B The accelerated expansion of the universe; redshift and time/distance

[Enquerencia]

TL;DR Summary

If the objects in the universe further from us appear to be expanding faster than those closer to us, and the light we see from them is older, then isn't this consistent with a SLOWING expansion?

I am certain that my confusion here rests in a misunderstanding on my part and not in a mistake having been made by countless physical theorists. Nevertheless, I have had a hard time wrapping my head around it. Here is the crux:

We observe that light from distant objects is more redshifted than light from near objects. We also know that this light has traveled vast distances to reach us and therefore shows us the universe *as it was* rather than *as it is.* This indicates to me that the distant objects were moving away from us, faster than nearer objects are currently moving from us, at the time that this light was emitted. This seems more consistent with the idea that the expansion of the universe is slowing, rather than accelerating.

Those distant objects may be gone now for all we can know; the only evidence we have of their existence is, by nature of their distance, millions or even billions of years old. I fail to understand how we can assume that they are *currently* moving faster than the objects nearer to us. What seems to be the case is that objects once moved away from us more quickly than they do now.

I'm thinking my misunderstanding lies in my interpretation of how redshift is used to measure the speed of objects moving away from us. But again, I struggle to comprehend how light that has traveled for millions of years can tell us anything about the current state of the sources of that light.

I appreciate anyone who takes the time to help me understand this.

 

[phinds]

This misconception is explained here quite regularly. I suggest a forum search. Or ... take a look at the "related threads" linked to at the bottom of this page.

 

[PeroK]

We observe that light from distant objects is more redshifted than light from near objects. We also know that this light has traveled vast distances to reach us and therefore shows us the universe *as it was* rather than *as it is.* This indicates to me that the distant objects were moving away from us, faster than nearer objects are currently moving from us, at the time that this light was emitted. This seems more consistent with the idea that the expansion of the universe is slowing, rather than accelerating.

The "expansion" of the universe is a phenomenon that takes place on a cosmological scale where the distance between any two distant objects increases with time. The rate of increase is proportional to the distance between them. Further objects must be receding faster than near objects. That's common to all expansion scenarios. That itself doesn't tell you whether the expansion is accelerating, slowing down or staying the same.

The question is: how has the rate of expansion changed over the lifetime of the universe? To answer this question involves calculations based on all the available data. The initial prediction from the theory of General Relativity was that the rate of expansion should be slowing down. But, the data does not fit that model. Light reaching us that is less than four billion years old is more redshifted than it should be. And, light from longer ago, likewise, shows evidence that duirng its long journey, the rate of expansion began to increase about four billion years ago.

The model that best fits the data is that the expansion slowed for ten billion years or so but then began to increase. That's where the theory of an accelerated expansion comes from. It's directly from the data.

 

[Janus]

Summary:: If the objects in the universe further from us appear to be expanding faster than those closer to us, and the light we see from them is older, then isn't this consistent with a SLOWING expansion?

I am certain that my confusion here rests in a misunderstanding on my part and not in a mistake having been made by countless physical theorists. Nevertheless, I have had a hard time wrapping my head around it. Here is the crux:

We observe that light from distant objects is more redshifted than light from near objects. We also know that this light has traveled vast distances to reach us and therefore shows us the universe *as it was* rather than *as it is.* This indicates to me that the distant objects were moving away from us, faster than nearer objects are currently moving from us, at the time that this light was emitted. This seems more consistent with the idea that the expansion of the universe is slowing, rather than accelerating.

Those distant objects may be gone now for all we can know; the only evidence we have of their existence is, by nature of their distance, millions or even billions of years old. I fail to understand how we can assume that they are *currently* moving faster than the objects nearer to us. What seems to be the case is that objects once moved away from us more quickly than they do now.

I'm thinking my misunderstanding lies in my interpretation of how redshift is used to measure the speed of objects moving away from us. But again, I struggle to comprehend how light that has traveled for millions of years can tell us anything about the current state of the sources of that light.

I appreciate anyone who takes the time to help me understand this.

The thing to realize is the as long as the universe is expanding at all, whether that rate is decreasing with time, staying the same, or increasing with time, you are still going to see an increasing red shift with distance. This is a property of the expansion.
To determine whether the universe's expansion is slowing, remaining constant, or accelerating, you have to look at the exact relationship between distance and red shift.
With a universe expanding at a constant rate, you would measure a direct relationship (double the distance, double the red shift). With the other two cases, the relationship will not be proportional. The Red shift could increase slightly more than double at twice the distance, or slightly less than double.
Depending on which one you find determines whether the expansion is slowing or accelerating.
The initial study which discovered the acceleration was fully expecting to find the opposite( that the expansion slowed over time). What they were hoping to learn was whether or not the rate of slowing was enough to ever bring the expansion to a complete halt or not. The results that indicated an accelerating expansion came as a complete surprise.
The fact that red shift increased with distance was known for decades before this study was done. It was just the first to measure the exact distance to red shift ratio accurately enough to reach any conclusion on how the expansion rate changed over time.