205 research outputs found
Intrinsic uncertainty on the nature of dark energy
We argue that there is an intrinsic noise on measurements of the equation of
state parameter from large-scale structure around us. The presence
of the large-scale structure leads to an ambiguity in the definition of the
background universe and thus there is a maximal precision with which we can
determine the equation of state of dark energy. To study the uncertainty due to
local structure, we model density perturbations stemming from a standard
inflationary power spectrum by means of the exact Lema\^{i}tre-Tolman-Bondi
solution of Einstein's equation, and show that the usual distribution of matter
inhomogeneities in a CDM cosmology causes a variation of -- as
inferred from distance measures -- of several percent. As we observe only one
universe, or equivalently because of the cosmic variance, this uncertainty is
systematic in nature.Comment: 12 pages, 3 figures. Version as accepted for publication in Physics
of the Dark Universe (Open Access
A back-reaction approach to dark energy
This thesis is mainly about how to set up and carry out in a physically
meaningful way the idea of back-reaction, according to which dark energy could
be an effective source. There are, broadly speaking, two distinct approaches.
One is focused on how cosmological observables are affected by inhomogeneities,
while the other is focused on a theoretical description of the inhomogeneous
universe by means of a mean-field description. Both approaches, however, share
the idea of smoothing out inhomogeneities. We developed this duality in the
interpretation of the back-reaction by means of toy models based on the
Lemaitre-Tolman-Bondi solution of Einstein's equations. In particular we
focused on voids expanding faster than the background solution.Comment: 128 pages, 52 figures; PhD thesi
Cosmic variance and the measurement of the local Hubble parameter
There is an approximately 9% discrepancy, corresponding to 2.4sigma, between
two independent constraints on the expansion rate of the universe: one
indirectly arising from the cosmic microwave background and baryon acoustic
oscillations, and one more directly obtained from local measurements of the
relation between redshifts and distances to sources. We argue that by taking
into account the local gravitational potential at the position of the observer
this tension - strengthened by the recent Planck results - is partially
relieved and the concordance of the standard model of cosmology increased. We
estimate that measurements of the local Hubble constant are subject to a cosmic
variance of about 2.4% (limiting the local sample to redshifts z>0.010) or 1.3%
(limiting it to z>0.023), a more significant correction than that taken into
account already. Nonetheless, we show that one would need a very rare
fluctuation to fully explain the offset in the Hubble rates. If this tension is
further strengthened, a cosmology beyond the standard model may prove
necessary.Comment: 5 pages, 4 figures. v2: added one figure, improved results and
discussion, added references. v3: updated to Planck results. v4: matches
version accepted for publication in PR
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