Hypothetical astrophysics question

"Joseph Lazio" wrote in message
...
"JG" == John Griffin writes:

JG I was thinking of a universe as another collection of galaxies and
JG stuff, with its own origin (big bang or whatever) occupying
JG another part of the infinite space this one is in.

This sounds like Martin Rees' Multiverse concept. He suggests that
multiple universes may in fact exist. In this concept, though,
universes are distinct. One cannot travel from one universe to
another.

JG It seems to me that you're saying light emitted by a universe
JG stops or vanishes at some kind of boundary. (...) I don't know if
JG that's a consequence of the big bang theory, common sense, or just
JG a conjecture. If it has to do with the curvature of space, can
JG the path of a photon that doesn't collide with another object in
JG the galaxy be described?

I think it is a consequence of general relativity. Others have
mentioned "expanding space" and the like. I dislike the notion of
"expanding space" because it suggests that space is this mystical,
almost ether-like stuff. A better description is to consider objects
separated by distances and ask how those objects move and how the
distances between them change in response to the total mass-energy of
the objects. General relativity provides a formalism to answer this
question, but I think that a consequence of this is that one has to
treat all objects between which one can measure a distance as being
within the same universe.

Well, that makes sense, but then why couldn't we just define
the universe as a scattering of giant clumps of stuff, each
arising from a big bang, in which stars have formed? There
is no apparent (to the layman) reason why there couldn't be
another such blob of gas a quadrillion light years away.
This could also explain where God went, for those who
believe in such things.

JG I don't understand why this universe wouldn't be visible from
JG "outside."

How would one get "outside" our Universe to view it?

That would be easy (conceptually) if what we call the universe
is just a collection of visible matter in an otherwise empty container.
You just pack for a long trip and head out in a direction where there
are no galaxies in your way. Keep going until the last one is behind
you. If you find yourself in familiar territory, having travelled for
billions
of years without doing anything to alter the direction of travel and
having never found a place where there are no more galaxies ahead,
you have some tangible evidence for the antisocial universe..

It just seems way too parochial to say that our universe is all
there is, even from our own perspective. I think it's as likely
that there is a "god's eye view," as Funke called it, as that
there isn't. Defining that as unknowable should mean that
we can't say anything at all about it, including saying that it
doesn't exist in any way we can comprehend.

The Gott theory is looking better every day, because it's
easy to understand, and it claims that what we know as
a universe is a "bubble" in...something. Maybe "out there"
is a "perfect solid" as opposed to the perfect vacuum.
(Gott referred to it as an "infinitely dense vibration" in the
Scientific American article I read about 20 years ago.) That
would sure as hell stop radiation from travelling between
universes. Gott might be (have been?) a Sarfatti lookalike,
except that Gott's idea is comprehensible and coherent.

I'm slowly plodding through the links Matthew Funke provided.
Everything you guys have said that shoots down my original
hypothetical question is beyond my mental event horizon at
this time. Besides that, there's a possibility that all of the
cosmologists' understanding will be overturned within a few
centuries. Therefore, I'm going to avoid the rush and reject
all of it now, start with a completely open mind, and read
all that stuff as many times as it takes to begin to understand
it. Stick around for several years and I'll be back with some
more questions.

"John Griffin" wrote in message ...

"Joseph Lazio" wrote in message
...
"JG" == John Griffin writes:

JG I was thinking of a universe as another collection of galaxies and

Astronomers refer to these as 'clusters'. This is a map of
clusters within 200 million light years:

http://www.anzwers.org/free/universe/200mill.html

JG stuff, with its own origin (big bang or whatever) occupying
JG another part of the infinite space this one is in.

'Universe' is generally taken to mean not only the galaxies but
also the infinite space within which they reside.

This sounds like Martin Rees' Multiverse concept. He suggests that
multiple universes may in fact exist. In this concept, though,
universes are distinct. One cannot travel from one universe to
another.

Joseph, can you say if that is similar to the ideas in
this paper by Alan Guth?

http://www.arxiv.org/abs/astro-ph/0301199

JG It seems to me that you're saying light emitted by a universe
JG stops or vanishes at some kind of boundary. (...) I don't know if
JG that's a consequence of the big bang theory, common sense, or just
JG a conjecture. If it has to do with the curvature of space, can
JG the path of a photon that doesn't collide with another object in
JG the galaxy be described?

