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71
Modified Gravity and Cosmology
, 2012
"... In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, ScalarTensor, EinsteinAether, and Bimetric theories, as well as TeVeS, f(R), general higherorder theo ..."
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Cited by 26 (0 self)
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In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, ScalarTensor, EinsteinAether, and Bimetric theories, as well as TeVeS, f(R), general higherorder theories, HořavaLifschitz gravity, Galileons, Ghost Condensates, and models of extra dimensions including KaluzaKlein, RandallSundrum, DGP, and higher codimension braneworlds. We also review attempts to construct a Parameterised PostFriedmannian formalism, that can be used to constrain deviations from General Relativity in cosmology, and that is suitable for comparison with data on the largest scales. These subjects have been intensively studied over the past decade, largely motivated by rapid progress in the field of observational cosmology that now allows, for the first time, precision tests of fundamental physics on the scale of the observable Universe. The purpose of this review is to provide a reference tool for researchers and students in cosmology and gravitational physics, as well as a selfcontained, comprehensive and uptodate introduction to the subject as a whole.
The phenomenology of DvaliGabadadzePorrati cosmologies
 Phys. Rept
, 2006
"... Cosmologists today are confronted with the perplexing reality that the universe is currently accelerating in its expansion. Nevertheless, the nature of the fuel that drives today’s cosmic acceleration is an open and tantalizing mystery. There exists the intriguing possibility that the acceleration i ..."
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Cosmologists today are confronted with the perplexing reality that the universe is currently accelerating in its expansion. Nevertheless, the nature of the fuel that drives today’s cosmic acceleration is an open and tantalizing mystery. There exists the intriguing possibility that the acceleration is not the manifestation of yet another mysterious ingredient in the cosmic gas tank (dark energy), but rather our first real lack of understanding of gravity itself, and even possibly a signal that there might exist dimensions beyond that which we can currently observe. The braneworld model of Dvali, Gabadadze and Porrati (DGP) is a theory where gravity is altered at immense distances by the excruciatingly slow leakage of gravity off our threedimensional Universe and, as a modifiedgravity theory, has pioneered this line of investigation. I review the underlying structure of DGP gravity and those phenomenological developments relevant to cosmologists interested in a pedagogical treatment of this intriguing model.
Dark Energy as a Modification of the Friedmann Equation
, 2003
"... Dark energy could actually be the manifestation of a modification to the Friedmann equation arising from new physics (e.g., extra dimensions). Writing the correction as (1 − ΩM)H α /H α−2 0, we explore the phenomenology and detectability of such. We show that: (i) α must be ∼ < 1; (ii) such a cor ..."
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Dark energy could actually be the manifestation of a modification to the Friedmann equation arising from new physics (e.g., extra dimensions). Writing the correction as (1 − ΩM)H α /H α−2 0, we explore the phenomenology and detectability of such. We show that: (i) α must be ∼ < 1; (ii) such a correction behaves like dark energy with equationofstate weff = −1 + α 2 in the recent past (104> z ≫ 1) and w = −1 in the distant future and can mimic w < −1 without violating the weakenergy condition; (iii) weff changes, dz/dwz∼0.5 ∼ O(0.2), which is likely detectable; and (iv) a future supernova experiment like SNAP that The discovery that the expansion of the Universe is speeding up and not slowing down [1, 2] has presented cosmologists and particle physicists with a profound (and wonderful) puzzle. In the context of general relativity this surprising result can be accounted for by the existence
Exactly Solvable SFT Inspired Phantom Model
 MATH. PHYS
, 2006
"... An exact solution to the Friedmann equations with a string inspired phantom scalar matter field is constructed and the absence of the “Big Rip” singularity is shown explicitly. The notable features of the concerned model are a ghost sign of the kinetic term and a special polynomial form of the effec ..."
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Cited by 12 (7 self)
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An exact solution to the Friedmann equations with a string inspired phantom scalar matter field is constructed and the absence of the “Big Rip” singularity is shown explicitly. The notable features of the concerned model are a ghost sign of the kinetic term and a special polynomial form of the effective tachyon potential. The constructed solution is stable with respect to small fluctuations of the initial conditions and special deviations of the form of the potential.
Approaches to understanding cosmic acceleration
"... Abstract. Theoretical approaches to explaining the observed acceleration of the universe are reviewed. We briefly discuss the evidence for cosmic acceleration, and the implications for standard General Relativity coupled to conventional sources of energymomentum. We then address three broad methods ..."
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Abstract. Theoretical approaches to explaining the observed acceleration of the universe are reviewed. We briefly discuss the evidence for cosmic acceleration, and the implications for standard General Relativity coupled to conventional sources of energymomentum. We then address three broad methods of addressing an accelerating universe: the introduction of a cosmological constant, its problems and origins; the possibility of dark energy, and the associated challenges for fundamental physics; and the option that an infrared modification of general relativity may be responsible for the largescale behavior of the universe. Approaches to Understanding Cosmic Acceleration 2 1.
