The Duration of the Big Bang: Exploding Myths and Misconceptions

[Brunolem33]

Do we actually know if the Big Bang was an instantaneous event, that is an event not measurable in time?
Or, would it be possible that the Big Bang had a duration, a length in time?
In other words, was the Big Bang more like, say, an explosion or more like a volcanic eruption, and how do we know that?

 

[phinds]

Do we actually know if the Big Bang was an instantaneous event, that is an event not measurable in time?
Or, would it be possible that the Big Bang had a duration, a length in time?
In other words, was the Big Bang more like, say, an explosion or more like a volcanic eruption, and how do we know that?

There really are two meaning to the phrase "big bang". The first, the one you mean, is more properly referred to as the "big bang singularity" and the second is "the cosmology theory that describes the evolution of the universe after, and not including, the singularity".

The word singularity in this context does not at all mean "point", it means "the place where our model gives nonphysical results and we don't know WHAT was happening". The big bang singularity happened everywhere at once and it's size and duration are unknown. It might have been infinite or finite. Mathematically it is taken as an instant in time but that's just the math. We have no idea, really.

EDIT: Oh, and it was NOT an "explosion". Those happen at a point. The BB singularity did not.

 

[Brunolem33]

There really are two meaning to the phrase "big bang". The first, the one you mean, is more properly referred to as the "big bang singularity" and the second is "the cosmology theory that describes the evolution of the universe after, and not including, the singularity".

The word singularity in this context does not at all mean "point", it means "the place where our model gives nonphysical results and we don't know WHAT was happening". The big bang singularity happened everywhere at once and it's size and duration are unknown. It might have been infinite or finite. Mathematically it is taken as an instant in time but that's just the math. We have no idea, really.

EDIT: Oh, and it was NOT an "explosion". Those happen at a point. The BB singularity did not.

There really are two meaning to the phrase "big bang". The first, the one you mean, is more properly referred to as the "big bang singularity" and the second is "the cosmology theory that describes the evolution of the universe after, and not including, the singularity".

The word singularity in this context does not at all mean "point", it means "the place where our model gives nonphysical results and we don't know WHAT was happening". The big bang singularity happened everywhere at once and it's size and duration are unknown. It might have been infinite or finite. Mathematically it is taken as an instant in time but that's just the math. We have no idea, really.

EDIT: Oh, and it was NOT an "explosion". Those happen at a point. The BB singularity did not.

Thanks for your input, but allow me to go further.
The Big Bang was a release of energy, lots of it.
What I mean by "instantaneous event" is that all the energy available in our universe today was released at once.
On the other hand, if the Big Bang was, or is, a "continuing event", the energy could have been released incrementaly.
There comes my analogy with a volcanic eruption.
First, under enormous pressure, a huge amount of energy is released, together with a toxic cloud and so on.
Then, things get quieter and the lava keeps flowing along the sides of the volcano...the volcano is still erupting, but it not quite the same thing as the initial explosion.
Back to the Big Bang, there could have been an initial massive release of energy, but then things didn't stop, they only got quieter...and invisible, yet detectable.
Under the initial conditions, matter was created and together with it the observable universe.
Then, after the initial burst, much more energy was released, but it didn't meet the required conditions to be turned into matter.
Instead, part of this energy became what we call dark matter and the rest remained as what we call dark energy.

 

[PeterDonis]

The Big Bang was a release of energy, lots of it.

This is true if by "Big Bang" you mean what happened at the end of the inflation era, when the matter and energy in our current universe was first formed in a very hot, very dense, rapidly expanding state. But this event is not an "initial singularity" despite what many pop science presentations say. See below.

Under the initial conditions, matter was created and together with it the observable universe.

This is not quite correct. The matter and energy that appeared at the end of inflation wasn't created out of nothing; it came from the energy contained in the field that caused inflation. Whether we say our observable universe was "created" at the end of inflation depends on what form of inflation theory we use.

The end of inflation also did not happen everywhere in the universe at the same instant; it happened over a period of time (a very short one by our current standards, but still a period of time).

Then, after the initial burst, much more energy was released, but it didn't meet the required conditions to be turned into matter.
Instead, part of this energy became what we call dark matter and the rest remained as what we call dark energy.

I'm not aware of any cosmological theory that says this. All the theories I'm aware of agree that ordinary matter and radiation, dark matter, and dark energy have all existed since the end of inflation.

