12 research outputs found
Validity of the kink approximation to the tunneling action
Coleman tunneling in a general scalar potential with two non-degenerate
minima is known to have an approximation in terms of a piecewise linear
triangular-shaped potential with sharp 'kinks' at the place of the local
minima. This approximate potential has a regime where the existence of the
bounce solution needs the scalar field to 'wait' for some amount of Euclidean
time at one of the 'kinks'. We discuss under which conditions a kink
approximation of locally smooth 'cap' regions provides a good estimate for the
bounce action.Comment: 7 pages, 4 figures, title changed in version 2 to match published
versio
Roulette Inflation with K\"ahler Moduli and their Axions
We study 2-field inflation models based on the ``large-volume'' flux
compactification of type IIB string theory. The role of the inflaton is played
by a K\"ahler modulus \tau corresponding to a 4-cycle volume and its axionic
partner \theta. The freedom associated with the choice of Calabi Yau manifold
and the non-perturbative effects defining the potential V(\tau, \theta) and
kinetic parameters of the moduli bring an unavoidable statistical element to
theory prior probabilities within the low energy landscape. The further
randomness of (\tau, \theta) initial conditions allows for a large ensemble of
trajectories. Features in the ensemble of histories include ``roulette
tractories'', with long-lasting inflations in the direction of the rolling
axion, enhanced in number of e-foldings over those restricted to lie in the
\tau-trough. Asymptotic flatness of the potential makes possible an eternal
stochastic self-reproducing inflation. A wide variety of potentials and
inflaton trajectories agree with the cosmic microwave background and large
scale structure data. In particular, the observed scalar tilt with weak or no
running can be achieved in spite of a nearly critical de Sitter deceleration
parameter and consequently a low gravity wave power relative to the scalar
curvature power.Comment: Version submitted to Phys.Rev.D. 29 pages, 12 Figures, minor change
Surprising phenomena in a rich new class of inflationary models
We report on a new class of fast-roll inflationary models. In a huge part of
its parameter space, inflationary perturbations exhibit quite unusual phenomena
such as scalar and tensor modes freezing out at widely different times, as well
as scalar modes reentering the horizon during inflation. In another, narrower
range of parameters, this class of models agrees with observations. One
specific point in parameter space is characterized by extraordinary behavior of
the scalar perturbations. Freeze-out of scalar perturbations as well as
particle production at horizon crossing are absent. Also the behavior of the
perturbations around this quasi-de Sitter background is dual to a quantum field
theory in flat space-time. Finally, the form of the primordial power spectrum
is determined by the interaction between different modes of scalar
perturbations.Comment: 12 pages, 5 figures, 1 table, references + comments added, errors
corrected, conclusions unchanged, version published in JCA
The Virtues of Frugality - Why cosmological observers should release their data slowly
Cosmologists will soon be in a unique position. Observational noise will
gradually be replaced by cosmic variance as the dominant source of uncertainty
in an increasing number of observations. We reflect on the ramifications for
the discovery and verification of new models. If there are features in the full
data set that call for a new model, there will be no subsequent observations to
test that model's predictions. We give specific examples of the problem by
discussing the pitfalls of model discovery by prior adjustment in the context
of dark energy models and inflationary theories. We show how the gradual
release of data can mitigate this difficulty, allowing anomalies to be
identified, and new models to be proposed and tested. We advocate that
observers plan for the frugal release of data from future cosmic variance
limited observations.Comment: 5 pages, expanded discussion of Lambda and of blind anlysis, added
refs. Matches version to appear in MNRAS Letter
CMBPol Mission Concept Study: Probing Inflation with CMB Polarization
We summarize the utility of precise cosmic microwave background (CMB)
polarization measurements as probes of the physics of inflation. We focus on
the prospects for using CMB measurements to differentiate various inflationary
mechanisms. In particular, a detection of primordial B-mode polarization would
demonstrate that inflation occurred at a very high energy scale, and that the
inflaton traversed a super-Planckian distance in field space. We explain how
such a detection or constraint would illuminate aspects of physics at the
Planck scale. Moreover, CMB measurements can constrain the scale-dependence and
non-Gaussianity of the primordial fluctuations and limit the possibility of a
significant isocurvature contribution. Each such limit provides crucial
information on the underlying inflationary dynamics. Finally, we quantify these
considerations by presenting forecasts for the sensitivities of a future
satellite experiment to the inflationary parameters.Comment: 107 pages, 14 figures, 17 tables; Inflation Working Group
contribution to the CMBPol Mission Concept Study; v2: typos fixed and
references adde
An Exact Tunneling Solution in a Simple Realistic Landscape
We present an analytical solution for the tunneling process in a piecewise
linear and quadratic potential which does not make use of the thin-wall
approximation. A quadratic potential allows for smooth attachment of various
slopes exiting into the final minimum of a realistic potential. Our tunneling
solution thus serves as a realistic approximation to situations such as
populating a landscape of slow-roll inflationary regions by tunneling, and it
is valid for all regimes of the barrier parameters. We shortly comment on the
inclusion of gravity.Comment: RevTeX 4.1, 4 pages, 5 figure
Constraints on the Topology of the Universe: Extension to General Geometries
We present an update to the search for a non-trivial topology of the universe
by searching for matching circle pairs in the cosmic microwave background using
the WMAP 7 year data release. We extend the exisiting bounds to encompass a
wider range of possible topologies by searching for matching circle pairs with
opening angles 10 degree < \alpha < 90 degree and separation angles 11 degree <
\theta < 180 degree. The extended search reveal two small anomalous regions in
the CMB sky. Numerous pairs of well-matched circles are found where both
circles pass through one or the other of those regions. As this is not the
signature of any known manifold, but is a likely consequence of contamination
in those sky regions, we repeat the search excluding circle pairs where both
pass through either of the two regions. We then find no statistically
significant pairs of matched circles, and so no hints of a non-trivial
topology. The absence of matched circles increases the lower limit on the
length of the shortest closed null geodesic that self-intersects at our
location in the universe (equivalently the injectivity radius at our location)
to 98.5% of the diameter of the last scattering surface or approximately 26
Gpc. It extends the limit to any manifolds in which the intersecting arcs of
said geodesic form an angle greater than 10^o.Comment: 11 pages, 11 figure
The Overshoot Problem in Inflation after Tunneling
We show the absence of the usual parametrically large overshoot problem of
small-field inflation if initiated by a Coleman-De Luccia (CDL) tunneling
transition from an earlier vacuum in the limit of small inflationary scale
compared to the tunneling scale. For low-power monomial exit potentials
, we derive an expression for the amount of overshoot.
This is bounded from above by the width of the steep barrier traversed after
emerging from tunneling and before reaching a slow-roll region of the
potential. For we show that overshooting is entirely absent. We
extend this result through binomials to a general potential written as a series
expansion, and to the case of arbitrary finite initial speed of the inflaton.
This places the phase space of initial conditions for small-field and
large-field inflation on the same footing in a landscape of string theory vacua
populated via CDL tunneling.Comment: 14 pages, 4 figure