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16
Towards inflation in string theory
 JCAP
, 2003
"... We investigate the embedding of brane inflation into stable compactifications of string theory. At first sight a warped compactification geometry seems to produce a naturally flat inflaton potential, evading one wellknown difficulty of braneantibrane scenarios. Careful consideration of the closed ..."
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Cited by 82 (9 self)
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We investigate the embedding of brane inflation into stable compactifications of string theory. At first sight a warped compactification geometry seems to produce a naturally flat inflaton potential, evading one wellknown difficulty of braneantibrane scenarios. Careful consideration of the closed string moduli reveals a further obstacle: superpotential stabilization of the compactification volume typically modifies the inflaton potential and renders it too steep for inflation. We discuss the nongeneric conditions under which this problem does not arise. We conclude that brane inflation models can only work if restrictive assumptions about the method of volume stabilization, the warping of the internal space, and the source of inflationary energy are satisfied. We argue that this may not be a real problem, given the large range of available fluxes and background geometries in string theory.
Proeyen, FayetIliopoulos terms in supergravity and cosmology
"... We clarify the structure of N = 1 supergravity in 1+3 dimensions with constant Fayet–Iliopoulos (FI) terms. The FI terms gξ induce nonvanishing Rcharges for the fermions and the superpotential. Therefore the Dterm inflation model in supergravity with constant FI terms has to be revisited. We pres ..."
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Cited by 59 (3 self)
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We clarify the structure of N = 1 supergravity in 1+3 dimensions with constant Fayet–Iliopoulos (FI) terms. The FI terms gξ induce nonvanishing Rcharges for the fermions and the superpotential. Therefore the Dterm inflation model in supergravity with constant FI terms has to be revisited. We present all corrections of order gξ/M 2 P to the classical supergravity action required by local supersymmetry and provide a gaugeanomalyfree version of the model. We also investigate the case of the socalled anomalous U(1) when a chiral superfield is shifted under U(1). In such a case, in the context of string theory, the FI terms originate from the derivative of the Kähler potential and they are inevitably fielddependent. This raises an issue of stabilization of the relevant field in applications to cosmology. The recently suggested equivalence between the Dterm strings and Dbranes of type II theory shows that braneantibrane systems produce FI terms in the effective 4d theory, with the RamondRamond axion shifting under the U(1) symmetry. This connection gives the possibility to interpret many unknown properties of D− ¯ D systems in the more familiar language of 4d supergravity Dterms, and vice versa. For instance, the shift of the axion field in both cases restricts the possible
An inflaton mass problem in string inflation from threshold corrections to volume stabilization
 LETT. B
, 2005
"... Inflationary models whose vacuum energy arises from a Dterm are believed not to suffer from the supergravity eta problem of Fterm inflation. That is, Dterm models have the desirable property that the inflaton mass can naturally remain much smaller than the Hubble scale. We observe that this advan ..."
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Cited by 15 (1 self)
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Inflationary models whose vacuum energy arises from a Dterm are believed not to suffer from the supergravity eta problem of Fterm inflation. That is, Dterm models have the desirable property that the inflaton mass can naturally remain much smaller than the Hubble scale. We observe that this advantage is lost in models based on string compactifications whose volume is stabilized by a nonperturbative superpotential: the Fterm energy associated with volume stabilization causes the eta problem to reappear. Moreover, any shift symmetries introduced to protect the inflaton mass will typically be lifted by threshold corrections to the volumestabilizing superpotential. Using threshold corrections computed by Berg, Haack, and Körs, we illustrate this point in the example of the D3D7 inflationary model, and conclude that inflation is possible, but only for finetuned values of the stabilized moduli. More generally, we conclude that inflationary models in stable string compactifications, even Dterm models with shift symmetries, will require a certain amount of finetuning to avoid this new contribution to the eta problem
Coupling hybrid inflation to moduli
 JCAP 0609 (2006) 012 [hepth/0606140
"... Abstract. Hybrid inflation can be realised in lowenergy effective string theory, as described using supergravity. We find that the coupling of moduli to the hybrid inflation sector in supergravity leads to a slope and a curvature for the inflaton potential. The ǫ and η parameters receive contributi ..."
