Time and Motion

Published 16 years, 1 month past

I was reading an article on cosmology, as I am sometimes wont to do, and it brought back to me one of those questions that I’ve had for a while now, concerning the redshifting of light from distant galaxies as it relates to the history and expansion of the universe.

For those of you not familiar with this topic, the general idea here is that when we look at galaxies outside our own, the light they give off is shifted toward the redder end of the electromagnetic spectrum, which means the wavelengths are getting longer.  According to our present understanding of physics, the simplest explanation for this observation that the further away a galaxy is, the faster it is receding from us—thus redshifting the light it gives off, thanks to the Doppler effect.  It turns out that the amount of redshifting is directly and linearly proportional to the distance of the galaxy, a ratio named the Hubble constant in honor of Edwin Hubble, the man who first made this observation.  (He’s also the namesake of the Hubble Space Telescope, of course.)

It seems to me that this explanation  either overlooks or glosses over one kind of important point: we don’t see those galaxies as they are right now.  In fact, we’re seeing them as they were in the past, and the further out we look, the further back in time we’re looking.  If a galaxy is a five million light-years distant, then we see it as it was five million years ago.  Double the distance, and double the amount of time involved, which would seem to mean that greater redshifts are as much a product of how far back in time we’re looking as they are distance.

So why is it that distance is regarded as the primary factor here?  Why don’t we assume that the universe’s expansion is actually slowing down, given that the closer things are (and therefore the more recent they are), the less quickly they’re receding, whereas the really distant (and therefore much, much older) galaxies were receding more quickly back then?

I’ve no doubt this has been explained one way or another by people way smarter than me, but some Googling yielded no decent results—just about everything I came up with challenged the Hubble constant on various and sundry grounds, not all of them sensical (at least to me).  Nothing I found addressed this specifically.  Though I figure the explanation is straightforward enough, I don’t seem to be using the right search terms to find it.  Anyone got any help for me here?

Comments (31)

  1. Don’t quote me on this but could it be that the farther away they are, the faster they must have been accelerating when the light reaching us now was emitted?

  2. That’s an interesting question. I’m not sure if this will exactly answer it, but you may want to check out the podcast Astronomy Cast. They have has several recent shows covering the expansion, size, shape and center of the universe (well, there actually is no center as they explain).

    I’ve listened to most of the episodes, but I must admit I need to re-listen to them – they come up with great analogies to explain very complex theories.

  3. I think it has something to do with the fact that certain elements common in galaxies give off a certain wavelength of light… but that’s just me guessing.

    You could ask Bad Astronomer (although he gets lots of email, so he may take a long time to reply) : http://www.badastronomy.com/. He also addresses the issue here: http://www.badastronomy.com/bablog/2006/08/08/wait-a-sec-how-big-is-the-universe-again/ … read the comments as well.

  4. From reading Stephen Hawking’s the Briefer History of Time (the newer one, as opposed to the one with all the maths in that everyone bought and no-one read), it’s also entirely possible that the universe is contracting.

    The theory being that if the universe is in a contracting phase (?), time travels backward but we would still experience it as though it were expanding because we’d be viewing it in the other direction (if you see what I mean)… so we’d still see redshift as opposed to blueshift.

    I wonder if therefore distance is chosen as the primary ‘measure’ simply because talking about ‘time’ starts to throw up this sort of question and therefore distance is just a convenient yardstick?

    PS if you didn’t follow this, send me an email and I’ll try to find the relevant bit in the book to explain in more detail…

  5. That’s exactly my point, Andrew: if expansion rates were larger further back in time, that seems to imply that expansion rates are slower now, which in turn implies that cosmic expansion has been slowing down throughout cosmic history.

    Which seems just way too obvious an inference, so I’m assuming it’s come up before and been explained. I just couldn’t find any explanations of that type.

  6. Hello Eric.

    The age of the galaxies is factored out of the equation using various methods, but this still leaves a redshift. This is the absolute expansion rate.

    This expansion rate has been confirmed in a variety of ways, but my favourite is the measurement of the energy left over from the Big Bang. As this radiation was present at the beginning of time, it has been stretched the most. Science predicts this radiation would be so stretched – we would now see it in the microwave form. This is what we see when we look at any direction of the sky. Uniform (albeit lumpy) microwave radiation (google Arno Penzias and Robert Wilson). Interestingly – when you turn your TV on and detune it – around 10% of that white noise static is from the Big Bang microwave radiation. Wow!

    As we are looking at old galaxies in the distant past, we are measuring ‘old light’. Perhaps they have all suddenly stopped moving? (we wouldn’t know for 5 billion years!) Maybe the reshift is caused by the light losing energy as it bumps into intervening matter on it’s way to us?

