Conformal cosmology with no cosmological constant

Mannheim, Philip D.

doi:10.1007/bf00756278

Cited by 136 publications

(164 citation statements)

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“…To test their method, they compared the ΛCDM prediction with that of a conformal gravity model [e.g. 278], where the Universe's expansion has always accelerated. They found that, when properly analyzed, current data are consistent with distance moduli of GRBs and Type Ia supernovae that can be, in a variant of conformal gravity, ∼ 15 and ∼ 3 magnitudes fainter than in ΛCDM, respectively.…”

Section: Gravity In Cosmologymentioning

confidence: 99%

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

420

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: 99%

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

420

210

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“…The cosmology associated with the conformal gravity theory was first presented in [6] where it was shown to possess no flatness problem, to thus release conformal cosmology from the need for the copious amounts of cosmological dark matter required of the standard theory. Subsequently [7,8], the cosmology was shown to also possess no horizon problem, no universe age problem, and, through negative spatial curvature, to naturally lead to cosmic repulsion. [9] Finally, it was shown [10,11] that even after the conformal symmetry is spontaneously broken by a Λ inducing cosmological phase transition, the cosmology is still able to control the contribution of the induced cosmological constant to cosmic evolution even in the event that Λ is in fact as big as particle physics suggests, to thereby provide a completely natural solution to the cosmological constant problem.…”

Section: The Hubble Plot Of Conformal Cosmologymentioning

confidence: 99%

How Recent Is Cosmic Acceleration?

Mannheim

2003

Int. J. Mod. Phys. D

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Possibly the most peculiar expectation of the standard fine-tuned cosmological paradigm is that cosmic acceleration is to only be a very recent (z < 1) phenomenon, with the universe being required to be decelerating at all higher redshifts. Detailed exploration of the Hubble plot out to z = 2 or so will not only provide an absolute test of this expectation but will also allow for testing of conformal gravity, a non fine-tuned alternate cosmological theory which provides equally good fitting to the current z < 1 Hubble plot data while requiring the universe to be accelerating at higher z instead. With both standard and conformal gravity being found to be compatible with a very recently reported z = 1.7 data point, additional data points will thus be needed to determine whether the universe is accelerating or decelerating above z = 1. I. THE HUBBLE PLOT OF STANDARD COSMOLOGYA variety of recent observational measurements [1][2][3][4] have sharply constrained the space available to the standard model cosmological parameters. Specifically, phenomenological data fitting using the standard Einstein-Friedmann cosmological evolution equatioṅ(where Ω M (t) = 8πGρ M (t)/3c 2 H 2 (t) is due to ordinary ρ M (t) ∼ 1/R n (t) matter and where Ω Λ (t) = 8πGΛ/3cH 2 (t) is due to a cosmological constant cΛ) is found to favor current era values Ω M (t 0 ) ≃ 0.3 and Ω Λ (t 0 ) ≃ 0.7, with the current era deceleration parameter q 0 = q(t 0 ) = (n/2 − 1)Ω M (t 0 ) − Ω Λ (t 0 ) then having to be of order −1/2. While these values are very supportive of the flat Ω k (t) = −kc 2 /Ṙ 2 (t) = 1 − Ω M (t) − Ω Λ (t) = 0 inflationary universe paradigm [5], they are nonetheless extremely troubling. Specifically, solving Eq. (1) for ρ M (t) = B/R 3 (t) and k = 0 yields R(t) = (B/cΛ) 1/3 sinh 2/3 (3D 1/2 t/2) where D = 8πGΛ/3c, so that Ω Λ (t) = D/H 2 (t) = 1 − Ω M (t) = tanh 2 (3D 1/2 t/2).(2) *

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“…In the fourth order Weyl theory of gravity [1,2,3,4,5,6,7,8,9], the Einstein-Hilbert action is replaced by a term proportional to the square of the Weyl tensor, and the action of the standard model of particle physics is modified to make it be conformally invariant. For simplicity and following Ref.…”

Section: Introductionmentioning

confidence: 99%

Fourth order Weyl gravity

Flanagan

2006

Phys. Rev. D

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The fourth order Weyl gravity theory of Mannheim and Kazanas is based on replacing the Einstein-Hilbert action with the square of the Weyl tensor, and on modifying the matter action of the standard model of particle physics to make it conformally invariant. This theory has been suggested as a model of both dark matter and dark energy. We argue that the conformal invariance is not a fundamental property of the theory, and instead is an artifact of the choice of variables used in its description. We deduce that in the limit of weak fields and slow motions the theory does not agree with the predictions of general relativity, and is therefore ruled out by Solar System observations. PACS numbers: 95.36.+x,04.50.+h,95.35.+d

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Conformal cosmology with no cosmological constant

Cited by 136 publications

References 7 publications

BeyondΛCDM: Problems, solutions, and the road ahead

BeyondΛCDM: Problems, solutions, and the road ahead

How Recent Is Cosmic Acceleration?

Fourth order Weyl gravity

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