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Cited by 8 (5 self)
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Abstract. Hybrid inflation can be realised in lowenergy effective string theory, as described using supergravity. We find that the coupling of moduli to the hybrid inflation sector in supergravity leads to a slope and a curvature for the inflaton potential. The ǫ and η parameters receive contributions at tree and one loop level which are not compatible with slow roll inflation. Furthermore the coupling to the moduli sector can even prevent inflation from ending at all. We show that introducing shift symmetries in the inflationary sector and taking the moduli sector to be noscale removes most of these problems. If the moduli fields are fixed during inflation, as is usually assumed, it appears that viable slowroll inflation can then be obtained with just one finetuning of the moduli sector parameters. However, we show this is not a reasonable assumption, and that the small variation of the moduli fields during inflation gives a significant contribution to the effective inflaton potential. This typically implies that η ≈ −6.
Moduli stabilisation and applications in IIB string theory,” Fortsch. Phys
"... Cambridge University 2006Per varios casus, per tot discrimina rerum tendimus in Latium; sedes ubi fata quietas ostendunt; illic fas regna resurgere Troiae. Durate, et vosmet rebus servate secundis. Aeneid I:2047Acknowledgements I have many people to thank. I am particularly grateful to my superviso ..."
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Cited by 4 (0 self)
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Cambridge University 2006Per varios casus, per tot discrimina rerum tendimus in Latium; sedes ubi fata quietas ostendunt; illic fas regna resurgere Troiae. Durate, et vosmet rebus servate secundis. Aeneid I:2047Acknowledgements I have many people to thank. I am particularly grateful to my supervisor Fernando Quevedo for the physics he has taught me, for his continual encouragement and for his generosity of time and spirit in nurturing me in the world of science. On the subjects contained in this thesis I have learned many things from my collaborators Vijay Balasubramanian, Per Berglund, Fernando Quevedo and Kerim Suruliz. I am also grateful to Ralph Blumenhagen and Kerim Suruliz for collaboration on the unrelated paper [1] not described in this thesis. Within Cambridge I have had stimulating discussions on various topics with many people including
SUITP04/01 hepth/0402047 Volume Stabilization and the Origin of the Inflaton Shift Symmetry in String Theory
, 2004
"... One of the main problems of inflation in string theory is finding models with a flat potential while simultaneously stabilizing the volume of the compactified space. This can be achieved in theories where the potential has (an approximate) shift symmetry in the inflaton direction. We will identify a ..."
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One of the main problems of inflation in string theory is finding models with a flat potential while simultaneously stabilizing the volume of the compactified space. This can be achieved in theories where the potential has (an approximate) shift symmetry in the inflaton direction. We will identify a class of models where the shift symmetry uniquely follows from the underlying mathematical structure of the theory. It is related to the symmetry properties of the corresponding coset space and the period matrix of special geometry, which shows how the gauge coupling depends on the volume and the position of the branes. In particular, for type IIB string theory on K3 × T 2 /Z2 with D3 or D7 moduli belonging to vector multiplets,) the shift symmetry is a part of SO(2,2+n) symmetry of the coset space
Update of D3/D7Brane Inflation on K3 . . .
, 2008
"... We update the D3/D7brane inflation model on K3×T 2 /Z2 with branes and fluxes. For this purpose, we study the low energy theory including gs corrections to the gaugino condensate superpotential that stabilizes the K3 volume modulus. The gauge kinetic function is verified to become holomorphic when ..."
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We update the D3/D7brane inflation model on K3×T 2 /Z2 with branes and fluxes. For this purpose, we study the low energy theory including gs corrections to the gaugino condensate superpotential that stabilizes the K3 volume modulus. The gauge kinetic function is verified to become holomorphic when the original N = 2 supersymmetry is spontaneously broken to N = 1 by bulk fluxes. From the underlying classical N = 2 supergravity, the theory inherits a shift symmetry which provides the inflaton with a naturally flat potential. We analyze the fate of this shift symmetry after the inclusion of quantum corrections. The field range of the inflaton is found to depend significantly on the complex structure of the torus but is independent of its volume. This allows for a large kinematical field range for the inflaton. Furthermore, we show that the D3/D7 model may lead to a realization of the recent CMB fit by Hindmarsh et al. with an 11 % contribution from cosmic strings and a spectral index close to ns = 1. On the other hand, by a slight change of the parameters of the model one can strongly suppress the cosmic string contribution and reduce the spectral index ns to fit the WMAP5 data in the absence of cosmic strings. We also demonstrate that the inclusion of quantum corrections allows for a regime of eternal D3/D7 inflation.