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Tucson_Tim
2013-Nov-17, 06:23 PM
I was reading Timothy Ferris's book Seeing in the Dark (2003) and he stated that the space contained within a galaxy super cluster (like Virgo or Centaurus) does not expand while the space between the super clusters does expand. It was implied, I believe, that space that is within a gravitationally attractive group of galaxies doesn't expand. I assume this conforms with current observations? If this is true then surrounding a super cluster there is a boundary line between expanding space and non-expandable space. This boundary line is troubling to me. Is it a binary on/off type of boundary or is it a gradual boundary that may be light-years in width? The latter would seem most likely. Non-expanding space obviously exists or we would have detected it between solar system objects. And expanding space exists -- it is a tenet of the BB Theory and goes all the way back to Hubble and his assistant Humason.

Can someone explain how they view the boundary between expanding space and non-expanding space?

NOTE: Just to clarify . . . I really didn't mean that there are two different "types" of space. I know there is just one "kind" of space that sometimes expands and sometimes doesn't.

Shaula
2013-Nov-17, 06:40 PM
I'd take issue with his statement to be honest. It is fairer to say that there are areas of space in which we can easily observe the effects we model with metric expansion and there are areas of space for which we cannot. If you take the expanding space explanation (what we actually use to model what we see is metric expansion, which is slightly different because it does not posit an underlying physical mechanism for what is seen) then you can picture is as space expanding but the force of gravity dragging things back. Remember that expansion is very, very weak if you model it as a force. It only really obvious over huge distances.

So there isn't really a hard and fast boundary and there are not really two types of space, in this conceptual model.

Tucson_Tim
2013-Nov-17, 06:46 PM
So there isn't really a hard and fast boundary and there are not really two types of space, in this conceptual model.
That's my fault. Timothy Ferris didn't make this statement. I made this statement, paraphrasing incorrectly from his book. I really didn't mean that there are two different "types" of space. I know there is one "kind" of space that sometimes expands and sometimes doesn't. I'm curious about where the boundary might lie.

Tucson_Tim
2013-Nov-17, 07:55 PM
Another thought . . .

If the expansion of space can be slowed, or even stopped, by the presence of mass and its gravitational effect, then an equally valid assumption is that we should see the contraction of space in more intense gravitational fields. Shouldn't we see this effect in our own solar system?

PetTastic
2013-Nov-17, 08:02 PM
This is one that I also have trouble with.

Some of the large clusters are many Mpc across, and my understanding is they are gravitationally bound.
Does this mean they are expanding slower than the space lying parallel to them but outside the cluster?

I am happy with distant galaxies receding faster than the speed of light being stationary in their local space.
However, if you assume the same thing across the size of the cluster, you get a sheering of the local space coordinates, between inside and outside?

Van Rijn
2013-Nov-17, 11:24 PM
Another thought . . .

If the expansion of space can be slowed, or even stopped, by the presence of mass and its gravitational effect, then an equally valid assumption is that we should see the contraction of space in more intense gravitational fields. Shouldn't we see this effect in our own solar system?

No. Imagine a uniform anti-gravitational effect applied to Earth. If it were equivalent to a substantial fraction of the Earth's gravity, the Earth would be significantly less dense, and surface gravity and escape velocity would be noticeably lower. However, if this "anti-gravity" was very small compared to Earth's gravity, than the effect on surface gravity and other things near Earth would be below what instruments can measure.

But consider what happens if you move away from Earth. Earth's gravity decreases with distance. Even so, if the Earth were the only world in the universe, and if there was nothing to oppose it, then no matter the distance from Earth, Earth's gravity would still be felt.

Now add a uniform anti-gravitational effect. Even if it is very small, eventually, as you get further from Earth, the effect will be as powerful as Earth's gravity at that distance. At even greater distances, Earth's gravity would be weaker than this anti-gravity, so things could be pulled away from Earth.

That's essentially what's happening here: This is such a small factor that it is undetectable within galaxy clusters, but the universe is vast, and at great distances, gravitational attraction is insufficient to counter it.

Jeff Root
2013-Nov-18, 02:42 AM
There are different kinds of boundaries.

You can imagine a boundary in which space is different on
one side from what it is on the other side. There is no such
boundary between a region in which matter is gravitationally
bound together, and so is not moving apart, and another
region in which matter is not gravitationally bound to the
matter in the first region.

You can also imagine a boundary like the dividing line
between watersheds. Rain falls on the ground and then
flows downhill. It flows in opposite directions on opposite
sides of a watershed, because downhill is in opposite
directions. That is like what is observed with the motions
of galaxy clusters. Gravity of galaxies which are close to
one another puts them in the same gravitational watershed,
so the cosmic expansion isn't able to pull them apart.
Galaxies which are far apart from one another are not in
the same gravitational watershed, so the cosmic expansion
is able to make them move even farther apart.

Whether expansion of space is occurring within clusters of
galaxies, within individual galaxies, within the Solar System,
and even within my apartment is completely unknown. The
expansion has only been observed on the scale of tens of
millions of light years, and larger. It may well be operating
at all scales, but it is too weak to overcome the gravity and
other forces that affect matter at smaller scales, so we
can't detect it, and we don't know.

-- Jeff, in Minneapolis

.

Cougar
2013-Nov-18, 01:06 PM
...Now add a uniform anti-gravitational effect. Even if it is very small, eventually, as you get further from Earth, the effect will be as powerful as Earth's gravity at that distance. At even greater distances, Earth's gravity would be weaker than this anti-gravity, so things could be pulled away from Earth.

That's essentially what's happening here: This is such a small factor that it is undetectable within galaxy clusters, but the universe is vast, and at great distances, gravitational attraction is insufficient to counter it.

I think this explains it well.

TooMany
2013-Nov-18, 04:09 PM
I believe that idea that the expansion is a force that pulls things apart is incorrect. The expansion is inertial (ignoring dark energy). Things are just floating apart rather than being pulled apart. If nearby masses interact via gravity, the separating inertia of expansion is transferred into an orbital inertia of the bodies so they cease to move further apart, but remain in a constant orbit.

caveman1917
2013-Nov-18, 09:54 PM
I believe that idea that the expansion is a force that pulls things apart is incorrect. The expansion is inertial (ignoring dark energy). Things are just floating apart rather than being pulled apart. If nearby masses interact via gravity, the separating inertia of expansion is transferred into an orbital inertia of the bodies so they cease to move further apart, but remain in a constant orbit.

That's correct, but doesn't that answer your own question? The boundary for some object is the boundary of the set of stuff that is not in an escape trajectory from that object. This is not the same thing as the boundary where the force of gravity and the force of dark energy cancel, because galaxies aren't stationary and hence can, and generally do, have kinetic energies that allow them to overcome that potential maximum.

