Models Of Brane Inflation Research Articles

Effects of scale-dependent non-Gaussianity on cosmological structures

The detection of primordial non-Gaussianity could provide a powerful means to test variousinflationary scenarios. Although scale-invariant non-Gaussianity (often described by thefNL formalism) is currently best constrained by the CMB, single-field models with changingsound speed can have strongly scale-dependent non-Gaussianity. Such models could evadethe CMB constraints but still have important effects at scales responsible for theformation of cosmological objects such as clusters and galaxies. We compute the effectof scale-dependent primordial non-Gaussianity on cluster number counts as afunction of redshift, using a simple ansatz to model scale-dependent features. Weforecast constraints on these models achievable with forthcoming datasets. We alsoexamine consequences for the galaxy bispectrum. Our results are relevant forthe Dirac–Born–Infeld model of brane inflation, where the scale dependence ofthe non-Gaussianity is directly related to the geometry of the extra dimensions.

Cosmic (Super)String Constraints from 21 cm Radiation

We calculate the contribution of cosmic strings arising from a phase transition in the early Universe, or cosmic superstrings arising from brane inflation, to the cosmic 21 cm power spectrum at redshifts z > or =30. Future experiments can exploit this effect to constrain the cosmic string tension G mu and probe virtually the entire brane inflation model space allowed by current observations. Although current experiments with a collecting area of approximately 1 km2 will not provide any useful constraints, future experiments with a collecting area of 10(4)-10(6) km2 covering the cleanest 10% of the sky can, in principle, constrain cosmic strings with tension G mu > or = 10(-10)-10(-12) (superstring/phase transition mass scale >10(13) GeV).

Energy radiation by cosmic superstrings in brane inflation

The dominant method of energy loss by a loop of cosmic D-strings in models of warped brane inflation is studied. It is shown that the energy loss via Ramond-Ramond field radiation can dominate by many orders of magnitude over the energy radiation via gravitational wave emission. The ratio of these two energy loss mechanisms depends on the energy scale of inflation, the mass scale of string theory, and whether it is a single-throat or a multithroat inflationary scenario. This can have important consequences for the detection of cosmic superstrings in the near future. It is argued that the bounds from cosmic microwave background anisotropies and big bang nucleosynthesis are the dominant cosmological sources to constrain the physical parameters of the network of cosmic superstrings, whereas the role of the gravitational wave-based experiments may be secondary.

Comparing infrared Dirac-Born-Infeld brane inflation to observations

We compare the infrared Dirac-Born-Infeld brane inflation model to observations using a Bayesian analysis. The current data cannot distinguish it from the $\ensuremath{\Lambda}\mathrm{CDM}$ model but is able to give interesting constraints on various microscopic parameters including the mass of the brane moduli potential, the fundamental string scale, the charge or warp factor of throats, and the number of the mobile branes. We quantify some distinctive testable predictions with stringy signatures, such as the large non-Gaussianity, and the large, but regional, running of the spectral index. These results illustrate how we may be able to probe aspects of string theory using cosmological observations.

Multifield Dirac-Born-Infeld inflation and non-Gaussianities

We analyze the trajectories for multifield Dirac-Born-Infeld inflation, which can arise in brane inflation models, and show that the trajectories are the same as in typical slow roll inflation. We calculate the power spectrum and find that the higher derivative terms of the Dirac-Born-Infeld action lead to a suppression of the contribution from the isocurvature perturbations. We also calculate the bispectrum generated by the isocurvature perturbation, and find that it leads to distinctive features.

