Distant light sources today

  • #1
Since more distant objects are older, and the farthest objects we receive light from are billions of years old, or rather, their light is billions of years old, in what way, or how different would the distant universe be, if we could see the light it generates today? For that matter does light change as it travels extreme distances, or as it "ages?"

In other words, can we confidently assume that distant regions have evolved more or less along the lines of closer regions, or is there any reason to believe the speed of light prevents us from seeing distant developments that could be different from what might be expected?
 

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  • #2
Ibix
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Our cosmological models rely on the assumption that everything's the same everywhere. So the short answer is that we assume that all parts of the universe "today" look more or less the same as the regions nearby. We can have no direct evidence.

That assumption fits the data we have very well, though. If you look one way then the exact opposite way, everything looks the same at the same distance - so the question would be why would they diverge at later times where we can't see yet? In our experience, similar things in similar circumstances develop in similar ways.

I don't think there's any evidence of change in travelling light. "Tired light" was an early attempt to explain cosmological redshift, but it fell by the wayside and I don't know much about it.
 
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  • #3
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How much of "everything's the same everywhere" is bare observation vs adjusted measurement? Is there a difference between "everything's the same everywhere" and "everything looks the same at the same distance"?

What I mean is, if the mean density was greater in the past then distant objects in the past sent their light from closer proximity to each other... how is it that progressively more distant and older objects don't show increasing density with distance? In order to observe today that the mean distances between distant old objects compare uniformly with local objects, it seems that the density of those old objects back then would have had to be comparable to our local present density now... they being further apart from each other back then than local objects back then.

So does "everything looks the same" include adjustments or corrections for density changes in time, or is that the bare observation? If it includes adjustments, then those would be accounting for the distant objects being presently further apart now than local objects, but if it is the bare observation, then those distant objects would be further apart now than than local objects.
 
  • #4
Ibix
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How much of "everything's the same everywhere" is bare observation vs adjusted measurement?
Sorry: I should have said we model everything the same everywhere at any given cosmological time. That's always true. The density of the universe at earlier times was higher, though, so the universe looks different the further away we look.

The point I was trying to make was that if you pick a particular redshift and observe objects with that redshift anywhere in the sky they look broadly similar. That's consistent with "everything's the same at a given cosmological time, and everything changes more or less the same as time advances".
 
  • #5
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The idea of tired light has long been discarded.because the prediction of light having a constant speed has been measured and checked countless times.
Red shift explains things simply and elegantly.
 
  • #6
timmdeeg
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Today astronomers found evidence that the universe expands accelerated. About e.g. 7 billion years back they would have found that the universe expands decelerated. So the light received now and then shows some difference.
 
  • #7
On what scale is the expansion of the universe observable? Presumably we are not farther from local objects on the scale of our galaxy. Is the distance between galaxies observably greater over a given time period, and is it possible to say how much? Presumably, there is no center, all points move away from all others expanding evenly?
 
  • #8
Little typo there: presumably we are not becoming farther from local objects on the scale of the galaxy
 
  • #9
kimbyd
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Since more distant objects are older, and the farthest objects we receive light from are billions of years old, or rather, their light is billions of years old, in what way, or how different would the distant universe be, if we could see the light it generates today? For that matter does light change as it travels extreme distances, or as it "ages?"

In other words, can we confidently assume that distant regions have evolved more or less along the lines of closer regions, or is there any reason to believe the speed of light prevents us from seeing distant developments that could be different from what might be expected?
In principle it is possible for distant regions to have evolved differently. In practice, probably not. At least, on large scales.

The fundamental issue is this: on large scales, the universe is incredibly homogeneous. With reference to this post, this means two things:
1) At the same redshift, all of the large-scale parameters, such as the number of galaxies and amount of matter, are close to constant as long as we're looking on scales larger than about 250 million light years or so.
2) Across different redshifts, the results of those parameters are consistent with what would expect. For instance, the density changes with expansion exactly as we would expect, and nearby galaxies really do look like evolved versions of far-away galaxies (e.g. more heavy elements, less star formation). For the parameters we can simulate effectively, they are all consistent with uniform evolution.

That observed homogeneity suggests a question: if different regions of the universe were to evolve differently, what could induce them to do so? You'd really need some kind of difference between two regions for them to evolve differently over time. And all of our observations strongly suggest that there are no such significant differences to draw on (again, as long as you're looking at large scales). Furthermore, if it were possible for them to evolve differently over time, why wouldn't they have done so already?

So yes, ultimately the fact that we can't observe anything outside of that narrow cone into our past means that we can't be certain that things outside that cone are the same. But the the universe would make very, very little sense if they were different.
 
