[jeffreyH (OP)]

There is a limit on the amount of the universe we can see. Since the universe is theorised to have started with the big bang. Therefore, if we look out far enough, shouldn't we be able to see only half a galaxy at the boundary of the visible universe? Would we only be able
detect this at some point in our future when the galaxy starts to become visible to us? Due to the vast distances between galaxies.

[Halc]

There is a limit on the amount of the universe we can see. Since the universe is theorised to have started with the big bang. Therefore, if we look out far enough, shouldn't we be able to see only half a galaxy at some point? Would this be thousands or millions of years in the future? Due to the vast distances between galaxies.

Not sure what you mean by half a galaxy.  All the really distant galaxies we see (including those currently beyond our event horizon) are light emitted vastly in the past, not the future.  So the light from say GN-z11 for instance was emitted around 12 billion years in the past. Light 'currently' emitted from there will never reach our galaxy.

Our own galaxy is half a galaxy.  The far side is completely obscured by clouds since we're looking at it edge on, so a good percentage of it is completely unmapped.  We can only see some of the central objects via extremely long wavelength light.

[jeffreyH (OP)]

Then you missed the point of the question entirely.

[jeffreyH (OP)]

I have modified my first post to make it clearer what I mean.

[evan_au]

shouldn't we be able to see only half a galaxy at the boundary of the visible universe?

The most distant galaxies we can see are limited by:
- Their extreme red shift. That means you need an infra-red telescope to see them (like the James Webb Space Telescope) rather than visible light (eg the Hubble Space Telescope). When we exhaust galaxies visible with infra-red telescopes, we will need to look for galaxies at TeraHertz wavelengths (a very challenging part of the electromagnetic spectrum).
- Limited Observing Time on expensive telescopes. The Hubble Ultra Deep field took 11 days exposure on the highest-demand telescope, and revealed far more galaxies than had been seen previously. If they could afford a 100-day exposure, it would reveal even more galaxies!
- Limited angular resolution of real telescopes. The most distant galaxies known are "quasars": Quasi-Stellar objects. They look like a point source (like a star), because our current telescopes don't have enough size, sensitivity and angular resolution to see the faint galaxy surrounding the very active central black hole.

So, whether we can detect a particular galaxy or not depends on the physical limitations of real telescopes, not on the physics that might cause half a galaxy to fall off the edge of the observable universe.

See: https://en.wikipedia.org/wiki/Quasar
https://www.spacetelescope.org/images/heic0611b/

[Halc]

Galaxies don't slowly move into view over time.  For one, galaxies disappear over the event horizon. They pass a point where light can no longer reach us, when it could before.  GN-z11 is such a galaxy. The view of it is much like a star falling into a black hole, sort of freezing in time but never completely disappearing.

Any galaxy that is not visible to us now will never be more visible later on.  They fade over time in fact, shifting to lower frequency light, but never completely disappearing. All galaxies will eventually fade from view, except arguably Andromeda which will become our galaxy once it eats us. Some models predict that any object will fade from view eventually, like even my left hand.

[hamdani yusuf]

That means you need an infra-red telescope to see them

For galaxies at the boundary of the visible universe, we need radio telescopes.

[Bored chemist]

It may be instructive to consider what we see without telescopes.
Across flat land I can see people at a distance of a mile or so. (never mind the exact distance, there's some "cut off".
But that doesn't mean that , if there's someone exactly a mile away, I can only see the tip of their nose and their toes.

[Bill S]

  But that doesn't mean that , if there's someone exactly a mile away, I can only see the tip of their nose and their toes.

I interpreted Jeffrey’s “shouldn't we be able to see only half a galaxy at the boundary of the visible universe?” as meaning something like: If he boundary of the visible universe coincided with the centre line of a galaxy, would this mean that, in principle, only half of the galaxy would be visible, if we had the ability to see it. 

I suspect that, at that distance, any galaxy would appear so small that the point is moot; but Jeffrey’s question still stands.

[Halc]

I interpreted Jeffrey’s “shouldn't we be able to see only half a galaxy at the boundary of the visible universe?” as meaning something like: If he boundary of the visible universe coincided with the centre line of a galaxy, would this mean that, in principle, only half of the galaxy would be visible, if we had the ability to see it. 

OK, I see the meaning of the question, but it assumes not a distant galaxy, but one at a specific moment in time, in which case  the question is relevant.
So as an example, we'll consider the Milky Way, which is a distant galaxy to some observer X, who is on the far side of our galaxy, billions of LY away.  Light from the far side of our galaxy (star R) today (our frame) will eventually reach X and he can in theory see it.  Light from our sun today (star S) will never reach X ever.  So that sort of answers the question when worded that way.

