• flyguy posted an update 4 months, 3 weeks ago

    Where is the center of the Universe? Reading a copy of Focus magazine, it says…

    “As the Universe may not have a physical edge, there is no sense in the idea of an ‘absolute’ position. Hence, it is meaningless to think of the ‘centre’ of the Universe; something of infinite extent has no ‘centre’ as the point at which it began is also meaningless. The Big Bang happened everywhere at once and the Universe has been expanding ever since. Every point can be regarded as being the ‘centre’ of this expansion. So, the centre of the Universe is nowhere, and everywhere!”

    A couple of points:

    How can something infinite expand?
    How can it be that something might have a physical edge?

    It doesn’t sound to me like anyone knows what’s going on.

    It’s discombobulating.

    • Think about where is the center of the surface of a sphere.

    • For a simple 2D model draw some dots on a party balloon and inflate. All dots will increase their distance from each other. Now imagine that the balloon was initially so small that all dots overlapped.

      Also, popular science is often sloppy in distinguishing between “infinite in size” and “having no boundary”, these are two independent properties: The surface of the balloon has no boundary (you can draw a line going around it without interrupting) but is clearly finite in size. Pop the balloon, and you end up with a plane that is still 2D but does have an edge.

      • At the risk of sounding like a dullard (and I appreciate it might not be possible to explain all aspects of physics to non scientists such as myself) – how do we know that the Universe isn’t shaped like a rubber glove?

    • The center of the knowable universe is centered on the observer, no matter where they are. The knowable universe is limited by the fact that the universe is expanding (in all directions, from all observers as best we can tell) – faster than light speed (note that’s space expanding faster than light speed, NOT that galaxies are moving in space faster than light speed, which is not possible).

      This means that beyond a a certain distance, galaxies are receding from us too fast for their light to get to us so they are not in our knowable universe, but for someone else their knowable universe would be different

      • Not at all sure about this, if the universe began with the big bang, and nothing can travel faster than the speed of light, the suggestion that galaxies are receding to quickly for their light to reach us to me implies that they are traveling faster than light…

        • No, it implies that space is expanding faster than light speed. Nothing is moving in that space faster than light speed. Bit of a mind bender, really.

        • “if the universe began with the big bang, and nothing can travel faster than the speed of light, the suggestion that galaxies are receeding to quickly for their light to reach us to me implies that they are traveling faster than light…”

          They are traveling faster than light away from us in the sense that (increase in separation)/(increase in time) gives a value greater than the speed of light.

          But, that’s not a problem. The statement “nothing can travel faster than light” is more pedantically stated as “no thing can travel from one location in space to another location in space at faster than light”.

          In the above scenario the galaxies are not moving from one region of space to another, they are stationary within their local bit of space, but the space between them is expanding.

    • “How can something infinite expand?”

      Even if the universe is infinite*, there is no reason why it should not be expanding – all points in space would just be moving apart.

      If it is infinite, then it must always have been infinite, since something cannot expand from nothing to infinity in a finite time. So it must have been infinite at the time of the big bang, so the big bang would have happened at all points in infinite space simultaneously – which seems a bit weird but I believe there is nothing known which rules this possibility out.

    • “How can it be that something might have a physical edge?”

      Basically, in the case of the universe, because we can’t just nip over there and have a look. We have to come up with mathematical models for how the whole operation might work, and then see whether a) they accurately describe the stuff that we _can_ see and b) whether the universe that they predict has an edge and ultimately c) whether we can prove that any reasonable model that does a) properly will give the same answer for b). But given that we’re still working on a), c) is probably some way off.

    • I just can’t accept the universe is infinite, or what I should say is that my limited understanding of the evidence for it is not enough to convince me it’s true.

      By infinite, I mean containing a truly never ending number of things whether that be atoms, stars or galaxies however thinly spread they might be. The implications would be that everything physically possible would exist right now and there would be infinite copies of it all as well, including an unlimited number of atom perfect copies of earth and everyone on it as well as every possible variation. It’s just too much to comprehend for me at this stage.

      • Why are you unconvinced about this?

        • I know it’s not very scientific but it just seems too mind blindingly far out to be true. If eventually we find indisputable proof that it truly is the case then okay but right now I think you need some pretty extraordinary proof for such extraordinary claims.

          Odds or rarity would no longer have any meaning, if the chance of it happening is more than absolutely zero then it is happening all the time and there are an infinite number of them. You’d get things like near god like beings (within the bounds of physics) and freak quantum things happening such as super powered brains spontaneously popping into existence and the like. In fact everything that could ever be will happen constantly.