    This is a debate which is known as ‘Tired Light’ or ‘Light Friction’. My problem with this is that it doesn’t explain the blueshift of light (which should also be redshifted if the tired light theory is correct. Google Halton Arp).

    The Big Bang theory is by no means established orthodoxy, notwithstanding it’s almost ubiquitous support from most scientists. There are a few holes, and problems to be filled (as you mentioned) but that’s the great thing about Science… there’s always more to learn!

    Hope this was of some help.


  7. I remember having read somewhere (that I can’t find again right now) that accelerated expansion over time is measured for objects far nearer to us – within a few light-years or so. This should indicate that the accelerated expansion is real.

    The whole issue is of course just theories based on a limited set of observations over a short time-period, so it is still open for questioning, modifications and counter-theories.

    One interesting theory is that the universe is accelerating because it is pulled outwards around the bended space towards its own center, which means it’ll eventually collapse forward and end where they think it started.

  8. Thanks, katy; the comments on that post did contain an explanation. However, I’m not sure it makes any more sense, because it relies on cosmic expansion to produce the redshift. That is, the light is said to have redshifted because space expands and so stretches out the lightwaves because they’re part of spacetime.

    But if cosmic expansion stretches out lightwaves then it should also stretch out observers (which are after all part of spacetime), so why would we perceive any change in those light?

    Maybe I will write Dr. Bad and see if he’s got an answer. Thanks!

  9. Imagine blowing up a balloon using a constant flow of air. The early life balloon appears to grow very fast. The later life balloon much slower. Yet the balloon is always expanding. So the observations, at first glance could fit the theory that Universe is expanding at a constant rate.

    Another thought experiment is to consider the escape velocity of the earth and rockets that are close to it, some rockets get slower and slower and eventually come back. some get slower and slower, but never come back.

    As I understood it whether we are in a universe that will continue expanding forever, or eventually start contracting was still an open question.

  10. According to Wikipedia, the Cosmological red shift has more to do w/ the expansion of space between the galaxies than it does w/ their physical velocity. The cosmological red shift is due to the photon being physically stretched by the expanding space and how fast it’s being stretched rather than the velocity of the photon at its emission from the light source. So, since the red shift associated w/ a particular light source is an artifact imparted to its photons from how fast it’s being stretched by the space it was emitted in, it’s not time-dependent in the sense of which you’re thinking of it.

    That is…if I understand Wikipedia and cosmology correctly. Frankly, I’d rather try to decode the intricacies of the W3C box model while intoxicated than try to comprehend cosmology.

  11. From what I remember of the theory, the expansion of spacetime (i.e. the universe) isn’t even. The presence of matter restricts the expansion so that it is primarily the space between matter (ex. observers) that expands.

  12. But if cosmic expansion stretches out lightwaves then it should also stretch out observers (which are after all part of spacetime), so why would we perceive any change in those light?

    First, why would the fact that we’re being stretched change our perception? I don’t follow. Second, let’s assume it WOULD change our perception. We’re being stretched at a different location in space time, and thus at a different rate. So we would still be able to perceive a change in something that is being stretched at a different location in spacetime.

  13. I think Chris gave an excellent explanation, but here’s mine.

    You question is: how do we know we are perceiving recessional velocity and not simply time?

    time = distance x speed of light

    recessional velocity = distance x Hubble parameter

    If you double the distance then it would seem that you double both the time and the recessional velocity. Both seem perfectly linear. They may even seem equal because, for nearby galaxies, we can use the speed of light in place of the Hubble parameter and get a very close estimate.

    But it is not perfectly linear because the Hubble parameter is not a constant. It is dependent on time. For distant galaxies, velocity can’t be determined from distance alone, we have to construct the Hubble parameter by knowing a lot about how the object (or its light) changes with time. (How time affects the parameter is where you employ a theory about how far the universe expanded while the photon was moving through space.)

    Look at those equations again: if you double the distance you double the time, but then that new time also has an effect on the Hubble parameter so that doubling the distance does not exactly double the recessional velocity. You might casually say it is linear but it is not.

  14. @ Bryce

    The light has become stretched over many hundreds of thousands, millions, or even billions of light years’ travel. The expansion of spacetime per unit area is relatively small, but if you have 5 billion light years distance to travel – you will eventually notice the effect.

    That small rate of expansion of spacetime, when applied to an observer – is negligable, and probably unmeasurable.

    It’s like saying “Why don’t I get completely soaked when I take a drop of water from the sea and put it on my head?”

