395 research outputs found
Early structure formation from cosmic string loops
We examine the effects of cosmic strings on structure formation and on the
ionization history of the universe. While Gaussian perturbations from inflation
are known to provide the dominant contribution to the large scale structure of
the universe, density perturbations due to strings are highly non-Gaussian and
can produce nonlinear structures at very early times. This could lead to early
star formation and reionization of the universe. We improve on earlier studies
of these effects by accounting for high loop velocities and for the filamentary
shape of the resulting halos. We find that for string energy scales G\mu >
10^{-7} the effect of strings on the CMB temperature and polarization power
spectra can be significant and is likely to be detectable by the Planck
satellite. We mention shortcomings of the standard cosmological model of galaxy
formation which may be remedied with the addition of cosmic strings, and
comment on other possible observational implications of early structure
formation by strings.Comment: 22 pages, 10 figures. References adde
Bubbles of Nothing in Flux Compactifications
We construct a simple AdS_4 x S^1 flux compactification stabilized by a
complex scalar field winding the extra dimension and demonstrate an instability
via nucleation of a bubble of nothing. This occurs when the Kaluza -- Klein
dimension degenerates to a point, defining the bubble surface. Because the
extra dimension is stabilized by a flux, the bubble surface must be charged, in
this case under the axionic part of the complex scalar. This smooth geometry
can be seen as a de Sitter topological defect with asymptotic behavior
identical to the pure compactification. We discuss how a similar construction
can be implemented in more general Freund -- Rubin compactifications.Comment: 16 pages, 5 figures References adde
Large parallel cosmic string simulations: New results on loop production
Using a new parallel computing technique, we have run the largest cosmic
string simulations ever performed. Our results confirm the existence of a long
transient period where a non-scaling distribution of small loops is produced at
lengths depending on the initial correlation scale. As time passes, this
initial population gives way to the true scaling regime, where loops of size
approximately equal to one-twentieth the horizon distance become a significant
component. We observe similar behavior in matter and radiation eras, as well as
in flat space. In the matter era, the scaling population of large loops becomes
the dominant component; we expect this to eventually happen in the other eras
as well.Comment: 23 pages, 10 figures, 2 tables. V2: combine 3 figures, add 1 table,
better discussion + citation of prev. wor
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