  We investigate the effects of pre-hydrodynamic evolution on final-state
observables in heavy-ion collisions using state-of-the art event simulations
coupled to different pre-hydrodynamic scenarios, which include the
recently-developed effective kinetic transport theory evolution model KoMPoST.
Flow observables are found to be insensitive to the details of pre-hydrodynamic
evolution. The main effect we observe is in the $p_T$ spectra, particularly the
mean transverse momentum. However, at least part of this effect is a
consequence of the underlying conformal invariance assumption currently present
in such approaches, which is known to be violated in the temperature regime
probed in heavy-ion collisions. This assumption of early time conformal
invariance leads to an artificially large out-of-equilibrium bulk pressure when
switching from (conformal) pre-hydrodynamic evolution to hydrodynamics (using
the non-conformal QCD equation of state), which in turn increases the
transverse momentum. Our study indicates that a consistent treatment of
pre-hydrodynamic evolution in heavy-ion collisions requires the use of
non-conformal models of early time dynamics.
