Why is there an observable universe?

Click For Summary

Discussion Overview

The discussion revolves around the concept of the observable universe, particularly addressing why it is possible to observe regions that seem to exceed the age of the universe in light years. Participants explore the implications of cosmic expansion, the nature of the initial singularity, and the limitations of observational astronomy.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question how the observable universe can extend beyond the age of the universe, suggesting confusion about the implications of cosmic expansion and the nature of the initial singularity.
  • Others argue that the universe's expansion means that light from regions that were once close can be receding faster than light, complicating the notion of what is observable.
  • A participant clarifies that the term "singularity" does not imply an infinitesimal point, but rather a breakdown in current mathematical models, suggesting that the original state of the universe may have been infinite in extent.
  • Some contributions emphasize that the observable universe is larger than the distance light could have traveled in the age of the universe due to the expansion of space itself.
  • There are discussions about the Cosmic Microwave Background (CMB) and how photons emitted from the early universe are now reaching us after billions of years, with some participants noting that regions of the universe may become unobservable over time due to redshift.
  • One participant proposes a model where any point in the universe can be considered a center, leading to the observation of the CMB from all directions, while others challenge this view by questioning the implications of initial conditions and inflation.
  • There is a mention of the possibility of detecting gravitational waves from the early universe, which raises further questions about the nature of observational limits.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of the observable universe and the implications of cosmic expansion. There is no consensus on the interpretation of the initial singularity or the implications of inflation, indicating ongoing debate and uncertainty.

Contextual Notes

Participants highlight limitations in current understanding, particularly regarding the nature of the singularity and the effects of cosmic expansion on observability. The discussion reflects a variety of assumptions about the initial conditions of the universe and the implications of inflationary theory.

fsujoseph
Messages
18
Reaction score
0
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.
 
Last edited:
Space news on Phys.org
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.

See also https://www.physicsforums.com/forums/cosmology-faq.206/
 
Last edited by a moderator:
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.
 
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:

horizon_part.jpg


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.
 
Last edited:
  • Like
Likes   Reactions: timmdeeg and aboro
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.
 
bapowell said:
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:

horizon_part.jpg


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!
 
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.
 
Chronos said:
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 spatial 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.
 
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.
 
  • #10
edpell said:
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"?
 
  • #11
I think this graphic has a great pedagogical value
bapowell said:
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.
 
  • Like
Likes   Reactions: bapowell
  • #13
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
 

Similar threads

Undergrad The Boundary of the Observable Universe
  • · Replies 44 ·
2
Replies
44
Views
6K
Undergrad Observable Universe Size in Different Perspectives
  • · Replies 42 ·
2
Replies
42
Views
6K
High School Cosmological Redshift in Simulated Universe
  • · Replies 39 ·
2
Replies
39
Views
7K
High School Clarification on how old the Universe is
  • · Replies 11 ·
Replies
11
Views
3K
High School Question about "our observed universe"
  • · Replies 15 ·
Replies
15
Views
2K
High School How can the Observable Universe be a closed system?
  • · Replies 37 ·
2
Replies
37
Views
6K
High School Big Bang and Infinite Universe
  • · Replies 20 ·
Replies
20
Views
2K
High School The observable and non-observable parts of the Universe
  • · Replies 38 ·
2
Replies
38
Views
7K
Undergrad Questions about the Location of the Big Bang and the Age of the Universe
  • · Replies 8 ·
Replies
8
Views
3K
High School Doesn't there have to be more than one observable universe?
  • · Replies 41 ·
2
Replies
41
Views
7K