String (In)Stability Issues with Broken Supersymmetry

Physical Review D, 2003

It has been suggested that the observed value of the cosmological constant is related to the supersymmetry breaking scale M susy through the formula Λ ∼ M 4 p (M susy /M p) 8. We point out that a similar relation naturally arises in the codimension two solutions of warped spacetime varying compactifications of string theory in which non-isotropic stringy moduli induce a small but positive cosmological constant.

2021

We investigate the effects of the leading tadpole potentials of 10D tachyon-free non-supersymmetric strings in warped products of flat geometries of the type M p+1 × R × T 10−p−2 depending on a single coordinate. In the absence of fluxes and for p < 8, there are two families of these vacua for the orientifold disk-level potential, both involving a finite internal interval. Their asymptotics are surprisingly captured by tadpole-free solutions, isotropic for one family and anisotropic at one end for the other. In contrast, for the heterotic torus-level potential there are four types of vacua. Their asymptotics are always tadpole-dependent and isotropic at one end lying at a finite distance, while at the other end, which can lie at a finite or infinite distance, they can be tadpole-dependent isotropic or tadpole-free anisotropic. We then elaborate on the general setup for including symmetric fluxes, and present the three families of exact solutions that emerge when the orientifold potential and a seven-form flux are both present. These solutions include a pair of boundaries, which are always separated by a finite distance. In the neighborhood of one, they all approach a common supersymmetric limit, while the asymptotics at the other boundary can be tadpole-free isotropic, tadpole-free anisotropic or again supersymmetric. We also discuss corresponding cosmologies, with emphasis on their climbing or descending behavior at the initial singularity. In some cases the toroidal dimensions can contract during the cosmological expansion.

Nuclear Physics B, 2009

We extend the analysis of the recently obtained stringy cosmological solutions induced by thermal and quantum effects, once space-time supersymmetry is spontaneously broken by geometrical fluxes. Cases in which more than one modulus participating in the supersymmetry breaking mechanism are investigated. The free energy is obtained at the full string level. In the intermediate cosmological region where the temperature and the supersymmetry breaking scale are sufficiently smaller than the Hagedorn temperature, the quantum and thermal corrections are under control and calculable. The reason is that the contributions to the effective potential of the moduli that are not participating in the supersymmetry breaking are exponentially suppressed. The backreaction on the initially flat background results in many cases into cosmological evolutions, where the dynamics of all complex structure moduli is frozen. The solutions describe effectively a radiation dominated era, where thermal effects are never negligible, even if the temperature tends to zero at late times. We analyze several types of supersymmetry breaking patterns and examine the stability of the corresponding radiation era.

Nucl.Phys. B877 (2013) 1028-1106

"We build a number of integrable one–scalar spatially flat cosmologies, which play a natural role in inflationary scenarios, examine their behavior in several cases and draw from them some general lessons on this type of systems, whose potentials involve combinations of exponential functions, and on similar non–integrable ones. These include the need for the scalar to emerge from the initial singularity while climbing up sufficiently steep exponential potentials (“climbing phenomenon”) and the inevitable collapse in a big Crunch whenever the scalar tries to settle at negative extrema of the potential. We also elaborate on the links between these types of potentials and “brane supersymmetry breaking”, a mechanism that ties together string scale and scale of supersymmetry breaking in a class of orientifold models. We show that, under some assumptions that are spelled out in the text, the extended objects of these vacua can inject inflationary phases with discrete values of the spectral index that are determined by the number of unwrapped dimensions of the branes and by the inverse power with which the string coupling gs enters their world–volume actions. An NS fivebrane, which is interestingly unstable in this class of models, when wrapped on a small internal cycle would yield a spectral index that is amusingly close to the experimentally favored PLANCK value ns ≈ 0.96."

Arxiv preprint astro-ph/0303499, 2003

The second string revolution, which begin around 1995, has led to a drastic alteration in our perception of the universe, perhaps even more so then did the first string revolution of 1984. That is, extending 10-dimensional string theory to 11-dimensional M -theory has had more profound implications than did the original extension of 4-dimensional quantum mechanics and relativity to 10-dimensional string theory. After a brief review of M -theory, I discuss some implications of large extra dimensions. I then consider astronomical evidence for, and constraints on, large compactified dimensions. I conclude with a possible resolution to the apparent inconsistency between the MSSM scale and string scale in the weak coupling limit.

Nuclear Physics B, 2008

In the framework of superstring compactifications with N = 1 supersymmetry spontaneously broken, (by either geometrical fluxes, branes or else), we show the existence of new inflationary solutions. The time-trajectory of the scale factor of the metric a, the supersymmetry breaking scale m ≡ m(Φ) and the temperature T are such that a m and a T remain constant. These solutions request the presence of special moduli-fields: i) The universal "no-scale-modulus" Φ, which appears in all N = 1 effective supergravity theories and defines the supersymmetry breaking scale m(Φ).

2015

String theory is one of the most active branches of theoretical physics and constitutes the most promising candidate for a consistent fundamental theory describing our Universe. Its structure provides a uni ed description of all known particles and forces of Nature, including gravity, in a single quantum mechanical framework. Its low-energy limit, determined by compacti cations of extra dimensions on Calabi-Yau orientifolds, yields an e ective theory which represents the key tool in order to make contact with experiments and observable physics. This theory is Supergravity, with the natural presence of a moduli hidden sector, which consists in a collection of gravitationally coupled scalar elds inherited from KaluzaKlein dimensional reduction. In order to avoid non-observed long range fth-forces mediated by massless moduli, these elds have to develop a large mass via the process of moduli stabilisation. The stabilisation of the moduli provides also a dynamical supersymmetry-breaking ...

2009

We review recent progress in string cosmology, where string dualities are applied so as to obtain complete cosmological evolutions, free of any essential singularities. Two classes of models are analyzed. The first class consists of string gas cosmologies associated to certain thermal configurations of type II N=(4,0) models. Finite temperature is introduced along with non-trivial &quot;gravito-magnetic&quot; fluxes that lift the Hagedorn instabilities of the canonical ensemble and restore thermal T-duality symmetry. At a critical maximal temperature additional thermal states become massless sourcing stringy S-branes, which facilitate a bounce between the two dual, asymptotically cold phases. Unlike previous incarnations of pre-Big Bang cosmologies, the models remain perturbative throughout the cosmological evolution. The second class consists of exact solutions to classical string theory that admit a Euclidean description in terms of compact parafermionic worldsheet systems. The Eu...

Advances in Space Research, 2005

The second string revolution, which begin around 1995, has led to a drastic alteration in our perception of the universe, perhaps even more so then did the first string revolution of 1984. That is, extending 10-dimensional string theory to 11-dimensional M -theory has had more profound implications than did the original extension of 4-dimensional quantum mechanics and relativity to 10-dimensional string theory. After a brief review of M -theory, I discuss some implications of large extra dimensions. I then consider astronomical evidence for, and constraints on, large compactified dimensions. I conclude with a possible resolution to the apparent inconsistency between the MSSM scale and string scale in the weak coupling limit.