The discussion revolves around the concept of the expansion of space, exploring questions about the nature of space, gravity, and the implications of an expanding universe. Participants delve into theoretical frameworks, observational evidence, and philosophical considerations related to cosmology.
Participants do not reach a consensus, as multiple competing views and interpretations of the expansion of space and related concepts remain present throughout the discussion.
Limitations include unresolved assumptions about the nature of space and gravity, as well as the dependence on definitions of distance and measurement standards in cosmology.
Starship said:This brought me to the question: How can space expand? Does it mean that i can push off the vacuum? Is space some kind of aether of energetic particles? Do we really know what gravity is (i mean except the curvature of spacetime by mass-energy)? If space has a finite shape but is unbound then what lies outside it?
lawtonfogle said:scientist thought at first it will expand, slow down due to gravity pull form all matter, and then stop, then it would again begin to move, but it would move backwords until all matter colided.
what would happen next? It does not matter, because galaxies are actually acellerating apart. After so long we will hit the speed of light. I heard that matter, when it goes the speed of light, converts to pure energy.
Phobos said:You're thinking of matter moving through space. The separation of galaxies due to the expansion of space itself is different. The galaxies are still moving through space at sub-light speeds, but their relative velocities compared to us will keep increasing until they're out of sight.
cosmoboy said:We would not be knowing about the expansion of the universe if everything were expanding, however, we know that bound objects do not expand so we can feel the expansion of the universe. My question is that since gravity is scale free then how this scale comes, below
which objects do not expand.
Plese look somewhere else this is an important properySpaceTiger said:I'm not sure what you mean by gravity being scale-free.
cosmoboy said:Plese look somewhere else this is an important propery
of grvaitational intercation unlike to other fundmental
intercations gravity shows this property.
No actually, space as a hypersurface within space-time is said to be expanding, space-time may suffer curvature, or not, depending on the average density of the universe, but it cannot itself expand as that is movement which requires the passage of time, and time is part of the space-time that you have said is expanding!Starship said:I think i am wrong here. It is not "space" (by space i meant vacuum and not distance) which is expanding, it's space-time which is expanding.
Garth said:No actually, space as a hypersurface within space-time is said to be expanding, space-time may suffer curvature, or not, depending on the average density of the universe, but it cannot itself expand as that is movement which requires the passage of time, and time is part of the space-time that you have said is expanding!
It depends on the perspective from which you look at it. We experience a (3+1)D universe which is expanding; a fully relativistic perspective is to 'see' 4D space-time, which has to be a static view.
Garth
No, what we percieve as a recession velocity isn't a real speed. Its a manifestation of the expansion of space between the galaxies. Locally, galaxies are very nearly at rest.lawtonfogle said:Um, i was talking about galaxies and all matter in them. are not they increasing in speed as they move from us.
Or maybe i just misread something.
russ_watters said:No one replied to this: No, what we percieve as a recession velocity isn't a real speed. Its a manifestation of the expansion of space between the galaxies. Locally, galaxies are very nearly at rest.
What about the Unruh affect? Isn't this where accelerating frames feel a temerature? Then what about space very far away from us, aren't we accelerating with respect to that that frame? Shouldn't we see a temperature difference somewhere due to acceleration? Thanks.Phobos said:Just to help clarify this...
We're talking about two kinds of motion here. One, galaxies move through space based on gravitational interactions. Two, galaxies move apart from each other (except in local groups) due to the expansion of space.
lawtonfogle - When you were talking about the relativistic effects of moving at the speed of light, that applies to an object moving through space, not to the expansion of space itself. As an example, note that during the first instants of the universe, it appears that space expanded faster than the speed of light...this does not violate relativity, which applies to things moving through space.
As far as I know, general relativity is scale invariant (conformally invariant, guv -> k guv) if no matter is included in the theory. If matter is considered and one wants to preserve conformal symmetry then there are some conditions to be imposed on the energy-momentum tensor, or one has to leave the framework of general relativity. Correct?SpaceTiger said:I'm not sure what you mean by gravity being scale-free. In the Newtonian limit it goes as 1/r2, meaning it's stronger at smaller distances. As for the scale on which gravity is dominant over expansion, that depends on the age of the universe. In the CDM paradigm, as the universe gets older, more matter falls into gravitationally bound structures, making them larger, both in comoving units and physical units.cosmoboy said:We would not be knowing about the expansion of the universe if everything were expanding, however, we know that bound objects do not expand so we can feel the expansion of the universe. My question is that since gravity is scale free then how this scale comes, below which objects do not expand.
However, this depends on the parameters of the universe as well. If it continues to accelerate under the influence of a cosmological constant, then structure basically freezes into place and no further growth occurs, setting a sort of special scale for structure in the universe. I don't know what the currently predicted number is for this, but I suspect it's in the ballpark of 100 Mpc.
You can still detect a kind of doppler shift (that's where the idea of expanding space first came from), but that still doesn't mean its the same type of motion.Mike2 said:What about the Unruh affect? Isn't this where accelerating frames feel a temerature? Then what about space very far away from us, aren't we accelerating with respect to that that frame? Shouldn't we see a temperature difference somewhere due to acceleration? Thanks.
Nereid said:Welcome to Physics Forums, Starship!
Perhaps the easiest way to answer your excellent (primary) question is to ask you another one - what is 'space'? It looks simple, doesn't it?
Let's take a closer look at what might be behind the question.
First, why do we say 'the universe is expanding'? Because there are lots of good observations showing that objects such as galaxies and quasars are moving away from us, and a quick bit of simple math shows that they are all very likely moving away from each other.
