  The baryon acoustic oscillation (BAO) analysis from the first year of data
from the Dark Energy Spectroscopic Instrument (DESI), when combined with data
from the cosmic microwave background (CMB), has placed an upper-limit on the
sum of neutrino masses, $\sum m_\nu < 70$ meV (95%). In addition to excluding
the minimum sum associated with the inverted hierarchy, the posterior is peaked
at $\sum m_\nu = 0$ and is close to excluding even the minumum sum, 58 meV at
2$\sigma$. In this paper, we explore the implications of this data for
cosmology and particle physics. The sum of neutrino mass is determined in
cosmology from the suppression of clustering in the late universe. Allowing the
clustering to be enhanced, we extended the DESI analysis to $\sum m_\nu < 0$
and find $\sum m_\nu = - 160 \pm 90$ meV (68%), and that the suppression of
power from the minimum sum of neutrino masses is excluded at 99% confidence. We
show this preference for negative masses makes it challenging to explain the
result by a shift of cosmic parameters, such as the optical depth or matter
density. We then show how a result of $\sum m_\nu =0$ could arise from new
physics in the neutrino sector, including decay, cooling, and/or time-dependent
masses. These models are consistent with current observations but imply new
physics that is accessible in a wide range of experiments. In addition, we
discuss how an apparent signal with $\sum m_\nu < 0$ can arise from new long
range forces in the dark sector or from a primordial trispectrum that resembles
the signal of CMB lensing.
