Universe expansion velocity question?

[porsche11]

Hello,

I am wondering about the expansion of the universe and how it is expanding faster the farther we look out into space. If the farther we look into space the earlier back in time we see, doesn't this mean we are seeing the very distant galaxies, that are moving away from us faster than closer galaxies, at the velocity they USED to be moving away from us.

In other words, wouldn't the closer galaxies be a better representation at the speed the galaxies are moving away from us because we are not looking so far in the past and that the expansion has actually SLOWED. I'm probably missing a fundamental point here but thanks!!

[EdEarl]

Are you asking if the accuracy of speed measurement is better for nearby than distant observations?

[Archimedes]

and that the expansion has actually SLOWED.

 

The universe is definitely expanding at an increasing rate. If you want mathematical terms, the cosmic scale factor has a positive second derivative. The velocity that we see a galaxy moving away from us is continuously increasing as time goes on.

 

wouldn't the closer galaxies be a better representation at the speed the galaxies are moving away from us

 

Are you asking about our accuracy to measure the speed? (EDIT - I see EdEarl already asked this).

 

-Arch

Edited July 7, 2013 by Archimedes

[porsche11]

Are you asking if the accuracy of speed measurement is better for nearby than distant observations?

No I am not asking about accuracy. I was curious about the fact that we are seeing the very distant galaxies as they were billions of years ago and they are moving away VERY fast....but the closer galaxies are not moving away as fast. Is it possible the very distant galaxies have since slowed down?

[ACG52]

No I am not asking about accuracy. I was curious about the fact that we are seeing the very distant galaxies as they were billions of years ago and they are moving away VERY fast....but the closer galaxies are not moving away as fast. Is it possible the very distant galaxies have since slowed down?

No, because the're not really moving through space at those velocities. The metric expansion of space means that the further something is from us, the more the space between us is stretched (or the more new space is created, whichever you like best), and so the faster the apparent recession rate is.

[EdEarl]

From studies of Type 1a Supernova, the velocity of expansion of space is increasing (accelerating), which will carry distant galaxies with it until they exceed the speed of light and disappear. The cause of this acceleration is attributed to dark energy, of which very little is known. Cosmologists expect that in time all galaxies will disappear, except ones in our local cluster, because of the acceleration of space caused by DE.

[zapatos]

I was curious about the fact that we are seeing the very distant galaxies as they were billions of years ago and they are moving away VERY fast....but the closer galaxies are not moving away as fast. Is it possible the very distant galaxies have since slowed down?

It is important to remember that distant galaxies are not moving away quickly (compared to nearby galaxies), but instead the distance between us and is increasing at a high rate. If you were on that distant galaxy looking toward the Milky Way, you would notice that the galaxies near to you do not appear to be receding rapidly, but those at a great distance, such as the Milky Way, DO appear to be receding a rapidly.

 

Which ones are receding quickly and which ones are receding slowly is completely dependent on your vantage point.

[porsche11]

No, because the're not really moving through space at those velocities. The metric expansion of space means that the further something is from us, the more the space between us is stretched (or the more new space is created, whichever you like best), and so the faster the apparent recession rate is.

 

OK I think I understand that thanks. The only reason we know this is because of red shift correct? Does red shift also rely on speed of light travel too tho? If a galaxy a billion light years away were to immediately stop and start moving towards us, would it take a billion years to see the blue shift? Thanks

It is important to remember that distant galaxies are not moving away quickly (compared to nearby galaxies), but instead the distance between us and is increasing at a high rate. If you were on that distant galaxy looking toward the Milky Way, you would notice that the galaxies near to you do not appear to be receding rapidly, but those at a great distance, such as the Milky Way, DO appear to be receding a rapidly.

 

Which ones are receding quickly and which ones are receding slowly is completely dependent on your vantage point.

Ahhh! Okay that really hit home with me now thanks!

[zapatos]

OK I think I understand that thanks. The only reason we know this is because of red shift correct?