      • I have more trouble getting my head round a finite universe than an infinite one!

        • I’m not a scientist, but I thought this idea of a centre of the universe, or of some kind of possibly infinite ‘body’, was something we were meant to have got out of our heads, post-Einstein (and, at the other extreme, post quantum physics, for that matter). Because it still makes us think of the universe as some kind of huge, expanding Newtonian ping-pong ball. From a philosophical point of view, it’s a very odd question too, because it has no conceivable interest beyond the totally theoretic. With the obvious concomitant mistake that the ‘centre’ has some kind of special importance, and that everything revolves around it (as in the Ptolemaic and Copernican way of seeing things). I suspect, when applied to ‘the universe’ (whatever we quite mean by that), it makes a lot less sense than even something like the North Pole, which is a famously disappointing place to visit. In terms of vacuity, it’s probably on a par with the medieval quest to find ‘the number of angels you can get on a pin-head’.

        • There are privileged positions on the number line. If we consider multiplication for example, 0 is the only number that gives you the same result no matter what you multiply it by, and 1 is the only number that always gives you back the other number. -83.24 whilst I’m sure interesting doesn’t as far as I’m aware fulfill any of these special functions.

          Re the universe being infinite, my understanding (particle physicist not cosmologist, so I defer to more appropriately learned colleagues if they come and contradict me) is that it’s not infinite, it’s been expanding from a size of effectively zero at finite rate for a finite time, so it can’t be infinite. However, it does extend out past the distance we can see because at some points in the history of the universe the rate of expansion was faster than the speed of light (look up inflation for more details). Because we can’t see past this barrier we don’t really know whether there’s an edge or whether it just wraps around to the other side. There are consequences of the universe having some particular shapes, but my understanding was that we weren’t currently able to disambiguate between thet with available data/models.

          • “Re the universe being infinite, my understanding […] is that it’s not infinite, …”

            Well we don’t really know, since we can’t see further than the observable horizon. The default model assumes that it is infinite (but that just means “large compared to the observable horizon”).

            ” … it’s been expanding from a size of effectively zero at finite speed for a finite time, so it can’t be infinite.”

            Not really. We don’t have physics that would apply at time=0, size=0, since we know that currently known physics breaks down at the Planck scale and we don’t have working quantum-gravity theories. The default model would thus assume an infinite extent of stuff round about the Planck time.

            “However, it does extend out past the distance we can see because at some points in the history of the universe the rate of expansion was faster than the speed of light (look up inflation for more details).”

            Expansion always obeys Hubble’s law (expansion velocity being linear with distance), so at *all* times we can find regions expanding faster than the speed of light away from us (by making them a long way away), and at all times sufficiently near regions will not be expanding faster than light away from us.

            • “Well we don’t really know, since we can’t see further than the observable horizon. The default model assumes that it is infinite (but that just means “large compared to the observable horizon”).”

              Exactly, just like no one in particle physics believes that the results of the Standard Model of particle physics will hold up to infinite energy scales, I presume no one in cosmology believes that the Standard Model of the universe actually works perfectly outside the region we have data.

              “Not really. We don’t have physics that would apply at time=0, size=0, since we know that currently known physics breaks down at the Planck scale and we don’t have working quantum-gravity theories. The default model would thus assume an infinite extent of stuff round about the Planck time.”

              Yes, you’re right I was being imprecise when I said “effectively zero”. Starting to model at some epsilon size and epsilon time (often as you say the Planck scale) and propagating using the laws we do have would be a more precise.

              “Expansion always obeys Hubble’s law (expansion velocity being linear with distance), so at *all* times we can find regions expanding faster than the speed of light away from us (by making them a long way away), and at all times sufficiently near regions will not be expanding faster than light away from us.”

              Whether that’s actually physical or a model artifact that only occurs outside the causally connected bit depends on the scales involved doesn’t it? Is the point where it happens inside or outside the visible universe with our current Hubble constant measurements?

    • “As the Universe may not have a physical edge, there is no sense in the idea of an ‘absolute’ position. Hence, it is meaningless to think of the ‘centre’ of the Universe; something of infinite extent has no ‘centre’ as the point at which it began is also meaningless.”

      That seems a bit casual. Take the integer number line, it is infinite in extent but there are absolute positions and it does have a centre at ‘0’.

      • I’ve arbitrarily decided that the centre of my number line is at -83.24. But you stick with 0 if you like. I actually think it has no centre.