[evan_au]

An event is a mathematical point which has no extent.  Sure, this is an idealization.

For many purposes an approximation is fine. eg the Earth is an extended object, thousands of km across, which you could approximate as the center of the Earth; knowing this location within 1km is fine (unless you are a GPS satellite).

Similarly, for a cat entering a supermassive black hole, measuring the position of a cat within 100m may be enough. If you want more accuracy, you could measure from the center of mass of the cat, or the center of the pupil of its left eye.

But is the cat dead or alive?

For extended objects like a cat, there is not only an uncertainty in space, there is an uncertainty in time as well.

Quantum theory says that there will always be some uncertainty in space and/or time...

[alancalverd]

But is the cat dead or alive?

To summarise quantum physics in one word, "yes".

[jeffreyH]

But is the cat dead or alive?

To summarise quantum physics in one word, "yes".

Now that answer I like.

[PmbPhy]

I understand that an "event" is used to mean a point on a spacetime coordinate system  or  it can be one element  of  the set of aĺl events (which doesn't help).

But if we take  an event as a physical oçcurence , what can we say about it?...

It is something which has a spatial point location at an instant of time. Events are pretty much fictional in practice on a large scale because real objects are not point objects and have spatial extent. They apply more aptly in particle physics. This does not mean that events don't occur and can't be described on the large scale though. They're approximations in that case like a circle is not exactly what the top of a tuna can is.

[pensador]

I understand that an "event" is used to mean a point on a spacetime coordinate system  or  it can be one element  of  the set of aĺl events (which doesn't help).

But if we take  an event as a physical oçcurence , what can we say about it?...

It is something which has a spatial point location at an instant of time. Events are pretty much fictional in practice on a large scale because real objects are not point objects and have spatial extent. They apply more aptly in particle physics. This does not mean that events don't occur and can't be described on the large scale though. They're approximations in that case like a circle is not exactly what the top of a tuna can is.

An event is definitely something that happens in a region of space and time, and can encompass multiple interactions, as a result of an initial trigger event. The inflationary stage preceding the Big Bang was an event, that covered a period of time whereby particles came into existence, before resulting in a hot big bang and the slowing of the inflationary stage, which is now accelerating as g >0.

I am not so sure a tin of tuna can be described as an event although I suppose they happen to come into existence at a point in space and time, which could be a little chaotic as some have circular lids and others rectangular. The contents may result in different flavours on a galactic scale

[PmbPhy]

An event is definitely something that happens in a region of space and time,

That's not how the term event is defined. An event is a point in space at an instant in time, i.e. = (t, x, y, z)  is an event.

The inflationary stage preceding the Big Bang was an event, ..

Wrong. Please read the definition of event at
https://en.wikipedia.org/wiki/Event_(relativity)

I am not so sure a tin of tuna can be described as an event ..

It can't. Its an analogy of how a geometrical object only approximates something in reality. That's why I said that the top of  a tuna can is not a circle even though it looks like one or is measured to be one on a macroscopic scale.

[Halc]

The inflationary stage preceding the Big Bang was an event, that covered a period of time whereby particles came into existence, before resulting in a hot big bang and the slowing of the inflationary stage, which is now accelerating as g >0.

I don't think any cosmologist would refer to the inflationary stage (which followed the big bang) an event.  It is an epoch, which implies a significant period of time.  It is no more an event than is the Holocene epoch which is currently ending.  And while the inflation epoch was certainly of shorter duration than the Holocene, it represented a far greater percentage of the entirety of the history of time while it was going on.

I am not so sure a tin of tuna can be described as an event

It was being compared to something that is approximately round, just like the sinking of the Titanic is treated approximately as a point event.

[pensador]

I don't think any cosmologist would refer to the inflationary stage (which followed the big bang) an event.  It is an epoch, which implies a significant period of time.  It is no more an event than is the Holocene epoch which is currently ending.  And while the inflation epoch was certainly of shorter duration than the Holocene, it represented a far greater percentage of the entirety of the history of time while it was going on.

This is of thread, but I may start a thread to discuss this.

