Suppressing Quantum Fluctuations in Classicalization

We study vacuum quantum fluctuations of simple Nambu-Goldstone bosons - derivatively coupled single scalar-field theories possessing shift-symmetry in field space. We argue that quantum fluctuations of the interacting field can be drastically suppressed with respect to the free-field case. Moreover, the power-spectrum of these fluctuations can soften to become red for sufficiently small scales. In quasiclassical approximation, we demonstrate that this suppression can only occur for those theories that admit such classical static backgrounds around which small perturbations propagate faster than light. Thus, a quasiclassical softening of quantum fluctuations is only possible for theories which classicalize instead of having a usual Lorentz invariant and local Wilsonian UV- completion. We illustrate our analysis by estimating the quantum fluctuations for the DBI-like theories.Comment: 6 pages, no figures, published version, more general discussion of uncertainty relation in QFT, improved and more general derivation of the main resul

Comment on superluminality in general relativity

General relativity provides an appropriate framework for addressing the issue of sub- or superluminality as an apparent effect. Even though a massless particle travels on the light cone, its average velocity over a finite path measured by different observers is not necessarily equal to the velocity of light, as a consequence of the time dilation or contraction in gravitational fields. This phenomenon occurs in either direction (increase or depletion) irrespectively of the details and strength of the gravitational interaction. Hence, it does not intrinsically guarantee superluminality, even when the gravitational field is reinforced.Comment: 6 page

Price for Environmental Neutrino-Superluminality

We ask whether the recent OPERA results on neutrino superluminality could be an environmental effect characteristic of the local neighborhood of our planet, without the need of violation of the Poincar\'e-invariance at a fundamental level. This explanation requires the existence of a new spin-2 field of a planetary Compton wave-length that is coupled to neutrinos and the rest of the matter asymmetrically, both in the magnitude and in the sign. Sourced by the earth this field creates an effective metric on which neutrinos propagate superluminally, whereas other species are much less sensitive to the background. Such a setup, at an effective field theory level, passes all immediate phenomenological tests and its natural prediction is an inevitable appearance of a testable long-range gravity-type fifth force. We then prove that under the assumption of the weakly-coupled Poincar\'e-invariant physics, the asymmetrically-coupled second massive graviton is the only possible environmental explanation. Despite phenomenological viability, the sign asymmetry of the coupling we identify as the main potential obstacle for a consistent UV-completion. We also discuss the possible identification of this field with a Kaluza-Klein state of an extra dimension in which neutrino can propagate.Comment: 5 pages, added references and discussion of strong coupling, corrected typos, matches the published versio

Stability of Closed Timelike Curves in a Galileon Model

Recently Burrage, de Rham, Heisenberg and Tolley have constructed eternal, classical solutions with closed timelike curves (CTCs) in a Galileon model coupled to an auxiliary scalar field. These theories contain at least two distinct metrics and, in configurations with CTCs, two distinct notions of locality. As usual, globally CTCs lead to pathologies including nonlocal constraints on the initial Cauchy data. Locally, with respect to the gravitational metric, we use a WKB approximation to explicitly construct small, short-wavelength perturbations without imposing the nonlocal constraints and observe that these perturbations do not grow and so do not lead to an instability.Comment: 10 pages, no figure

Vainshtein in the UV and a Wilsonian analysis of derivatively coupled scalars

In the first part of this paper we critically examine the ultra-violet implications of theories that exhibit Vainshtein screening, taking into account both the standard Wilsonian perspective as well as more exotic possibilities. Aspects of this discussion draw on results from the second part of the paper in which we perform a general study of derivatively coupled scalar theories using non–perturbative exact renormalisation group techniques, which are of interest independently of their application to modified gravity. In this context, we demonstrate the suppression of quantum corrections within the Vainshtein radius and discuss the potential relation with the classicalisation conjecture. We question whether the latter can be considered a realistic candidate for UV completion of large-scale modifications of gravity on account of a dangerously low classicalisation/strong coupling scale

Vacuum structure for scalar cosmological perturbations in Modified Gravity Models

We have found for the general class of Modified Gravity Models f(R,G) a new instability which can arise in vacuum for the scalar modes of the cosmological perturbations if the background is not de Sitter. In particular, the short-wavelength modes, if stable, in general have a group velocity which depends linearly in k, the wave number. Therefore these modes will be in general superluminal. We have also discussed the condition for which in general these scalar modes will be ghost-like. There is a subclass of these models, defined out of properties of the function f(R,G) and to which the f(R) and f(G) models belong, which however does not have this feature.Comment: 17 pages, 1 figure, uses RevTeX, references adde

The Imperfect Fluid behind Kinetic Gravity Braiding

We present a standard hydrodynamical description for non-canonical scalar field theories with kinetic gravity braiding. In particular, this picture applies to the simplest galileons and k-essence. The fluid variables not only have a clear physical meaning but also drastically simplify the analysis of the system. The fluid carries charges corresponding to shifts in field space. This shift-charge current contains a spatial part responsible for diffusion of the charges. Moreover, in the incompressible limit, the equation of motion becomes the standard diffusion equation. The fluid is indeed imperfect because the energy flows neither along the field gradient nor along the shift current. The fluid has zero vorticity and is not dissipative: there is no entropy production, the energy-momentum is exactly conserved, the temperature vanishes and there is no shear viscosity. Still, in an expansion around a perfect fluid one can identify terms which correct the pressure in the manner of bulk viscosity. We close by formulating the non-trivial conditions for the thermodynamic equilibrium of this imperfect fluid.Comment: 23 pages plus appendices. New version includes extended discussion on diffusion and dynamics in alternative frames, as well as additional references. v3 reflects version accepted for publication in JHEP: minor comments added regarding suitability to numerical approache

A Braneworld Dark Energy Model with Induced Gravity and the Gauss-Bonnet Effect

We construct a holographic dark energy model with a non-minimally coupled scalar field on the brane where Gauss-Bonnet and Induced Gravity effects are taken into account. This model provides a wide parameter space with several interesting cosmological implications. Especially, the equation of state parameter of the model crosses the phantom divide line and it is possible to realize bouncing solutions in this setup.Comment: 20 pages, 3 eps figures, to appear in IJT

Cosmological constraints on the dark energy equation of state and its evolution

We have calculated constraints on the evolution of the equation of state of the dark energy, w(z), from a joint analysis of data from the cosmic microwave background, large scale structure and type-Ia supernovae. In order to probe the time-evolution of w we propose a new, simple parametrization of w, which has the advantage of being transparent and simple to extend to more parameters as better data becomes available. Furthermore it is well behaved in all asymptotic limits. Based on this parametrization we find that w(z=0)=-1.43^{+0.16}_{-0.38} and dw/dz(z=0) = 1.0^{+1.0}_{-0.8}. For a constant w we find that -1.34 < w < -0.79 at 95% C.L. Thus, allowing for a time-varying w shifts the best fit present day value of w down. However, even though models with time variation in w yield a lower chi^2 than pure LambdaCDM models, they do not have a better goodness-of-fit. Rank correlation tests on SNI-a data also do not show any need for a time-varying w.Comment: 19 pages, 11 figures, JCAP format, typos corrected, references update