Next, in terms of how we understand physical reality - in the large, not at the microscopic, quantum level - what does 'moving away' mean? No matter how you gnaw on this, you quickly end up acknowledging General Relativity - whether you run with it, or rebel against it, the ideas you work with (in your head, or in equations) have GR in their bones. So, 'how can space expand?' becomes 'within GR, what does the concept of the expansion of space mean?' or 'apart from GR, what theoretical framework is there within which the expansion of space can be understood?'
As to whether 'we really know what gravity is', you quickly get into philosophy, or you get something like 'gravity is whatever GR says it is, because GR is a highly successful theory' or "'gravity' is a theoretical concept; it means whatever the theory you are looking at says is means; for example, in LQG, 'gravity' is [insert], in String Theory, 'gravity' is [insert]".
For the 'if space has a finite shapre but is unbound' question, please take the time to read some of the many threads here in PF on this topic.
If one had another way to explain that, and CMB, what else is there??..The evidence for space expansion is the Hubble red shift. ..Garth
I took a look at the thread, and found this.russ_watters said:Welcome to PF. This is a very common topic, most recently discussed here: https://www.physicsforums.com/showthread.php?t=152773&highlight=space+expanding
..
Fred Hoyle and others have suggested it could be due to atomic particles having less mass in the past, thus they would emit photons at lower energies and these would be observed as red shifted. See On the origin of the microwave background F. Hoyle, ApJ. 196, 661-670, 1975.dad said:If one had another way to explain that, and CMB, what else is there??..
Interesting, but I have some very different ideas.Garth said:Fred Hoyle and others have suggested it could be due to atomic particles having less mass in the past, thus they would emit photons at lower energies and these would be observed as red shifted. See On the origin of the microwave background F. Hoyle, ApJ. 196, 661-670, 1975.
"In SCC there are two frames of interpretation of observational data, which depend on whether energy or energy-momentum is to be conserved and whether photons or atoms respectively are chosen as the invariant standards of measurement. In the former frame the universe is stationary and eternal with exponentially shrinking rulers and accelerating atomic clocks, while in the latter, and more familiar, frame the universe is 'freely coasting', and expands linearly from a Big Bang with rigid rulers and regular atomic clocks." (your link)A recent theory that explains red shift in its Jordan conformal frame in the same way is http://en.wikipedia.org/wiki/Self-creation_cosmology (That Wikipedia article has not (yet) been trashed!)
Bingo. And does redshift have something to do with determining those distances?And yes the standard model does assume SN Ia are standard candles over cosmological distances, an assumption, which by it nature, cannot be tested unless there is another degeneracy free method of measuring their distances.
So, where the z equals the redshift. Again, all depends on the explanation for the redshift.The surface brightness of distant objects is proportional to (1 + z)2. Only the first two effects you mention are significant, the third does not make sense, distant objects are at the distance they are! ("when the object was closer"?)
Well, still, cool , but at least an attempt at getting warm. Thanks.I hope this helps.
Garth
dad said:So it depends on the starting assumptions. My concern is beyond assumption, and in regards to what we actually, really do know.
You can observe the redshift, but the assumptions are in what it really means. The other things, as I pointed out, depend on those assumptions, whatever the measurements!Garth said:It does not depend on assumptions but the definition of the method of measurement.
What we actually do know is what we can observe and measure.
Good question. Since it is claimed it is expanding I assume there would be an answer?So, how do we measure mass, length and time at the far reaches of the universe to say whether we "actually really do know" that the universe is expanding or not?
The millions of years of time come in the assuming a constant past, and future, when science also claims that the present is the key to the future, and the past. The calculations rest on that premise, I think. In other words, IF things were the same, THEN this means that, and it woulda, could have should have taken so long for this or that to happen, etc.Measurements are a comparison of the properties of those far objects with some set of laboratory defined standard units of M, L, & T that does not change across the intervening millions of light years of space and millions of years of time. In other words we need a conservation principle, something that is conserved and does not change over cosmological distances.
Right, and I assume it does. But to also assume it will in the future, for example, would only be speculation. To assume it did since creation, also, is assumption, since we really deal only with how it now is, and has been since the hundreds, or thousands of years we have observed.The standard GR model conservation principle is that of energy-momentum, or in other words, 'rest' mass. The mass of an atom, and therefore its size and atomic frequencies, is defined to be constant and atomic rulers and clocks are thus defined to be standard 'rigid rulers' and 'regular clocks'. In this frame the universe expands around a fixed ruler.
Right, there are various theories, all are, of course based on the same basic precept. The constant future and past. - With which, all things are attempted to be explainedHowever in a mass field theory, such as Hoyle's, the ruler itself changes size, and clocks 'speed up', relative to the laboratory standard. In SCC in the Jordan conformal frame where the conservation principle is that of energy, an eternal universe is static with exponentially shrinking rulers and 'speeding up' clocks. The ruler shrinks in a static universe.
It was mentioned that it was not assumptions but calculations. I pointed out that it was assumptions, and that really isn't disputable. Consider it a correction.complexPHILOSOPHY said:To Dad:
I am not exactly sure what you are trying to dispute, ..
So, you assume that it was the same, I see. Also, you prefer no additional assumptions. OK.It is perfectly natural to assume that the physics in the past are causally related to the physics of today and the physics of the future, because it requires an additional assumption to consider otherwise.
Oh, that is easy, but I prefer to stick to what we do know, rather than get too much into theories.You would actually have to construct an additional postulate, in that the physics of the universe were some how different than they are today and then engineer a model which describes this. We are merely inducing from observation (from several different methods) that the physics are the same.
However, if you want to dispute the notion of causality altogether, that is a philosophical discussion of science initiated by the great David Hume and not relevant to the discussion of pure science. If you are merely asking for the causal relations between the scientific discoveries and theories, that is perfectly acceptable and I am sure you will receive more than enough information from our knowledgeable PFers!