Tucson_Tim
2013-Nov-20, 11:18 AM
Thanks for the responses.

galacsi
2013-Nov-20, 05:19 PM
No. Imagine a uniform anti-gravitational effect applied to Earth. If it were equivalent to a substantial fraction of the Earth's gravity, the Earth would be significantly less dense, and surface gravity and escape velocity would be noticeably lower. However, if this "anti-gravity" was very small compared to Earth's gravity, than the effect on surface gravity and other things near Earth would be below what instruments can measure.

But consider what happens if you move away from Earth. Earth's gravity decreases with distance. Even so, if the Earth were the only world in the universe, and if there was nothing to oppose it, then no matter the distance from Earth, Earth's gravity would still be felt.

Now add a uniform anti-gravitational effect. Even if it is very small, eventually, as you get further from Earth, the effect will be as powerful as Earth's gravity at that distance. At even greater distances, Earth's gravity would be weaker than this anti-gravity, so things could be pulled away from Earth.

That's essentially what's happening here: This is such a small factor that it is undetectable within galaxy clusters, but the universe is vast, and at great distances, gravitational attraction is insufficient to counter it.

I read again this post and I am not convinced.

First : The size of galaxy clusters is not so tiny compared to the size of the universe we can observe.

Two : I don't see why this space expansion will not expand the orbits of Galaxies around cluster and even the orbits of stars around galaxies and even planets around stars. Yes these bodies remain attached to their parents because the gravity force is strong enough to keep it that way but space expand anyway. There is only one space.

Tucson_Tim
2013-Nov-20, 06:08 PM
There is only one space.
This was established in the first post. The inference that there are two different kinds of space was the result of my inaccurate paraphrasing of what Timothy Ferris stated in his book.

Strange
2013-Nov-20, 06:22 PM
First : The size of galaxy clusters is not so tiny compared to the size of the universe we can observe.

A large cluster is, say, 20 million light years in diameter. The observable universe is about 93 billion light years across. And of course, the larger universe is many times larger than that. So, as a minimum, a cluster is 1/4000th of the observable universe and an even tinier fraction of the whole universe.


Two : I don't see why this space expansion will not expand the orbits of Galaxies around cluster and even the orbits of stars around galaxies and even planets around stars. Yes these bodies remain attached to their parents because the gravity force is strong enough to keep it that way but space expand anyway.

You appear to have answered your own doubts.

TooMany
2013-Nov-20, 06:41 PM
I don't see why this space expansion will not expand the orbits of Galaxies around cluster and even the orbits of stars around galaxies and even planets around stars. Yes these bodies remain attached to their parents because the gravity force is strong enough to keep it that way but space expand anyway. There is only one space.

Maybe because the "expansion of space" is a poor model for the theory. Objects in the universe are moving apart but are not being forced apart by the "expansion of space" (again ignoring dark energy). If space is expanding, it's only in the sense that everything is moving apart so the volume occupied by objects in the universe increases. Gravity doesn't overcome some force of expansion, rather it changes the trajectories of nearby objects such that the volume they occupy becomes constant and ceases to expand. If frictional forces are present, then nearby objects can get closer.

caveman1917
2013-Nov-20, 07:33 PM
That's correct, but doesn't that answer your own question?

It appears i mixed up Tucson_Tim and TooMany here, apologies to both.

galacsi
2013-Nov-20, 09:01 PM
A large cluster is, say, 20 million light years in diameter. The observable universe is about 93 billion light years across. And of course, the larger universe is many times larger than that. So, as a minimum, a cluster is 1/4000th of the observable universe and an even tinier fraction of the whole universe.

93 billion LY ? Really ? Because I believe the farthest observed galaxies are about 13 billions LY away. Stranger and stranger . . . (No pun intended )

And even if this is true maybe we can observe (or not) a difference of density between the nearest cluster and the farthest.

Strange
2013-Nov-20, 09:05 PM
93 billion LY ? Really ? Because I believe the farthest observed galaxies are about 13 billions LY away.

Apparently, yes: http://en.wikipedia.org/wiki/Observable_universe#Size

galacsi
2013-Nov-20, 09:12 PM
You appear to have answered your own doubts.

What I find , yes, strange, is not the expansion per se but the distinguo between the expanding space and the not expanding space. If space expansion is a cosmological effect derived from relativity ,it must apply at all scales , no ?
May be the Earth orbit has expanded from its birth and is keeping expanding even today ? From my limited knowledge I cannot see why not ?

galacsi
2013-Nov-20, 09:21 PM
Apparently, yes: http://en.wikipedia.org/wiki/Observable_universe#Size

Excerpt :
The figures quoted above are distances now (in cosmological time (http://en.wikipedia.org/wiki/Cosmological_time)), not distances at the time the light was emitted. For example, the cosmic microwave background radiation that we see right now was emitted at the time of photon decoupling (http://en.wikipedia.org/wiki/Recombination_%28cosmology%29), estimated to have occurred about 380,000 years after the Big Bang,[17] (http://en.wikipedia.org/wiki/Observable_universe#cite_note-wmap7parameters-17)[18] (http://en.wikipedia.org/wiki/Observable_universe#cite_note-18) which occurred around 13.8 billion years ago. This radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from us.

Observable and calculated are not really the same. By the way I don't understand how this is computed ,is space expanding only after the light has departed and behind it ? No expansion between the starting point of the light and us ? All this is quite mysteriousto me.

Noclevername
2013-Nov-20, 10:50 PM
What I find , yes, strange, is not the expansion per se but the distinguo between the expanding space and the not expanding space. If space expansion is a cosmological effect derived from relativity ,it must apply at all scales , no ?
May be the Earth orbit has expanded from its birth and is keeping expanding even today ? From my limited knowledge I cannot see why not ?

Let me explain it as I understand it. Picture an elastic sheet, with objects on it. The logical conclusion is that if stretched out, the objects would all move apart evenly. But these objects are bound together with untied strings wrapped around them. Where the friction of the strings around the objects is stronger than the force of the stretching, the objects remain together while the sheet slides out from under them. Where the strings are too loose or have too few loops holding on, they unravel and the objects move with the sheet. So you would get clumps of objects, sliding around in groups.

It isn't a perfect analogy, but I think it helps visualize the constant tug-o-war between local gravity and spatial expansion.

Van Rijn
2013-Nov-21, 12:08 AM
May be the Earth orbit has expanded from its birth and is keeping expanding even today ? From my limited knowledge I cannot see why not ?

Earth's orbit would only expand if the accelerated expansion effect itself accelerated. Though even then this would be (so far) much too small to be detected.

