The Cosmological Constant Problem: Why it's hard to get Dark Energy from Micro-physics

Abstract

These notes present a brief introduction to `naturalness' problems in cosmology, and to the Cosmological Constant Problem in particular. The main focus is the `old' cosmological constant problem, though the more recent variants are also briefly discussed. Several notions of naturalness are defined, including the closely related ideas of technical naturalness and `t Hooft naturalness, and it is shown why these naturally arise when cosmology is embedded within a framework --- effective field theories --- that efficiently captures what is consistent with what is known about the physics of smaller distances. Some care is taken to clarify conceptual issues, such as the relevance or not of quadratic divergences, about which some confusion has arisen over the years. A set of minimal criteria are formulated against which proposed solutions to the problem can be judged, and a brief overview made of the general limitations of most of the approaches. A somewhat more in-depth discussion is provided of what I view as the most promising approach. These notes are aimed at graduate students with a basic working knowledge of quantum field theory and cosmology, but with no detailed knowledge of particle physics.

Lectures given to the Les Houches Summer School "Post-Planck Cosmology," 8 July - 2 August 2013. 52 pages

Cited by in corpus (18)

• Unimodular Gravity vs General Relativity: A status report
• A small cosmological constant from Abelian symmetry breaking
• Model-independent reconstruction of $f(T)$ teleparallel cosmology
• Alleviating the cosmological constant problem from particle production
• Yoga Dark Energy: Natural Relaxation and Other Dark Implications of a Supersymmetric Gravity Sector
• Absence of cosmological constant problem in special relativistic field theory of gravity
• Comparison of different approaches to the quasi-static approximation in Horndeski models
• Strained Bilayer Graphene, Emergent Energy Scales, and Moire Gravity
• Supersymmetry Breaking and the Cosmological Constant
• Who's Afraid of the Supersymmetric Dark? The Standard Model vs Low-Energy Supergravity
• Fine-tuning with Brane-Localized Flux in 6D Supergravity
• Noether charge formalism for Weyl transverse gravity
• Cosmological bounds on TeV-scale physics and beyond
• Can Compactifications Solve the Cosmological Constant Problem?
• Splitting the source term for the Einstein equation to classical and quantum parts
• Conformal couplings of Galileons to other degrees of freedom
• Technically Natural Vacuum Energy at the Tip of a Supersymmetric Teardrop
• Multiscalar-metric gravity: cosmological constant screening and emergence of massive-graviton dark components