Cosmic Acceleration and a New Concordance from Causal Backreaction

Abstract: A phenomenological formalism is presented in which the apparent acceleration of the universe is generated by large-scale structure formation, thus eliminating the magnitude and coincidence fine-tuning problems of the Cosmological Constant in the Concordance Model, as well as potential instability issues with dynamical Dark Energy. The observed acceleration results from the combined effect of innumerable local perturbations due to individually virializing systems, overlapping together in a smoothly-inhomogeneous adjustment of the FRW metric, in a process governed by the causal flow of inhomogeneity information outward from each clumped system. After explaining why arguments from the literature claiming to place restrictive limits upon backreaction are not applicable in a physically realistic cosmological analysis, we present a selection of simply-parameterized models which are capable of fitting the luminosity distance data from Type Ia supernovae essentially as well as the best-fit flat $\Lambda$CDM model, without resort to Dark Energy, any modification to gravity, or a local void. Simultaneously, these models can reproduce measured cosmological parameters such as the age of the universe, the matter density required for spatial flatness, the present-day deceleration parameter, and the angular scale of the Cosmic Microwave Background to within a reasonable proximity of their Concordance values. A potential observational signature for distinguishing this cosmological formalism from $\Lambda$CDM may be a cosmic jerk parameter significantly in excess of unity.