantom
equations of state induce the formation of a dark energy halo, damping the
growth of structures; phantom models, on the other hand, generate dark energy
voids, enhancing structure growth. Finally, we employ the Press-Schechter
formalism to compute how dark energy affects the number of massive objects as a
function of redshift.Comment: 21 pages, 8 figures. Matches published version, with caption of Fig.
6 correcte
Textures and Semi-Local Strings in SUSY Hybrid Inflation
Global topological defects may account for the large cold spot observed in
the Cosmic Microwave Background. We explore possibilities of constructing
models of supersymmetric F-term hybrid inflation, where the waterfall fields
are globally SU(2)-symmetric. In contrast to the case where SU(2) is gauged,
there arise Goldstone bosons and additional moduli, which are lifted only by
masses of soft-supersymmetry breaking scale. The model predicts the existence
of global textures, which can become semi-local strings if the waterfall fields
are gauged under U(1)_X. Gravitino overproduction can be avoided if reheating
proceeds via the light SU(2)-modes or right-handed sneutrinos. For values of
the inflaton- waterfall coupling >=10^-4, the symmetry breaking scale imposed
by normalisation of the power spectrum generated from inflation coincides with
the energy scale required to explain the most prominent of the cold spots. In
this case, the spectrum of density fluctuations is close to scale-invariant
which can be reconciled with measurements of the power spectrum by the
inclusion of the sub-dominant component due to the topological defects.Comment: 29 page
Sensitivity and Insensitivity of Galaxy Cluster Surveys to New Physics
We study the implications and limitations of galaxy cluster surveys for
constraining models of particle physics and gravity beyond the Standard Model.
Flux limited cluster counts probe the history of large scale structure
formation in the universe, and as such provide useful constraints on
cosmological parameters. As a result of uncertainties in some aspects of
cluster dynamics, cluster surveys are currently more useful for analyzing
physics that would affect the formation of structure than physics that would
modify the appearance of clusters. As an example we consider the Lambda-CDM
cosmology and dimming mechanisms, such as photon-axion mixing.Comment: 24 pages, 8 eps figures. References added, discussion of scatter in
relations between cluster observables lengthene
Effects of Scale-Dependent Non-Gaussianity on Cosmological Structures
The detection of primordial non-Gaussianity could provide a powerful means to
test various inflationary scenarios. Although scale-invariant non-Gaussianity
(often described by the formalism) is currently best constrained by
the CMB, single-field models with changing sound speed can have strongly
scale-dependent non-Gaussianity. Such models could evade the CMB constraints
but still have important effects at scales responsible for the formation of
cosmological objects such as clusters and galaxies. We compute the effect of
scale-dependent primordial non-Gaussianity on cluster number counts as a
function of redshift, using a simple ansatz to model scale-dependent features.
We forecast constraints on these models achievable with forthcoming data sets.
We also examine consequences for the galaxy bispectrum. Our results are
relevant for the Dirac-Born-Infeld model of brane inflation, where the
scale-dependence of the non-Gaussianity is directly related to the geometry of
the extra dimensions.Comment: 43 pages, 9 figures; references added, submitted to JCAP; typo
corrected in Table 1, minor changes to the tex