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

In summary, the conversation discusses the confusion around the concept of the expansion of the universe and how it is measured using red shift. The speaker questions how we can assume that distant objects are currently moving faster than nearer objects, and argues that this seems more consistent with a slowing expansion rather than an accelerating one. However, experts explain that the expansion itself will always result in an increasing red shift with distance, and to determine the true nature of the expansion, one must examine the relationship between distance and red shift.
  • #1
Enquerencia
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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.
 
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  • #2
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.
 
  • #3
Enquerencia said:
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.
 
  • #4
Enquerencia said:
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.
 
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Related to The accelerated expansion of the universe; redshift and time/distance

1. What is the accelerated expansion of the universe?

The accelerated expansion of the universe refers to the observation that the rate at which the universe is expanding is increasing over time. This means that the distance between galaxies is increasing at a faster rate than it has in the past.

2. What is redshift and how does it relate to the accelerated expansion of the universe?

Redshift is a phenomenon in which light from distant objects appears to be shifted towards the red end of the electromagnetic spectrum. This is caused by the expansion of the universe, as the space between the source of light and the observer is stretched, causing the wavelength of light to increase. The accelerated expansion of the universe is responsible for the increasing redshift observed in distant galaxies.

3. How does time and distance play a role in the accelerated expansion of the universe?

As the universe expands, the distance between galaxies increases. This means that it takes longer for light to travel from one galaxy to another, resulting in a longer time for us to observe the light. This is why we see more redshift in distant galaxies, as the light has had more time to be stretched by the expanding universe.

4. What evidence supports the theory of the accelerated expansion of the universe?

One of the main pieces of evidence for the accelerated expansion of the universe is the observation of Type Ia supernovae in distant galaxies. These supernovae have a known brightness, and by measuring their apparent brightness, we can determine their distance. The observations of these supernovae have shown that they are further away than expected, indicating that the expansion of the universe is accelerating.

5. What are the implications of the accelerated expansion of the universe?

The accelerated expansion of the universe has several implications for our understanding of the universe. It suggests that there is a mysterious force, known as dark energy, that is causing the expansion to accelerate. It also has implications for the ultimate fate of the universe, as it suggests that the expansion will continue to accelerate and eventually lead to a "Big Freeze" scenario, in which the universe becomes cold and dark.

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