Did I miss something?

[ponderer]

I just read a post on a closed thread.

 

It stated that the rate of the universe is expanding, because, distant galaxies and cosmololgical features are receeding faster than local galaxies.

 

This makes no sense to me. It seems to me that being in the past, that they would be moving away faster if the galaxy was expanding faster in the past.

 

I don't get it.

 

My understanding was that the rate of expansion was determined as a relative rate of increase in reference distance, per time period.

 

One would expect that if the rate of expansion was increasing, extactly the opposite would occur, local galaxies would receed faster that middle distance galaxies, but not faster than the most distant cosmological features.

 

What have I got wrong?

[Sisyphus]

That's a bit mixed up. More distant objects receding faster would be a consequence of any expansion at all, not just accelerating expansion. So before worrying about whether the rate of expansion is speeding or slowing down, first just imagine what a constant rate of expansion would look like.

 

If every light year of space increases by x every year, then an object 10 light years away recedes at a rate of 10x per year, an object 20 light years away recedes at 20x per year, etc. Hence, farther away objects are receding faster. Make sense so far?

 

Another thing to consider that might be helpful. It's true that looking at farther away things is looking into the past, in that the events you are seeing happened longer ago. However, also remember that you're still seeing it in the present, and that all that time between then and now, the light has been traveling through space from there to here, while the space it was traveling through was expanding. So light that has been traveling for longer has also had more time to "stretch" and redshift.

[ponderer]

That's a bit mixed up. More distant objects receding faster would be a consequence of any expansion at all, not just accelerating expansion. So before worrying about whether the rate of expansion is speeding or slowing down, first just imagine what a constant rate of expansion would look like.

 

If every light year of space increases by x every year, then an object 10 light years away recedes at a rate of 10x per year, an object 20 light years away recedes at 20x per year, etc. Hence, farther away objects are receding faster. Make sense so far?

 

Another thing to consider that might be helpful. It's true that looking at farther away things is looking into the past, in that the events you are seeing happened longer ago. However, also remember that you're still seeing it in the present, and that all that time between then and now, the light has been traveling through space from there to here, while the space it was traveling through was expanding. So light that has been traveling for longer has also had more time to "stretch" and redshift.

 

Not in the least. The more distant a object, the further back it is in time. If the universe was expanding faster in the past, and has slowed down since, viewing distant galaxies in the past, we will see them receeding faster than nearby galaxies in comparison. The nearby galaxies are in the present, and experiencing a slower growth rate.

 

Seems elimentary to me.

 

All distant galaxies can show us is what happened in the distant past.

[Sisyphus]

Again, don't worry about increasing or decreasing rates. Does it make sense that in an expanding universe, farther away objects are receding faster?

[ponderer]

Again, don't worry about increasing or decreasing rates. Does it make sense that in an expanding universe, farther away objects are receding faster?

 

It's like annual compound interest. Any increased growth rate that exceeds that of the expected compound interest means the growth rate is accelerating.

 

However, if the growth rate has varied over time, say on a parobolic sort of curve, you have to take that into account, as well as the fact that you are slicing through time.

 

Big bang theories all show a very rapid growth rate at the beginning of time, which slowed in growth in a parabolic looking sort of plot of 4D radius vs. time.

 

This curve must be factored in for any observations made of distant cosmological objects.

 

Only once these things have been factored in can any current or recent growth rate be shown to be accelerating through observations of local galaxies.

 

Let me put it this way. When you look billions of light years away, those were the days when galaxies were real galaxies and quasars were real quasars. They were young fresh out of big bang college, and looking to spread their wings. Now galaxies have gotten old and have been weighed down by their seeming black holes of troubles, and just don't seem to get around as much any more.

 

You can't say because you look way back in time at the younger universe sipping around in hot cars, that the 60's are back, because you are driving a gas mizer sedan now.

 

See that doesn't make sense to me.

 

If you want to say the the rate of the universe is accelerating, then all the galaxies in the neighbourhood just got a mid-life crisis, and all went out and got Ferrari's and Harley's. It started with that Andromeda galaxy you know. Then everyone had to have one.

Edited February 4, 2010 by ponderer