Understanding the Expansion of Space: A Scientific Perspective

[Starship]

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?

 

[Nereid]

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.

 

[cosmoboy]

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?

Like you and me and I think many others are bothered by expansion of space. There are many questions related to space some of them are as follows:

a). What is means by space ?
b). In case the universe is finite what lies beyond it ?
c). Apart form the fact that matter or energy needs space for their existence is there any other meaning of space ?

I think there is no independent existence of space. It exists only with matter/energy, I mean whenever there is matter/energy there is space. I think there is no effect of non local space on any matter/energy.

 

[lawtonfogle]

im guessing he means space as the stars and their moons (planets), and their moons

all space is expanding

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. So their is a place, X number of miles away from where all matter began to move, where a event-horizon lies. It might not be a perfect sphere for some 'areas' of the universe have more matter, so the pull of gravity will afect the accelleration.

Now i have a question pertaining to this. Does a event horizon have a with?

 

[Garth]

"Is space really expanding? " It depends how you measure distances. Edit: That is: "What standard do you define to be the measure of distance?"

The evidence for space expansion is the Hubble red shift.

Also the next question is: "What has it expanded from?" The expanding solution to Einstein's GR cosmological case teaches us that it has expanded from a singularity, or near singularity, called the Big Bang. Nucleo-synthesis in the BB then predicts that the universe should be 3/4 hydrogen and 1/4 helium with very little of anything else. This ties in with the observed cosmic makeup once the elements created in the nuclear furnaces of stellar cores have been taken into account. Also the Cosmic Microwave Background is good evidence of the radiation given off by that BB, now red shifted into the microwave region of the electromagnetic spectrum.

We can say that if our rulers, made of atomic material, are defined to be of fixed size, and this means defining the rest mass of their atoms to be constant, then the Hubble red shift is Doppler in nature and the galaxies are rushing away from us; the universe itself is expanding everywhere because the space in which all stars, galaxies etc. are embedded is itself expanding.

However if we instead choose a different ruler, then the universe may indeed be static in size and it is atoms that are shrinking within it. An expanding universe with fixed rulers is replaced, or interpreted equivalently, as a static universe with shrinking rulers.

A ruler, or rather the atoms it is made of, may shrink over time because they are gaining mass over cosmological time. Edit: (Because other things being equal, the radius of an atom is inversely proportional to its mass) Hubble red shift is then understood to be an effect caused by the increasing mass of the apparatus used to measure it rather than a Doppler effect.

A second effect of atomic rest masses increasing is that atomic clocks would speed up. Edit: (The frequency of an 'atomic' vibration is proportional to its mass) This would have the effect of projecting the BB back, as measured by this new time, into the infinite past.

These ideas were first mooted by Fred Hoyle in his 'Mass field theory', a conformal gravity theory, in which he tried to explain the microwave background in a steady state model. ( Hoyle, F. "On the origin of the microwave background" Ap. J. 196:661-670, 1975 March 15). They also appear again in Self Creation Cosmology

Garth

 

[Phobos]

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.

Well, that wasn't "the first" scientific cosmology, but yes, that "closed universe" model is losing ground vs. an eternally expanding universe (although the closed model is not out of the running yet!)

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.

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.

 

[lawtonfogle]

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.

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.

 

[cosmoboy]

expansion

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.

 

[SpaceTiger]

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.

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.

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.

 

[cosmoboy]

I'm not sure what you mean by gravity being scale-free.

Plese look somewhere else this is an important propery
of grvaitational intercation unlike to other fundmental
intercations gravity shows this property.

 

[SpaceTiger]

Plese look somewhere else this is an important propery
of grvaitational intercation unlike to other fundmental
intercations gravity shows this property.

I can't tell if you're a crackpot or just very poor at expressing yourself. Would you mind pointing me to some references that can show me what you mean?

 

[Starship]

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. Since we can't visualize a four dimensional universe, Riemannian geometry and tensors are the only way to demonstrate it.

Space in theoretical physics has various definitions:

1. The structure defined by the set of "spatial relationships" between objects.

2. A manifold defined by a coordinate system where an object can be located.

3. The entity that stops all objects in the universe from touching one another.

4. The condition within the conceptual field of Existence that provides the 'ground' for any manifested form and as such it enables the movement and all physical dynamics.

