Q: How far away is the edge of the universe?

Physicist: If you ever hear a physicist talking about “the edge of the universe”, what they probably mean is “the edge of the visible universe”.  The oldest light (from the most distant sources) is around 15 billion years old.  Through a detailed and very careful study of cosmic inflation we can estimate that those sources should now be about 45 billion light years away.  So if you define the size of the visible universe as the present physical distance (in terms of the “co-moving coordinates” which are stationary with respect to the cosmic microwave background) to the farthest things we can see, then the edge of the visible universe is 45 billion light years away (give or take).  However, that “edge” doesn’t mean a lot.  It’s essentially a horizon, in that it only determines how far you can see.

Of course, if you wanted to know “how far can we see?” you would have asked that.  The picture of the universe that most people have is of a universe enclosed in some kind of bubble.  That is, the picture that most people have is of a universe that has an edge.  However, there are some big problems with assuming that there’s a boundary out there.

If you decide that space suddenly ends at an edge, then you have to figure out how particles would interact with it.  Obviously they can’t keep going, but bouncing off or stopping both violate conservation of momentum, and disappearing violates conservation of mass/energy.  Moreover, if you say that spacetime has a definite edge at a definite place then you’re messing with relativistic equivalence (all of physics works the same in all positions and velocities).  It may seem easy to just put an asterisk on relativity and say that there’s an exception where the edge of the universe is concerned, but the math isn’t nearly as forgiving.

The nicest theories today suggest that there is no boundary to the universe at all.  This leads to several options:

Three possibilities for a homogeneous (same everywhere), edgeless universe.

1) A negatively curved, infinite universe. This option has been ruled out by a study of the distribution of the Cosmic Microwave Background.

2) A flat (non-curved), infinite universe. The measurements so far (devotees may already know how to do these measurements) show that space is flat, or very very nearly flat.  However, infinite universes make everyone nervous.  An infinite universe will repeat everything in the visible universe an infinite number of times, as well as every possible tiny variation, as well as every vastly different variation.  All philosophy aside, what really bothers physicists is that an infinite (roughly homogeneous) universe will contain an infinite amount of matter and energy.  Also, the big bang (assuming that the Big Bang happened) would have had to happen everywhere at once.  As bad as the mathematical descriptions of the Big Bang traditionally are, an infinitely large Big Bang is much worse.

3) A curved, finite universe. This is the best option.  You can think of the universe as being a 3-dimensional space that is the surface of a 4-dimensional ball, in the same way that the surface of a balloon is a 2-dimensional space wrapped around a 3-dimensional ball.  Of course, this immediately begs the question “what’s inside the ball?”.  Well, keep in mind that what defines a space is how the things inside it relate to each other (the only thing that defines space is rulers).  So even if you turned the “balloon” inside-out you’d still have the same space.  Or, if you’re not a topologist, then remember that there’s nothing outside of space, and the surface of the 4-d sphere is space.  Now, be warned, the “3-d surface of a 4-d ball” description isn’t entirely accurate.  Right of the bat, we don’t live in 3 dimensions, we live in 3+1 dimensions (not “space” but “spacetime”), and the metric for that is a little weird.  Also, when you talk about “the shape of the universe”, you probably mean “the shape of the universe right now”, and sadly there’s no way to universally agree on what “now” means in a universe with any rotating stuff in it.  That being said, the “surface of a sphere” thing is still a good way to talk about the universe.

Since our best measurements show that space is very flat, if the universe has taken the 3rd “curved, finite” path (it probably has), then it must be really really big.  This is for the same reason that you can easily show that a ball is curved, but may have some difficulty showing that the Earth is curved.

Also, to answer the original question: the universe doesn’t have an edge.

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61 Responses to Q: How far away is the edge of the universe?

  1. Eric says:

    Yes it would look different based on different perspectives in the universe … and yes the relationship of the expanding and accelerating universe is also related to the motion of the constituents within … imagine the universe more like an ocean …

  2. earnieP says:

    If every bit of a hologram contains all the information as every other bit, how would the view change to a hologram dweller regardless of which bit his view originated?

  3. Steven Wheeler says:

    Thanks for your reply. I’m afraid I still don’t think I understand correctly though. If the observable universe is different on Alpha Centauri, and I actually travel there and observe the outer edge of the observable universe, I will be seeing things that were previously unobservable to me on Earth because they were travelling away from me at the speed of light. How can that be possible when I have travelled slower than light?