AFAIK, that is correct.

 

Does red shift also rely on speed of light travel too tho?

Yes.

 

If a galaxy a billion light years away were to immediately stop and start moving towards us, would it take a billion years to see the blue shift?

Basically, yes.

 

The reason for the qualifier is because other factors may be in play (as long as we are just speculating). For example, remember that the distance between us and distant galaxies is increasing not because the distant galaxies are moving through space away from us, but because the amount of space between us is increasing.

Let's say a galaxy is one billion light years from us when it begins to shine. Because the space between us is increasing, the light may end up traveling two billion light years to get to us (and of course, it will have taken two billion years).

So (back to the speculation about the sudden change of direction of the distant galaxy from 'away' to 'towards') if its sudden reversal of direction is due to the 'contraction' of space (although I just made that up as the opposite of 'expansion'), the light may have to travel less than one billion light years to get to us, and thus we will see its blue shift in less than one billion years.

[porsche11]

AFAIK, that is correct.

 

Yes.

 

Basically, yes.

 

The reason for the qualifier is because other factors may be in play (as long as we are just speculating). For example, remember that the distance between us and distant galaxies is increasing not because the distant galaxies are moving through space away from us, but because the amount of space between us is increasing.

Let's say a galaxy is one billion light years from us when it begins to shine. Because the space between us is increasing, the light may end up traveling two billion light years to get to us (and of course, it will have taken two billion years).

So (back to the speculation about the sudden change of direction of the distant galaxy from 'away' to 'towards') if its sudden reversal of direction is due to the 'contraction' of space (although I just made that up as the opposite of 'expansion'), the light may have to travel less than one billion light years to get to us, and thus we will see its blue shift in less than one billion years.

 

Awesome explanation I understand now thank you!

Edited July 8, 2013 by porsche11

[Airbrush]

If the farther we look into space the earlier back in time we see, doesn't this mean we are seeing the very distant galaxies, that are moving away from us faster than closer galaxies, at the velocity they USED to be moving away from us.

 

Yes, what we see is the red shift indicating how fast it WAS moving away from us at the moment the light LEFT the distant galaxy a Billion years ago. NOW we believe it is moving away much faster than the red shift told us.

[Iggy]

Hello,

I am wondering about the expansion of the universe and how it is expanding faster the farther we look out into space. If the farther we look into space the earlier back in time we see, doesn't this mean we are seeing the very distant galaxies, that are moving away from us faster than closer galaxies, at the velocity they USED to be moving away from us.

In other words, wouldn't the closer galaxies be a better representation at the speed the galaxies are moving away from us because we are not looking so far in the past and that the expansion has actually SLOWED. I'm probably missing a fundamental point here but thanks!!

The universe decelerated expansion for the first seven billion years or so. Initially the rate of expansion was extremely high. It slowed more and more until the onset of acceleration six and some odd billion years ago. Since then the rate of expansion has increased more and more.

 

To visualize it, imagine a single galaxy that used to be quite close to us. It started out moving away from us very fast, but it was decelerating. If you look at the history of the speed between us and that single galaxy, the speed lessens over the first few billion years. At first it decelerated pretty quickly, like someone applying the brakes to that galaxy quite hard. Over time the rate of deceleration lessened (like someone letting up on the brake pedal) until the rate of deceleration hits zero (seven billion years ago) and that specific galaxy was just coasting away at a constant velocity. The rate of deceleration does not, however, stop at zero. It goes negative. 'Negative deceleration' means acceleration, and for the past few billion years the galaxy has increased its rate of recession more and more -- like someone pressing the gas pedal more and more.

 

Another sticky point is that when cosmologists talk about the rate of expansion (in terms of hubble's law) they are talking about the current change in the current distance to the galaxy as it currently exists. We don't see the distant galaxy as it exists in the present, but we know it is there and we can model how fast it is receding from us, currently.