Maximal freedom at minimum cost: linear large-scale structure in general modifications of gravity

Bellini, Emilio; Sawicki, Ignacy

doi:10.1088/1475-7516/2014/07/050

Cited by 373 publications

(801 citation statements)

References 145 publications

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“…The validity of the QS approximation imposes strong requirements on the dark energy theory however -in particular, that its sound speed be close to c [211]. An alternative approach, inspired by effective field theory, that exploits the diffeomorphism-invariant structure of GR [181,468,469,470] allows for a parameterization that does not depend on the assumption of quasi-staticity (although it does depend on modeling the dark energy sector as containing a single extra degree of freedom).…”

Section: Model-independent Tests Of λCdm Discussion Session Chairs: Ementioning

confidence: 99%

BeyondΛCDM: Problems, solutions, and the road ahead

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Akrami

Adamek

et al. 2016

Physics of the Dark Universe

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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: Model-independent Tests Of λCdm Discussion Session Chairs: Ementioning

confidence: 99%

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

Self Cite

420

206

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“…The problem is made more tractable by applying constraints on the scale dependence of MG parameters as discussed above (in particular the parameter γ and µ in equations (10.41) and (10.42)). This takes us from two functions of time and scale to five functions of time, which are significantly easier to constrain from observations (see also [941,980,981]). …”

Section: Model Independent Approaches To Cosmological Analysismentioning

confidence: 99%

Beyond the cosmological standard model

et al. 2015

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After a decade and a half of research motivated by the accelerating universe, theory and experiment have a reached a certain level of maturity. The development of theoretical models beyond Λ or smooth dark energy, often called modified gravity, has led to broader insights into a path forward, and a host of observational and experimental tests have been developed. In this review we present the current state of the field and describe a framework for anticipating developments in the next decade. We identify the guiding principles for rigorous and consistent modifications of the standard model, and discuss the prospects for empirical tests.We begin by reviewing recent attempts to consistently modify Einstein gravity in the infrared, focusing on the notion that additional degrees of freedom introduced by the modification must "screen" themselves from local tests of gravity. We categorize screening mechanisms into three broad classes: mechanisms which become active in regions of high Newtonian potential, those in which first derivatives of the field become important, and those for which second derivatives of the field are important. Examples of the first class, such as f (R) gravity, employ the familiar chameleon or symmetron mechanisms, whereas examples of the last class are galileon and massive gravity theories, employing the Vainshtein mechanism. In each case, we describe the theories as effective theories and discuss prospects for completion in a more fundamental theory. We describe experimental tests of each class of theories, summarizing laboratory and solar system tests and describing in some detail astrophysical and cosmological tests. Finally, we discuss prospects for future tests which will be sensitive to different signatures of new physics in the gravitational sector.The review is structured so that those parts that are more relevant to theorists vs. observers/experimentalists are clearly indicated, in the hope that this will serve as a useful reference for both audiences, as well as helping those interested in bridging the gap between them.

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“…In alternative theories of gravity, additional polarizations may propagate, each with potentially different velocities, attenuations and effective masses [7]. This issue has been well studied in cosmology, and has been a topic of discussion in connection to the early [8][9][10][11] and the late Universe [12][13][14][15]. There are fairly model-independent tests for effects caused by additional polarizations [16], damping [17][18][19], mass [20], and Lorentz symmetry violations [21,22].…”

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confidence: 99%

Speed of gravitational waves and the fate of scalar-tensor gravity

et al. 2017

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The direct detection of gravitational waves (GWs) is an invaluable new tool to probe gravity and the nature of cosmic acceleration. A large class of scalar-tensor theories predict that GWs propagate with velocity different than the speed of light, a difference that can be O(1) for many models of dark energy. We determine the conditions behind the anomalous GW speed, namely that the scalar field spontaneously breaks Lorentz invariance and couples to the metric perturbations via the Weyl tensor. If these conditions are realized in nature, the delay between GW and electromagnetic (EM) signals from distant events will run beyond human timescales, making it impossible to measure the speed of GWs using neutron star mergers or other violent events. We present a robust strategy to exclude or confirm an anomalous speed of GWs using eclipsing binary systems, whose EM phase can be exquisitely determined. he white dwarf binary J0651+2844 is a known example of such system that can be used to probe deviations in the GW speed as small as cg/c − 1 2 · 10 −12 when LISA comes online. This test will either eliminate many contender models for cosmic acceleration or wreck a fundamental pillar of general relativity.PACS numbers: 04.30.Nk 04.50. Kd, 95.36.+x, Introduction and summary. The direct detection of gravitational radiation [1,2] has initiated a new era for astronomy, astrophysics and fundamental physics. The observed gravitational wave (GW) events and the ones to come will usher in novel ways to test the nature of gravity [3]. Here, we will argue that probing the speed of GWs will be a decisive test for gravity and dark energy models.

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Maximal freedom at minimum cost: linear large-scale structure in general modifications of gravity

Cited by 373 publications

References 145 publications

BeyondΛCDM: Problems, solutions, and the road ahead

BeyondΛCDM: Problems, solutions, and the road ahead

Beyond the cosmological standard model

Speed of gravitational waves and the fate of scalar-tensor gravity

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