We don't know what space-time is. Space-time has been treated as a question of geometry, in this case it's a Minkowski space.

 

[Garth]

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.

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

 

[Starship]

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

Maybe something like http://people.hofstra.edu/faculty/Stefan_Waner/diff_geom/pics/LightCone.gif .

 

[russ_watters]

No one replied to this:

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.

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.

 

[Starship]

More accurately, the metric space is the one which is expanding. Or when viewed as a Riemannian metric the distance in the metric is the one which is growing.

 

[Phobos]

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.

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.

 

[Mike2]

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.

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.

 

[hellfire]

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.

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.

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.

As far as I know, general relativity is scale invariant (conformally invariant, g_uv -> k g_uv) 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?

 

[russ_watters]

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.

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.

 

[misskitty]

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.

This is a pretty good explanation Nereid. I have a few questions though after reading everyone's posts. What are LQG and I've heard of string theory, but I don't know anything about it. Also didn't Einstein develop tensor analysis to help explain the way the universe is expanding (or contracting however you desire to view it) and the behavior for some of the cosmological phenomenon? Or am I completely off base and way out in left field?

 

[dad]

The evidence for space expansion is the Hubble red shift. ..Garth

If one had another way to explain that, and CMB, what else is there??..

 

[russ_watters]

Welcome to PF. This is a very common topic, most recently discussed here: https://www.physicsforums.com/showthread.php?t=152773&highlight=space+expanding

There is a host of other information on the Web as well, available with a Google. Here's what Wik has to say: http://en.wikipedia.org/wiki/Big_bang_theory

 

[dad]

Welcome to PF. This is a very common topic, most recently discussed here: https://www.physicsforums.com/showthread.php?t=152773&highlight=space+expanding
..

I took a look at the thread, and found this.

"Additionally to redshift, there are other proofs for the expansion of space, as for example the cosmological time dilation of the supernova luminosity curves, the Tolman surface brightness test, the number density of galaxies and their evolution with distance, the variation of the temperature of the cosmic microwave background, the cosmic microwave background itself, the light elements abundances, etc. All of these are based on the validity of general relativity..."

Digging a little deeper, I see this.
"(1) Studies of relatively nearby Type Ia supernovae (SNe Ia) have shown that their intrinsic luminosities can be accurately determined; (2) New research techniques have made it possible to schedule the discovery and follow-up observations of distant supernovae, producing well over 50 very distant (z = 0.3 -- 0.7) SNe Ia to date. These distant supernovae provide a record of changes in the expansion rate over the past several billion years. By making precise measurements of supernovae at still greater distances, and thus extending this expansion history back far enough in time, we can distinguish the slowing caused by the gravitational attraction of the universe's mass density Omega_M from the effect of a possibly inflationary pressure caused by a cosmological constant Lambda"
http://xxx.lanl.gov/abs/astro-ph/9712212

It doesn't mention how the changes between the perceived 'several billion years' away, and the closer SN are determined.
Also, again, the Tolman surface brightness test is wholly dependant on the classical conception of what redshift is, and was caused by.
My original question was what ELSE do we have.
"In a simple (static and flat) universe, the light received from an object drops inversely with the square of its distance, but the apparent area of the object also drops inversely with the square of the distance, so the surface brightness would be independent of the distance. In an expanding universe, however, there are two effects that reduce the power detected coming from distant objects. First, the rate at which photons are received is reduced because each photon has to travel a little farther than the one before. Second, the energy of each photon observed is reduced by the redshift. At the same time, distant objects appear larger than they really are because the photons observed were emitted at a time when the object was closer. Adding these effects together, the surface brightness in a simple expanding universe (flat geometry and uniform expansion over the range of redshifts observed) should decrease with the fourth power of (1+z).

To date, the best investigation of the relationship between surface brightness and redshift was carried out using the 400-inch Keck telescope to measure nearly a thousand galaxies' redshifts and the 94-inch HST to measure those galaxies' surface brightness.[1] The exponent found is not 4 as expected in the simplest expanding model, but 2.6 or 3.4, depending on the frequency band"
http://en.wikipedia.org/wiki/Tolman_surface_brightness_test

I guess, then, aside form redshift based ideas, and the CMB, there is nothing else that says the universe is expanding. (?)

