How Delicate is Brane-Antibrane Inflation?

We systematically explore the parameter space of the state-of-the-art brane-antibrane inflation model (Baumann {\it et al}., arXiv:0706.0360, arXiv:0705.3837) which is one of the most rigorously derived from string theory, applying the cosmic background explorer normalization and constraint on the spectral index. We improve on previous treatments of uplifting by antibranes and show that the contributions from noninflationary throats play an important role in achieving a flat inflationary potential. To quantify the degree of fine-tuning needed by the model, we define an effective volume in the part of parameter space which is consistent with experimental constraints, and using Monte Carlo methods to search for a set of optimal parameters, we show that the degree of fine-tuning is alleviated by 8 orders of magnitude relative to a fiducial point which has previously been considered. In fact, close to the optimal parameter values, fine-tuning is no longer needed for any of the parameters. We show that in this natural region of the parameter space, larger values of $n_s$ close to 0.99 (still within 2$\sigma$ of the WMAP5 central value) are favored, giving a new aspect of testability to the model.Comment: 33 pages, 12 figures. v2: added discussion of an alternate measure of fine-tuning; published versio

Testing for Features in the Primordial Power Spectrum

Well-known causality arguments show that events occurring during or at the end of inflation, associated with reheating or preheating, could contribute a blue component to the spectrum of primordial curvature perturbations, with the dependence k^3. We explore the possibility that they could be observably large in CMB, LSS, and Lyman-alpha data. We find that a k^3 component with a cutoff at some maximum k can modestly improve the fits (Delta chi^2=2.0, 5.4) of the low multipoles (l ~ 10 - 50) or the second peak (l ~ 540) of the CMB angular spectrum when the three-year WMAP data are used. Moreover, the results from WMAP are consistent with the CBI, ACBAR, 2dFGRS, and SDSS data when they are included in the analysis. Including the SDSS galaxy clustering power spectrum, we find weak positive evidence for the k^3 component at the level of Delta chi' = 2.4, with the caveat that the nonlinear evolution of the power spectrum may not be properly treated in the presence of the k^3 distortion. To investigate the high-k regime, we use the Lyman-alpha forest data (LUQAS, Croft et al., and SDSS Lyman-alpha); here we find evidence at the level Delta chi^2' = 3.8. Considering that there are two additional free parameters in the model, the above results do not give a strong evidence for features; however, they show that surprisingly large bumps are not ruled out. We give constraints on the ratio between the k^3 component and the nearly scale-invariant component, r_3 < 1.5, over the range of wave numbers 0.0023/Mpc < k < 8.2/Mpc. We also discuss theoretical models which could lead to the k^3 effect, including ordinary hybrid inflation and double D-term inflation models. We show that the well-motivated k^3 component is also a good representative of the generic spikelike feature in the primordial perturbation power spectrum.Comment: 23 pages, 6 figures; added new section on theoretical motivation for k^3 term, and discussion of double D-term hybrid inflation models; title changed, added a new section discussing the generic spikelike features, published in IJMP

Dynamical Fine Tuning in Brane Inflation

We investigate a novel mechanism of dynamical tuning of a flat potential in the open string landscape within the context of warped brane-antibrane inflation in type IIB string theory. Because of competing effects between interactions with the moduli stabilizing D7-branes in the warped throat and anti-D3-branes at the tip, a stack of branes gives rise to a local minimum of the potential, holding the branes high up in the throat. As branes successively tunnel out of the local minimum to the bottom of the throat the potential barrier becomes lower and is eventually replaced by a flat inflection point, around which the remaining branes easily inflate. This dynamical flattening of the inflaton potential reduces the need to fine tune the potential by hand, and also leads to successful inflation for a larger range of inflaton initial conditions, due to trapping in the local minimum.Comment: 23 pages, 9 figures. v2: Updated D3-dependence in potential, small changes to numerical result

