Dynamically avoiding fine-tuning the cosmological constant: the "Relaxed Universe"

Abstract

We demonstrate that there exists a large class of action functionals of the scalar curvature and of the Gauss-Bonnet invariant which are able to relax dynamically a large cosmological constant (CC), whatever it be its starting value in the early universe. Hence, it is possible to understand, without fine-tuning, the very small current value of the CC as compared to its theoretically expected large value in quantum field theory and string theory. In our framework, this relaxation appears as a pure gravitational effect, where no ad hoc scalar fields are needed. The action involves a positive power of a characteristic mass parameter, M, whose value can be, interestingly enough, of the order of a typical particle physics mass of the Standard Model of the strong and electroweak interactions or extensions thereof, including the neutrino mass. The model universe emerging from this scenario (the "Relaxed Universe") falls within the class of the so-called LXCDM models of the cosmic evolution. Therefore, there is a "cosmon" entity X (represented by an effective object, not a field), which in this case is generated by the effective functional and is responsible for the dynamical adjustment of the cosmological constant. This model universe successfully mimics the essential past epochs of the standard (or "concordance") cosmological model (LCDM). Furthermore, it provides interesting clues to the coincidence problem and it may even connect naturally with primordial inflation.

LaTeX, 63 pp, 8 figures. Extended discussion. Version accepted in JCAP

References in corpus (19)

• Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology
• Dynamics of dark energy
• Dark energy in modified Gauss-Bonnet gravity: late-time acceleration and the hierarchy problem
• Conditions for the cosmological viability of f(R) dark energy models
• Are f(R) dark energy models cosmologically viable ?
• Avoiding Dark Energy with 1/R Modifications of Gravity
• String-inspired Gauss-Bonnet gravity reconstructed from the universe expansion history and yielding the transition from matter dominance to dark energy
• Effective Action of Vacuum: Semiclassical Approach
• Dark Interactions and Cosmological Fine-Tuning
• Ghosts, Instabilities, and Superluminal Propagation in Modified Gravity Models
• Cosmological Dark Energy: Prospects for a Dynamical Theory
• LXCDM: a cosmon model solution to the cosmological coincidence problem?
• Relaxing a large cosmological constant
• The solution of the cosmological constant problem from the inhomogeneous equation of state - a hint from modified gravity?
• Effective growth of matter density fluctuations in the running LCDM and LXCDM models
• Planck Scale Cosmology in Resummed Quantum Gravity
• On Cosmological Implications of Gravitational Trace Anomaly
• Pseudo-redundant vacuum energy
• Resummed Quantum Gravity

Cited by in corpus (3)

• Hubble expansion and structure formation in the "running FLRW model" of the cosmic evolution
• Zero-point quantum fluctuations and dark energy
• Filtering out the cosmological constant in the Palatini formalism of modified gravity