A constant effect would cause a fixed offset to the effect of the sun's gravity, so solar orbital velocity at the same distance would be (very slightly) lower than it would without the effect. But in that case, the orbital velocity would never change, so the Earth's orbit would not expand.

If the effect accelerated, it could eventually lead to the Big Rip scenario, where eventually it would be stronger than any force. But in that case, it wouldn't have a large effect on the Earth's orbit until near the end.

See, for instance:

http://en.wikipedia.org/wiki/Big_Rip

galacsi
2013-Nov-22, 09:34 AM
Let me explain it as I understand it. Picture an elastic sheet, with objects on it. The logical conclusion is that if stretched out, the objects would all move apart evenly. But these objects are bound together with untied strings wrapped around them. Where the friction of the strings around the objects is stronger than the force of the stretching, the objects remain together while the sheet slides out from under them. Where the strings are too loose or have too few loops holding on, they unravel and the objects move with the sheet. So you would get clumps of objects, sliding around in groups.

It isn't a perfect analogy, but I think it helps visualize the constant tug-o-war between local gravity and spatial expansion.

Yes , I understand, but there is a difference between saying ; expansion is not strong enough to rip gravity connected objects apart and saying ; where there is gravity there is no expansion.

As Shaula wrote :
So there isn't really a hard and fast boundary and there are not really two types of space, in this conceptual model.

Ken G
2013-Nov-22, 02:42 PM
If the expansion of space can be slowed, or even stopped, by the presence of mass and its gravitational effect, then an equally valid assumption is that we should see the contraction of space in more intense gravitational fields. Shouldn't we see this effect in our own solar system?Yes, that's what we see when we see planets orbit the Sun. I think the key thing that has not been pointed out as clearly as necessary is that "what is happening to space" (really spacetime), and "what gravity does", are exactly the same thing. So the fact that Earth orbits the Sun, and the fact that the universe is expanding, is all about gravity. The boundaries between expansion and contraction are like the boundaries between dimples on a golf ball, there's no sudden transition but you can tell there are dimples in some places but not others, and you can tell that the ball as a whole curves differently from the dimples. So I wouldn't say that we don't see universal expansion in our solar system because it is too small, I'd say we don't see it here because we do see a very different gravitational environment here due to the Sun and the galaxy, etc. These are all things gravity is doing, and since the mass distributions are on different scales, gravity is doing different things on different scales.

The fundamental reason for that is instability-- gravity in some sense "wants" the whole universe, everywhere, to expand uniformly, but it isn't stable at doing that, so many small pockets do the opposite.

caveman1917
2013-Nov-22, 03:40 PM
"what is happening to space" (really spacetime)

I think you do mean space, nothing can be happening to spacetime, at least in the sense of "something happening" meaning that some variable changes over time.


The fundamental reason for that is instability-- gravity in some sense "wants" the whole universe, everywhere, to expand uniformly, but it isn't stable at doing that, so many small pockets do the opposite.

We really should make the distinction here between expansion and accelerated expansion. Disregarding accelerated expansion, it doesn't seem that gravity "wants" anything to expand. If we were to suddenly set the force of gravity (in the sense of the inverse-square attraction bit) to zero then the things that were gravitationally bound will not start expanding uniformly. I suppose in a certain sense they would since stuff would start flying apart due to its previous orbital velocity and inertia, but that's not what can be taken as gravity wanting the universe to expand uniformly. Perhaps better would be to say that inertia "wants" the whole universe to expand uniformly, and i guess one could make an argument that inertia is really gravity, but that seems to be stretching an explanation quite far.

The expansion is included in the initial conditions, it does not follow generically from gravitational interaction per se.

Noclevername
2013-Nov-22, 08:58 PM
Yes , I understand, but there is a difference between saying ; expansion is not strong enough to rip gravity connected objects apart and saying ; where there is gravity there is no expansion.

As Shaula wrote :

That's what I meant, that there is only one kind of space and it expands everywhere, but we can't measure/observe it because we can't directly detect space, only the matter/energy that occupies space. At least that's my inexpert interpretation.

Jeff Root
2013-Nov-22, 10:00 PM
galacsi,

I usually explain what you are asking about with the same
balloon analogy used to explain several other things about
the cosmic expansion. It's like Noclevername's elastic sheet,
but with sprinkles.

Imagine a spherical rubber balloon. (Darn, that reminds me
I still have a huge, rubber weather balloon in storage. It is
supposedly sealed in a nitrogen-filled container so it might
still be useable after all these decades....)

Imagine a spherical rubber balloon. Cover it with a thin layer
of sticky oil or grease. Now get some metallic sequins, and
magnetize them. Sprinkle the sequins all over the balloon so
that they stick to the oil or grease. The sequins represent bits
of matter, or maybe galaxies, in the expanding Universe. Now
expand the Universe -- I mean, blow up the balloon. As the
balloon inflates, the sequins get farther apart from one another.
But they are magnetized, so sequins that are near one another
attract each other and stick together. They don't move apart,
even though the balloon is expanding under them. The sequins
will end up in clusters and strings, with large voids between.

Just an analogy, but I think it works well.

-- Jeff, in Minneapolis

neilzero
2013-Nov-22, 10:07 PM
Perhaps logic is inappropriate, but it seems to me: The nearest thing to a boundary is a point where the expansion is maximum. Slightly less expansion occurs as you move many light years from that point that is near the center of the void. The expansion falls to negligible, sometimes passes zero, locally near the outskirts of the galaxies which boarder on the larger voids. I suppose my hypothesis is unproveable. :)

TooMany
2013-Nov-22, 10:33 PM
galacsi,

Imagine a spherical rubber balloon. Cover it with a thin layer
of sticky oil or grease. Now get some metallic sequins, and
magnetize them. Sprinkle the sequins all over the balloon so
that they stick to the oil or grease. The sequins represent bits
of matter, or maybe galaxies, in the expanding Universe. Now
expand the Universe -- I mean, blow up the balloon. As the
balloon inflates, the sequins get farther apart from one another.
But they are magnetized, so sequins that are near one another
attract each other and stick together. They don't move apart,
even though the balloon is expanding under them. The sequins
will end up in clusters and strings, with large voids between.

Just an analogy, but I think it works well.

-- Jeff, in Minneapolis

One minor addition to the analogy is that the surface of the balloon is friction-less. Sequins cannot move in a direction normal to the surface but they float freely along the surface.

Can anyone suggest ways in which this analogy fails to match theoretical expansion model?

Amber Robot
2013-Nov-22, 11:33 PM
One minor addition to the analogy is that the surface of the balloon is friction-less. Sequins cannot move in a direction normal to the surface but they float freely along the surface.