 

[Garth]

If one had another way to explain that, and CMB, what else is there??..

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.

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!)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.

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"?)

I hope this helps.

Garth

 

[dad]

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.

Interesting, but I have some very different ideas.

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!)

"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)

So it depends on the starting assumptions. My concern is beyond assumption, and in regards to what we actually, really do know.

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.

Bingo. And does redshift have something to do with determining those distances?
If so, again, my concern is only things other than redshift, and CMB. I have other explanations for that.

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"?)

So, where the z equals the redshift. Again, all depends on the explanation for the redshift.

I hope this helps.

Garth

Well, still, cool , but at least an attempt at getting warm. Thanks.

 

[Garth]

So it depends on the starting assumptions. My concern is beyond assumption, and in regards to what we actually, really do know.

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.

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?

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.

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.

However 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.

"Is space really expanding?" - It depends on how you measure it.

So, is it possible to actually measure such a change in the standards of measurement? In SCC this is possible by comparing the atom against a representative photon, such as one sampled from the CMB.

Such a photon when measured by such an atom will appear to exhibit red shift, not because the photon has lost energy but because the atom has gained mass.

I have included this in answer to the title question of this thread, however, further discussion on the theory I restrict to the Self Creation Cosmology thread.

Garth

 

[dad]

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.

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!

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?

Good question. Since it is claimed it is expanding I assume there would be an answer?

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.

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.
This is why I was wondering what else, besides redhift, and the CMB, and things based on them, science had to support claims such as the universe expanding.

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, 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.

However 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.

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 explained

 

[complexPHILOSOPHY]

To Dad:

I am not exactly sure what you are trying to dispute, perhaps you could organize your responses in a more logical manner? You seem to display the mentality that everything is assumed, speculated and conjectured without evidence. If one wants to dispute the notion of past and future, one should evaluate the logic.

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.

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!

 

[dad]

To Dad:

I am not exactly sure what you are trying to dispute, ..

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.

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.

So, you assume that it was the same, I see. Also, you prefer no additional assumptions. OK.

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.

Oh, that is easy, but I prefer to stick to what we do know, rather than get too much into theories.

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!

Well, as far as space expanding, I guess that redshift, and the CMB just about all science has at the moment to base a claim of an expanding universe on, then. (?)

Interesting.

 

[dad]

It does not depend on assumptions but the definition of the method of measurement.

We shall see.

What we actually do know is what we can observe and measure.

We shall see.

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?

That is the question, yes.

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.

I know. But you miss my point. The state of this present universe may not have been the same in the far past, therefore, using only present observation only tells us about the present.

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.

So you seem to think. But, aside from redshift, and CMB, how do we actually know this??

However 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.

Interesting speculation. I was more concerned about what we know.

"Is space really expanding?" - It depends on how you measure it.

So, is it possible to actually measure such a change in the standards of measurement? In SCC this is possible by comparing the atom against a representative photon, such as one sampled from the CMB.

Such a photon when measured by such an atom will appear to exhibit red shift, not because the photon has lost energy but because the atom has gained mass.

"Self-creation cosmology (SCC) theories are gravitational theories in which the mass of the universe is created out of its self-contained gravitational and scalar fields,"
Speculation, in other words on how mass came about in our universe.

Not a way we know that the universe is expanding. That is simply one of a number of theories quasi relating to the issue.

 

[dad]

To Dad:

I am not exactly sure what you are trying to dispute, perhaps you could organize your responses in a more logical manner? You seem to display the mentality that everything is assumed, speculated and conjectured without evidence. If one wants to dispute the notion of past and future, one should evaluate the logic.

Some feel that there is an afterlife, for example. A place, as we know, most call heaven. If there really were such a place, the universe would have to undergo a complete state change. Things here, we know, are not eternal, not even the sun. In fact, a new heavens and Earth are claimed to be on the way by some.

We cannot evidence that, or say true or false. It is out of the scope and concern of science. If there were such a state in the far past, basically a different universe, we likewise could not tell. In other words, we cannot prove with science that the state of the universe in the past was as it now is!
Personally, I suspect the universe was in a different state, one that included the spiritual, and physical. Something changed, and we were left in this physical only state, separated from the spiritual. A state that is literally, temporary.
That is why, looking at light far away, or redshift, or the CMB, etc. is not looking in the far past after all. It is simply looking far away. We have assumed that light coming from there, taking billions of years meant that it took that long for the light to get here. In effect, that is simply assuming that it was always as it now is.
Light in the former state could have gotten here in days, for example. A different light, in a different state universe. As it was changed, we were left with the slow light we now have. Similarly, such a universal state change could leave light redshifted, in a pattern as we now see (more shifted the further we get out). Same thing with the CMB.
Far as I know, there is nothing science can do to prove, or support, observe, evidence, etc that the past was this same state. That is nothing but an assumption.
So, that would leave us in a temporary present state, with the future, and past being in different states. Anything but a 'steady state'. This could explain a lot, even things quantum. (If waves do go to and from the different past and future, we in the present would consider their behaviour hard to understand)
So, yes I know full well that the past state is but assumed. -No?

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.

Well, if one did not want to use any other assumption, it matters not. The only thing that does matter is to be able to defend the ones we do use! That cannot be done by science for or against a same or different state past or future.

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.

That is already done in a model where the spiritual and physical are together in an eternally stable state.

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.

No, under normal observed limits, the same past (recent) is very evidenced.

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!

I explained where I was coming from. I think it is impossible to prove a same past, that all science depends on, and assumes as a foundation for past ideas.
So, how do we know the universe is expanding?? (aside from redshift, and CMB, for the reasons I outlined)

 

[InfinityDelta]

The universe is gaining speed as it expands, yet I personally dislike this because it destroys my 2nd theory for the Big Bang, unless the last universe did collapse.

 

[dad]

The universe is gaining speed as it expands, yet I personally dislike this because it destroys my 2nd theory for the Big Bang, unless the last universe did collapse.

You think you know it is gaining speed how? Redshifted light??

 

[russ_watters]

You already know the answer to that question, dad. We only have the best information we have. No we aren't certain of anything, but we draw the most reasonable conclusions based on the available evidence.

 

[InfinityDelta]

Yes, the universe is, because the average density of space is less than 3 hydrogen atoms per cubic meter, so the universe will keep expanding forever.

 

[dad]

Yes, the universe is, because the average density of space is less than 3 hydrogen atoms per cubic meter, so the universe will keep expanding forever.

You sound fairly certain of that. Russ seems a little less certain. So, what is it you think that that the present spacing out of hydrogen atoms per cubic meter has to mean, and why?

 

[MeJennifer]

We cannot evidence that, or say true or false. It is out of the scope and concern of science. If there were such a state in the far past, basically a different universe, we likewise could not tell. In other words, we cannot prove with science that the state of the universe in the past was as it now is!
Personally, I suspect the universe was in a different state, one that included the spiritual, and physical. Something changed, and we were left in this physical only state, separated from the spiritual. A state that is literally, temporary.
That is why, looking at light far away, or redshift, or the CMB, etc. is not looking in the far past after all. It is simply looking far away. We have assumed that light coming from there, taking billions of years meant that it took that long for the light to get here. In effect, that is simply assuming that it was always as it now is.
Light in the former state could have gotten here in days, for example. A different light, in a different state universe. As it was changed, we were left with the slow light we now have. Similarly, such a universal state change could leave light redshifted, in a pattern as we now see (more shifted the further we get out). Same thing with the CMB.
Far as I know, there is nothing science can do to prove, or support, observe, evidence, etc that the past was this same state. That is nothing but an assumption.
So, that would leave us in a temporary present state, with the future, and past being in different states. Anything but a 'steady state'. This could explain a lot, even things quantum. (If waves do go to and from the different past and future, we in the present would consider their behaviour hard to understand)
So, yes I know full well that the past state is but assumed. -No?

Let me ask you this: would you agree that if we had a universe without mass and energy that such universe would expand?

Think of a very simple model of a universe that starts with a point where we emit photons in all directions, so that we get an expanding sphere.

Then consider the following situations:

A universe without mass and energy will expand forever, since the area of the expanding sphere will continue to increase.

