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mieral
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Before the period of the the accelerated expansion started about 5 billion years ago.. where was the dark energy and what was it doing. Or could it only got produced 5 billion years ago and non-existent before that?
It was there and it was exerting the same force per unit volume that it is now (give or take a very small amount) ... BUT, it was not ENOUGH of a force to overcome gravity. Only about 5 to 6 Billion years ago did the expansion weaken gravitational attractions enough on cosmological scales that dark energy started being stronger than the gravitational attraction.mieral said:Before the period of the the accelerated expansion started about 5 billion years ago.. where was the dark energy and what was it doing. Or could it only got produced 5 billion years ago and non-existent before that?
phinds said:It was there and it was exerting the same force per unit volume that it is now (give or take a very small amount) ... BUT, it was not ENOUGH of a force to overcome gravity. Only about 5 to 6 Billion years ago did the expansion weaken gravitational attractions enough on cosmological scales that dark energy started being stronger than the gravitational attraction.
As far as I'm aware, it's the latter.mieral said:What was the proof the dark energy was there before the expansion? Anything to do with the total mass energy from the Big Bang and the expansion? Or just assuming it was there because it couldn't just appear 5 billion years ago?
There may be some evidence from the effect of dark energy on large scale structure at relatively early times, such as the Integrated Sachs-Wolfe effect, but it's necessarily weak because the density is so low.mieral said:What was the proof the dark energy was there before the expansion? Anything to do with the total mass energy from the Big Bang and the expansion? Or just assuming it was there because it couldn't just appear 5 billion years ago?
mieral said:Negative curvature is tidal gravity where things antigravitate (geodesic converge).
mieral said:Dark matter expand spacetime
PeterDonis said:No. Negative curvature means geodesics diverge. That is what dark energy (not dark matter--see below) does.
No. "Expand spacetime" doesn't make sense (neither does "metric expansion"). Dark matter gravitates the same way ordinary matter gravitates. That means, heuristically, that it causes positive curvature in tangential directions (geodesics falling radially inward at different angular coordinates will converge), and negative curvature in the radial direction (geodesics falling radially inward at the same angular coordinates, but starting from different radial coordinates, will diverge).
Did you mean dark energy?
The rest of your post just seems confused.
mieral said:I think negative curvature doesn't automatically mean there is expansion
mieral said:only in the presence of dark energy is there expansion
mieral said:can't we say whenever there is negative curvature, there is very very minimum expansion (say close to distances in the Planck length)?
mieral said:What's the right word for "metric expansion"?
PeterDonis said:They are two separate concepts. Whether there is a connection between them depends on the specific spacetime. There are spacetimes that have negative curvature but no expansion; and there are spacetimes that have expansion but no negative curvature. There are also spacetimes that have both.
If you mean, only in the presence of dark energy is there negative curvature and expansion, that's not correct either, at least not with the usual definition of "dark energy", where it has constant density everywhere. There is a much wider range of possible models which have negative curvature and expansion, but allow the density of whatever-it-is that is causing the expansion and negative curvature to vary.
No. See above.
It's better to describe the specific model you are using; general terms like this are too vague.
mieral said:I thought it was emphasized in physicsforums that space couldn't expand, shrink and curve
mieral said:only the metric could do that
mieral said:What should you really use when so many phrases can't be used?
PeterDonis said:Yes.
Spacetime can curve, but it can't expand or shrink. What expands or shrinks are families of worldlines.
The family of worldlines describing "comoving" observers--observers who always see the universe as homogeneous and isotropic--is expanding. That means these observers are moving apart.
Khashishi said:Dark energy is assumed to have a fixed density in the universe, regardless of the scale of the universe. This is in contrast to matter and radiation, which decrease in density as the universe expands. (The matter and energy get spread out over more space, decreasing the density. Moreover, energy is also redshifted as the universe expands.) That's because dark energy isn't a material that can be spread out over space, but rather some kind of property of space itself. Probably. We really are just guessing with this dark energy business.
So in the early universe, dark energy was still present, but the matter and radiation densities were much larger and the dark energy could be ignored.
mieral said:When you used the word "interpretation" above. Did you mean describing space as expanding was a valid interpretation like Bohmian mechanics where particles have trajectories?
mieral said:Is Bohmian mechanics an "actual physical model"?
mieral said:can "space expanding" not a valid interpretation at all like BM, MWI, etc.?
mieral said:Is it a mainstream view that dark energy can either be on the left or right side of the Einstein Equations?
mieral said:Were you describing in on the left or right side.
PeterDonis said:They are two separate concepts. Whether there is a connection between them depends on the specific spacetime. There are spacetimes that have negative curvature but no expansion; and there are spacetimes that have expansion but no negative curvature. There are also spacetimes that have both.
If you mean, only in the presence of dark energy is there negative curvature and expansion, that's not correct either, at least not with the usual definition of "dark energy", where it has constant density everywhere. There is a much wider range of possible models which have negative curvature and expansion, but allow the density of whatever-it-is that is causing the expansion and negative curvature to vary.
No. See above.
It's better to describe the specific model you are using; general terms like this are too vague.
mieral said:what spacetime have expansion but no negative curvature??
martinbn said:Also it is probably not a good idea to think of curvature causing geodesics to diverge or converge. That can happen with zero curvature.