According to a couple of speculative theories I have read, the inflationary stage came first and caused virtual particles to become separated and become real particles. A little like Hawking radiation. Further more these particles decayed to become stable particles, giving off lots of heat creating the hot big bang. The concept of Aeons conformal cyclic cosmology by Roger Penrose tends to side step singularities and Big Bangs, but does require inflation as g>0.

Do you have a reference for your comment above. The term Big Bang is term which seems to have evolved since Hoyle coined the phrase and means a lot to different people. In particular I am interested in reference to the time duration of the inflationary stage  ie what do you mean by Significant time

[pensador]

An event is definitely something that happens in a region of space and time,

That's not how the term event is defined. An event is a point in space at an instant in time, i.e. = (t, x, y, z)  is an event.

The inflationary stage preceding the Big Bang was an event, ..

Wrong. Please read the definition of event at
https://en.wikipedia.org/wiki/Event_(relativity)

Correct link https://en.wikipedia.org/wiki/Event_(relativity)

From your link  "an event is the instantaneous physical situation or occurrence associated with a point in spacetime"

From the oxford English dictionary an event in physics is
"
1.3Physics A single occurrence of a process, e.g. the ionization of one atom.

Example sentences
‘The aim of induction is to find series of events whose frequency of occurrence converges toward a limit.’
‘Consider our passage through time as riding the shockwave of the bang event.’
‘In the lower graph the current scale is amplified so that single channel events can be distinguished.’
"
The Big Bang according to the oxford English dictionary was an event

https://en.oxforddictionaries.com/definition/event

[PmbPhy]

An event is definitely something that happens in a region of space and time,

That's not how the term event is defined. An event is a point in space at an instant in time, i.e. = (t, x, y, z)  is an event.

The inflationary stage preceding the Big Bang was an event, ..

Wrong. Please read the definition of event at
https://en.wikipedia.org/wiki/Event_(relativity)

Correct link https://en.wikipedia.org/wiki/Event_(relativity)

From your link  "an event is the instantaneous physical situation or occurrence associated with a point in spacetime"

From the oxford English dictionary an event in physics is
"
1.3Physics A single occurrence of a process, e.g. the ionization of one atom.

Example sentences
‘The aim of induction is to find series of events whose frequency of occurrence converges toward a limit.’
‘Consider our passage through time as riding the shockwave of the bang event.’
‘In the lower graph the current scale is amplified so that single channel events can be distinguished.’
"
The Big Bang according to the oxford English dictionary was an event

https://en.oxforddictionaries.com/definition/event

Strict definitions in physics should never be taken from a dictionary. I wrote the glossary to a GR text before so I know how event is defined in SR/GR. I know SR very well and GR nearly as well. Look in any SR or GR text and you'll see that I'm write. Being a physicist myself the precise definition is second nature to me. Wikipedia is a lot more dependable than a dictionary when it comes to science.

[PmbPhy]

Do you have a reference for your comment above. The term Big Bang is term which seems to have evolved since Hoyle coined the phrase and means a lot to different people. In particular I am interested in reference to the time duration of the inflationary stage  ie what do you mean by Significant time

A knowledge of what an event and epoch is is proof enough. E.g., from Spacetime and Geometry by Sean Carroll  page 4

Spacetime is a four-dimensional set, with elements labeled by three dimensional of space and one of time.
...
A point in spacetime is called an event.

Notice the qualifier "point".

[Halc]

I don't think any cosmologist would refer to the inflationary stage (which followed the big bang) an event.
..
 And while the inflation epoch was certainly of shorter duration than the Holocene, it represented a far greater percentage of the entirety of the history of time while it was going on.

This is of thread, but I may start a thread to discuss this.
Under that model, it is quite debatable as to which moment to assign the term 'big bang'.

According to a couple of speculative theories I have read, the inflationary stage came first and caused virtual particles to become separated and become real particles. A little like Hawking radiation. Further more these particles decayed to become stable particles, giving off lots of heat creating the hot big bang. The concept of Aeons conformal cyclic cosmology by Roger Penrose tends to side step singularities and Big Bangs, but does require inflation as g>0.

Do you have a reference for your comment above.

