Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Class of viable modified $f(R)$ gravities describing inflation and the
  onset of accelerated expansion   A general approach to viable modified $f(R)$ gravity is developed in both the
Jordan and the Einstein frames. A class of exponential, realistic modified
gravities is introduced and investigated with care. Special focus is made on
step-class models, most promising from the phenomenological viewpoint and which
provide a natural way to classify all viable modified gravities. One- and
two-steps models are explicitly considered, but the analysis is extensible to
$N$-step models. Both inflation in the early universe and the onset of recent
accelerated expansion arise in these models in a natural, unified way.
Moreover, it is demonstrated that models in this category easily pass all local
tests, including stability of spherical body solution, non-violation of
Newton's law, and generation of a very heavy positive mass for the additional
scalar degree of freedom.

2
The categories of this paper are: math
11
The categories of this paper are: hep-lat
16
The categories of this paper are: stat
21
The categories of this paper are: adap-org
10
The categories of this paper are: q-bio
27
The categories of this paper are: comp-gas
36
The categories of this paper are: cmp-lg
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
26
The categories of this paper are: bayes-an
0
The categories of this paper are: hep-ph
1
The categories of this paper are: cs
28
The categories of this paper are: alg-geom
29
The categories of this paper are: funct-an
13
The categories of this paper are: quant-ph
15
The categories of this paper are: nucl-ex