Constraints on brane inflation and cosmic strings

By considering simple, but representative, models of brane inflation from a singlebrane–antibrane pair in the slow roll regime, we provide constraints on the parametersof the theory imposed by measurements of the cosmic microwave background(CMB) anisotropies by WMAP (Wilkinson Microwave Anisotropy Probe) includinga cosmic string component. We find that inclusion of the string component iscritical in constraining parameters. In the most general model studied, whichincludes an inflaton mass term, as well as the brane–antibrane attraction, valuesns<1.02 are compatiblewith the data at 95% confidence level. We are also able to constrain the volume of the warped throat region(modulo factors dependent on the warp factor) and the value of the inflaton field to be<0.66MP at horizon exit. We also investigate models with a mass term.These observational considerations suggest that such models haver<2 × 10−5, whichcan only be circumvented in the fast roll regime, or by increasing the number of antibranes. Such a valueof r would not be detectable in any CMB polarization experiment likely in the near future, but theB-mode signal from the cosmic strings could be detectable. We present forecasts of what a similaranalysis using PLANCK data would yield and find that it should be possible to rule outGμ>6.5 × 10−8 usingjust the TT, TE and EE power spectra.

Inflation from wrapped branes

We show that the use of higher dimensional wrapped branes can significantly extend the inflaton field range compared to brane inflation models which use $D3$-branes. We construct a simple inflationary model in terms of 5-branes wrapping a 2-cycle and traveling towards the tip of the Klebanov-Strassler throat. Inflation ends when the branes reach the tip of the cone and self-annihilate. Assuming a quadratic potential for the brane it is possible to match the cosmic microwave background data in the Dirac-Born-Infeld regime, but we argue that the backreaction of the brane is important and cannot be neglected. This scenario predicts a strong non-Gaussian signal and possibly detectable gravitational waves.

Wilson line inflation

We present a general setup for inflation in string theory where the inflaton field corresponds to Wilson lines in compact space in the presence of magnetic fluxes. T-dualities and limits on the value of the magnetic fluxes relate this system to the standard D-brane inflation scenarios, such as brane-antibrane inflation, D3/D7 brane inflation and different configurations of branes at angles. This can then be seen as a generalised approach to inflation from open string modes. Inflation ends when the Wilson lines achieve a critical value and an open string mode becomes tachyonic. Then hybrid-like inflation, including its cosmic string remnants, is realized in string theory beyond the brane annihilation picture. Our formalism can be incorporated within flux-induced moduli stabilisation mechanisms in type IIB strings. Also, contrary to the standard D-brane separation, Wilson lines can be considered in heterotic string models. We provide explicit examples to illustrate similarities and differences of our mechanism to D-brane inflation. In particular we present an example in which the η problem present in brane inflation models is absent in our case. We have examples with both blue and red tilted spectral index and remnant cosmic string tension \(G\mu \lesssim 10^{-7}\) .

P-term potentials from 4-D supergravity

P-term inflation arises in some models of brane inflation. Within N = 2 supersymmetry the scalar potential contains a vector of Fayet-Iliopoulos (FI) terms ξi. Depending on the direction of this vector it is possible to get D-term and F-term inflation or a mix of these models. In this paper we review the problems of embedding the P-term model in supergravity and show how these can be solved by considering the truncation from an N = 2 theory to N = 1. We show that with a simple gauging the scalar potential can include F-term or D-term parts but not both. The gauging can be altered so that both F-terms and D-terms containing FI constants can be included. In all cases we display the inflationary trajectory and, if it exists, the supersymmetric minimum.

Predictions of dynamically emerging brane inflation models

We confront the recent proposal of emerging brane inflation with $\mathrm{WMAP}3+\mathrm{SDSS}$, finding a scalar spectral index of ${n}_{s}={0.9659}_{\ensuremath{-}0.0052}^{+0.0049}$ in excellent agreement with observations. The proposal incorporates a preceding phase of isotropic, nonaccelerated expansion in all dimensions, providing suitable initial conditions for inflation. Additional observational constraints on the parameters of the model provide an estimate of the string scale. A graceful exit to inflation and stabilization of extra dimensions is achieved via a string gas. The resulting preheating phase shows some novel features due to a redshifting potential, comparable to effects due to the expansion of the universe itself. However, the model at hand suffers from either a potential overproduction of relics after inflation or insufficient stabilization at late times.

