Is "Dark Energy" a Quantum Vacuum Energy? (2111.12576v2)

Abstract: We review the origins, motivations, and implications for cosmology and black holes, of our proposal that "dark energy" is not a quantum vacuum energy, but rather arises from a Weyl scaling invariant nonderivative component of the gravitational action.

• The paper challenges the conventional vacuum energy model by proposing that dark energy arises from a Weyl scaling invariant non-derivative gravitational action.
• The paper leverages a preferred CMB rest frame to bypass the cosmological constant fine-tuning and modify cosmic perturbation dynamics.
• The paper discusses observable implications, including altered black hole horizon behavior and modifications in dark energy evolution that may resolve the Hubble tension.

Is "Dark Energy" a Quantum Vacuum Energy?

The paper authored by Stephen L. Adler critically examines the nature of dark energy, challenging the conventional interpretation of it being a quantum vacuum energy. Adler proposes an alternative explanation: dark energy emerges from a Weyl scaling invariant non-derivative component of the gravitational action, deviating from the widely held view that it is attributable to a cosmological constant term in the gravitational action.

Analysis of Conventional Understanding and the Cosmological Constant Problem

The conventional understanding posits dark energy as a cosmological vacuum energy, modeled through Einstein's cosmological constant in general relativity. However, this leads to the profound cosmological constant problem—commonly referred to as the fine-tuning problem—where the observed value of the vacuum energy is vastly smaller than theoretical predictions from quantum mechanics at the Planck scale. Such disparities necessitate extreme fine-tuning, which becomes philosophically and theoretically unsatisfactory.

Weyl Scaling Invariant Approach

Adler introduces an innovative approach, employing Weyl's concept of scale invariance—originally intended as a local symmetry in gravitational physics—which has regained attention in modern theoretical studies. By leveraging global Weyl scaling invariance, particularly for the non-derivative gravitational action, the proposed framework circumvents the cosmological constant problem by negating the need to interpret dark energy as a vacuum energy. Instead, it arises from a frame-dependent action that respects Weyl invariance.

Preferred Frame and Breakdown of General Covariance

A pivotal aspect of Adler’s argument is the introduction of a preferred frame, identified with the rest frame of the Cosmic Microwave Background (CMB), contrasting with the traditional general covariance in general relativity. While disappearing in frameworks assuming four-dimensional general coordinate invariance, the preferred frame uniquely contributes to three-dimensional invariance based approaches. Therefore, while preserving the large-scale homogeneity and isotropy of the universe, this model incorporates potential frame-dependent phenomena, observable through cosmic variance on smaller scales.

Theoretical and Observational Implications

1. Cosmological Perturbations: The paper explores first-order cosmological perturbation implications. Adler demonstrates that scalar gravitational waves remain non-propagating due to three-space coordinate invariance, maintaining consistency with observational data while allowing the model to retain novel features at small perturbations.
2. Black Holes: Crucially, in this model, black holes do not manifest traditional event horizons due to the intricacies of the $g_{00}^{-2}$ factor in the modified action. Consequently, potential leaky behavior or "black hole winds" could arise, potentially influencing phenomena such as star formation close to supermassive black holes and feeding matter into surrounding galactic disks.
3. Late-time Cosmology and Hubble Tension: Adler’s model contributes to addressing discrepancies such as the Hubble tension, suggesting modifications in the effective dark energy density evolution and cosmological parameters. The dynamic interpretation of perturbation allows examination of dark energy's role in the accelerated cosmic expansion observed today.

Future Directions and Open Questions

Adler's proposition necessitates exploring several open questions. These include deeper theoretical understanding of the pre-quantum dynamics leading to a Weyl invariant action, computational simulations to validate implications on cosmic structures and extension to a rotating black hole framework. Moreover, observable consequences of the proposed leaky black holes and associated particle fluxes could lead to groundbreaking insights that push our understanding of cosmological and astrophysical processes forward.

In conclusion, Adler’s work provides a compelling alternative narrative to the dark energy enigma, calling for an intersectional investigation bridging theoretical advancements and empirical observations. Through reevaluating foundational assumptions about symmetry and invariance in gravitational physics, this approach holds potential to resolve longstanding inconsistencies and enhance our comprehension of the universe’s fundamental structure.