155 research outputs found
Is Brane Inflation Eternal?
In this paper, we show that eternal inflation of the random walk type is
generically absent in the brane inflationary scenario. Depending on how the
brane inflationary universe originated, eternal inflation of the false vacuum
type is still quite possible. Since the inflaton is the position of the
D3-brane relative to the anti-D3-brane inside the compactified bulk with finite
size, its value is bounded. In DBI inflation, the warped space also restricts
the amplitude of the scalar fluctuation. These upper bounds impose strong
constraints on the possibility of eternal inflation. We find that eternal
inflation due to the random walk of the inflaton field is absent in both the
KKLMMT slow roll scenario and the DBI scenario. A more careful analysis for the
slow-roll case is also presented using the Langevin equation, which gives very
similar results. We discuss possible ways to obtain eternal inflation of the
random walk type in brane inflation. In the multi-throat brane inflationary
scenario, the branes may be generated by quantum tunneling and roll out the
throat. Eternal inflation of the false vacuum type inevitably happens in this
scenario due to the tunneling process. Since these scenarios have different
cosmological predictions, more data from the cosmic microwave background
radiation will hopefully select the specific scenario our universe has gone
through.Comment: 32 pages; v2: references and comments adde
Comparing Brane Inflation to WMAP
We compare the simplest realistic brane inflationary model to recent
cosmological data, including WMAP 3-year cosmic microwave background (CMB)
results, Sloan Digital Sky Survey luminous red galaxies (SDSS LRG) power
spectrum data and Supernovae Legacy Survey (SNLS) Type 1a supernovae distance
measures. Here, the inflaton is simply the position of a -brane which is
moving towards a -brane sitting at the bottom of a throat (a warped,
deformed conifold) in the flux compactified bulk in Type IIB string theory. The
analysis includes both the usual slow-roll scenario and the Dirac-Born-Infeld
scenario of slow but relativistic rolling. Requiring that the throat is inside
the bulk greatly restricts the allowed parameter space. We discuss possible
scenarios in which large tensor mode and/or non-Gaussianity may emerge. Here,
the properties of a large tensor mode deviate from that in the usual slow-roll
scenario, providing a possible stringy signature. Overall, within the brane
inflationary scenario, the cosmological data is providing information about the
properties of the compactification of the extra dimensions.Comment: 45 pages 11 figure
Cosmology of the Tachyon in Brane Inflation
In certain implementations of the brane inflationary paradigm, the exit from
inflation occurs when the branes annihilate through tachyon condensation. We
investigate various cosmological effects produced by this tachyonic era. We
find that only a very small region of the parameter space (corresponding to
slow-roll with tiny inflaton mass) allows for the tachyon to contribute some
e-folds to inflation. In addition, non-adiabatic density perturbations are
generated at the end of inflation. When the brane is moving relativistically
this contribution can be of the same order as fluctuations produced 55 e-folds
before the end of inflation. The additional contribution is very nearly
scale-invariant and enhances the tensor/scalar ratio. Additional
non-gaussianities will also be generated, sharpening current constraints on
DBI-type models which already predict a significantly non-gaussian signal.Comment: 30 pages, 2 figures; v3, minor revision, JCAP versio
Duality Cascade in Brane Inflation
We show that brane inflation is very sensitive to tiny sharp features in
extra dimensions, including those in the potential and in the warp factor. This
can show up as observational signatures in the power spectrum and/or
non-Gaussianities of the cosmic microwave background radiation (CMBR). One
general example of such sharp features is a succession of small steps in a
warped throat, caused by Seiberg duality cascade using gauge/gravity duality.
