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About time, size and light speed

  1. Feb 20, 2009 #1
    Yesterday I saw an online news saying that scientists spotted a huge gamma-ray blast 12.2 bln light-years from earth....if this is true then the blast should have happened 12.2 billion years ago when the whole universe were still in quite an early stage according to the modern cosmolgy......then I would suppose at that time the size of the universe would NOT be the same as today, therefore, the distance of that blast spot from the earth at that time should NOT be the same as the distance of that spot from the earth today. This raises a few questions: 1) when the scientists say "the spot is 12.2 billion light-years from the earth" do they mean the current distance or the distance back to 12.2 billion years ago? 2) during the expansion of the universe, does the speed of light change? 3) during the expansion of the universe since the very beginning of the BIG BANG, does time rate changes (getting slow or fast)? 4) if the current distance of that spot from the earth is 12.2 billion light-years, then how to calculate the actual time for the light to have taken to travel to the earth considering the change of the distance during this 12.2 billion years?

    What makes this issue more complicated is that during this 12.2 billion years, not only the size of the universe has changed but the structure of the universe has changed greatly. The earth was not formed yet at 12.2 billion years ago, and that blasted object might not be there today.....

    Last edited: Feb 20, 2009
  2. jcsd
  3. Feb 20, 2009 #2

    The summery is that distances do not reflect time the light took to reach us. This is due to the fact that the Universe is expanding. We can measure distances much farther than the age of the Universe would seem to allow.
    Last edited: Feb 20, 2009
  4. Feb 20, 2009 #3
    Thanks DaleSwanson...I had a quick look at the link you gave (http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN) and I might need some more time to digest....there comes some more questions based on what I read from the link which I would appreciate any reply:

    1) when there was any explosion in the space, I would imagine that the local speed of any exploding part would be much larger than the speed of the moving velocity of the stary object (say, a star), and plus that we might not be sure that any explosion would be isotropic or evenly distributed in direction, plus that the light we observe on our earth was emitted from some exploding part with very high local speed, then how could we be sure that the redshift we could observe from the light is an accurate measure of the distance of the blasted object?

    2) one major difference between the Einstein relativity and the Galileon relativity is that the speed of light does not change with the moving body that emits the light.....therefore, if at time instance t the distance between two space objects is d, then no matter how fast these two objects are moving apart from each other, as long as their relative velocity is much smaller that the speed of light, it would take d/c time for the light from one of them to reach the other one. Therefore, it does not matter how much the universe has expanded during the past 12.2 billion years, when the light was emitted from the blasting spot, the distance between that spot and the spot where the earth was formed later on should be approximately 12.2 billion light-years, isn't it? In case there relative departing velocity is close to the speed of light (very unlikely at almost 2 billion years after the BIG BANG), v, then iy would take less than d/(c-v) for the light to travel between, isn't it?

    3) Time does get slower or faster depends on the density of mass in the vicinity space, right? For example, the time on the ground is a bit different from the time on the top of the highest mountain, right? If this is true, then considering the density of the universe was much much greater than today during the early ages of the universe, why should we assume the time unit never changed during the past 14 billion years of the history of universe?

    4) if the light-year measure does not mean the years the light takes to travel between two objects, then what does 12.2 billion light-years exactly mean? As I know that when we say the distance between the Sun and earth is 12 light-hours, we do mean that it takes 12 hours for the light to travel from the Sun to the earth, if the 12.2 billion light-years does not mean it took 12.2 billion years for the light to reach us and does not mean the current actual distance either, then does it mean anything or not?

    Last edited: Feb 20, 2009
  5. Feb 20, 2009 #4
    Honestly I'm not an expert here, but I will try to answer the best I can, hopefully someone will come along and correct us both.

    1. First the universe can expand faster than the speed of light, as no information is being transmitted this is allowed. While you are right that local speeds will always be faster than the expansion (it doesn't really happen on less than the galactic scale), the total speed measured by redshift relative to us is would be dependent on the combined speed from expansion and the explosion. I see where this could cause problems with measuring, some possible solutions might be to wait until the explosion slows, or measure the different parts of the explosion and average the red/blue shifts.

    2. The light year is the distance light travels in one year (in a vacuum). I think you misunderstood, what I was saying is that just because something is 12.2 billion light years away, doesn't mean that it took the light 12.2 billion years to get here. I know this is confusing, but it is because space itself is expanding. So the GRB is 12.2 billion light years away today, but when it happened it was much closer. Since the space it was traveling through was itself expanding the journey took much longer than it should have. I can't come up with a good analogy because space expanding isn't something we deal with from day to day.

    I'll try to use the standard balloon analogy. Imagine a large balloon, with two dots a foot apart on it. An ant sets out, from dot A to dot B, at a rate of one foot per minute (1 foot = 1 ant minute). You would expect the ant to reach the second dot in a minute. However, the balloon is being inflated, and because of this the surface itself is expanding. All points on the surface are moving apart from each other. After a minute, the ant has traveled one foot. However, the dots are now farther apart than a foot, thus the ant still has some distance to go. How long it actually takes the ant depends on specifics like how fast the two points are moving apart, but for this example it doesn't matter. Let's say that when the ant finally reaches dot B the two dots are 2 feet apart (or 2 ant minutes). The time it took will be longer than 1 minute (dots were 1 ant minute apart at start), but less than 2 minutes (dots are currently 2 ant minutes apart). If you think about it from the ant's perspective it didn't travel 1 ant minute (or a foot), he traveled further than that. But, he also didn't travel 2 ant minutes (2 feet) he traveled less than that. Hopefully this helps.

    3. Gravity does slow down time, however this is only significant in a deep gravity well. For the vast majority of the photon's life it was in intergalactic space where the effects of gravity, while present, were extremely small.

    4. See 2.
  6. Feb 20, 2009 #5
    DaleSwanson: Thanks so much...your kind response is very helpful...I think my main confusion is caused by lack of good understanding of space expansion itself......

    For question 3, I still have a bit concern: during the very beginning of the BIG BANG, I would suppose the density of the universe is much bigger than the density of the earth right now, so the effect on the density of mass on time should be much bigger than the gravity well of earth, isn't it?

  7. Feb 20, 2009 #6


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    Well, yes but the comparison is somewhat meaningless. It was so hot in the moments after the BB that the laws of physics as we know them were not even fixed yet.

    Not only was it far too hot and energetic for matter (protons or electrons) to exist, but it was so hot that the 4 fundamental forces - one of which is gravity - had not separated out yet.

    I guess you could say there was no gravity in the very first moments of the BB. Gravity is actually a consequence of the cooling and expansion after the BB.
  8. Feb 20, 2009 #7
    Thanks DaveC426913:

    I agree with you for the very very beginning when it was very very hot....but I think there should a period (say one minutes or even longer after the BB) when it was not so extremely hot but the density was still terribly high....at that time we already know all the laws, but the density should be still higher than the earth (right?)...why don't we consider the effect of that dense mass on time?

  9. Feb 20, 2009 #8


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    What makes you think we're not? What are you proposing should be different than current theory dictates?
  10. Feb 20, 2009 #9

    It seems to me that it is implied in the link given by DaleSwanson that currently scientists are not considering the change of time unit....that's why I am asking....I got this question only after I read that link......so...it seems that you are saying the time unit is NOT considered unchanged during the Universe expansion, is that true?
    Last edited: Feb 20, 2009
  11. Feb 20, 2009 #10
    There probably was a time when the average density of the universe was high enough to have a noticeable effect on the passage of time. The GRB you mentioned happened at least a couple billions years after the big bang though, so by that time there was plenty of empty space. A GRB comes at the end of a star's life, so this had to be quite a while after the universe began functioning as it we know it. I'm sure that scientists who study the events in the hours after the Big Bang have to compensate for lots of things that don't really apply anymore.
  12. Feb 20, 2009 #11
    Thanks again for your answer which is helpful....the only things left on this issue are: 1) how exactly the magnitude difference between the local exploding speed and the expansion velocity is taken into consideration when determining the distance; 2) I am still not very clear about what exactly the 12.2 billion LY tells anyone if it is neither the time for the light to travel nor the real distance (considering the possiblity that the bursting object might not have co-existed with the earth at all, even if it refers to the distance, it would be some imaginary distance)....It seems that it misses some clear physical meaning here.....

    Last edited: Feb 20, 2009
  13. Feb 20, 2009 #12
    As for knowing the true redshift due solely to expansion goes, I just posted in another thread that I think taking the average from different areas of the explosion would work well. Secondly the measurement 12.2 billion light years tells us that the GRB is 12.2 billion light years away right now. Parsecs are another unit of distance used in astronomy, maybe using them will help clarify by removing the confusing unit of time (1ly = 0.3 parsecs). This GRB is 3.74 billion parsecs away today. The GRB doesn't exist today, but there is something there in it's place, probably a black hole, which is 3.74 billion parsecs away right now.

    At the risk of adding more confusion, if you were to travel to the GRB/black hole at the speed of light today, from Earth. It would take longer than 12.2 billion years to get there, because the space between would further expand during your journey. You could argue that the measurement 12.2 billion light years is of limited value. But, it does describe the current state of the Universe, even if we won't be able to use the measurement in any practical way.
  14. Feb 20, 2009 #13
    Thanks...I think this might clear issue.... by the way, what do you personally think is causing the universe expansion?....is there any clear explanation except for the cosmological constant?...thanks
  15. Feb 20, 2009 #14


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    These are good questions to be asking!
    I answered several of them here:

    The current distance to the GRB object is not 12.2 billion lightyears, as I believe someone in this thread said. The current distance is 24.6 billion lightyears.

