[Paper Review] What Drives Our Accelerating Universe?

S. A. Bludman|ArXiv.org|Feb 2, 2007

Cosmology and Gravitation Theories14 references3 citations

TL;DR

This paper argues that cosmological observations of homogeneous expansion alone cannot distinguish between a static cosmological constant, dynamic dark energy, or modified gravity (dark gravity). It proposes that growth of structure—measured via weak lensing, CMB, and large-scale clustering—can differentiate these mechanisms, with weak lensing surveys offering the most sensitive probe of dynamical dark energy versus modified gravity.

ABSTRACT

The homogeneous expansion history H(z) of our universe measures only kinematic variables, but cannot fix the underlying dynamics driving the recent acceleration: cosmographic measurements of the homogeneous universe, are consistent with either a static finely-tuned cosmological constant or a dynamic `dark energy' mechanism, which may be material Dark Energy or modified gravity (Dark Gravity). Resolving the composition and foreground noise to reduce their large systematic errors.dynamics of either kind of `dark energy', will require complementing the homogeneous expansion observations with observations of the growth of cosmological fluctuations. Because the 'dark energy' evolution is at least quasi-static, any dynamical effects on the fluctuation growth function g(z) will be minimal. They will be best studied in the weak lensing convergence of light from galaxies at 0

Motivation & Objective

• To clarify the distinction between kinematic expansion measurements (cosmography) and the underlying dynamics driving cosmic acceleration.
• To investigate whether 'dark energy' is a static cosmological constant or a dynamic field, and whether it arises from new material or modified gravity.
• To evaluate the potential of weak lensing and large-scale structure surveys to distinguish between dynamic dark energy and modified gravity.
• To address the Cosmological Constant Problem by reinterpreting vacuum energy as intrinsic spacetime curvature rather than quantum vacuum fluctuations.
• To assess the viability of modified gravity models like DGP braneworld cosmology and their observational signatures in cosmological and local gravity tests.

Proposed method

• Uses cosmographic analysis of H(z) to describe homogeneous expansion without fixing underlying dynamics.
• Applies the Friedmann equation and its modifications in general relativity and alternative gravity models (e.g., DGP, f(R), TeVeS).
• Analyzes the growth of cosmological fluctuations g(z) as a probe of dynamics, emphasizing weak lensing convergence from galaxies (z<5), HI (6<z<20), and CMB (z=1089).
• Evaluates the Vainshtein screening mechanism in DGP gravity, where gravity deviates from Einstein’s theory at intermediate scales r_S << r_* << H_0^{-1}.
• Compares the DGP model’s modified Friedmann equation H² + H/r_c = κ²ρ/3 with standard ΛCDM and assesses its observational viability.
• Considers the role of the cosmological constant as a geometric property (Ricci curvature R_dS) rather than a quantum vacuum energy, avoiding the cosmological constant problem.

Experimental results

Research questions

• RQ1 Can cosmographic measurements of H(z) alone distinguish between a static cosmological constant and dynamic dark energy?
• RQ2 To what extent can the growth of structure in the universe differentiate between dynamic dark energy and modified gravity?
• RQ3 What observational signatures does the DGP braneworld model produce in weak lensing and large-scale structure?
• RQ4 How does the Vainshtein screening mechanism suppress deviations from general relativity in modified gravity models?
• RQ5 Can the cosmological constant be interpreted geometrically as intrinsic spacetime curvature, avoiding the quantum vacuum energy problem?

Key findings

• Cosmographic observations of H(z) are insufficient to determine whether the acceleration is driven by a static cosmological constant or dynamic dark energy.
• The DGP model with β = 1.39 predicts late-time acceleration starting at z_acc ≈ 0.58, matching observations.
• The Vainshtein scale r_* ≈ (r_S r_c²)^{1/3} determines the transition scale where modified gravity departs from general relativity, with r_* ≪ H_0^{-1}.
• Weak lensing surveys of galaxies at 0 < z < 5 and HI at 6 < z < 20 offer the most promising route to distinguish dynamic dark energy from modified gravity.
• The DGP model’s modified Friedmann equation interpolates between Einstein-de Sitter (α=2) and ΛCDM (α=0) limits, with w_DE = -1 + α/2 for general α.
• The original DGP model is indistinguishable from the flat DGP model in cosmological fits, but both require fine-tuning to match current data.