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, tachyacoustic, and explicitly construct examples of noncanonical 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 (noninflationary) 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.

We study DBI inflation based upon a general model characterized by a power-law flow parameter ǫ(φ) ∝ φ α and speed of sound cs(φ) ∝ φ β , where α and β are constants. We show that in the slow-roll limit this general model gives rise to distinct inflationary classes according to the relation between α and β 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 noncanonical 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 tensorto-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.PACS numbers: 98.80.Cq

Abstract.We study in this paper three different theories of gravitation with massive gravitons -the modified Fierz-Pauli model, Massive Gravity and the bimetric theory proposed by Visser -in linear perturbation theory around a Minkowski and a flat FriedmannRobertson-Walker background. For the transverse-traceless tensor perturbations we show that the three theories give rise to the same dynamical equations, to the same form of the tensor Sachs-Wolfe effect, and consequently to the same form of the Boltzmann equations for the radiative transfer in General Relativity.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 Fierz-Pauli theory the vector gravitational-wave polarization modes (Ψ 3 amplitudes in the Newman-Penrose formalism) do not decay too fast as it happens in the former case. Rather, we show that such Ψ 3 polarization modes give rise to an unusual vector Sachs-Wolfe effect, leaving a signature in the quadrupole form Y 2,±1 (θ, ϕ) on the Cosmic Microwave Background Radiation polarization. We then derive the details for the Thomson scattering of CMB photons for these Ψ 3 modes, and then construct the correspondent Boltzmann equations. Based upon these results we then qualitatively show that Ψ 3 -mode vector signatures -if they do exist -could clearly be distinguished on the CMB polarization from the usual Ψ 4 tensor modes.We also estimate that the graviton mass limit for the vector modes is m = 10 −66 g ∼ 10 −29 cm −1 , so that vector modes with masses below this limit exhibit the same dynamical evolution as the massless gravitons.We argue at the end of this paper that CMB polarization experiments can be decisive to test alternative theories of gravitation by measuring CMB polarization in the E-mode.PACS numbers: 04.50.+h, 95.36.+x, 95.30.Sf

Here we present a status report of the first spherical antenna project equipped with a set of parametric transducers for gravitational detection. The Mario Schenberg, as it is called, started its commissioning phase at the Physics Institute of the University of São Paulo, in September 2006, under the full support of FAPESP. We have been testing the three preliminary parametric transducer systems in order to prepare the detector for the next cryogenic run, when it will be calibrated. We are also developing sapphire oscillators that will replace the current ones thereby providing better performance. We also plan to install eight transducers in the near future, six of which are of the two-mode type and arranged according to the truncated icosahedron configuration. The other two, which will be placed close to the sphere equator, will be mechanically non-resonant. In doing so, we want to verify that if the Schenberg antenna can become a wideband gravitational wave detector through the use of an ultra-high sensitivity non-resonant transducer constructed using the recent achievements of nanotechnology.

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.

The spectrum and amplitude of the stochastic background of relic gravitons produced in a bouncing universe is calculated. The matter content of the model consists of dust and radiation fluids, and the bounce occurs due to quantum cosmological effects when the universe approaches the classical singularity in the contracting phase. The resulting amplitude is very small and it cannot be observed by any present and near future gravitational wave detector. Hence, as in the ekpyrotic model, any observation of these relic gravitons will rule out this type of quantum cosmological bouncing model. PACS numbers: 98.80.Cq

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