Hi all,

Is there evidence for dark energy over modified gravity to explain the accelerating expansion of the universe? Or, is it still a toss-up?


M74

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I honestly don't know, M74. We really don't know anything about dark energy other than the fact that it appears to be most akin to Einstein's cosmological constant. And we don't really understand gravity--at least not on the quantum level. Maybe someone more well-versed on the subject can answer your question. Sorry.

Eric

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There are some ideas floating around out there... They don't actually counter mainstream theory, but they're not yet a part of it, either, if they ever will be.

I wasn't the first to propose this, by the way. Dyson was.

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Within the subject of astronomy I as a rule avoid the word exciting. It conjures up all the wrong images... Without the mathematics I can see this idea. That does not however give it credibility. A logical conclusion does not require mathematical proof. The formulation of the explanation of gravity and its apparent effect on space time. Invites a little exitment... but ... Sigh, but. Without real testable theorem its not going to fly... is it ?
I could try arm waving or, shouting loudly... but no. So I wait. and smile

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It's still a toss-up whether there is dark energy, let alone its nature; the only reason we think there might be some force at work increasing universal expansion is because the universe appears to be expanding faster than we thought it should be, and we think it might be accelerating. Period. That's it. That's all we actually know.

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In a way, dark energy is modified gravity.

The default dark energy measured in the main research programs out there is a vacuum energy that might couple with gravity but that is indistinguishable from a modified Einstein Field Equation with a constant thrown in.

Most of these same research projects are attempting to get a finer measurement of the way dark energy might change its influence over time. This could indicate the presence of a vacuum energy that couples with gravity in a way that is distinguishable from a constant. (These projects seek to determine the equation of state of dark energy.) So far, it looks like the dark energy out there is the indistinguishable kind.

I think there might be a misunderstanding by the OP. Typically, the debate is whether the problem of galactic rotation curves is solved by dark matter or modified gravity. It seems possible that OP was thinking of this.

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It seems that there is more direct evidence for dark matter than dark energy.

EG: http://dsc.discovery.com/news/2008/0...er-giants.html

Actually, this is why I thought to phrase my OP wrt dark energy.


Thanks, I had the feeling that it was considered much more of a filler than dark matter (for which it seems there is evidence).


In a way, dark energy is modified gravity.

The default dark energy measured in the main research programs out there is a vacuum energy that might couple with gravity but that is indistinguishable from a modified Einstein Field Equation with a constant thrown in.

It would be a real shame if it were determined to be indistinguishable. Would this imply that it would forever be unverifiable?



Thanks,
M74

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I believe that John Earman wrote a paper on this somewhere, but I no longer have a reference. If I remember correctly, the conclusion of the paper was that it may be that the dark energy vs. pure cosmological constant metaphysics is a real example of a scientific case that could never be empirically distinguished.