There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

We review in detail a number of approaches that have been adopted to try and explain the remarkable observation of our accelerating universe. In particular we discuss the arguments for and recent progress made towards understanding the nature of dark energy. We review the observational evidence for the current accelerated expansion of the universe and present a number of dark energy models in addition to the conventional cosmological constant, paying particular attention to scalar field models such as quintessence, K-essence, tachyon, phantom and dilatonic models. The importance of cosmological scaling solutions is emphasized when studying the dynamical system of scalar fields including coupled dark energy. We study the evolution of cosmological perturbations allowing us to confront them with the observation of the Cosmic Microwave Background and Large Scale Structure and demonstrate how it is possible in principle to reconstruct the equation of state of dark energy by also using Supernovae Ia observational data. We also discuss in detail the nature of tracking solutions in cosmology, particle physics and braneworld models of dark energy, the nature of possible future singularities, the effect of higher order curvature terms to avoid a Big Rip singularity, and approaches to modifying gravity which leads to a late-time accelerated expansion without recourse to a new form of dark energy.

Dynamics of dark energy

Physics welcomes the idea that space contains energy whose gravitational effect approximates that of Einstein's cosmological constant, \ensuremath{\Lambda}; today the concept is termed dark energy or quintessence. Physics also suggests that dark energy could be dynamical, allowing for the arguably appealing picture of an evolving dark-energy density approaching its natural value, zero, and small now because the expanding universe is old. This would alleviate the classical problem of the curious energy scale of a millielectron volt associated with a constant \ensuremath{\Lambda}. Dark energy may have been detected by recent cosmological tests. These tests make a good scientific case for the context, in the relativistic Friedmann-Lema\^{\i}tre model, in which the gravitational inverse-square law is applied to the scales of cosmology. We have well-checked evidence that the mean mass density is not much more than one-quarter of the critical Einstein--de Sitter value. The case for detection of dark energy is not yet as convincing but still serious; we await more data, which may be derived from work in progress. Planned observations may detect the evolution of the dark-energy density; a positive result would be a considerable stimulus for attempts at understanding the microphysics of dark energy. This review presents the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.

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The Cosmological Constant and Dark Energy

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160)  Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

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The Observation of Gravitational Waves from a Binary Black Hole Merger

Modified gravity theories have received increased attention lately due to combined motivation coming from high-energy physics, cosmology, and astrophysics. Among numerous alternatives to Einstein's theory of gravity, theories that include higher-order curvature invariants, and specifically the particular class of $f(R)$ theories, have a long history. In the last five years there has been a new stimulus for their study, leading to a number of interesting results. Here $f(R)$ theories of gravity are reviewed in an attempt to comprehensively present their most important aspects and cover the largest possible portion of the relevant literature. All known formalisms are presented---metric, Palatini, and metric affine---and the following topics are discussed: motivation; actions, field equations, and theoretical aspects; equivalence with other theories; cosmological aspects and constraints; viability criteria; and astrophysical applications.

f ( R ) theories of gravity

1. Extended gravity: a primer.- 2. Mathematical Tools.- 3. The landscape beyond Einstein gravity.- 4. Spherical symmetry.- 5. Weak-field limit.- 6. Qualitative analysis and exact solutions in cosmology.- 7. Cosmology.- 8. From the early to the present universe.- A. Physical constants and astrophysical and cosmological parameters.- B. The Noether symmetry approach to f(R) gravity.- C. The weak-field limit of metric f(R) gravity.- References.- Index.

Beyond Einstein Gravity: A Survey of Gravitational Theories for Cosmology and Astrophysics