But X still sees our entire galaxy since he can see our sun as it was a million years ago when it was well inside his event horizon, so X never really sees half a Milky Way.  Similarly, we never see any edge-on galaxy as it is in a moment in time since the far side we see is some 100,000 years older than the near side.  It just takes light that long to cross a galaxy.

Another way of looking at it is that star R (100 KLY away) can see our sun, so light from the sun is there, and if that light is there, it is going to get to distant observer X just the same as light from star R, it's just older light.

[jeffreyH (OP)]

I interpreted Jeffrey’s “shouldn't we be able to see only half a galaxy at the boundary of the visible universe?” as meaning something like: If he boundary of the visible universe coincided with the centre line of a galaxy, would this mean that, in principle, only half of the galaxy would be visible, if we had the ability to see it. 

OK, I see the meaning of the question, but it assumes not a distant galaxy, but one at a specific moment in time, in which case  the question is relevant.
So as an example, we'll consider the Milky Way, which is a distant galaxy to some observer X, who is on the far side of our galaxy, billions of LY away.  Light from the far side of our galaxy (star R) today (our frame) will eventually reach X and he can in theory see it.  Light from our sun today (star S) will never reach X ever.  So that sort of answers the question when worded that way.

But X still sees our entire galaxy since he can see our sun as it was a million years ago when it was well inside his event horizon, so X never really sees half a Milky Way.  Similarly, we never see any edge-on galaxy as it is in a moment in time since the far side we see is some 100,000 years older than the near side.  It just takes light that long to cross a galaxy.

Another way of looking at it is that star R (100 KLY away) can see our sun, so light from the sun is there, and if that light is there, it is going to get to distant observer X just the same as light from star R, it's just older light.

According to your reasoning we should see all galaxies no matter how far away. The light from the furthest half of a distant galaxy may not yet have reached us while the light from the nearest half may have. What Bill said about resolution still stands. It may be too far away to matter.

[Halc]

According to your reasoning we should see all galaxies no matter how far away. The light from the furthest half of a distant galaxy may not yet have reached us while the light from the nearest half may have.

I didn't say that.  I said referred to light 'at a given moment in time'.

What Bill said about resolution still stands. It may be too far away to matter.

That's a technological limitation, and it just says that all we see is a pixel (one detected photon if it is aimed well), but it has nothing to do with whether that photon came from the near or far edge.

There is very much a limit, and it is closer than the CMB, which is something that can be seen with our 'limited' technology.

If a galaxy has width (emits light from the near side and far side), and the light from the far side has not yet reached us, then light from that same far side but emitted earlier in time has reached us.  So it kind of comes down to how far back in time we can reach when said galaxy started emitting light.  If it does so over a large region all at once, then we indeed might only ever see the near side since there is no past light emission from when the galaxy was closer.  I don't think galaxies light up like that.  They start glowing (emitting photons) only in the middle where they're hottest, and that glow spreads to the edges as the thing forms.  That means there is no near and far side, there is just a theoretical first-glow of some proto-galaxy that we can see or not, but there is not yet a near and far side to it.  We'd get one very red-shifted photon at best, and then it would be gone to our view.

[hamdani yusuf]

OK, I see the meaning of the question, but it assumes not a distant galaxy, but one at a specific moment in time, in which case  the question is relevant.
So as an example, we'll consider the Milky Way, which is a distant galaxy to some observer X, who is on the far side of our galaxy, billions of LY away.  Light from the far side of our galaxy (star R) today (our frame) will eventually reach X and he can in theory see it.  Light from our sun today (star S) will never reach X ever.  So that sort of answers the question when worded that way.

But X still sees our entire galaxy since he can see our sun as it was a million years ago when it was well inside his event horizon, so X never really sees half a Milky Way.  Similarly, we never see any edge-on galaxy as it is in a moment in time since the far side we see is some 100,000 years older than the near side.  It just takes light that long to cross a galaxy.

Another way of looking at it is that star R (100 KLY away) can see our sun, so light from the sun is there, and if that light is there, it is going to get to distant observer X just the same as light from star R, it's just older light.

Let's take spacetime expansion into account. The space between an observer and near side of a galaxy is expanding below speed of light which makes it visible, but the space between the observer and far side of the galaxy is expanding above speed of light which makes it invisible.

