# Where is the edge of the universe

## Main Question or Discussion Point

Where is the edge/boundary of the universe may sound like a simple question but I can see 3 possibilities.

1 Observed, this puts us inside a sphere looking out towards an edge which is beyond visible range, some estimates appear to give up to around 50 bly away.

2 Expanding universe theory, a timeline of expansion clearly puts us on the edge of an expanding sphere looking inwards.

3 The no edge scenario

1 and 2 are opposites, 3 could fit in with 1 if the universe was infinitely old and large.
So which one is correct or is it possible to merge them all together

mfb
Mentor
There are no hints of any edge in space - the universe looks the same in all directions, and theories without an edge can describe this perfectly. The big bang could be an "edge in time".

An edge would make no sense at all. This suggest that our usual notion of space and time makes no sense, which of course it doesn't, since it gives rise to the question you've asked.

phinds
Gold Member
2019 Award
Where is the edge/boundary of the universe may sound like a simple question but I can see 3 possibilities.

1 Observed, this puts us inside a sphere looking out towards an edge which is beyond visible range, some estimates appear to give up to around 50 bly away.
No, it puts us exactly where we are, which is a place of no particular significance (other than to us) in an expanding universe that has no center and no edge. If you mean the observable universe (the 50BLY radius), then we are at the center of it and it has an "edge" of a sort but only in the same sense that there is an "edge" to a sphere that I define as having a 10ft radius centered on my left eyeball.

2 Expanding universe theory, a timeline of expansion clearly puts us on the edge of an expanding sphere looking inwards.
No, it does not put us "looking inward", it has exactly the same as the answer above

3 The no edge scenario
Yes, that is correct

1 and 2 are opposites
No, they are not. They both describe the existing universe.

3 could fit in with 1 if the universe was infinitely old and large.
No, 3 fits in with 1 quite nicely and certainly does not require an infinitely old universe.

Please describe a shape that has no edge or boundary or are you saying the universe is shapeless.
How do you measure expansion without an edge or boundary to relate that expansion to. You can use any point you like and the boundary is the point the expansion has reached, no boundary gives rise to infinite space.

one shape that has no edge is the surface of a ball much like an ant on a beachball. The surface of the beachball looks flat. However this would hold for a finite curved universe.
Were reasonably confident that the universe is flat so one
of the few shapes is a torus.

Keep in mind that no one knows if the universe is finite or infinite.
How do we measure expansion? We judge the rate of expansion by the movement of galaxies by measuring its redshift. In every direction non gravitationally bound galaxies are moving apart.
Take a 3 dimensional grid. With each vertical, horizontal and Z directional line intersection as a coordinate.
Each coordinate is moving apart regardless of direction or cross direction.
This essentially means the space between coordinates is increasing. Or space between galaxies.

phinds
Gold Member
2019 Award
Please describe a shape that has no edge or boundary or are you saying the universe is shapeless.
How do you measure expansion without an edge or boundary to relate that expansion to. You can use any point you like and the boundary is the point the expansion has reached, no boundary gives rise to infinite space.
I think your problem here is that you think of the universe as expanding INTO something, which is not the case. The universe is all there is. It is not expanding into something outside of itself. If it were, THEN there would be an edge, but it isn't so there isn't.

The FAQ subforum has some decent articles on this. Also the balloon analogy in the sticky threads also has related material. I believe Phinds signature links to a balloon analogy.
Please read those mentioned articles they will clear up a lot of misconceptions.

Thanks Phinds it would be nice to get our signatures back lol.

phinds
Gold Member
2019 Award
Thanks Phinds it would be nice to get our signatures back lol.
The price we pay for "progress" ?

Chronos
Gold Member
You could argue that all three are true, but, meaningless. 1] The universe is about 13.8 billion years old and the 'beginning' is currently at a proper distance from us of a little under 50 billion light years. Obviously you cant see the universe before it was born so the observable universe is effectively a sphere a little under 100 billion light years in diameter. 2] We reside at the oldest place in the observable universe. Everything else is younger than us, therefore, we reside at the temporal edge of the universe. 3] The is no edge of the universe. An edge implies the existence of something 'beyond' or 'outside' the universe, which [as already discussed] is forbidden by definition. It is also true the question is meaningless is the universe is infinite. That's the easy way out and the odds are very good you will never be proven wrong.

2] We reside at the oldest place in the observable universe.
I like this, does it help when I add 'as measured using the youngest photons'.

assuming you can get those baby(youngest photons) to cooperate

Chronos
Gold Member
The point is when you talk about the 'edge' of the universe, you can only frame it in temporal terms - from the birth of the universe to the present. That is like talking about the 'edge' of Monday. The universe does not recognize our definition of 'edges'.

It occurs to me that if we have no edge or nowhere then a 4th option of Everywhere should be included if only for consideration.

While I agree any 3d shape can have a surface without edge it still has boundaries. With the earth gravity stops you walking to the moon and its surface stops you falling towards the center, so we have a 2d edgeless surface with boudaries. I can see edgeless in these circumstances but not edgeless with no boundaries.
The balloon analogy is well known as is its brother the fruit bun to explain the expansion of space,

Post 7 I do not think the universe is expanding into anything unless you regard time as something, it is pretty obvious that space cannot expand into space.
Chronos post 12 point 2 are you absolutely sure I thought that today would be the youngest place as yesterday would be older than today etc

It occurs to me that if we have no edge or nowhere then a 4th option of Everywhere should be included if only for consideration.
Edge, center and boundary are defined states and restricted to space. For instance, I have a ruler. I can say it has an edge and center bec. i can subjectively define a limit to the ruler when in fact it isn't the case especially when you zoom it in. So what we knew as edge is limited to our observation and remain temporal since nothing is certain. Same is true with our universe with a temporal edge(SLS).

Chronos
Gold Member
... Chronos post 12 point 2 are you absolutely sure I thought that today would be the youngest place as yesterday would be older than today etc
Are you younger today than you were yesterday? Due to the finite speed of light, everything we observe is a consequence of events that occured in our past - i.e., when the universe was younger than it is at present.

Edge, center and boundary are defined states and restricted to space. For instance, I have a ruler. I can say it has an edge and center bec.
Restricted to space/time, each edge, center, and boundary with a defined state are temporal in nature. Using the photon as our ruler, with all photons traveling outward from emission, I would think the younger photons show us a outer edge of the universe we see as smaller. Yet what about the surface of last scattering if we were to call an inside edge of the universe it must be the view we see from the oldest photons that we see from all directions in space.

Big Bang came from nothing ,to understand the"edge" science have to explain how that is possible....

The "big bang" is a rather ambigous term here. Seom people describe it as you say but a more responsible approach it to say the standard model of cosmology (known as LCDM) describes the universe as expanding froma hot dense state for the last 13.8 bio year . What happened before that is an open question.

Chronos is today older than yesterday.

So concensus is that the universe has no edge, I find this most unsatisfactory and is not the first time I have found I cant explain it so it does not exist.

martinbn