    The distance to the object when it exploded was 4.6 billion lightyears.

    The light travel time was 12.2 billion years.

    These are just numbers gotten from the standard cosmo model---the LambdaCDM.
    This involves no exotic assumptions like variable speed of light,
    or changes in the rate that physical processes occur or that "time passes".

    No need to worry about any of that stuff, unless you want to. The standard model is very straightforward.

    They got the redshift by looking at the afterglow. :biggrin: Lots of nice hot atoms to look at the emission/absorption spectral lines of. They found that the redshift was z = 4.35.

    What that means is that distances expanded by a factor of 5.35 while the light was in transit. z+1 is always the factor by which the light's wavelength is expanded and also the factor by which the universe expands during transit.

    Our matter and the distant matter that exploded are essentially not moving (relative to CMB or equivalently to the Hubble flow) so the factor 5.35 shows up as the ratio of distance now divided by distance then.
    now/then = 24.6/4.6 = 5.35 (approximately)

    Not right. Not a good guess. Have to be careful with how the Public Outreach people write up the news. The PR department, the online magazines, journalists etc. Can be very misleading. They often take the light travel time and convert it naively to a pseudo distance. Because the astronomer tell them the light took 12 billion years, they jump to the conclusion that the distance is 12 billion lightyears and report that to the public. A lot of intelligent lay readers get confused by this. I sympathize:frown:
    Last edited: Feb 20, 2009
  16. Feb 20, 2009 #15
    My mistake. I apologize for stating that it was 12.2 ly, I had assumed from the news reports that was the current distance. It seems strange to me that they would use that figure for the distance, instead of just saying it happened 12.2 billion years ago.

    To ronald_dai, most of what I said still applies, just the numbers were wrong.
  17. Feb 20, 2009 #16

    Thanks so much for the clarification and help!

    By the way, what do you think is causing the Space Expansion?

  18. Feb 20, 2009 #17

    Thanks to your very kind response and help as well!
  19. Feb 20, 2009 #18


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    You are very welcome. Good questions! The answers to several basic cosmology questions are most likely emerging from the research area called quantum cosmology.

    If you want to look at the raw technical articles (together with occasional introductory general survey article) here is a keyword search for articles written since 2006 ranked by a measure of importance (how many times scholars cited them).
    http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+and+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

    Classical nonquantum 1915 vintage GR works well for a lot of things. It is a dynamical theory of geometry. It says that you have no right to expect distances to stay the same or triangles to add up to 180 degrees or geometry to be like Euclid thought it was. Geometry evolves as matter moves around and geometry is governed by differential equations.
    Which means that if once distances get started expanding they will keep on. And the rate of expansion can only change gradually. That's how differential equations work. So the universe doesn't need a present cause to make it continue expanding all it needs is to have gotten started.
    Essentially we already knew that in 1915. The puzzles concern less why it continues than how it got started.

    The reason quantum cosmology is interesting is it studies conditions in very early universe where classical nonquantum GR breaks down and doesnt apply. So QC may provide some answers to questions like how did the expansion get started?

    People are running some pretty interesting models of this these days, and also coming up with ideas for testing them by observing and measuring features of the CMB.

    Books are being written about possible ways the expansion could have gotten started.
    You asked what I think. Well first how about you take a glance at the titles that keyword search comes up with. And if you want a brief summary of any article just click on where it says "abstract" under the title and authors. It's a way of getting a quick impression of that field of research. Full text is usually available free for download as well but I'm not talking about spending a lot of time, just kind of scanning the abstracts.
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  20. Feb 21, 2009 #19

    Thanks so much for your kind reply.....I think it would take me a while to digest since I never thought geomtry could make things "move"....I always thought that "moving" is a dynamical thing.....and geometry does not need dynamics to be valid....

    However, I still have some concern your following statement:

    basically what I can see from what you said here is that the GEOMETRY itself is the one of the most BASIC LAW's here, which means no matter what some of the geometrical feature of the universe should NEVER change, and in order to keep this feature unchanged while the matters are moving around, the only answer is to expand....right?.........However, my concern is that the Space Expansion actually happens mostly at empty space where no matter is moving around.....therefore it seems that the space expansion is not really caused by the movement of matter within it............

    Then I have to take the statement of "Space expand because matter is moving around" from a descriptive point of view, meaning that since when matter moving around the vicinity space has to expand so that certain Geometric feature of universe would be preserved, and also because we believe that the feature of the universe should be SAME everywhere, therefore, space should be expanding everywhere............Well, I could accept this kind of view only for the sake of mathematical consistence but not as a dynamical reason.......somehow I feel that there must be some Global or Local dynamic reason to cause the space to expand.........what do think?

  21. Jan 19, 2011 #20
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