6 research outputs found
On three dimensions as the preferred dimensionality of space via the Brandenberger-Vafa mechanism
In previous work it was shown that, in accord with the Brandenberger-Vafa
mechanism, three is the maximum number of spatial dimensions that can grow
large cosmologically from an initial thermal fluctuation. Here we complement
that work by considering the possibility of successive fluctuations. Suppose an
initial fluctuation causes at least one dimension to grow, and suppose
successive fluctuations occur on timescales of order alpha'^{1/2}. If the
string coupling is sufficiently large, we show that such fluctuations are
likely to push a three-dimensional subspace to large volume where winding modes
annihilate. In this setting three is the preferred number of large dimensions.
Although encouraging, a more careful study of the dynamics and statistics of
fluctuations is needed to assess the likelihood of our assumptions.Comment: 20 pages, 4 figures. v2: additional references, various improvements
for clarity. v3: more improvements, version to appear in PR
Bouncing and cyclic string gas cosmologies
We show that, in the presence of a string gas, simple higher-derivative
modifications to the effective action for gravity can lead to bouncing and
cyclic cosmological models. The modifications bound the expansion rate and
avoid singularities at finite times. In these models the scale factors can have
long loitering phases that solve the horizon problem. Adding a potential for
the dilaton gives a simple realization of the pre-big bang scenario. Entropy
production in the cyclic phase drives an eventual transition to a
radiation-dominated universe. As a test of the Brandenberger-Vafa scenario, we
comment on the probability of decompactifying three spatial dimensions in this
class of models.Comment: 35 pages, LaTeX, 9 figures. v2: additional references. v3: comments
on Einstein frame, version to appear in PR
Dynamical Decompactification and Three Large Dimensions
We study string gas dynamics in the early universe and seek to realize the
Brandenberger - Vafa mechanism - a goal that has eluded earlier works - that
singles out three or fewer spatial dimensions as the number which grow large
cosmologically. Considering wound string interactions in an impact parameter
picture, we show that a strong exponential suppression in the interaction rates
for d > 3 spatial dimensions reflects the classical argument that string
worldsheets generically intersect in at most four spacetime dimensions. This
description is appropriate in the early universe if wound strings are heavy -
wrapping long cycles - and diluted. We consider the dynamics of a string gas
coupled to dilaton-gravity and find that a) for any number of dimensions the
universe generically stays trapped in the Hagedorn regime and b) if the
universe fluctuates to a radiation regime any residual winding modes are
diluted enough so that they freeze-out in d > 3 large dimensions while they
generically annihilate for d = 3. In this sense the Brandenberger-Vafa
mechanism is operative.Comment: 20 pages, 2 figures, minor changes, updated figures, as will appear
in Phys.Rev.
Gravitational Lensing as Signal and Noise in Lyman-alpha Forest Measurements
In Lyman-alpha forest measurements it is generally assumed that quasars are
mere background light sources which are uncorrelated with the forest.
Gravitational lensing of the quasars violates this assumption. This effect
leads to a measurement bias, but more interestingly it provides a valuable
signal. The lensing signal can be extracted by correlating quasar magnitudes
with the flux power spectrum and with the flux decrement. These correlations
will be challenging to measure but their detection provides a direct measure of
how features in the Lyman-alpha forest trace the underlying mass density field.
Observing them will test the fundamental hypothesis that fluctuations in the
forest are predominantly driven by fluctuations in mass, rather than in the
ionizing background, helium reionization or winds. We discuss ways to
disentangle the lensing signal from other sources of such correlations,
including dust, continuum and background residuals. The lensing-induced
measurement bias arises from sample selection: one preferentially collects
spectra of magnified quasars which are behind overdense regions. This
measurement bias is ~0.1-1% for the flux power spectrum, optical depth and the
flux probability distribution. Since the effect is systematic, quantities such
as the amplitude of the flux power spectrum averaged across scales should be
interpreted with care.Comment: 22 pages, 8 figures; v2: references added, discussion expanded,
matches PRD accepted versio