"the whole mess will start over again ..."

[lawcat]

Gosh, that is a rather heterodox treatment of the topic!!!!

 

I understand. I do not agree that it is quite heterodox. You indicated that the universe is expanding and the spacetime between galaxies may be finite and expanding or not. However:

The dynamics of a galaxy are not like the dynamics of the overall universe; they are quite independent of each other.

 

And my post suggests the same: That energy and spacetime are independent of each other.

This is also a discussion that has been going on with Michael Mooney on the problem of Big Bang. I hope Modest can review since it is so heterodox, but in essence my view does not negate the big bang. Rather, it is a fact that background radiation is everywhere in the observable universe.

 

Since we live and operate in the observable universe, we only operate in radiation, and we can not perceive anything independently of radiation: We do not know universe independent of radiation.

 

Modest's currently accepted explanation, although I will fail to do it justice here, is in essence: Since Universe is expanding, then it had origin; since it had origin, it was big bang; since there was big bang, there could be big crunch. (this is from another thread which I will regratably not take time to find.)

 

Mooney's problem was the impossibility of everything obeservable to be contained in a point, which is effectively the size of an imaginary volumeless, lengthless point.

 

But if there was Big Bang, it does not mean there even should be or could be a Big Cruch. The Big Bang could be the energy big bang, instead of the universe Big Bang. Why not? Since energy changes form, and all forms do not behave equally, and energy moves inertially, there is no reason why there could be a big crunch.

 

Radiation is expanding, and since radiation does not bind matter, there is no reason to believe that inertially expanding radiation could ever bind matter into a big crunch. Nor is there a reason to believe that, all matter could collapse into a point, when it is currently separated by intertially expanding radiation.

 

The premise of Big Crunch argument presupposes: (1) nature of radiation not consistent with facts, or (2) nature of spacetime in likeness of matter. Neither makes any sense.

[modest]

I hope Modest can review since it is so heterodox

 

Not entirely sure how to take that

 

Modest's currently accepted explanation, although I will fail to do it justice here, is in essence: Since Universe is expanding, then it had origin; since it had origin, it was big bang; since there was big bang, there could be big crunch. (this is from another thread which I will regratably not take time to find.)

 

Mooney's problem was the impossibility of everything obeservable to be contained in a point, which is effectively the size of an imaginary volumeless, lengthless point.

 

Yes, I probably know what conversation you're talking about. My insistence was simply that our current laws of physics break down at the singularity so trying to speculate as to what 'preceded' the proposed singularity is bound to come with lots of speculation.

 

But, I don't think that has much to do with your quandary.

 

And my post suggests the same: That energy and spacetime are independent of each other.

This is also a discussion that has been going on with Michael Mooney on the problem of Big Bang. I hope Modest can review since it is so heterodox, but in essence my view does not negate the big bang. Rather, it is a fact that background radiation is everywhere in the observable universe.

 

Since we live and operate in the observable universe, we only operate in radiation, and we can not perceive anything independently of radiation: We do not know universe independent of radiation...

 

...But if there was Big Bang, it does not mean there even should be or could be a Big Cruch. The Big Bang could be the energy big bang, instead of the universe Big Bang. Why not? Since energy changes form, and all forms do not behave equally, and energy moves inertially, there is no reason why there could be a big crunch.

 

Radiation is expanding, and since radiation does not bind matter, there is no reason to believe that inertially expanding radiation could ever bind matter into a big crunch. Nor is there a reason to believe that, all matter could collapse into a point, when it is currently separated by intertially expanding radiation.

 

The premise of Big Crunch argument presupposes: (1) nature of radiation not consistent with facts, or (2) nature of spacetime in likeness of matter. Neither makes any sense.

 

I don't really follow what you're saying, but I'll blather on anyway...

 

There are more than one kind of spacetime which depend on how you define distance and time. Depending then on the spacetime used, the appropriate laws of physics are different. For example, if you use special relativistic coordinates then SR is the correct physics to use. If you treat time a little differently then you get a Robertson-Walker metric and it's then appropriate to use the Friedmann equation.