Can anyone suggest ways in which this analogy fails to match theoretical expansion model?

Just the typical way that the balloon analogy fails: you don't need to embed the space in a higher dimension to model the expansion. Most lay people, when presented with the balloon analogy always ask about the inside of the balloon.

Cougar
2013-Nov-22, 11:45 PM
Can anyone suggest ways in which this analogy fails to match theoretical expansion model?

Well, for one thing, the 2-dimensional surface of the balloon fails to match the 3-dimensional space of the Universe. But that's usually pointed out in the introduction to the analogy.

Also, if the sequins represent galaxies, the fact that they are magnetized is not going to produce a cluster of orbiting galaxies (even though the actions of electromagnetism and gravity are both representable by inverse-square laws).

Probably others.

Jeff Root
2013-Nov-23, 12:07 AM
The principal, huge advantage of a balloon over a flat sheet
is that a balloon is very easy for a person with good lungs to
cause to expand evenly over the entire surface. I've done it,
but I didn't go so far as to use grease and sequins. That's a
refinement I thought of later. A balloon covered with grease
is not a safe object.

-- Jeff, in Minneapolis

Ken G
2013-Nov-23, 04:16 AM
I think you do mean space, nothing can be happening to spacetime, at least in the sense of "something happening" meaning that some variable changes over time.
I actually think it's fine to imagine that something is happening to spacetime. It's much like if a wind blows a baseball, we might easily say that something happened to the trajectory-- even though a trajectory has time embedded in it. Things can still be dynamical, even when time is included. The problem with saying that something happened to space is that space by itself is always just a coordinate, so nothing can actually happen to space itself in any physically invariant way.


We really should make the distinction here between expansion and accelerated expansion.No, I would not find that distinction important. It' was all gravity before we used a cosmological constant, and it's still all gravity after we invoke one.

Disregarding accelerated expansion, it doesn't seem that gravity "wants" anything to expand.I can't agree. In my mind, gravity has just one purpose-- to establish what the force-free (inertial) paths in spacetime are. That's all gravity does, and once it has done that, it is finished. Other forces then step in and determine the paths that objects actually take through that spacetime, self-consistently with the gravitational effects, but those forces have no idea how to act until gravity has established what the baseline inertial paths are. So any time we are discussing purely inertial motion, as in cosmology, it's all gravity-- everything that happens is what gravity "wants," including expansion. i think you are imagining that I am saying gravity always wants expansion, but I have not said that-- I said that's what it wants in our universe. It can mediate other kinds of behavior, including contraction, but that's not how it acts in our universe-- for unknown reasons (the best we can do is say it is all due to the iniitial condition, but there really is no such thing as an initial condition, that's a device of the physicist). The point is, all inertial motion is just gravity.

caveman1917
2013-Nov-23, 09:03 PM
I actually think it's fine to imagine that something is happening to spacetime. It's much like if a wind blows a baseball, we might easily say that something happened to the trajectory-- even though a trajectory has time embedded in it.

The trajectory is just a curve in spacetime, how can something happen to it? If the wind blows a baseball then nothing happened to the trajectory, it just happens to be the case that its trajectory deviates from what you expected its trajectory to be. There is only one trajectory and it's always the same, always has and always will.


The problem with saying that something happened to space is that space by itself is always just a coordinate, so nothing can actually happen to space itself in any physically invariant way.

But that's exactly how it goes. Does space expand in comoving coordinates? Does space "fall" towards a black hole in coordinates other than Gullstrand-Painleve? No, these are all coordinate stories, it is more important to recognize that than to imagine that it is somehow something that "happens to" spacetime in an invariant way.


In my mind, gravity has just one purpose-- to establish what the force-free (inertial) paths in spacetime are.

But there are plenty of inertial paths from every point in spacetime. What decides which of those paths will be taken is the velocity of the object under consideration. It is part of the initial conditions that a galaxy "chooses" the inertial path that will take it away from another galaxy rather than an inertial path that will take it closer.

Ken G
2013-Nov-24, 01:55 PM
The trajectory is just a curve in spacetime, how can something happen to it? Well of course it's simply a matter of semantics, but I'm happy saying that a "happening" can refer to some property of a path in spacetime. For example, if an object falls into a black hole, we can say that what "heppened" to the world line is that it joined a black hole. It's not particularly important to associate a particular time with that occurrence, or say where it occurred, because those are coordinates we might use to distinguish various events along that worldline, but the most important thing about the world line, that it goes to a black hole, is a kind of topological property of the worldline itself. Similarly, you could have a world line that is straight for awhile, and then curves, and then what "happened" to the world line is that it went from straight to curved.

There's no point in saying a word choice is right or wrong, the only issue is what meaning does it convey-- the meaning I wish to convey is that anything that happens to an inertial trajectory is just due to gravity, it was Newtonian thinking that inertial paths were one thing and gravity was something different.
Does space expand in comoving coordinates? Does space "fall" towards a black hole in coordinates other than Gullstrand-Painleve? No, these are all coordinate stories, it is more important to recognize that than to imagine that it is somehow something that "happens to" spacetime in an invariant way.
Yes, that is also what I am saying, seems to me you are just agreeing.


But there are plenty of inertial paths from every point in spacetime. Yup, and gravity determines them all.

What decides which of those paths will be taken is the velocity of the object under consideration.Actually, I would say that something completely mysterious determines which path an object will take, but we have found a way to organize that mysteriousness and give it a name associated with a class of observations we call velocity observations. The point is, that part is something different from gravity. So that's why I organized my post to talk about the parts that are due to gravity, and the parts that are not, and the parts that are refer to the question in the OP-- there is no need to say space is doing something different across some kind of physical boundary, it's all just gravity, and gravity can make nearby trajectories converge and also make distant trajectories diverge, it is like a sculptor of spacetime that can produce all kinds of structures.

It is part of the initial conditions that a galaxy "chooses" the inertial path that will take it away from another galaxy rather than an inertial path that will take it closer.Again, I would say that "initial conditions" are an analysis tool of a physicist. They are the way we connect information when we do physics-- we have information X, and we want information Y separated by some time. The order isn't so important-- we can know the final conditions and infer the initial conditions, or vice versa, but neither means that one really determines the other in any absolute or physically invariant way. All we get are constraints on what can happen, and the way we apply those constraints to "tell the story" of what happened is very much a question of how we like to think about things, and what kinds of questions we are interested in answering. My point is that one of the most important lessons of relativity is that our stories include all kinds of elements that are not "really there" in the invariant sense. What is "happening to space," without time being in there, is one of those "storytelling" issues that does not seem to be really there, though it is true that there does appear to be something special about comoving-frame coordinates that might eventually spawn a theory that gives them special status. GR is not that theory, however.

caveman1917
2013-Nov-24, 09:24 PM
Yes, that is also what I am saying, seems to me you are just agreeing.

