Phys.Lett.B575:165-171,2003 We investigate tachyon dynamics with an inverse power-law potential $V(\phi)
\propto \phi^{-\alpha}$. We find global attractors of the dynamics leading to a
dust behaviour ...for $\alpha > 2$ and to an accelerating universe for $0 < \alpha
\le 2$. We study linear cosmological perturbations and we show that metric
fluctuations are constant on large scales in both cases. In presence of an
additional perfect fluid, the tachyon with this potential behaves as dust or
dark energy.
We show that the thermal Sunyaev-Zeldovich effect caused by hot electrons in the Local Supercluster (LSC) can explain the abnormal quadrupole and octopole of the cosmic microwave background (CMB) ...that were measured by WMAP and COBE. The distortion needed to account for the low observed quadrupole is a spot in the direction of the LSC with a temperature decrease of order \Delta T \approx - 7 \mu K for \nu ~ 20 -- 90 Ghz photons. The temperature and density of the hot gas which can generate such an effect are consistent with observations of the X-ray background. If this hypothetic foreground is subtracted from the WMAP data, we find that the amplitude of the quadrupole (l=2) is substantially increased, and that the ``planarity'' of both the quadrupole and the octopole (l=3) are weakened. For smaller scales the effect decays and, at least in our simplified model, it does not affect the angular power spectrum at l>10. Moreover, since the Sunyaev-Zeldovich effect increases the temperature of photons with frequencies above 218 GHz, observations sensitive in that range (such as PLANCK's HFI) will be able to confirm whether the LSC indeed affects the CMB.
We describe how a survey of the polarization of the cosmic microwave background induced by Compton scattering in galaxy clusters can be used to make a full spatial reconstruction of the primordial ...(z~1089) matter distribution inside our surface of last scattering. This "polarization tomography" can yield a spatial map of the initial state of the Universe just as gravitational collapse was beginning to drive structure formation. We present a transparent method and simple formulas from which one can compute the 3D primordial map in real and in Fourier space, given a 3D map of the polarization due to galaxy clusters. The advantage of the real space reconstruction is that it is free from the statistical uncertainties which are inherent in the Fourier space reconstruction. We discuss how noise, partial sky covering and depth of the survey can affect the results.
Phys.Lett. B549 (2002) 13-19 We show that Einstein's gravity coupled to a non-minimally coupled scalar
field is stable even for high values of the scalar field, when the sign of the
Einstein-Hilbert ...action is reversed. We also discuss inflationary solutions and
a possible new mechanism of reheating.
We show that phantom dark energy, if it is described by a K-essence theory, has three fundamental problems: first, its hamiltonian is unbounded from below. Second, classical stability precludes the ...equation of state from crossing the ``Lambda-barrier'', \(w_\Lambda=-1\). Finally, both the equation of state and the sound speed are unbounded -- the first, from below, the second, from above -- if the kinetic term is not bounded by dynamics.
We study the back reaction of cosmological perturbations on the evolution of the universe. The object usually employed to describe the back reaction of perturbations is called the effective ...energy-momentum tensor (EEMT) of cosmological perturbations. In this formulation, the problem of the gauge dependence of the EEMT must be tackled. We advance beyond traditional results that involve only high frequency perturbations in vacuo, and formulate the back reaction problem in a gauge invariant manner for completely generic perturbations. We give a quick proof that the EEMT for high-frequency perturbations is gauge invariant which greatly simplifies the pioneering approach by Isaacson. As applications we analyze the back reaction of gravitational waves and scalar metric fluctuations in Friedmann-Robertson-Walker background spacetimes. We investigate in particular back reaction effects during inflation in the Chaotic scenario. Fluctuations with a wavelength much bigger than the Hubble radius during inflation contribute a negative energy density, and in that case back reaction counteracts any pre-existing cosmological constant. Finally, we set up the equations of motion for the back reaction on the geometry and on the matter, and show how they are perfectly consistent with the Bianchi identities and the continuity equations.
Phys.Rev. D70 (2004) 063517 We study the CMB constraints on two Dark Energy models described by scalar
fields with different Lagrangians, namely a Klein-Gordon and a Born-Infeld
field. The speed of ...sound of field fluctuations are different in these two
theories, and therefore the predictions for CMB and structure formation are
different. Employing the WMAP data on CMB, we make a likelihood analysis on a
grid of theoretical models. We constrain the parameters of the models and
compute the probability distribution functions for the equation of state. We
show that the effect of the different sound speeds affects the low multipoles
of CMB anisotropies, but is at most marginal for the class of models studied
here.
Phys.Rev.D74:063506,2006 We investigate the large scale anomalies in the angular distribution of the
cosmic microwave background radiation as measured by WMAP using several tests.
These tests, based ...on the multipole vector expansion, measure correlations
between the phases of the multipoles as expressed by the directions of the
multipole vectors and their associated normal planes. We have computed the
probability distribution functions for 46 such tests, for the multipoles l=2-5.
We confirm earlier findings that point to a high level of alignment between l=2
(quadrupole) and l=3 (octopole), but with our tests we do not find significant
planarity in the octopole. In addition, we have found other possible anomalies
in the alignment between the octopole and the l=4 (hexadecupole) components, as
well as in the planarity of l=4 and l=5. We introduce the notion of a total
likelihood to estimate the relevance of the low-multipoles tests of
non-gaussianity. We show that, as a result of these tests, the CMB maps which
are most widely used for cosmological analysis lie within the ~ 10% of randomly
generated maps with lowest likelihoods.
We investigate tachyon dynamics with an inverse power-law potential \(V(\phi) \propto \phi^{-\alpha}\). We find global attractors of the dynamics leading to a dust behaviour for \(\alpha > 2\) and to ...an accelerating universe for \(0 < \alpha \le 2\). We study linear cosmological perturbations and we show that metric fluctuations are constant on large scales in both cases. In presence of an additional perfect fluid, the tachyon with this potential behaves as dust or dark energy.
Phys.Rev. D67 (2003) 027301 We investigate the robustness of some recent results obtained for homogeneous
and isotropic cosmological models with conformally coupled scalar fields. For
this purpose, ...we investigate anisotropic homogeneous solutions of the models
described by the action $$ S=\int d^4x \sqrt{-g}\left\{F(\phi)R -
\partial_a\phi\partial^a\phi -2V(\phi) \right\}, $$ with general $F(\phi)$ and
$V(\phi)$. We show that such a class of models leads generically to geometrical
singularities if for some value of $\phi$, $F(\phi)=0$, rendering previous
cosmological results obtained for the conformal coupling case highly unstable.
We show that stable models can be obtained for suitable choices of $F(\phi)$
and $V(\phi)$. Implications for other recent results are also discussed.
