The Location of Celestial Objects

[Slinkey]

When we look into the night sky we are looking back in time, for we do not see the Universe as it is but as it was. If, for example, we look at the Andromeda Galaxy - I believe it is something like 2.2mil Ly away - the light from Andromeda takes 2.2million years to reach us. Thus we see an apparent universe.

 

Because it takes light time to travel the Andromeda Galaxy may have exploded 1million years ago but we wouldn't know for another 1.2million years.

 

Consider: Six billion years from now we receive a communication from a region of the universe six billion light years away. The communication is a map of the Universe and the apparent distribution of galaxies as seen from the location of the message sender six billion years previously. We go to our databanks and seek out an ancient archive - 6 billion years old - that contains a map of the apparent distribution of galaxies as seen from earth 6 billion years ago.

 

We compare the two maps.... do they match up?

[Klaynos]

No.

 

The other map was done somewhere else...

[Slinkey]

That's not entirely correct I think. They would agree in certain areas but not in others. They would also be able to deduce similarites anywhere the maps over lap. For example, a distant quasar that appears to be directly behind where the commincation came from and some 10bil ly distant from us, would be how far from the source of the alien map? 6bil ly closer?

[Klaynos]

would the maps be 3D so give distances or just 2D images as you see when you look upwards?

[Slinkey]

I'm assuming the maps are as good as ours. ie. we can measure the locations and etimate their distances using similar means as the aliens used to create their map and from this can create 3-D maps that we run on computers to fly us around.

[Klaynos]

Well in that case the stars furthest from earth heading in their direction would appear younger and the ones in the other direction older, around the 3bill ly midway point between us and them they would be identicle...

[Slinkey]

Except that we weren't six billion light years apart when the message left the source. It took the message six billion years to get here. In that time the universe has expanded. I agree however, that there would be a midpoint where we would agree on what we see.

[Klaynos]

True, the distance would be about 50% greater than it is now, if you think the expansion is accelerating so it'd be a bit more than that but consider a finite minimal size so it'd be a bit less than that 50% isn't a bad esstimate, so that would be 4billion ly...

[antimatter]

Except that we weren't six billion light years apart when the message left the source. It took the message six billion years to get here. In that time the universe has expanded. I agree however, that there would be a midpoint where we would agree on what we see.

 

Right, but wouldn't most of it still be agreeable, I mean it would just be the outer parts of the map that has expanded, and the space betweem galaxies, but towards the center of the map, there should be significantly more midpoints.

[Klaynos]

Some bits of it where 6billion years older or younger when the maps where drawn though...

[antimatter]

So there are parts of space, that time doesn't affect?

other than black holes, which is an interesting point too,

the places where black holes are situated would stay the same.

[Klaynos]

So there are parts of space, that time doesn't affect?

other than black holes, which is an interesting point too,

the places where black holes are situated would stay the same.

 

Why would they stay the same? and no all bits of space are changing... it's just the places between us are 3billion ly away to both planets so as the maps are drawn at the same time, 6b years before they are received on earth, they would be the same.

[antimatter]

Don't black holes warp the space-time fabric into an infinite loop?

[Klaynos]

The create a singularity in GR :|

 

But that's no reason why they would be stationary, infact IIRC they are mostly formed in binary pairs, so they will orbit a mid point with their pair...

[antimatter]

Yeah, but when something passes the event horizon it appears to be frozen in place, that's what it does to time, it loops it.

 

What do you mean...

two black holes will orbit a midpoint?

[Klaynos]

Yeah, but when something passes the event horizon it appears to be frozen in place, that's what it does to time, it loops it.

 

What do you mean...

two black holes will orbit a midpoint?

 

No the other object is not a black hole, it's a more normal star...

 

Your above description is relative to a coordinate frame where the bh is at rest.

[antimatter]

So what you're saying is that a black hole and a star actually orbit, and between them there is a midpoint? How would they orbit, wouldn't the bh just pull in the star?

 

I understand that, but it shows that the fabric around it is warped, it's just an example

[Klaynos]

http://www.physorg.com/newman/gfx/news/2006/page1_1.jpg

 

An artists impression.

 

A blackhole (not a super massive just a normal one), is formed from a type II supernovae, to create one of these you need a binary star system, the stars are orbiting each other.

 

Once the white dwarf goes supernovae it leaves behind a black hole (well can do it can leave behind several things), this black hole will continue to orbit with the original binary pair star, if it's left behind which they are known to do.

 

The artists impression shows the large star on the right, with an acrecian disk around the blackhole, which is right at the very centre, until you get quite close to the bh their are stable kelparian orbits, the same as any other massive object.

[antimatter]

Alright, that makes a lot of sense now,

thanks...again, for like the hundredth time for explaining a concept to me.

[Klaynos]

Alright, that makes a lot of sense now,

thanks...again, for like the hundredth time for explaining a concept to me.

 

lol, np, I sat through a lecture on this about a week ago....

[D H]

Sans the expansion of the universe, any two maps of the universe taken from two different points in space and in time would have to agree at at least point per the Brouwer fixed point theorem (e.g., Stir a good cup of hot tea and let it come to rest. Ignoring the discrete nature of matter, at least one point in the tea will have come back to its original position.)

[antimatter]

Sans the expansion of the universe, any two maps of the universe taken from two different points in space and in time would have to agree at at least point per the Brouwer fixed point theorem (e.g., Stir a good cup of hot tea and let it come to rest. Ignoring the discrete nature of matter, at least one point in the tea will have come back to its original position.)

 

Right, those are the midpoints, where there are common areas in the the two Universes

[D H]

Suppose some remote civilization constructed a star map and transmitted the map shortly after they made it. Suppose we receive that transmission today, 6 billion years after it was sent due to the separation between the Earth and that civilization, the map arrives at Earth today. A star map made from Earth today will agree with that 6 billion year old chart in the immediate vicinity of the remote civilization, not the midpoint.

[Klaynos]

Suppose some remote civilization constructed a star map and transmitted the map shortly after they made it. Suppose we receive that transmission today, 6 billion years after it was sent due to the separation between the Earth and that civilization, the map arrives at Earth today. A star map made from Earth today will agree with that 6 billion year old chart in the immediate vicinity of the remote civilization, not the midpoint.

 

True, but we're talking about a map that was create, then sent to us and compared to one we made 6b years ago.