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  • #10
I would be very interested to learn something about the scale at which the expansion of the universe can be measured. Our galaxy is not expanding, correct?
Gravity holds local objects in their current relationships? Is it the space between galaxies that is growing? at what rate? Presumably all points are expanding from each other, in other words there is no center to the expansion?
 
  • #11
Bandersnatch
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I would be very interested to learn something about the scale at which the expansion of the universe can be measured. Our galaxy is not expanding, correct?
Gravity holds local objects in their current relationships? Is it the space between galaxies that is growing? at what rate? Presumably all points are expanding from each other, in other words there is no center to the expansion?
You want to be looking at the scale of galactic clusters and above. Systems smaller (=closer together) than that tend to be in a bound state, so not expanding. There is no sharp delineation, since it depends on the particulars of a system whether it's bound strongly enough or not.

The current rate of expansion is given by the Hubble constant. It's ~68 km/s/Mpc (as per PLANCK mission), which is equivalent to all relevant (i.e. large-scale) distances increasing by 1/144 % per million years.
That rate is changing with time - asymptotically decreasing towards a value of approx 1/173% per million years.

Where all distances are increasing at the same rate, there can be no centre.


As a side note - There has been a number of threads introducing the concepts of expansion, so searching the cosmology section for earlier discussions might prove productive. There's also a few well-written blog posts in the 'Insights' section of the forum.
 
  • #12
I found Brian Powell's Inflationary Misconceptions and the Basics of Cosmological Horizons, Aug. 28, 2015, PF Insights, helped quite a bit to get my head around these ideas. I'm not quite sure how badly it may try the patience of physicists to explain to us boneheads a bit about the meaning of things like the expansion of the universe, nor precisely why they should bother, altho, a work like A Brief History of Time, makes a wonderful attempt. A paragraph or so into the Powell article the mathematics are too thick for me, but the balloon analogy was quite helpful. Sooooo, space itself is expanding, yet an object like our galaxy is not due to its gravity. Obviously, the scales involved are another obstacle to some kind of bare understanding of apparently very basic implications of physics and cosmology.
 
  • #13
phinds
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I found Brian Powell's Inflationary Misconceptions and the Basics of Cosmological Horizons, Aug. 28, 2015, PF Insights, helped quite a bit to get my head around these ideas. I'm not quite sure how badly it may try the patience of physicists to explain to us boneheads a bit about the meaning of things like the expansion of the universe, nor precisely why they should bother, altho, a work like A Brief History of Time, makes a wonderful attempt. A paragraph or so into the Powell article the mathematics are too thick for me, but the balloon analogy was quite helpful. Sooooo, space itself is expanding, yet an object like our galaxy is not due to its gravity.
I recommend the link in my signature, although I think you've got it now.
Obviously, the scales involved are another obstacle to some kind of bare understanding of apparently very basic implications of physics and cosmology.
Not clear what you mean by this but it doesn't sound right.
 
  • #15
kimbyd
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You want to be looking at the scale of galactic clusters and above. Systems smaller (=closer together) than that tend to be in a bound state, so not expanding. There is no sharp delineation, since it depends on the particulars of a system whether it's bound strongly enough or not.

The current rate of expansion is given by the Hubble constant. It's ~68 km/s/Mpc (as per PLANCK mission), which is equivalent to all relevant (i.e. large-scale) distances increasing by 1/144 % per million years.
That rate is changing with time - asymptotically decreasing towards a value of approx 1/173% per million years.

Where all distances are increasing at the same rate, there can be no centre.


As a side note - There has been a number of threads introducing the concepts of expansion, so searching the cosmology section for earlier discussions might prove productive. There's also a few well-written blog posts in the 'Insights' section of the forum.
Another way to ballpark it is to compare the Hubble velocity to typical galaxy velocities. In dense galaxy clusters, galaxy velocities within the cluster can get up to a few thousand km/s. So, a reasonable cutoff at which the vast majority of the redshift would be due to the Hubble flow and not to the motion of the galaxy in its local environment would be around 10,000 km/s.

If we round the Hubble expansion rate to 70km/s/Mpc, then that means that objects at a distance greater than (10,000 km/s) / (70 km/s/Mpc) = ~140 Mpc, or around 470 million light years. This distance corresponds to a redshift of about ##z=0.03##. In most contexts, physicists think that ~80Mpc is good enough, so this is a really conservative cutoff. Anything that is further than this distance will have most of its redshift made up by its recession velocity.

You could still measure the expansion accurately by using more nearby sources, but you'd need a large number of them.
 
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