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# Astrophysics/Hubbles law and red shift

Question
QUESTION: I was wondering about redshift and the Galaxy farthest away traveling fastest.
My Question is How does Time  Inter into the equation? A galaxy 13 million light years away would be traveling faster because it would of been closer to the time of big bang.The red shift that we see would of been 13 million years ago.
Mike

ANSWER: The universe is expanding at an accelerating pace, meaning it's moving away from its other parts faster and faster as time goes on.  That makes the Doppler shift in wavelength further and further into the red as time goes on because light takes time to reach us from the last location and we're moving away from it faster the further away it came from.

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QUESTION: What I have a hard time wrapping my brain around is that the red shift that we observe is from light that is 13.7  billion years old and would not be actually what is going on at the edge of the observable universe . Just as we would not know if a distant star went super nova we wouldn't actually find out until that light reached us. Just a the deep field images show young galixey formation, all that time ago , Is not that red shift just showing the movement soon after the Big Bang when everything was moving faster?

You're making the classic mistake when you say that light that is 13.7 billion years old "would not" represent what "is" going on at the edge of the observable universe.  You have to get past notions like that to understand relativity and cosmology.  Of course it doesn't represent our notion of time, because our instinctive notion of time does not account for the speed of light.  Since light from great distances won't reach us for a long period of time, what "is" going on to us is defined by the light that is reaching us now.  It is not defined by light yet to reach us.  Our notions of time are determined by the fact that our everyday world is close enough by that the speed of light is practically infinite.  It's a difficult concept to get used to.  Read Einstein's original book "Relativity."  He's still the master at explaining it.  It's a cheap book, and probably in your local library somewhere.  Redshifts are determined by both the relative velocity of ourselves and the object that emitted the light and by the expansion of space itself between us and that light.  That last part is complicated, but mostly what we see when we see young stars are stars that emitted light at a high velocity, yes, from early in the universe...but we're also accelerating away from their original position.  That means the redshift is partly due to our velocity in space now.

Astrophysics

Volunteer

#### Dr. Stephen O. Nelson

##### Expertise

Fusion, solar flares, cosmic rays, radiation in space, and stellar physics questions. Generally, nuclear-related astrophysics, but I can usually point you in the right direction if it's not nuclear-related or if it's nuclear but not astrophysics.

##### Experience

Just moved from being a physics professor at the University of Texas of the Permian Basin into government work. Doctoral dissertation was on a reaction in CNO-cycle fusion, worked in gamma-ray astronomy in the space science division of the naval research laboratory in the high-energy space environment branch.

Organizations
Government work as a physical scientist with a nuclear focus.

Education/Credentials
Ph.D. in physics, research was on nuclear fusion reactions important in stellar fusion, further work on space telescope technology.