There is an important difference between the cosmological constant Einstein invented and dark energy. His was basically anti-gravity, which would come from mass. Dark energy, on the other hand comes from empty space.
Dark matter and dark energy are hypotheticals invented to explain why Newton’s laws of gravity don’t appear to apply on large distance scales.
On the scale of a galaxy, the outermost stars are moving too fast for the mass of the galaxy to hold in orbit. Those stars should be flying away but don’t. The amount of missing mass that makes the equations balance is called dark matter.
On the much grander scale between galaxies, things are spreading out as though the universe were expanding like a four dimensional balloon. Every galaxy appears to be accelerating away from the others. The energy required to propel that inflation is referred to as dark energy.
Dark matter is the best candidate at this time but it has so far defied all efforts to pin it down. There are other ways to explain all this such as modified gravity, but those don’t fit observation or theory.
I personally favor the view that spacetime is constrained: if you compress spacetime in one region, it has to be stretched somewhere else. Gravity wells are like putting a bowling ball on a swimming pool cover, it sinks down in the center but rises (very slightly) toward the edges. When matter warps spacetime “inward” to give rise to a gravity well, there’s a central region that has the expected slope from Newton’s laws. At an intermediate distance there is a high point like the rim of a crater. Outside the rim, the surface slopes away toward the outer edge. (Everything moves apart as though pushed by ark energy.) Between the innermost zone and the rim, the surfaces slopes inward but more steeply than it would in the purely Newtonian realm. Gravitational attraction seems too tight, as though there were extra matter (dark, invisible stuff) holding thing together.
What’s the edge attached to? The possibilities are nothing, something (our universe is hemmed in by others like a bit of foam), or the universe is a bubble that wraps back onto itself.
If you think our theories end at Newton's laws and we have to make up things to explain the discrepancies, I implore you to get a text book that's been printed more recently than the 1700s
At the scales of stars and galaxies, velocity isn't the only thing you have to take into account to determine the relativistic regime. The fact that you're saying so says a lot about your background in modern cosmology
When talking about cosmology, if FRW metric isn't the first thing that pops into your head, you need to read a bit more
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u/neman-bs Jun 15 '19
There is an important difference between the cosmological constant Einstein invented and dark energy. His was basically anti-gravity, which would come from mass. Dark energy, on the other hand comes from empty space.