Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Horizon surface gravity as 2d geodesic expansion   The surface gravity of any Killing horizon, in any spacetime dimension, can
be interpreted as a local, two-dimensional expansion rate seen by freely
falling observers when they cross the horizon. Any two-dimensional congruence
of geodesics invariant under the Killing flow can be used to define this
expansion, provided that the observers have unit Killing energy.

1
The categories of this paper are: cs
8
The categories of this paper are: hep-ex
21
The categories of this paper are: adap-org
23
The categories of this paper are: solv-int
17
The categories of this paper are: q-fin
18
The categories of this paper are: econ
37
The categories of this paper are: supr-con
12
The categories of this paper are: nucl-th
30
The categories of this paper are: q-alg
26
The categories of this paper are: bayes-an
27
The categories of this paper are: comp-gas
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
20
The categories of this paper are: acc-phys
6
The categories of this paper are: astro-ph
35
The categories of this paper are: mtrl-th