Intro to Big Bang and Infinity Concepts - Comments

[PAllen]

Hi @Arman777

I like your presentation, but I feel it would be improved by mentioning the following point.

Here is a quote.

On the surface of the sphere, we could move in some direction and we may find ourselves to the point that we are started.​

​

In an actual finite universe, which would typically be expanding or contracting, one might have to travel faster than the speed of light in the expanding case in order for the mover to arrive at the same spatial point. It may also be useful to mention choosing the point of interest as fixed in co-moving coordinates.

Here is a suggestion.

On the surface of a sphere which is expanding in the same manner as our universe, if we started at a fixed point in co-moving coordinates and traveled at some faster than light speed in any direction along a great circle of the sphere, we could eventually find ourselves back at the point where we started.​

Regards,
Buzz

I’m not sure this is true. I think, even with accelerated expansion of a closed, finite, simply connected universe, there is some timelike path from a given starting event on a comoving world line to a future event on the same world line. Further, there is such a path for every starting direction from the given starting event.

This basically follows from the fact that light always moves at c relative to any comoving world line. Thus light always makes progress relative to comoving world lines treated as having fixed coordinates in standard cosmological coordinates. And timelike paths can explore the complete interior of a light cone.

 

[Eirhead]

Just a quick shower thought:

- if the Universe is finite, then c is the speed limit
- if the Universe is infinite, then c is not a speed limit, but moving faster than c relative to something would determine if you can observe it or not.

 

[PeterDonis]

- if the Universe is finite, then c is the speed limit
- if the Universe is infinite, then c is not a speed limit,

This is wrong. c is the speed limit whether the universe is (spatially) finite or infinite.

 

[Eirhead]

This is wrong. c is the speed limit whether the universe is (spatially) finite or infinite.

Well, based on currently accepted physics. Fine I get it. But perhaps time dilation formulae needs to be revisited as we understand what is happening at cosmological distances / unobservable.

 

[Ibix]

Well, based on currently accepted physics.

You mean, over a century of experimental evidence? Bertozzi's experiment demonstrating electron speed approaching but never exceeding $c$ is even on YouTube if you want to see a practical demonstration.

But perhaps time dilation formulae needs to be revisited as we understand what is happening at cosmological distances / unobservable.

The time dilation formula is not relevant to cosmology. It's a special relativity concept that most definitely does not apply to the curved spacetime used in cosmological models.

I rather suspect that you are speculating without actually understanding the mathematics of the models. That's a bit like trying to add a second hand to a watch by drawing it on the face with a pen. It won't do anything helpful because you aren't working with the important part of the theory. Taylor and Wheeler's Spacetime Physics is a good place to start if you want to learn.

 

[timmdeeg]

Well, based on currently accepted physics. Fine I get it. But perhaps time dilation formulae needs to be revisited as we understand what is happening at cosmological distances / unobservable.

This reasoning is irrelevant because c is measured locally. For the same reason the result of the measurement doesn't depend on the size of the universe.

 

[Buzz Bloom]

This is wrong. c is the speed limit whether the universe is (spatially) finite or infinite.

Hi Peter:

I think the above quote might be a bit confusing to @Eirhead. Consider two objects, each stationary relatative to a different co-moving coordinate. If the distance between them is great enough, the expansion of the univefrse will cause the velocity of one relative to the other to be greater than c. I understand that the concept of "speed limit" does not apply to this motion, but I think this example will help Eirhead understand the topic more clearly.

Regards,
Buzz

 

[PAllen]

Hi Peter:

I think the above quote might be a bit confusing to @Eirhead. Consider two objects, each stationary relatative to a different co-moving coordinate. If the distance between them is great enough, the expansion of the univefrse will cause the velocity of one relative to the other to be greater than c. I understand that the concept of "speed limit" does not apply to this motion, but I think this example will help Eirhead understand the topic more clearly.

Regards,
Buzz

No, that is actually a false statement. True is:

1) What you refer to is recession rate which is wholly different than relative velocity. In particular, if you set up a cosmological style of coordinates in pure SR, you can have a recession rate between comoving timelike world lines that is arbitrarily large despite absence of curvature. However in flat spacetime, there will be a separate, well defined, relative velocity less than c.

2) Actual relative velocity between distant objects is inherently ambiguous in GR, because of the path dependence of parallel transport (you compare vectors by parallel transport of one to the other). However, despite the ambiguity, one can argue that the ambiguous relative velocity in GR is always less than c, because this is true no matter which transport path you pick.

 

[PeterDonis]

based on currently accepted physics

That's what we discuss here.

perhaps time dilation formulae needs to be revisited as we understand what is happening at cosmological distances / unobservable

Personal speculations are out of bounds here.

 

[PeterDonis]

Consider two objects, each stationary relatative to a different co-moving coordinate. If the distance between them is great enough, the expansion of the univefrse will cause the velocity of one relative to the other to be greater than c.

