Nah, that's the result of an explosion; a single expansive push from one point. I get a much more constant-push self-sustaining type of expansion feel from the Halo pulse. It has to spread enormous distances across the galaxy.

Either the initial blast is so utterly powerful that the energy bubble literally does expand with only that initial push (I find that impossible though to accomplish the end result, since it would only go as fast as light, at best), or the pulse is somehow co-existent in slipspace allowing it kind of 'rake' along normal space at faster than light speed, or somehow the Halos blast something that affects more than just physical space (and not slipspace) that affects its radius based on its strength with immediate propagation (not physical in nature, bound by light speed)... fun thoughts!

*and then he checks the 'pedia, cuz his memory is very shady...*

Already answered:

When activated, the Halo rings release a burst of cross-phased supermassive neutrinos. ...

Like virtually all Forerunner technology, the Halos are powered by vacuum energy. When the main weapon is fired, vacuum energy is siphoned from local space-time to near-depletion. The pulse of the main weapon is amplified by a series of phase pulse generators and channeled toward the Halo's center. A concentration of energy collects into the hub of the installation, which, when the weapon is fully charged, blasts outward in all directions.

The radiation is propelled at superluminal speeds and will eventually propagate at a near-infinite velocity. This was known to generate causal paradoxes when the rings first fired, with two of the Halos reporting pre-echoes of the combined activation before the rings had been fired. Once activated from Installation 00, all installations will cumulatively trigger one another as their radiation fields intersect, amplifying the effects of each individual Halo.

So yep, faster than light expansion, with space/time side-effects. Gotta love those Forerunners.