The Missing Link in Gravitational-Wave Astronomy: Discoveries waiting in
  the decihertz range
  The gravitational-wave astronomical revolution began in 2015 with LIGO's
observation of the coalescence of two stellar-mass black holes. Over the coming
decades, ground-based detectors like LIGO will extend their reach, discovering
thousands of stellar-mass binaries. In the 2030s, the space-based LISA will
enable gravitational-wave observations of the massive black holes in galactic
centres. Between LISA and ground-based observatories lies the unexplored
decihertz gravitational-wave frequency band. Here, we propose a Decihertz
Observatory to cover this band, and complement observations made by other
gravitational-wave observatories. The decihertz band is uniquely suited to
observation of intermediate-mass ($\sim 10^2$-$10^4 M_\odot$) black holes,
which may form the missing link between stellar-mass and massive black holes,
offering a unique opportunity to measure their properties. Decihertz
observations will be able to detect stellar-mass binaries days to years before
they merge and are observed by ground-based detectors, providing early warning
of nearby binary neutron star mergers, and enabling measurements of the
eccentricity of binary black holes, providing revealing insights into their
formation. Observing decihertz gravitational-waves also opens the possibility
of testing fundamental physics in a new laboratory, permitting unique tests of
general relativity and the Standard Model of particle physics. Overall, a
Decihertz Observatory will answer key questions about how black holes form and
evolve across cosmic time, open new avenues for multimessenger astronomy, and
advance our understanding of gravitation, particle physics and cosmology.
