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Why is the light speed delay ignored by cosmologists?

by Peter Watkins
Tags: cosmologists, delay, light, speed
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Peter Watkins
#1
Feb4-09, 01:39 PM
P: 111
By which I mean; if we see a galaxy at a distance of 1 billion light years, then we see that galaxy as, and where, it was 1 billion years ago. This is something that I presume no-one would disagree with. If there should be a galaxy situated at the same distance from us, but in the opposite direction it can be said that 1 billion years ago, these two galaxies were 2 billion light years apart. Equally, two similarly configured galaxies that are each 5 billion light years distant from us, were 10 billion light years apart, 5 billion years ago. The Hubble Space Telescope can now see some 12 billion light years of distance in most, though not quite all, directions. This can only mean that 12 billion years ago the, (visible to us), universe was not a small expanding cluster of newly formed, post radiant matter, but was instead, vast, with a 24 billion light year spread. This is not wishful thinking, day-dreaming or imagination. The evidence can be seen, quite literally, all around us. In short, the further back in time that we look, via distance, the larger the universe becomes.The universe is said to have been little more than a speck 13.7 billion years ago, therefore, if the universe is still expanding, to look back in time should be to see a smaller universe. Why then, is that which can be seen, completely ignored?
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DaveC426913
#2
Feb4-09, 01:50 PM
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No. Wrong from the get-go.

In the intervening billion years since the light left that galaxy, the space between it and us has expanded. Thus, though we see it a billion ly away, it might have only been a half billion ly away back then when the light left.

When we look back as far as possible, we are looking at light that, due to the expansion factor, has taken 13 billion years to reach us, even though when it started out it might have only been 1 billion light years from us.
marcus
#3
Feb4-09, 02:21 PM
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Quote Quote by DaveC426913 View Post

When we look back as far as possible, we are looking at light that, due to the expansion factor, has taken 13 billion light years to reach us, even though when it started out it might have only been 1 billion light years from us.
Absolutely! When we look back as far as possible that means looking at the matter that radiated the CMB now reaching us. That matter, at the time the light started out, was only 42 million light years from us (from the matter that eventually became the solar system.)
So your rough estimate of "only 1 billion" is generous.

dj1972
#4
Feb4-09, 03:11 PM
P: 24
Why is the light speed delay ignored by cosmologists?

Let me see if I get this, as an example, if the light left say 1 billion light years ago, and that light took 13 billion light years to get to us cause of expansion, that tells me were either going much slower then the other galaxy or were going away from that galaxy or space in between us is expanding faster then we are traveling through space which is causing a 12 billion light year spread? So, if were going in that direction roughly at the same speed as the other galaxy but because of a expanding universe its taking us a while to make up the ground before the light gets to us? Right? If that's is right im really scared cause I think I understood that. LOL
marcus
#5
Feb4-09, 04:19 PM
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Quote Quote by dj1972 View Post
Let me see if I get this, as an example, if the light left say 1 billion light years ago, and that light took 13 billion light years to get to us cause of expansion,...

... traveling through space ...
Dj, DaveC may have more to say about this in response to you. I hope he does because there is quite a bit of explaining needed here.

Just as a partial response let me urge you
1. use years as a measure of time and lightyears as a measure of distance.
Don't say "light left 1 billion lightyears ago". That sounds crazy. Say "light left 1 billion years ago", if that is what you mean.
Or, if the light took 13 billion years to get here then it left 13 billion years ago.
Be consistent, don't say things like "13 billion lightyears ago"!

2. don't picture Big Bang cosmology as an explosion, with galaxies moving thru space. The explosion idea is a popular misconception. I have a Scientific American article link in my signature that deals with several popular misconceptions.
http://www.astro.princeton.edu/~aes/...onsBigBang.pdf
Princeton astronomy department keeps a copy at their website because they use it for course reading, and it's more convenient than getting it from the SciAm archives.

3. don't think of galaxies as traveling thru space, as if moving away from a central explosion point. Try looking at the balloon analogy sticky thread. Watch the Ned Wright animation of the balloon expanding. In that analogy space is 2D and all existence is concentrated on a spherical surface. At each point, there are no other directions or dimensions in space, besides the directions lying in the surface at that point. The galaxies are not moving, but you can see photons, drawn as wrigglers, traveling across the surface from one galaxy towards another.

