Is it possible that the universe could be eternal??...

[polymath257]

Per the Heisenberg Uncertainty Principle, how could matter not exist, as matter & energy can be converted one to another.  Zero matter, zero fields and zero energy is a precisely defined state which would violate the HUP.

Negative energy and positive energy can cancel out?.   Perhaps global aggregate state is zero but local deviation is possible?

One problem is how to define the 'energy of curvature', in other words, the energy of gravity. This has to be done in a way that is frame independent.

For compact spacetime (finite, negatively curved), this is possible and gives a 'total energy' of zero. For non-compact space, this is still a problematic thing.