Cosmological inflation and the primordial power spectrum

In this thesis, we discuss two important aspects of cosmological inflation, which is a prominent part of modern cosmology.Since inflation typically occurs at ultrahigh energy scales, string theory is a popular framework to achieve inflation. We investigate the fine-tuning problem in string theoretic inflation models, based on a brane-antibrane potential. We phenomenologically quantify the degree of fine-tuning by defining an effective volume in the part of parameter space which is consistent with observational constraints. Moreover, we use Monte Carlo methods to illustrate that the degree of fine-tuning can be substantially alleviated.We also explore a novel mechanism of dynamical tuning of a flat potential in brane-antibrane inflation. Because of competing effects between its interactions with D7-branes and anti-D3-branes, a stack of D3-branes gives rise to a local minimum in the potential. As branes successively tunnel out, the potential barrier is eventually replaced by a flat inflection point, around which the remaining branes easily inflate. We give an example where this time-dependent flattening of the inflaton potential is possible.Another important aspect of inflation is the primordial power spectrum. In this thesis, we consider a primordial power spectrum with a k^3 component, which can be generated through different mechanisms and is a good representative of the generic spikelike features. We explore the possibility that the features could be observably large. Although there is no strong evidence, large spikes are not ruled out by the data, and we give constraints on the k^3 component.Dans cette thèse, nous discutons deux aspects importants de l'inflation cosmologique, qui est une partie importante de la cosmologie moderne.Dans la mesure où l'inflation se produit généralement à des échelles ultra-haute d'énergie, la théorie des cordes est un cadre populaire pour atteindre l'inflation. Nous étudions le problème de mise au point dans les modèles d'inflation provenant de la théorie des cordes, basé sur un potentiel de Brane-antibrane. Phénoménologiquement, nous quantifions le degré de mise au point en définissant un volume effectif dans la partie de l'espace des paramètres qui est compatible avec les contraintes observationnelles. En outre, nous utilisons des méthodes de Monte Carlo pour illustrer le fait que le degré de mise au point peut être considérablement allégé.Nous explorons aussi un nouveau mécanisme de réglage dynamique d'un potentiel plat dans l'inflation Brane-antibrane. En raison des effets de compétition entre ses interactions avec les D7-branes et les anti-D3-branes, une pile de D3-branes donne lieu à un minimum local du potentiel. Comme les branes franchissent la barrière de potentiel l'une après l'autre, cette dernière est finalement remplacée par un point d'inflexion plat, autour duquel les branes restantes s'étendent avec facilité. Nous donnons un exemple où cet aplatissement du potentiel de l'inflaton qui dépend du temps est possible.Un autre aspect important de l'inflation est le spectre de puissance primordial. Dans cette thèse, on considère un spectre de puissance primordial avec un terme en k^3 pouvant être généré par différents mécanismes et qui est un bon représentant des caractéristiques génériques des pics. Nous explorons la possibilité que ces caractéristiques pourraient être observationnellement larges. Bien qu'il n'existe aucune preuve solide, des grands pics ne sont pas exclus par les données, et nous donnent des contraintes sur la composante en k^3

Inflationary spectral indices and potential reconstruction

The Inflationary power spectrum plays an important role in modern cosmology. In this thesis, we studied both the experimental and theoretical aspects of the inflationary spectral index. By exploring the recent WMAP data, we found that the evidence for the running of the spectral index mainly comes from multipoles near l = 40. This fact allows a partial running spectrum to give as good a fit as the WMAP running spectrum. We gave some simple formulae for the inflationary spectral indices based on the Hamilton-Jacobi formulation of inflation. These simple formulae agree with the exact solutions in some special cases. The Hamilton-Jacobi formulation of inflation was also applied to reconstruct inflaton potentials from a given power spectrum. A simple and accurate reconstruction formulation was presented. All analytic potentials giving a constant spectral index are derived, which show that a nearly scale-invariant spectrum can give rise to slow-roll inflation during 60 e-foldings within sub-Planckian inflaton field values and a potential energy V1/4 ~ 1015GeV. Potentials for large running of the spectral index and large tensor-to-scalar ratio were also constructed, which need super-Planckian field values and require that the slow-roll approximation breaks down before reaching 60 e-foldings. We have shown that for the cosmologically interesting scales, a renormalizable potential fits the reconstructed potential for a large running spectrum very well. Our reconstruction formulation also produces a self-consistent tensor spectrum once a scalar spectrum and the tensor-to-scalar ratio are given. Higher order corrections to the slow-roll approximation are also considered. We showed that they can be incorporated straightforwardly into our formulae for spectral indices and the reconstruction formalism