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> This isn't gravitational repulsion. My layman’s understanding matches up with what you said, and my curiosity about this is what brought me to the comments.

> This isn't gravitational repulsion. My layman’s understanding matches up with what you said, and my curiosity about this is what brought me to the comments. What I don’t get is that the author appears to be an astrophysicist who must surely understand the difference. Can someone shed more light on this? E.g. is there a version of the theory where these formulations are equivalent?

jmyeet 4y ago
Yeah, I found this strange too. I too am a layman and that prompted me to comment: why is an astrophysics professor making statements that dark energy is gravity repulsion? The most charitable interpretation is that the author has dumbed down the theory to make it digestible by the layman and in doing so has used a bad analogy. But I think you can correctly describe the core concepts in simpler terms that are a lot closer to taht theory.
- mr_mitm 4y ago
  Because it's true. When I got my PhD in cosmology, this is what everyone kept saying, including my thesis advisor, who is an accomplished professor in the field. So it's not just the author of this article. Gravity works a bit different at the scales we are talking about, but generally we would expect an expanding universe filled with only matter and radiation to slow down and eventually collapse back into itself. Or at least it would slow down and approach some constant velocity. Not unlike a ball that you kick into the air with below or above the escape velocity. But what we actually observed in the late 90ies is that is indeed not slowing down, but accelerating, as if some force was pushing it apart. The cause of this - whatever it may be - has been dubbed "dark energy". At these scales we don't think of gravity as a "force" like we did in Newtonian physics, so I don't think it's wrong at all to say that dark energy has the opposite effect of matter and is pushing the universe apart, i.e. has an repelling effect.

jmyeet 4y ago

Yeah, I found this strange too. I too am a layman and that prompted me to comment: why is an astrophysics professor making statements that dark energy is gravity repulsion? The most charitable interpretation is that the author has dumbed down the theory to make it digestible by the layman and in doing so has used a bad analogy. But I think you can correctly describe the core concepts in simpler terms that are a lot closer to taht theory.

mr_mitm 4y ago
Because it's true. When I got my PhD in cosmology, this is what everyone kept saying, including my thesis advisor, who is an accomplished professor in the field. So it's not just the author of this article. Gravity works a bit different at the scales we are talking about, but generally we would expect an expanding universe filled with only matter and radiation to slow down and eventually collapse back into itself. Or at least it would slow down and approach some constant velocity. Not unlike a ball that you kick into the air with below or above the escape velocity. But what we actually observed in the late 90ies is that is indeed not slowing down, but accelerating, as if some force was pushing it apart. The cause of this - whatever it may be - has been dubbed "dark energy". At these scales we don't think of gravity as a "force" like we did in Newtonian physics, so I don't think it's wrong at all to say that dark energy has the opposite effect of matter and is pushing the universe apart, i.e. has an repelling effect.