Expansion and less visibility of objects

[Earl]

As we have had the opportunity to measure expansion due to when we are, when would objects outside of our galaxy become invisible to us? im guess high end billions of years or is this a not really thing?

[scotty38]

Can I also throw in another but related question?

I understand the concept of looking back in time and that's fine but what about duration? When we do look further back, say with the JWST, I assume, for what seems obvious to me, we will only see things that have occurred for a substantial period of time?

Let's say we want to look at the big bang itself and let's also say that event hasn't reached us yet but when it does and if it only lasts for a second then we'll need to be pretty sure we capture it otherwise it's gone forever, correct?

Then, my next question is let's say planet earth existed 2 seconds after the big bang then of course by that point it's already gone and passed us by so 13.5Bn years later we have no chance of EVER seeing it, correct?

Therefore, we will only ever see anything that happened far enough back in time that the light still hasn't reached us yet (including the big bang for example) plus also has a duration long enough that we don't blink and miss it (so to speak)?

Hope that makes sense but I think I know what I'm trying to ask 🙂

[vlaiv]

https://en.wikipedia.org/wiki/Future_of_an_expanding_universe#Timeline

10 minutes ago, scotty38 said:

Let's say we want to look at the big bang itself and let's also say that event hasn't reached us yet but when it does and if it only lasts for a second then we'll need to be pretty sure we capture it otherwise it's gone forever, correct?

"Big bang event" is continuously reaching us. This is because big bang was everywhere. We can't really see the big bang - but we can see first event that can be seen after that - and that is cosmic microwave background. It can be seen now and it was seen 1 billion years ago. However - that does not mean it lasted billion years.

If there was planet at the moment of big bang - or for sake of having any sort of merit - say we have localized event that had finite duration. Such event "projects" a sphere of certain thickness that travels at speed of light and expands. Thickness is given by duration of event x speed of light and radius at any point in time after event is - how much time passed after event x speed of light (this is simplified as you really need to do the same in 4d space-time to get correct results).

In order to observe such event - you need to be at certain time space coordinates when this event shell is passing by.

It's a bit like agreeing to meet. Say we agree to meet in pub in Island Inn pub in West Bromwich. Odds are - we will miss each other. If we agree to meet at 4:13 pm - we will miss each other. But if we agree to meet at the pub at 4:13pm - then we will meet. You need both space and time coordinates to coincide in order to meet.

Ok, back to the big bang - big bang is not event that is localized in space, although it is localized in time. This means it is everywhere - and therefore "sphere" of detection is no longer expanding sphere. If we could detect big bang - we could detect it any time in all directions. This is in fact how we detect similar event that is causing cosmic microwave background. It was also localized in time but happened everywhere.

[iantaylor2uk]

There wouldn't have been a planet, or any atoms either, at the time of the big bang - the temperatures are so high it is thought there was just a quark-gluon plasma. When the temperatures cooled off, the first atoms/molecules to form would have been hydrogen - all other elements then formed from nuclear reactions in stars. 

[scotty38]

Thanks Vlaiv, I was rather hoping you'd reply but I need to read that again as I'm not quite sure it's sunk in yet

[scotty38]

1 minute ago, iantaylor2uk said:

There wouldn't have been a planet, or any atoms either, at the time of the big bang - the temperatures are so high it is thought there was just a quark-gluon plasma. When the temperatures cooled off, the first atoms/molecules to form would have been hydrogen - all other elements then formed from nuclear reactions in stars. 

Yes I realise that it's why I said "Let's say it existed" as a means of discussion....

[andrew s]

As we look back we can see transitory events particularly super or hyper novae. 

One milestone will be to see the first generation of massive stars explode.

Regards Andrew 

[scotty38]

38 minutes ago, vlaiv said:

https://en.wikipedia.org/wiki/Future_of_an_expanding_universe#Timeline

"Big bang event" is continuously reaching us. This is because big bang was everywhere. We can't really see the big bang - but we can see first event that can be seen after that - and that is cosmic microwave background. It can be seen now and it was seen 1 billion years ago. However - that does not mean it lasted billion years.

If there was planet at the moment of big bang - or for sake of having any sort of merit - say we have localized event that had finite duration. Such event "projects" a sphere of certain thickness that travels at speed of light and expands. Thickness is given by duration of event x speed of light and radius at any point in time after event is - how much time passed after event x speed of light (this is simplified as you really need to do the same in 4d space-time to get correct results).