    I realise both this observable universe, and the one on Alpha Centauri, are expanding during my travel time so the answer must have something to do with this. What confuses me is that since the two universes have different centres I don’t see how they can have the same perimeters?

  4. The Physicist The Physicist says:

    @Steven Wheeler
    This is a remarkably subtle question!
    If you move four light years away, you’ll see events on one side of the universe four years earlier and events on the other side four years later. So, in that sense, you’re observable universe will be different. However, if you turned around to rush home to tell everyone what you saw, they’d have already seen it by the time you got home (since you’ll be traveling slower than light).

  5. Steven Wheeler says:

    Thanks – I think it’s starting to make more sense now. You said I’d see things four years earlier if I looked away from home on my new star. Just to clarify does that means I would literally be able to see objects that my partner who stayed on Earth wouldn’t be able to see? I know there weren’t a lot of objects 4 years after the big bang but in principle if I travelled far enough would I begin to see new stars that weren’t visible before – i.e. ones that were outside of the original universe?

    Or is the case that as I am moving slower than light the outer objects I can see will always be the same ones as they are travelling away from me at the speed of light?

    Sorry for the very long question but to simplify it a bit what I’m trying to work out is whether it is in principle possible to see things outside of the (currently) observable universe given enough time or whether it will always be the same things but at a different stages in time.

  6. Eric says:

    …observing water ripples as they expand out across the lake … ripples emanate out from many different sources and places … the ripples interact with each other, peaks and troughs, … your statement involves the concept of a gods eye view in which all these ripples are being viewed from an above perspective. To then portal in to a specific place in the universe a view that perspective … is like viewing a beautiful piece of art , one that changes over time in complex fractal pattern … to zoom back out and look down on it all ripples from various sources , Rivers of Entropy flowing along the gradient and resulting in the concentration of atoms, star formation and the existence of life … zoom back in to another region and view a completely different piece Art , as before growing and scaling with time in fractal pattern. … all dice have rolled, all cards have been drawn, our perception of reality is just enough to allow survival and not so much to confuse and destroy. 10^138 coins spinning on the table , the combination of heads and tails at each moment in time creates a pattern that we perceive as reality of that moment. In fact its important to realize that when we say universe we are really refering to the observable universe … we can’t obverse Dark Matter and yet we see its effects all arounds us … then we should also not limit our thinking and distinguish between the observable universe and that which we can not detect.

    Imagine the environment inside the universe is a positive charge and the ocean around the universe is negative … then perhaps Black holes are tears in the foamy fabric where the vacuum of super universal space draws out the concentrated inner atmosphere of our universe …

  7. earnieP says:

    That was absolutely beautifully written. Excellent!
    You are extremely talented. Thank you so much!

  8. David Medlyn says:

    So now I’m interested in the real edge of the Universe, as it compares to the edge of the visible Universe. And in this, I would love to know the answer to two initial questions:

    (a) At what time (or distance from the Big Bang) is it calculated that the outer rim of the expanding Universe reached light speed?
    (b) Am I correct in assuming that all that matter beyond that point is now invisible to us on Earth?


  10. Ron Williams says:

    We can only speculate on the “size” of the universe since our ability to view it is limited and because it is constantly expanding. It is a comfort to me to consider that the universe that we occupy is infinite or alternatively that it is only one of an infinite number of universes. I am satisfied with the idea that our limitation to “view” the “limits” of our universe also limits our ability to conceive of whether is has an “edge” or that it is immeasurably large and possibly getting larger. Also I am satisfied that what we can now observe is NOT the remnants of something that started a calculable number of billions of years ago from a “point” in “nothing” for some inconceivable reason and then progressed from a lot of excited, disassociated atomic particles into hydrogen atoms and on to all the other elements that we now know about. The components of the observable universe are still being formed (the Pillars of Creation) and are not merely decaying into “black holes” (whatever they are) and supernovae, etc. It is my contention that it is beyond human ability to “know” the answer to the many questions about the universe, either observable or infinite, because we don’t know enough about what constitutes the “entire” universe and whether it was formed or has always existed and is indeed infinite. Our understanding and tools with which to calculate its dimensions and construction are too inadequate.

  11. Ranjit singh says:

    Are black holes sucking out all the visible matter out of this universe into the super universe?

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