Stephen C. Davis

N=1 Supersymmetric Cosmic Strings

Abstract We investigate the microphysics of supersymmetric cosmic strings. In particular we focus on the vortices admitted by N = 1 supersymmetric abelian Higgs models. We give the vortex solutions and demonstrate that the two simplest supersymmetric cosmic string models admit fermionic superconductivity. Further, by using supersymmetry transformations, we show how to solve for the fermion zero-modes giving rise to string superconductivity in terms of the background string fields. We note that this constrains all supersymmetric grand unified theories with abelian strings.

Supergravity chaotic inflation and moduli stabilization

Chaotic inflation predicts a large gravitational wave signal which can be tested by the upcoming Planck satellite. We discuss a supergravity implementation of chaotic inflation in the presence of moduli fields, and find that inflation does not work with a generic Kachru–Kallosh–Linde–Trivedi moduli stabilization potential. A viable model can be constructed with a fine-tuned moduli sector, but only for a very specific choice of Kahler potential. Our analysis also shows that inflation models satisfying for all inflation sector fields i can be combined successfully with a fine-tuned moduli sector.

Cosmic strings, zero modes, and supersymmetry breaking in non-Abelian N=1 gauge theories

We investigate the microphysics of cosmic strings in non-Abelian gauge theories with N=1 supersymmetry. We give the vortex solutions in a specific example and demonstrate that fermionic superconductivity arises because of the couplings and interactions dictated by supersymmetry. We then use supersymmetry transformations to obtain the relevant fermionic zero modes and investigate the role of soft supersymmetry breaking on the existence and properties of the superconducting strings. {copyright} {ital 1998} {ital The American Physical Society}

Cosmic String Zero Modes and Multiple Phase Transitions

The zero modes and current carrying capability of a cosmic string formed at one phase transition can be modified at subsequent phase transitions. A new, generalised index theorem is derived that is applicable to theories with multiple phase transitions. This enables us to investigate the fate of string zero modes during sequences of phase transitions in a variety of models. Depending on the couplings that the breaking introduces, the zero modes may be destroyed and the superconductivity of the string removed, and thus vortons dissipate. We discuss the features of the theory that are required to produce this behaviour and consider the implications of spectral flow.

Warping and F-term uplifting

We analyse the effective supergravity model of a warped compactification with matter on D3 and D7-branes. We find that the main effect of the warp factor is to modify the F-terms while leaving the D-terms invariant. Hence warped models with moduli stabilisation and a small positive cosmological constant resulting from a large warping can only be achieved with an almost vanishing D-term and a F-term uplifting. By studying string-motivated examples with gaugino condensation on magnetised D7-branes, we find that even with a vanishing D-term, it is difficult to achieve a Minkowski minimum for reasonable parameter choices. When coupled to an ISS sector the AdS vacua is uplifted, resulting in a small gravitino mass for a warp factor of order 10−5.

The Robustness of n(s) <~ 0.95 in racetrack inflation

A spectral index appears to be a generic prediction of racetrack inflation models. Reducing a general racetrack model to a single-field inflation model with a simple potential, we obtain an analytic expression for the spectral index, which explains this result. By considering the limits of validity of the derivation, possible ways to achieve higher values of the spectral index are described, although these require further fine-tuning of the potential.A spectral index appears to be a generic prediction of racetrack inflation models. Reducing a general racetrack model to a single-field inflation model with a simple potential, we obtain an analytic expression for the spectral index, which explains this result. By considering the limits of validity of the derivation, possible ways to achieve higher values of the spectral index are described, although these require further fine-tuning of the potential.

Mimicking general relativity in the solar system

In order for a modified gravity model to be a candidate for cosmological dark energy, it has to pass stringent local gravity experiments. We find that a Brans-Dicke (BD) theory with well-defined second order corrections that include the Gauss-Bonnet term possesses this feature. We construct the generic second order theory that gives, to linear order, a BD metric solution for a pointlike mass source. We find that the Eddington parameter {gamma}, heavily constrained by time delay experiments, can be arbitrarily close to the general relativity value of 1, with an arbitrary BD parameter {omega}{sub BD}. We find the region where the solution is stable to small timelike perturbations.

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.

Microphysics of SO(10) cosmic strings

We uncover a rich microphysical structure for SO(10) cosmic strings. For the abelian string the electroweak symmetry is restored around it in a region depending on the electroweak scale. A rich structure of nonabelian strings is found. Some of these also restore the electroweak symmetry. We investigate the zero mode structure of our strings. Whilst there are right handed neutrino zero modes for the abelian string, they do not survive the electroweak phase transition. In general the nonabelian strings do not have fermion zero modes. We consider the generalisation of our results to other theories and consider cosmological consequences of them.

Cosmic D-strings and vortons in supergravity

Abstract Recent developments in string inspired models of inflation suggest that D-strings are formed at the end of inflation. Within the supergravity model of D-strings there are 2 ( n − 1 ) chiral fermion zero modes for a D-string of winding n. Using the bounds on the relic vorton density, we show that D-strings with winding number n > 1 are more strongly constrained than cosmic strings arising in cosmological phase transitions. The D-string tension of such vortons, if they survive until the present, has to satisfy 8 π G N μ ≲ p × 10 −26 where p is the intercommutation probability. Similarly, D-strings coupled with spectator fermions carry currents and also need to respect the above bound. D-strings with n = 1 do not carry currents and evade the bound. We discuss the coupling of D-strings to supersymmetry breaking. When a single U ( 1 ) gauge group is present, we show that there is an incompatibility between spontaneous supersymmetry breaking and cosmic D-strings. We propose an alternative mechanism for supersymmetry breaking, which includes an additional U ( 1 ) , and might alleviate the problem. We conjecture what effect this would have on the fermion zero modes.