A universe with some mass and energy will expand forever as well but the area of the sphere increases slower than in the first case. The decreasingly negative contribution to the area of the sphere by the curvature of space-time reduces the rate of increase of the area of the expanding sphere but overall the area still increases. Effectively the expansion accelerates here since the negative contribution to the area of the sphere decreases over time.

A universe with exactly a critical amount of mass and energy will seize to expand, the area of the sphere remains constant. The negative contribution to the area of the sphere by the curvature of space-time completely compensates for the expansion of the sphere.

A universe with more than a critical amount of mass and energy will seize to expand, the area of the expanding sphere decreases. The negative contribution to the area of the sphere by the curvature of space-time actually reduces the area of the expanding sphere and the surface is trapped, it can only get smaller.

 

[dad]

E=MeJennifer;1279000]Let me ask you this: would you agree that if we had a universe without mass and energy that such universe would expand?

Think of a very simple model of a universe that starts with a point where we emit photons in all directions, so that we get an expanding sphere.

Then consider the following situations:

A universe without mass and energy will expand forever, since the area of the expanding sphere will continue to increase.

A universe with some mass and energy will expand forever as well but the area of the sphere increases slower than in the first case. The decreasingly negative contribution to the area of the sphere by the curvature of space-time reduces the rate of increase of the area of the expanding sphere but overall the area still increases. Effectively the expansion accelerates here since the negative contribution to the area of the sphere decreases over time.

Hypothetical questions, all. The basic assumption is here, that all we had to begin with was matter similar to the kind we now have. Then, you imagine how it 'must' have worked. But stick to what we know. Do you think you know that matter in the past was as now? What if the state of the universe was different then? In the example I gave, with BOTH, the spiritual, and physical together. If that 'forever state' did exist, rather than the physical only state we now see, none of your ideas apply.

A universe with exactly a critical amount of mass and energy will seize to expand, the area of the sphere remains constant.

Again you are talking of physical only matter, as we are familiar with. Wouda, could have shoulda, and what if.

The negative contribution to the area of the sphere by the curvature of space-time completely compensates for the expansion of the sphere.

Our present space time. What can you offer to prove that the state of the universe will be, or was the same? Let me answer for you. NOTHING. Therefore your whole scenario is based on a same state past. It is based, in other words, on something that is nothing at all more than an assumption. An assumption that cannot be supported by fact, observation of man, evidence, or science.

A universe with more than a critical amount of mass and energy will seize to expand, the area of the expanding sphere decreases.

No! A physical only state universe as we now know it, would expand...etc.

The negative contribution to the area of the sphere by the curvature of space-time actually reduces the area of the expanding sphere and the surface is trapped, it can only get smaller.

So, what are you saying? Do you think that the universe is really getting smaller?

 

[MeJennifer]

Hmm, I actually attempted for you to understand by giving some examples, I seem to have failed miserably.

 

[Garth]

Hmm, I actually attempted for you to understand by giving some examples, I seem to have failed miserably.

Don't waste your time MeJennifer, you tried.

 

[dad]

Hmm, I actually attempted for you to understand by giving some examples, I seem to have failed miserably.

Well, in the context of normal physics, assuming that it was this state that existed at creation, your scenario is good. But what can get us from here to there??
"a very simple model of a universe that starts with a point where we emit photons in all directions, so that we get an expanding sphere."
If we look at claims of the future, where some envision a new universe, where there is no state of decay, and a different light, etc etc,- how would that be measured by this temporary universe? In other words, of course we really don't know the state of the universe in the future.
Like you have tried to do with the past, one can say, IF it were the same in the future, then we would see the sun burn out, galaxies crash, etc etc.
Same with the far past. IF the universe were just physical at the time, it had to have come from ..such and such.
So, how do you know it was in this same state, any more than the future will be?? My point is that all we really know is the present state of the universe. (some thousands of years of observation). The rest is pure belief. How could anyone really call that science??

Science, in other words, has real limits. Dreams of what the future would be like, or what a far past was like cannot be proved, supported, observed, tested, etc.

 

[russ_watters]

This thread is done.

Dad, this is a physics site. As such, discussions must be grounded in physics. This is not a place for religious speculation, or worse, religiously motivated attacks on science.