Richard Madden

Towards a singularity - free inflationary universe?

Abstract We consider the problem of constructing a non-singular inflationary universe in stringy gravity via branch changing, from a previously superexponentially expanding phase to an FRW-like phase. Our approach is based on the phase space analysis of the dynamics, and we obtain a no-go theorem which rules out the efficient scenario of branch changing catalyzed by dilaton potential and stringy fluid sources. We furthermore consider the effects of string-loop corrections to the gravitational action in the form recently suggested by Damour and Polyakov. These corrections also fail to produce the desired branch change. However, focusing on the possibility that these corrections may decouple the dilaton, we deduce that they may lead to an inflationary expansion in the presence of a cosmological constant, which asymptotically approaches Einstein-de Sitter solution.

Axions and the graceful exit problem in string cosmology

Abstract We reexamine the graceful exit problem in the Pre-Big-Bang inflationary scenario. The dilaton-gravity action is generalized by adding the axion and a general axion/dilaton potential. We provide a phase space analysis of the dynamics which leads us to extend the previous no-go theorem and rule out the branch change necessary for graceful exit in this context.

Corrections to bino annihilation. I. Sfermion mixing

We consider corrections to bino annihilation due to sfermion mixing. Within the standard model of electroweak interactions, the only plausible dark matter candidate is the neutrino. A neutrino (ντ , for example) with a mass of 10-30 eV would make a significant contribution to the overall mass density of the Universe. A light neutrino, however, is a hot dark matter candidate and as such cannot account for the observed structure on small (galactic) scales. Cold dark matter alone also has its difficulties. It is difficult to make large scale structure and conflicts with observation of the microwave background anisotropies. At present, the best mix of dark matter and baryons seems to be 60-70% cold, about 30% hot, and the remainder in baryons (see [1] and references therein). Cold dark matter still appears to be a necessity. Before their demise, heavy (masses > ∼ a few GeV) neutrinos were by far the simplest cold dark matter candidates. The minimal supersymmetric standard model (MSSM) is perhaps the simplest logical extension which offers a cold dark matter candidate. Because of an unbroken discrete symmetry, R-parity, the lightest supersymmetric partner (LSP) is expected to be stable. This makes the LSP a natural candidate for role of the non-baryonic dark matter. The identity of the LSP is concealed by the relatively large number of unknown parameters of the MSSM. Nevertheless, by a combination of i) theoretical presumptions (eg that the MSSM will eventually be contained in a grand unified theory), ii) limits from accelerator experiments and iii) cosmological bounds, it is possible to identify the more plausible regions of the parameter space. In previous studies, the lightest neutralino, a linear combination of the supersymmetric partners of the neutral SU(2) gauge boson (neutral wino), U(1) hypercharge gauge boson (bino), and the two neutral components of the Higgs doublets (higgsinos) has been suggested as the most likely candidate for the LSP [2]. The mass matrix for these states contains three unknown parameters: (1) M2, the supersymmetry breaking SU(2)L gaugino mass; (2) ǫ, the higgsino mixing mass; and (3) tanβ = v1/v2, the ratio of the vacuum expectation values of the Higgs doublets. In a large portion of this parameter space, the least massive eigenstate is a nearly pure bino [3]. Restricting our attention to the bino B̃ as the candidate LSP, it is straightforward to use its couplings to the other fields in the MSSM to compute the relic abundance. These calculations were previously performed under a variety of assumptions: degenerate squark and

Physical properties of four-dimensional superstring gravity black hole solutions

Abstract We consider the physical properties of four-dimensional black hole solutions to the effective action describing the low energy dynamics of the gravitational sector of heterotic superstring theory. We compare the properties of the external field strengths in the perturbative solution to the full O(α′) string effective action equations, to those of exact solutions in a truncated action for charged black holes, and to the Kerr-Newman family of solutions of Einstein-Maxwell theory. We contrast the numerical results obtained in these approaches, and discuss limitations of the analyses. Finally we discuss how the new features of classical string gravity affect the standard tests of general relativity.

Corrections to bino annihilation II: one-loop contribution to B̃B̃ → Z☆

Abstract We calculate the one-loop contribution to the bino annihilation rate due to the process B B → Z ∗ , which vanishes at tree level.