Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Opposite of Dark Energy: Limits on $w = 2/3$ Ultralight Energy in
  the Early Universe   Might stable energy species "lighter" than radiation, with $w > 1/3$, exist?
A dimensional expansion of the cosmological Friedmann Equation of energy has a
clear place for them. Such energies would affect the universe much differently
than dark energies, and so are here dubbed "ultralight." As the universe
expands, ultralight dilutes even faster than light. Although any specie of
energy can be mimicked by a properly evolving scalar field, ultralight energy
species are hypothesized here to be stable and not related to dynamics of a
scalar field. Ultralight is not considered a candidate to make a significant
contribution to the energy budget of the universe today, although ultralight
might have affected the universe in the distant past. In particular, the
$w=2/3$ ultralight energy specie appears to have relatively mundane physical
attributes. A discussion of properties and falsifiable attributes of ultralight
is given. The duration of primordial nucleosynthesis is extrapolated to limit
the present density of $w=2/3$ ultralight to below one part in 100 billion of
the critical density.

18
The categories of this paper are: econ
21
The categories of this paper are: adap-org
7
The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th
9
The categories of this paper are: nlin
1
The categories of this paper are: cs
15
The categories of this paper are: nucl-ex
16
The categories of this paper are: stat
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
4
The categories of this paper are: cond-mat
2
The categories of this paper are: math
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
19
The categories of this paper are: eess
0
The categories of this paper are: hep-ph