[Paper Review] Dark Matter and Dark Energy: A Physicist's Perspective

Michael Dine|ArXiv.org|Jul 30, 2001

Radioactive Decay and Measurement Techniques3 references4 citations

TL;DR

This paper presents a physicist's perspective on dark matter and dark energy, arguing that string theory offers the only viable framework for understanding these phenomena. It explores how supersymmetry, axions, and the cosmological constant may be connected through string-theoretic mechanisms, with holography and anthropic principles offering potential resolutions to deep cosmological puzzles.

ABSTRACT

For physicists, recent developments in astrophysics and cosmology present exciting challenges. We are conducting "experiments" in energy regimes some of which will be probed by accelerators in the near future, and others which are inevitably the subject of more speculative theoretical investigations. Dark matter is an area where we have hope of making discoveries both with accelerator experiments and dedicated searches. Inflation and dark energy lie in regimes where presently our only hope for a fundamental understanding lies in string theory.

Motivation & Objective

• To assess the role of particle physics and string theory in addressing unresolved questions about dark matter and dark energy.
• To investigate whether string theory can provide a consistent theoretical foundation for explaining the cosmological constant and the nature of dark energy.
• To evaluate the viability of anthropic and holographic principles in resolving fundamental cosmological puzzles.
• To explore the potential of accelerator experiments and cosmological observations in testing theoretical models of dark matter and dark energy.

Proposed method

• Analyzes energy scales from QCD to the Planck scale to identify regimes where new physics may emerge.
• Applies string theory as a unifying framework to examine dark matter candidates such as the LSP and axions.
• Examines holographic principles, including the Bekenstein-Hawking entropy formula and AdS-CFT correspondence, to constrain degrees of freedom in quantum gravity.
• Evaluates the role of supersymmetry breaking and the cosmological constant in holographic theories, particularly in de Sitter space.
• Considers the implications of multiple vacua and anthropic reasoning in string theory for explaining the observed value of the cosmological constant.
• Reviews the potential of future experiments—like the LHC and SNAP—to test theoretical predictions on dark matter and dark energy.

Experimental results

Research questions

• RQ1 Can string theory provide a consistent framework for understanding dark matter candidates such as the LSP and axions?
• RQ2 What role does holography play in limiting the number of degrees of freedom and constraining the cosmological constant?
• RQ3 Is the observed coincidence between dark matter density and the cosmological constant a sign of anthropic selection, and can string theory rule out or support this?
• RQ4 How might the scale of supersymmetry breaking be related to the cosmological constant in a holographic quantum gravity framework?
• RQ5 Can string theory offer a predictive mechanism for the small value of the cosmological constant, or is it inherently non-predictive?

Key findings

• String theory is the only known framework that can, in principle, calculate the cosmological constant and accommodate both axion and LSP dark matter candidates.
• The holographic principle, supported by AdS-CFT and matrix models, suggests that the number of physical degrees of freedom in a volume is proportional to its surface area, not its volume.
• The proposal by Banks that the number of states in de Sitter space is finite and determined by a single parameter offers a novel, though speculative, explanation for the cosmological constant and supersymmetry breaking scale.
• Holographic reasoning suggests that the cosmological constant may not arise from a simple volume integral over momentum modes, but from a much smaller set of surface degrees of freedom.
• Anthropic explanations for the smallness of the cosmological constant are not ruled out by current string theory, but the lack of a predictive underlying theory limits their scientific status.
• The remarkable coincidence between the present-day values of dark matter density and the cosmological constant is highly suggestive of an anthropic explanation, though no complete theory yet supports it.