Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I How fast is the Universe expanding?

  1. Aug 10, 2017 #1
    hello there,

    I am totally into all astrophysics.

    so I want to know, how fast is the known universe at the most outer edge moving? or in other words, how fast is the most outer edge expanding away from earth. - and whats the rate of acceleration?
    I mean even if, as I understand it, the space between galaxies is increasing, there should be a know parameter to this effect by now.

    is there?
    I wonder why no one is ever mentioning the speed. again lets say the relative to earth (or milkyway) speed. or .. the rate of expansion - generated space over time.

    courious what comes now..
  2. jcsd
  3. Aug 10, 2017 #2


    User Avatar
    Gold Member

    The edge of the OU is receding from us at about 3c. No one mentions this as a "speed" because it is NOT a speed in the normal sense, as there is no proper motion involved, it's just a change in geometry.
  4. Aug 10, 2017 #3
    thanks. i understand. so u say 3 times lightspeed, right? thats kinda scary but ok.
    and how do u grab the acceleration of that movement (which I know isnt actual movement in space) in numbers?
    or in other words, at what rate is 3c getting a bigger number ?
  5. Aug 10, 2017 #4


    User Avatar
    Gold Member

    As I recall, recession distances change by something like 1/144th of a percent per million years.

    To quote from my Insights article:

    Expansion, even with acceleration, is so staggeringly slow on small scales that it might as well not be happening. Over cosmological distances, it has a huge effect, but here’s my favorite analogy to show local effect. Even though the universe is expanding, it’s still going to be hard to find a parking place. This is just a simple-minded way of thinking about the local effects of the expansion. If you could go out into intergalactic space and magically draw a set of parking place lines, it would be about TWENTY BILLION YEARS before they had moved far enough apart to let you park a second car. Now, I’m willing to circle the block a couple of times to get a parking place, but twenty billion years is just too much. I’d be late for the movie.

  6. Aug 13, 2017 #5
    1. thanks but I must admit, I dont quite understand the parking place analogy knowing now that spacetime expandes with a lets say 'virtual speed' of 3c or more.
    maybe I am missing something that early in the morning.

    2. the muffin-analogy extends your balloon-analogy, doesnt it.

    3. besides I am wondering how time behaves in empty space. do we know? when I hear scientists talk about flat spacetime, do I understand that right by thinking even in absolute vaccuum time will not speed up. is it prooven that like mass/matter slows down time, vaccuum doesnt accellerate it? is that whats meant by flat spacetime? and how can we be sure? how can we know whats happening to (space)time when there is really nothing around, like in the big voids in between galaxy structures.
    I mean there is lots of polarity in our universe as we know it. even if some examples are greatly based on subjective human experience. for example, +/-, light/dark, hot/cold, up/down, etc..
    isnt it obvious that if there is something (matter) that curves spacetime lets say down (like in simulations), there must be circumstances or effects or maybe things that do the exact opposite.

    4. one last thing. I see you are a friend of analogies, so am I.
    I want to share this one. to better imagine the fact that visible matter and energy is only 5% of all energy and matter in the universe. I like to think about the ocean waves surface where the foam is sitting. visible stuff is the foam. everything else, the liquid volume of the ocean, the waves traveling through it, all the kintetic energy, is the rest we call dark these days. some waves break and create foam. thats it. thats how the stuff was made. and it may be that the foam on the surface is slowly vanishing back into the water, dissapearing, being water again until the next wave breaks.
    I think we must consider something much bigger than we can ever imagine.

    with regards,
    Last edited by a moderator: Aug 13, 2017
  7. Aug 13, 2017 #6


    User Avatar
    Gold Member

    Yes, you haven't thought it through. Suppose you had a million mile long rod that was expanding at a rate of 1% per year. In one year, the ends would recede from each other by 10,000 miles. Now that's a hefty amount, but what you really care about is section right next to you that is 10 inches long. IT only expands by .1" and it will take 100 years to double to 20" (forgetting for the moment about compound interest)

    poetic but not at all scientific or helpful. You need to study the basics before making such a sweeping statement. That is, it is not useful to comment on what we don't know until you have some idea what we DO know.
    Last edited: Aug 13, 2017
  8. Aug 13, 2017 #7
    yes u are right I got carried away a little too much.
    however, thanks for the answers. I apologize. all I seek is understanding.
    u didnt comment on the vaccuum tho. and my flatness question.
  9. Aug 13, 2017 #8


