Matter cools when universe expands. Where does that energy go to? I would expect that delta E = delta V (with some clear way to represent volume in Joules). Shouldn't dark energy density also fall?
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Physicists now question the fate of the Universe
31–40 of 84 posts
Re: Physicists now question the fate of the Universe
#32TL;DR: the Big Rip is likely not to be the fate of the Universe as dark energy term in lambda-CDM cosmology is not a constant but weakens as time moves forward. Seems like Heat Death or Big Crunch options are back on the menu!
I think that's a slightly incorrect TL;DR. Here's an updated version based on my reading:
TL;DR: a heat death (Big Freeze) due to eternal acceleration was the previously favored candidate for the fate of the universe; but there's now evidence that dark energy varies, so acceleration may not be eternal; we may still get a Big Freeze (if it stop varying soon) or we may get a Big Crunch (if it keeps going far enough in the direction it has been going) or we may get a Big Rip (if it starts going in the opposite direction that it has been going)
Re: Physicists now question the fate of the Universe
#33This kind of research is non-falsifiable and I think it's just a waste of these scientists' talents. You will hear never-ending theories of the Big Bang, black holes, and the end of the universe because all this research is non-falsifiable and just people playing with math and coming up with different ideas that humans find interesting, but will not affect your quality of life at all. Science has to be falsifiable or…
Technically, by this logic, isn't estimating when a bridge or building will collapse also non-falsifiable? Because it is in the future? But we do use math plus current understandings to make predictions.
Re: Physicists now question the fate of the Universe
#34TL;DR: the Big Rip is likely not to be the fate of the Universe as dark energy term in lambda-CDM cosmology is not a constant but weakens as time moves forward. Seems like Heat Death or Big Crunch options are back on the menu!
You seem to have mixed up your scenarios.
In the standard model, i.e. constant dark energy, the universe ends in heat death. Big Crunch or Big Rip were never off the menu in the sense that our measurements couldn't exclude a value of w!=-1. But it's a critical value, you'll never be able to exclude the other scenarios if it's truly -1, because you always have a positive measurement uncertainty.
The standard model is the standard model because it fits the data best. This can change as measurements become more precise. Now new data hints at the standard model being incorrect (to the surprise of basically no one in cosmology), making a big crunch more likely, but still not strongly ruling out the other scenarios.
Re: Physicists now question the fate of the Universe
#35The premise comes off as a bit of a surprise to me. My understanding had always been that physicists have not reached any majority agreement on which of the outcomes was likely, precisely because of the lack of understanding of the exact nature of dark energy.
It's because these people all sit and talk with each other coming up with more high concept sci-fi rigamarole. occasionally, they share whatever they come up with other physicists but it kind of seems like everyone just shrugs and goes back to whatever they were working on.
I have a very different reading of the 60 years of theories and measurements detailed in the article. The results from the article come from a $50M instrument developed in the past 10 years for this exact purpose.
Re: Physicists now question the fate of the Universe
#36When I hear debates about whether or not the universes are black holes nested inside each other, or what the fate of the cosmos is, I often have an instinctive reaction to question whether or not concepts like "inside", "nested", or "fate" have actual fundamental qualities at the scales and timeframes under discussion.
Re: Physicists now question the fate of the Universe
#37Matter cools when universe expands. Where does that energy go to? I would expect that delta E = delta V (with some clear way to represent volume in Joules). Shouldn't dark energy density also fall?
> Matter cools when universe expands. More precisely, the energy density of matter decreases as the universe expands. > Where does that energy go to? Stress-energy density is locally conserved; there is no energy being created or destroyed. In fact that local conservation of stress-energy density is what requires the energy density of matter (which in cosmology-speak means a perfect fluid with zero pressure) to decre…
> In fact that local conservation of stress-energy density is what requires the energy density of matter (which in cosmology-speak means a perfect fluid with zero pressure) to decrease as the universe expands (as the cube of the scale factor).
That makes total sense!
Does this meak we know how much the total density of matter has to decrease in order for universe to expand one cubic meter? What's the energy equivalent of one new cubic meter of space, in Joules? Do we know that? Does it fit the observed matter density, with and without dark matter?
It looks to me that "dark energy" / cosmological constant is just a measure of preference for matter+volume (or at least energy+volume) to take the form of volume rather than matter, in an otherwise conserving process.
Re: Physicists now question the fate of the Universe
#38This kind of research is non-falsifiable and I think it's just a waste of these scientists' talents. You will hear never-ending theories of the Big Bang, black holes, and the end of the universe because all this research is non-falsifiable and just people playing with math and coming up with different ideas that humans find interesting, but will not affect your quality of life at all. Science has to be falsifiable or…
> This kind of research is non-falsifiable
Do you mean that we will not be able to witness the fate of the Universe? But this very article shows the case when the theory about a Universe fate was falsified or at least came close to it. If a theory is making predictions, and one can check them, then it is a falsifiable theory. Simple models of Universe make predictions, like "you will get the same rate of an expansion however you measure it", and oops... different methods give different results.
I believe you are confusing a falsifiable theory and pragmatic knowledge. We are really cannot predict any practical applications for these kind of theories. So they are not pragmatic. But they are falsifiable.
Re: Physicists now question the fate of the Universe
#39Looking at the chart, it appears they’ve narrowed it down to absolutely anything.
You are making me reach Olympic levels coffee snorting.
Re: Physicists now question the fate of the Universe
#40The premise comes off as a bit of a surprise to me. My understanding had always been that physicists have not reached any majority agreement on which of the outcomes was likely, precisely because of the lack of understanding of the exact nature of dark energy.
Years it was said that we were going to end up with the Big Crunch. Then in the past decade or two, it was said that we’d end up with a heat death. Now it seems like we’re heading away from that direction.
Naturally, if you were immersed in the actual research, I’m sure the question would have seemed a lot more unsolved than it was presented. But it wasn’t just presented this way by pop science books; even scientists, when talking to laymen, tend to oversimplify things to a degree that they can become misleading. We’ve seen this discussed when it comes to things like string theory and quantum mechanics. Too many times people are saying “this is what happens” when they should be saying “we have no idea, there are a bunch of different theories, but we fundamentally don’t know.”
Whatever happens to the universe, it won’t happen for billions of years. What people think will happen today might not be what they think will happen in ten, a hundred, or a thousand years from now. The research is interesting, but I have to assume that any answer we have now could be completely wrong.