What would the dark energy value weakening do to something like Penrose's Conformal Cyclic Cosmology? Seems like it should falsify that idea, as C3 depended on a point where there was nothing left but photons that could never meet causing the end of time and a boundary for another big bang. (If my impression is correct.)
Physicists now question the fate of the Universe
51–60 of 84 posts
Re: Physicists now question the fate of the Universe
#52I have some questions:
1. When they say "70% of the universe is made up of dark energy, the rest is matter and dark matter", in what sense is the universe "made up" of those things? Or, perhaps, what is the "universe" that is made up of those things? If dark energy is this cosmological constant, in what sense does it "make up" 70% of the universe?
2. If the value of w is changing, and w is this cosmological constant that is dark energy (I'm probably not using the right words here), and dark energy is part of what makes up the sum total of the universe, did the decrease in dark energy go somewhere? Did it show up as an increase of something else? If so, what?
3. If w is changing over time, even if very slowly, should we suspect that other things might also be changing? The speed of light? The fine structure constant? For that matter, if w changes, does that require something else to change as a cause?
4. How did w change? Was it a step function, a corner (discontinuous first derivative), or a smooth change? Is it now constant, or is it continuing to decrease? (Yeah, I know - we just discovered this at all, I'm almost certainly asking for more precision than we have data for yet...)
Re: Physicists now question the fate of the Universe
#53When 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.
In other words, we don't know shit.
Not quite. I don't ascribe completely to the notion that we can't figure stuff out. But we do need to be careful about assuming that things like 3d topology (aka "inside", "outside", when the mathematical spaces we're looking might have no equivalent concept), cause/effect -- or even the spatial dimensions themselves -- are fundamental. Particularly when we're talking about black holes. It's hard, because our brains are optimized to deal with this very specific slice of spacetime we've evolved in, so we keep falling back into the trap of our words.
Re: Physicists now question the fate of the Universe
#54When 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.
In other words, we don't know shit.
It's approximations based on measured observations all the way down.
Keep in mind, we cannot explain what matter and energy are. We measure and work in terms of waves. What is "waving?" We literally do not know.
The thing is NOT the wave. Period. But everywhere we look we detect that literally everything is vibrating/oscillating/waving. Each improved microscope/observation/meaurement reveals space and some type of vibration.
However, we rely on the observed, measured properties to do things. Generate (harness) and use electricity and perform (encourage) chemical transitions and combinations, for instance. We have a significant well of practical knowledge.
But the nature of existence and the grand cosmological scheme? Probably not going to crack that. It's very fun to try and you learn practical things along the way, though.
Re: Physicists now question the fate of the Universe
#55I have some questions: 1. When they say "70% of the universe is made up of dark energy, the rest is matter and dark matter", in what sense is the universe "made up" of those things? Or, perhaps, what is the "universe" that is made up of those things? If dark energy is this cosmological constant, in what sense does it "make up" 70% of the universe? 2. If the value of w is changing, and w is this cosmological constant…
1. It's the energy content. The stuff you can see (baryonic matter) is 5%, dark matter is 25%, the rest is dark energy. Technically you should take radiation into account, but this is negligible in today's universe. The types of energy can be characterized by how it changes as the universe expanda. If it becomes larger by a factor of a, matter density goes down as a^3, radiation density goes down as a^4 and dark energy density stays constant. Energy is not conserved in an expanding universe, so don't worry about that.
2. W is not the cosmological constant, but a related parameter. It does not go anywhere, the density just changes.
3. These things are unrelated. We do look for changes in these constants either way as good as we can, because imagine the excitement! But so far they all look very constant.
4. No one knows. It's probably always been close to -1, or else the universe would look very different, but you can play around with all kinds of time parameterization of w. The math gets complex very quickly, so the most common parameterization is a linearization. Simply measuring that is a challenge, so measuring this quantity in a model independent way is extremely difficult.
Re: Physicists now question the fate of the Universe
#56Earlier quoted context omitted.
> 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 equivalen…
> Does this meak we know how much the total density of matter has to decrease in order for universe to expand one cubic meter?
It doesn't work that way. Remember that GR is a theory of spacetime geometry. When we say "the universe is expanding", that's really shorthand for "the spacetime geometry of the universe has a particular shape". It doesn't mean that the universe "adds cubic meters" over time.
