Avoiding Dark Energy with 1/R Modifications of Gravity

Abstract

Scalar quintessence seems epicyclic because one can choose the potential to reproduce any cosmology (I review the construction) and because the properties of this scalar seem to raise more questions than they answer. This is why there has been so much recent interest in modified gravity. I review the powerful theorem of Ostrogradski which demonstrates that the only potentially stable, local modification of general relativity is to make the Lagrangian an arbitrary function of the Ricci scalar. Such a theory can certainly reproduce the current phase of cosmic acceleration without Dark Energy. However, this explanation again seems epicyclic in that one can construct a function of the Ricci scalar to support any cosmology (I give the technique). Models of this form are also liable to problems in the way they couple to matter, both in terms of matter's impact upon them and in terms of the long range gravitational force they predict. Because of these problems my own preference for avoiding Dark Energy is to bypass Ostrogradski's theorem by considering the fully nonlocal effective action built up by quantum gravitational processes during the epoch of primordial inflation.

30 pages, no figures, conference proceedings for the 3rd Aegean Summer School, Chios, 26 September - 1 October, 2005. Replaced version corrects some typoes and adds references

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• Dark Energy and Dark Gravity
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• Dynamics with Infinitely Many Derivatives: The Initial Value Problem
• Reconstructing the Distortion Function for Nonlocal Cosmology
• How do Black Holes Spin in Chern-Simons Modified Gravity?
• Higher order gravity theories and their black hole solutions
• The viability of theories with matter coupled to the Ricci scalar
• Vacuum structure for scalar cosmological perturbations in Modified Gravity Models
• Black hole entropy in modified gravity models
• 6+1 lessons from f(R) gravity
• Energy-momentum complexes in f(R) theories of gravity
• Stellar configurations in f(R) theories of gravity
• Various Hamiltonian formulations of f(R) gravity and their canonical relationships
• Relaxing a large cosmological constant
• Dynamics of holographic vacuum energy in the DGP model
• Dynamics and Stability of Light-Like Tachyon Condensation
• Effect of the Trace Anomaly on the Cosmological Constant
• Pseudo-redundant vacuum energy
• Stable, Accelerating Universes in Modified Gravity
• Can cosmic acceleration be caused by exotic massless particles?
• Backreaction of superhorizon perturbations in scalar field cosmologies
• On the problem of inflation in nonlinear multidimensional cosmological models
• Maximal symmetry and metric-affine f(R) gravity