We study the cosmological observational consequences of these steps in brane
inflation. Since the steps come in a series, the prediction of other steps and
their properties can be tested by future data and analysis. It is also possible
that the steps are too close to be resolved in the power spectrum, in which
case they may show up only in the non-Gaussianity of the CMB temperature
fluctuations and/or EE polarization. We study two cases. In the slow-roll
scenario where steps appear in the inflaton potential, the sensitivity of brane
inflation to the height and width of the steps is increased by several orders
of magnitude comparing to that in previously studied large field models. In the
IR DBI scenario where steps appear in the warp factor, we find that the
glitches in the power spectrum caused by these sharp features are generally
small or even unobservable, but associated distinctive non-Gaussianity can be
large. Together with its large negative running of the power spectrum index,
this scenario clearly illustrates how rich and different a brane inflationary
scenario can be when compared to generic slow-roll inflation. Such distinctive
stringy features may provide a powerful probe of superstring theory.Comment: Corrections in Eq.(5.47), Eq (5.48), Eq(5.49) and Fig
On the Slow Roll Expansion for Brane Inflation
One possibility for identifying the inflaton in the framework of string
theory is that it is a -brane modulus. This option involves a specific,
non-canonical form of the kinetic energy -- the Dirac-Born-Infeld action. This
note investigates the applicability of the slow roll approximation in
inflationary models of this type. To this end the slow roll expansion of
Liddle, Parsons and Barrow is derived for the case of the DBI action. The
resulting slow roll conditions augment the standard ones valid in the case of
canonical kinetic terms. It is also shown that in DBI models inflation does not
require that the potential dominate the energy density.Comment: References adde
Structures in the Gauge/Gravity Duality Cascade
We study corrections to the anomalous mass dimension and their effects in the
Seiberg duality cascade in the Klebanov-Strassler throat, where
supersymmetric gauge theory with bifundamental chiral
superfields and a quartic tree level superpotential in four dimensions is dual
to type IIB string theory on background. Analyzing the
renormalization group flow of the couplings on the gauge theory side, we
propose specific corrections to the anomalous mass dimension. Applying
gauge/gravity duality, we then show that the corrections reveal structures on
the supergravity side with steps appearing in the running of the fluxes and the
metric. The "charges" at the steps provide a gravitational source for Seiberg
duality transformations. The finiteness of these corrections suggests that the
theory flows to a baryonic branch rather than to a confining branch. The
cosmological implication of the duality cascade and the gauge/gravity duality
on the brane inflationary scenario and the cosmic microwave background
radiation is pointed out.Comment: v3: 40 pages, minor changes and typos fixe
Multi-field Inflation with a Random Potential
Motivated by the possibility of inflation in the cosmic landscape, which may
be approximated by a complicated potential, we study the density perturbations
in multi-field inflation with a random potential. The random potential causes
the inflaton to undergo a Brownian motion with a drift in the D-dimensional
field space. To quantify such an effect, we employ a stochastic approach to
evaluate the two-point and three-point functions of primordial perturbations.
We find that in the weakly random scenario the resulting power spectrum
resembles that of the single field slow-roll case, with up to 2% more red tilt.
The strongly random scenario, leads to rich phenomenologies, such as primordial
fluctuations in the power spectrum on all angular scales. Such features may
already be hiding in the error bars of observed CMB TT (as well as TE and EE)
power spectrum and can be detected or falsified with more data coming in the
future. The tensor power spectrum itself is free of fluctuations and the tensor
to scalar ratio is enhanced. In addition a large negative running of the power
spectral index is possible. Non-Gaussianity is generically suppressed by the
growth of adiabatic perturbations on super-horizon scales, but can possibly be
enhanced by resonant effects or arise from the entropic perturbations during
the onset of (p)reheating. The formalism developed in this paper can be applied
to a wide class of multi-field inflation models including, e.g. the N-flation
scenario.Comment: More clarifications and references adde
Multi-field Inflation with a Random Potential
Motivated by the possibility of inflation in the cosmic landscape, which may
be approximated by a complicated potential, we study the density perturbations
in multi-field inflation with a random potential. The random potential causes
the inflaton to undergo a Brownian motion with a drift in the D-dimensional
field space. To quantify such an effect, we employ a stochastic approach to
evaluate the two-point and three-point functions of primordial perturbations.
We find that in the weakly random scenario the resulting power spectrum
resembles that of the single field slow-roll case, with up to 2% more red tilt.
The strongly random scenario, leads to rich phenomenologies, such as primordial
fluctuations in the power spectrum on all angular scales. Such features may
already be hiding in the error bars of observed CMB TT (as well as TE and EE)
power spectrum and can be detected or falsified with more data coming in the
future. The tensor power spectrum itself is free of fluctuations and the tensor
to scalar ratio is enhanced. In addition a large negative running of the power
spectral index is possible. Non-Gaussianity is generically suppressed by the
growth of adiabatic perturbations on super-horizon scales, but can possibly be
enhanced by resonant effects or arise from the entropic perturbations during
the onset of (p)reheating. The formalism developed in this paper can be applied
to a wide class of multi-field inflation models including, e.g. the N-flation
scenario.Comment: More clarifications and references adde
The (p,q) String Tension in a Warped Deformed Conifold
We find the tension spectrum of the bound states of p fundamental strings and
q D-strings at the bottom of a warped deformed conifold. We show that it can be
obtained from a D3-brane wrapping a 2-cycle that is stabilized by both electric
and magnetic fluxes. Because the F-strings are Z_M-charged with non-zero
binding energy, binding can take place even if (p,q) are not coprime.
Implications for cosmic strings are briefly discussed.Comment: 17 pages, 1 figur
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