Closing the Universe by relaxing the cosmological constant

We propose a string-inspired model which correlates several aspects of particle physics and cosmology. Inspired by the flat directions and the absence of adjoint Higgs representations found in typical string models, we consider a no-scale $SU(5)\times U(1)$ supergravity model. This model entails well determined low-energy phenomenology, such as the value of the neutralino dark matter relic abundance and a negative contribution to the vacuum energy. A positive contribution to the vacuum energy is also typically present in string theory as a consequence of the running of the fundamental constants towards their fixed point values. If these two contributions cancel appropriately, one may end up with a vacuum energy which brings many cosmological observations into better agreement with theoretical expectations. The present abundance of neutralinos would then be fixed. We delineate the regions of parameter space allowed in this scenario, and study the ensuing predictions for the sparticle and Higgs-boson masses in this model.Comment: 5 pages, 2 figures (included), Latex, CTP-TAMU-06/9

A supergravity explanation of the CDF eeγγee\gamma\gamma event

We present a unified no-scale supergravity model with a light gravitino that can naturally explain the observed $ee\gamma\gamma$ event at CDF via right-handed selectron pair-production. The full spectrum of our model can be described in terms of a single parameter and can be distinguished from alternative proposals in the literature. Ongoing and future runs at LEP 2 should be able to probe the full allowed parameter space via acoplanar diphoton events from $e^+e^-\to\chi^0_1\chi^0_1$ production.Comment: 10 pages, LaTeX, 4 figures (included

RbR_b in supergravity models

We compute the supersymmetric contribution to $R_{b}\equiv \Gamma (Z\to b{\bar b})/\Gamma (Z\to {\rm hadrons})$ in a variety of supergravity models. We find R^{\rm susy}_b\lsim0.0004, which does not shift significantly the Standard Model prediction ($R^{\rm SM}_b=0.2162$ for m_t=160\GeV). An improvement in experimental precision by a factor of four would be required to be sensitive to such an effect.Comment: 5 pages, LaTeX, 1 figure (included)

Renormalized Stress Tensor for trans-Planckian Cosmology

Finite expressions for the mean value of the stress tensor corresponding to a scalar field with a generalized dispersion relation in a Friedman--Robertson--Walker universe are obtained using adiabatic renormalization. Formally divergent integrals are evaluated by means of dimensional regularization. The renormalization procedure is shown to be equivalent to a redefinition of the cosmological constant and the Newton constant in the semiclassical Einstein equations.Comment: 14 pages. Minor changes; version published in Physical Review