  In this work I reason that in expanding space only those quantum modes
contribute to the measured vacuum energy that do not transcend the observable
volume. Since all quantised field modes have various observable consequences,
when a gravitational horizon causally confines an observer to a finite volume
quantised modes should be restricted to the observable patch to remain
consistent with gravity.
  Within the observable patch of Friedmann-Lemaitre-Robertson-Walker (FLRW)
space the vacuum expectation value of the energy-momentum tensor can be
expressed as a sum over discrete field modes. Friedmann's first equation
provides a straightforward ultraviolet cut-off allowing only a finite number of
modes in the sum. The finite volume acts as an infrared regulator and the
calculation of the vacuum energy density is tractable without regularisation
and renormalisation.
  To test the validity of this idea I quantise a scalar field on an FLRW
background and calculate its vacuum energy density in the vacuum dominated,
conformal, holographic limit. In this limit I show that the quantum vacuum
energy density scales with the square of the Hubble parameter, consistently
with gravity. In this example quantum vacuum expands space while the horizon of
the expanding space limits the energy density of the vacuum to the observed
value.