Ok then. I just wanted to make sure that people wouldn't be left thinking that it is actually spacetime that expands, it is quite distinctly space (as in proper distance over cosmological time) and is a coordinate story.


Actually, I would say that something completely mysterious determines which path an object will take, but we have found a way to organize that mysteriousness and give it a name associated with a class of observations we call velocity observations. The point is, that part is something different from gravity. So that's why I organized my post to talk about the parts that are due to gravity, and the parts that are not, and the parts that are refer to the question in the OP-- there is no need to say space is doing something different across some kind of physical boundary, it's all just gravity, and gravity can make nearby trajectories converge and also make distant trajectories diverge, it is like a sculptor of spacetime that can produce all kinds of structures.

I'm still not following here. Consider a point in spacetime. Gravity decides on the set of inertial paths from that point, an uncountable set (a smooth transformation of \mathbb{S}^2 \times \mathbb{R} to be exact), and the initial velocity decides on which of those paths an object at that point will take. Expanding space is solely a function of the fact that galaxies over time get further away, which is itself solely a function of which path that object takes (further away from the others or not) from the set of inertial paths. How is then expanding space due to what gravity "wants", rather than due to the initial velocity distribution? The issue is not if the galaxies are on an inertial path (rather than a non-inertial one), but which one they are on.


Again, I would say that "initial conditions" are an analysis tool of a physicist.

Fair enough, let's just say that the galaxies are "choosing" at each point the inertial path that takes them away from eachother because they have done that in the past.

Jeff Root
2013-Nov-24, 10:47 PM
If space is changed into time by mass-energy, is time
changed into space in the cosmic expansion, in which
the density of mass-energy constantly decreases?

-- Jeff, in Minneapolis

caveman1917
2013-Nov-24, 10:50 PM
If space is changed into time by mass-energy

What do you mean by this?

Ken G
2013-Nov-24, 11:20 PM
Ok then. I just wanted to make sure that people wouldn't be left thinking that it is actually spacetime that expands, it is quite distinctly space (as in proper distance over cosmological time) and is a coordinate story.What I'm saying is, whatever gravity is doing, it is doing to spacetime, because that's all it does. Also, I'm saying that all motions that are inertial are governed by nothing but gravity. Put those two fads together any way you like.


I'm still not following here. Consider a point in spacetime. Gravity decides on the set of inertial paths from that point, an uncountable set (a smooth transformation of \mathbb{S}^2 \times \mathbb{R} to be exact), and the initial velocity decides on which of those paths an object at that point will take.The way I would put it is, gravity decides on the possible inertial paths, and something else decides which inertial path is actually taken by what objects, and whether or not there are other forces that cause deviation from the inertial paths. If we constrain the velocity at some point, then we have already chosen such a path, by asserting that constraint. This does not mean the velocity determines the path, it is more like the name of the path. That's all specifying the velocity does-- it is how you are picking out that path. What makes the object actually follow that path, instead of some other path, is a kind of sociological story, that in principle has to be traced all the way back to the Big Bang, which of course cannot be done. So that's why it is fundamentally mysterious why any particle follows any path. Nevertheless, we can predict other data, given some data, and appropriate laws. That's what physics is for, but it is not for saying what determines the paths taken, as that is still a mystery but can be manipulated.

Expanding space is solely a function of the fact that galaxies over time get further away, which is itself solely a function of which path that object takes (further away from the others or not) from the set of inertial paths. How is then expanding space due to what gravity "wants", rather than due to the initial velocity distribution? What people informally call expanding space is actually a function of the curvature of spacetime. It is part of the dynamics of gravity. It has virtually nothing to do with velocity, and indeed the simplest way to understand cosmology is to imagine that nothing ever moves, except the light that we use to see that nothing is moving. Of course there are redshifts, but that is part of the dynamical evolution of the universe. The equation of gravity cannot by itself tell us we will see redshifts, but that is typical of dynamical equations in physics. They only decide the possibilitiies, we need to actually look to see what is happening. Again I repeat, I do not mean that what gravity "wants" is any particular thing that we could know in advance, I do mean that anything that happens that is inertial is what gravity wants, because the entire purpose of GR is to find out what the inertial paths are.


Fair enough, let's just say that the galaxies are "choosing" at each point the inertial path that takes them away from eachother because they have done that in the past.
It's not even clear that one can single out the past to make that statement. All we can really say is that there is some fundamentally mysterious reason why the galaxies are on the paths they are on. We could as easily look to the future to explain why the past is the way it was, the dynamical equations of GR allow either type of reasoning. But in our own lives, as physicists and humans, we single out the past as a decider of the future. Most likely, this is just how our minds work.

caveman1917
2013-Nov-24, 11:42 PM
What people informally call expanding space is actually a function of the curvature of spacetime. It is part of the dynamics of gravity. It has virtually nothing to do with velocity

Every constant-time surface in the FLRW metric is a cauchy surface, therefor the information on such a surface determines uniquely the spacetime on either side of it. If we change the sign of time all we do is change the sign of the velocities (or rather momentum), yet we go from an expanding to a contracting space. How can then the velocity have virtually nothing to do with it?


It's not even clear that one can single out the past to make that statement.

It doesn't matter, past to future or future to past, it's the same thing.

Jeff Root
2013-Nov-25, 12:20 AM
If space is changed into time by mass-energy, is time
changed into space in the cosmic expansion, in which
the density of mass-energy constantly decreases?
I mean, since time is dilated at Earth's surface compared
to what it is in geosynchronous orbit, because the surface
is deeper in the gravity well, so that going from GEO to
the surface would be a change (which of course can't be
detected or measured locally because it is only relative),
would going from deep in the gravity well of the dense
early Universe to the flatter space later on (like now)
mean going from a place of more time dilation and less
space to a place of less time dilation but more space?

Sorry for that marathon sentence.

-- Jeff, in Minneapolis

Ken G
2013-Nov-25, 01:46 AM
Every constant-time surface in the FLRW metric is a cauchy surface, therefor the information on such a surface determines uniquely the spacetime on either side of it. If we change the sign of time all we do is change the sign of the velocities (or rather momentum), yet we go from an expanding to a contracting space. How can then the velocity have virtually nothing to do with it?It depends on what one means by velocity. Normally, velocity is defined as the relative motion of two objects passing each other, or at least in the same locally observable tangent space too small for tidal gravity effects to enter (i.e., the realm of SR), that's the only invariant vector there. Most cosmological models have zero velocity, they just have dynamics of spacetime itself. Nothing is passing anything else, not on cosmological scales, so the velocities are all zero. There are other meanings of velocity, but they are coordinate dependent, so are not "real" meanings of the term.