Brane inflation after Wilkinson Microwave Anisotropy Probe three year results

The Wilkinson Microwave Anisotropy Probe (WMAP) three year data favour a red powerspectrum at the level of two standard deviations, which provides a stringent constraint on theinflation models. In this note we use these data to constrain brane inflation models and findthat the KKLMMT (Kachru–Kallosh–Linde–Maldacena–McAllister–Trivedi) model cannot fitWMAP+SDSS (Sloan Digital Sky Survey) data at the level of one standard deviation and a fine-tuning,eight parts in a thousand at least, is needed at the level of two standard deviations.

Fermionic zero modes of supergravity cosmic strings

Recent developments in string theory suggest that cosmic strings could be formed at the end of brane inflation. Supergravity provides a realistic model to study the properties of strings arising in brane inflation. Whilst the properties of cosmic strings in flat space-time have been extensively studied there are significant complications in the presence of gravity. We study the effects of gravitation on cosmic strings arising in supergravity. Fermion zero modes are a common feature of cosmic strings, and generically occur in supersymmetric models. The corresponding massless currents can give rise to stable string loops (vortons). The vorton density in our universe is strongly constrained, allowing many theories with cosmic strings to be ruled out. We investigate the existence of fermion zero modes on cosmic strings in supergravity theories. A general index theorem for the number of zero modes is derived. We show that by including the gravitino, some (but not all) zero modes disappear. This weakens the constraints on cosmic string models. In particular, winding number one cosmic D-strings in models of brane inflation are not subject to vorton constraints. We also discuss the effects of supersymmetry breaking on cosmic D-strings.

Erratum: Observational constraints on cosmic string production during brane inflation [Phys. Rev. D68, 23506 (2003)

Overall, brane inflation is compatible with the recent analysis of the WMAP data. Here we explore the constraints of WMAP and 2dFGRS data on the various brane inflationary scenarios. Brane inflation naturally ends with the production of cosmic strings, which may provide a way to distinguish these models observationally. We argue that currently available data cannot exclude a non-negligible contribution from cosmic strings definitively. We perform a partial statistical analysis of mixed models that include a sub-dominant contribution from cosmic strings. Although the data favor models without cosmic strings, we conclude that they cannot definitively rule out a cosmic-string-induced contribution of $\sim 10 %$ to the observed temperature, polarization and galaxy density fluctuations. These results imply that $G\mu \lesssim 3.5\times 10^{-7}(\lambda/0.25)\sqrt{B/0.1}$, where $\lambda$ is a dimensionless parameter related to the interstring distance, and $B$ measures the importance of perturbations induced by cosmic strings. We argue that, conservatively, the data available currently still permit $B\lesssim 0.1$. Precision measurements sensitive to the B-mode polarization produced by vector density perturbation modes driven by the string network could provide evidence for these models. Accurate determinations of $n_s(k)$, the scalar fluctuation index, could also distinguish among various brane inflation models.

Cycloops: dark matter or a monopole problem for brane inflation?

We consider cosmic loop production by long string interactions in cosmological models with compact extra dimensions. In the case that the compact manifold is not simply connected, we focus on the possibility of loops wrapping around non-trivial cycles and becoming topologically trapped. Such loops, denoted cycloops, behave like matter in the radiation era, posing a potential monopole problem. We calculate the number distribution and the energy density of these objects as functions of cosmic time and use them to study cosmological constraints imposed on simple brane inflation models. For typical choices of parameters we find that to avoid cycloop domination before the matter-radiation transition, the strings must be unacceptably light, namely $G\mu<10^{-18}$, unless some mechanism to dilute the cycloops is provided. By exploring the full parameter space however, we are able to find models with $G\mu \sim 10^{-14}$, which is marginally consistent with brane inflation. In such models the cycloop could provide an interesting dark matter candidate.

