87 research outputs found
Quantum Evolution of the Bianchi Type I Model
The behaviour of the flat anisotropic model of the Universe with a scalar
field is explored within the framework of quantum cosmology. The principal
moment of the account of an anisotropy is the presence either negative
potential barrier or positive repelling wall. In the first case occur the above
barrier reflection of the wave function of the Universe, in the second one
there is bounce off a potential wall. The further evolution of the Universe
represents an exponential inflating with fast losses of an anisotropy and
approach to the standard cosmological scenario.Comment: Latex, 18 pages, 5 figure
Homogeneous Solutions of Quadratic Gravity
It is believed that soon after the Planck time, Einstein's general relativity
theory should be corrected to an effective quadratic theory. In this work we
present the 3+1 decomposition for the zero vorticity case for arbitrary
spatially homogenous spaces. We specialize for the particular Bianchi
diagonal case. The 3- curvature can be understood as a generalized potential,
and the Bianchi case is a limiting case where this potential is negligible
to the dynamics. The spirit should be analogous, in some sense to the BKL
solution. In this sense, a better understanding of the Bianchi case could
shed some light into the general Bianchi case.Comment: talk presented in the 8th Friedmann Seminar, 30 May - 03 June 2011,
Rio de Janeiro, Brazi
The Power of General Relativity
We study the cosmological and weak-field properties of theories of gravity
derived by extending general relativity by means of a Lagrangian proportional
to . This scale-free extension reduces to general relativity when
. In order to constrain generalisations of general relativity of
this power class we analyse the behaviour of the perfect-fluid Friedmann
universes and isolate the physically relevant models of zero curvature. A
stable matter-dominated period of evolution requires or . The stable attractors of the evolution are found. By considering the
synthesis of light elements (helium-4, deuterium and lithium-7) we obtain the
bound We evaluate the effect on the power spectrum of
clustering via the shift in the epoch of matter-radiation equality. The horizon
size at matter--radiation equality will be shifted by for a value of
We study the stable extensions of the Schwarzschild
solution in these theories and calculate the timelike and null geodesics. No
significant bounds arise from null geodesic effects but the perihelion
precession observations lead to the strong bound assuming that Mercury follows a timelike geodesic. The combination of
these observational constraints leads to the overall bound on theories of this type.Comment: 26 pages and 5 figures. Published versio
Disappearing cosmological constant in f(R) gravity
For higher-derivative f(R) gravity where R is the Ricci scalar, a class of
models is proposed which produce viable cosmology different from the LambdaCDM
one at recent times and satisfy cosmological, Solar system and laboratory
tests. These models have both flat and de Sitter space-times as particular
solutions in the absence of matter. Thus, a cosmological constant is zero in
flat space-time, but appears effectively in a curved one for sufficiently large
R. A 'smoking gun' for these models would be small discrepancy in values of the
slope of the primordial perturbation power spectrum determined from galaxy
surveys and CMB fluctuations. On the other hand, a new problem for dark energy
models based on f(R) gravity is pointed which is connected with possible
overproduction of new massive scalar particles (scalarons) arising in this
theory in the very early Universe.Comment: 8 pages, footnote clarified, grammatical typo corrected, references
added, final version to be published in JETP
Isotropic Loop Quantum Cosmology
Isotropic models in loop quantum cosmology allow explicit calculations,
thanks largely to a completely known volume spectrum, which is exploited in
order to write down the evolution equation in a discrete internal time. Because
of genuinely quantum geometrical effects the classical singularity is absent in
those models in the sense that the evolution does not break down there,
contrary to the classical situation where space-time is inextendible. This
effect is generic and does not depend on matter violating energy conditions,
but it does depend on the factor ordering of the Hamiltonian constraint.
Furthermore, it is shown that loop quantum cosmology reproduces standard
quantum cosmology and hence (e.g., via WKB approximation) to classical behavior
in the large volume regime where the discreteness of space is insignificant.
Finally, an explicit solution to the Euclidean vacuum constraint is discussed
which is the unique solution with semiclassical behavior representing quantum
Euclidean space.Comment: 30 page
A Cosmological Theory without Singularities
A theory of gravitation is constructed in which all homogeneous and isotropic
solutions are nonsingular, and in which all curvature invariants are bounded.
All solutions for which curvature invariants approach their limiting values
approach de Sitter space. The action for this theory is obtained by a higher
derivative modification of Einstein's theory. We expect that our model can
easily be generalized to solve the singularity problem also for anisotropic
cosmologies.Comment: 25 pages, 11 figures (available as hard copies from the authors),
uses phyzzx, BROWN-HET-89
Emergent Universe with Exotic Matter
A general framework for an emergent universe scenario has been given which
makes use of an equation of state. The general features of the model have also
been studied and possible primordial composition of the universe have been
suggested.Comment: 11 pages, no fi
- …