Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Why we need dark energy   It is argued that dark energy -or something dynamically equivalent at the
background level- is necessary if the expanding universe is to behave as an
ordinary macroscopic system; that is, if it is to tend to some thermodynamic
equilibrium state in the long run.

34
The categories of this paper are: chem-ph
30
The categories of this paper are: q-alg
10
The categories of this paper are: q-bio
0
The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
21
The categories of this paper are: adap-org
20
The categories of this paper are: acc-phys
37
The categories of this paper are: supr-con
29
The categories of this paper are: funct-an
24
The categories of this paper are: dg-ga
27
The categories of this paper are: comp-gas
22
The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th
33
The categories of this paper are: plasm-ph
18
The categories of this paper are: econ
15
The categories of this paper are: nucl-ex