I think it is a consequence of general relativity. Others have
mentioned "expanding space" and the like. I dislike the notion of
"expanding space" because it suggests that space is this mystical,
almost ether-like stuff. A better description is to consider objects
separated by distances and ask how those objects move and how the
distances between them change in response to the total mass-energy of
the objects. General relativity provides a formalism to answer this
question, but I think that a consequence of this is that one has to
treat all objects between which one can measure a distance as being
within the same universe.

Well, that makes sense, but then why couldn't we just define
the universe as a scattering of giant clumps of stuff, each
arising from a big bang, in which stars have formed? There
is no apparent (to the layman) reason why there couldn't be
another such blob of gas a quadrillion light years away.

The 'Big Bang' refers to expansion from a state where all
the blobs were a mass of gas that filled all the space. The
fact that there are gaps now when there weren't any to start
with is one reason why people talk of 'space expanding'. If
you think of the BB as referring to expansion of the space in
which the blobs reside, you may understand some of the
discussions better.

I'm slowly plodding through the links Matthew Funke provided.

Ned Wright's tutorial is almost a standard work in this
group.

Everything you guys have said that shoots down my original
hypothetical question is beyond my mental event horizon at
this time. Besides that, there's a possibility that all of the
cosmologists' understanding will be overturned within a few
centuries. Therefore, I'm going to avoid the rush and reject
all of it now, start with a completely open mind, and read
all that stuff as many times as it takes to begin to understand
it. Stick around for several years and I'll be back with some
more questions.

I started out asking dumb questions about 8 years ago. Some
effort is required to expand your perspectives, but IMHO it
is well worth it.

Have fun
George

"CeeBee" replied to Jeff Root:

How the heck can you possibly know that light doesn't go
between universes?

It's not as much a matter of "knowing", as [...] following
from a theory that you use to predict things to happen or to
be observed. If the predictions and observations coincide with
what follows from your theory, the validity of it grows, and
so with it all other predictions coming from it.

Yes, of course, but how the heck can you possibly know that
light doesn't go between universes? What theory predicts that
light doesn't go between universes, and what observations is
that theory derived from?

Why is it impossible for space, time, and/or spacetime to exist
outside of a universe? What observations support that idea?

Given that the Universe we know and love came into existence
about 14 billion years ago, and is now seen to be extravagantly
enormous, with no end or edge even hinted at, what leads you
to think that other universes could not have come about in a
similar manner, and have some spatial and temporal relationship
to ours?

It's _perfectly_ possible. There could be a gazillion universes
each with their own space-time. However there's no need for those
universes to be "embeddded into something" that is able to transmit
information.

Since there is no "need" for such hypothesized universes to be
"embedded into something" (are you quoting someone???) that is
able to transmit information, it is therefore impossible for
light to go from one such universe to another? I think that
what you really mean to say is that it *might* be impossible
for light to go from one such universe to another.

And the information we talk about here (light, energy, whatever)
was created inside a self-contained universum. For that information,
as for us, there is no outside, just "our" space.

Is "universum" a term I should look up somewhere? If you are
introducing it as a way to distinguish between "everything" and
"everything involved in the Big Bang", you should define or at
least describe the intent of the term.

Cosmology seems to defy common sense even more than it coincides
with it, yet if a theory is postulated, and it's capable of
explaining and predicting the vast majority of phenomenon, we
have to cope with defying our common sense

I haven't seen any theory which predicts observed phenomenae
and also predicts that light can't reach us from a "universe"
other than our own. Or, at least, I haven't seen such a theory
and been made aware that it was such a theory.

-- Jeff, in Minneapolis

..

(Jeff Root) wrote in sci.astro:

Yes, of course, but how the heck can you possibly know that
light doesn't go between universes? What theory predicts that
light doesn't go between universes, and what observations is
that theory derived from?


The big bang theory doesn't describe "universes". It just describes our
universe, the development of it, and the phenomenon in it.


Since there is no "need" for such hypothesized universes to be
"embedded into something" (are you quoting someone???) that is
able to transmit information, it is therefore impossible for
light to go from one such universe to another? I think that
what you really mean to say is that it *might* be impossible
for light to go from one such universe to another.


The Big Bang theory "predicts" that. It tells us that space and time
were created simultaneously with the Big Bang, and all matter and energy
in it. It predicts how the state of the universe was, is and will be. If
observations are in accordance with the predictions of the theory, the
theory gains validity.