 

[Brunolem33]

This is true if by "Big Bang" you mean what happened at the end of the inflation era, when the matter and energy in our current universe was first formed in a very hot, very dense, rapidly expanding state. But this event is not an "initial singularity" despite what many pop science presentations say. See below.
This is not quite correct. The matter and energy that appeared at the end of inflation wasn't created out of nothing; it came from the energy contained in the field that caused inflation. Whether we say our observable universe was "created" at the end of inflation depends on what form of inflation theory we use.

The end of inflation also did not happen everywhere in the universe at the same instant; it happened over a period of time (a very short one by our current standards, but still a period of time).
I'm not aware of any cosmological theory that says this. All the theories I'm aware of agree that ordinary matter and radiation, dark matter, and dark energy have all existed since the end of inflation.

Thanks for your extensive reply and explanations.

Since you mention time...very short periods of it...I must say this is also something that makes me wonder.
My question is: did time really exist from the very beginning, or said differently did the clock start ticking with the Big Bang?
It is not because time, or at least a measurement of it, is required for our mathematical models that it existed in the absolute.
Time appears to be rather elastic, stretching from nothing if you are a photon to the passage of time as we experiment it.
Yet, it seems that time is somehow related to mass, since there is no time for massless objects traveling at light speed.
At the very beginning of the universe, there was no mass, so how was there time...at least from the point of view of the observed, the early universe, not from our human point of view?

 

[Brunolem33]

Thanks for your input, but allow me to go further.
The Big Bang was a release of energy, lots of it.
What I mean by "instantaneous event" is that all the energy available in our universe today was released at once.
On the other hand, if the Big Bang was, or is, a "continuing event", the energy could have been released incrementaly.
There comes my analogy with a volcanic eruption.
First, under enormous pressure, a huge amount of energy is released, together with a toxic cloud and so on.
Then, things get quieter and the lava keeps flowing along the sides of the volcano...the volcano is still erupting, but it not quite the same thing as the initial explosion.
Back to the Big Bang, there could have been an initial massive release of energy, but then things didn't stop, they only got quieter...and invisible, yet detectable.
Under the initial conditions, matter was created and together with it the observable universe.
Then, after the initial burst, much more energy was released, but it didn't meet the required conditions to be turned into matter.
Instead, part of this energy became what we call dark matter and the rest remained as what we call dark energy.

This is true if by "Big Bang" you mean what happened at the end of the inflation era, when the matter and energy in our current universe was first formed in a very hot, very dense, rapidly expanding state. But this event is not an "initial singularity" despite what many pop science presentations say. See below.
This is not quite correct. The matter and energy that appeared at the end of inflation wasn't created out of nothing; it came from the energy contained in the field that caused inflation. Whether we say our observable universe was "created" at the end of inflation depends on what form of inflation theory we use.

The end of inflation also did not happen everywhere in the universe at the same instant; it happened over a period of time (a very short one by our current standards, but still a period of time).
I'm not aware of any cosmological theory that says this. All the theories I'm aware of agree that ordinary matter and radiation, dark matter, and dark energy have all existed since the end of inflation.

If the universe is infinite, then it contains an infinite amount of energy.
But how could a Big Bang happening in a finite and extremely short amount of time release an infinite amount of energy?
In other words, for the universe to be infinite, isn't it necessary that the Big Bang is also infinite in time?
Going further, if at some point, during its early expansion phase, the universe had the size, say, of a golf ball, that means it was finite in size.
But then, how could something finite in size evolve into something infinite in size?

 

[Bandersnatch]

If the universe is infinite, then it contains an infinite amount of energy.
But how could a Big Bang happening in a finite and extremely short amount of time release an infinite amount of energy?
In other words, for the universe to be infinite, isn't it necessary that the Big Bang is also infinite in time?

All you need, is to consider the universe at BB, whatever it was, to be infinite as well, so that in every finite unit volume there is always finite energy content.

Going further, if at some point, during its early expansion phase, the universe had the size, say, of a golf ball, that means it was finite in size.
But then, how could something finite in size evolve into something infinite in size?

The 'golf ball', or any other size approximation for the early universe you might see or hear about describes only the observable universe at some specified time in its evolution. It should never be taken to mean the entirety of the universe, as its extent is unknown. Both infinite and finite (but very large) extent is possible, and whichever of the two is true of the universe now, was likely true of the universe always.

