Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Acoustic instabilities at the transition from the radiation-dominated to
  the matter-dominated universe   The transition from acoustic noise in the radiation-dominated universe to the
density structures in the matter dominated epoch is considered. The initial
state is a stochastic field of sound waves moving in different directions. The
construction of the initial state is compatible with the hyperbolic type of
propagation equation for density perturbations, and parallel to the theory of
stochastic background of gravitational waves. Instantaneous transition between
the cosmological epochs is assumed, and Darmois-Israel joining conditions are
applied to match solutions for sound waves with growing or decaying modes at
the decoupling. As a result a substantial amplification of the low scale
structures is obtained.

32
The categories of this paper are: atom-ph
23
The categories of this paper are: solv-int
37
The categories of this paper are: supr-con
26
The categories of this paper are: bayes-an
2
The categories of this paper are: math
3
The categories of this paper are: physics
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
21
The categories of this paper are: adap-org
4
The categories of this paper are: cond-mat
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
30
The categories of this paper are: q-alg
28
The categories of this paper are: alg-geom