PeterDonis said:To clarify, by "converge" and "diverge" I mean relative acceleration of nearby geodesics, not relative velocity. Dark energy causes nearby geodesics to accelerate away from each other; ordinary matter causes nearby geodesics (inside the matter) to accelerate towards each other. In flat spacetime, neither of those things can happen.
PeterDonis said:Yes, if you also understand that I meant that neither is worth spending time thinking about, IMO. Interpretations are not physics. You can't use them to make predictions or analyze experiments. You have to first make predictions or analyze experiments using the actual physical theory--the actual math. Then, if you really feel the need, you can use an "interpretation" to make up a story after the fact about what is going on. But I don't see the point of the last part; it doesn't actually tell you anything you don't already know. But it can mislead you into thinking you know something new, which is not good.
No. It's an interpretation. See above.
All of those are interpretations; I don't see the point of asking whether an interpretation is "valid" or "invalid", since all interpretations of a given physical theory (QM for BM/MWI, the FRW spacetime in cosmology for "space expanding") make the same predictions, so as far as physics is concerned they're all the same.
mieral said:So you are saying to talk to laymen quickly... It's indeed not wrong to say "space expands".
mieral said:Also I need to know what is the official interpretations of Spacetime called
Why, when one wants to share quantum mechanics with layman.. is it not possible to start by sharing them about Bohmian Mechanics? A person I initially shared it said of course particles had trajectories. I countered by telling him there was no trajectories.. but on second thought I remembered BM mentioning there was trajectories. So Is there something wrong by sharing with laymen directly BM and saying there were trajectories?PeterDonis said:Not if you don't mind correcting them as soon as they start to make erroneous inferences from that statement. Which they will do very quickly. We have many, many threads here on PF that attest to that.
The two most common interpretations of GR are the curved spacetime interpretation, and the "spin-2 field in flat spacetime" interpretation.
mieral said:Why, when one wants to share quantum mechanics with layman.. is it not possible to start by sharing them about Bohmian Mechanics?
mieral said:in standard General Relativity, it is curved spacetime too
PeterDonis said:Of course it's possible to do this. You can tell people whatever you want. I just don't think it's a good idea. I don't think it's a good idea to start with any interpretation. You should start with the actual physics: what model we use to make predictions, what we actually observe, and how the two match up.
That way you are sure you're not telling people anything they will have to unlearn, and you're not telling them anything they can use to make erroneous inferences. As soon as you go beyond that into an interpretation, those things are no longer true.
Experts in the field use interpretations to communicate with each other because they know their limitations. They don't use them to actually make predictions. They use them in order to have handy shorthand in ordinary language for things where all the other experts they're talking to know the underlying mathematical model and what the ordinary language terms in the interpretation refer to in that model. But this only works because everybody knows all those things, so they aren't being misled.
Occasionally, you find an expert who is able to use interpretations to communicate with lay people without misleading them, but it's rare. Feynman comes to mind as an example: if you read one of his pop science books, such as QED: The Strange Theory of Light and Matter, or The Character of Physical Law, you will find him using interpretations (though sparingly), but you will also find him constantly cautioning about the things he is leaving out and not to take what he is saying too literally. Most scientists who do pop science, however, are not that careful.
Similar comments apply to the "space expanding" interpretation of cosmological models.
Not really. Curved spacetime is actually an interpretation. It's just so common that it's the one that is almost always used in textbooks and papers to describe things in words, so it has filtered down into pop science books on the subject as though it were the actual theory. But the underlying math of GR does not have to be interpreted as curved spacetime; that should be evident since, as you agree, there is another interpretation, the spin-2 field in flat spacetime interpretation, of the same math. Most GR textbooks talk about this, although not many spend a lot of time on the spin-2 field interpretation (Weinberg's book, as I understand it, is an exception; I don't have it so I can't say from my own experience.)
For an example of a pop science book that actually explains this in connection with GR, try Kip Thorne's Black Holes and Time Warps. (Thorne also mentions a third interpretation, which he calls the "membrane paradigm", but it's not really a complete interpretation of GR because it doesn't use the same math, it only uses an approximation that's valid in some cases but not all.) But again, for a pop science book to go to that trouble is rare.
mieral said:is there anything (other examples) in physics where the Map is itself the Territory?
mieral said:in computer programs.. there are only maps, no territory
The origin of dark energy is still a mystery to scientists. Some theories suggest that it has been present since the beginning of the universe, while others propose that it is a result of the interactions between particles in the vacuum of space.
It is believed that dark energy has always been present, but its effects were not noticeable until about 5 billion years ago when the expansion of the universe began to accelerate.
Scientists have observed the expansion of the universe using various methods, such as measuring the distance and brightness of distant supernovae. These observations have shown that the expansion of the universe was decelerating until about 5 billion years ago, indicating that dark energy was not present before then.
The exact cause of the accelerated expansion is still unknown. However, many scientists believe that it is due to the presence of dark energy, which has a repulsive effect on matter and causes the expansion to accelerate.
Based on current observations, it is believed that dark energy will continue to cause the expansion of the universe to accelerate. However, the rate of acceleration may change over time, depending on the amount and properties of dark energy.