15 research outputs found
Chasing Brane Inflation in String-Theory
We investigate the embedding of brane anti-brane inflation into a concrete
type IIB string theory compactification with all moduli fixed. Specifically, we
are considering a D3-brane, whose position represents the inflaton , in a
warped conifold throat in the presence of supersymmetrically embedded D7-branes
and an anti D3-brane localized at the tip of the warped conifold cone. After
presenting the moduli stabilization analysis for a general D7-brane embedding,
we concentrate on two explicit models, the Ouyang and the Kuperstein
embeddings. We analyze whether the forces, induced by moduli stabilization and
acting on the D3-brane, might cancel by fine-tuning such as to leave us with
the original Coulomb attraction of the anti D3-brane as the driving force for
inflation. For a large class of D7-brane embeddings we obtain a negative
result. Cancelations are possible only for very small intervals of
around an inflection point but not globally. For the most part of its motion
the inflaton then feels a steep, non slow-roll potential. We study the
inflationary dynamics induced by this potential.Comment: 34 pages, 4 figures. Final version published in JCA
DBI Inflation using a One-Parameter Family of Throat Geometries
We demonstrate the possibility of examining cosmological signatures in the
DBI inflation setup using the BGMPZ solution, a one-parameter family of
geometries for the warped throat which interpolate between the Maldacena-Nunez
and Klebanov-Strassler solutions. The warp factor is determined numerically and
subsequently used to calculate cosmological observables including the scalar
and tensor spectral indices, for a sample point in the parameter space. As one
moves away from the KS solution for the throat the warp factor is qualitatively
different, which leads to a significant change for the observables, but also
generically increases the non-Gaussianity of the models. We argue that the
different models can potentially be differentiated by current and future
experiments.Comment: 17 pages, 10 figures; v2: section 4 expanded, references added; v3:
typos fixe
Universality in D-brane Inflation
We study the six-field dynamics of D3-brane inflation for a general scalar
potential on the conifold, finding simple, universal behavior. We numerically
evolve the equations of motion for an ensemble of more than 7 \times 10^7
realizations, drawing the coefficients in the scalar potential from statistical
distributions whose detailed properties have demonstrably small effects on our
results. When prolonged inflation occurs, it has a characteristic form: the
D3-brane initially moves rapidly in the angular directions, spirals down to an
inflection point in the potential, and settles into single-field inflation. The
probability of N_{e} e-folds of inflation is a power law, P(N_{e}) \propto
N_{e}^{-3}, and we derive the same exponent from a simple analytical model. The
success of inflation is relatively insensitive to the initial conditions: we
find attractor behavior in the angular directions, and the D3-brane can begin
far above the inflection point without overshooting. In favorable regions of
the parameter space, models yielding 60 e-folds of expansion arise
approximately once in 10^3 trials. Realizations that are effectively
single-field and give rise to a primordial spectrum of fluctuations consistent
with WMAP, for which at least 120 e-folds are required, arise approximately
once in 10^5 trials. The emergence of robust predictions from a six-field
potential with hundreds of terms invites an analytic approach to multifield
inflation.Comment: 28 pages, 9 figure
Brane inflation and the WMAP data: a Bayesian analysis
The Wilkinson Microwave Anisotropy Probe (WMAP) constraints on string
inspired ''brane inflation'' are investigated. Here, the inflaton field is
interpreted as the distance between two branes placed in a flux-enriched
background geometry and has a Dirac-Born-Infeld (DBI) kinetic term. Our method
relies on an exact numerical integration of the inflationary power spectra
coupled to a Markov-Chain Monte-Carlo exploration of the parameter space. This
analysis is valid for any perturbative value of the string coupling constant
and of the string length, and includes a phenomenological modelling of the
reheating era to describe the post-inflationary evolution. It is found that the
data favour a scenario where inflation stops by violation of the slow-roll
conditions well before brane annihilation, rather than by tachyonic
instability. Concerning the background geometry, it is established that log(v)
> -10 at 95% confidence level (CL), where "v" is the dimensionless ratio of the
five-dimensional sub-manifold at the base of the six-dimensional warped
conifold geometry to the volume of the unit five-sphere. The reheating energy
scale remains poorly constrained, Treh > 20 GeV at 95% CL, for an extreme
equation of state (wreh ~ -1/3) only. Assuming the string length is known, the
favoured values of the string coupling and of the Ramond-Ramond total
background charge appear to be correlated. Finally, the stochastic regime
(without and with volume effects) is studied using a perturbative treatment of
the Langevin equation. The validity of such an approximate scheme is discussed
and shown to be too limited for a full characterisation of the quantum effects.Comment: 65 pages, 15 figures, uses iopart. Shortened version, updated
references. Matches publication up to appendix B kept on the arXi
Towards an Explicit Model of D-brane Inflation
We present a detailed analysis of an explicit model of warped D-brane
inflation, incorporating the effects of moduli stabilization. We consider the
potential for D3-brane motion in a warped conifold background that includes
fluxes and holomorphically-embedded D7-branes involved in moduli stabilization.