 

[Brunolem33]

All you need, is to consider the universe at BB, whatever it was, to be infinite as well, so that in every finite unit volume there is always finite energy content.

The 'golf ball', or any other size approximation for the early universe you might see or hear about describes only the observable universe at some specified time in its evolution. It should never be taken to mean the entirety of the universe, as its extent is unknown. Both infinite and finite (but very large) extent is possible, and whichever of the two is true of the universe now, was likely true of the universe always.

Thanks for your explanations.
Regarding the Big Bang, isn't there some kind of contradiction in saying that the BB happened in an already infinite universe?
I agree that in order to be infinite, the universe had to be infinite from the get go, but the BB is at the origin of the universe, thus it predates the universe and cannot happen inside it.
Or am I wrong, or not understanding what you mean?
Regarding the golf ball, things get even more complicated, at least for me.
What you say would mean that there is an observable and finite universe expanding inside an infinite universe.
Things could be explained differently if time, the fourth dimension, didn't exist from the very beginning of the universe, but that the clock only started ticking when matter and mass appeared.
What would be the meaning of time before that, other than for mathematical purpose that is?
So, if time was not turned on in the early stages of the universe, because everything was moving at the speed of light (no mass), the universe was able to expand from a point to infinity...instantaneously, even if from our human point of view, we measure things differently.

 

[PeterDonis]

did time really exist from the very beginning, or said differently did the clock start ticking with the Big Bang?

There is no way to answer this question. "Time" is not an absolute thing, and there is no one "clock" that applies to the entire universe.

 

[Johninch]

Regarding the Big Bang, isn't there some kind of contradiction in saying that the BB happened in an already infinite universe?
I agree that in order to be infinite, the universe had to be infinite from the get go, but the BB is at the origin of the universe, thus it predates the universe and cannot happen inside it.
Or am I wrong, or not understanding what you mean?
Regarding the golf ball, things get even more complicated, at least for me.
What you say would mean that there is an observable and finite universe expanding inside an infinite universe.

I am following the discussion with interest.

Could someone please clarify what we mean by “infinite” in this context? Is it not so, that the universe is everything, in its broadest possible meaning, disregarding multiverses? In this case, the universe must be infinite and was always infinite. The problem seems to be that we can’t imagine infinity, so we are always imagining a finite sphere or other shape, which is wrong. Otherwise I am not understanding what “universe” means.

Could someone clarify the terms please?

 

[Bandersnatch]

Regarding the Big Bang, isn't there some kind of contradiction in saying that the BB happened in an already infinite universe?
I agree that in order to be infinite, the universe had to be infinite from the get go, but the BB is at the origin of the universe, thus it predates the universe and cannot happen inside it.

You're probably thinking of the BB as a point in pre-existing space, right? A place from which all matter emerged to fill the universe? Is that the correct representation of your perception of BB?

 

[Bandersnatch]

I am following the discussion with interest.

Could someone please clarify what we mean by “infinite” in this context? Is it not so, that the universe is everything, in its broadest possible meaning, disregarding multiverses? In this case, the universe must be infinite and was always infinite. The problem seems to be that we can’t imagine infinity, so we are always imagining a finite sphere or other shape, which is wrong. Otherwise I am not understanding what “universe” means.

Could someone clarify the terms please?

No, it doesn't have to be infinite.
We're talking about its spatial extent. If it is infinite, it means that the metric space of the universe is unbounded. That is, you can go in at least one direction infinitely, and never get back to the starting point*. Or, equivalently, there is no volume large enough to contain all of the space in the universe.
The space of the universe could be of a closed shape (e.g. 4-dimensional torus or sphere), making it finite (i.e., bounded, but without a boundary). That is, in such a universe you could draw a sphere of volume large enough to contain all of space, or move in any direction and eventually get back to your starting point.

So, to put in yet another way, the everything in the broadest possible sense might turn out to be finite.

*in an expanding universe this is true also of a bounded space, due to the limitations of the travel speed (c). What I mean here, is a 'magical' travel with arbitrary speed, or travel in a 'frozen' universe where you don't have to worry about cosmological horizons.

