Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The relation between general relativity and a class of Ho\v{r}ava
  gravity theories   Violations of Lorentz (and specifically boost) invariance can make gravity
renormalizable in the ultraviolet, as initially noted by Ho\v{r}ava, but are
increasingly constrained in the infrared. At low energies, Ho\v{r}ava gravity
is characterized by three dimensionless couplings, $\alpha$, $\beta$ and
$\lambda$, which vanish in the general relativistic limit. Solar system and
gravitational wave experiments bound two of these couplings ($\alpha$ and
$\beta$) to tiny values, but the third remains relatively unconstrained
($0\leq\lambda\lesssim 0.01-0.1$). Moreover, demanding that (slowly moving)
black-hole solutions are regular away from the central singularity requires
$\alpha$ and $\beta$ to vanish {\it exactly}. Although a canonical constraint
analysis shows that the class of khronometric theories resulting from these
constraints ($\alpha=\beta=0$ and $\lambda\neq0$) cannot be equivalent to
General Relativity, even in vacuum, previous calculations of the dynamics of
the solar system, binary pulsars and gravitational-wave generation show perfect
agreement with General Relativity. Here, we analyze spherical collapse and
compute black-hole quasinormal modes, and find again that they behave {\it
exactly} as in General Relativity, as far as {\it observational} predictions
are concerned. Nevertheless, we find that spherical collapse leads to the
formation of a regular {\it universal} horizon, i.e. a causal boundary for
signals of arbitrary propagation speeds, inside the usual event horizon for
matter and tensor gravitons. Our analysis also confirms that the additional
scalar degree of freedom present alongside the spin-2 graviton of General
Relativity remains strongly coupled at low energies, even on curved
backgrounds. These puzzling results suggest that any further bounds on
Ho\v{r}ava gravity will probably come from cosmology.

21
The categories of this paper are: adap-org
29
The categories of this paper are: funct-an
12
The categories of this paper are: nucl-th
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
35
The categories of this paper are: mtrl-th
9
The categories of this paper are: nlin
0
The categories of this paper are: hep-ph
27
The categories of this paper are: comp-gas
30
The categories of this paper are: q-alg
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
4
The categories of this paper are: cond-mat
28
The categories of this paper are: alg-geom
8
The categories of this paper are: hep-ex
3
The categories of this paper are: physics