BeyondΛCDM: Problems, solutions, and the road ahead

Abstract

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, ΛCDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of ΛCDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.Keywords: cosmology -dark energy -cosmological constant problem -modified gravitydark matter -early universe Cosmology has been both blessed and cursed by the establishment of a standard model: ΛCDM. On the one hand, the model has turned out to be extremely predictive, explanatory, and observationally robust, providing us with a substantial understanding of the formation of large-scale structure, the state of the early Universe, and the cosmic abundance of different types of matter and energy. It has also survived an impressive battery of precision observational tests -anomalies are few and far between, and their significance is contentious where they do arise -and its predictions are continually being vindicated through the discovery of new effects (B-mode polarization [1] and lensing [2,3] of the cosmic microwave background (CMB), and the kinetic Sunyaev-Zel'dovich effect [4] being some recent examples). These are the hallmarks of a good and valuable physical theory.On the other hand, the model suffers from profound theoretical difficulties. The two largest contributions to the energy content at late times -cold dark matter (CDM) and the cosmological constant (Λ) -have entirely mysterious physical origins. CDM has so far evaded direct detection by laboratory experiments, and so the particle field responsible for it -presumably a manifestation of "beyond the standard model" particle physics -is unknown. Curious discrepancies also appear to exist between the predicted clustering properties of CDM on small scales and observations. The cosmological constant is even more puzzling, giving rise to quite simply the biggest problem in all of fundamental physics: the question of why Λ appears to take such an unnatural value [5,6,7]. Inflation, the theory of the very early Universe, has also been criticized for being fine-tuned and under-predictive [8], and appears to leave many problems either unsolved or fundamentally unresolvable. These problems are indicative of a crisis.From January 14th-17th 2015, we held a conference in Oslo, Norway to surve...

Inflation and reheating in theories with spontaneous scale invariance symmetry breaking

“…This automatically introduces in the theory a mass scale that can be identified with ϕ 0 . A similar mechanism was studied in [21], where the quadratic term in R is replaced by another dynamical scalar field, 1 The Lagrangian is not invariant since it transforms as a density. Precisely because we have a two-parameter group, we can break scale symmetry without spoiling diffeomorphism invariance.…”

“…The wealth of recent observational data has dramatically reduced the number of viable inflationary models and opened a debate on the possibilities that are left [1]. For instance, in the context of single-field inflation, the data from Planck [2] basically exclude all potentials of the type V ∼ ϕ p with p ≥ 2, thus leaving only a handful of feasible alternatives.…”

Breaking the Vainshtein screening in clusters of galaxies

“…Nevertheless, it is undisputable that it also has many problems and caveats; a non exhaustive list can be found in [26] and references therein.…”

“…Unfortunately, extension or modification of GR can be performed in too many ways; an interesting summary of most of the approaches still on study, is in Fig. 3 of [26] and in [30]. The main problem is that GR is a very well-tested theory at Solar System scales [106], and this poses very strong and limiting bounds on any possible extension.…”

Tests of Gravity Theories Using Supermassive Black Holes

“…The study of infrared modifications of general relativity (GR) has undergone a renaissance in the last decade, driven partly by the dark energy mystery and partly by recent developments in the construction of healthy higher-derivative scalar-tensor theories (see Clifton et al (2012); Joyce et al (2014); Koyama (2015); Berti et al (2015); Bull et al (2016); Joyce et al (2016); Burrage & Sakstein (2016) for recent reviews), which have opened up a new realm of possibilities for driving the acceleration of the cosmic expansion. On small scales, consistency with tests of GR is achieved by utilising screening mechanisms, which use non-linear effects to hide the modifications of gravity in the solar system.…”