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it's my understanding that dark matter is another word for the indirect observation of matter that we can't otherwise detect, through gravity. I don't know if c

it's my understanding that dark matter is another word for the indirect observation of matter that we can't otherwise detect, through gravity. I don't know if calling it a placeholder is the proper parlance. Dark energy is something predicted by general relativity and iirc it's what is behind the expansion of space (and acceleration of expansion).

pdonis 7y ago
> Dark energy is something predicted by general relativity Not really. GR can accommodate it just fine, but GR did not predict it. In fact, Einstein, who originally introduced the cosmological constant (which was the term then for what we now call "dark energy") in order to allow a static solution to his equation, thereby missed the chance to predict that the universe was expanding, a decade before Hubble's observations showed that it was. Once it became clear that expanding models matched the data, and did not (then) require a cosmological constant to do so, Einstein dropped it, calling it "the biggest blunder of my life". Only in the 1990s, when evidence for accelerating expansion became available, was the cosmological constant (now called "dark energy") put back into the equations so they would match the data. > it's what is behind the expansion of space (and acceleration of expansion) The expansion itself, no. The acceleration, yes.
- galangalalgol 7y ago
  How much would the fate of the universe page on Wikipedia change with linear expansion? Do we have a bit longer before the stretch?
  - pdonis 7y ago
    I'm not sure what you mean by "linear expansion". If you mean that the rate of change of the scale factor with respect to time never changes at all, that is not possible except in the edge case of a universe containing zero matter or energy and zero cosmological constant. (This edge case is often called the Milne universe.) Which is obviously not the universe we live in. If you just mean "no dark energy", i.e., the model that most cosmologists used before the 1990s when accelerating expansion was discovered, it wouldn't change the universe's long term fate very much. It would still expand forever and become more and more diluted; the expansion would just slow down forever instead of accelerating forever. There would be some differences as far as our observable universe--we would not eventually become isolated from all other galaxies and unable to see them--but that's about all.

pdonis 7y ago

> Dark energy is something predicted by general relativity Not really. GR can accommodate it just fine, but GR did not predict it. In fact, Einstein, who originally introduced the cosmological constant (which was the term then for what we now call "dark energy") in order to allow a static solution to his equation, thereby missed the chance to predict that the universe was expanding, a decade before Hubble's observations showed that it was. Once it became clear that expanding models matched the data, and did not (then) require a cosmological constant to do so, Einstein dropped it, calling it "the biggest blunder of my life". Only in the 1990s, when evidence for accelerating expansion became available, was the cosmological constant (now called "dark energy") put back into the equations so they would match the data. > it's what is behind the expansion of space (and acceleration of expansion) The expansion itself, no. The acceleration, yes.

galangalalgol 7y ago
How much would the fate of the universe page on Wikipedia change with linear expansion? Do we have a bit longer before the stretch?
- pdonis 7y ago
  I'm not sure what you mean by "linear expansion". If you mean that the rate of change of the scale factor with respect to time never changes at all, that is not possible except in the edge case of a universe containing zero matter or energy and zero cosmological constant. (This edge case is often called the Milne universe.) Which is obviously not the universe we live in. If you just mean "no dark energy", i.e., the model that most cosmologists used before the 1990s when accelerating expansion was discovered, it wouldn't change the universe's long term fate very much. It would still expand forever and become more and more diluted; the expansion would just slow down forever instead of accelerating forever. There would be some differences as far as our observable universe--we would not eventually become isolated from all other galaxies and unable to see them--but that's about all.