The link to the short movie can be gotten by googling "wright balloon model"
or, if you prefer, here it is
http://www.astro.ucla.edu/~wright/Balloon2.html

Here's a sidebar from the SciAm article. Just 2 or 3 sentences and a some pictures, gets the idea across about distances expanding without galaxies traveling thru space (if anything is expanding call it space, since space just the sum total collection of all the distances)
http://www.sciam.com/media/inline/00...7F0147_p39.gif
dj1972
#6
Feb4-09, 04:59 PM
P: 24
Hey thanks that makes a lot of sense between light years and years. Really don't know why I wrote that I after I just reread it. LOL
So let me get this straight then, one more time, the space between the two galaxies is expanding, therefore the light traveling from that spot to where we are will take longer because of the space between the two is expanding. Interesting, that's not what I learned, they should teach that in high school cause supposedly everything came from one point because gravity couldn't hold it together any more and it exploded. You know big bang. (Never did like High School) LOL Wow, I will take your advice and go have a look, thanks. Either way I knew what I was trying to get at and its still scary because I actually understood it.
DaveC426913
#7
Feb5-09, 09:38 AM
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Quote Quote by dj1972 View Post
Interesting, that's not what I learned, they should teach that in high school
As with much of science in school, they teach highly-simplified versions of things to the general student body. Those who are interested in pursuing it will read on their own and take more in-depth courses. Or seek out online forums to ask questions.
heldervelez
#8
Feb5-09, 10:04 AM
P: 253
The photons have energy, proportional to the frequency.
As, they say, they are reddening as time goes by, then they are loosing energy.
Then someone as to tell what happened to the lost energy. Energy does not vanish.
But, if energy could vanish, we can think that also matter coud be vanishing (*).
E=mc^2 implies a strong correlation (better, equivalence) between both forms of energy.
(*) we do not have evidence of a yes or a no on that hypo.
DaveC426913
#9
Feb5-09, 10:19 AM
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Quote Quote by heldervelez View Post
As, they say, they are reddening as time goes by, then they are loosing energy.
Then someone as to tell what happened to the lost energy. Energy does not vanish.
The energy isn't vanishing, it's just being spread over a wider distance. Locally, the energy may be less, but over its entire journey (which is longer now) it sums up the same.
dj1972
#10
Feb5-09, 10:58 AM
P: 24
So, its safe to assume the way im interpreting this is, the universe didnt come from an explosion or a point from an explosion, it just started from unknown circumstances "through out space". So were not riding the rip tides of the explosion causing the expansion but we are being pushed about by expanding space that is expanding rapidly causing the universe to expand even larger.
So, if everything has to react to this such as us being pushed about, what happens if the space around the galaxy expands? Will this reaction cause the galaxy to expand as well or is there to much gravitational effect within the galaxy to stop the expansion on itself? Thats kind of a neat point, "if" the universe is expanding rapidly or at a high rate of speed, the galaxy would have enough gravitational force to stop itself from being streched apart. But I kinda doubt that.
marcus
#11
Feb5-09, 11:31 AM
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Quote Quote by dj1972 View Post
... So we're not riding the rip tides of [the wrong concept]
...
what happens if the space around the galaxy expands? Will this reaction cause the galaxy to expand as well or is there to much gravitational effect within the galaxy to stop the expansion on itself? .

The latter. Good thinking. A galaxy has plenty enough gravity to resist expansion, and it does!
The current rate of expansion is 1/140 percent every million years.
So without its size being stabilized by its own gravity, our galaxy, in a million years, would increase in size by 1/140 of one percent.
But it will already have adjusted to that.

You shouldn't think of the universe as expanding at some particular speed. Distances expand at a percentage rate. So a very large distance like 14 billion lightyears is expanding by a million lightyears per million years, that is, it is is expanding at the speed of light.
But a small distance like 140,000 lightyears (about the diameter of our galaxy) only expands by 10 lightyears in a million years, so it is expanding at only 1/100,000 of the speed of light.