I fail to see the relevance between universes "embedded" into something
conducting light and me quoting someone, but I might miss your point.


Is "universum" a term I should look up somewhere? If you are
introducing it as a way to distinguish between "everything" and
"everything involved in the Big Bang", you should define or at
least describe the intent of the term.

I'm Dutch. It's a typo. Universum = universe. I'm no native speaker.


I haven't seen any theory which predicts observed phenomenae
and also predicts that light can't reach us from a "universe"
other than our own. Or, at least, I haven't seen such a theory
and been made aware that it was such a theory.


That's because you fail to understand the Big Bang theory. It says that
our universe comprises everything. It says you won't meet a boundary
with "space outside" because there isn't such. There is nothing outside,
as there is no outside, hence nothing to travel _through_.
You can't seem to get yourself unhooked from the idea that there's
_nothing_ outside the universe because there's _no_ outside.

Light can't reach the boundaries of the universe because there are no
boundaries. It can't go outside, it can't go inside. It can't cross a
boundary, because there is no boundary.

A simple thought experiment is the line that makes up a circle. A line
is one-dimensional. It has no boundary. If you're a one-dimensional
living on the circle you will run around forever, never finding a
boundary, a one dimensional beginning or end. Don't try to translate it
to higher dimensions; it's just a thought experiment to show how
"finity" and "no boundary" can go together.

You repeatedly suggest that "no theory predicts"; however it's more "I
don't get the theory".

http://www.astro.ucla.edu/~wright/cosmology_faq.html
http://imagine.gsfc.nasa.gov/docs/as...s/961202c.html


BTW the Big Bang theory isn't a religion. There are new idea's about the
creation of our universe, handsomely called the "ekpyrotic" model:
http://www.sciencenews.org/20010922/bob9.asp



--
CeeBee


Uxbridge: "By God, sir, I've lost my leg!"
Wellington: "By God, sir, so you have!"


Google CeeBee @ www.geocities.com/ceebee_2

(Matthew F Funke) wrote in message ...
John Griffin wrote:

Two tiny, fuzzy points of light are observed. Observations of
each object indicate that it's a globular galaxy ten billion light
years distant. The first one is exactly that, but the second one
happens to be another entire universe. It's 100,000 times as
far away as the galaxy, and it's emiting ten billion times as
much light. Is there anything that would distinguish these
objects from one another?

Redshift. The universe is expanding, and there is more space in
between us and the object 100,000 times as far away; it would appear to be
more redshifted.

Let me to reformulate it.
Small star - 10 light years.
Huge Quasar – 10 billion light years.
Huge Quasar moves toward us with exact speed to compensate redshift.
For this arrangement, the redshift is not enough to distinguish them.

--George

"CeeBee" replied to Jeff Root:

Yes, of course, but how the heck can you possibly know that
light doesn't go between universes? What theory predicts that
light doesn't go between universes, and what observations is
that theory derived from?

The big bang theory doesn't describe "universes". It just
describes our universe, the development of it, and the phenomenon
in it.

Yes! Exactly!

Since there is no "need" for such hypothesized universes to be
"embedded into something" (are you quoting someone???) that is
able to transmit information, it is therefore impossible for
light to go from one such universe to another? I think that
what you really mean to say is that it *might* be impossible
for light to go from one such universe to another.

The Big Bang theory "predicts" that.

But you just said that the Big Bang theory doesn't say anything
about other universes, only our own! In your next sentence you
contradict that and say that it predicts what can and can't
happen outside our universe! Which is it???

It tells us that space and time were created simultaneously
with the Big Bang,

Actually, the Big Bang theory says nothing about the origin of
space and/or time. It does say that at some time about 13.7
billion years ago everything in the Universe was squished real
close together, was very hot, and was expanding real fast.
It also says that a moment before that, something completely
unknown happened, which no theory is yet able to describe in
any way that makes sense. So, space and time *may* have been
created at that time, or they may not have. There's no way
to say, as yet.

and all matter and energy in it. It predicts how the state
of the universe was, is and will be. If observations are in
accordance with the predictions of the theory, the theory
gains validity.

I fail to see the relevance between universes "embedded" into
something conducting light and me quoting someone, but I might
miss your point.

I asked about your use of the term "embedded" because Greg
used it prominently in a message posted less than two hours
after yours; I don't think the term had been used previously;
and I see no particular reason for its use here.

