[Paper Review] Cosmic Acceleration from Causal Backreaction in a Smoothly Inhomogeneous Universe

B. Bochner|arXiv (Cornell University)|Sep 22, 2011

Cosmology and Gravitation Theories127 references3 citations

TL;DR

This paper proposes that cosmic acceleration arises not from dark energy or modified gravity, but from causal backreaction in a smoothly inhomogeneous universe, where the cumulative effect of innumerable virialized structures generates apparent acceleration. It presents phenomenological models that fit Type Ia supernova data as well as flat ΛCDM without dark energy, while reproducing key cosmological parameters such as age, matter density, and CMB angular scale within observational bounds.

ABSTRACT

A phenomenological formalism is presented in which the apparent acceleration of the universe is generated by large-scale structure formation, thus eliminating the coincidence and magnitude 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 virialized 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. We discuss several arguments from the literature claiming to place sharp limits upon the strength of backreaction-related effects, and show why such arguments are not applicable in a physically realistic cosmological analysis. A selection of simply-parameterized models are presented, including several which are capable of fitting the luminosity distance data from Type Ia supernovae essentially as well as the best-fit flat $Λ$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. We conclude by considering potential observational signatures for distinguishing this cosmological formalism from $Λ$CDM or Dark Energy, as well as the possible long-term fate of such a universe with ever-spreading spheres of influence for its increasingly superposed perturbations.

Motivation & Objective

• To address the fine-tuning and coincidence problems of the cosmological constant in the ΛCDM model by replacing dark energy with structure-induced backreaction.
• To demonstrate that apparent cosmic acceleration can emerge from the causal propagation of inhomogeneity information from virialized structures in a smoothly inhomogeneous spacetime.
• To construct phenomenological models that reproduce key observational constraints—such as luminosity distance, matter density, age, and CMB angular scale—without invoking dark energy or modified gravity.
• To challenge prior claims that backreaction effects are too weak to explain cosmic acceleration, showing those arguments are inapplicable in realistic cosmological settings.
• To identify observational signatures distinguishing this causal backreaction paradigm from ΛCDM and dynamical dark energy models.

Proposed method

• Develops a phenomenological formalism for causal backreaction in a smoothly inhomogeneous Friedmann-Robertson-Walker (FRW) metric, where perturbations from virialized systems overlap and propagate causally.
• Models the cumulative effect of innumerable local perturbations using a smoothed, inhomogeneous adjustment of the FRW metric, governed by the causal flow of inhomogeneity information outward from each clumped system.
• Introduces simply parameterized models that simulate the backreaction effect without requiring full nonlinear cosmological simulations.
• Uses Type Ia supernova luminosity distance data to constrain model parameters and compare performance to the flat ΛCDM model.
• Computes the observed jerk parameter j₀^Obs to test model consistency with data, finding values significantly above unity.
• Analyzes long-term cosmic fate under causal backreaction, emphasizing self-limiting, stop-and-go acceleration and extreme tidal forces due to distant inhomogeneities.

Experimental results

Research questions

• RQ1 Can cosmic acceleration be generated purely by backreaction from large-scale structure formation, without invoking dark energy or modified gravity?
• RQ2 Why are previous arguments limiting the strength of backreaction effects invalid in a physically realistic cosmological context?
• RQ3 To what extent can phenomenological models based on causal backreaction reproduce key cosmological observables such as luminosity distance, matter density, and CMB angular scale?
• RQ4 What observational signatures distinguish this backreaction-based model from ΛCDM and dynamical dark energy models?
• RQ5 How does the long-term evolution of a universe dominated by causal backreaction differ from standard dark energy scenarios?

Key findings

• The causal backreaction model produces apparent cosmic acceleration through the cumulative, overlapping effects of innumerable virialized structures, eliminating the need for dark energy.
• Several parameterized models fit Type Ia supernova luminosity distance data as well as the best-fit flat ΛCDM model, without requiring dark energy, modified gravity, or a local void.
• The models reproduce the observed age of the universe, the matter density required for spatial flatness, and the present-day deceleration parameter within reasonable proximity to Concordance Model values.
• The observed jerk parameter j₀^Obs is predicted to be in the range 2.5–5.5 (with best runs at 2.6–3.8), significantly higher than the ΛCDM value of 1, and supported by weak trends in current data.
• The model suggests a self-limiting, stop-and-go acceleration fate rather than a runaway Big Rip, due to the internal dynamics of structure formation driving the acceleration.
• The model implies extreme, asymmetrical tidal forces on bound systems due to long-range, causally propagated inhomogeneity effects, leading to a more chaotic cosmic evolution than in standard models.