Although the D7-branes significantly modify the inflaton potential, they do not
correct the quadratic term in the potential, and hence do not cause a uniform
change in the slow-roll parameter eta. Nevertheless, we present a simple
example based on the Kuperstein embedding of D7-branes, z_1=constant, in which
the potential can be fine-tuned to be sufficiently flat for inflation. To
derive this result, it is essential to incorporate the fact that the
compactification volume changes slightly as the D3-brane moves. We stress that
the compactification geometry dictates certain relationships among the
parameters in the inflaton Lagrangian, and these microscopic constraints impose
severe restrictions on the space of possible models. We note that the shape of
the final inflaton potential differs from projections given in earlier studies:
in configurations where inflation occurs, it does so near an inflection point.
Finally, we comment on the difficulty of making precise cosmological
predictions in this scenario. This is the companion paper to arXiv:0705.3837.Comment: 68 pages, 6 figures; v2: fixed typos, added refs and clarifications;
v3: expanded discussion of inflection point inflatio
Constraining Running Non-Gaussianity
The primordial non-Gaussian parameter fNL has been shown to be
scale-dependent in several models of inflation with a variable speed of sound.
Starting from a simple ansatz for a scale-dependent amplitude of the primordial
curvature bispectrum for two common phenomenological models of primordial
non-Gaussianity, we perform a Fisher matrix analysis of the bispectra of the
temperature and polarization of the Cosmic Microwave Background (CMB) radiation
and derive the expected constraints on the parameter nNG that quantifies the
running of fNL(k) for current and future CMB missions such as WMAP, Planck and
CMBPol. We find that CMB information alone, in the event of a significant
detection of the non-Gaussian component, corresponding to fNL = 50 for the
local model and fNL = 100 for the equilateral model of non-Gaussianity, is able
to determine nNG with a 1-sigma uncertainty of Delta nNG = 0.1 and Delta nNG =
0.3, respectively, for the Planck mission. In addition, we consider a Fisher
matrix analysis of the galaxy power spectrum to determine the expected
constraints on the running parameter nNG for the local model and of the galaxy
bispectrum for the equilateral model from future photometric and spectroscopic
surveys. We find that, in both cases, large-scale structure observations should
achieve results comparable to or even better than those from the CMB, while
showing some complementarity due to the different distribution of the
non-Gaussian signal over the relevant range of scales. Finally, we compare our
findings to the predictions on the amplitude and running of non-Gaussianity of
DBI inflation, showing how the constraints on a scale-dependent fNL(k)
translate into constraints on the parameter space of the theory.Comment: 37 pages, 14 figure
Lectures on Cosmic Inflation and its Potential Stringy Realizations
These notes present a brief introduction to Hot Big Bang cosmology and Cosmic
Inflation, together with a selection of some recent attempts to embed inflation
into string theory. They provide a partial description of lectures presented in
courses at Dubrovnik in August 2006, at CERN in January 2007 and at Cargese in
August 2007. They are aimed at graduate students with a working knowledge of
quantum field theory, but who are unfamiliar with the details of cosmology or
of string theory.Comment: 68 pages, lectures given at Dubrovnik, Aug 2006; CERN, January 2007;
and Cargese, Aug 200
Warped Wilson Line DBI Inflation
We propose a novel inflationary scenario in string theory in which the
inflaton field is a 'Wilson line' degree of freedom in the worldvolume of a
probe Dp-brane, in a warped flux compactification. Kinetic terms for Wilson
line fields on the world volume of a D-brane take a nonstandard
Dirac-Born-Infeld (DBI) form. Thus, we work in the framework of DBI inflation.
This extends the original slow roll Wilson line inflationary scenario, where
only the quadratic piece was considered. Warped DBI Wilson line inflation
offers an attractive alternative to ordinary (position field) DBI inflation,
inasmuch as observational and theoretical constraints get considerably relaxed.
Besides the standard large non-Gaussianities in DBI scenarios, it is also
possible to achieve an observable amount of gravitational waves.Comment: v3: Typos corrected, Published in JCAP; 21 page