Strings at the bottom of the deformed conifold

We present solutions of the equations of motion of macroscopic F and D strings extending along the non compact 4D sections of the conifold geometry and winding around the internal directions. The effect of the Goldstone modes associated with the position of the strings on the internal manifold can be seen as a current on the string that prevents it from collapsing and allows the possibility of static 4D loops. Its relevance in recent models of brane inflation is discussed.

Braneworld flux inflation

We propose a geometrical model of brane inflation where inflation is driven by the fluxgenerated by opposing brane charges and terminated by the collision of the branes, withcharge annihilation. We assume the collision process is completely inelastic and the kineticenergy is transformed into the thermal energy after collision. Thereafter the two branescoalesce together and behave as a single-brane universe with zero effective cosmologicalconstant. In the Einstein frame, the four-dimensional effective theory changesabruptly at the collision point. Therefore, our inflationary model is necessarilyfive-dimensional in nature. As the collision process has no singularity in five-dimensionalgravity, we can follow the evolution of fluctuations during the whole history of theuniverse. It turns out that the radion field fluctuations have a steeply tilted, redspectrum, while the primordial gravitational waves have a flat spectrum. Instead,primordial density perturbations could be generated by a curvaton mechanism.

The fate of massive F-strings

We calculate the semi-inclusive decay rate of an average string state with compactification both in the bosonic string theory and in the superstring theory. We also apply this calculation to a brane-inflation model in a warped geometry and find that the decay rate is greatly suppressed if final strings are all massive and enhanced with one final string massless, which may pose a challenge to this class of models.

Brane necklaces and brane coils

We investigate the evolution of the networks of the cosmic strings in angled brane inflation. We show how they can be distinguished from the conventional ones. The cosmic strings in angled inflation are the daughter $D_{p-2}$ branes that are extended between the mother $D_p$ branes. In the effective action, the strings should have a moduli, since the endpoints of the $D_{p-2}$ branes can move freely on the $D_p$ branes. Then naturally the position of the $D_{p-2}$ branes, which corresponds to the moduli of the $(1+1)$-dimensional effective action, can vary along the cosmic strings. The variation of the moduli results in the peculiar $(1+1)$-dimensional kink configurations. The kinks are the monopoles on the strings. Therefore, the cosmic strings in angled inflation become necklaces. The loops of the necklaces can shrink to produce stable winding states, which look like coils. We show why the cosmological implications of the brane necklaces are important. We point out that the cosmic strings in generic models of brane inflation should become necklaces, depending on the structure of the compactified space and the effective potential of the model.

Relic gravitons on Kasner-like branes

We discuss the cosmological amplification of tensor perturbations in a simple example of brane-world scenario, in which massless gravitons are localized on a higher-dimensional Kasner-like brane embedded in a bulk AdS background. Particular attention is paid to the canonical normalization of the quadratic action describing the massless and massive vacuum quantum fluctuations, and to the exact mass-dependence of the amplitude of massive fluctuations on the brane. The perturbation equations can be separated. In contrast to de Sitter models of brane inflation, we find no mass gap in the spectrum and no enhancement for massless modes at high curvature. The massive modes can be amplified, with mass-dependent amplitudes, even during inflation and in the absence of any mode-mixing effect.

Moduli stabilization with the string Higgs effect

We review the notion of the Higgs effect in the context of string theory. We find that by including this effect in time dependent backgrounds, one is led to a natural mechanism for stabilizing moduli at points of enhanced gauge symmetry. We consider this mechanism for the case of the radion (size of the extra dimensions) and find that as decompactification of the large spatial dimensions takes place the radion will remain stabilized at the self-dual radius. We discuss how this mechanism can be incorporated into models of string cosmology and brane inflation to resolve some outstanding problems. We also address some issues regarding which string states should be included when constructing low energy actions in string cosmology.