How Do We Distinguish Red Shift Changes Over Time in Distant Objects?

AI Thread Summary
The discussion centers on understanding how redshift measurements help us infer the movement and distance of distant celestial objects. It highlights that redshift indicates an object's speed relative to us, with further objects receding faster, allowing us to observe the universe's past. The conversation also addresses how the universe's expansion rate has changed over time, with evidence suggesting that it has recently accelerated. Participants emphasize that redshift measurements alone are insufficient; they must be considered alongside other factors, such as distance and Einstein's equations, to build a comprehensive understanding of cosmic expansion. Overall, the dialogue underscores the complexity of interpreting redshift in the context of the universe's evolution.
kinman
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I have always been puzzled by how we see an object from the past and can tell what it is doing now if it is billions of light years away.

Given that;

1. the shift in the measurement of the light spectrum indicates the speed at which an object is retreating or getting closer.

2. that the furthest objects are retreating quicker than the nearest.

3. that when we look far away we are looking into the past.

Q1> How do we distinguish that an observable shift in the light spectrum is greater or less now than at any other time in a distant object's time line?

Q2> If the universe expanded quicker in the early stages of its existence and then progressively slower, would we not be observing the light that is reaching us now from that period as red shifted more than from objects closer to us?
 
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kinman said:
Q1> How do we distinguish that an observable shift in the light spectrum is greater or less now than at any other time in a distant object's time line?

When we measure redshift, we are only measuring a single moment in time. Since the redshift tells us that the object is moving away, we infer that it was closer to us in the past. Since closer objects have less redshift, we infer that the closer something was to us the slower it was receding. Does that make sense?

Q2> If the universe expanded quicker in the early stages of its existence and then progressively slower, would we not be observing the light that is reaching us now from that period as red shifted more than from objects closer to us?

Actually, I believe something like this has happened. Our measurements of supernova redshift has told us that the expansion, which WAS slowing down, has started to accelerate. I think that when we look out we can cut the universe into "slices" of time the further away they are. IE looking at an object 1 billion years in the past is x distance away. (It is not 1 billion light years away, but far more due to expansion) This let's us observe the redshift and look for differences between all these slices, which let's us see how the universe was at different periods in the past. I believe this is how we discovered that the expansion was accelerating.
 
Drakkith said:
This let's us observe the redshift and look for differences between all these slices, which let's us see how the universe was at different periods in the past. I believe this is how we discovered that the expansion was accelerating.

Yep, that's what I've read in several places.
 
Thanks for taking the time to reply Drakkith

I guess my confusion comes from focusing on one single element of the argument for expansion, measurements of red shift. Alone it doesn't make the case. It is only when you add it to other elements that the total argument becomes convincing. For instance, the fact that expansion is proportional to distance and that observation is consistent with expansion solutions that have been derived from Einstein's equations.

You mention:
Our measurements of supernova redshift has told us that the expansion, which WAS slowing down, has started to accelerate.

Can you point me towards a source for my reading please?
 
Thank you
 
https://en.wikipedia.org/wiki/Recombination_(cosmology) Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in context of the calculation of the observable universe radius, where the time integral of the inverse of the scale factor is multiplied by the constant speed of light ##c##.
The formal paper is here. The Rutgers University news has published a story about an image being closely examined at their New Brunswick campus. Here is an excerpt: Computer modeling of the gravitational lens by Keeton and Eid showed that the four visible foreground galaxies causing the gravitational bending couldn’t explain the details of the five-image pattern. Only with the addition of a large, invisible mass, in this case, a dark matter halo, could the model match the observations...
Why was the Hubble constant assumed to be decreasing and slowing down (decelerating) the expansion rate of the Universe, while at the same time Dark Energy is presumably accelerating the expansion? And to thicken the plot. recent news from NASA indicates that the Hubble constant is now increasing. Can you clarify this enigma? Also., if the Hubble constant eventually decreases, why is there a lower limit to its value?
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