10 research outputs found
Non-Gaussianity in three fluid curvaton model
The generation of non-gaussianity is studied in a three fluid curvaton model.
By utilizing second order perturbation theory we derive general formulae for
the large scale temperature fluctuation and non-gaussianity parameter,
, that includes the possibility of a non-adiabatic final state. In the
adiabatic limit we recover previously known results. The results are applied to
a three fluid curvaton model where the curvaton decays into radiation and
matter. We find that the amount of non-gaussianity decreases as the final state
of the system becomes more adiabatic and that the generated non-gaussianity in
the scenario is small, .Comment: 10 pages, 2 figure
Static spherically symmetric perfect fluid solutions in theories of gravity
Static spherically symmetric perfect fluid solutions are studied in metric
theories of gravity. We show that pressure and density do not uniquely
determine ie. given a matter distribution and an equation state, one
cannot determine the functional form of . However, we also show that
matching the outside Schwarzschild-de Sitter-metric to the metric inside the
mass distribution leads to additional constraints that severely limit the
allowed fluid configurations.Comment: 5 page
Cosmological expansion and the uniqueness of gravitational action
Modified theories of gravity have recently been studied by several authors as
possibly viable alternatives to the cosmological concordance model. Such
theories attempt to explain the accelerating expansion of the universe by
changing the theory of gravity, instead of introducing dark energy. In
particular, a class of models based on higher order curvature invariants,
so-called gravity models, has drawn attention. In this letter we show
that within this framework, the expansion history of the universe does not
uniquely determine the form of the gravitational action and it can be radically
different from the standard Einstein-Hilbert action. We demonstrate that for
any barotropic fluid, there always exists a class of models that will
have exactly the same expansion history as that arising from the
Einstein-Hilbert action. We explicitly show how one can extend the
Einstein-Hilbert action by constructing a theory that is equivalent on
the classical level. Due to the classical equivalence between theories
and Einstein-Hilbert gravity with an extra scalar field, one can also hence
construct equivalent scalar-tensor theories with standard expansion.Comment: 4 page
Stellar configurations in f(R) theories of gravity
We study stellar configurations and the space-time around them in metric
theories of gravity. In particular, we focus on the polytropic model of
the Sun in the model. We show how the stellar configuration in
the theory can, by appropriate initial conditions, be selected to be
equal to that described by the Lane-Emden -equation and how a simple scaling
relation exists between the solutions. We also derive the correct solution
analytically near the center of the star in theory. Previous analytical
and numerical results are confirmed, indicating that the space-time around the
Sun is incompatible with Solar System constraints on the properties of gravity.
Numerical work shows that stellar configurations, with a regular metric at the
center, lead to outside the star ie. the
Schwarzschild-de Sitter -space-time is not the correct vacuum solution for such
configurations. Conversely, by selecting the Schwarzschild-de Sitter -metric as
the outside solution, we find that the stellar configuration is unchanged but
the metric is irregular at the center. The possibility of constructing a
theory compatible with the Solar System experiments and possible new
constraints arising from the radius-mass -relation of stellar objects is
discussed.Comment: 8 pages, 7 figures; typos corrected, reference adde
Constraints on the three-fluid model of curvaton decay
A three fluid system describing the decay of the curvaton is studied by
numerical and analytical means. We place constraints on the allowed interaction
strengths between the fluids and initial curvaton density by requiring that the
curvaton decays before nucleosynthesis while nucleosynthesis, radiation-matter
equality and decoupling occur at correct temperatures. We find that with a
continuous, time-independent interaction, a small initial curvaton density is
naturally preferred along with a low reheating temperature. Allowing for a
time-dependent interaction, this constraint can be relaxed. In both cases, a
purely adiabatic final state can be generated, but not without fine-tuning.
Unlike in the two fluid system, the time-dependent interactions are found to
have a small effect on the curvature perturbation itself due to the different
nature of the system. The presence of non-gaussianity in the model is
discussed.Comment: 9 pages, 10 figure
Numerical simulations of fragmentation of the Affleck-Dine condensate
We present numerical simulations of fragmentation of the Affleck-Dine
condensate in two spatial dimensions. We argue analytically that the final
state should consist of both Q-balls and anti-Q-balls in a state of maximum
entropy, with most of the balls small and relativistic. Such a behaviour is
found in simulations on a 100x100 lattice with cosmologically realistic
parameter values. During fragmentation process, we observe filament-like
texture in the spatial distribution of charge. The total charge in Q-balls is
found to be almost equal to the charge in anti-Q-balls and typically orders of
magnitude larger than charge asymmetry. Analytical considerations indicate
that, apart from geometrical factors, the results of the simulated two
dimensional case should apply also to the fully realistic three dimensional
case.Comment: 28 pages, 39 figure
Spherically symmetric solutions in f(R)-gravity via Noether Symmetry Approach
We search for spherically symmetric solutions of f(R) theories of gravity via
the Noether Symmetry Approach. A general formalism in the metric framework is
developed considering a point-like f(R)-Lagrangian where spherical symmetry is
required. Examples of exact solutions are given.Comment: 17 pages, to appear in Class. Quant. Gra
Constraining Newtonian stellar configurations in f(R) theories of gravity
We consider general metric theories of gravity by solving the field
equations in the presence of a spherical static mass distribution by analytical
perturbative means. Expanding the field equations systematically in \cO(G),
we solve the resulting set of equations and show that theories which
attempt to solve the dark energy problem very generally lead to
in the solar system. This excludes a large class of theories
as possible explanations of dark energy. We also present the first order
correction to and show that it cannot have a significant effect.Comment: 4 pages; v2: added references, modified abstract and introduction,
conclusions unchange
Energy-momentum complexes in f(R) theories of gravity
Despite the fact that modified theories of gravity, in particular the f(R)
gravity models have attracted much attention in the last years, the problem of
the energy localization in the framework of these models has not been
addressed. In the present work the concept of energy-momentum complexes is
presented in this context. We generalize the Landau-Lifshitz prescription of
calculating the energy-momentum complex to the framework of f(R) gravity. As an
important special case, we explicitly calculate the energy-momentum complex for
the Schwarzschild-de Sitter metric for a general f(R) theory as well as for a
number of specific, popular choices of f(R).Comment: 11 pages, no figures, LaTeX; v2: 9 pages now, rearranged Sections,
references added, no changes in physics and results, version to appear in CQ