Question about detection of very far away objects

SolidHelix

I was thinking, when we see headlines about new galaxy found to be oldest yet, or furthest away.... is it possible that we could be seing some of these as earlier positions of previously found nearer objects?

DaveC426913

What do you mean, like we saw them once then forgot they were there, then found them again in a different place?
1] All deep sky objects are cataloged. Messier has been doing it since 1771.
2] We've only been seeing deep sky objects for 300 years. How far do you think they move in 300 years?

Simon Bridge

Why would we see the earlier position after the later position?
I realise you are thinking that the later position is further away and so it takes light longer to get here from there. However, the object must be travelling slower than light. Can you think of any way that an object can go from a far position to a nearer one, emmitting light the whole time, so that the light from the near position arrives before the light from the distant one? (And so that the distant object looks like a new discovery and not, say, the previously discovered object getting farther away?)

More to the point - we can check to see if any of the older discoveries are still there.

SolidHelix

OK, I understand... I was thinking hypothetically that if the said object was moving toward us, that since it was further away in the earlier position it would take the light longer to reach us and maybe it would just be reaching us now. but I guess if this were to be the case, it would have to be traveling faster than light....

DaveC426913

Except for some local objects, the vast bulk of deep sky objects are receding from us. The farther away they are, the faster they are receding.

Picture a rapidly expanding balloon covered in wandering ants. All ants are receding from us as the balloon expands, but because they all have their own (small) motions, some are receding fast, others slower. A very few right nearby might have local motion towards us even as the balloon is expanding, causing the distance to us to slightly decrease over time.

SolidHelix

Yeah I understand, while on the topic and i'm not getting bashed for asking stupid questions, since the universe as we accept (i guess generally) that the universe is expanding like a balloon as you said, and we and all objects are the ants, if one object were to exit away from everything else, in a direction where there were no known objects and keep going would it's "presence" expand the universe? or would it have to end up following the curvature of the balloon and end up meeting something other object in our universe? I know long winded and probably hard to understand my meaning.

DaveC426913

There are no dumb questions except unasked questions.

The universe is not a bubble that is expanding into a void. There is no void. The universe has no boundary.

This is where the balloon analogy works to make a good point. The universe does behave somewhat like a balloon in that sense. As the balloon expands its surface gets larger in area. But its surface has no boundary. The ants do not see any edge. No ant is closer to any edge than any other ant.

SolidHelix

So... are you saying whatever direction we choose to go in if we go far enough and faster than the expansion rate of the universe, we will end up in the same place we left off? please let me know if i'm on track... and if these questions were asked before, which i would guess they were, you can at any point, point me to the correct thread i should be reading :) the thing i'm thinking is, we are self sustained machines with our own agenda and arent bound to the natural flow of all the moving bodies in the universe and can choose to exercise the "bubble". If we are expanding, it seems to me, it wasnt a big bang, but some force outside our universe pulling us apart. i'm starting to feel crazy. again, you can point me to another thread..

Chronos

You might end up back where you started if the universe is closed. Of course you will have no clue you have circumnavigated the universe since it will have evolved and expanded for billions of years while you were out circumnavigating it.

Simon Bridge

That's pretty much what he's saying all right. The more promising models involve space being closed on itself so that, indeed, travel far enough in any direction and you'll end up back where you started.

The way it works, there does not need to be some "force outside the Universe pulling us apart" ... there is no "outside" to the Universe anyway. The whole thing has to be self-contained, but in a way that does not involve circular reasoning. Maybe what we percieve of as "the Universe" is actually part of some meta-Universe which can provide an "outside" in a meaningful way but there is no need for one so far.

I can see the attraction for theories that have one - they have a surface simplicity that is beguiling. However these ideas usually end up more convoluted than the weird math things we have now.

You are right in that these are very common questions. But they bear repeating - and each time we see the question we have to make slight changes to how we present the answers, which deepens our understanding of the concepts involved. This is why so many people have a lot of patience with this sort of thing.

DaveC426913

Well said.

surajt88

I remember reading somewhere, most probably written by Stephen hawking, that it can be proven that it would be impossible to circumnavigate the universe (in the closed and unbounded model of it) given the light speed as the universal speed limit and the FTL expansion of space over large distances. Do I recall correctly?

Chronos

Yes, circumnavigating the universe is impossible according to physics as currently known.

Simon Bridge

It's not so much that the speed of light is a "speed limit" as such, just that the Universe is so big bits of it are moving away from you faster than you can ever catch up. You need a smaller Universe or a slower expansion. The idea, though, helps people visualize the setup. Best out of three :)