 

[Johninch]

No, it doesn't have to be infinite.
We're talking about its spatial extent. If it is infinite, it means that the metric space of the universe is unbounded. That is, you can go in at least one direction infinitely, and never get back to the starting point*. Or, equivalently, there is no volume large enough to contain all of the space in the universe.
The space of the universe could be of a closed shape (e.g. 4-dimensional torus or sphere), making it finite (i.e., bounded, but without a boundary). That is, in such a universe you could draw a sphere of volume large enough to contain all of space, or move in any direction and eventually get back to your starting point.

So, to put in yet another way, the everything in the broadest possible sense might turn out to be finite.

The idea of getting back to the starting point seems to be without foundation. It is not possible because the starting point would cease to exist and would not be discoverable. I am saying that I can go in one direction infinitely because I will never find a boundary. If the universe has no boundary, then it goes on forever, and that is why I wrote that it is infinite. Similarly, I think that if I did this shortly after the BB, the result would be the same.


I don’t understand what you mean by a “closed” shape, nor what you mean by “draw a sphere of volume large enough to contain all of space”. This is impossible. Calling on geometrical shapes does not solve the problem. It even makes it worse. How can you have a universe with a “shape” when the universe is everything?


I don’t mean to hijack the thread, I am only wanting to clarify the size of the BB issue.

 

[Bandersnatch]

I don’t understand what you mean by a “closed” shape, nor what you mean by “draw a sphere of volume large enough to contain all of space”. This is impossible. Calling on geometrical shapes does not solve the problem. It even makes it worse. How can you have a universe with a “shape” when the universe is everything?

Let's take it one dimension down, and imagine a 2D universe. This is to make the shape easy to imagine for us 3-dimensional beings.

If the shape of the 2D universe is that of a flat plane, it is infinite. It has no boundaries, since you can't point to a place that is not on the 2D plane (remember, the 3rd dimension doesn't exist as part of the universe in this analogy). It is not bounded, since no matter how large a circle you draw on that plane, there will always be points not encompassed by this circle.

If the shape of this 2D universe was that of a sphere (again, we're talking only about the 2D part of the sphere, i.e. its surface), then the universe would have no boundaries, but be bounded. That is, by the definition of a bounded set, you can draw a circle on the surface of a sphere of radius large enough to encompass all the points on the surface of this sphere. In this case, the radius needs to be equal to half the circumference of the sphere.

Going back to where you started on a surface of a sphere should be also easy to imagine.

An important point to note here, is that while the third dimension in which we embedded the sphere in this analogy, to help with visualisation, is not necessary for the 2D surface to have a well-defined shape. Even without embedding, if your 2D surface has certain properties, like the sum of angles in all triangles adding up to more than 180 degrees, and all parallel lines eventually intersecting, this 2D surface will have the shape of a sphere.

This works in exactly the same way for any number of higher dimensions.

 

[Johninch]

Let's take it one dimension down, and imagine a 2D universe.

Thank you for the analogy, which has been quoted before. Some students have remarked that the 2D space analogy does not help them, and I also belong to this group.

I prefer to discuss the 4D universe which we seem to live in. The OP asked about the timing of the BB and inflation and he later added a question about the spatial aspect.

I see it like this, based on my limited knowledge. I am grateful for your expert comments.

BB theory and its timing depends heavily on our observation of the increasing expansion of the visible universe. We extrapolate backwards and arrive at the BB about 13.8 bn years ago. Possible criticisms of the calculation include that the sample of galaxies is a long way off 100% and that we don’t have much data of previous rates of expansion. I am aware of research that indicates a slower rate of expansion in the past. Nevertheless the calculation of 13.8 bn years is widely accepted.

Decoupling occurred at about 380 million years after the BB. During this period matter was created, photons were confined and then released as CMBR.

So the first question which comes up is, how do we know that the formation of the universe until decoupling took 380 million years and if it did, what does this mean? PeterDonis already remarked, “"Time" is not an absolute thing, and there is no one "clock" that applies to the entire universe.” So is it not also true that there is no one “clock* that applies to the universe at all times in its history? If there is not, can we conclude from this that we have little idea about the timing of the early formation of the universe in terms of today’s earthly clock. Does the 380 million years really mean anything in today’s terms?

