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from https://www.space.com/astronomy/is-the-universe-infinite-or-does-it-have-a-limit

Is the universe infinite, or does it have a limit?
News
By Paul Sutter published 22 hours ago
If the universe is expanding, then what is it expanding into, and what
is it expanding from?

Comments (42)
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a red field of bright gas on a starry background
Where's the center of the universe, and where is its edge? (Image
credit: Roberto Machado Noa/Getty Images)

After a century of observations spanning the breadth of the cosmos and theoretical insights that push humanity's vision of the universe to its
utmost limits, we can finally, confidently say that the universe is
infinite.

Or not. It's complicated.

Let's start with something we can say for certain: We live in an
expanding universe. But if the universe is expanding, then what is it expanding into? And what is it expanding from? Where's the edge of the universe, and where is its center?

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It's easy to imagine an expanding universe, and there are plenty of
analogies to help guide our thinking. We can imagine drawing little
galaxies on the surface of a balloon and inflating that balloon to see
the galaxies getting farther apart. We can imagine baking a loaf of
bread with raisins in it and seeing how, as the bread rises, the raisins
get farther apart.

But the balloon has both a center and an edge. And the bread has a
center and a crust. So where's the center of the universe, and where is
its edge?

Here's the uncomfortable answer: The Big Bang has no center, and it has
no edge. How can this make any sense?

Let's start with the center. Where did the Big Bang start? Right here.
And right over there. And in the next galaxy over. The Big Bang happened everywhere, all at once. It had to happen everywhere, because everywhere
is, by definition, part of the universe. It was not an explosion that
occurred somewhere in space. It was an explosion of space rCo when the expansion of the universe first got started. It was not a place but a time.

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Now what about the other side of the coin? If the universe is expanding,
what is it expanding into? Where's the crust in our expanding loaf of
bread, and what's the oven we're sitting in?

A blue cylinder with a flared end on a black background

An illustration that shows the timeline of our universe, from the Big
Bang to today. (Image credit: RubinObs/NOIRLab/SLAC/NSF/DOE/AURA)
This is going to get weird. I don't even want to say something like "the universe isn't expanding into anything," because that still conjures up
the wrong mental image. It's too tempting to imagine a wall or boundary,
with galaxies and stuff on one side and nothingness on the other, with
the universe expanding to fill that nothingness.

But that's wrong. Even the vacuum of space is something. There are still points, locations and existence. There's no "outside" of the universe
because "outside" implies existence, even an empty one. But the universe
is, by definition, all there is. There is nothing to physical reality
except the universe. Walls separate one region from another, but the
universe comprises all of the regions simultaneously.

If there were an edge, you could imagine working hard enough to get
outside that edge. But that's not possible. There is no outside; there
is no side. There is just the universe.

Join our Space Forums to keep talking space on the latest missions,
night sky and more! And if you have a news tip, correction or comment,
let us know at: community@space.com.

Paul Sutter
Paul Sutter
Space.com Contributor
Paul M. Sutter is a cosmologist at Johns Hopkins University, host of Ask
a Spaceman, and author of How to Die in Space.
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All Comments
42

NEWEST

All Comments
Comment by christophe_dupin.

john.miles.fimeche
46 min ago
We we accept our universe as all there is, we must also accept the fact
that man has done for thousands of years. Science cannot work with what
it cannot see. Talk is of a balloon or baking bread, not of the big bang
being one of a million such bang happening every second, somewhere.

All we can hope to do, whilst constrained by the limit to the speed of
light, is study what we observe, to interpret where parts of our
universe are being affected by events outside our universe, and where
parts of our universe are being infiltrated by external matter, not of
our Big Bang.

Take the loaf if bread analogy. A ball of dough the size of the solar
system, with a grenade exploding at its centre our Big Bang, still
witnesses expanding raisins. And the expansion will come to a halt. And
the universe will collapse again.

Better still though, imagine a night sky full, horizon to horizon
exploding with massive chrysanthemum fireworks. As they explode, their
matter gravitates into another mass causing another chrysanthemum
explosion, perpetuating across the sky throughout the night.

reply

aarongrayson84
2 hrs ago
Depends on the mind observing it.

sw

shane watt
6 hrs ago
There is no infinity in another dimension.

