7 research outputs found
Horava-Lifshitz gravity: a status report
This is intended to be a brief introduction and overview of Horava-Lifshitz
gravity. The motivation and all of the various version of the theory (to date)
are presented. The dynamics of the theory are discussed in some detail, with a
focus on low energy viability and consistency, as these have been the issues
that attracted most of the attention in the literature so far. Other properties
of the theory and developments within its framework are also covered, such as:
its relation to Einstein-aether theory, cosmology, and future perspectives.Comment: 17 pages, no figures, based on talk given at the 14th Conference on
Recent Developments in Gravity (NEBXIV), Ioannina, Greece, 8-11 Jun 2010; v2:
minor changes to match published version, references adde
Remarks on the Scalar Graviton Decoupling and Consistency of Horava Gravity
Recently Horava proposed a renormalizable gravity theory with higher
derivatives by abandoning the Lorenz invariance in UV. But there have been
confusions regarding the extra scalar graviton mode and the consistency of the
Horava model. I reconsider these problems and show that, in the Minkowski
vacuum background, the scalar graviton mode can be consistency decoupled from
the usual tensor graviton modes by imposing the (local) Hamiltonian as well as
the momentum constraints.Comment: Some clarifications regarding the projectable case added, Typos
corrected, Comments (Footnote No.9, Note Added) added, References updated,
Accepted in CQ
Lorentz violating kinematics: Threshold theorems
Recent tentative experimental indications, and the subsequent theoretical
speculations, regarding possible violations of Lorentz invariance have
attracted a vast amount of attention. An important technical issue that
considerably complicates detailed calculations in any such scenario, is that
once one violates Lorentz invariance the analysis of thresholds in both
scattering and decay processes becomes extremely subtle, with many new and
naively unexpected effects. In the current article we develop several extremely
general threshold theorems that depend only on the existence of some energy
momentum relation E(p), eschewing even assumptions of isotropy or monotonicity.
We shall argue that there are physically interesting situations where such a
level of generality is called for, and that existing (partial) results in the
literature make unnecessary technical assumptions. Even in this most general of
settings, we show that at threshold all final state particles move with the
same 3-velocity, while initial state particles must have 3-velocities
parallel/anti-parallel to the final state particles. In contrast the various
3-momenta can behave in a complicated and counter-intuitive manner.Comment: V1: 32 pages, 6 figures, 3 tables. V2: 5 references adde
Quantum gravity without Lorentz invariance
There has been a significant surge of interest in Horava's model for 3+1
dimensional quantum gravity, this model being based on anisotropic scaling at a
z=3 Lifshitz point. Horava's model, and its variants, show dramatically
improved ultra-violet behaviour at the cost of exhibiting violation of Lorentz
invariance at ultra-high momenta. Following up on our earlier note,
[arXiv:0904.4464 [hep-th]], we discuss in more detail our variant of Horava's
model. In contrast to Horava's original model, we abandon "detailed balance"
and restore parity invariance. We retain, however, Horava's "projectability
condition" and explore its implications. Under these conditions, we explicitly
exhibit the most general model, and extract the full classical equations of
motion in ADM form. We analyze both spin-2 and spin-0 graviton propagators
around flat Minkowski space. We furthermore analyze the classical evolution of
FLRW cosmologies in this model, demonstrating that the higher-derivative
spatial curvature terms can be used to mimic radiation fluid and stiff matter.
We conclude with some observations concerning future prospects.Comment: 36 pages. V2: three references added, technical changes to
gauge-fixing discussion, minor edits. V3: some additional discussion and
references, closely matches final published versio
Perturbative instabilities in Horava gravity
We investigate the scalar and tensor perturbations in Horava gravity, with
and without detailed balance, around a flat background. Once both types of
perturbations are taken into account, it is revealed that the theory is plagued
by ghost-like scalar instabilities in the range of parameters which would
render it power-counting renormalizable, that cannot be overcome by simple
tricks such as analytic continuation. Implementing a consistent flow between
the UV and IR limits seems thus more challenging than initially presumed,
regardless of whether the theory approaches General Relativity at low energies
or not. Even in the phenomenologically viable parameter space, the tensor
sector leads to additional potential problems, such as fine-tunings and
super-luminal propagation.Comment: 21 pages, version published at Class. Quant. Gra
Analogue Gravity
Analogue models of (and for) gravity have a long and distinguished history dating back to the earliest years of general relativity. In this review article we will discuss the history, aims, results, and future prospects for the various analogue models. We start the discussion
by presenting a particularly simple example of an analogue model, before exploring the rich history and complex tapestry of models discussed in the literature. The last decade in particular
has seen a remarkable and sustained development of analogue gravity ideas, leading to some hundreds of published articles, a workshop, two books, and this review article. Future prospects for the analogue gravity programme also look promising, both on the experimental
front (where technology is rapidly advancing) and on the theoretical front (where variants of analogue models can be used as a springboard for radical attacks on the problem of quantum
gravity)