The second question relates to the spatial dimensions of the universe, which I already commented in part. My understanding is that during the inflationary period, the universe was a hot, dense, crowded-with-matter-and-energy place. Further, that the inflation happened everywhere, meaning in the whole universe. Is it not the case, that the universe was everything then and is still everything now? So everywhere you travel and for however long you travel, you will remain in the universe, because that is all there is (ignoring multiverses). It doesn’t matter what imagined shape or size the universe has. Since the universe is expanding faster than the speed of light, you can travel infinitely. If you somehow run out of matter to overtake, you will expand the universe yourself.

Putting it all together, I don’t see how we can say that the universe could be finite. Furthermore, if we were able to imagine a spatially finite universe now, then we would have to imagine a spatially finite early universe in its formation too. I think that this goes against current theory, or at least against current opinion. It comes back to your question to the OP, “You're probably thinking of the BB as a point in pre-existing space, right?”

 

[Brunolem33]

You're probably thinking of the BB as a point in pre-existing space, right? A place from which all matter emerged to fill the universe? Is that the correct representation of your perception of BB?

Not at all.
Space was created from the BB, so the BB could not take place in pre-existing space.
Yet the BB could either start from a point, or happen everywhere, not in, but with the infinite universe.
If time, the fourth dimension, did not exist at the BB and until there was matter and mass, then the universe could start from a point and expand, or inflate, infinitely, in no time!
Thus, it was both point sized and infinite, at the same time...so to speak.

 

[Brunolem33]

There is no way to answer this question. "Time" is not an absolute thing, and there is no one "clock" that applies to the entire universe.

I agree with you, but yet science assumes, mostly for mathematical purposes, that time existed from the get go, and maybe such assumption leads to wrong conclusions.
According to the models, at the very beginning, inflation seems to happen in a fraction of a fraction of a second, which is mindboggling.
But maybe it is not, because if the clock is not ticking, inflation has all the time in the world, so to speak, in order to take place.
With our mostly mathematical tools, we try to observe something that happened at a time, so to speak, when there were no observers, and the conditions were not met to harbor observers anyway.
I am not convinced that this is possoble..

 

[Brunolem33]

Thank you for the analogy, which has been quoted before. Some students have remarked that the 2D space analogy does not help them, and I also belong to this group.

I prefer to discuss the 4D universe which we seem to live in. The OP asked about the timing of the BB and inflation and he later added a question about the spatial aspect.

I see it like this, based on my limited knowledge. I am grateful for your expert comments.

BB theory and its timing depends heavily on our observation of the increasing expansion of the visible universe. We extrapolate backwards and arrive at the BB about 13.8 bn years ago. Possible criticisms of the calculation include that the sample of galaxies is a long way off 100% and that we don’t have much data of previous rates of expansion. I am aware of research that indicates a slower rate of expansion in the past. Nevertheless the calculation of 13.8 bn years is widely accepted.

Decoupling occurred at about 380 million years after the BB. During this period matter was created, photons were confined and then released as CMBR.

So the first question which comes up is, how do we know that the formation of the universe until decoupling took 380 million years and if it did, what does this mean? PeterDonis already remarked, “"Time" is not an absolute thing, and there is no one "clock" that applies to the entire universe.” So is it not also true that there is no one “clock* that applies to the universe at all times in its history? If there is not, can we conclude from this that we have little idea about the timing of the early formation of the universe in terms of today’s earthly clock. Does the 380 million years really mean anything in today’s terms?

The second question relates to the spatial dimensions of the universe, which I already commented in part. My understanding is that during the inflationary period, the universe was a hot, dense, crowded-with-matter-and-energy place. Further, that the inflation happened everywhere, meaning in the whole universe. Is it not the case, that the universe was everything then and is still everything now? So everywhere you travel and for however long you travel, you will remain in the universe, because that is all there is (ignoring multiverses). It doesn’t matter what imagined shape or size the universe has. Since the universe is expanding faster than the speed of light, you can travel infinitely. If you somehow run out of matter to overtake, you will expand the universe yourself.

Putting it all together, I don’t see how we can say that the universe could be finite. Furthermore, if we were able to imagine a spatially finite universe now, then we would have to imagine a spatially finite early universe in its formation too. I think that this goes against current theory, or at least against current opinion. It comes back to your question to the OP, “You're probably thinking of the BB as a point in pre-existing space, right?”

You are not hijacking the thread and I agree with you regarding the measure of time beyond a certain point.
I just posted a couple of answers about this issue.
Yet, you make me raise another question which is: what is the meaning of an expanding, yet infinite universe?
How can something infinite expand?
Isn't that contradictory?

