(adsbygoogle = window.adsbygoogle || []).push({}); That would be normal positive pressure. A model where a gas is expanding and pushing things with it. That's not what's happening here. rede96 said: ↑Thanks for the link. If we imagine galaxies plotted on some come cosmic size coordinate system then yes I understand that the galaxies are at rest wrt that coordinate system (comoving galaxies) and the points on the coordinate grid are moving further apart (expansion).

But what I was interested in is what is powering the expansion of the grid? I understood dark energy to be a sort of negative 'pressure' that caused expansion but it seems most tend to model expansion as objects "free falling" like in a gravitational field, just away from each other.

What confused me is that in Leonard Susskind's lectures on cosmology he does describe dark energy as a kind of negative pressure and not as a type of anti-gravity. In fact he describes all bound systems being very slightly and equally effected by expansion in that the the point of equilibrium the systems end up in is very slightly more expanded then it would be without dark energy. (excuse my poor terminology!)

Hence my confusion to this: as that's exactly how I understood it. That dark energy was applying a negative pressure to matter / energy and causing matter in non bound systems to move apart.

Negative pressure means that energy is required to expand the vacuum - rather than energy required to compress it.

Note that vacuum energy is positive: if a vacuum expands and its energy density remains constant - as it must, assuming vacuum is vacuum and you can't dilute a vacuum - then that implies more total energy.

The other energies (matter and radiation) have the obvious property that as space expands, their energy densities must decrease. Vacuum energy has what seems the extraordinary property that this does not happen and, as space expands, the total vacuum energy goes up and up.

As I understand it, that is where we reach the mystery of dark energy.

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# B Gravity and expansion

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