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40
Noncommutative Geometry Modified NonGaussianities of Cosmological Perturbation
, 707
"... We investigate the noncommutative effect on the nonGaussianities of primordial cosmological perturbation. In the lowest order of string length and slowroll parameter, we find that in the models with small speed of sound the noncommutative modifications could be observable if assuming a relatively ..."
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We investigate the noncommutative effect on the nonGaussianities of primordial cosmological perturbation. In the lowest order of string length and slowroll parameter, we find that in the models with small speed of sound the noncommutative modifications could be observable if assuming a relatively low string scale. In particular, the dominant modification of nonGaussianity estimator fNL could reach O(1) in DBI inflation and Kinflation. The corrections are sensitive to the speed of sound and the choice of string length scale. Moreover the shapes of the corrected nonGaussianities are distinct from that of ordinary ones. 1
Cosmology of the Tachyon in Brane Inflation
, 2006
"... Abstract: 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 (c ..."
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Abstract: 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 slowroll with tiny inflaton mass) allows for the tachyon to contribute some efolds to inflation. In addition, nonadiabatic 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 efolds before the end of inflation. The additional contribution is very nearly scaleinvariant and enhances the tensor/scalar ratio. Additional nongaussianities will also be generated, sharpening current constraints on DBItype
Beyond the Cosmological Standard Model
, 2014
"... After a decade and a half of research motivated by the accelerating universe, theory and experiment have a reached a certain level of maturity. The development of theoretical models beyond Λ or smooth dark energy, often called modified gravity, has led to broader insights into a path forward, and a ..."
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After a decade and a half of research motivated by the accelerating universe, theory and experiment have a reached a certain level of maturity. The development of theoretical models beyond Λ or smooth dark energy, often called modified gravity, has led to broader insights into a path forward, and a host of observational and experimental tests have been developed. In this review we present the current state of the field and describe a framework for anticipating developments in the next decade. We identify the guiding principles for rigorous and consistent modifications of the standard model, and discuss the prospects for empirical tests. We begin by reviewing recent attempts to consistently modify Einstein gravity in the infrared, focusing on the notion that additional degrees of freedom introduced by the modification must “screen” themselves from local tests of gravity. We categorize screening mechanisms into three broad classes: mechanisms which become active in regions of high Newtonian potential, those in which first derivatives of the field become important, and those for which second derivatives of the field are important. Examples of the first class, such as f(R) gravity, employ the familiar chameleon or symmetron mechanisms, whereas examples of the last class are galileon
hepth/0605045 Observational Signatures and NonGaussianities of General Single Field Inflation
"... We perform a general study of primordial scalar nonGaussianities in single field inflationary models. We consider models where the inflaton Lagrangian is an arbitrary function of the scalar field and its first derivative, and the sound speed is arbitrary. We find that under reasonable assumptions, ..."
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We perform a general study of primordial scalar nonGaussianities in single field inflationary models. We consider models where the inflaton Lagrangian is an arbitrary function of the scalar field and its first derivative, and the sound speed is arbitrary. We find that under reasonable assumptions, the nonGaussianity is completely determined by 5 parameters. In special limits of the parameter space, one finds distinctive “shapes ” of the nonGaussianity. In models with a small sound speed, several of these shapes would become potentially observable in the near future. Different limits of our formulae recover various previously known results. Submitted to JCAP
Reconstructing Single Field Inflationary Actions From CMBR Data. ∗
, 804
"... This paper describes a general program for deriving the action of single field inflation models with nonstandard kinetic energy terms using CMBR power spectrum data. This method assumes that an action depends on a set of undetermined functions, each of which is a function of either the inflaton wave ..."
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This paper describes a general program for deriving the action of single field inflation models with nonstandard kinetic energy terms using CMBR power spectrum data. This method assumes that an action depends on a set of undetermined functions, each of which is a function of either the inflaton wave function or its time derivative. The scalar, tensor and nongaussianity of the curvature perturbation spectrum are used to derive a set of reconstruction equations whose solution set can specify up to three of the undetermined functions. The method is then used to find the undetermined functions in various types of actions assuming power law type scalar and tensor spectra. In actions that contain only two unknown functions, the third reconstruction equation implies a consistency relation between the nongaussianty, sound speed and slow roll parameters. In particular we focus on reconstructing a generalized DBI action with an unknown potential and warp factor. We find that for realistic scalar and tensor spectra, the reconstructed warp factor and potential are very similar to the theoretically derived result. Furthermore, physical consistency of the reconstructed warp factor and potential imposes strict constraints on the scalar and tensor spectral indices. 1
Internal Examiner: Dr. Robert Crittenden Contents
, 2008
"... The thesis is submitted in partial fulfilment of the requirements ..."
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Dynamics of D3D7 Brane Inflation in Throats
, 807
"... Abstract: Dynamics of D3branes in models of warped D3D7 inflationary set up is studied where perturbative α ′ correction to the Kähler potential and the nonperturbative corrections to the superpotential are included. It is shown that a dS minimum can be obtained without introducing antibranes. So ..."
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Abstract: Dynamics of D3branes in models of warped D3D7 inflationary set up is studied where perturbative α ′ correction to the Kähler potential and the nonperturbative corrections to the superpotential are included. It is shown that a dS minimum can be obtained without introducing antibranes. Some specific configurations of D7branes embedding were studied. After stabilizing the angular directions, it is shown that the resulting D3D7 potential of the radial position of the D3brane is too steep to allow slowroll inflation.
MITCTP3961 FineTuning in DBI Inflationary Mechanism
, 807
"... We show a modelindependent finetuning issue in the DBI inflationary mechanism. DBI inflation requires a warp factor h small enough to sufficiently slow down the inflaton. On the other hand, the Einstein equation in extra dimensions under the inflationary background deforms the warp space in the IR ..."
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We show a modelindependent finetuning issue in the DBI inflationary mechanism. DBI inflation requires a warp factor h small enough to sufficiently slow down the inflaton. On the other hand, the Einstein equation in extra dimensions under the inflationary background deforms the warp space in the IR side. Generically these two locations coincide with each other, spoiling the DBI inflation. The origin and tuning of this “hproblem ” is closely related, through the AdS/CFT duality, to those of the wellknown “ηproblem ” in the slowroll One important motivation for the DBI (DiracBornInfeld) inflation [1,2] is to circumvent the ηproblem in the slowroll inflation [3]. Namely, the required inflaton masssquared for the slowroll inflation, � O(0.01H 2), is generically corrected to O(H 2) due to the backreaction of the inflationary background. This leads to a potential too steep to support the slowroll
Warped KaluzaKlein Dark Matter
, 908
"... Abstract: Warped compactifications of type IIB string theory contain natural dark matter candidates: KaluzaKlein modes along approximate isometry directions of long warped throats. These isometries are broken by the full compactification, including moduli stabilization; we present a thorough survey ..."
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Abstract: Warped compactifications of type IIB string theory contain natural dark matter candidates: KaluzaKlein modes along approximate isometry directions of long warped throats. These isometries are broken by the full compactification, including moduli stabilization; we present a thorough survey of KaluzaKlein mode decay rates into light supergravity modes and Standard Model particles. We find that these dark matter candidates typically have lifetimes longer than the age of the universe. Interestingly, some choices for embedding the Standard Model in the compactification lead to decay rates large enough to be observed, so this dark matter sector may provide constraints on the parameter space of the compactification.