Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 4G: Pure fourth-order gravity   Einstein field equations are second-order differential equations. In this
paper, we propose a new gravity theory with pure fourth-order field equations,
which we call 4G for brevity. We discuss the applications of 4G in cosmology,
gravitational waves, and local gravitational systems. 4G predicts the scale
factor $a\propto t^{4/3}$ for the matter-dominated universe, and $a\propto t$
for the radiation-dominated universe. The former can explain the late-time
acceleration without suffering from the coincidence and fine-tuning problems,
while the latter can solve the horizon problem. 4G is a massless gravity, which
means the speed of gravitational waves equals to the speed of light. Based on
the discussions about exact vacuum solution and weak field approximation, we
argue that Schwarzschild metric should be the real physical metric to describe
solar system in 4G.

13
The categories of this paper are: quant-ph
29
The categories of this paper are: funct-an
20
The categories of this paper are: acc-phys
22
The categories of this paper are: chao-dyn
17
The categories of this paper are: q-fin
21
The categories of this paper are: adap-org
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
11
The categories of this paper are: hep-lat
9
The categories of this paper are: nlin
34
The categories of this paper are: chem-ph
0
The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom
27
The categories of this paper are: comp-gas