The not-so-well understood dark energy is cited as the reason for distant galaxies drifting even further away from each other – or, rather, perpetually causing an additional length of empty space proportional to the current distance between the galaxies to “add” into the distance.

But, based on the current knowledge, how exactly does dark energy act on matter? If, in theory, we had a steel rod – billions of light years long – would dark energy pull the rod apart and make it snap? (Presuming it wouldn’t come in contact with stars, planets or other big objects that would break it…) Or would the rod stay intact, while other celestial objects drifted away from it? Why/why not? What if the rod was made of softer and more brittle material?

Would this question have real life relevance on structural integrity of Dyson sphere scale objects?

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    $\begingroup$ This is relevant: math.ucr.edu/home/baez/physics/Relativity/GR/… $\endgroup$ – Rococo Sep 8 '16 at 23:05
  • $\begingroup$ It seems to me that it should be possible to define a minimum binding energy, as a function of radial separation, that separates objects that stay bound from those that are pulled apart by the expansion of the universe. But I'm very much an amateur for cosmology. $\endgroup$ – Rococo Sep 8 '16 at 23:07

Dark matter and dark energy are not "things" like we're used to in physics. They're error terms phrased as though there was no error. We don't know the answers to your questions.

When you do calculations in cosmology, based on our known universe, made up of matter and energy, you get results that disagree with the observed data. They just don't line up, suggesting that our current understanding of the universe is incomplete (shocker, I know!).

It has been observed that if there was more "stuff" that acted something like matter, and there was more "stuff" that acted like energy, the equations line up with empirical observation better. So, for all intents and purposes, science assumes that there must be more energy and more matter out there in the universe. We're just having trouble observing it. It's kind of an application of Occam's Razor to the problem.

This "stuff" has been called dark matter and dark energy because we believe it is there, based on our observations and our equations, but we have seen no direct evidence of it.

We can't say how it behaves because we've never observed it. The perplexing part of it is that we simply don't know what it truly is, so there's no way to say exactly how it acts on the universe, other than in an "energy-like" way.

I believe this is comparable to the Lorentz transform from the early days of relativity. We knew from experimental observations that Maxwell's equations were not perfectly describing the behaviors we saw, and the Lorentz transform was a mathematical fit which corrected for those observations. We could say how it behaved, but we didn't know why. When Einstein proposed that spacetime warped, it showed a very simple rationale for why we saw the effects of the Lorentz transform.

In the world of cosmology, we're between those steps. We've seen the mathematical model which solves our problem, and it involves tweaking some factors -- mass and energy. We are still working towards a rationale which explains why these effects occur.


Dark energy does not break matter. It just accelerates the expansion of the universe. In the expansion of the universe, space expands.The celestial bodies do not break apart from each other. Its just that the space between two celestial bodies increases due to expansion of the universe.

  • $\begingroup$ If, instead of a long rod, we had a sphere with a radius of billions of light years, made of matter and, thus, enclosing a vast space within. Would the space inside the sphere expand? If so, what would that mean for the sphere? $\endgroup$ – miikkas Sep 8 '16 at 20:02

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