In order to observe such event - you need to be at certain time space coordinates when this event shell is passing by.

It's a bit like agreeing to meet. Say we agree to meet in pub in Island Inn pub in West Bromwich. Odds are - we will miss each other. If we agree to meet at 4:13 pm - we will miss each other. But if we agree to meet at the pub at 4:13pm - then we will meet. You need both space and time coordinates to coincide in order to meet.

Ok, back to the big bang - big bang is not event that is localized in space, although it is localized in time. This means it is everywhere - and therefore "sphere" of detection is no longer expanding sphere. If we could detect big bang - we could detect it any time in all directions. This is in fact how we detect similar event that is causing cosmic microwave background. It was also localized in time but happened everywhere.

Thanks again Vlaiv... Ok, let's say this event lasted 1 second at the time and then that event travels as you say so 13.5 billion years later we "see" it because it now lasts, let's say again, a million years. Is that sort of what you're saying with the obvious comment I'm "ignoring" the real maths to come up with my million years duration but you get what i mean?

[AstroKeith]

An interesting phenomena of transitory early events will be created by gravitational lensing.

As we look further back/away the likelyhood that our targets will be gravitational lensed increases. Thus the radiation from a supernova say, will travel over many different path lengths to reach us. This will create 'echos' of the event,  spreading out from the apparent direction of the object. Such echos have already been seen for 'nearer' objects.

[andrew s]

3 hours ago, Earl said:

As we have had the opportunity to measure expansion due to when we are, when would objects outside of our galaxy become invisible to us? im guess high end billions of years or is this a not really thing?

This is not  a simple question 

If I have understood this correctly it is, for non gravitationally bound objects, the particle horizon which will get to more than 60 glyrs in the current cosmological model and as time tends to infinity.  So we will see objects fade away as they get more and more red shifted.

Regards Andrew 

PS the link is to a pdf that downloads . 

Edited December 1, 2021 by andrew s

[andrew s]

43 minutes ago, scotty38 said:

Thanks again Vlaiv... Ok, let's say this event lasted 1 second at the time and then that event travels as you say so 13.5 billion years later we "see" it because it now lasts, let's say again, a million years. Is that sort of what you're saying with the obvious comment I'm "ignoring" the real maths to come up with my million years duration but you get what i mean?

Distant events are time dilated but on anything, like that scale. It is seen in the shape of supernova light curves.

Regards Andrew 

Edited December 1, 2021 by andrew s

[vlaiv]

5 hours ago, scotty38 said:

Thanks again Vlaiv... Ok, let's say this event lasted 1 second at the time and then that event travels as you say so 13.5 billion years later we "see" it because it now lasts, let's say again, a million years. Is that sort of what you're saying with the obvious comment I'm "ignoring" the real maths to come up with my million years duration but you get what i mean?

Not quite.

If event lasted 1 second and was localized in single point - we will now see it lasting 1 second. It is a bit like this:

So event happened at some point and light (information) from it starts to expand in every direction. We are some distance from that point and eventually that front hits us - and then we see the event. If event lasted 1s - we see front for one second.

This all holds true for localized event that happened at single point. We need to be at right distance to see event now at some point and that event will last 1s to us as well.

Big bang is not like that - big bang did not happen at single point.

Big bang is more like this:

It was happening in every point and in every point we have a circle that started out - so we don't have to wait for particular moment for front to arrive - at any given moment, some front will be passing us. In each second - front from points one light second further will be passing us.

That is why we see cosmic microwave background radiation in every direction - in every second light from event that happened everywhere is passing by us - but each second we have front from a different place passing us.

Makes sense?

 

[scotty38]

ok I see, so my initial 1 second event theory stands but the big bang wasn't a 1 second event... Ok that's so good so far but I don't get the "happened everywhere type point" more thinking needed 🙂 I mean I get the multiple points so we can see it at any time but I don't get why it was multiple points versus the single local event.....

[vlaiv]

2 minutes ago, scotty38 said:

I don't get why it was multiple points versus the single local event.....

We don't actually know what was it like - but all points to type of event similar to recombination (source of CMB).

We can't say that it was in single point as at that time - space was not well defined. What we can say is that it most likely happened "all over the universe". This implies sort of causality arrow which I would prefer to be pointed the other way - something like "universe was created all over the big bang" - but point is - there is 1:1 mapping between big bang and "all over the place".

As such - it is different than single point event and it leaves effects similar to happening in every point in space.

[scotty38]

ok thank you, much appreciated.

