  The null energy condition (NEC) is a cornerstone of general relativity, and
its violation could leave observable imprints in the cosmic gravitational wave
spectrum. Theoretical models suggest that NEC violations during inflation can
amplify the primordial tensor power spectrum, leading to distinct features in
the stochastic gravitational wave background (SGWB). In this work, we search
for these NEC-violating signatures in the SGWB using data from Advanced LIGO
and Advanced Virgo's first three observing runs. Our analysis reveals no
statistically significant evidence of such signals, allowing us to place
stringent upper limits on the tensor power spectrum amplitude, $P_{T,2}$,
during the second inflationary stage. Specifically, we find that $P_{T,2}
\lesssim 0.15$ at a $95\%$ confidence level. Notably, this upper limit is
consistent with constraints derived from pulsar timing array observations,
reinforcing the hypothesis that NEC violations during inflation could explain
the signal detected by pulsar timing arrays. Our findings contribute to a
deeper understanding of the early Universe and highlight the potential of
current and future gravitational wave experiments in probing the physics of
inflation and NEC violations.