    It’s a question of scale ;-)


  15. 42

  16. Your precise question was answered in Astronomy Cast Episode 27 (which I happen to have listened to this past week…).

    Here’s the question that was asked:
    “How do we know that the redshift we observe in more distant galaxies isn’t simply due to the speed of the galaxies at the time they’re seen? So, to explain, when any explosion goes off, it starts off fast and slows with time. The further back in time, the faster galaxies will be moving at that time. Isn’t the increase in redshift in distant galaxies in fact prove that the Universe is slowing down?”

    You can read the transcript here. Search down for “redshift”.

    Or for even more detail go to Episode 11 regarding Dark Energy.

  17. Eric, there are several factors involved here, which you haven’t picked up in your post. The redshift is only being used as a measure of distance because of a lot of painstaking work that went into establishing the distances in the first place. Hubble is credited with some of the early work on this. There are some five or six different methods for establishing distances to celestial objects, and the scales for which they work best overlap. The overlap gives astronomers a way to establish a chain of measurement indicators that reach out to the very distant galaxies.

    After the distance have been established, one can inquire into possible relationships between distance and redshift. So it’s distance first, redshift second. Of course, if one has good confidence in the relationship, one can then use redshift as a proxy for distance, if one can be pretty sure there are no other causes for a redshift.

    Next, redshift is determined by the shift of specific spectral lines, not just by a general reddening of the light. The only way those lines could change from early times til now is for something in the physics to change – most likely, some of the fundamental constants of nature. Whether or not such constants can change has been a periodic subject for debate and theorizing , but there isn’t currently any evidence or theory that has been widely accepted.

    A redshift could also be observed, if the light from a distant object were absorbed and then re-emitted by an intervening body (i.e., a mass of gas). This could modify the redshift if the state of motion of the absorber were different from that of the emitter. So astronomers tend to inspect their spectra with care to try to rule confounding effects like this (e.g., intervening absorbers generally add other characteristic changes to spectra, which can often be detected).

    This kind of work is amazingly difficult and painstaking, especially to establish the basic phenomena in the first place. But it does seem to be on fairly firm ground in general. I wouldn’t hang my hat on any exact estimates on the degree of expansion or contraction just yet, though.

  18. You are mixing up some things. Galaxies are not in fact receding from us. They are comparatively stationary, but as space expands, they become more distant from us. It is the expansion of space that whisks them away – they”re not actually moving like that.

    So the cosmological red shift is in fact due to the stretching of space.

    Now, that does not mean that objects in expand. As space expands, there is more of it – more room in which objects of the same size can exist. This expansion is driven by something that”s much weaker than gravity (whatever it is), which in turn is unfathomably much weaker than the weak interaction, in turn a lot weaker than electromagnetism, which finally is a good deal weaker than the strong force. (Gravity is considered 10^40 times weaker than the strong force; electromagnetism is 1/137th the strength of the strong force. In actual fact, this scale is a construct of fiction, as the forces cannot really be compared like that. But it”s a useful fiction nonetheless.) For something to expand as a consequence of the expansion of space, it would have to be unbound even gravitationally. Since the electromagnetic force (which is what”s responsible for nearly all structure we see at scales below the size of a planet) is unimaginably much stronger than whatever it is that drives the expansion of space, humans (and houses and tables and trees and rocks and blowfishes) do not expand just because space is expanding. The expansive force may be pulling things apart, but gravity, electromagnetism and the strong force pull them back.

  19. Curious, but if all galaxies are expending, wouldn’t they effectively be more closer together (from edge to edge)? Or are galaxies moving away at the same rate they are expanding? If so, from what core object are they moving away from?

  20. There is not centre to the expansion – if you moved to the edge if the visible universe (about 12-15 billion light years away from where we are right now) you would see the exact same thing – space all round for 12-15 billion light years, and the same expansion and cosmological redshift…

    This is the problem when using the dots on the expanding balloon analogy for cosmological expansion – the surface of a balloon is a 2-dimensional space expanding within a 3-dimensional context… the universe is a 3-dimensional space expanding within an even higher context (not sure how much higher, though).

  21. It actually has more to do with the amount of time the light took to get here. The farther away a galaxy is, the longer (time) the light has to travel, and thus the more time it is exposed to the expanding space between. In other words, it is the fact that the light is traveling through expanding space that it shifts red. The more distance to travel, the more space expansion the light is exposed to, and thus the redder it gets.

  22. Here’s a dilemma for you Eric.

    For redshift to occur and object must be moving faster than the speed of light. If the universe is now in a contracting phase those galaxies that seem very far away are really just around the corner since theoretically the galaxies are moving towards us faster than the light that they emit.

    Hope to speak to you soon before the bang!