Cosmological dynamics and dark energy from nonlocal infrared modifications of gravity,” Int.J.Mod.Phys. A29
, 2014
"... We perform a detailed study of the cosmological dynamics of a recently proposed infrared modification of the Einstein equations, based on the introduction of a nonlocal term constructed with m2gµν2 −1R, where m is a mass parameter. The theory generates automatically a dynamical dark energy componen ..."
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Cited by 8 (2 self)
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We perform a detailed study of the cosmological dynamics of a recently proposed infrared modification of the Einstein equations, based on the introduction of a nonlocal term constructed with m2gµν2 −1R, where m is a mass parameter. The theory generates automatically a dynamical dark energy component, that can reproduce the observed value of the dark energy density without introducing a cosmological constant. Fixing m so to reproduce the observed value ΩDE ' 0.68, and writing w(a) = w0 + (1−a)wa, the model provides a neat prediction for the equation of state parameters of dark energy, w0 ' −1.042 and wa ' −0.020. We show that, because of some freedom in the definition of 2−1, one can extend the construction so to define a more general family of nonlocal models. However, in a first approximation this turns out to be equivalent to adding an explicit cosmological constant term on top of the dynamical dark energy component. This leads to an extended model with two parameters, ΩΛ and m. Even in this extension the EOS parameter w0 is always on the phantom side, in the range −1.33<∼w0 ≤ −1, and there is a prediction for the relation between w0 and wa. ar
The IRCompletion of Gravity: What happens at Hubble Scales?
, 907
"... We have recently proposed an “UltraStrong ” version of the Equivalence Principle (EP) that is not satisfied by standard semiclassical gravity. In the theory that we are conjecturing, the vacuum expectation value of the (bare) energy momentum tensor is exactly the same as in flat space: quartically ..."
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Cited by 7 (3 self)
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We have recently proposed an “UltraStrong ” version of the Equivalence Principle (EP) that is not satisfied by standard semiclassical gravity. In the theory that we are conjecturing, the vacuum expectation value of the (bare) energy momentum tensor is exactly the same as in flat space: quartically divergent with the cutoff and with no spacetime dependent (subleading) terms. The presence of such terms seems in fact related to some known difficulties, such as the black hole information loss and the cosmological constant problem. Since the terms that we want to get rid of are subleading in the highmomentum expansion, we attempt to explore the conjectured theory by “IRcompleting ” GR. We consider a scalar field in a flat FRW Universe and isolate the first IRcorrection to its Fourier modes operators that kills the quadratic (next to leading) time dependent divergence of the stress energy tensor VEV. Analogously to other modifications of field operators that have been proposed in the literature (typically in the UV), the present approach seems to suggest a breakdown (here, in the IR, at large distances) of the metric manifold description. We show that corrections to GR are in fact very tiny, become effective at distances comparable to the inverse curvature and do not contain any adjustable parameter. Finally, we derive some cosmological implications. By studying the consistency of the canonical commutation relations, we infer a correction to the distance between two comoving observers, which grows as the scale factor only when small compared to the Hubble length, but gets relevant corrections otherwise. The corrections to cosmological distance measures are also calculable and, for a spatially flat matter dominated Universe, go in the direction of an effective positive acceleration.
Decoding the bispectrum of singlefield inflation
 JCAP
"... Abstract. Galileon fields arise naturally from the decoupling limit of massive gravities, and possess special selfinteractions which are protected by a spacetime generalization of Galilean symmetry. We briefly revisit the inflationary phenomenology of Galileon theories. Working from recent computat ..."
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Cited by 6 (3 self)
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Abstract. Galileon fields arise naturally from the decoupling limit of massive gravities, and possess special selfinteractions which are protected by a spacetime generalization of Galilean symmetry. We briefly revisit the inflationary phenomenology of Galileon theories. Working from recent computations of the fluctuation Lagrangian to cubic order in the most general model with secondorder equations of motion, we show that a distinct shape is present but with suppressed amplitude. A similar shape has been found in other higherderivative models. It may be visible in a theory tuned to suppress the leadingorder shapes, or if the overall bispectrum has large amplitude. Using a partialwave expansion of the bispectrum, we suggest a possible origin for the frequent appearance of this shape. It follows that models with very disparate microphysics can produce very similar bispectra. We argue that it may be more profitable to distinguish these models by searching for relations between the amplitudes of these common shapes. We illustrate this method using the examples of DBI and kinflation.
The Cosmological Constant and the Deconstruction of Gravity
, 2002
"... Witten has presented an argument for the vanishing of the cosmological constant in 2 + 1 dimensions. This argument is crucially tied to the specific properties of (2 + 1)dimensional gravity. We argue that this reasoning can be deconstructed to 3 + 1 dimensions under certain conditions. Our observat ..."
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Cited by 5 (1 self)
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Witten has presented an argument for the vanishing of the cosmological constant in 2 + 1 dimensions. This argument is crucially tied to the specific properties of (2 + 1)dimensional gravity. We argue that this reasoning can be deconstructed to 3 + 1 dimensions under certain conditions. Our observation is also tied to a possibility that there exists a welldefined UV completion of (3 + 1)dimensional gravity.