 

In the former it would be inappropriate to say that space is expanding (by definition: it is not). In the latter way of doing things it is quite appropriate to say that space is expanding (the metric expansion of space).

 

I know of two good links talking about this at a basic level:

 

What Causes the Hubble Redshift?

 

Cosmology Tutorial - Part 2

 

But, there's no reason to get caught up on this. Either the galaxy out there is moving away from us through space or the space between us and the galaxy is expanding. Either way, the two are moving away from one another meaning they used to be closer in the past. Redshift is a very good indication that everything on a intergalactic scale is moving away from everything else.

 

It then becomes a simple matter of logic that if you go far enough back in time everything must have been balled up quite tightly. After redshift and Hubble's law were discovered that was the question cosmologists asked themselves and they came up with two good predictions based on that premise. If the universe used to be a "primeval atom" then we would expect to find CMBR and a certain abundance of light elements. Both of these predictions were later verified by astronomical observations.

 

This rather long preamble means two things. First, that it is exceedingly difficult at this point to deny the primeval atom (ie "Big Bang") scenario. It seems an inescapable conclusion (based on observation) that the visible universe used to be a very much 'smaller' place and everything in our observable universe has since gotten further away from everything else.

 

It also means (with somewhat less certainty) that our cosmic model that predicted all of this (relativistic cosmology) can't be too far off. If standard cosmology can correctly model the past evolution of the universe then we can have some confidence that it can model the future evolution of the universe.

 

According to standard cosmology the future of the universe is determined by two variables. Ignoring one of them for brevity, if the universe has a certain density then the expansion of the universe will slow, stop, and collapse (ie big crunch). If the density is less than this critical density then expansion will continue indefinitely (ie heat death).

 

So, coming at last to your question (or perhaps my butchered version of it)... what if relativistic cosmology is wrong? What if things are moving through space and space is *not* expanding? Does this 'critical density' still exist, still work the same way?

 

The answer is yes. Even if we throw out expanding space and consider only Newton's laws of motion and Newtonian gravity we can still come to this critical density -> big crunch solution. Not only that, but the formula for finding the critical density is exactly the same which I recently solved in this post:

 

Critical density for predicting a big crunch

 

Long story short, if the laws of physics that work so accurately in our neighborhood of the universe can be applied on a larger scale then a big crunch is at least a possibility (depending on the density of the universe).

 

~modest

[lawcat]

Modest,

 

Thank you for the well thought out response. I have three response points to above.

 

Unity of Energy and Space

 

First, I do not see how we can consider space and energy as one entity, to draw a conclusion that space-energy in the current expanding state started as one from the big bang. This unity of space and energy is impossible to prove or infer for us. Moreover, evidence suggests that they are separate: space is continuous even though energy is discontinous in the space (matter, radiation, black holes.) There is evidence that the two are completely independent. In addition, the known universe is filled with radiation, and radiation simply propagates. For the unity of space and energy to be true, it would have to be true that radiation can force the expansion of space. But this is inconsistent with everything we now about waves. Moreover, even if true that space and energy were one, we would not know it because at the boundary space and radiation would propagate equally; or there would some indication of refraction if the two were of different speed of propagation. There is simply nothing to indicate that energy and space are one, to conlude that the big bang caused the space to expand.

 

Homogeneity of All Energy

 

Second, the big crunch theory is premised on the homogeneity of energy density of the observable universe. But this is a fallacy because it implies that we can group radiation and matter into one energy entity for the purposes of deriving conclusions about expansion and contraction of the universe (and here I mean the observable energy-universe, even though the theory includes space.).

 

Radiation, although energy, does not create gravity, does not bind matter--it simply propagates at the speed of light. Matter on the other hand, binds matter, and let's pressupose radiation as well, and creates gravity, and moves at less than the speed of light. The nature of the two energies--radiation and matter--is different, and we can not group them together to form a common energy density of the universe, to say that there is some binding force on the energy of the universe. The two are not one phase for this purpose--there is no energy density homogeneity.