  23. I think I’ve seen this problem before on Intergalactic Explorer 6. A workaround: you have to set overflow:hidden and width:100% (height will work too) to every galaxy with floating elements inside its gravitational field. This will auto-clear the galaxy.

    The cleanest way to apply this hack is by using the Universal selector hack (* html).

    The question is… the universe… hasLayout?

  24. Referencing what Chris (comment #6) wrote, we are really all in the dark so to speak. The light reaching us now is old, and increasingly out of date the further we look out. Unless we (science) find a way to observe the universe by something faster than light (instantaneous, really) we will never be able to form a complete and current picture of the cosmos.

    I think your observation is very interesting and not one I had really thought of. I had posted not too long ago a topic regarding red shift which perhaps you may find interesting:


  25. I would email this specific question to Dr. Pamela Gay at Astronomy Cast, assuming the podcast referenced does not explain it. That podcast is amazing, although, I do have to re-listen to some to get it to soak in.

    Unless I missed something, light over time can theoretically be impacted by black holes it passes by, which may distort the wavelength. One of the things I wonder when thinking about light moving toward us is what has it passed that may have impacted the perceived time, the actual wavelength, and the color shift accordingly. Can a black hole bend light to another spectrum?

  26. @Maniquí

    That is not quite true in Intergalactic Explorer 6 since overflow:hidden does not trigger hasLayout. This only happens in Intergalactic Explorer 7 which is good since the Universal selector hack * html.doesn’t work unless quirks is triggered. We need to add a adjacent sibling selector to the equation. Maybe Intergalactic Explorer 8 can really solve this equation since it delves deep into these Atoms and analyzes the quirks ops quarks behavior. We can no longer solve the equation by hasLayout since is no longer present.

    According to the laws of position:relativity or is that special relativity which give us the equation E = mc2. Appearing on this page is this.

    Conservation of energy implies that in any reaction a decrease of the sum of the masses of particles must be accompanied by an increase in kinetic energies of the particles after the reaction.

    This means that for an observer who is position:fixed any object that reaches the speed of light will have zero mass but infinite energy. In theory when the Big Bang happened mass was expelled form a singular point of no mass and infinite energy in all directions. Once matter begin to slow the energy would decrease but the mass will increase. This in theory would indicate that the expansion of the universe is decreasing or decelerating and must reach a point of time when expansion stops and contraction begins.

  27. Eric, if I understand your question correctly, you’re asking if we’re seeing, not the current speed of distant objects, but their speed aeons ago, how do we know that their current speed is greater than closer objects?, well, from my reading, the ‘greater speed farther away’ is simply the most common alternative in our current cosmological understanding (which does not, as far as I know, include elephants standing on giant turtles)

    Hawkings makes it clear that the expansion is indeed slowing. Seems to make it more logical that objects farther away from us are receding at the same speed as objects closer to us, but we haven’t measured that yet ’cause we don’t have anything fast enough to measure it with.

    Or, as Paul suggests, the answer may just be 42.

  28. There is a good explanation of what is happening at Scientific American Online .

    Includes why Woody Allen’s mother was right. And why we can see light from galaxies that are moving faster than the speed of light.

  29. What bothers me with redshift lies in the very definition of light. It is composed of particles with a wave like behaviour. Wouldn’t anything traveling distances so great lose energy to surrounding gravitational fields, interfering particle collisions, black holes, white dwarfs and what have you. Funny how Physics can make abstraction of incremental factors to fit a not so perfect model. We think of gravity in terms of Newton and classical mechanics here locally, we apply Maxwell and Einstein to space. What if there were still missing information? What’s with the 3K background? Don’t blackholes contradict E=MC2… Some interesting alternate readings on cosmology. http://redshift.vif.com/Apeiron_Home.htm

  30. In an article (28 april 2008) in New Scientist they write:

    Once upon a time, time was different. Supernova explosions in the early universe appear to age more slowly than today’s supernovae, as if time itself was running slower back then, according to a recent series of astronomical observations. This cosmic time warp is exactly what should be produced by the expansion of the universe, confirming conventional big bang theory.

    The article seems to answer your question.

    We cannot calculate the way in which time passes for the observer who is moving. We can calculate only the way in which the passage of time is perceived (seen) by him.

    Light doesn”t travel at a constant speed in relation to time, spectrum waves have different speeds over long distances. Red waves travel at a different speed to violet. Pure white light has to be split into the 6 colors or the non visual spectrum, radio (yellow), microwave (orange), infrared, ultra violet, x ray (blue) and Gamma (green)

    I Think 0 time is coming very soon, and we will start to go into reverse. Maybe the Maya were correct.

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