68 research outputs found
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
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
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
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
Non-Vacuum Bianchi Types I and V in f(R) Gravity
In a recent paper \cite{1}, we have studied the vacuum solutions of Bianchi
types I and V spacetimes in the framework of metric f(R) gravity. Here we
extend this work to perfect fluid solutions. For this purpose, we take stiff
matter to find energy density and pressure of the universe. In particular, we
find two exact solutions in each case which correspond to two models of the
universe. The first solution gives a singular model while the second solution
provides a non-singular model. The physical behavior of these models has been
discussed using some physical quantities. Also, the function of the Ricci
scalar is evaluated.Comment: 15 pages, accepted for publication in Gen. Realtiv. Gravi
Loitering universe models in light of the CMB
Spatially flat loitering universe models have recently been shown to arise in
the context of brane world scenarios. Such models allow more time for structure
formation to take place at high redshifts, easing, e.g., the tension between
the observed and predicted evolution of the quasar population with redshift.
While having the desirable effect of boosting the growth of structures, we show
that in such models the position of the first peak in the power spectrum of the
cosmic microwave background anisotropies severely constrains the amount of
loitering at high redshifts.Comment: 4 pages, 3 figures. Included discussion of the linear growth factor.
Matches version accepted for publication in PR
Reheating as a surface effect
We describe a new mechanism for reheating the Universe through evaporation of
a surface charge of a fragmented inflaton condensate. We show that for a range
of Yukawa coupling of the inflaton to the matter sector evaporation gives rise
to a much smaller reheat temperature compared to the standard perturbative
decay. As a consequence, reheating through a surface effect could solve the
gravitino and moduli over production problem in inflationary models without
fine tuning the Yukawa sector.Comment: 4 page
Q-ball dynamics from atomic Bose-Einstein condensates
Relativistic scalar field theories with a conserved global charge Q possess
often (meta)stable spherically symmetric soliton solutions, called Q-balls. We
elaborate on the perfect formal analogy which exists between Q-balls, and
spherically symmetric solitons in certain non-relativistic atomic Bose-Einstein
condensates, for which the dominant interatomic interaction can be tuned
attractive. In a harmonic trap, present in existing experiments, the Q-ball
solution is modified in an essential way. If the trap is significantly
prolongated in one direction, however, then genuine solitons do appear, and
actual experimental data can be obtained for some of the Q-ball properties
studied numerically in the relativistic cosmological context, such as their
formation and collisions. We also suggest conditions under which the same
cosmologically relevant analogies could be extended to the fully
three-dimensional case.Comment: 16 pages. v2: clarifications and references adde
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