Gibsense
3 hrs ago
Reply to shane watt - view message
I am not sure what you mean. In flat space (no curvature) a hypothetical
line (dimension) is infinite no matter what label (1,2,3 or4) you
assign. If curved, like the surface of a sphere, then it could be
endless (going around and around) but limited to the sphere. If you wish
put your statement in a context

0
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a425couple wrote in alt.astronomy, rec.aviation.military, and alt.fan.heinlein ^^^^^^^^^^
Your real name belongs there. And please avoid excessive crossposting
(without Followup-To).

from https://www.space.com/astronomy/is-the-universe-infinite-or-does-it-have-a-limit

Is the universe infinite, or does it have a limit?

By Paul Sutter published 22 hours ago

Paul M. Sutter is an astrophysicist, but there he wrote a
*popular*-scientific article, and he fell into the trap of oversimplifying
or simply made wrong statements.

If the universe is expanding, then what is it expanding into, and what
is it expanding from?

[...]
After a century of observations spanning the breadth of the cosmos and theoretical insights that push humanity's vision of the universe to its utmost limits, we can finally, confidently say that the universe is infinite.

Or not. It's complicated.

Correct.

Let's start with something we can say for certain: We live in an
expanding universe.

Correct.

But if the universe is expanding, then what is it expanding into? And
what is it expanding from? Where's the edge of the universe, and where
is its center?

In a nutshell:

Our universe is not necessarily expanding into anything, and certainly not expanding from anything; our universe probably *is* its edge, and it has no center.

[...]
It's easy to imagine an expanding universe, and there are plenty of analogies to help guide our thinking. We can imagine drawing little
galaxies on the surface of a balloon and inflating that balloon to see
the galaxies getting farther apart. We can imagine baking a loaf of
bread with raisins in it and seeing how, as the bread rises, the raisins
get farther apart.

Correct.

But the balloon has both a center and an edge. And the bread has a
center and a crust. So where's the center of the universe, and where is
its edge?

Here's the uncomfortable answer: The Big Bang has no center, and it has
no edge. How can this make any sense?

It does not make sense, at least partially. The Big Bang is not the same as our universe; it is its expansion. So it makes sense to speak of a center
of the Big Bang (and the non-existence of that), but it does not make sense
to speak of an edge of the Big Bang. For what is the edge of an expansion supposed to be?

It would make sense to speak of an edge of our universe, but that is where
the balloon analogy stops working. Our universe is analogous to the edge of
an expanding rubber balloon.

Let's start with the center. Where did the Big Bang start? Right here.
And right over there. And in the next galaxy over. The Big Bang happened everywhere, all at once.

It _is happening_ everywhere instead (we think).

But "the next galaxy over" did not exist yet when the expansion began: Our universe had to expand so that its temperature could decrease enough to
enable the formation of galaxies.

It had to happen everywhere, because everywhere is, by definition, part
of the universe.

Oversimplified.

It was not an explosion that occurred somewhere in space.

Correct.

It was an explosion of space rCo

No. It _is_ the _expansion_ of space.

when the expansion of the universe first got started.

No, the Big Bang is the expansion; a process, NOT an event.

It was not a place but a time.

No.

Dr. Becky:
Astrophysicist explains why JWST HASN'T "disproved Big Bang theory" | Night
Sky News Aug 22 (2022)
<https://www.youtube.com/watch?v=Fqfap3v0xxw&t=791s>

[...]
Now what about the other side of the coin? If the universe is expanding, what is it expanding into? Where's the crust in our expanding loaf of
bread, and what's the oven we're sitting in?

[...]
This is going to get weird. I don't even want to say something like "the universe isn't expanding into anything," because that still conjures up
the wrong mental image. It's too tempting to imagine a wall or boundary, with galaxies and stuff on one side and nothingness on the other, with
the universe expanding to fill that nothingness.

But that's wrong. Even the vacuum of space is something. There are still points, locations and existence. There's no "outside" of the universe because "outside" implies existence, even an empty one. But the universe
is, by definition, all there is. There is nothing to physical reality
except the universe. Walls separate one region from another, but the universe comprises all of the regions simultaneously.

If there were an edge, you could imagine working hard enough to get
outside that edge. But that's not possible. There is no outside; there
is no side. There is just the universe.

IOW, our universe, more precisely the *space* of our universe (which is described by *spacetime*), is analogous to the edge of the rubber balloon.

In the raising bread analogy, the space of our universe is the bread.