The idea of universes being embedded in anything sounds goofy.
I agree with you that there is no "need" for such universes to
be "embedded" in anything, so I wonder why you brought it up.
Why did you bring it up?

Is "universum" a term I should look up somewhere? If you are
introducing it as a way to distinguish between "everything" and
"everything involved in the Big Bang", you should define or at
least describe the intent of the term.

I'm Dutch. It's a typo. Universum = universe. I'm no native speaker.

Okay. I wish that I could learn *any* other language 1/10th as
well as you've mastered English. BTW, my understanding is that
my surname, "Root", is Dutch, so presumeably I'm Dutch, too!

I haven't seen any theory which predicts observed phenomenae
and also predicts that light can't reach us from a "universe"
other than our own. Or, at least, I haven't seen such a theory
and been made aware that it was such a theory.

That's because you fail to understand the Big Bang theory. It says
that our universe comprises everything.

You have said that it is possible that other universes could
exist. In this post you have said that the Big Bang theory
only describes *our* universe, not others. Now you say the Big
Bang theory asserts that our universe comprises everything.
Contradictions! Which statements should I believe?

It says you won't meet a boundary with "space outside" because
there isn't such.

There certainly is no indication of any such boundary, and I'm
quite sure that if one exists, neither I nor any other human
will ever "meet" it, because it would be too far away.

There is nothing outside, as there is no outside,

How do you know that? What observations support the idea?
I am not aware of any.

hence nothing to travel _through_.

One doesn't need to travel "through" anything in order to
travel. One just needs to go from one place to another.

You can't seem to get yourself unhooked from the idea that
there's _nothing_ outside the universe because there's _no_
outside.

I'd like to know what observations support the assertion that
there is no outside. I can certainly understand that there
*might* not be any outside, but the assertion that there *is*
no outside seems wildly beyond what is known.

Light can't reach the boundaries of the universe because there
are no boundaries. It can't go outside, it can't go inside. It
can't cross a boundary, because there is no boundary.

Okay, okay. What observations support the assertion that there
is no boundary?

You repeatedly suggest that "no theory predicts"; however it's
more "I don't get the theory".

I'm nearly certain that, as you said at the beginning of your
post, the Big Bang theory describes our universe only, and
predicts nothing about other universes or possible relationships
between our universe and other universes.

http://www.astro.ucla.edu/~wright/cosmology_faq.html

I have read a significant chunk of it, and will continue to
work through it as best I can.

http://imagine.gsfc.nasa.gov/docs/as...s/961202c.html

This basically just refers to the balloon model. I used the
balloon model in a talk to my class in 12th grade. I snuck out
of school during lunch hour to buy the balloons, and only had
time to draw galaxies on a white balloon. If I had more time I
would have glued white dots to a black balloon so the galaxies
wouldn't expand along with the balloon.

I think the balloon model is flawed in that the balloon is a
material object which stretches and on which the galaxies are
fixed, whereas there is probably nothing material between the
real galaxies which stretches or expands or holds them in place.
In reality, it is just the distances between galaxies which are
increasing because the matter which comprises the galaxies has
momentum, imparted to it early in the history of the Universe.

The balloon model is also flawed in that the surface of the
balloon is strongly curved. The latest observations show the
Universe to be indistinguishable from flat, overall. No
indication of any overall curvature.

BTW the Big Bang theory isn't a religion. There are new idea's
about the creation of our universe, handsomely called the
"ekpyrotic" model: http://www.sciencenews.org/20010922/bob9.asp

Thanks for the link to the Science News article! Ron Cowen
always does an excellent job of explaining complex ideas of
physics and mathematics.

The article contradicts most of your above assertions.
Of course, the main ideas of the article are an alternative
to inflation and a way for the Big Bang to be initiated,
which naturally contradicts those theories in many ways.

I consider inflation, string theory, and brane theory all to
be largely ad hoc and disconnected from observational evidence.
That doesn't mean I reject them, but I doubt them very much.

I have my own looney speculation which is far less developed
than those theories, but which may be capable of answering
some of the same questions far more simply. It has a serious
failing which is exactly the same as those other theories have:
A shortage of observational evidence. Two observations could
be made to support or refute my speculation.

