  We investigate how dark energy affects atom-field interaction. To this end,
we consider acceleration radiation of a freely falling atom close to a
Schwarzschild black hole (BH) in the presence of dark energy characterized by a
positive cosmological constant $\Lambda$. The resulting spacetime is endowed
with a BH and a cosmological (or de Sitter) horizon. Our consideration is a
\textit{nonextremal} $(1+1)$-dimensional geometry with horizons far apart,
giving rise to a flat Minkowski-like region in between the two horizons.
Assuming a scalar ($\text{spin}-0$) field in a Boulware-like vacuum state, and
by using a basic quantum optics approach, we numerically achieve excitation
probabilities for the atom to detect a photon as it falls toward the BH
horizon. It turns out that the nature of the emitted radiation deeply drives
its origin from the magnitude of $\Lambda$. In particular, radiation emission
is enhanced due to dilation of the BH horizon by dark energy. Also, we report
an oscillatory nonthermal spectrum in the presence of $\Lambda$, and these
oscillations, in a varying degree, also depend on BH mass and atomic excitation
frequency. We conjecture that such a hoedown may be a natural consequence of a
constrained motion due to the bifurcate Killing horizon of the given spacetime.
The situation is akin to the Parikh-Wilzcek tunneling approach to Hawking
radiation where the presence of extra contributions to the Boltzmann factor
deforms the thermality of flux. It apparently hints at field satisfying a
modified energy-momentum dispersion relation within classical regime of general
relativity arising as an effective low energy consequence of an underlying
quantum gravity theory. Our findings may signal new ways of conceiving the
subtleties surrounding the physics of dark energy.