It doesn't matter, past to future or future to past, it's the same thing.Yes, but my point is, we have no idea what causes what, or what determines what, we just have some constraints and some data, and we use it to predict and understand some other data. The concept of an "initial condition" is not a physical statement, it's just a technique for making certain kinds of predictions. No initial condition is actually determining anything in a physically true way, it is just how physicists manipulate information. We have no idea how the universe does this, and there is really no such thing as an initial condition for the universe. The beginning just kind of disappears into a big question mark, it's just a device of manipulating information that we can choose some data that we have observed at some epoch in the evolution, and take that as our "initial condition", but that cannot be said to be the cause of the expansion. No one has any idea what is the cause of the expansion, although some speculations exist that I can't say if we should believe.

Noclevername
2013-Nov-25, 01:56 AM
I mean, since time is dilated at Earth's surface compared
to what it is in geosynchronous orbit, because the surface
is deeper in the gravity well, so that going from GEO to
the surface would be a change (which of course can't be
detected or measured locally because it is only relative),
would going from deep in the gravity well of the dense
early Universe to the flatter space later on (like now)
mean going from a place of more time dilation and less
space to a place of less time dilation but more space?


I don't know if we can answer that, as time dilation is relative to a reference frame. If all space had the same gravatic energy level at t=0, can we really call it a gravity well? If all places have the same time passage, how can we measure time dilation?

Jeff Root
2013-Nov-25, 02:13 PM
My understanding -- which I'm not at all sure about -- is that
a small portion -- maybe 1 or 2 percent -- of the redshift of
the CMBR is due to the light climbing out of the gravity well
from that time to now.

Caveman was arguing against the notion that space itself
does anything (as though it were a sort of aether) but rather
that the cosmic expansion is a coordinate effect. Yet the
clusters of galaxies clearly are getting farther apart from
each other over time. That is motion. The region around
a black hole is described as the space flowing inward toward
the center. At the event horizon, space is flowing inward
at the speed of light relative to the location of that horizon.
Which is to say, anything that doesn't feel acceleration
(any inertial observer) falls across the event horizon at the
speed of light. It can't orbit the black hole, but is pulled
down as though the space it is in is flowing down.

The descriptions I've read say that space near a black hole
is rotated into the time dimension. I take it that at the
event horizon, it is rotated 45 degrees, so that space and
time are in some way "equal", while at the singularity the
rotation would be 90 degrees, so that the only direction is
forward in time, there is no space at all, and in perfectly
"flat" Minkowski space, there would be no rotation in the
time direction, and there would be only space, no time.

So I figure that as the Universe expands, and there is more
and more space, there should be less and less time -- or
less and less time dilation.

I was hoping for a response from Ken on this, rather than
from caveman, though it was caveman who prompted me
to ask.

-- Jeff, in Minneapolis

Strange
2013-Nov-25, 02:22 PM
The descriptions I've read say that space near a black hole
is rotated into the time dimension. I take it that at the
event horizon, it is rotated 45 degrees, so that space and
time are in some way "equal", while at the singularity the
rotation would be 90 degrees, so that the only direction is
forward in time, there is no space at all, and in perfectly
"flat" Minkowski space, there would be no rotation in the
time direction, and there would be only space, no time.

My understanding was that at the event horizon one of the spacial dimensions (radial) had become swapped with the time dimension. Hence, when you cross the event horizon, the singularity is now in your future, which is why you can't turn back (it would require going back in time). I think I got that from some of Grant H's comments.

Jeff Root
2013-Nov-25, 02:54 PM
Yes, the event horizon is an inflection surface above which
space is dominant and below which time is dominant. At the
surface they would be equal. There must be a smooth,
continuous transition from flat, Minkowski boring heaven to
infinitely curved, singularity inferno hell. A person falling in
wouldn't notice that he was going to hell and had no way to
prevent it if he didn't keep careful track of where he was in
relation to the outside Universe. Nothing happens at the
event horizon that can be detected locally by the infaller.

-- Jeff, in Minneapolis

caveman1917
2013-Nov-25, 03:02 PM
There are other meanings of velocity, but they are coordinate dependent, so are not "real" meanings of the term.

Yes, but "expanding space" is also not a real meaning of the dynamics, it's just a coordinate picture. Other coordinate systems have different pictures for the same thing, for instance in comoving coordinates space does not expand but the speed of light gets smaller over time. When someone asks why space is expanding, they would mean why the proper distance between galaxies is increasing over cosmological time - certainly a coordinate picture - but one that defines a specific velocity, namely proper distance over cosmological time. And within that picture the only answer as to why those distances are increasing is "because that's what they were doing earlier". Granted, that's not much of an answer, but it is the one consistent with the chosen picture.

The reason i think it is important to point this out is that it seems that a lot of Aristotelian thinking occurs when thinking about expanding space, as if there needs to be some ongoing "reason" for galaxies to keep getting away from eachother.


The concept of an "initial condition" is not a physical statement, it's just a technique for making certain kinds of predictions.

In a sense you are correct of course, but we need to distinguish between what is the "ultimate" cause for expansion and what is the cause for seeing galaxies move away from eachother at any one time. In the latter case the only cause for them to move away at this time is because that's what they did earlier, in as much as one can call that a "cause" in the first place. Obviously, this leads to infinite regress, which is solved by saying that the "they move away from eachother" is part of the initial conditions of the big bang. I agree that's not much of a solution for the "ultimate" cause of expansion, saying that it is contained in the initial conditions is really nothing more than a fancy way of saying "we don't know".

I presume our discussion here is because we are approaching the subject from two different perspectives, you're talking about the cause of the expansion in the sense of the "ultimate" cause of it, where i am just trying to counter the Aristotelian thinking that came up in the earlier answers (such as where the boundary was placed on where two forces cancel).

caveman1917
2013-Nov-25, 03:07 PM
I mean, since time is dilated at Earth's surface compared
to what it is in geosynchronous orbit, because the surface
is deeper in the gravity well, so that going from GEO to
the surface would be a change (which of course can't be
detected or measured locally because it is only relative),
would going from deep in the gravity well of the dense
early Universe to the flatter space later on (like now)
mean going from a place of more time dilation and less
space to a place of less time dilation but more space?

Sorry for that marathon sentence.

-- Jeff, in Minneapolis

So you are asking if there is time dilation between events in the past and present in an expanding universe? The answer to that is yes, in fact the observed time dilation of distant (and hence far in the past) supernovae is one of the lines of evidence that disproved tired-light models.

caveman1917
2013-Nov-25, 03:10 PM
I don't know if we can answer that, as time dilation is relative to a reference frame.

