# Observable Universe question

If we can only see things that traveled 14 billion light years away (I think) or less, then why is there a so called observable universe? If everything started at a singularity, wouldn't we have already seen everything? It's hard to put this question into words. How can something be farther away in light years than the Universe is old? So confusing, sorry if this has been answered. I also do understand inflation, but it just seems like when the universe first cooled and galaxies were formed at that point light from everything would've already reached each other y'know because it was all so close? Unless the singularity was INFINITELY big then I guess I could see that

And again, sorry if parts of my question are wrong etc.

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Orodruin
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You have to take into account that the Universe is expanding. Even if something was very close to us at an early time, it is not certain that it would have reached us by now because if the expansion was fast enough, the distance would have initially increased even if the light was traveling towards us.

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Chronos
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The only point, so to speak, generally agreed upon is the origin of the universe was NOT a singularity in the sense of an infinitesimal point. The term singularity means our mathematical formulas cease to yield sensible results. For example, a vortex of water going down a drain theoretically has a singularity in its throat where the water achieves infinite velocity. That, of course, is obviously not realized in nature. The same applies to the putative cosmic singularity from which the universe arose. We do not know the 'size' of the region from which the universe originated. It may have been infinite. I use the word size parenthetically because that is a misleading way of looking at it.

bapowell
Regarding the singularity: as Chronos says, the singularity associated with the big bang was not a point in space. If anything, it was a *surface* in space. I say "if anything" because the singularity signals a breakdown of the mathematical theory -- it is certainly possible (expected, even) that a more complete theoretical understanding of the origin of the universe will resolve the singularity.

Regarding the size of the observable universe, the universe is larger than it is old times the speed of light because of the expansion, as Orodruin says. Here is a graphic I've put together to hopefully clarify:

The top row shows an expanding shell of light from a central point (say, our position in the universe) for the case of a static universe (the grid doesn't grow as time progresses, from left to right). The size of this shell grows simply as t*c, where t is time and c is the speed of light. The bottom row shows this same expanding shell when the universe expands (notice how the grid does grow as time progresses, from left to right). Since light travels through space, as space expands it carries the light along with it. The growth in size of the light shell due to the expansion alone is given by the dashed yellow line in each frame in the bottom row; the additional growth in going from dashed to solid in each frame is due to the light traveling relative to the expanding space. So the light in the bottom row grows by time t to a size greater than t*c.

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timmdeeg and aboro
Chronos
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The CMB photons we currently observe were emitted at a proper distance 'then' of about 42 million light years. They are just now reaching us after an interval of about 13.7 billion years. Assuming inflation theory is correct, it is entirely possible we could detect 'gravitons' [gravity waves] from the big band emitted when the universe was only milliseconds old and at a proper distance 'then' of virtually zero. There is, however, a horizon beyond which no signal traveling at c can ever reach us. See Davis and Lineweaver; Expanding Confusion, http://arxiv.org/abs/astro-ph/0310808, for a rigorous explanation. Note that this 'horizon' is not located at some point, or proper distance 'then' in space, it is a point in time. For all practical purposes, there is no place in the universe that is observationally inaccessible to us. There are, however, places that have, and will redshift beyond our observational grasp over time - i.e. there are distant galaxies whose photons emitted 'today' will never reach us due to expansion.

Regarding the singularity: as Chronos says, the singularity associated with the big bang was not a point in space. If anything, it was a *surface* in space. I say "if anything" because the singularity signals a breakdown of the mathematical theory -- it is certainly possible (expected, even) that a more complete theoretical understanding of the origin of the universe will resolve the singularity.

Regarding the size of the observable universe, the universe is larger than it is old times the speed of light because of the expansion, as Orodruin says. Here is a graphic I've put together to hopefully clarify:

The top row shows an expanding shell of light from a central point (say, our position in the universe) for the case of a static universe (the grid doesn't grow as time progresses, from left to right). The size of this shell grows simply as t*c, where t is time and c is the speed of light. The bottom row shows this same expanding shell when the universe expands (notice how the grid does grow as time progresses, from left to right). Since light travels through space, as space expands it carries the light along with it. The growth in size of the light shell due to the expansion alone is given by the dashed yellow line in each frame in the bottom row; the additional growth in going from dashed to solid in each frame is due to the light traveling relative to the expanding space. So the light in the bottom row grows by time t to a size greater than t*c.

A great way to explain this visually!!

ChrisVer
Gold Member
Since the universe is described by an homogenous and isotropic universe, you are able to take any point on it and see it as the "center" of universe.
If you take the center of the universe to be the earth, and look outside, you are going to see the CMB all over the sky, something that happens, as the furthest in the past object observable.
It would be a bad image, but imagine you being in the dot-center of a sphere, and the sphere around you being the CMB.
The same can happen to an object that is not "causally" connected to you (because you can take it as a center) so it's unobservable.

I would like some critisism in this comment by the way, if there is any.

there is no place in the universe that is observationally inaccessible to us.
Cronos, I do not follow. If the initial condition had a large spacial extent how is this possible.

If we start at one point and we have expansion faster than the speed of light (inflation, something I consider to be a fudge) then I can see that all could see all at the first instant and then parts of the visible universe "fall of the edge of the observable light bubble". But expansion slows down and now the Hubble expansion at the edge of the light bubble is slower than the speed of light the observable universe is growing? Not sure about that statement it is more a question.

Chronos
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Bear in mind we are discussing the observable universe. What I was saying is all regions of the universe that were ever observable, are still observable. Remote regions are heavily time dilated and appear as they were when the universe was much younger. For example CMB photons were emitted when the universe was about 380,000 years old. Due to expansion, they are just now arriving 13.8 billion years later. We will never observe photons emitted from these regions 'today' because they have long since receded beyond our horizon. We will merely observe these regions redshift beyond detectability.

bapowell
If we start at one point and we have expansion faster than the speed of light (inflation, something I consider to be a fudge)
What does inflation have to do with "expansion faster than the speed of light"?

timmdeeg
Gold Member
I think this graphic has a great pedagogical value
This is a popular misconception. The expansion of space does not proceed at a given speed (it is really a rate per distance), and so it's never correct to say that the expansion was faster than the speed of light. What is typically meant by this, I'll grant, is that the expansion can be such that that objects comoving with the expansion attain speeds surpassing that of light. This is true, but it's always true in any expanding cosmology, at both early and late epochs. The distance at which objects attain a superluminal recession velocity is called the Hubble distance.
and could clarify superluminal recession velocity as well by interchanging the order of the dashed and solid lines.

I have never seen such a graph, it seems a good complement to the well known spacetime diagrams of Davis&Lineweaver.

bapowell
I understand that the speed of light limit applies to (ultimately) quantum particles traveling THROUGH the fabric of space-time...

But not to the fabric of space-time itself...

Which can expand, or stretch, at hypothetically arbitrary speeds ...

Sometimes I think that imaging the fabric of space-time to be embedded in a higher dimensional hyperspace ("bulk") can be helpful ...

In that picture, the "hypervelocity" of space-time itself through hyperspace is NOT limited...

Only the speed of wave functions propagating inside of, and through, the fabric of space-time, as measured relative to said fabric, is limited