Closing the Universe by relaxing the cosmological constant JORGE

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) × 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 cosmolog-ical 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. There are two fundamental problems in cosmology today which are intimately related to particle physics: the dark matter problem and the cosmological constant problem. Indeed, almost any extension of the minimal Standard Model (SU(3) × SU(2) × U(1)) predicts some new particles which can be identified as viable candidates for hot (e.g., massive neutrinos and axions) or cold (e.g., neutralinos and cryptons) dark matter. Concerning the cosmological constant problem, there are not that many satisfactory solutions. Recent developments at the cosmological and particle physics fronts indicate a possible correlation between dark matter and the cosmological constant (Λ). In this note we allow the cosmological constant to contribute to the cosmic energy density, thus reducing the corresponding dark matter contribution such that the latest cosmological observations are best fit. The standard cold dark matter model (with Ω = 1 and h = 0.5) has had great success, but as observations have improved various discrepancies with the data have started to appear [1]. Recent observations of X-rays from the gas surrounding galaxy clusters indicate that Ω (visible plus dark) is O(0.2), implying Ω CDM < ∼ 0.2 [2]. On the other hand, an Ω = 1 Universe is not only appealing in the inflationary scenario, but it may also arise under more general circumstances (as we discuss below). Thus, the phenomenological suggestion has been made that a Universe with Ω CDM ≈ 0.2, Ω Λ ≈ 0.8, and h ≈ 1 should be seriously considered [3]. Indeed, such …