In cosmology one conventionally takes cosmological time as time coordinate, and either proper distance or comoving distance as spatial coordinates. This is just convention, any coordinates will work, but one can consider discussions about cosmology to take place in either of those coordinate systems unless otherwise noted.

blueshift
2013-Nov-26, 12:25 AM
The analogy I usually use when this issue comes up is to draw a parallel to how gravitational forces exists in all matter and there is a puzzle as to why all things are not round like people, potatoes, trees. The reason is that the gravitational forces are too weak to overcome the electrical forces that shape us into what we are but the difference between those forces in odd shaped things becomes less and less the more and more massive things become. Once the gravitational forces exceed the electrical forces then the object loses its strange shape and begins to round out. Likewise with spatial expansion. It is very weak locally but it is still there. Gravity locally has the upper hand. Over great distances expansion becomes stronger and continues getting stronger with distance until the force of spatial expansion exceeds the force of gravity.

Sums up what a lot have mentioned already, just with a different illustration. That is my two cents anyway.

Ken G
2013-Nov-26, 12:41 AM
Yes, but "expanding space" is also not a real meaning of the dynamics, it's just a coordinate picture. Other coordinate systems have different pictures for the same thing, for instance in comoving coordinates space does not expand but the speed of light gets smaller over time. I know, that's why I resist saying that space is expanding. Whatever the cosmological dynamics are, they are the dynamics of spacetime. They are "happening" to spacetime.

The reason i think it is important to point this out is that it seems that a lot of Aristotelian thinking occurs when thinking about expanding space, as if there needs to be some ongoing "reason" for galaxies to keep getting away from eachother.I agree, what is happening is fundamentally mysterious, but the only constraint it has to obey is that which gravity imposes. That's what I mean by gravity "wanting it" to be so, and nothing else being involved other than the mysterious causes that we do not know, or maybe cannot know.


In a sense you are correct of course, but we need to distinguish between what is the "ultimate" cause for expansion and what is the cause for seeing galaxies move away from eachother at any one time. In the latter case the only cause for them to move away at this time is because that's what they did earlier, in as much as one can call that a "cause" in the first place.That is certainly what we often mean when talking about causes, all I'm saying is that this is how we think. We are imposing our thought processes on nature. But nature doesn't actually work that way, in the sense that the laws of physics don't assert that. It is a kind of physics sociology that asserts that things do what they used to be doing. As for the laws, they are just as happy to say that things are constrained to have done what they are doing now.
I agree that's not much of a solution for the "ultimate" cause of expansion, saying that it is contained in the initial conditions is really nothing more than a fancy way of saying "we don't know".Right, that's all I'm saying. Gravity is a kind of constraint, and it's the closest we get to an explanation of what is going on in cosmology, but gravity is all we have-- it's all gravity.


I presume our discussion here is because we are approaching the subject from two different perspectives, you're talking about the cause of the expansion in the sense of the "ultimate" cause of it, where i am just trying to counter the Aristotelian thinking that came up in the earlier answers (such as where the boundary was placed on where two forces cancel).
Yet that is what I am doing as well. I am fighting the tendency to imagine that there are two different kinds of gravity, the kind of gravity that relates to expanding space, and the kind of gravity that relates to the Earth orbiting the Sun. But it's all the same gravity, it's just inertial motion, and inertial motion is all ruled by just one thing: gravity. Alternatively, we could say that gravity is ruled by just one thing, inertial motion. I don't know which causes which-- they are the same thing.

Ken G
2013-Nov-26, 12:44 AM
Likewise with spatial expansion. It is very weak locally but it is still there. Gravity locally has the upper hand. Over great distances expansion becomes stronger and continues getting stronger with distance until the force of spatial expansion exceeds the force of gravity. But in my view, this is essentially a misconception. There is no need for the expanding universe to overcome gravity: the expanding universe is gravity, gravity is all there is when we are talking about the motions of objects in space. All kinds of motions, all of orbital mechanics and all of cosmology is the study of gravity. We do use different techniques and assumptions when we do gravity on different scales, but we should not let that confuse us into thinking we are dealing with two different things, which gets right back to the OP question.

Ken G
2013-Nov-26, 12:54 AM
I mean, since time is dilated at Earth's surface compared
to what it is in geosynchronous orbit, because the surface
is deeper in the gravity well, so that going from GEO to
the surface would be a change (which of course can't be
detected or measured locally because it is only relative),
would going from deep in the gravity well of the dense
early Universe to the flatter space later on (like now)
mean going from a place of more time dilation and less
space to a place of less time dilation but more space?
Yes, I believe that one can model essentially all the cosmological redshift as being due to clocks running faster now than they did in the past (and rulers are getting shorter). That's how I like to think about it, I don't like to imagine that space is expanding because space isn't a thing, but clocks and rulers are. Still, my approach is just another coordinate choice, it's not a physical statement.

blueshift
2013-Nov-26, 12:18 PM
But in my view, this is essentially a misconception. There is no need for the expanding universe to overcome gravity: the expanding universe is gravity, gravity is all there is when we are talking about the motions of objects in space. All kinds of motions, all of orbital mechanics and all of cosmology is the study of gravity. We do use different techniques and assumptions when we do gravity on different scales, but we should not let that confuse us into thinking we are dealing with two different things, which gets right back to the OP question.I take it that you are seeing things from the perspective that MOND holds, or a touch like it? And should I further take it that you see gravity existing right away at the point of the BB and you don't see it freezing out? Just clarifying here on my part. I see it freezing out and not existing at the instant of the BB, since there would be no density difference anywhere. From my understanding the density remained the same and didn't change right away.

Ken G
2013-Nov-26, 07:22 PM
I take it that you are seeing things from the perspective that MOND holds, or a touch like it? No, I'm motivated just by GR. Indeed, MOND-type theories are the ones that try to section gravity into different effects happening at different scales. GR completely unifies gravity-- it is all one thing, that controls the geometry of spacetime, by which I mean it determines all the inertial paths. Since in cosmology and orbital mechanics, pretty much everything follows inertial paths, so their dynamics are all one thing. The expansion of the universe and the orbit of the Earth are the tail and the trunk of the same elephant.


And should I further take it that you see gravity existing right away at the point of the BB and you don't see it freezing out?Yes, I'm using only GR, not any other possible ways that gravity might work.

