Higuchi ghosts and gradient instabilities in bimetric gravity

Könnig, Frank

doi:10.1103/physrevd.91.104019

Cited by 55 publications

(89 citation statements)

References 63 publications

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“…109,110,111,112,113,114,115,116]. Unfortunately, although the simplest bigravity models are able to provide viable self-accelerating background expansions, all such models suffer from ghost and/or gradient instabilities [117,112,114,118]. While it is possible to push these instabilities back to unobservably early times, beyond the regime of validity of the theory, without losing self-acceleration and obtaining a technically natural acceleration parameter [119], the theory becomes observationally indistinguishable from ΛCDM in this case.…”

Section: Gravity In Cosmologymentioning

confidence: 98%

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

459

210

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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...

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Section: Gravity In Cosmologymentioning

confidence: 98%

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

459

210

View full text Add to dashboard Cite

show abstract

“…(IIc) Exotic branches: These are all other branches which generically lead to bouncing cosmologies or static universes in the asymptotic past or future [210]. Even though it is not impossible to make such scenarios compatible with observations, these branches correspond to rather unconventional cosmologies and have not been studied in great detail so far.…”

Section: Classification Of Solutionsmentioning

confidence: 99%

“…= are free of scalar instabilities [208], in general the perturbations around this class of FLRW backgrounds suffer from ghosts in both the scalar and tensor sectors [209,210,217]. Due to these pathologies, branch IIa solutions do not correspond to consistent backgrounds and must be discarded on theoretical grounds.…”

Section: Classification Of Solutionsmentioning

confidence: 99%

Recent developments in bimetric theory

Schmidt-May

Strauss

2016

J. Phys. A: Math. Theor.

198

240

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This review is dedicated to recent progress in the field of classical, interacting, massive spin-2 theories, with a focus on ghost-free bimetric theory. We will outline its history and its development as a nontrivial extension and generalisation of nonlinear massive gravity. We present a detailed discussion of the consistency proofs of both theories, before we review Einstein solutions to the bimetric equations of motion in vacuum as well as the resulting mass spectrum. We introduce couplings to matter and then discuss the general relativity and massive gravity limits of bimetric theory, which correspond to decoupling the massive or the massless spin-2 field from the matter sector, respectively. More general classical solutions are reviewed and the present status of bimetric cosmology is summarised. An interesting corner in the bimetric parameter space which could potentially give rise to a nonlinear theory for partially massless spin-2 fields is also discussed. Relations to higher-curvature theories of gravity are explained and finally we give an overview of possible extensions of the theory and review its formulation in terms of vielbeins.

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“…It is quite natural to begin this program from the first part. Indeed, it is highly nontrivial to construct IR modifications of GR that are cosmologically viable, as has been learned from experience with the DGP model [2][3][4][5][6][7][8][9], the dRGT theory of massive gravity [10][11][12][13][14][15][16], Hassan-Rosen bigravity [17][18][19][20][21][22][23][24][25][26][27], or nonlocal models of the Deser-Woodard type [28][29][30][31]. Indeed, none of these attempts has yet produced a viable competitor to ΛCDM.…”

Section: Introductionmentioning

confidence: 99%

Conformal symmetry and nonlinear extensions of nonlocal gravity

Cusin¹,

Foffa²,

Maggiore³

et al. 2016

Phys. Rev. D

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We study two nonlinear extensions of the nonlocal R□ −2 R gravity theory. We extend this theory in two different ways suggested by conformal symmetry, either replacing □ −2 with ð−□ þ R=6Þ −2 , which is the operator that enters the action for a conformally-coupled scalar field, or replacing □ −2 with the inverse of the Paneitz operator, which is a four-derivative operator that enters in the effective action induced by the conformal anomaly. We show that the former modification gives an interesting and viable cosmological model, with a dark energy equation of state today w DE ≃ −1.01, which very closely mimics ΛCDM and evolves asymptotically into a de Sitter solution. The model based on the Paneitz operator seems instead excluded by the comparison with observations. We also review some issues about the causality of nonlocal theories, and we point out that these nonlocal models can be modified so to nicely interpolate between Starobinski inflation in the primordial universe and accelerated expansion in the recent epoch.

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Higuchi ghosts and gradient instabilities in bimetric gravity

Cited by 55 publications

References 63 publications

BeyondΛCDM: Problems, solutions, and the road ahead

BeyondΛCDM: Problems, solutions, and the road ahead

Recent developments in bimetric theory

Conformal symmetry and nonlinear extensions of nonlocal gravity

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