The Phenomenology of Dvali–gabadadze–porrati Cosmologies

Cosmologists today are confronted with the perplexing reality that the universe is currently accelerating in its expansion. Nevertheless, the nature of the fuel that drives today’s cosmic acceleration is an open and tantalizing mystery. There exists the intriguing possibility that the acceleration is not the manifestation of yet another mysterious ingredient in the cosmic gas tank (dark energy), but rather our first real lack of understanding of gravity itself, and even possibly a signal that there might exist dimensions beyond that which we can currently observe. The braneworld model of Dvali, Gabadadze and Porrati (DGP) is a theory where gravity is altered at immense distances by the excruciatingly slow leakage of gravity off our three-dimensional Universe and, as a modified-gravity theory, has pioneered this line of investigation. I review the underlying structure of DGP gravity and those phenomenological developments relevant to cosmologists interested in a pedagogical treatment of this intriguing model. E-mail: [email protected] I. THE CONTEMPORARY UNIVERSE Cosmology in this decade is said to be thriving in a golden age. With the cornucopia of observational data from both satellite and ground-based surveys, an increasingly coherent phenomenological and theoretical picture is now emerging. And while our understanding of cosmic expansion, primordial nucleosynthesis, the microwave background and other phenomena allows particle physics and cosmology to use the very vastness of our Universe to probe the most incomprehensibly high energies, this golden age of cosmology is offering the first new data regarding the physics on immense scales in of themselves. In other words, while modern cosmology is a ratification of the notion of the deep connection between the very small and the very large, it offers also the opportunity to challenge fundamental physics itself at the lowest of energies, an unexplored infrared domain. A central example highlighting this theme is that physicists are currently faced with the perplexing reality that the Universe is accelerating in its expansion [1,2]. That startling reality is only driven home with the observation of the onset of this acceleration [3]. The acceleration represents, in essence, a new imbalance in the governing gravitational equations: a universe filled only with ordinary matter and dark matter (ingredients for which we have independent corroboration) should decelerate in its expansion. What drives the acceleration thus remains an open and tantalizing question. Instructively, physics historically has addressed such imbalances in the governing gravitational equation in either one of two ways: either by identifying sources that were previously unaccounted for (e.g., Neptune and dark matter) or by altering the governing equations (e.g., general relativity). Standard cosmology has favored the first route to addressing the imbalance: a missing energy-momentum component. Indeed, a “conventional” explanation exists for the cause of that acceleration — in general relativity, vacuum energy provides the repulsive gravity necessary to drive accelerated cosmological expansion. Variations on this vacuum-energy theme, such as quintessence, promote the energy density to the potential energy density of a dynamical field. Such additions to the roster of cosmic sources of energy-momentum are collectively referred to as dark energy. If it exists, this mysterious dark energy would constitute the majority of the energy density of the universe today. However, one may also entertain the alternative viewpoint. Can cosmic acceleration be the first new signal of a lack of understanding of gravitational interactions? I.e., is the cosmic acceleration the result, not of the contents of the the cosmic gas tank, as it were, but a consequence of the engine itself. This is the question that intrigues and excites us, and more importantly, how we can definitively answer that question. How can one definitively