 

[phinds]

Thus, it was both point sized and infinite, at the same time...so to speak.

Uh ... I'm thinking you'd better find a different way to speak. That doesn't really make sense.

 

[phinds]

According to the models, at the very beginning, inflation seems to happen in a fraction of a fraction of a second, which is mindboggling.

Yes, it certainly is.

But maybe it is not, because if the clock is not ticking, inflation has all the time in the world, so to speak, in order to take place.

No, when we say it takes a fraction of a second we are using the current definition of a second. That's what it takes a fraction of.

With our mostly mathematical tools, we try to observe something that happened at a time, so to speak, when there were no observers, and the conditions were not met to harbor observers anyway.
I am not convinced that this is possoble..

The universe never cares what we think, it just does what it does.

 

[Brunolem33]

Yes, it certainly is.No, when we say it takes a fraction of a second we are using the current definition of a second. That's what it takes a fraction of.
The universe never cares what we think, it just does what it does.

I understand that we are using the current definition of a second, but what does it mean in a context where there is no time?
What would be the meaning of one of our seconds for a photon?

 

[Brunolem33]

Uh ... I'm thinking you'd better find a different way to speak. That doesn't really make sense.

It doesn't make sense from our actual point of view, in the same way that a particle being everywhere at the same time, when not observed, doesn't make sense.
But how do we know that our actual theories, which rely on the existence of matter, and time, are still valid in a context where there is no matter, and maybe no time, because it or they didn't exist yet?
These theories are our tools, developed to deal with our present conditions.
How do we know that they are adapted to deal with totally different conditions, such as those immediately following the BB and preceeding the creation of matter?

 

[PeterDonis]

These theories are our tools, developed to deal with our present conditions.
How do we know that they are adapted to deal with totally different conditions, such as those immediately following the BB and preceeding the creation of matter?

We don't. But if they're not, then we're out of luck, because we don't have anything else to go on.

 

[Brunolem33]

It doesn't make sense from our actual point of view, in the same way that a particle being everywhere at the same time, when not observed, doesn't make sense.
But how do we know that our actual theories, which rely on the existence of matter, and time, are still valid in a context where there is no matter, and maybe no time, because it or they didn't exist yet?
These theories are our tools, developed to deal with our present conditions.
How do we know that they are adapted to deal with totally different conditions, such as those immediately following the BB and preceeding the creation of matter?

After thinking more about it, I will try to explain things in a better way, as you suggest.
Imagine light emitted in all directions from a single point.
It will spread infinitely, in no time from its point of view, but only at 300,000 km per second, or so, from our point of view.
Now, replace this point by the BB and replace light by a mix of energy and space expanding, or inflating, at light speed, because there is no matter and mass to slow things down.
Without us, or anyone else, to observe, everything happens in no time.
Things go from an infinitely small point to an infinitely vast universe in no time.

 

[Brunolem33]

We don't. But if they're not, then we're out of luck, because we don't have anything else to go on.

Maybe not.
We have thought experiments, upon which new theories can be built.
I obviously don't know the details, but I understand that we can't reconcile quantum theory and relativity, or that these theories provide unsatisfying results when used in the conditions supposedly prevailing right after the BB.
Maybe it is because these theories are not adapted to such conditions and only work under our current conditions.

 

[Johninch]

No, when we say it takes a fraction of a second we are using the current definition of a second. That's what it takes a fraction of.

Yes, I agree. But is our current definition of a second applicable at the BB event? I think not.

Here we have to consider what time means. The best way of describing it which I have come across is that it relates to the passage of events. Without events there is no time. That is why time started with the BB, the first event.

In the first inflation period, which lasted a fraction of a second, there was an incredible number of events, such that this so-called fraction of a second is totally meaningless in our terms of today's time or passage of events. In today's terms, inflation took far, far longer than the quoted fraction of a second.

Do you agree?

 

[Johninch]

Yet, you make me raise another question which is: what is the meaning of an expanding, yet infinite universe?
How can something infinite expand? Isn't that contradictory?

This thread is getting like the BB. To answer your question, I would interpret the expansion of the universe as the dilation of space. So the universe is getting "bigger" in the sense of the increase of distances between galaxy super-clusters. (I choose super-clusters to be on the safe side, because I am not familiar with the current theory about the expansion of space locally.)