I see it freezing out and not existing at the instant of the BB, since there would be no density difference anywhere.It is an important aspect of GR that gravity still exists even if there are no density variations anywhere. Indeed, that is the core assumption of cosmology, the "cosmological principle." It is one of the main places where Newton went astray-- he thought that if the density was constant everywhere, the force of gravity would have nowhere to point, so that force had to be zero. But in GR, gravity is not a force, so does not need to point anywhere-- the pointing of the acceleration of gravity is a function of the coordinates you use, gravity is something different that can point in different directions in different coordinates, and exists even when the density is the same everywhere.

blueshift
2013-Nov-26, 08:29 PM
No, I'm motivated just by GR. Indeed, MOND-type theories are the ones that try to section gravity into different effects happening at different scales. GR completely unifies gravity-- it is all one thing, that controls the geometry of spacetime, by which I mean it determines all the inertial paths. Since in cosmology and orbital mechanics, pretty much everything follows inertial paths, so their dynamics are all one thing. The expansion of the universe and the orbit of the Earth are the tail and the trunk of the same elephant.

Yes, I'm using only GR, not any other possible ways that gravity might work.
It is an important aspect of GR that gravity still exists even if there are no density variations anywhere. Indeed, that is the core assumption of cosmology, the "cosmological principle." It is one of the main places where Newton went astray-- he thought that if the density was constant everywhere, the force of gravity would have nowhere to point, so that force had to be zero. But in GR, gravity is not a force, so does not need to point anywhere-- the pointing of the acceleration of gravity is a function of the coordinates you use, gravity is something different that can point in different directions in different coordinates, and exists even when the density is the same everywhere.Ken, I will be rechecking my own sources. In the meantime, what source or sources can you point to that influenced that view?

Ken G
2013-Nov-26, 08:53 PM
Ken, I will be rechecking my own sources. In the meantime, what source or sources can you point to that influenced that view?Which view do you mean, the view that GR involves equations that induce universal dynamics even if the density is the same everywhere? That is simply the cosmological principle, all the cosmology I have ever learned invoked that fact.

blueshift
2013-Nov-27, 03:10 AM
Which view do you mean, the view that GR involves equations that induce universal dynamics even if the density is the same everywhere? That is simply the cosmological principle, all the cosmology I have ever learned invoked that fact.
Perhaps I am misreading something here:

http://en.wikipedia.org/wiki/Cosmological_principle

I do see mentions of a global scale and even some text suggesting that Newton was right. Being homogeneous and isotropic? Is that what suggests that to you?

Ken G
2013-Nov-27, 04:08 AM
The part where it says "In modern physical cosmology, the cosmological principle is an axiom that embodies the working assumption or premise that the distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale" suggests to me that the density is uniform everywhere, that is correct. It's the central concept in all of cosmology, a huge simplification.

WayneFrancis
2013-Nov-27, 06:34 AM
I mean, since time is dilated at Earth's surface compared
to what it is in geosynchronous orbit, because the surface
is deeper in the gravity well, so that going from GEO to
the surface would be a change (which of course can't be
detected or measured locally because it is only relative),
would going from deep in the gravity well of the dense
early Universe to the flatter space later on (like now)
mean going from a place of more time dilation and less
space to a place of less time dilation but more space?

Sorry for that marathon sentence.

-- Jeff, in Minneapolis

The difference is there is a gradient of gravity between the Earth's surface and geosynchronous orbit that is there. Over time the observer at geosynchronous and the Earth's surface can clearly see that the clock run at different rates. 2 observers expanding with the universe are being lifted out of ...well it isn't a gravity well as much as it is a gravity plane. All observers will observe the same effect for all other points when you factor in distance between those 2 points. Essentially the "centre is every where" while the Earth and Geosynchronous orbit will measure things differently. Not only time but distance. Sure there is a similar effect with space expanding but if space stopped expanding at different points in time then 2 clocks otherwise at rest with each other would measure the same thing. Take the following thought experiment

f\left(t_{1}\right) the universe is not expanding
Points A & B would measure each others clocks the same.
f\left(t_{2} \right) the universe expands by 100x and stops
f\left(t_{3}\right) the universe is not expanding
Points A & B would measure each others clocks the same.

even though they both exist in a lower mass-energy density universe. The key is that the mass-energy density of the 2 is the same. In the geosynchronous orbit and Earth surface example the mass-energy density is not the same and more importantly is not uniformly distributed.

blueshift
2013-Nov-27, 01:30 PM
The part where it says "In modern physical cosmology, the cosmological principle is an axiom that embodies the working assumption or premise that the distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale" suggests to me that the density is uniform everywhere, that is correct. It's the central concept in all of cosmology, a huge simplification.I agree with that part but the scale at a BB singularity is not a large enough scale in my head. Granted there is a lot of mass in the universe and the energy of the inflation field was stretching gluons to a point of snapping, converting that energy into matter over and over but I see a universe collapsing on itself with gravity existing at those early moments and I don't see it collapsing on itself without gravity existing.

To support your view, only GR exists on the global scale while both special and Galilean relativity break down. Now if I am in error it could be that I am seeing the universe expanding into something on a subtle level while GR is clearly stating that inflation doesn't matter and only exists from a special relativistic or Galilean relativistic viewpoint.

Or, worse yet, this could be the anomaly between quantum physics, in operation at the singularity, at odds with GR and no resolution is seen yet. I think I better go back to my telescope. The clouds just cleared out with my solar scope staring at me.

Cougar
2013-Nov-29, 02:16 PM
Yes, I believe that one can model essentially all the cosmological redshift as being due to clocks running faster now than they did in the past (and rulers are getting shorter). That's how I like to think about it, I don't like to imagine that space is expanding because space isn't a thing, but clocks and rulers are. Still, my approach is just another coordinate choice, it's not a physical statement.

Shouldn't such a thought be in ATM? :lol: How do you know it's not a physical statement? And BTW, don't you think, as Rocky Kolb said, "the job of a scientist is to understand nature"? It's not just calculatin' and making predictions, is it? Or as someone else said, "the heart of science is a kind of shrewd honesty that springs from really wanting to know what the hell is going on!"

Ken G
2013-Nov-29, 05:19 PM
Shouldn't such a thought be in ATM? :lol: How do you know it's not a physical statement?Because it is just a different interpretaiton of the exact same theory. No experiment could distinguish anything I said from the standard interpretation of expanding space. GR certainly doesn't care, it only predicts the observations, and its guts are all coordinate-free.

And BTW, don't you think, as Rocky Kolb said, "the job of a scientist is to understand nature"? Certainly I agree. But here's the tricky part: what we test are observations. What happens when your "understanding" can involve two very different sounding interpretations that are not distinguishable by observation? So I would expand Rocky's statement to "the job of a scientist is to understand nature in as many ways as are consistent with observation, subject to Occam's razor, but noting that Occam's razor is often a subjective principle that admits to alternative interpretations." A bit longer, grant you, but more accurate to the mission.