Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Avoiding the cosmological constant issue in a class of
  phenomenologically viable $F(R,{\cal G})$ theories   In this paper we investigate a class of phenomenologically viable $F(R,{\cal
G})$ theories that are able to avoid the cosmological constant issue. While the
absence of ghosts and other kinds of instability issues is of prime importance,
other reasonable requirements such as vanishing effective (low curvature)
cosmological constant, including the flat space as a stable vacuum solution,
are also imposed on the viable models. These are free of the cosmological
constant problem thanks to the following outstanding feature: the de Sitter
space is an attractor of the asymptotic cosmological dynamics, with the
resulting constant Hubble rate being unrelated both to the energy density of
vacuum and to the low-curvature effective cosmological constant.

20
The categories of this paper are: acc-phys
3
The categories of this paper are: physics
17
The categories of this paper are: q-fin
30
The categories of this paper are: q-alg
24
The categories of this paper are: dg-ga
8
The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
26
The categories of this paper are: bayes-an
25
The categories of this paper are: patt-sol
36
The categories of this paper are: cmp-lg
31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
15
The categories of this paper are: nucl-ex
4
The categories of this paper are: cond-mat
19
The categories of this paper are: eess