Gravitating systems like the solar system (or a galaxy) have a certain resilience. The solar system planets are going just fast enough to keep them on track at their current distance from Sol and interaction amongst the planets favors circularized orbits. If you magically pulled each one a little farther out, then they woudn't be going fast enough to stay and would tend to gradually relax back and get re-circularized back where they belonged.
DaveC426913
#12
Feb5-09, 11:45 AM
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Quote Quote by dj1972 View Post
So, its safe to assume the way im interpreting this is, the universe didnt come from an explosion or a point from an explosion, it just started from unknown circumstances "through out space". So were not riding the rip tides of the explosion causing the expansion
No, but it did start out very, very small - much smaller than a proton - and did expand very rapidly.
heldervelez
#13
Feb5-09, 12:59 PM
P: 253
Quote Quote by DaveC426913 View Post
The energy isn't vanishing, it's just being spread over a wider distance. Locally, the energy may be less, but over its entire journey (which is longer now) it sums up the same.
In the framework of standard model, it is known that energy is not conservative (*):
By definition of photon energy, proportional to frequency, we know that Photons, as we measure them now, have less energy then when emitted.
I've never heard of :Space 'eat' energy, and I do not know how to integrate along the path the photons.
In the framework of standard model 'photon energy is vanishing'.

(*) in fact it is not widely known.
dj1972
#14
Feb5-09, 01:22 PM
P: 24
Hmmm, that is interesting.
I may have more questions pertaining to this, I think I hi-jacked a thread so I'll start a new post asking more later.
marcus
#15
Feb5-09, 01:24 PM
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Quote Quote by heldervelez View Post
The photons have energy, proportional to the frequency.
As, they say, they are reddening as time goes by, then they are loosing energy.
Then someone as to tell what happened to the lost energy. Energy does not vanish.
...
This is not a trivial question---I think you realize this. There is no easy pat answer.

How do you know that energy is conserved? Can you prove conservation laws in curved spacetime? When you say "as time goes by", what time coordinate are you using?

It's not all that obvious and there are interesting questions. Conservation laws come about by Noether's theorem from symmetry---e.g. spatial and temporal translation symmetry. But in highly irregular geometry there is no such symmetry. The conservation laws become weaker and more iffy. They require help from unfamiliar extra assumptions.

I think there is something about this at John Baez website or at some Relativity FAQ.

I remember seeing a proof that something like an amoeba could move around in curved space vacuum just by morphing its shape---extending and contracting. This was a technical math journal type paper. The conclusion was surprising, seemed to contradict conservation of momentum---although the amoeba didn't necessarily end up with any momentum it just blobbed over to a new location.
heldervelez
#16
Feb5-09, 05:42 PM
P: 253
I think that this subject on Energy deserves a OP.
Mr. Marcus tanks for pointing Noether's theorem.

Energy has dimensions M ( L / T ) , or Mv .
Also Temperature is a measure of energy (usually on a macroscopic level)
and also (in EM) by Planck equation E = hν (where h is the Planck's constant and ν the frequency) where Planck Constant has dimensions M L T^(-1).

I do not know of any local experiment where energy is not conserved.
Energy is only exchanged, not vanish.
But at a Cosmological scale things could be not so clear.

The dimensions M ( L / T ) is determinant to say what could, or not, happen to energy. If one considers that it could not be conserved along time than, what could be the change, also in time, reflected on M or or L or T ?.
Tricky.

If it was the case that energy is not conserved in a curved-space than problably the quest about Dark Matter (near Galaxys where space is more curved) could possibly have and end.

sidenote:
Only on the last year I found in a FAQ of a web site of some conceituated university the mention to the non conservation of photon energy. But by Planck equation this is out of controversy.
marcus
#17
Feb5-09, 06:24 PM
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Quote Quote by heldervelez View Post
...sidenote:
Only on the last year I found in a FAQ of a web site of some conceituated university the mention to the non conservation of photon energy. But by Planck equation this is out of controversy.
How so? You know that as time goes on a photon loses energy. By Planck equation this means its frequency declines and the associated wavelength increases.

Each of the photons of the CMB has lost approximately 999/1000 of its original energy by the time we detect it.

It is a puzzle by what mechanism this energy could have gone somewhere, but I have not heard any satisfactory explanation.
Emreth
#18
Feb9-09, 03:02 AM
P: 99
Yeah i agree marcus. It would make some sense if the photon energy was actually energy density, then the energy would be conserved, the spreading by redshift,etc.Also that would explain why the energy increased with frequency, because actually the energy density would be increasing. Unfortunately photon energy has the unit of energy.I remember some other threads on where this energy was going after redshifting, i guess the expansion of the universe is sucking up their energy.


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