Insights Intro to Big Bang and Infinity Concepts - Comments

[Arman777]

Greg Bernhardt submitted a new PF Insights post

Intro to Big Bang and Infinity Concepts

Continue reading the Original PF Insights Post.

 

[Buzz Bloom]

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

 

[Arman777]

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.

That's a really good point. I never thought that when I was trying to explain the concept...Thanks :)

 

[haushofer]

Hi,

you say the BB can be thought of as an event, but a singularity does not belong to spacetime, the union of all events (plus metric). So that can be confusing :)

 

[phinds]

Good job.

I would also add that I believe your statement that the universe is temporally finite only applies to the Big Bang model and we KNOW that that model is incomplete because it has a singularity in the math. Since we don't know what that singularity IS, we cannot say with confidence that there was no time before it. SO ... I would say "In the Big Bang model of cosmology the universe is temporally finite" rather than a categorical statement that it is.

 

[Arman777]

Hi,

you say the BB can be thought of as an event, but a singularity does not belong to spacetime, the union of all events (plus metric). So that can be confusing :)

I thought that, the Big Bang is just not an initial singularity but also "an event" (it's really hard to explain it without using the word event) that universe emerged. Like these are bounded and cannot be separated. But yes I understand your point. Is big bang just a name for the initial singularity? But not the part of the "emerging universe"?

 

[Orodruin]

I thought that, the Big Bang is just not an initial singularity but also "an event" (it's really hard to explain it without using the word event) that universe emerged. Like these are bounded and cannot be separated. But yes I understand your point. Is big bang just a name for the initial singularity? But not the part of the "emerging universe"?

The word ”event” has a very precise meaning in relativity. It is a point in space-time.

What is usually referred to as the standard Big Bang is actually not a priori related to the singularity. It is the expansion of the Universe from a hot dense homogeneous state. Essentially the physics we know occurred. The singularity likely only occurs if you extrapolate this to earlier times using nothing but GR.

 

[Arman777]

Good job.

I would also add that I believe your statement that the universe is temporally finite only applies to the Big Bang model and we KNOW that that model is incomplete because it has a singularity in the math. Since we don't know what that singularity IS, we cannot say with confidence that there was no time before it. SO ... I would say "In the Big Bang model of cosmology the universe is temporally finite" rather than a categorical statement that it is.

Hmm, that's a good point. But still the Big Bang model is a strong model and even we don't understand the singularity can't we say the universe is finite in time?
In other cases where the universe is infinite in time, the universe got created and destroyed repeatedly? (I remember some universe models like that). It also doesn't seem to fit our universe since it expands?

Or am I missing something ?

 

[phinds]

Hmm, that's a good point. But still the Big Bang model is a strong model and even we don't understand the singularity can't we say the universe is finite in time?

No, not categorically. We don't know.

 

[nikkkom]

Hmm, that's a good point. But still the Big Bang model is a strong model and even we don't understand the singularity can't we say the universe is finite in time?

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

 

[Arman777]

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

That's really nice, I didn't know that.

 

[haushofer]

The word ”event” has a very precise meaning in relativity. It is a point in space-time.

What is usually referred to as the standard Big Bang is actually not a priori related to the singularity. It is the expansion of the Universe from a hot dense homogeneous state. Essentially the physics we know occurred. The singularity likely only occurs if you extrapolate this to earlier times using nothing but GR.

Yes, I guess it depends on what people call "THE big bang". This is also confusing from literature which, e.g., places the BB after inflation. I was referring to the initial singularity, of course.

 

[phinds]

Yes, I guess it depends on what people call "THE big bang". This is also confusing from literature which, e.g., places the BB after inflation. I was referring to the initial singularity, of course.

I always find it less confusing to spell that out specifically as "The Big Bang Singularity" so that there's no confusion with the BB Theory

 

[Arman777]

Yes, I guess it depends on what people call "THE big bang". This is also confusing from literature which, e.g., places the BB after inflation. I was referring to the initial singularity, of course.

Well yes I see. In the first lines I was referring to general idea but just not as a singularity. I ll make the proper changes

 

[phinds]

Well yes I see. In thise first lines I was referring to general idea but just not as a singularity. I ll make the proper changes

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

 

[Arman777]

I made the proper changes. Hope its better now.

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

2 and 3 are not in the Big Bang Theory?

 

[phinds]

I made the proper changes. Hope its better now.
2 and 3 are not in the Big Bang Theory?

2 is not, 3 IS the Big Bang Theory. If you meant are 1 and 2 not in the BB Theory, then that is correct.

 

[Arman777]

2 is not, 3 IS the Big Bang Theory. If you meant are 1 and 2 not in the BB Theory, then that is correct.

oh wait I should have just said 2. But why exactly it's not considered as in the Big Bang theory ?