[Halc]

Let's take spacetime expansion into account. The space between an observer and near side of a galaxy is expanding below speed of light which makes it visible, but the space between the observer and far side of the galaxy is expanding above speed of light which makes it invisible.

This isn't true. We see plenty of galaxies that are currently beyond the event horizon.  We see them in the past when they were not.  Similarly, the far side of your galaxy had just now crossed over said horizon and we can always see it in the past, just like I can always see a light source dropped into a black hole.

[A-wal]

Let's take spacetime expansion into account. The space between an observer and near side of a galaxy is expanding below speed of light which makes it visible, but the space between the observer and far side of the galaxy is expanding above speed of light which makes it invisible.

This isn't true. We see plenty of galaxies that are currently beyond the event horizon.  We see them in the past when they were not.  Similarly, the far side of your galaxy had just now crossed over said horizon and we can always see it in the past, just like I can always see a light source dropped into a black hole.

Not true. We see no galaxies that are currently beyond the event horizon. There is no amount of time on our watches that could pass which would allow us to say that they are now beyond the horizon.

This is very easily demonstrated in the case of a black hole. Any object close to an event horizon can always in principle accelerate away from it. This is why I don't think black hole event horizons are reachable from any perspective. There may be a time on a faller's watch when they would reach the horizon but the black hole will always have evaporated before their watch reaches that time. An infinite amount of time has to pass on any more distant watch in order for a falling observer to reach an event horizon, never going to happen.

[Halc]

We see plenty of galaxies that are currently beyond the event horizon.  We see them in the past when they were not.  Similarly, the far side of your galaxy had just now crossed over said horizon and we can always see it in the past, just like I can always see a light source dropped into a black hole.

Not true. We see no galaxies that are currently beyond the event horizon.

Take GN-z11 for instance, which may still hold the record for the most distant object visible.  It is currently 32 billion light years away, about double the distance to the event horizon of about 16 BLY.  It crossed our event horizon when the universe was about 4 billion years old, and the image we see today is from when the universe was only .3 - .5 BY old.


See the dotted line that crosses the blue 'now' line at around 31 BLY?  That's approximately the path of GN-z11.  You can see it cross the event horizon at 4BY, and since the red light cone crosses its worldline much lower down, the galaxy is visible to us.

[A-wal]

We see plenty of galaxies that are currently beyond the event horizon.  We see them in the past when they were not.  Similarly, the far side of your galaxy had just now crossed over said horizon and we can always see it in the past, just like I can always see a light source dropped into a black hole.

Not true. We see no galaxies that are currently beyond the event horizon.

Take GN-z11 for instance, which may still hold the record for the most distant object visible.  It is currently 32 billion light years away, about double the distance to the event horizon of about 16 BLY.  It crossed our event horizon when the universe was about 4 billion years old, and the image we see today is from when the universe was only .3 - .5 BY old.


See the dotted line that crosses the blue 'now' line at around 31 BLY?  That's approximately the path of GN-z11.  You can see it cross the event horizon at 4BY, and since the red light cone crosses its worldline much lower down, the galaxy is visible to us.

Assuming it works the same way as a black hole event horizon, we will never see it or any other galaxy reach our horizon. All galaxies in the universe are in principle visible to us and always will be at a time before they reach the horizon, but that's not what I've heard from some cosmologists. Maybe they mean galaxies pass a point where we could ever realistically see them, but could in principle if we had a good enough instrument.

[hamdani yusuf]

This isn't true. We see plenty of galaxies that are currently beyond the event horizon.  We see them in the past when they were not.  Similarly, the far side of your galaxy had just now crossed over said horizon and we can always see it in the past, just like I can always see a light source dropped into a black hole.

If X is the furthest galaxy we can currently see. It means that X is inside an effective boundary of currently observable universe. Then any other galaxy further away than the boundary is currently invisible to us. If there is a galaxy located right at that boundary, it would be partially visible for now.

[Halc]

If there is a galaxy located right at that boundary, it would be partially visible for now.

It wouldn't be visible at all since we would see it at an age when it had yet to begin to glow.  The gravity of part of the thing may in theory have had an influence, but no light can come from that distance.  The universe wasn't even transparent back then.

[hamdani yusuf]

Take GN-z11 for instance, which may still hold the record for the most distant object visible.  It is currently 32 billion light years away, about double the distance to the event horizon of about 16 BLY.  It crossed our event horizon when the universe was about 4 billion years old, and the image we see today is from when the universe was only .3 - .5 BY old.

In the future, GN-z11 is expected to be invisible due to its crossing our event horizon. When it happens, the far side would become invisible first while the near side is still visible for some time.