https://biblio.ugent.be/publication/685594/file/1130605.pdf

Review of Particle Physics

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

http://risa.stanford.edu/teaching/362/particledarkmatter.pdf

Particle dark matter: Evidence, candidates and constraints

Modified Gravity and Cosmology

Over the past decade, f(R) theories have been extensively studied as one of the simplest modifications to General Relativity. In this article we review various applications of f(R) theories to cosmology and gravity - such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds. We present a number of ways to distinguish those theories from General Relativity observationally and experimentally. We also discuss the extension to other modified gravity theories such as Brans-Dicke theory and Gauss-Bonnet gravity, and address models that can satisfy both cosmological and local gravity constraints.

f(R) theories

In models with dynamical supersymmetry breaking in the hidden sector, the gaugino masses in the observable sector have been believed to be extremely suppressed (below 1 keV), unless there is a gauge singlet in the hidden sector with specific couplings to the observable sector gauge multiplets. We point out that there is a pure supergravity contribution to gaugino masses at the quantum level arising from the superconformal anomaly. Our results are valid to all orders in perturbation theory and are related to the ‘exact’ beta functions for soft terms. There is also an anomaly contribution to the A terms proportional to the beta function of the corresponding Yukawa coupling. The gaugino masses are proportional to the corresponding gauge beta functions, and so do not satisfy the usual GUT relations.

/pdf/gaugino-mass-without-singlets-32pzor5lp4.pdf

Gaugino mass without singlets

We propose a theoretically consistent modification of gravity in the infrared, which is compatible with all current experimental observations. This is an analog of the Higgs mechanism in general relativity, and can be thought of as arising from ghost condensation — a background where a scalar field has a constant velocity, = M2. The ghost condensate is a new kind of fluid that can fill the universe, which has the same equation of state, ρ = −p, as a cosmological constant, and can hence drive de Sitter expansion of the universe. However, unlike a cosmological constant, it is a physical fluid with a physical scalar excitation, which can be described by a systematic effective field theory at low energies. The excitation has an unusual low-energy dispersion relation ω2 ~ 4/M2. If coupled to matter directly, it gives rise to small Lorentz-violating effects and a new long-range 1/r2 spin dependent force. In the ghost condensate, the energy that gravitates is not the same as the particle physics energy, leading to the possibility of both sources that can gravitate and anti-gravitate. The newtonian potential is modified with an oscillatory behavior starting at the distance scale MPl/M2 and the time scale MPl2/M3. This theory opens up a number of new avenues for attacking cosmological problems, including inflation, dark matter and dark energy.

https://iopscience.iop.org/article/10.1088/1126-6708/2004/05/074/pdf

Ghost condensation and a consistent infrared modification of gravity

The model of Dvali, Gabadadze, and Porrati (DGP) gives a simple geometrical setup in which gravity becomes 5-dimensional at distances larger than a length scale lambda(DGP). We show that this theory has strong interactions at a length scale lambda(3) similar to (lambda(DGP)(2)/M-P)(1/3). If lambda(DGP) is of order the Hubble length, then the theory loses predictivity at distances shorter than lambda(3) similar to 1000 km. The strong interaction can be viewed as arising from a longitudinal 'eaten Goldstone' mode that gets a small kinetic term only from mixing with transverse graviton polarizations, analogous to the case of massive gravity. We also present a negative-energy classical solution, which can be avoided by cutting off the theory at the same scale scale lambda(3). Finally, we examine the dynamics of the longitudinal Goldstone mode when the background geometry is curved.

/pdf/strong-interactions-and-stability-in-the-dgp-model-1xfygoydzg.pdf

Strong Interactions and Stability in the DGP Model

We present a consistent effective theory that violates the null energy condition (NEC) without developing any instabilities or other pathological features. The model is the ghost condensate with the global shift symmetry softly broken by a potential. We show that this system can drive a cosmological expansion with > 0. Demanding the absence of instabilities in this model requires H2. We then construct a general low-energy effective theory that describes scalar fluctuations about an arbitrary FRW background, and argue that the qualitative features found in our model are very general for stable systems that violate the NEC. Violating the NEC allows dramatically non-standard cosmological histories. To illustrate this, we construct an explicit model in which the expansion of our universe originates from an asymptotically flat state in the past, smoothing out the big-bang singularity within control of a low-energy effective theory. This gives an interesting alternative to standard inflation for solving the horizon problem. We also construct models in which the present acceleration has w<−1; a periodic ever-expanding universe; and a model with a smooth ``bounce'' connecting a contracting and expanding phase.

http://users.ictp.it/~pub_off/preprints-sources/2006/IC2006034P.pdf

Starting the Universe: Stable Violation of the Null Energy Condition and Non-standard Cosmologies

If right-handed Majorana neutrinos are added to the standard model, then lepton-number-violating out-of-equilibrium decays of right-handed neutrinos combined with anomalous electroweak processes can generate the baryon number of the Universe. We analyze this mechanism in detail, and determine the ranges of parameters for which the correct baryon number is generated. We find that the scenario works for a wide range of parameters in the neutrino sector, including right-handed neutrino masses ranging from \ensuremath{\sim}1 TeV to \ensuremath{\sim}${10}^{19}$ GeV, depending on the assumptions made about the structure of the neutrino mass matrices.

Markus A. Luty

Papers

Gaugino mass without singlets

Ghost condensation and a consistent infrared modification of gravity

Strong Interactions and Stability in the DGP Model

Starting the Universe: Stable Violation of the Null Energy Condition and Non-standard Cosmologies

Baryogenesis via leptogenesis