 

[phinds]

oh wait I should have just said 2. But why exactly it's not considered as in the Big Bang theory ?

The BB Theory is defined as starting AFTER the Inflationary Period. I don't make the definitions, I just tell'm like they are.

 

[Arman777]

The BB Theory is defined as starting AFTER the Inflationary Period. I don't make the definitions, I just tell'm like they are.

It's understandable but kind of awkward, well thanks.

 

[AlexCaledin]

Why the BB is always considered as an event that has actually happened ?? To me, it comes as merely the temporal limit of the theoretical spacetime in the "past" direction.

 

[rootone]

initial singularity

That is only stating that before physics we know of there must have been physics we don't know.
It is not supposing a physical object which is a primary cause of everything.

 

[bapowell]

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

Actually, eternal inflation does not extend infinitely far into the past, but only the infinite future. See work by Borde, Guth, and Vilenkin: https://arxiv.org/abs/gr-qc/0110012

 

[timmdeeg]

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

I found this list quite helpful:

http://www.earlyearthcentral.com/early_universe_page.html

The Planck Era (Big Bang To 10^-43 Seconds)
The GUT Era (10^-43 To 10^-38 Seconds)
Electroweak Era (10^-38 To 10^-10 Seconds)
Inflation (10^-38 To 10^-35 Seconds)
Reheating (10^-35 to About 10^-10 Of A Second)
The Particle Era (10^-10 To 10^-3 Seconds)
Big Bang Nucleosynthesis (10^-3 Seconds to 3 Minutes)
The Nuclei Era (20 Minutes to 380 Thousand Years)

 

[Arman777]

Why the BB is always considered as an event that has actually happened ?? To me, it comes as merely the temporal limit of the theoretical spacetime in the "past" direction.

You can look #7

 

[dkamarinchev]

Great article - quite well explained. However the idea of a finite universe that envelopes everything and due to this there is no such thing as "outside" of the it bothers me somewhat. I can't quite imagine how this would work. If we take thermodynamics and imagine a perfectly insulated system that can neither gain or give both heat and work then its state will stay forever unchanged. There could be no changes in volume or even temperature. Well actually temperature changes would be possible if a combustion reaction is initiated inside, but work exchange will not be possible even under these conditions due to the rigidity of the system boundaries. So ultimately if the system is to undergo changes it is supposed to be able to exchange heat and work with its surroundings, or at least work? However if we assume the infinite universe without an "outside" then doesn't that break the thermodynamic laws? The universe represents a system if I am not mistaken and if it is expanding then how can it do so without interaction with the surroundings? Doesn't that violate basic thermodynamic principles? Or am I perhaps missing something critical? I am no physicist and I might be looking in the wrong direction, and that's why I couldn't help myself but ask.

 

[phinds]

he universe represents a system if I am not mistaken and if it is expanding then how can it do so without interaction with the surroundings? Doesn't that violate basic thermodynamic principles?

There ARE NO "surroundings".

If you don't like the fact that the universe encompasses everything there is, then, to quote Feynman, Go somewhere else, to another universe where the rules are simpler, philosophically more pleasing, more psychologically easy.

 

[PeterDonis]

if we assume the infinite universe without an "outside" then doesn't that break the thermodynamic laws?

No.

Or am I perhaps missing something critical?

Yes.

What you are missing is that the thermodynamic laws are more general than the particular cases you are used to. You are used to seeing them applied to cases like a gas in a pressure vessel, where there is a clear boundary between "system" and "everything else". But that does not mean the laws are limited to those particular cases. They work for any case where you can define some kind of state space for the system and some kind of coarse graining of the state space according to thermodynamic variables. That can be done for models of the entire universe like the ones used in cosmology. There are some subtleties because of gravity/curved spacetime, but they are not insurmountable.

Wikipedia actually has a decent article on this:

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

 

[dkamarinchev]

No.
Yes.

What you are missing is that the thermodynamic laws are more general than the particular cases you are used to. You are used to seeing them applied to cases like a gas in a pressure vessel, where there is a clear boundary between "system" and "everything else". But that does not mean the laws are limited to those particular cases. They work for any case where you can define some kind of state space for the system and some kind of coarse graining of the state space according to thermodynamic variables. That can be done for models of the entire universe like the ones used in cosmology. There are some subtleties because of gravity/curved spacetime, but they are not insurmountable.

Wikipedia actually has a decent article on this:

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

OK, thanks for clarifying and even bigger thank for the link! I'll definitely check it out. Just to point, I was not challenging any theory that currently exists - I was merely trying to figure out what's going on and why. Once again thanks for the info and for pointing in the right direction.

 

[kurros]

It's understandable but kind of awkward, well thanks.

It's a logical definition if you look back historically and see that the Big Bang model arose long before the idea of inflation.