Gravitational microlensing is not required to explain quasar variability

In a recent Letter to Nature, Hawkins presents a statistical analysis of about 300 quasar light curves which appears to indicate that high- redshift quasar variability is the result of microlensing by intervening compact substellar-mass objects (i.e., brown dwarfs). This contradicts the general belief that high-redshift quasar variability is predominantly intrinsic. Furthermore, Hawkins states that the density of lensing objects required to produce the observed light curves must be at least 0.1 of the critical density needed to close the universe. Thus a prominent constituent of the total mass of the universe would be in the form of brown dwarf-sized baryonic dark matter, rather than the more exotic particles being proposed by theoretical particle physicists to explain the "dark matter" problem. In this Letter, we describe how two well-known effects can be combined to account for Hawkins's observations within the context of intrinsic variability models. We show that observed timescales of intrinsic quasar variability at a fixed observed wavelength should not be expected to correlate linearly with (1 + z), contrary to common perception. We conclude that the evidence at this time does not require the gravitational microlensing hypothesis to explain high- redshift quasar variability.