  Jet opening angles of long gamma-ray bursts (lGRBs) appear to evolve in
cosmic time, with lGRBs at higher redshifts being on average more narrowly
beamed than those at lower redshifts. We examine the nature of this
anti-correlation in the context of collimation by the progenitor stellar
envelope. First, we show that the data indicate a strong correlation between
gamma-ray luminosity and jet opening angle, and suggest this is a natural
selection effect - only the most luminous GRBs are able to successfully launch
jets with large opening angles. Then, by considering progenitor properties
expected to evolve through cosmic time, we show that denser stars lead to more
collimated jets; we argue that the apparent anti-correlation between opening
angle and redshift can be accounted for if lGRB massive star progenitors at
high redshifts have higher average density compared to those at lower
redshifts. This may be viable for an evolving IMF - under the assumption that
average density scales directly with mass, this relationship is consistent with
the form of the IMF mass evolution suggested in the literature. The jet
angle-redshift anti-correlation may also be explained if the lGRB progenitor
population is dominated by massive stars at high redshift, while lower redshift
lGRBs allow for a greater diversity of progenitor systems (that may fail to
collimate the jet as acutely). Overall, however, we find both the jet
angle-redshift anti-correlation and jet angle-luminosity correlation are
consistent with the conditions of jet launch through, and collimation by, the
envelope of a massive star progenitor.