No. it won't, because there is no well-defined concept of "the velocity of one relative to the other" (@PAllen explained why). Relative velocity in a curved spacetime is only well-defined locally.

 

[Eirhead]

Personal speculations are out of bounds here.

I mean, I tend to wonder a lot more about gravity from a quantum perspective lately, and have been consumed by black hole physics and observable universe physics. It's all stuff that is kind of beyond the framework of relativity. And there are things happening there that still require further definition. I'm not so much proposing answers as I am proposing conceptual questions inherent to event horizons and an infinite universe.

 

[PeterDonis]

I tend to wonder a lot more about gravity from a quantum perspective lately, and have been consumed by black hole physics and observable universe physics. It's all stuff that is kind of beyond the framework of relativity. And there are things happening there that still require further definition. I'm not so much proposing answers as I am proposing conceptual questions inherent to event horizons and an infinite universe.

All of this is off topic here as personal speculation unless you can give references. Can you?

 

[Eirhead]

All of this is off topic here as personal speculation unless you can give references. Can you?

It's largely a theoretical discussion, but I can hardly claim these are my own personal speculations. Boundaries of AdS/CFT correspondence should adjust based on our relative position and motion in an infinite universe. And matter outside those boundaries need not obey a speed limit.

 

[PeterDonis]

I can hardly claim these are my own personal speculations.

You're not the one who is saying they are. i am. I am saying that because, despite repeated requests, you have provided no references to back up your claims. Either do so, or you will receive a warning and a thread ban.

 

[Eirhead]

You're not the one who is saying they are. i am. I am saying that because, despite repeated requests, you have provided no references to back up your claims. Either do so, or you will receive a warning and a thread ban.

Whatever, I'm over it. Go look at the wikipedia entry for "observable universe" and see what it means to be outside the observable universe. I thought this was pretty basic stuff.

 

[PeterDonis]

I thought this was pretty basic stuff.

The fact that there is an observable universe, that it is not the entire universe, and that coordinate velocities in a curved spacetime can be greater than $c$, are all pretty basic stuff, yes.

None of that implies the things that you were saying.

 

[Catastrophe]

Hi @Arman777

I like your presentation, but I feel it would be improved by mentioning the following point.

Here is a quote.

On the surface of the sphere, we could move in some direction and we may find ourselves to the point that we are started.​

​

In an actual finite universe, which would typically be expanding or contracting, one might have to travel faster than the speed of light in the expanding case in order for the mover to arrive at the same spatial point. It may also be useful to mention choosing the point of interest as fixed in co-moving coordinates.

Here is a suggestion.

On the surface of a sphere which is expanding in the same manner as our universe, if we started at a fixed point in co-moving coordinates and traveled at some faster than light speed in any direction along a great circle of the sphere, we could eventually find ourselves back at the point where we started.​

Regards,
Buzz

"On the surface of a sphere which is expanding in the same manner as our universe . . . . . . "

It seems to me that you are adopting a flatlander analogy where the flatlander is living on the surface of the sphere, and has no comprehension of up or down. Thus you are postulating (quite correctly) traveling (I am English) on great circles. As your sphere's surface expands (= expansion of flatlander universe) the flatlander is unaware of the increasing radius of the sphere, which increase, however, is observable to an observer of (n + 1) dimensions (where the flatlander is n dimensional - I am avoiding time for the moment).
This means, of course, that (n + 1) can travel along a chord between two points on the surface. It also means that (n + 1) can appreciate expansion of his universe (I am purposely using small 'u's here) in a dimension beyond the comprehension of n (n being the flatlander). To 'n' it might appear that (n + 1) had traveled faster than light, or even instantaneously.

I appreciate that this is only an analogy, and proves nothing, but it helps me to understand comprehension of dimensions from different dimensional points of view. It is capable of further development.

Catastrophe (aka Cat)

 

[elcaro]

There are classes of plausible theories in which time extends infinitely far into the past. One of my favorites is

https://en.wikipedia.org/wiki/Eternal_inflation

The eternal inflating universe model is future eternal, but not past eternal. Vilenkin and Guth proved that generically an inflating spacetime is geodesical incomplete to the past timeline, which means that any two particles in an inflating space must have been arbitrarily close together in the past. So, strictly speaking, eternal inflation is not eternal to the past.

However, since eternal inflation it is future eternal, the point in time in which eternal inflation must have started, can be an arbitrariry amount of time in the past, without bound. Likely inflation already started in earlier times, before our universe was born.

Further, there is no restriction for eternal inflation to have started only once, so theoretically, eternal inflation could have been going on before our time line or branch of inflation that created our universe, has started, but that would be a completely different space time, not causally connected to our spacetime, and about which nothing can be known.

