42 research outputs found
Non-linear general relativistic effects in the observed redshift
We present the second-order expression for the observed redshift, accounting
for all the relativistic effects from the light propagation and from the frame
change at the observer and the source positions. We derive the generic
gauge-transformation law that any observable quantities should satisfy, and we
verify our second-order expression for the observed redshift by explicitly
checking its gauge transformation property. This is the first time an explicit
verification is made for the second-order calculations of observable
quantities. We present our results in popular gauge choices for easy use and
discuss the origin of disagreements in previous calculations.Comment: 44 pages, 1 figure, Version published in JCA
Observation angles, Fermi coordinates, and the Geodesic-Light-Cone gauge
We show that the angular directions locally measured by a static geodesic
observer in a generic cosmological background and expressed in the system of
Fermi Normal Coordinates always coincide with those expressed in the
Geodesic-Light-Cone (GLC) gauge, up to a local transformation which exploits
the residual gauge freedom of the GLC coordinates. This is not the case for
other gauges - like, for instance, the synchronous and longitudinal gauge -
commonly used in the context of observational cosmology. We also make an
explicit proposal for the GLC gauge-fixing condition that ensures a full
identification of its angles with the observational ones.Comment: 14 pages, version accepted for publication on JCA
CMB-lensing beyond the leading order: temperature and polarization anisotropies
We investigate the weak lensing corrections to the CMB temperature and
polarization anisotropies. We consider all the effects beyond the leading
order: post-Born corrections, LSS corrections and, for the polarization
anisotropies, the correction due to the rotation of the polarization direction
between the emission at the source and the detection at the observer. We show
that the full next-to-leading order correction to the B-mode polarization is
not negligible on small scales and is dominated by the contribution from the
rotation, this is a new effect not taken in account in previous works.
Considering vanishing primordial gravitational waves, the B-mode correction due
to rotation is comparable to cosmic variance for , in
contrast to all other spectra where the corrections are always below that
threshold for a single multipole. Moreover, the sum of all the effects is
larger than cosmic variance at high multipoles, showing that higher-order
lensing corrections to B-mode polarization are in principle detectable.Comment: 32 pages, 6 figures. New results about the signal-to-noise amplitude
for next-to-leading order corrections, further clarifications about the
polarization rotation and references added. Version accepted for publication
in Physical Review
Fractal universe and cosmic acceleration in a Lema\^itre-Tolman-Bondi scenario
In this paper we attempt to answer to the question: can cosmic acceleration
of the Universe have a fractal solution? We give an exact solution of a
Lema\^itre-Tolman-Bondi (LTB) Universe based on the assumption that such a
smooth metric is able to describe, on average, a fractal distribution of
matter. While the LTB model has a center, we speculate that, when the fractal
dimension is not very different from the space dimension, this metric applies
to any point of the fractal structure when chosen as center so that, on
average, there is not any special point or direction. We examine the observed
magnitude-redshift relation of type Ia supernovae (SNe Ia), showing that the
apparent acceleration of the cosmic expansion can be explained as a consequence
of the fractal distribution of matter when the corresponding space-time metric
is modeled as a smooth LTB one and if the fractal dimension on scales of a few
hundreds Mpc is .Comment: 6 pages, 4 figures, accepted for publication in Classical and Quantum
Gravit