Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ambiguities in second-order cosmological perturbations for non-canonical
  scalar fields   Over the last few years, it was realised that non-canonical scalar fields can
lead to the accelerated expansion in the early universe. The primordial
spectrum in these scenarios not only shows near scale-invariance consistent
with CMB observations,but also large primordial non-Gaussianity. Second-order
perturbation theory is the primary theoretical tool to investigate such
non-Gaussianity. However, it is still uncertain which quantities are
gauge-invariant at second-order and their physical understanding therefore
remains unclear. As an attempt to understand second order quantities, we
consider a general non-canonical scalar field, minimally coupled to gravity, on
the unperturbed FRW background where metric fluctuations are neglected a
priori. In this simplified set-up, we show that there arise ambiguities in the
expressions of physically relevant quantities, such as the effective speeds of
the perturbations. Further, the stress tensor and energy density display a
potential instability which is not present at linear order.

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The categories of this paper are: hep-ex
32
The categories of this paper are: atom-ph
2
The categories of this paper are: math
16
The categories of this paper are: stat
29
The categories of this paper are: funct-an
15
The categories of this paper are: nucl-ex
21
The categories of this paper are: adap-org
24
The categories of this paper are: dg-ga
9
The categories of this paper are: nlin
12
The categories of this paper are: nucl-th
27
The categories of this paper are: comp-gas
4
The categories of this paper are: cond-mat
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
10
The categories of this paper are: q-bio
0
The categories of this paper are: hep-ph