A thought on the expansion of the Universe

[Igottaknow]

One thing that has always puzzled me is that the farther we look out into the universe we see that galaxies are moving away from us faster than galaxies that are closer to us, but also the farther we look out into space the farther back in time we are observing.

I have trouble understanding this since we know that what we are observing when we view a galaxy 4 billion light years away is what was happening 4 billion years ago. What is it about that light that we can say for certain that that galaxy is moving away from us at any specific rate as it is in relation to us now?

I do understand that it is is because of the red shift of this 4 billion year old light and the idea of the Doppler effect of that light that we have determined that galaxy's rate of movement but...how do we know for certain anything about what is happening with that galaxy right now in that region of space?

I guess i am trying to understand how we can determine for certain any thing about what we observe in the universe not being anything other than what was happening at the time the light left that object.

 

[.Scott]

If something was moving away from us at half the speed of light 4 Billion years ago, we can look at what was around it and determine what kind of gravitational pull is being applied to it. When can then determine whether it is going to continue moving away at the same speed, slow down but continue moving away, or slow down and reverse direction.

... and it's all going to continue moving away.

 

[rootone]

We can't say with certainty what will have happened to a distant galaxy which we are seeing as it was billions of years ago.
There could be unforeseeable events due to gravitational interactions with other galaxies.
However we can make reasonable predictions from models and rule out some possibilities, such as that galaxy spontaneously disintegrates.
It's the same in principal as a weather forecast; we can say that given the present data rain can be expected tomorrow and there is no chance at all of frost.