In a previous post I already expalined why the universe is "infinite". So there you have it: the universe is getting stretched out. It was always infinite. I don't see any contradiction.

 

[Johninch]

Decoupling occurred at about 380 million years after the BB.

Correction: Decoupling occurred at about 380 thousand years after the BB.

 

[Brunolem33]

According to Wiki: Time is a measure in which events can be ordered from the past through the present into the future, and also the measure of durations of events and the intervals between them.

I agree with Wiki. The existence of the time dimension does not depend on the existence of matter, it depends on occurrence of events.

I am not sure that there is A definition of time.
The Wiki definition is more of a philosophical than scientifical nature.
One would need to define what exactly constitute an event.
A photon living the sun to reach the Earth is an event for us observers, because the trip takes 8 minutes, so there is a past, present and future.
But it is not an event for the photon because it makes the trip in no time.
Now can we consider as events what was happening around the BB period, since there were no observers and there couldn't have been any, because the matter necessary to the existence of observers didn't exist.
In other words don't we somehow distort things by introducing an observer, and his mathematical tools, in an environment where there should be none?

 

[PeterDonis]

in no time from its point of view

No, this is not correct. There is no such thing as a "point of view" for light, and the concept of "elapsed time" does not apply to light. We have a FAQ on this:

https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

A photon living the sun to reach the Earth is an event for us observers

An "event" is a single point in spacetime. What you are talking about is a "worldline", composed of a continuous series of events forming a curve in spacetime; in this case, the curve runs from the event of the photon leaving the sun to the event of the photon arriving at Earth. Because this curve is the worldline of a photon, it has a spacetime "length" of zero (such a curve is called a "null" or "lightlike" curve); but it is still a curve, not a point. In a pseudo-Riemannian metric, such as spacetime has, curves consisting of distinct points can have zero length; that is a key difference from Riemannian metrics, which our intuitions are more familiar with.

it is not an event for the photon because it makes the trip in no time.

No. The curve is a curve, regardless of how we choose coordinates to describe it. Its length (zero) is invariant, but so is the fact that it is a curve composed of distinct points.

can we consider as events what was happening around the BB period, since there were no observers and there couldn't have been any

Events--points in spacetime--and worldlines--curves in spacetime--do not require observers to exist.

 

[PeterDonis]

what is the meaning of an expanding, yet infinite universe?

It means that the distance between objects in the infinite universe is increasing with time. You have to be very careful with infinities; they don't work the way many of our intuitions about finite things would indicate. An infinite universe (more precisely, a spatially infinite one) with objects moving apart with time is perfectly consistent.

 

[PeterDonis]

Does the 380 million years really mean anything in today’s terms?

Yes. When I said there isn't one clock that applies to the entire universe, I was referring to the fact that observers moving on different worldlines through spacetime can experience different elapsed times between events. I was not saying that the concept of "time" has no meaning at all. I was only saying that you need to specify which observers are measuring the time (note that the "observers" can be hypothetical and don't have to be conscious beings, hypothetical atomic clocks or light clocks work just fine).

In the case of the 380 thousand years (per your correction from 380 million in a later post), that means 380 thousand years as a hypothetical observer on a "comoving" worldline (meaning, an observer who always sees the universe as homogeneous and isotropic) would measure time. This is perfectly well-defined; but it applies only to that particular family of observers. Observers in different states of motion would measure different elapsed times.

 

[PeterDonis]

Could someone please clarify what we mean by “infinite” in this context?

It means "spatially infinite"--the spatial volume of the universe is infinite at any instant of time. Here "time" means "comoving time", i.e., the time experienced by "comoving" observers who always see the universe as homogeneous and isotropic.

 

[PeterDonis]

In the first inflation period, which lasted a fraction of a second, there was an incredible number of events

What do you mean by "number of events"? It is true that the universe was expanding much, much faster, but how does that translate into "more events"? Remember that, according to classical GR (we are leaving out speculations about quantum gravity in this discussion), spacetime is a continuum, so there are exactly as many points in a line segment one femtometer long as there are in all of spacetime (namely, $C$ of them, where $C$ denotes the cardinality of the continuum).

 

[PeterDonis]

I have reopened the thread and posted some clarifications on points that it seemed were causing confusion. Please keep the discussion on the topic of the original question (which I think has been answered by now, but there may be further follow-up questions).