18,776 research outputs found
Cosmological perturbations in teleparallel Loop Quantum Cosmology
Cosmological perturbations in Loop Quantum Cosmology (LQC) are usually
studied incorporating either holonomy corrections, where the Ashtekar
connection is replaced by a suitable sinus function in order to have a
well-defined quantum analogue, or inverse-volume corrections coming from the
eigenvalues of the inverse-volume operator.
In this paper we will develop an alternative approach to calculate
cosmological perturbations in LQC based on the fact that, holonomy corrected
LQC in the flat Friedmann-Lema\^itre-Robertson-Walker (FLRW) geometry could be
also obtained as a particular case of teleparallel gravity (teleparallel
LQC). The main idea of our approach is to mix the simple bounce provided by
holonomy corrections in LQC with the non-singular perturbation equations given
by gravity, in order to obtain a matter bounce scenario as a viable
alternative to slow-roll inflation.
In our study, we have obtained an scale invariant power spectrum of
cosmological perturbations. However, the ratio of tensor to scalar
perturbations is of order , which does not agree with the current
observations. For this reason, we suggest a model where a transition from the
matter domination to a quasi de Sitter phase is produced in order to enhance
the scalar power spectrum.Comment: version accepted for publication in JCA
Gravitational production of dark matter in the Peebles-Vilenkin model
The gravitational production of superheavy dark matter, in the
Peebles-Vilenkin quintessential inflation model, is studied in two different
scenarios: When the particles, whose decay products reheat the universe after
the end of the inflationary period, are created gravitationally, and when are
produced via instant preheating. We show that the viability of both scenarios
requires that the mass of the superheavy dark matter to be approximately
between 10^{16} and 10^{17} GeV.Comment: 10 pages, 5 figures. Version accepted for publication in EPJ
Bicultural and Bilingual Americans: A Need for Understanding
published or submitted for publicatio
How can holonomy corrections be introduced in gravity?
We study the introduction of holonomy corrections in gravity. We will
show that there are infinitely many ways, as many as canonical transformations,
to introduce this kind of corrections, depending on the canonical variables
(two coordinates and its conjugate momenta) used to obtain the Hamiltonian. In
each case, these corrections lead, at effective level, to different modified
holonomy corrected Friedmann equations in gravity, which are in practice
analytically unworkable, i.e. only numerical analysis can be used to understand
its dynamics. Finally, we give arguments in favour of one preferred set of
variables, the one that conformally maps to Einstein gravity, because
for these variables the dynamics of the system has a clear physical meaning:
the same as in standard Loop Quantum Cosmology, where the effective dynamics of
a system can be analytically studied
Gravitationally induced adiabatic particle production: from big bang to de Sitter
In the background of a flat homogeneous and isotropic space–time, we consider a scenario of the Universe driven by the gravitationally induced 'adiabatic' particle production with constant creation rate. We have shown that this Universe attains a big bang singularity in the past and at late-time it asymptotically becomes de Sitter. To clarify this model Universe, we performed a dynamical analysis and found that the Universe attains a thermodynamic equilibrium in this late de Sitter phase. Finally, for the first time, we have discussed the possible effects of 'adiabatic' particle creations in the context of loop quantum cosmology.Peer ReviewedPostprint (author's final draft
Bouncing cosmologies in geometries with positively curved spatial sections
Background boucing cosmologies in the framework of General Relativity, driven
by a single scalar field filling the Universe, and with a quasi-matter
domination period, i.e., depicting the so-called Matter Bounce Scenario, are
reconstructed for geometries with positive spatial curvature. These cosmologies
lead to a nearly flat power spectrum of the curvature fluctuations in co-moving
coordinates for modes that leave the Hubble radius during the quasi-matter
domination period, and whose spectral index and its running, which are related
with the effective Equation of State parameter given by the quotient of the
pressure over the energy density, are compatible with observational data.Comment: Version accepted for publication in PL
Spacetime and Physical Equivalence
In this essay I begin to lay out a conceptual scheme for: (i) analysing
dualities as cases of theoretical equivalence; (ii) assessing when cases of
theoretical equivalence are also cases of physical equivalence. The scheme is
applied to gauge/gravity dualities. I expound what I argue to be their
contribution to questions about: (iii) the nature of spacetime in quantum
gravity; (iv) broader philosophical and physical discussions of spacetime.
(i)-(ii) proceed by analysing duality through four contrasts. A duality will be
a suitable isomorphism between models: and the four relevant contrasts are as
follows:
(a) Bare theory: a triple of states, quantities, and dynamics endowed with
appropriate structures and symmetries; vs. interpreted theory: which is endowed
with, in addition, a suitable pair of interpretative maps.
(b) Extendable vs. unextendable theories: which can, respectively cannot, be
extended as regards their domains of application.
(c) External vs. internal intepretations: which are constructed,
respectively, by coupling the theory to another interpreted theory vs. from
within the theory itself.
(d) Theoretical vs. physical equivalence: which contrasts formal equivalence
with the equivalence of fully interpreted theories.
I apply this scheme to answering questions (iii)-(iv) for gauge/gravity
dualities. I argue that the things that are physically relevant are those that
stand in a bijective correspondence under duality: the common core of the two
models. I therefore conclude that most of the mathematical and physical
structures that we are familiar with, in these models, are largely, though
crucially never entirely, not part of that common core. Thus, the
interpretation of dualities for theories of quantum gravity compels us to
rethink the roles that spacetime, and many other tools in theoretical physics,
play in theories of spacetime.Comment: 25 pages. Winner of the essay contest "Space and Time After Quantum
Gravity" of the University of Illinois at Chicago and the University of
Genev
- …