Galactus already contains infinite energy,
we know this because when he was "killed" (or rather when he returned to his natural state)
he became a Star that would radiate energy forever.
(he needs to feed to keep his armor in check ... stable sortaspeak
why he can't do that with the energies inside him is a pis way to keep the character interesting)
When Johnny Storm became Galactus' herald, his awareness expanded,
this allowed Johnny to realize that Galactus contained a cosmos within him.
Galactus recently was said to have no dimensions or boundaries. (again signifying infinity)
During Annihilation, Galactus was being used to obliterate the 616 Reality,
in addition to the Negative Zone, which just happens to contain the 'Crossroads to Infinity',
which is a freakin infinite Nexus to all realities connected to the prime Multiverse.
Galactus could've fed on Hyperspace for all eternity,
but he disconnected when the Hyperstorm threat was gone.
Lastly, let's not forget friends,
Galan became Galactus by merging with GOD! (the Infinity Being)
*** This is why the BCA feat is not surprising,
because it is well within Galactus' makeup to be capable of such grandeur when putting his historical characterization into perspective.
Taimut NEVER augmented Galactus' power (that claim is false)
Taimut only tampered with Galactus' ability to stop himself from feeding,
nothing more, this made Galactus an insatiable absorption machine,
but the capacity necessary to absorb all that space-time is Galactus' own ability.
This is why during the 'Last Galactus Story' (a possible future)
... Byrne had Galactus destroying the entire Marvelverse while killing Ecce,
then Galactus used the energy within him to re-create the Marvelverse again.
Anyhow,
BCA Galactus absorbs Eternity as easily as he was eating all space-time in the story.