17 research outputs found
Attractor Solutions in Tachyacoustic Cosmology
We study the dynamical stability of "tachyacoustic" cosmological models, in
which primordial perturbations are generated by a shrinking sound horizon
during a period of decelerating expansion. Such models represent a potential
alternative to inflationary cosmology, but the phase-space behavior of
tachyacoustic solutions has not previously been investigated. We numerically
evaluate the dynamics of two non-canonical Lagrangians, a cuscuton-like
Lagrangian and a Dirac-Born-Infeld Lagrangian, which generate a scale-invariant
spectrum of perturbations. We show that the power-law background solutions in
both cases are dynamical attractors.Comment: Some references and comments added. Accepted for publication in
Physical Review
Probing a cosmological model with a decaying-vacuum
In this work we study the evolution of matter-density perturbations for an
arbitrary model, and specialize our analysis to the particular
phenomenological law . We study the evolution
of the cosmic star formation rate in this particular dark energy scenario and,
by constraining the parameter using both the age of the universe and
the cosmic star formation rate curve, we show that it leads to a reasonable
physical model for .Comment: Published in Physical Review
A Gênese da Harmonia das Esferas no Antigo Pitagorismo
Nesse artigo apresento o conceito de Harmonia das Esferas conforme formulado inicialmente pelos pitagóricos. Partindo dos fragmentos de Filolau e das descrições de Aristóteles do pensamento pitagórico, traçarei um panorama geral das concepções pitagóricas acerca do número, do universo, e das consonâncias musicais, para então discutir como essas ideias geraram o conceito de Harmonia das Esferas.
Non-Gaussian signatures of Tachyacoustic Cosmology
I investigate non-Gaussian signatures in the context of tachyacoustic
cosmology, that is, a noninflationary model with superluminal speed of sound. I
calculate the full non-Gaussian amplitude , its size ,
and corresponding shapes for a red-tilted spectrum of primordial scalar
perturbations. Specifically, for cuscuton-like models I show that , and the shape of its non-Gaussian amplitude peaks for
both equilateral and local configurations, the latter being dominant. These
results, albeit similar, are quantitatively distinct from the corresponding
ones obtained by Magueijo {\it{et. al}} in the context of superluminal bimetric
models.Comment: Some comments and references added. Matches the version published in
JCA
Tachyacoustic Cosmology: An Alternative to Inflation
We consider an alternative to inflation for the generation of superhorizon
perturbations in the universe in which the speed of sound is faster than the
speed of light. We label such cosmologies, first proposed by Armendariz-Picon,
{\it tachyacoustic}, and explicitly construct examples of non-canonical
Lagrangians which have superluminal sound speed, but which are causally
self-consistent. Such models possess two horizons, a Hubble horizon and an
acoustic horizon, which have independent dynamics. Even in a decelerating
(non-inflationary) background, a nearly scale-invariant spectrum of
perturbations can be generated by quantum perturbations redshifted outside of a
shrinking acoustic horizon. The acoustic horizon can be large or even infinite
at early times, solving the cosmological horizon problem without inflation.
These models do not, however, dynamically solve the cosmological flatness
problem, which must be imposed as a boundary condition. Gravitational wave
modes, which are produced by quantum fluctuations exiting the Hubble horizon,
are not produced.Comment: 11 pages, LaTeX (V2: references added. Version submitted to PRD
Inflationary potentials in DBI models
We study DBI inflation based upon a general model characterized by a
power-law flow parameter and speed of
sound , where and are constants.
We show that in the slow-roll limit this general model gives rise to distinct
inflationary classes according to the relation between and and
to the time evolution of the inflaton field, each one corresponding to a
specific potential; in particular, we find that the well-known canonical
polynomial (large- and small-field), hybrid and exponential potentials also
arise in this non-canonical model. We find that these non-canonical classes
have the same physical features as their canonical analogs, except for the fact
that the inflaton field evolves with varying speed of sound; also, we show that
a broad class of canonical and D-brane inflation models are particular cases of
this general non-canonical model. Next, we compare the predictions of
large-field polynomial models with the current observational data, showing that
models with low speed of sound have red-tilted scalar spectrum with low
tensor-to-scalar ratio, in good agreement with the observed values. These
models also show a correlation between large non-gaussianity with low tensor
amplitudes, which is a distinct signature of DBI inflation with large-field
polynomial potentials.Comment: Minor changes, reference added. Version submitted to JCA
CMB Polarization and Theories of Gravitation with Massive Gravitons
We study in this paper three different theories of gravitation with massive
gravitons - the modified Fierz-Pauli (FP) model, Massive Gravity and the
bimetric theory proposed by Visser - in linear perturbation theory around a
Minkowski and a flat FRW background. For the TT tensor perturbations we show
that the three theories give rise to the same dynamical equations and to the
same form of the Boltzmann equations for the radiative transfer in General
Relativity (GR). We then analyze vector perturbations in these theories and
show that they do not give the same results as in the previous case. We first
show that vector perturbations in Massive Gravity present the same form as
found in General Relativity, whereas in the modified FP theory the vector
gravitational-wave (GW) polarization modes ( amplitudes in the
Newman-Penrose (NP) formalism) do not decay too fast as it happens in the
former case. Rather, we show that such polarization modes give rise
to an unusual vector Sachs-Wolfe effect, leaving a signature in the quadrupole
form on the CMB polarization. We then derive the
details for the Thomson scattering of CMB photons for these modes,
and then construct the correspondent Boltzmann equations. Based upon these
results we then qualitatively show that -mode vector signatures - if
they do exist - could clearly be distinguished on the CMB polarization from the
usual tensor modes.Comment: To appear in Classical and Quantum Gravit
“Poderia a ciëncias atual entender o inicio do universo?”
“Poderia a ciëncias atual entender o inicio do universo?