[saac]

In his latest series "Universe"  Prof Cox has a few wistful moments and  poetic musings .  One  such moment that I particularly liked is where he reflected upon how fortunate we are to live in  this particular period of the universe's evolution.  We are still able to receive light  from our neighbouring and more distant objects and hence we can decode the information in that light and become aware of our universe.  He then reflected upon a possible future in which our galaxy would be isolated, so far from any other that the universe outside would effectively be dark , forever  beyond our reach.  Any future inhabitants of the Milky Way would live in blissful ignorance of what lay outside their home galaxy.  We are indeed fortunate to be here now !

Jim  

[George Jones]

On 01/12/2021 at 09:12, vlaiv said:

If event lasted 1 second and was localized in single point - we will now see it lasting 1 second. 

In an expanding universe, I think that we will now see it lasting (1 second)*(a_now)/(a_then), where a_now is the present scale factor of the universe, a_then is the scale factor of the universe when the 1-second pulse was emitted.

[ollypenrice]

The universe didn't become transparent to light till it was about 380,000 years old (very young in cosmological terms) so we won't see anything before that.

I think that, if you're struggling with the idea for the BB starting everywhere, 1) you're not alone  and 2) you're thinking of it as being like an explosion. I think it's vital to rid your mind of this notion because it is utterly misleading. An explosion has a centre and moves out from it. Lots of TV documentaries are guilty of perpetuating this fallacy in their graphics, which show an explosion seen from the outside. Personally I think the trick is understand that there is no outside. It doesn't exist. Only allow yourself to think of the expanding universe from the inside. Pick a point in it and think of space expanding around you. Now pick a different point in it and think of space expanding around you. And then yet another point...  What is the difference between these points? There is no difference.  Each of these points will feel like the centre of the expansion but this simply demonstrates that there is no centre.

Olly

[vlaiv]

1 hour ago, ollypenrice said:

The universe didn't become transparent to light till it was about 380,000 years old (very young in cosmological terms) so we won't see anything before that.

Hopefully we will be able to see at least something before that epoch - but using other means than EM radiation - maybe gravity waves?

[vlaiv]

4 hours ago, George Jones said:

In an expanding universe, I think that we will now see it lasting (1 second)*(a_now)/(a_then), where a_now is the present scale factor of the universe, a_then is the scale factor of the universe when the 1-second pulse was emitted.

I think you are right, but I did not want to confuse matter further

 

[ollypenrice]

11 hours ago, vlaiv said:

Hopefully we will be able to see at least something before that epoch - but using other means than EM radiation - maybe gravity waves?

I'd settle for 'detect' something...!!!

lly

[George Jones]

On 01/12/2021 at 00:50, Earl said:

As we have had the opportunity to measure expansion due to when we are, when would objects outside of our galaxy become invisible to us? im guess high end billions of years or is this a not really thing?

Do mean in principle, or in practice? In principle, nothing that we can see now will become invisible to us. In fact, some, but not all, stuff that now is invisible to us will become visible. In practice, some stuff that we see now will become dimmer and dimmer, and more and more redshifted (as Andrew has said).

 

On 01/12/2021 at 04:06, andrew s said:

This is not  a simple question 

If I have understood this correctly it is, for non gravitationally bound objects, the particle horizon which will get to more than 60 glyrs in the current cosmological model and as time tends to infinity.  So we will see objects fade away as they get more and more red shifted.

 

[George Jones]

On 05/12/2021 at 10:29, George Jones said:

In practice, some stuff that we see now will become dimmer and dimmer, and more and more redshifted

I hove been careful here. Other stuff that we now see will become brighter and less red as we watch over centuries. This is very counterintuitive! As we watch, the universe expands, which seem like it should make all object dimmer and more red, because expansion increases distance and "stretches" light. This other stuff, will become less red, less red, ...., more  red, more red, more red, ...

[900SL]

On 01/12/2021 at 20:25, scotty38 said:

ok I see, so my initial 1 second event theory stands but the big bang wasn't a 1 second event... Ok that's so good so far but I don't get the "happened everywhere type point" more thinking needed 🙂 I mean I get the multiple points so we can see it at any time but I don't get why it was multiple points versus the single local event.....

You are confusing big bang with cosmic background. Cosmic background happened after big bang, so universe was expanding and had space. Cosmic background is the relic of when there was a phase change (like from hot soup to clear sky) and this happened everywhere in space, so we can see the Doppler shifted light as microwaves in every direction