Is deep Space deep?

[LaurieAG]

Originally Posted by sanctus

Nobody denies that but this doesn't imply that this two galaxies are also in causal contact...

Hi Sanctus,

How does this fit in with regards to the theory of the expanding universe and anti matter? If theres no causal implications between galaxies why would they be perceived as expanding?

On a lighter note. If light from one galaxy can reach another and vs a vs then there is also a potential for communications contact (possibly over vast time spans).

[freeztar]

Originally Posted by LaurieAG

Hi Sanctus,

How does this fit in with regards to the theory of the expanding universe and anti matter? If theres no causal implications between galaxies why would they be perceived as expanding?

I think what Sanctus was saying is that if we point a powerful telescope in one direction of the sky and find the galaxies whose light is from a time close to the big bang and then we point the telescope in the opposite direction (180 degrees) and view similar galaxies, then it is unreasonable to think that these galaxies could view each other.

On a lighter note. If light from one galaxy can reach another and vs a vs then there is also a potential for communications contact (possibly over vast time spans).

Vast time spans indeed. That's also allowing for such a strong signal that it does not degrade appreciably. Then there's the expansion problem, leading to something like Achilles and the tortoise.

[sanctus]

Exactly freez, that is what I meant.
Laurie we can say the galaxies are accelerating away from us for those galaxies where we can see their light, hence that are in causal contact with us. It is not important if there is causal contact between the considered galaxies. That we can see them both doesn't imply they have causal contact, consider Galaxy A and B both at the same distance from our galaxy, but one at \theata=0° and the other at \theata=180°. Now suppose that the distance is such that the light from A (and B too since we suppose them at the same distance) just reached us today. We see them both but the galaxy A has to wait another current age of the universe time to see galaxy B ( actually more due to expansion of space-time).

If you were asking why twwo galaxies which are in no causal contact (today) can behave in the same way (i.e. accelerating away from any other galaxy), one answer is inflation: in inflanationary cosmological models there was a time where things that are in no causal contact today were in the past. Look up Cosmic inflation - Wikipedia, the free encyclopedia
for example, but there are also other models without inflation, likes seed-models for example.

[modest]

Originally Posted by sanctus

Exactly freez, that is what I meant.
Laurie we can say the galaxies are accelerating away from us for those galaxies where we can see their light, hence that are in causal contact with us. It is not important if there is causal contact between the considered galaxies. That we can see them both doesn't imply they have causal contact, consider Galaxy A and B both at the same distance from our galaxy, but one at theata=0° and the other at theata=180°. Now suppose that the distance is such that the light from A (and B too since we suppose them at the same distance) just reached us today. We see them both but the galaxy A has to wait another current age of the universe time to see galaxy B ( actually more due to expansion of space-time).

Three past lightcones on expanding spacetime.



I'm sure this isn't exactly correct because I did it by hand - well, not exactly by hand... it's not the result of any code or equation.

The green lines are the world lines of our 3 galaxies. We are the center line. Our past lightcone is red and we can see A and B because our past lightcone includes their world line. A and B can't see each other because their past lightcone doesn't cross the other's world line - but they can see us.

I got the expanding spacetime layout from Ned Wright's cosmology page.

-modest