Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On Primordial Density Perturbation and Decaying Speed of Sound   The decaying speed of sound can lead to the emergence of the primordial
density perturbation in any expanding phase, even if the expansion is
decelerated. Recently, some proposals have been given to implement this
mechanism, in which it was found that the primordial spectrum of scalar
perturbation can be scale invariant. In this note, we will give more insights
for the details of this seeding mechanism.

2
The categories of this paper are: math
27
The categories of this paper are: comp-gas
35
The categories of this paper are: mtrl-th
18
The categories of this paper are: econ
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
6
The categories of this paper are: astro-ph
19
The categories of this paper are: eess
15
The categories of this paper are: nucl-ex
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The categories of this paper are: adap-org
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The categories of this paper are: hep-ex
22
The categories of this paper are: chao-dyn
14
The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph
3
The categories of this paper are: physics
9
The categories of this paper are: nlin