There is certainly a relationship between the density of matter (or more generally stress-energy density) and the geometric property that informally we refer to as "the rate of expansion of the universe". The equations that express that relationship are the Friedmann equations, which are just the Einstein Field Equation, the central equation of GR, specialized to the kind of spacetime geometry that "expanding universe" refers to. But it's not really useful to think of those relationships as saying that if the matter density decreases by this much, it makes the universe expand that much.
> 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.
No, that's not a good way to look at it.
A cosmological constant aka dark energy is best thought of as just an unusual form of perfect fluid whose pressure is minus its energy density. Then you just plug that into the Friedmann equations to get predictions for what we should expect to observe, such as "accelerating expansion".
Re: Physicists now question the fate of the Universe
#57Earlier quoted context omitted.
In other words, we don't know shit.
Not quite. I don't ascribe completely to the notion that we can't figure stuff out. But we do need to be careful about assuming that things like 3d topology (aka "inside", "outside", when the mathematical spaces we're looking might have no equivalent concept), cause/effect -- or even the spatial dimensions themselves -- are fundamental. Particularly when we're talking about black holes. It's hard, because our brains…
The thing is, I'm not sure how much an actor emerging from a system can learn about the system. Not saying we can't, 'cause I don't know, nobody does. It's just intriguing in it's own way.
Re: Physicists now question the fate of the Universe
#58Earlier quoted context omitted.
> 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 equivalen…
> Does this meak we know how much the total density of matter has to decrease in order for universe to expand one cubic meter? It doesn't work that way. Remember that GR is a theory of spacetime geometry. When we say "the universe is expanding", that's really shorthand for "the spacetime geometry of the universe has a particular shape". It doesn't mean that the universe "adds cubic meters" over time. There is certain…
I don't agree with you. Between us and a redshifted galaxy, for a specific cross-section "tube", there is a dV/dt of new space and a -dE/dt of energy from the redshift. It is possible to calculate both so at least the circumstantial "cubic metres for joules" metric is possible, and interesting. But it seems I'm not geting it out of this discussion.
You were talking about stress-energy tensor, and it did sound like density decrease is linked to the volume increase in a mathematical sense so that conservation happens. Now you say they are not related in that way. If the density decrease is not linked to the volume increase then you have to explain where that extra energy went.
"perfect fluid whose pressure is minus its energy density" does not make sense because you said it is expanding but keeping the same density. If a fluid is expanding but not becoming less dense, it wasn't a fluid in the first place.
Unfortunately what I can see is lack of understanding the mechanism and being able to productively reason about that, and stress-energy tensor is sufficiently complicated abstraction that one no longer feels the discomfort of not understanding.
Re: Physicists now question the fate of the Universe
#59Earlier quoted context omitted.
In other words, we don't know shit.
Not quite. I don't ascribe completely to the notion that we can't figure stuff out. But we do need to be careful about assuming that things like 3d topology (aka "inside", "outside", when the mathematical spaces we're looking might have no equivalent concept), cause/effect -- or even the spatial dimensions themselves -- are fundamental. Particularly when we're talking about black holes. It's hard, because our brains…
I think you have misused the word ascribe.
Re: Physicists now question the fate of the Universe
#60Earlier quoted context omitted.
Not quite. I don't ascribe completely to the notion that we can't figure stuff out. But we do need to be careful about assuming that things like 3d topology (aka "inside", "outside", when the mathematical spaces we're looking might have no equivalent concept), cause/effect -- or even the spatial dimensions themselves -- are fundamental. Particularly when we're talking about black holes. It's hard, because our brains…
The thing is, I'm not sure how much an actor emerging from a system can learn about the system. Not saying we can't, 'cause I don't know, nobody does. It's just intriguing in it's own way.
We may not be able to probe the system directly, but we can take guesses at some of its properties using a powerful tool: logic.
For instance, if it is true that something cannot come out of nothing (ex nihilo nihil fit), and its clearly true that something exists (cogito, ergo sum), then it must be the case that whatever base reality is, it must have always existed. This base reality could be god or some base physical laws or something else, but unless someone can show that the premise is incorrect, we can surmise the eternal nature of reality.