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alistair wrote:

[...]
shouldn't we
look at the possibility that some forms of energy do not curve
space-time?

Of course we should. This is a *major* experimental program,
with easily a dozen groups investigating the question of whether
composition affects gravitational fields.

So far, though, not one speck of evidence has been found to
suggest that ``some forms of energy do not curve space-time.''
There are strong limits for electrostatic and magnetstatic
energy, strong interaction energy, and gravitational binding
energy, with somewhat weaker limits on kinetic energy and the
energy of the parity-conserving part of the weak interactions.
For these, we have experimentally ruled out the possibility
that they don't gravitate.

Did you have some other kind of energy in mind?

Steve Carlip

I didn't have some other kind of energy in mind.I do think though,
that given how successful quantum mechanics has been over the
years,the
likelihood of there being 10^120 Joules/m^3 is high.If this energy
doesn't gravitate then perhaps this is because of the unusually high
energy density.
In other words,at very high energy densities space-time might not be
highly curved - good news if you want to stop a singularity from
forming at the beginning of the universe.