1. Scalar-Tensor Gravity.- 1 Introduction.- 2 Brans-Dicke theory.- 3 Brans-Dicke cosmology in the Jordan frame.- 4 The limit to general relativity.- 5 Relation to Kaluza-Klein theory.- 6 Brans-Dicke theory from Lyra's geometry.- 7 Scalar-tensor theories.- 7.1 Effective Lagrangians and Hamiltonians.- 8 Motivations for scalar-tensor theories.- 9 Induced gravity.- 10 Generalized scalar-tensor theories.- 11 Conformal transformation techniques.- 11.1 Conformal transformations.- 11.2 Brans-Dicke theory.- 11.3 Kaluza-Klein cosmology.- 11.4 Scalar-tensor theories.- 11.5 Generalized scalar-tensor theories.- 12 Singularities of the gravitational coupling.- 2. Effective Energy-Momentum Tensors and Conformal Frames.- 1 The issue of the conformal frame.- 1.1 The first viewpoint.- 1.2 The second viewpoint.- 1.3 The third viewpoint.- 1.4 Other viewpoints.- 1.5 Einstein frame or Jordan frame?.- 1.6 Energy conditions in relativistic theories.- 1.7 Singularity theorems and energy conditions.- 2 Effective energy-momentum tensors.- 2.1 Time-dependence of the gravitational coupling.- 2.2 Conservation equations for the various Tab(J) [oo].- 3. Gravitational Waves.- 1 Introduction.- 2 Einstein frame scalar-tensor waves.- 2.1 Gravitational waves in the Einstein frame.- 2.2 Corrections to the geodesic deviation equation.- 3 Gravitational lensing by scalar-tensor gravitational waves.- 3.1 Jordan frame analysis.- 3.2 Einstein frame analysis.- 3.3 Propagation of light through a gravitational wave background.- 4. Exact Solutions of Scalar-Tensor Cosmology.- 1 Introduction.- 2 Exact solutions of Brans-Dicke cosmology.- 2.1 K = 0 FLRW solutions.- 2.1.1 The O'Hanlon and Tupper solution.- 2.1.2 The Brans-Dicke dust solution.- 2.1.3 The Nariai solution.- 2.1.4 Other solutions with cosmological constant.- 2.1.5 Generalizing Nariai's solution.- 2.1.6 Phase space analysis for K = 0 and V(o) = 0.- 2.1.7 Phase plane analysis for K = 0 and V(o) = Ao.- 2.2 K = +-1 solutions and phase space for V = 0.- 2.3 Phase space for any K and V = m2o2/2.- 2.3.1 The Dehnen-Obregon solution.- 2.4 Bianchi models.- 2.4.1 Bianchi V universes.- 3 Exact solutions of scalar-tensor theories.- 5. The Early Universe.- 1 Introduction.- 2 Extended inflation.- 2.1 The original extended inflationary scenario.- 2.2 Alternatives.- 3 Hyperextended inflation.- 4 Real inflation?.- 5 Constraints from primordial nucleosynthesis.- 6. Perturbations.- 1 Introduction.- 2 Scalar perturbations.- 3 Tensor perturbations.- 7. Nonminimal Coupling.- 1 Introduction.- 1.1 Generalized inflation.- 1.2 Motivations for nonminimal coupling.- 1.3 Which value of.- 2 Effective energy-momentum tensors.- 2.1 Approach a la Callan-Coleman-Jackiw.- 2.2 Effective coupling.- 2.3 A mixed approach.- 2.4 Discussion.- 2.5 Energy conditions in FLRW cosmology.- 2.6 Nonminimal coupling and gravitational waves.- 3 Conformal transformations.- 4 Inflation and ? 0: the unperturbed universe.- 4.1 Necessary conditions for generalized inflation.- 4.1.1 Specific potentials.- 4.2 The effective equation of state with nonminimal coupling.- 4.3 Critical values of the scalar field.- 5 The slow-roll regime of generalized inflation.- 5.1 Derivation of the stability conditions.- 5.2 Slow-roll parameters.- 6 Inflation and ? 0: perturbations.- 6.1 Density perturbations.- 6.2 Tensor perturbations.- 7 Conclusion.- 8. The Present Universe.- 1 Present acceleration of the universe and quintessence.- 1.1 Coupled quintessence.- 1.2 Multiple field quintessence.- 1.3 Falsifying quintessence models.- 2 Quintessence with nonminimal coupling.- 2.1 Models using the Ratra-Peebles potential.- 2.2 Necessary conditions for accelerated expansion.- 2.3 Doppler peaks with nonminimal coupling.- 3 Superquintessence.- 3.1 An exact superaccelerating solution.- 3.2 Big Smash singularities.- 4 Quintessence in scalar-tensor gravity.- 5 Conclusion.- References.

Cosmology in Scalar-Tensor Gravity

Hawking has proven that black holes which are stationary as the end point of gravitational collapse in Brans-Dicke theory (without a potential) are no different than in general relativity. We extend this proof to the much more general class of scalar-tensor and f(R) gravity theories, without assuming any symmetries apart from stationarity.

Black holes in scalar-tensor gravity.

The nonminimal coupling (NMC) of the scalar field to the Ricci curvature is unavoidable in many cosmological scenarios. Inflation and quintessence models based on nonminimally coupled scalar fields are studied, with particular attention to the balance between the scalar potential and the NMC term $\ensuremath{\xi}R{\ensuremath{\varphi}}^{2}/2$ in the action. NMC makes acceleration of the universe harder to achieve for the usual potentials, but it is beneficial in obtaining cosmic acceleration with unusual potentials. The slow-roll approximation with NMC, conformal transformation techniques, and other aspects of the physics of NMC are clarified.

Valerio Faraoni

Papers

f ( R ) theories of gravity

Beyond Einstein Gravity: A Survey of Gravitational Theories for Cosmology and Astrophysics

Cosmology in Scalar-Tensor Gravity

Black holes in scalar-tensor gravity.

Inflation and quintessence with nonminimal coupling