83 research outputs found
An improved model of HII bubbles during the epoch of reionization
The size distribution of ionized regions during the epoch of reionization --
a key ingredient in understanding the HI power spectrum observable by 21cm
experiments -- can be modelled analytically using the excursion set formalism
of random walks in the smoothed initial density field. To date, such
calculations have been based on simplifying assumptions carried forward from
the earliest excursion set models of two decades ago. In particular, these
models assume that the random walks have uncorrelated steps and that haloes can
form at arbitrary locations in the initial density field. We extend these
calculations by incorporating recent technical developments that allow us to
(a) include the effect of correlations in the steps of the walks induced by a
realistic smoothing filter and (b) more importantly, account for the fact that
dark matter haloes preferentially form near peaks in the initial density. A
comparison with previous calculations shows that including these features,
particularly the peaks constraint on halo locations, has large effects on the
size distribution of the HII bubbles surrounding these haloes. For example,
when comparing models at the same value of the globally averaged ionized volume
fraction, the typical bubble sizes predicted by our model are more than a
factor 2 larger than earlier calculations. Our results can potentially have a
significant impact on estimates of the observable HI power spectrum.Comment: 13 pages, 6 figures; v2 - added clarifications and fixed typos.
Accepted in MNRA
The Averaging Problem in Cosmology
This thesis deals with the averaging problem in cosmology, which has gained
considerable interest in recent years, and is concerned with correction terms
(after averaging inhomogeneities) that appear in the Einstein equations when
working on the large scales appropriate for cosmology. It has been claimed in
the literature that these terms may account for the phenomenon of dark energy
which causes the late time universe to accelerate. We investigate the nature of
these terms by using averaging schemes available in the literature and further
developed to be applicable to the problem at hand. We show that the effect of
these terms when calculated carefully, remains negligible and cannot explain
the late time acceleration.Comment: 126 pages, PhD thesis; some typos fixed, one reference adde
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