[pensador]

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 now accepted, because no one has thought of a way to adduce objective physical evidence that such an event occurred.

Thankyou very much for that ?

As I understand it, the inflationary stage of space came first which caused the production of matter from virtual particles, a bit like hawking radiation around a black hole, (other possible mechanisms also exist). Do you have one or more preferred models for particle creation during the inflationary stage of the current universe?

[PmbPhy]

As I understand it, the inflationary stage of space came first which caused the production of matter from virtual particles, a bit like hawking radiation around a black hole, (other possible mechanisms also exist). Do you have one or more preferred models for particle creation during the inflationary stage of the current universe?

No. Sorry but I don't know much about quantum cosmology. I've only studied the dynamics of cosmological models.

[Bill S]

Peebles seems to subscribe to the “hot big bang model.  Page 7:

Thus the word “standard” is meant to express the fact that there is a very significant body of evidence indicating that the hot big bang model is a useful approximation of the real world.”

This book is over 25 years old.  There will have been much progress in the field in that time, so it looks as though this model is surviving well.

[Bill S]

An event is something that occurs at a point in space time.

I have no problem with this, as long as we take into consideration the fact that a point is defined as having no dimensions/extent; but an event has finite extent. Given that caveat, your definition of an event seems quite workable. 

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

Strictly speaking, the notion of an event is an idealization, in the sense that it specifies a definite time and place, whereas any actual event is bound to have a finite extent, both in time and in space.

Let’s go with that.

[guest4091]

Is an observer aware of any events without light (detectable em radiation)?

[Bill S]

Is an observer aware of any events without light (detectable em radiation)?

How about walking into a wall, in the dark?  That's an event you might be painfully aware of.

[geordief (OP)]

Is an observer aware of any events without light (detectable em radiation)?

How about walking into a wall, in the dark?  That's an event you might be painfully aware of.

Just because it is dark doesn't mean there is no em radiation ,does it?

Is low or high frequency em radiation  the same as low or high frequency "light" ?

[PmbPhy]

An event is not defined to require anything to happen there. Most events happen in simply empty space. Recall that an event is a point in spacetime and that does not require anything to be present at the position r for the event (r, t)

[PmbPhy]

Is an observer aware of any events without light (detectable em radiation)?

You're confusing the term observer with person. The observer could be a system of clocks and rods and detectors at the location at the time of the event. It may be a person but that's not necessary.

[geordief (OP)]

You're confusing the term observer with person. The observer could be a system of clocks and rods and detectors at the location at the time of the event. It may be a person but that's not necessary.

If it is a person ,is the person to be regarded as simply an outrageously sophisticated piece of machinery?

If the person or team of people  is collating results from a piece of measuring equipment  is he/she/they  " so to speak the mechanical observer of last resort"? (entirely dependent on em signals to do their work)

[PmbPhy]

You're confusing the term observer with person. The observer could be a system of clocks and rods and detectors at the location at the time of the event. It may be a person but that's not necessary.

If it is a person ,is the person to be regarded as simply an outrageously sophisticated piece of machinery?

If the person or team of people  is collating results from a piece of measuring equipment  is he/she/they  " so to speak the mechanical observer of last resort"? (entirely dependent on em signals to do their work)

You're getting into metaphysics and semantics now.

[geordief (OP)]

You're getting into metaphysics and semantics now.

That seems fair (metafluff   )

[guest4091]

Coordinates are a means of locating an event. Multiple observations are possible for the same event, yet with different coordinates.
An event is a change of state of a system, simple or complex, as Colin2b mentioned in #1.
Perception by an observer or proxy (video), requires a messenger (photon).
If you contact an object, sensory input (touch) is mediated by radiation via the electron cloud surrounding each nucleus. 

[PmbPhy]

An event is a change of state of a system, simple or complex, as Colin2b mentioned in #1.

Colin was wrong on that point. Don't get used to it since Colin rarely makes a boo boo.

Note that a car skidding to a stop allows observation of the event "car stopped" by locating the end of the skid mark which could have been located without light, e,g, the conductivity of the road mapped out would denote where it is.

[Halc]

An event is a change of state of a system, simple or complex, as Colin2b mentioned in #1.
Perception by an observer or proxy (video), requires a messenger (photon).

If Colin said that, then he's referencing a very different definition of event than the one used by physics.  WW II is that sort of event.

Coordinates are a means of locating an event. Multiple observations are possible for the same event, yet with different coordinates.

WW II has 'coordinates' of a 6-year swath of time on Earth.  My particular observation (from different coordinates) might be that I learned I am minus one uncle.  I did not make this observation via photon.

