  In addition to nonzero forces and nontrivial metrics, here I show that a
nonconstant Higgs expectation value, which endows elementary particles with
their masses, also leads to apparent universal particle accelerations and
photon frequency shifts. When effects of the Higgs is attributed to spacetime
curvatures, a spurious stress-energy tensor is required in Einstein's equation.
On cosmological scales, the spurious density coincides with the observed dark
energy density. On smaller scales, effects of the Standard Model Higgs
gradients are unlikely observable except near compact astrophysical bodies. To
estimate the experimental precision required to disambiguate causes of apparent
accelerations, I compare distinct effects of the force, metric, and Higgs
profiles that cause uniform acceleration of a test particle. When the
acceleration is caused by a force, the motion of all particles are hyperbolic
with the same acceleration. However, when the cause is a metric, only a
one-parameter family of particles undergo hyperbolic motion. In comparison,
when the cause is a Higgs gradient, the trajectory of all particles are
hyperbolic, but the acceleration is larger when the particle's energy is
higher. The discrepancies among the three causes are minuscule on laboratory
scales, which makes experimental tests very challenging.