It seems to be fairly common understanding.  A quick search picks this random page:
www.worldsciencefestival.com/2015/07/cosmological-inflation-primer/

For example, there are some puzzling features of our universe that can only be explained, some scientists argue, by the idea that our universe went through a short period (from somewhere in the vicinity of about 10⁻³³ to 10⁻³² seconds after the Big Bang) of rapid expansion (by what scientists believe to be around a factor of 10²⁶) after it emerged into existence. This idea is known as the theory of inflation, and is a widely accepted but still debated part of our universe’s beginnings.

My bold.  Picture depicts this.  Big bang is the singularity at left, the beginning of time as we know it.  After inflation, space has expanded by 10²⁶, which admittedly admits that the beginning wasn't a singularity or the size of the universe would be bounded.
It says it is debated, and there is eterial-inflation multiverse theory that says that inflation was outside the big bang, but bubbles of regular space-time condense out of that.  I think that might be what you reference.  Under that theory, inflation is still going on and did not terminate after this short time.
With eternal inflation, it becomes debatable which 'moment' or process to assign the term 'big bang'.

In particular I am interested in reference to the time duration of the inflationary stage  ie what do you mean by Significant time

Well, if you put time zero at the left point and have finite inflation epoch terminating after 10^-32 seconds, that short blip of time represents the entire history of the universe up to that point, as opposed to the Holocene which represents 2 millionths of Earth history.  So that's a long time on a percentage basis.

[pensador]

Strict definitions in physics should never be taken from a dictionary. I wrote the glossary to a GR text before so I know how event is defined in SR/GR. I know SR very well and GR nearly as well. Look in any SR or GR text and you'll see that I'm write(right . Being a physicist myself the precise definition is second nature to me. Wikipedia is a lot more dependable than a dictionary when it comes to science.

I do not doubt you. Being primarily English speaker and reading a lot of Pop Science as well as some good science, Am I wrong in thinking a Big Bang "event" happened 13.8 billion years ago.

I need to check this ! But I think it was Sean Carrol that published some interesting stuff recently, on Emergent Space time. Which does not have a hot big bang before the inflationary period. Further more a number of "speculatative" papers onm arxiv exist that indicate it was the inflationary stage of the universe that likely caused the Hot Big Bang.

[pensador]

It seems to be fairly common understanding.  A quick search picks this random page:
www.worldsciencefestival.com/2015/07/cosmological-inflation-primer/

For example, there are some puzzling features of our universe that can only be explained, some scientists argue, by the idea that our universe went through a short period (from somewhere in the vicinity of about 10⁻³³ to 10⁻³² seconds after the Big Bang) of rapid expansion (by what scientists believe to be around a factor of 10²⁶) after it emerged into existence. This idea is known as the theory of inflation, and is a widely accepted but still debated part of our universe’s beginnings.

My bold.  Picture depicts this.  Big bang is the singularity at left, the beginning of time as we know it.  After inflation, space has expanded by 10²⁶, which admittedly admits that the beginning wasn't a singularity or the size of the universe would be bounded.
It says it is debated, and there is eterial-inflation multiverse theory that says that inflation was outside the big bang, but bubbles of regular space-time condense out of that.  I think that might be what you reference.  Under that theory, inflation is still going on and did not terminate after this short time.
With eternal inflation, it becomes debatable which 'moment' or process to assign the term 'big bang'.

I am not sure how many infaltionary theories there are. I know there are more  than a few, starting out with Guth then onto Lindes later ideas. Penroses ideas are another take on inflation, whereby he introduces Aeons. Having the universe go through multiple inflationary stages, and Big Bangs, does away with a singularity and a beginning or end of time. 

Not being an expert, the number of different competing theories out there is mind boggling, I like Penroses theory because, it does away with singularities and a beginning to time, which does not ride well with me.

There also a number of competing theories as to what caused matter to come into existence. All the ones I have seen show it coming into existence in the inflationary stage, via the dynamic casimir effect, (recently proven) unruh effect (theoretical) similiar to Hawking radiation (theoretically plausible). That of course does not preclude other possible mechanisms, which I hope to learn more about.