 

[elcaro]

There really are three phases in cosmology as we understand it
1) t=0 the Big Bang Singularity where we don't know WHAT was going on
2) t = one Planck Time to something like t= 10E-32 seconds --- the Inflationary Period (hypothetical but likely)
3) t = the end of the Inflationary Period and onward --- the time of the Big Bang Theory

This kind of dividing cosmic history in three phases always make me wonder why - when assuming inflation - we still put the big bang singularity (purely hypothethical and only derived from GR without QM, which then only shows that GR is incomplete and one needs a full theory combining GR and QM to say something meaningfull about this) in front of this, while the inflationary period might have a totally different history (the history of the cosmos before inflation started) and is largely unknowable, as also is probably eternal into the past (there is no a priori reason to assume time itself or the universe as a whole had a finite history - with what could the universe have begun?).

Inflationary period: it depends on which model of inflation one talks here, but in many inflation models inflation is future eternal, and thus it can not simply be assumed that our universe happened to "condense out" from the inflationary space soon after inflation started, as it could as well have lasted an arbitrary amount of time after inflation started before our universe came to be.

 

[PeterDonis]

why - when assuming inflation - we still put the big bang singularity (purely hypothethical and only derived from GR without QM, which then only shows that GR is incomplete and one needs a full theory combining GR and QM to say something meaningfull about this) in front of this

Who is this "we" who does this? AFAIK this is not what is done in actual textbooks and peer-reviewed papers. Pop science presentations are another matter, but that's why we advise people here on PF not to try to learn science from pop science presentations.

 

[elcaro]

Who is this "we" who does this? AFAIK this is not what is done in actual textbooks and peer-reviewed papers. Pop science presentations are another matter, but that's why we advise people here on PF not to try to learn science from pop science presentations.

It does happen in scientific lectures too.

 

[PeterDonis]

It does happen in scientific lectures too.

Can you give an example?

 

[CultQuantum]

Whatever, I'm over it. Go look at the wikipedia entry for "observable universe" and see what it means to be outside the observable universe. I thought this was pretty basic stuff.

I baked some banana nut bread earlier today and watching it through the oven door I realized that all the matter and stuff of the universe is just like my dough ball and that the oven is the empty space...

Space and the vacuum isn't being created as the stuff of the universe is expanding. In fact space (the oven) has always been, we're just the banana nut loaf.

Why is it that every theoritician I hear speak on this issue always refer to space as something that is being created as we expand into it? Does the BB somehow clash with my Big Oven theory?

 

[phinds]

Space and the vacuum isn't being created as the stuff of the universe is expanding. In fact space (the oven) has always been, we're just the banana nut loaf.

This is not correct in the sense that the matter in the universe is NOT expanding into empty space the way the loaf bread is expanding into the space of the oven.

 

[Orodruin]

I baked some banana nut bread earlier today and watching it through the oven door I realized that all the matter and stuff of the universe is just like my dough ball and that the oven is the empty space...

Space and the vacuum isn't being created as the stuff of the universe is expanding. In fact space (the oven) has always been, we're just the banana nut loaf.

Why is it that every theoritician I hear speak on this issue always refer to space as something that is being created as we expand into it? Does the BB somehow clash with my Big Oven theory?

We do not expand into space. Space itself is expanding.

 

[Orodruin]

Well how are we supposed to learn?

By studying what the current best fit models that are the result of many experts dedicating their professional life to a subject have arrived at rather than making up your own and arguing ”probably the thousands of people that worked on this never had this one particular thought of mine”.

Learning is not done by making up your own speculative theory. Until you know what the current models actually state and why that is a good description of nature, you are just going to speculate wildly in all kinds of directions.

 

[PeterDonis]

how are we supposed to learn?

Not by personal speculation. And certainly not by hijacking a thread that isn't yours.

What can I do but laugh at this response.

What can you do? You can earn yourself a thread ban, which you just did.

I strongly suggest that you take some time to learn what our current Big Bang model actually says. You appear to have some serious misunderstandings.

 

[Orodruin]

What can I do but laugh at this response.

First of all, your attitude here is not a very good one if you are trying to learn. You are alienating the very people who possesses the knowledge that you seek.

Current models, Is that required belief? Because correct me if I am wrong but nobody knows for certain that's why they're ahem "models"

You are wrong. They are the current models because they are what best fits the observational data. They are typically very good fits to the data with a small amount of anomalies that call for further investigation. Therefore, if you are to have any chance whatsoever to understand what is going on you need to learn why the current models are in place, the observations they describe at high accuracy. You are not required to believe anything. You are required to know and understand how the theory works and why. Note that you will only achieve this from years of studying the professional literature, not by reading popularized accounts.

Your personal speculation here is made even worse by being unmeasurable when you stipulate that maybe something exists that isn’t perceivable or measurable. That is an unscientific statement.

Learning indeed involves interaction and guidance. Not wild personal speculation.