 

Dampened Motion Model

 

This to me looks like a dampened motion model, where the universe is behaving as a spring anchored at big bang. This makes no sense. First, as I alrady explained, radiation and matter can not be group into one, to model all universe as a homogeneous entity. Second, the model implies nice linear expansion from the center. But, the matter moves inertally, and upon collisions in different directions. After the first collision of matter, the center of gravity is no longer the big bang anchor. The spring is broken.

 

So in sum, the center of all matter is no longer big bang, and there is no biding force to bring matter towards the new center. Moreover, radiation propagates, and matter moves away inertially in accordance with primordial collisions. Big Crunch makes no sense on evidence.

[modest]

Modest,

 

Thank you for the well thought out response. I have three response points to above.

 

Unity of Energy and Space

 

First, I do not see how we can consider space and energy as one entity, to draw a conclusion that space-energy in the current expanding state started as one from the big bang. This unity of space and energy is impossible to prove or infer for us. Moreover, evidence suggests that they are separate: space is continuous even though energy is discontinous in the space (matter, radiation, black holes.) There is evidence that the two are completely independent. In addition, the known universe is filled with radiation, and radiation simply propagates. For the unity of space and energy to be true, it would have to be true that radiation can force the expansion of space. But this is inconsistent with everything we now about waves. Moreover, even if true that space and energy were one, we would not know it because at the boundary space and radiation would propagate equally; or there would some indication of refraction if the two were of different speed of propagation. There is simply nothing to indicate that energy and space are one, to conlude that the big bang caused the space to expand.

 

I honestly don’t know what you mean by “consider space and energy as one entity”. I think, perhaps, you’re talking about redshift...?

 

Homogeneity of All Energy

 

Second, the big crunch theory is premised on the homogeneity of energy density of the observable universe. But this is a fallacy because it implies that we can group radiation and matter into one energy entity for the purposes of deriving conclusions about expansion and contraction of the universe (and here I mean the observable energy-universe, even though the theory includes space.).

 

You are correct that big bang cosmology assumes (as a first order approximation) that the content of the universe is homogeneous. It does not, on the other hand, consider radiation and matter to be equal. Radiation and matter affect the scale factor similarly, but not equally. This is easiest to demonstrate with the omega form of the Friedmann equation:

 

The first Friedmann equation is often seen in a form with density parameters.

 

[math]\frac{H^2}{H_0^2} = \Omega_R a^{-4} + \Omega_M a^{-3} + \Omega_k a^{-2} + \Omega_{\Lambda}[/math]

 

Here [math]\Omega_R[/math] is the radiation density today (i.e. when [math]a=1[/math]), [math]\Omega_M[/math] is the matter (dark matter|dark plus baryonic) density today...

Friedmann equations - Wikipedia, the free encyclopedia

 

The matter and radiation content are both considered to be homogeneous in space, but they are not the same thing and they don't have exactly the same effect. The density parameters change over time. A very young universe would have been radiation dominated.

 

Radiation, although energy, does not create gravity, does not bind matter--it simply propagates at the speed of light.

 

Radiation does actually cause gravity.

 

Dampened Motion Model

 

This to me looks like a dampened motion model, where the universe is behaving as a spring anchored at big bang. This makes no sense. First, as I alrady explained, radiation and matter can not be group into one, to model all universe as a homogeneous entity. Second, the model implies nice linear expansion from the center. But, the matter moves inertally, and upon collisions in different directions. After the first collision of matter, the center of gravity is no longer the big bang anchor. The spring is broken.

 

So in sum, the center of all matter is no longer big bang, and there is no biding force to bring matter towards the new center. Moreover, radiation propagates, and matter moves away inertially in accordance with primordial collisions. Big Crunch makes no sense on evidence.

 

Where was the center of the Big Bang?

 

You should google “freely coasting cosmology” or "linear coasting cosmology". I think you’d find that very palatable.

 

~modest

[Qfwfq]

Indeed Lawcat I get the impression you have been elaborating on things expressed in dubious terms and perhaps shaky premisies too. Now that you mention Michael Mooney it seems a bit more understandable.

[lawcat]

Modest,

 

Thank you for the explanation and the links!