In the last few days, I’ve dived down a rabbit hole inspired by some new astrophysics papers suggesting that dark energy is actually black holes. I think I get it now. So let me explain. The universe is expanding, and instead of that expansion decelerating as was expected, we found a few decades ago that it is actually accelerating. And according to general relativity, this implies that a big component of the universe must have nearly maximal negative pressure, and be increasing in mass to keep its mass density constant. That is, on the margin this stuff is strongly attracted to itself, rather than repelled by itself. (We are ignoring gravity here, which yes attracts everything to everything, but is already accounted for in general relativity.)
I tend to agree with Sabine Hossenfelder: "The simplest explanation for dark energy is that it's a constant of nature, the cosmological constant, end of story. I don't know why people find this so hard to accept, it's the neatest possible law-of-nature that one can think of: A constant of nature!" (source: https://twitter.com/skdh/status/1626125552917569538)
The cosmological constant, which appears in a version of the Einstein's equations of general relativity (on which Einstein himself had mixed feelings), is the energy density of empty space. If we take it as a fundamental constant of nature there's no need to explain it further.
If this was true shouldn't we expect black hole gravity to be weaker than their mass implies, as they expand space around them?
Also shouldn't we see more expansion inside galaxies than between galaxies?
We seem to live in a quiet, leafy suburb of the Universe. The really interesting stuff is all happening downtown, but I wouldn’t want to live there. -- Bob (graboyes.substack.com)
Motion of massive objects causes pressure in the sense included in the stress-energy tensor. Just like the particles in a gas give it a positive pressure, so does the the motion of the edge of a flywheel. If you include this effect, the total stress in a flywheel would be about the same as one that simply had internal stress because its edge was longer than 2pi times its radius, so rotation shouldn't give a solution to negative total pressure.
Nitpick: I think it doesn’t make sense to say “in the buildings and machines around you, far more material by mass is under compression than tension”. A free standing wall has vertical compression, but horizontally internally it is mostly tension holding it together. A horizontal beam matches the tension at the bottom to the compression at the top, but also has molecular tension keeping the beam one solid piece. Loaded walls are similar.
I'm not understanding the mechanism here. How does negative energy inside a black hole cause the universe to expand? Is there a simpleish explanation for that, or do I just have to take it as a given?
I love the way you write about astronomy. Not often I get to say that I found a really complex line of reasoning nevertheless easy to follow!
This seems to be proposing a single point source (central galactic black holes) that both attracts stars in the galaxy and nearby local groups galaxies ... while simultaneously repelling very distant galaxies. How do those net forces work, from the same point source? Gravity falls off with an inverse square law. Is negative pressure supposed to be more far reaching than that, perhaps starting at a low strength but not following an inverse square law? How does the negative pressure force decline with distance?
Wouldn't this imply that expansion is 'lumpy', i.e. varies with the distribution of negative pressure stuff?
A typical 'exciting' move is to relate two enigmas together.
Examples - neutrinos and dark energy, dark matter and dark energy, Higgs boson and dark energy
So now another example is black holes and dark energy. Hide one puzzle inside of another puzzle.
No natural explanation why this mechanism keeps 'negative pressure' of dark energy constant as a function of Universe's size?!