Four odd (and probably naive) questions that confuse me. 1) I assume that the speed of light limits the size of the observable universe (by 'observable' I mean detectable in principle if not yet if fact), since light from the source has to have had time enough to get here. Is the size of the observable (in principle) universe thus always expanding? 2) When looking at very distant objects we see light that was emitted in the distant past when we were much closer to the object. Is there some mathematical formula relating time elapsed, distance at time of emmission and currently calculated distance? Or is this a misguided question? 3) If the actual universe is expanding faster than our observable universe then would this entail that matter is steadily disappearing from our observable universe? 4) How much faster would c be if the universe wasn't expanding?
Yes. Yes. The radius is believed to be about 43 billion light years now. Yes. For some formulas check out Ned Wright’s Cosmology Tutorial. This tutorial is comprehensive but isn’t in the easiest format. There are tons of other sites; you can try searching Google.com for “accelerating universe” or “expanding universe”. A book I like that is all in laymen’s terms and has the formulas is The Runaway Universe. I suppose, but the entire universe is thought to be expanding at the same rate everywhere. It is thought that regardless of whether the universe is expanding, c remains a constant. For example, the universe is thought to be 14 billion years old or so, so we shouldn’t be able to see objects 43 billion light years away, as light can travel only 14 billion light years since the Big Bang, right? Well the expansion of space gives light a “free ride,” so it travels more than one light year per year. But the light still travels at c within the expanding space. A rough analogy is when you walk within a moving train. Your speed is still 4 miles per hour, say.
@zanket Just a few corrections to your post 1) The radius is 13.7 billion light years. 2) The `event horizon` ( for better words ) recedes away from us at the speed of light. And i would just point out that it is <b>space</b> that is stretching/expanding at a rate of about a few centimetres per light year ( <i>from memory, please correct </i>). 3) As for the speed of light it is thought that it is ` independent` from the size of the universe and had the same speed in the early universe. However there are a few experiments that are in progress to verify that assumption...(<i> hehe, i have my own views on how it is dependant on the fine structure `constant`</i>)
It seems the texts make the cosmic horizon distance (the radius of the observable universe) more tricky than needed. One book I have mentions no distance other than the ~14 billion light years. Yet: which works out to 42 billion light years.
Very useful - thanks. One point - I understand that because things were closer together in the past light will have appeared to have travelled between points faster than they would otherwise (hence a radius of 42 billion light-years rather than 14 billion). However this is not a 'free-ride' for light. In the time it takes a photon to cover a set distance that distance will increase. It therefore seems logical to say that the speed of light is slowed down by expansion since if there was no expansion then adjacent points in space would remain closer together. (Imagine doing 30mph around the surface of a balloon as it was blown up - when it stops expanding you'll be doing 30mph plus a bit.) Is this right? Might light go faster in a non-expanding universe?
Hum, I stand corrected. It`s a interpretation thing , i think... My only problem with that interpretation of that idea is that we cannot receive the information/light from beyond that horizon... So even if the universe is a lot bigger that the observable ( which i think it is) then it really doesn`t influence our bit of the universe... It is really a question of which direction the information is going... For example a galaxy may exist 100 billion billion light years away ...the space/time there should be exactly similar to our `bit` of space/time...or there may be a `boundary condition/wall` just beyond our sight... it doesn`t matter... And i would question if the speed of light was influenced by the expansion of space, would it not just influence the frequency?
I think light effectively (not actually) goes slower in a non-expanding universe. In our expanding universe the light covered 42 billion light years in 13 billion years, an effective average speed of 3 times the speed of light, or 3c. In a non-expanding universe it would of course travel at c. I see what you’re saying with the balloon analogy. It works differently though. Suppose you take a 30-mile trip—as measured at the start of your trip—on the balloon at 30mph. If not inflating the trip takes 1 hour. While inflating the trip still takes 1 hour but meanwhile the original 30 miles has expanded to 60 miles, say. So you effectively did 60mph even though you actually did 30mph. How can I be sure that your trip still takes 1 hour while the balloon is inflating? Think in terms of percentages. At the start of your trip you have 100% of the trip to cover. Because an inflating balloon expands both behind and in front of you, after x% of the trip completed you’re x% complete, whether inflating or not, and x% can be so small as to make inflation negligible. So whether inflating or not, at 30mph it takes you 1 hour to complete a distance of 30 miles as measured at the start of your trip.
Check out that Scientific American article, about this topic. Yes, just the frequency, not the speed.
I presume that light must always travel at c by definition, but I see what you mean. But the distance travelled was not 42 billion miles, nor 13 billion. I can't do the maths but it must be a figure between these two I would have thought. In other words the distance wasn't 42 billion miles from origin to destination at the time the journey started, it became that by the time it arrived. This confuses me a bit. In this case what is the 'real' speed of light? What you say here is true if the light travels even a metre. Wouldn't this mean that light must always be measured going faster than it actually is since the distance it covers is always a bit more than we measured it to be? Or do all things including ourselves and our measuring tools expand at the same rate to compensate? This doesn't seem right to me but I'm working on it. At high rates of expansion the light could have further to go after x% of the trip than it had when it started out. In fact at very high rates of expansion the light would never arrive. (Relativity seems to confuse the issue even more). Blobrana's point about wavelength seems important. Can we assume that the increase in wavelength represents an expansion of photons that correlates to the expansion of space. (Does increase in wavelength correlate to amount of expansion, ie if distance doubles during time of trip does wavelength of light double during the trip?) I'm still not clear whether this really is a difficult issue or whether I'm just missing the point.
The correlation with frequency of light and the expansion of space should be a linear correlation. Although widely accepted, This has yet to be proved, perhaps with more/better observations of supernovas at diffing distances. I understand your point ( i think ) about measuring the true distance that light has travelled a given length ( because the length is constantly changing)...but ,i assume, since the change is a constant (acceleration ?) , it affects everything; so it doesn`t matter ( for all intents and purposes).... Please Register or Log in to view the hidden image! I think what is more influential , on the speed of light, is the <b>density</b> of the universe... In the early universe (just after the super-inflationary phase) the speed should have been slightly faster. Whether this minute variation can be detected from observations ( red-shift/supernova distance markers etc. ) remains to be seen.
I’m still thinking about this. I think it is a difficult issue for the layman. But it’s fun so I’ll get back to this thread after I'm clear on some stuff.
