Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Intermittent null energy condition violations during inflation and
  primordial gravitational waves   Primordial null energy condition (NEC) violation would imprint a blue-tilted
spectrum on gravitational wave background (GWB). However, its implications on
the GWB might be far richer than expected. We present a scenario, in which
after a slow-roll (NEC-preserving) inflation with Hubble parameter $H\simeq
H_{inf1}$, the Universe goes through an NEC-violating period and then enters
subsequent slow-roll inflation with a higher $H$ ($=H_{inf2}\gg H_{inf1}$). The
resulting primordial gravitational wave spectrum is nearly flat at the cosmic
microwave background band, as well as at the frequency $f\sim 1/{\rm yr}$ but
with higher amplitude (compatible with the recent NANOGrav result). It is also
highlighted that for the multi-stage inflation if the NEC violations happened
intermittently, we might have a Great Wall-like spectrum of the stochastic GWB
at the corresponding frequency band.

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The categories of this paper are: atom-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: nucl-th
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The categories of this paper are: cs
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The categories of this paper are: funct-an
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The categories of this paper are: dg-ga
0
The categories of this paper are: hep-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: cond-mat
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The categories of this paper are: solv-int
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The categories of this paper are: chao-dyn
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The categories of this paper are: econ
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The categories of this paper are: alg-geom
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The categories of this paper are: nlin