    User Avatar
    Gold Member

    Time behaves exactly the same EVERYWHERE. It ticks away at one second per second. The differences you hear about are relative to other observers and are only meaningful if you have a situation where two objects start out in the same place at the same time and then take different paths through space-time and then meet back up. In that kind of situation, they can easily have experienced a different number of one-second-per-second ticks. For example, traveling fast and then coming back or going deeper into or farther out of a gravity well and then coming back.
    Last edited: Aug 13, 2017
  10. Aug 14, 2017 #9
    well, I heard about relativistic effects in context of the expansion. in terms of, that supernovas at the observable border, which happen to have happened really long ago in fact, short after big bang maybe, but lets just say long ago. and as space expands not only space expands but spacetime does, which means also time expands and expanded since so called bigbang. correct me if I am wrong. so those observed supernovae I heard about, happened to explode at a different timeframe. to me thats 'meaningful' enough. thats in fact the only argument that convinces me of the theory of an accelerated expanding universe. not sure if timeframe is the right word. they seem to be happening faster.

    btw. strange, is there really only one person thinking about this with me?
    I mean why am I only talking to you.. I find this stuff so interesting, I hoped to hear different opinions.
    thanks for the answers tho, 3c is quite a happening I´d say.
    Last edited by a moderator: Aug 14, 2017
  11. Aug 14, 2017 #10


    User Avatar
    Science Advisor

    If you do not know whether they are faster or slower, then it is hard to put much faith in the argument for accelerated expansion that you have not stated.

    Initial upper case on sentences would make your prose more easily read, by the way. The only upper case I see appears to be what auto-correct applied.
  12. Aug 14, 2017 #11


    User Avatar
    Gold Member

    These concepts have been discussed on this forum thousands of times. Your apparent lack of knowledge about them and about the way specific words have specific meanings (and what those meanings are) indicates that this thread would not be a terribly fruitful addition to all of those other discussions.

    For example, there are no "relativistic effects in context of the expansion" because no proper motion is involved. Supernovae could not have happened "short [sic] after big bang" because there weren't even any stars shortly after the big bang (depending of course on what you call "shortly"). Further "as space expands not only space expands but spacetime does" is an incorrect statement. Space-time expands, space does not (things just get farther apart ... Google "metric expansion" or read the article pointed to in my signature).

    I don't even know what "so those observed supernovae I heard about, happened to explode at a different timeframe" means.
    Last edited: Aug 14, 2017
  13. Aug 14, 2017 #12
  14. Aug 14, 2017 #13


    User Avatar

  15. Aug 15, 2017 #14
    @phinds: relativistic effects can be mentioned in context of an expanding universe if they seem to proove it. thats all I said.

    @jbriggs444: I reread the article and corrected the post. its faster. supernovae in early universe happened faster, if we look at it today.

    for now, before it gets too messy or fruitless, I´m out.
  16. Aug 15, 2017 #15


    User Avatar
    Gold Member

    But they don't prove it. As I said, there ARE no relativistic effects in the recession because there is no proper motion.
  17. Aug 16, 2017 #16
    In that case, I am gonna have to shut up for a VERY long time...LOL!!!
  18. Aug 17, 2017 #17
    Oh , it really isn't that hard a study, if you are really interested. Take the links offered to get a start. Try to suspend your judgment regarding inconsistencies with your daily existence. I have been at cosmology and the Core Theory for about three and can understand within limits the conversation here. The universe is well worth studying, and its study can be astonishingly revealing. Good reading. Ron G.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted