When challenged about a possible horizon problem in his model, Hartnett deferred to an inability to understand the problem posed by his critic and did not oblige an answer. Humphreys' critic pointed out that the well-known equation for gravitational redshift/blueshift may countermand his model's efforts to achieve today’s observed redshift from cosmic sources, to which Humphreys countered by terming the gravitational redshift equation a "flawed equation" and became dismissive in his remarks about any potential applicability to his model. Since Humphreys relies heavily on the anomalous sunward acceleration of the Pioneer spacecraft to underscore a fundamental component of his cosmology, his critic was obliged to cite the findings of researchers from the Jet Propulsion Laboratory in California who attributed the apparent anomaly to the thermal recoil force acting on the spacecraft. In response, Humphreys adopted a wait-and-see attitude. Young Earth creationists typically reject standard accounts of stellar evolution and observational evidence of recent star formation. In particular, creationists dispute the widely accepted nebular hypothesis for star formation.