5 ms·

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 r

• pdonis 2y ago
  > I don't agree with you. Then you are disagreeing with our best current physical model of our universe, which does not say what you are saying. > I'm not geting it out of this discussion. You should be aware that General Relativity and modern cosmology are complex subjects, and cannot be understood all at once in the course of an Internet discussion. Do you know how long it took me to properly understand the model that I am giving you a very quick and abbreviated description of? Several decades. So if you're expecting a quick "sound bite" answer that will make sense to you, and you have no background whatever in those subjects, of course you are not going to get what you are looking for. I can't help that. All I can tell you is what our best current models say. > 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. No, that's not what I'm saying. What I'm saying is that the relationship is not a causal relationship, in which the density decrease causes the volume increase. It is just a relationship that is enforced by the overall relationship between the spacetime geometry and the stress-energy tensor. The general form of that relationship is the Einstein Field Equation, and the particular form that applies to our best current model of the universe is the Friedmann equations. Those equations most definitely do give a relationship between density decrease for matter and what you are calling "volume increase"--more precisely the rate of "volume increase" (but the term cosmologists actually use is "scale factor" and "rate of increase of the scale factor"), and that relationship most definitely does include a local conservation law for stress-energy. I apologize if that wasn't clear. > "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. I'm using "perfect fluid" here as a technical term for a particular form of the stress-energy tensor, where the only two things you need to specify are energy density and pressure. Terms like "matter" and "dark energy" are then just names for particular equations of state, i.e., particular relationships between energy density and pressure. "Matter" means pressure is zero. "Dark energy" means pressure is minus energy density. That is standard usage among cosmologists. As for why the dark energy density stays the same as the universe expands, that's what the Friedmann equations say when pressure is minus energy density. If you don't like the term "perfect fluid" to describe such a thing, that's fine, call it something else if you like. (But don't expect cosmologists to go along with you.) The important point is the physics, not the words we use to describe it. The physics is what I said. You can check it for yourself if you like by looking up the Friedmann equations. > what I can see is lack of understanding the mechanism What I can see, and I can't avoid saying this bluntly, is someone who has no background in physics but still presumes to tell someone who does have such a background what they do and do not understand. Sorry, but you, and again I can't avoid saying this bluntly, simply don't know what you are talking about. I do know what I am talking about. I'm sorry that I can't convey what I know to you in a quick sound bite that will make sense to you, but that's beyond human powers. There's a reason that people spend years or decades studying these subjects, and there's a reason that even the basics are taught in university courses that take multiple semesters to complete and require textbooks and working problem sets and so on.
  • thriftwy 2y ago
    I'm going to skip unnecessary stuff and: > What I'm saying is that the relationship is not a causal relationship, in which the density decrease causes the volume increase. It is just a relationship that is enforced by the overall relationship between the spacetime geometry and the stress-energy tensor. I'm not even sure why focus on casuality of the event. I'm more interested in whether that stress-energy tensor is a tool which lets us reason about any interesting info, such as: Can we imagine a noticeable area of space suddently becoming empty and devoid of matter - would it cause the inflation, as in elongation of paths between ourselves and some distant object with this area between us, to become faster? Slower? Just about the same? When you talk about a "local conservation law", do you mean that there's math which makes the energy difference go away, and then inflation goes at the same rate regardless of whether energy-losing matter (such as light) is present in the expanding area or not? Because that just does not sound satisfactory. Imagine driving over a $100 bill lying on the ground and once you've past it the bill is gone. You can reason that the bill wasn't significantly thick, and when you drove over it you created some tension and a measurable local depression, but once you went past the ground returned to the same level as it were. That is indeed true and scientifically sound. But where's that money now? That is a question that needs some answer. Maybe it's now stuck to your wheel, maybe it's under a film of dust now, or somebody have pocketed it. But there has to be some answer.
    • pdonis 2y ago
      > I'm not even sure why focus on casuality of the event. Because your original question made it seem like you were thinking of the density decrease caused the volume increase. If you understand that that's not how it works, that's good. > Can we imagine a noticeable area of space suddently becoming empty and devoid of matter Note that this can't happen except by matter flowing out of the area. The matter can't just disappear. That is what the local conservation law for stress-energy says. > would it cause the inflation, as in elongation of paths between ourselves and some distant object with this area between us, to become faster? Slower? Just about the same? As I understand it, cosmologists are working on models in which the density of matter does vary from place to place as well as with time (although even phrasing it that way introduces technical issues that I won't go into here), because our observations seem to indicate that, for example, there is a "void" a billion light-years or more wide in our general vicinity, where the density is significantly lower. Note that this only applies to ordinary matter, not dark energy. The general indications so far of those models, from what I understand, is that such "voids" result in a faster expansion in that area (though again I am ignoring significant technical issues with how this is phrased). However, this is still an open area of research, and the models become significantly more difficult to work with because you can't solve the equations analytically any more, the way you can when the density is constant everywhere in space. You have to do numerical simulations. > When you talk about a "local conservation law", do you mean that there's math which makes the energy difference go away, and then inflation goes at the same rate regardless of whether energy-losing matter (such as light) is present in the expanding area or not? I'm not sure what this means. What the local conservation law says is that stress-energy cannot be created or destroyed; it can change form (for example, matter can transfer some of its stress-energy to radiation by emitting it), and it can flow between regions of spacetime, but the flow cannot have any sources or sinks. The complication is that the "flow" has to take into account the spacetime geometry, which affects how you "count" the flow. For example, if we consider an expanding universe containing just ordinary matter, the energy density decreases like the cube of the scale factor, but that is not a violation of the local conservation law, it's because of the local conservation law--the spacetime geometry is changing as well as the energy density, and the combination of the two changes keeps the local conservation law satisfied--in terms of the local conservation law, the "flow" is conserved, there are no sources or sinks, even though the energy density is changing. This is one of those things that's really, really hard to explain in ordinary language. The mathematical statement is that the covariant divergence of the stress-energy tensor is zero. Physicists understand what that means and how to use it even if they can't express it very compactly in ordinary language.