Looking at the link you posted I still think inflation came before a hot big bang

[Bill S]

Chances are, you've all seen this, but just in case.......

https://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/hot-big-bang/

[PmbPhy]

Strict definitions in physics should never be taken from a dictionary. I wrote the glossary to a GR text before so I know how event is defined in SR/GR. I know SR very well and GR nearly as well. Look in any SR or GR text and you'll see that I'm write(right . Being a physicist myself the precise definition is second nature to me. Wikipedia is a lot more dependable than a dictionary when it comes to science.

I do not doubt you. Being primarily English speaker and reading a lot of Pop Science as well as some good science, Am I wrong in thinking a Big Bang "event" happened 13.8 billion years ago
.

Yes. That assumes there was an event called "big bang" but that's a myth. In modern cosmology there is no such event because there's no way to adduce such an event. Its based on an extrapolation. There are a lot of myths in pop physics. This one is explained in Peebles text on cosmology. Would you like me to quote that passage of his text?

[pensador]

Chances are, you've all seen this, but just in case.......

https://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/hot-big-bang/

Thanks for that Bill, according to the link inflation may have come first, resulting in the hot expansion.

"The Hot Big Bang may have begun when the universe became hot following a period of inflation, as explained in my article on the Era of Inflation. If so, the heat of the Hot Big Bang came from the dark energy that powered inflation, and the maximum temperature of the Hot Big Bang is related to the amount of dark energy that was available."

[Halc]

Thanks for that Bill, according to the link inflation may have come first, resulting in the hot expansion.

It seems that the hot big bang description just assigns the term 'big bang' to the hot part after the inflation instead of the compact part that precedes the inflation. The hot part is, after all, the part that 'bangs' the loudest, so to speak.
In that case, the term big bang is not being assigned to something that meets the definition of an event.

Surely the hot-big-bang terminology is also a different theory in some way, and not just a reassignment of terms, but I actually get from the linked article that it is merely a reassignment of terms and not anything new.  It does correct the notion that the universe was 'hot' during inflation.  The old text always described the initial singularity as an infinitely dense and hot point, but it is neither.  That old idea sort of went away when they realized the total energy of the universe was probably zero.

[pensador]

Am I wrong in thinking a Big Bang "event" happened 13.8 billion years ago
.

Yes. That assumes there was an event called "big bang" but that's a myth. In modern cosmology there is no such event because there's no way to adduce such an event. Its based on an extrapolation. There are a lot of myths in pop physics. This one is explained in Peebles text on cosmology. Would you like me to quote that passage of his text?

I havent read anyting knowingly by Peebles, a quick google flags up dozens of articles and papers by him. Where would you suggest I start. 

I have understood the original concept of Big Bang from a singularity proposed by LeMaitre was a myth, although it is still taught as fact by some. The inflationary models put forward by Guth and Linde were big improvements, and explained a lot. None of the inflationary models preclude a hot expansion stage of the universe. Which is what I am probably naively assuming is what people are referring to as the Big Bang today.

What I dont grasp is the multiverse idea. I can grasp the idea of Aeons put forward by Penrose, but find multiverses a little like singularities in the original Big Bang(LeMaitre) hard to believe.

If I am reading you correctly, extrapolating backwards an event can not be defined without evidence. Is the CMBR no longer considered as evidence for a hot expansion stage of the universe, ie a version of Big Bang? Are you referring to another theory I dont know anything about yet?

I am interested in anything you have to say, so if it is no bother if you have the Peebles text on Cosmology easily to hand, I would be interested to see what he has to say.

[PmbPhy]

I am interested in anything you have to say, so if it is no bother if you have the Peebles text on Cosmology easily to hand, I would be interested to see what he has to say.

From Principles of Physical Cosmology by Peebles, page 6

The familiar name for this picture, the "big bang" cosmological model, is unfortunate because it suggests we are identifying an event that triggered the expansion of the universe, and it may suggest the event was an explosion in space. Both are wrong. ... If there were an instant, at a "big bang," when our universe started expanding, it is not in the cosmology as ow accepted, because no one has thought of a way to adduce objective physical evidence that such an event occurred.