Neither analogy can properly depict the time of our universe other than both expand as time progresses.

Ultimately the only exact description is mathematical; the best cosmological model that we have (so far) is based on the Friedmann--Lema|<tre--Robertson--Walker (FLRW) metric:

ds^2 = -c^2 dt^2 + a^2(t) [1/(1 - k r^2) dr^2 + r^2 (d theta)^2 + r^2 sin^2(theta)],

where a(t) is the scale factor of our universe and t is cosmological time;
in an expanding universe, a(t) increases as t increases.

[...]
Paul M. Sutter is a cosmologist at Johns Hopkins University, host of Ask
a Spaceman, and author of How to Die in Space.

AISB. He did not do a very good job at science communication in that article.

All Comments
john.miles.fimeche
46 min ago
We we accept our universe as all there is,

We do not. There is the scientific idea of a multiverse, a larger structure that contains (in some sense) our universe and other (perhaps parallel) universes.

we must also accept the fact that man has done for thousands of years.

This sentence is gibberish; it is missing a word. *What* has man done for thousands of years?

Science cannot work with what it cannot see.

Incorrect. We are working with models that have no (simple/comprehensible) visual representation on a daily basis.

Talk is of a balloon or baking bread, not of the big bang
being one of a million such bang happening every second, somewhere.

We do not know that this happens, although it could happen. This statement corresponds to the cosmological models of "eternal inflation" and the multiverse.

All we can hope to do, whilst constrained by the limit to the speed of light, is study what we observe, to interpret where parts of our
universe are being affected by events outside our universe, and where
parts of our universe are being infiltrated by external matter,
not of our Big Bang.

Confused nonsense. There is no evidence of that happening.

Take the loaf if bread analogy. A ball of dough the size of the solar system, with a grenade exploding at its centre our Big Bang,

The Big Bang is not an explosion.

still witnesses expanding raisins.

But there is no reason to assume that we at the centre of our Big Bang.
If the history of cosmology has taught us anything, it is that we are not in any way special.

See also:

And the expansion will come to a halt. And the universe will collapse again.

It is possible, but we do NOT know that it *will* happen. According to our current model, that depends mostly on how much Dark Matter there is, and how the Dark Energy density is evolving: the larger the Dark Matter density compared to the Dark Energy density, the more likely for the expansion to
come to a halt, and for our universe to collapse again.

Better still though, imagine a night sky full, horizon to horizon
exploding with massive chrysanthemum fireworks. As they explode, their matter gravitates into another mass causing another chrysanthemum
explosion, perpetuating across the sky throughout the night.

The Big Bang is not an explosion, so all analogies with explosions fail.

aarongrayson84
2 hrs ago
Depends on the mind observing it.

No, it does not. Our universe is expanding regardless whether that is
observed (by a human). IOW, "our" universe could not care less about us/humans.

shane watt
6 hrs ago
There is no infinity in another dimension.

Pseudoscientific word salad. That person refers to a concept of "dimension" from fantasy and bad science-fiction; not mathematics/physics.

Gibsense
3 hrs ago
Reply to shane watt - view message
I am not sure what you mean. In flat space (no curvature) a hypothetical line (dimension)

A line is not a dimension; instead, it has one dimension, it is
one-dimensional (one number, one coordinate or parameter, suffices to locate
a point on a line).

is infinite

A line is not necessarily infinitely *long*. To state that a line "is infinite" is writing word salad.

no matter what label (1,2,3 or4) you assign.

Such a nonsense, it's not even wrong.

If curved, like the surface of a sphere,

A line can be curved, but the surface of a sphere is not a line. In fact,
they mean the surface of a _ball_; a sphere *is* the surface of a ball.

then it could be endless (going around and around) but limited
to the sphere.

Correct. This would be the case if our universe would be closed (had
curvature parameter k > 0 or curvature density parameter +-reu > 0): light could go around our universe in that sense.

Richard Dawkins Foundation for Reason & Science:
'A Universe From Nothing' by Lawrence Krauss, AAI 2009 <https://www.youtube.com/watch?v=7ImvlS8PLIo&t=1432s>

However, our universe is expanding, and there are points whose distance from each other increases faster than light could propagate between them. So
even if our universe would be closed, it would not be possible for light to arrive at its point of emission by going around our universe. Maybe that is the reason why this has never been observed.

Followup-To <news:sci.astro>
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