Understanding the Accelerating Expansion of the Universe

[keepit]

ok, the universe's expansion is accelerating. I assume the galaxies very far from us are moving apart faster than the galaxies close to us and so the galaxies out there are spaced out much more than our nearby galaxies. However, if one was sitting in one of those far out galaxies, one would think that a far out galaxy such as ours would be very spaced out from other galaxies but we know that we are not that spaced out. What's the deal, where am i going wrong here?

 

[Chalnoth]

ok, the universe's expansion is accelerating. I assume the galaxies very far from us are moving apart faster than the galaxies close to us and so the galaxies out there are spaced out much more than our nearby galaxies. However, if one was sitting in one of those far out galaxies, one would think that a far out galaxy such as ours would be very spaced out from other galaxies but we know that we are not that spaced out. What's the deal, where am i going wrong here?

The acceleration of the expansion is over time, not over space. The expansion rate is the same everywhere at any given time. We can see the expansion by looking far away because when we look far away, we are looking into the past.

 

[keepit]

Chalnoth,
Thank you much for very clear explanation.

 

[helios1]

This may have been asked and answered before but I could not find it so here goes: Based on current observations is the universe accelerating its expansion today or are we viewing a past period of expansion? This question struck me as I was viewing a traffic cam of the interstate where cars were definitely accelerating in the view...problem is there was a 90 second delay in the feed so the accelerating cars that I was looking at then were actually slowing down several miles ahead as the road narrows. How is this crude analogy reconciled by our current observations of distant galaxies with respect to time?

 

[Chalnoth]

This may have been asked and answered before but I could not find it so here goes: Based on current observations is the universe accelerating its expansion today or are we viewing a past period of expansion? This question struck me as I was viewing a traffic cam of the interstate where cars were definitely accelerating in the view...problem is there was a 90 second delay in the feed so the accelerating cars that I was looking at then were actually slowing down several miles ahead as the road narrows. How is this crude analogy reconciled by our current observations of distant galaxies with respect to time?

It's accelerating today. The acceleration began a few billion years ago, and has been increasing since (it is expected to limit out at slightly more acceleration than the current value).

 

[narrator]

...it is expected to limit out at slightly more acceleration than the current value

What's behind that expectation?

 

[Chalnoth]

What's behind that expectation?

Basically, the simplest model that explains the observations is the cosmological constant, which is currently at around 70% of the energy density of the observable universe. Other stuff will dilute away, but the density of the cosmological constant will stay the same. Once everything else has diluted away from the expansion, the acceleration will be a constant.

 

[helios1]

I see. So in my previous analogy, constant acceleration is inferred by the cosmological constant which holds that there is no narrowing of the road ahead, braking etc. just a foot on the pedal and unlimited free gas