: : : Yabbut, a vacuum is an extreme case of a decompressed mass. Vacuums are not sources of anti-gravity.

: What about the gravitational field due to vacuum energy? Theoretically it acts as a source of gravity (i.e. cosmilogical constant).

: : What you really need is negative mass-energy, negative pressures or negative stress-energy-momentum tensors (happy with that pmb).

: :-)

: : I found the two terms "energy-momentum tensor" and "stress-energy tensor" confusing since they refer to the same thing.

: In general - I'm happy if I'm clear.

: : This has never been observed in real life.

: What about the Casimir effect or the new experimental evidence of inflation and the accelerating expantion of the universe?

: : In physics every force (but gravity) has an equal but opposite.

: Do you mean by this that it has never been directly observed or is not allowed/predicted within general relativity?

: There is something that is very important in cosmology nowadays. It's known as a vacuum domain wall. They may have existed in the early universe. These objects are like walls but they repell matter rather than attact matter. A cosmic string neither repells nor attracts objects (i.e. gravitational force = 0) but it does alter the space around it making it non-Euclidean. However for straight cosmic strings the spacetime is flat. Also the spacetime near a vacuum domain wall is flat, yet there is a gravitational field.

: Pete

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