[Eternal Student]

Hi,

   I don't know how far back to start from.

   This forum post has meandered a bit from the question in the original post.    The people who send in questions to the podcast side of the Naked Scientists may not also use the forum but if Iain felt the post has gone too far off topic, they should just say so. 
  - - - - - - - - - - - - -

That is also my understanding of the current model - that dark energy opposes gravitation ....

An ordinary English dictionary may define "gravitation" like this.

Gravitation   
(noun)
in science, the force that attracts all objects towards one another

[Taken from the online Cambridge English dictionary.   https://dictionary.cambridge.org/dictionary/english/gravitation ]

    However, it's a bit of lie or oversimplification.   It's fine for the ordinary English language but it's not the definition usually used in physics at anything above school level.    A better definition of "gravitation" is that it's anything shown by or described in a theory of gravity.

    Look up  "gravitation" in the index of the textbook  "Spacetime and Geometry" by Sean carroll and it says....
  "see General Relativity".

   Wikipedia take this approach:
In physics, gravity (from Latin gravitas 'weight'), also known as gravitation or a gravitational interaction,[2] is a fundamental interaction, which may be described as the effect of a field that is generated by a gravitational source such as mass.

    ...which is suitable because it leaves the dooor open for the source to be something other than just ordinary mass, if you want.    Wikipedia has a whole section on general relativity at the end of that article.

   So, with this in mind,  dark energy shouldn't really be described as something which opposes gravitation.   It's just an unusual source of gravitation.   Vacuum energy,  which is all we need dark energy to be in the Einstein Field Equations of GR to explain an expanding universe,   is a source of gravitation that can be considered as a cosmological fluid.  A cosmological fluid is just the stuff we assume to be in the universe.  Radiation can be considered as a cosmological fluid, sometimes called a gas of photons.  A photon gas has some positive energy density and exerts a positive pressure.       Matter can also be considered as a cosmological fluid that exists throughout space.  Matter is usually called "dust" in this context because in a cosmological model we simplify and assume the matter is fairly evenly spread out through space like dust or like a fluid should be.   Dust is a fluid assumed to have some energy density (the rest energy of the particles, given by E=mc² as usual) but exerts no significant pressure.  Any matter particles with relativistic velocity are generally treated as radiation, e.g. neutrinos, and is therefore included as part of the radiation fluid component.   The dust component is just the slow particles and that's why we assume it contributes 0 pressure.
The vacuum energy fluid is perhaps the most unusual of the cosmological fluids -  this fluid would need to have a pressure that opposes the sign of its energy density.   Typically we assume it has a positive energy density but exerts a negative pressure.   People researching dark energy are free to assume their thing,  "dark energy" rather than "vaccum energy" could be the other way around, it may exert positive pressure but has a negative energy density.   Either way around, it would serve the purposes we need in Cosmology to explain why the universe is expanding.

    I'm a bit biased.   I'm interested in Cosmology and GR.  So I'd like dark energy to be a thing, let's say a particle, some thing we can consider as a cosmological fluid with the right properties and then GR and the Friedmann equations used in Cosmology can carry on much as it has been doing.    However, it's really just about gravity behaving badly or in a way that isn't adequately explained with just matter and radiation cosmological fluid components.   We want a fluid component with the right properties  BUT it could just as easily be that GR isn't quite right as a model of gravity.   So "dark energy" may not be some sort of particle,  it could be just a place holder or name for something that really turns out to be that our models of gravity were always just plain wrong.

   Anyway, with this background, I'm hoping that @evan_au and @paul cotter  might see some of their comments a bit differently.

(i)   "dark energy opposes gravitation..."  from @evan_au.      No, it doesn't.  It's just an unusual source of gravitation.

(ii)  "Surely the expansion is mediated by dark energy and not gravity..."   from @paul cotter.     The phrase  "mediated by" is quite different to just saying "it's caused by".   It is quite reasonable to say that the expansion of space may be "caused by" dark energy.     However "mediated by" implies that dark energy would be some fundamental or elementary particle,  something that should be added to the list of particles that appear on the standard model of particle physics.    So you're suggesting that what we currently think of as all just being gravity is really two distinct things, possibly two distinct forces and possibly needing two different force carrying particles in the standard model.     There's one thing that pulls stuff together  (like conventional Netwonian gravity)   and also  one thing that pushes stuff apart (such as in the Hubble expansion of space).
   Well... that is possible   but it's a quite a lot of physics that would need to be overhauled.   However, the standard model has grown many times in its life with the discovery of new particles.   If a suitable particle for dark energy was discovered, the standard model may need to be updated again.
    If, on the other hand, you simply meant to say that the expansion of space is "caused by" dark energy,  well that's probably fair enough.   Cosmological models based on the Friedmann equations want either a vacuum energy fluid component or a non-zero cosmological constant term in the EFE to explain expansion.    It turns out that a non-zero cosmological constant term would be equivalent in effect to having a vacuum energy fluid component  (for reasons that just aren't going to fit inside a small forum post).   So assuming we go with a vacuum energy fluid component instead of a non-zero cosmological constant, then yes... vacuum energy  (which is sometimes used as a term synonymous with dark energy because that's all that dark energy may need to be) is the source of gravitation that drives expansion. 

Best Wishes.

[Eternal Student]

Hi again,

That photon force creates a change in momentum of the interacting objects - and a change in the energy of the objects.

  I like it but I think you might have shot yourself in the foot when you went on to mention solar sails.     We can imagine a solar sail that is a good mirror,  it reflects light and doesn't absorb any of it.   Are the photons still changing the energy of the solar sail in the way you want them to?

Best Wishes.

[paul cotter]

No problem ES, I can go with "caused by" rather than "mediated by" but I am not going to edit my post as this would cause confusion. I accept that I am, at times loose with the use of language and in science accuracy and precision are so important.

[alancalverd]

Yes, ES, radiation pressure will (indeed does) cause the solar sail to accelerate away from the photon source. Reflection  still permits transfer of momentum: imagine a perfectly elastic collision between a bullet and  a target, or "Newton's Cradle".

[paul cotter]

That radiation that is propelling the sail must lose energy in the process for the conservation of energy to hold. Is the reflected light red shifted?

[alancalverd]

I presume so!

[paul cotter]

I was somewhat confused by Halc's contribution but having thought about it, it now makes perfect sense. We say that the electromagnetic fields are mediated by the photon but there is no evidence of photons in the field of a permanent magnet. If we now accelerate a permanent magnet we will detect radiation, ie photons. The bottom line here is that the field phenomena are mediated by virtual photons and real, detectable photons will appear if we accelerate the source(s). In an analogous manner with gravitation the "force" that is apparent is mediated by virtual gravitons and if we accelerate the source, as in the case of a black hole merger, we detect gravitational waves, ie gravitons. Okay, this is a bit muddled but I hope it is understandable.

[Halc]

That radiation that is propelling the sail must lose energy in the process for the conservation of energy to hold. Is the reflected light red shifted?

Both KE of the light sail and the energy of the photon are frame dependent.  Yes, in some frames, the reflected light is redshifted, and in other frames it is blueshifted.  In any one inertial frame, energy is conserved.