Acceleressence: Dark Energy from a Phase Transition at the Seesaw Scale

Simple models are constructed for “acceleressence” dark energy: the latent heat of a phase transition occurring in a hidden sector governed by the seesaw mass scale v/MPl, where v is the electroweak scale and MPl the gravitational mass scale. In our models, the seesaw scale is stabilized by supersymmetry, implying that the LHC must discover superpartners with a spectrum that reflects a low scale of fundamental supersymmetry breaking. Newtonian gravity may be modified by effects arising from the exchange of fields in the acceleressence sector whose Compton wavelengths are typically of order the millimeter scale. There are two classes of models. In the first class the universe is presently in a metastable vacuum and will continue to inflate until tunneling processes eventually induce a first order transition. In the simplest such model, the range of the new force is bounded to be larger than 25 μm in the absence of fine-tuning of parameters, and for couplings of order unity it is expected to be ≈ 100 μm. In the second class of models thermal effects maintain the present vacuum energy of the universe, but on further cooling, the universe will “soon” smoothly relax to a matter dominated era. In this case, the range of the new force is also expected to be of order the millimeter scale or larger, although its strength is uncertain. A firm prediction of this class of models is the existence of additional energy density in radiation at the eV era, which can potentially be probed in precision measurements of the cosmic microwave background. An interesting possibility is that the transition towards a matter dominated era has occurred in the very recent past, with the consequence that the universe is currently decelerating. 1 Dark Energy from a Phase Transition Cosmological observations of Type Ia supernovae, the cosmic microwave background radiation and large scale structure provide strong evidence that the universe is flat and composed of about 70% dark energy and 30% dark matter [1, 2, 3]. The dark energy, which is driving a recent acceleration in the expansion in the universe, has negative pressure and cannot be interpreted as matter or radiation. Rather, this unusual fluid may be some form of vacuum energy, with an energy density of order (10 eV). A crucial question is whether this vacuum energy is time independent – a “hard” cosmological constant, Λ – or evolves with time – a “soft” vacuum energy. An example of the latter is “quintessence”, a scalar field energy that evolves slowly over many decades of expansion of the universe [4]. However, theories of quintessence involve an unnaturally small mass scale of order the Hubble parameter, 10 eV, and do not explain why this field energy is just dominating the universe in the present epoch. A hard cosmological constant also suffers from this “Why now?” problem; are we really witnessing the transition to an era of eternal inflation? Our present understanding of the hot big bang is one of a succession of phase transitions interspersed with eras of smooth cooling. The phase transitions are the cosmological manifestation of symmetry breaking, as the underlying vacuum shifts from one stable minimum to another. Given the standard model of particle physics, it is extremely plausible that phase transitions occurred both as the temperature cooled through the electroweak scale, v, and through the scale of strong interactions, ΛQCD. At higher temperatures there may well have been other phase transitions associated, for example, with the breaking of lepton number (to generate right-handed neutrino masses and for leptogenesis), the breaking of grand unified gauge symmetries, and the generation of an early era of cosmic inflation. At each of these phase transitions it is likely that the universe was dominated for a period by the vacuum energy, or latent heat, of the associated change in vacuum state. It therefore seems plausible to us that the present cosmic expansion is fueled by the soft vacuum energy of some phase transition associated with an energy scale of 10 eV. We label this phenomenon acceleressence. At first sight it does not seem reasonable that there could be a phase transition in the universe with a vacuum energy of order (10 eV), because we have not discovered any particle physics symmetry breaking at the 10 eV scale. However, at low energies we know that interactions between particles can get very small, suppressed by inverse powers of a large mass scale, so that this new particle physics may be decoupled from us. For example, all interactions of the neutrino decouple at low energies, and its mass is often assumed to arise from inverse powers of the large right-handed neutrino mass MR, mν ≃ v/MR. Suppose that acceleressence occurs in some hidden sector that interacts with the standard model only by gravity. It could be that the mass