If you contact an object, sensory input (touch) is mediated by radiation via the electron cloud surrounding each nucleus.

This seems to be sidetracking to a discussion about how one system might have a causal influence on a second system (possibly a human), which is pretty far and gone from a discussion about what an event is, be it the physics definition or otherwise.

the conductivity of the road mapped out would denote where it is

Awesome!  You should put together a thread of needlessly weird ways to measure common things.  How might a blind person find North on an overcast day?  Grope up the moss on the trees, or climb a utility pole.

Concerning the latter, I have on occasion found myself lost in unfamiliar towns in New Jersey after being forced off the road by accidents or construction and such.  I don't have a compass, and it always seems to happen on a cloudy day.  But I find north because the power company puts one solar panel on more than half the power poles in the state, which always face south of course.  That's more reliable than the moss, which is hard to spot from a car.  Unfortunately it involves me spotting it with a photon, which prompted my suggestion of the blind man climbing the pole so he knew which way to drive home.   

[Colin2B]

An event is a change of state of a system, simple or complex, as Colin2b mentioned in #1.

Colin was wrong on that point. Don't get used to it since Colin rarely makes a boo boo.

Note that a car skidding to a stop allows observation of the event "car stopped" by locating the end of the skid mark which could have been located without light, e,g, the conductivity of the road mapped out would denote where it is.

It would appear that @phyti is misquoting me!

I was answering this question from @geordief:

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 occurrence , what can we say about it?

Taking this physical occurrence I replied:
“We define what an event is. Usually it is when something happens and we can define the point at which it happens as a spacetime coordinate. “
I didn’t go into events on the manifold as I assumed that was what @geordief meant by the coordinate system.
I was also clear that we are talking about a point not a ‘smear’. The examples I gave “The happening can be anything, an object passing a point, changing speed, hitting something, emitting a photon, etc” did not all imply a change of state. For example, an object passing a point requires no change of state.
To emphasise what @PmbPhy  is saying, the skid is not an event but a series of events starting with ‘brakes applied event’ and ending with ‘stops event’.

[guest4091]

goerdief;
"But if we take  an event as a physical occurrence ,"

Colin2B;
"Usually it is when something happens"
happen=occur

myself;
An event is a change of state of a system, simple or complex,
'complex' allows for sequential or prolonged events, a process of any duration.
An object passing a point requires observation/perception, which requires light emission, which qualifies as a change of state. It's why the photon is referred to as the messenger particle. It effectively says, 'something happened over here'. People in the dark use an artificial light source.
If all photon production were to stop, awareness, consciousness, and life itself would cease.
The coordinates of any event are not the same as the event, no more than a picture of an object is the same as the object.

[Colin2B]

goerdief;
"But if we take  an event as a physical occurrence ,"

Colin2B;
"Usually it is when something happens"
happen=occur

You still misquoted me.
I said “usually” this does not imply always as you seem to have interpreted it.
Also, happen ≠ occur, the 2 are different.

myself;
An event is a change of state of a system

Occurrences do not require a change of state.
I was walking in some hills recently, limestone occurs there and has done for millions of years.
Write out a sequence of “1s” = 11111111111 look at the 3rd, 4th and 5th occurrence of 1. No change of state there!

The definition used by @PmbPhy is the correct one.

[PmbPhy]

I said it before and I'll said it again: An even is merely a point in spacetime. It does not require anything to be at the location of the even nor does it require anything to happen at that location. Empty space for a long period of time is a good example of spacetime.

The more you attempt to use a dictionary for this term the more trouble you'll have learning it. Please don't do that since it makes our job harder. And the harder it will take for you to learn the actual physics of anything in physics.  And
I hate repeating myself.

[guest4091]

1905 paper, A. Einstein,

par.1, simultaneity
"If we wish to describe the motion of a material point, we give the values of its co-ordinates as functions of the time."

"We have to take into account that all our judgments in which time plays a part are always judgments of simultaneous events."

par. 3, transformation of coordinates
"To any system of values x, y, z, t, which completely defines the place and time of an event in the stationary system,"

[the coordinates are variables associated with an 'event', and the event is variation of position or 'motion' of a particle.]

[For accuracy, I prefer the original author of a publication. Wiki is correct some of the time]

Colin2B;
“Write out a sequence of “1s” = 11111111111 look at the 3rd, 4th and 5th occurrence of 1. No change of state there!”

[Not observing it in its current state, but the sequence changed as it was written. Now the total complex event is the transformation of a blank paper to one containing a sequence of 1’s, which required energy.]