First measurement of the correlation between cosmic voids and the
  Lyman-$\alpha$ forest
  We report the first detection of large-scale matter flows around cosmic voids
at a median redshift z = 2.49. Voids are identified within a tomographic map of
the large-scale matter density built from eBOSS Lyman-$\alpha$ (Lya) forests in
SDSS Stripe 82. We measure the imprint of flows around voids, known as
redshift-space distortions (RSD), with a statistical significance of 10
$\sigma$. The observed quadrupole of the void-forest cross-correlation is
described by a linear RSD model. The derived RSD parameter is $\beta = 0.52 \pm
0.05$. Our model accounts for the tomographic effect induced by the Lya data
being located along parallel quasar lines of sight. This work paves the way
towards growth-rate measurements at redshifts currently inaccessible to galaxy
surveys.
