It is a given that the universe is expanding.
It is said that the rate of this expansion is speeding up, by an unknown method scientists dubbed Dark Energy.
How did they determine this?
I would figure, the farther away an object… a galaxy…being seen in the distant past (due to the finite speed of light) would be moving slower that nearer objects moving faster.
But the farther away an object is moving away faster (as per the established Hubble expansion)
What’s the deal?
Remember there is no “center” to the universe. The Big Bang happened everywhere and not just in one spot from which all things are accelerating outward from.
Instead, it is space itself that is expanding. The galaxies are embedded in that space.
It is akin to having a yard stick increase in length by an inch every minute (example only). One yard stick increases in length by one inch per minute. But, a second one is also increasing by an inch every minute. Now line up many yard sticks and, eventually, it all adds up such that you see the far end receding at over light speed…much faster than things close to you.
That is waaay faster than the real universe which is expanding at about 73 kilometers per second/megaparsec (about 3.26 million light-years). But, the universe is very big so things far away are moving away very fast and there is a horizon beyond which we cannot see.
Again, remember there is no “center” to the universe. Someone in one of those distant galaxies sees us moving away from them. It’s all relative.
The standard Hubble expansion rate is linear with distance. So the further away a galaxy is the faster the recession rate, but in proportion to the distance. This would be true if the universe was expanding at the same speed since the early stages of its existence.
The newer observations found that it appears that this isn’t a linear relationship. The universe appears to be speeding up its expansion rate. Aka, it is accelerating. So something should be adding to the constant rate, and for want of imagination, this something got called dark energy.
I’ve tried to promote the term “crepuscular palpitation” but Physics Letters B has consistently refused to publish my submissions.
Stranger
I think the OP was perhaps asking about details of the observations supporting this.
Largely, if I understand correctly, this was based on observations of a particular type of supernova in distant galaxies. Theory has it that all such supernovae are likely to be of roughly similar luminosity and so serve as ‘standard candles’ allowing their distance to be calculated.
While the velocity is determined by the doppler shift of the light.
To be precise, the expansion, even before the recent acceleration, was caused by dark energy. The accelerating expansion is caused by an increase in the amount of dark energy. At least that’s my understanding.
Dark Energy will probably end up being akin to Einstein’s cosmological constant. Unless, that label will be slapped on whatever is causing the expansion.
Wiki:
In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on the largest scales.
Bastards!
No, all of our observations are consistent with a constant density of dark energy. A constant density of dark energy results in exponential expansion. What is relevant is that the density stays constant even as the Universe expands: Other sorts of energy get diluted with the expansion of the Universe. So the fraction of the total energy that’s due to dark energy is increasing.
No “probably” about it: It’s just a different name for the same thing. Or rather, the cosmological constant is the simplest case of dark energy, but that’s still well within the scope of “akin”.
If the density is constant, and the volume increases, doesn’t that mean that the total amount has increased?
Well, the total amount goes from being infinite to being infinite. That, unfortunately, makes it difficult to give a simple answer to that question.
Surprised there’s been no mention of the recent kerfluffle about the expansion acceleration and hence dark energy being an artifact:
https://nautil.us/these-physicists-want-to-ditch-dark-energy-1177085/
I thought we had a recent thread here on this, but my search fu came up empty.
Got it. That makes sense with an infinite universe*. I was assuming a universe of finite size that has been growing rather than merely stretching out. As for why I assume that, I admit that a universe of finite size feels more comfortable to me from a metaphysical / spiritual / whatever you want to call it standpoint than a universe of infinite size.
ETA. I misspoke. It doesn’t make sense to me. Which is a big part of why I prefer the idea of a universe that is really big but of finite size. But that’s a matter for a different thread.
We don’t really know enough about the universe to talk sensibly about infinites. Most people just apply a naive notion of flat Euclidean space to the universe. But we know that there is much freedom for it to be something much more interesting.
Space can and is warped. Within our local view it looks mostly flat. But even just the existence of expansion tells us that this isn’t the full story. Closed versus open universes was a question that touched on the problem.
Right now we would be just as justified drawing big signs on our maps of the visible universe saying “Here be dragons”. One dragon divided by another dragon may well be yet another dragon. Whether the measure of a dragon is defined, finite, or even meaningful is unknown. We do sort of hope we might make progress here. I’m going to bet a lot of the acceleration questions will get revised as we get ever more data from the JWST, and new deep sky surveys. But right now, when we want to talk about infinite universes, not much science lives here. Just dragons.
Probably because it is already a band name.
Hm. Well, science as a whole needs research like that… but I’d say that it’s very unlikely to be right. For one thing, that model needs very specific distributions of matter, both locally and at cosmological distances, to work, and it’s not clear that that matches up with the actual distribution. For another, while supernovae were the first evidence for the acceleration, they’re not the strongest evidence any more: The best evidence nowadays comes from the microwave background, so any alternative model needs to explain that, too.