Abstract
We present a model that naturally tunes a large positive cosmological constant to a small cosmological constant. A slowly rolling scalar field decreases the cosmological constant to a small negative value, causing the universe to contract, thus reheating it. An expanding universe with a small positive cosmological constant can be obtained, respectively, by coupling this solution to any model of a cosmological bounce and coupling the scalar field to a sector that undergoes a technically natural phase transition at the meV scale. A robust prediction of this model is a rolling scalar field today with some coupling to the standard model. This can potentially be experimentally probed in a variety of cosmological and terrestrial experiments, such as probes of the equation of state of dark energy, birefringence in the cosmic microwave background and terrestrial tests of Lorentz violation.
Highlights
The observed accelerated expansion of the Universe is well described by the existence of a small cosmological constant
This can be accomplished by coupling the massive vector to the normal matter through mixing with the photon or through higher dimensional operators, and allowing the Standard Model to thermalize at some point when the temperature is higher that the scale of big bang nucleosynthesis
The universe starts crunching and the energy density in these matter fields will blue-shift. This energy can be used to trigger a technically natural phase transition at the ∼meV4 scale, resulting in an addition to the vacuum energy and the cosmological constant (CC) changing from ∼ − meV4 to ∼ þ meV4. This transition is not fine-tuned so long as the CC is changed to a positive value of roughly the same size as, or greater than, the small negative value it had after relaxation
Summary
The observed accelerated expansion of the Universe is well described by the existence of a small cosmological constant. One might hope that the mysteries of quantum gravity hold the solution, but dangerous contributions come from very well-known physics at scales where spacetime curvature is weak (for example, finite corrections to vacuum energy from the electron mass) In this regard, the problem can be seen as one of fine-tuning, where contributions, known and unknown, conspire to cancel to generate the small value detected today. A more natural solution could come from the dynamical relaxation of the cosmological constant in the early universe via a slowly rolling field in a potential. Assumption that this sector can trigger dynamics that causes the scale factor to bounce at short distances, allowing the universe to expand and produce our observed cosmological history. This existence proof highlights the importance of short-distance descriptions of a cosmological bounce, and presents the opportunity to reimagine the source of the initial perturbations often credited to inflation
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