One is astronomical, involving gravitational lensing. The math
to quantitatively predict the effects of the lensing is beyond
me, but in broad terms I would expect to see very faint radial
lines where light of some distant galaxies or quasars has been
smeared out by lensing galaxies. The opposite of the lensing
actually seen so far, which is concentric and bright.

The other observation is of the response of antihydrogen atoms
to gravity. Experiments at CERN are now getting close to making
this observation. It has the tremendous advantage that a good,
clear result can totally demolish any possibility that my
speculation is correct. It's a real make or break experiment
for my pet notion.

-- Jeff, in Minneapolis

..

(George Buyanovsky) wrote
(Matthew F Funke) wrote
John Griffin wrote:

Two tiny, fuzzy points of light are observed. Observations of
each object indicate that it's a globular galaxy ten billion light
years distant. The first one is exactly that, but the second one
happens to be another entire universe. It's 100,000 times as
far away as the galaxy, and it's emiting ten billion times as
much light. Is there anything that would distinguish these
objects from one another?

Redshift. The universe is expanding, and there is more space in
between us and the object 100,000 times as far away; it would appear to be
more redshifted.

Let me to reformulate it.
Small star - 10 light years.
Huge Quasar ? 10 billion light years.
Huge Quasar moves toward us with exact speed to compensate redshift.
For this arrangement, the redshift is not enough to distinguish them.

I wish I had been more specific in my original question.

In the hypothetical situation, the observations I mentioned were that
radiation from the other universe had the same redshift as radiation
from a galaxy ten billion light years from "here." (The cosmologists
have me wondering if the word "here" is just some kind of strange
idea.) The equal redshift would be purely coincidental, since the
other universe's motion would be in no way related to the recession of
galaxies (just some shrapnel from the big bang) in this universe,
which from its perspective is a barely perceptible bang.

Also, I can't see any reason why the inverse square law wouldn't hold
between universes, so I used it to choose numbers that make the galaxy
and the universe have the same apparent brightness. I'm glad I said it
was hypothetical.

It turns out that the cosmologists have theories and some experimental
data that say the other universe can't exist. Twenty-first-century
mathematical models don't incorporate an outside of what they call the
universe, but which I think is actually just the current extent of a
blast zone.

Anyway, I've decided that if other universes exist, they can only
exist at points on a hypothetical fixed lattice, so the redshift of
the other universe would be zero. The astronomers would freak out
when they found it. At first, it would look like a weird star, with
the same composition as the universe. After they showed it was an
impossible distance away, the cosmologists would freak out.

"GD" == George Dishman writes:

"Joseph Lazio" wrote in message
...
"JG" == John Griffin writes:

JG stuff, with its own origin (big bang or whatever) occupying
JG another part of the infinite space this one is in.

This sounds like Martin Rees' Multiverse concept. He suggests
that multiple universes may in fact exist. In this concept,
though, universes are distinct. One cannot travel from one
universe to another.

GD Joseph, can you say if that is similar to the ideas in this paper
GD by Alan Guth?

GD http://www.arxiv.org/abs/astro-ph/0301199

It is certainly similar in spirit. The difference lies, I think, in
that Guth's idea describes one "connected" Universe while Rees' idea
describes "disconnected" universes. In Guth's idea, as I understand
it, one begins with a single spacetime region. A small chunk of that
inflates to form a "pocket universe." Sometime later, another small
chunk inflates, etc. I think it would be possible, in principle, to
travel from one pocket universe to another. (Though if this idea is
correct, the size of each pocket universe is much larger than the size
of our observable Universe.) Guth doesn't say this explicitly, but it
seems to me that a consequence of this idea is that the same physical
laws would apply everywhere.

In contrast, Rees suggests multiple, disconnected universes, each
potentially with different physical laws. A good analogy for the
Multiverse idea would be to ask, How do I drive to Venus? The answer
is, of course, that no matter where you start on the Earth's surface,
no matter what direction you travel, and no matter how long you drive,
you'll never make it to Venus. That's because to get to Venus you
have to go "up," but you cannot do so.

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

"JG" == John Griffin writes:

JG "Joseph Lazio" wrote in message
JG ...

JG I was thinking of a universe as another collection of galaxies and
JG stuff, with its own origin (big bang or whatever) occupying
JG another part of the infinite space this one is in.
This sounds like Martin Rees' Multiverse concept. He suggests
that multiple universes may in fact exist. In this concept,
though, universes are distinct. One cannot travel from one
universe to another.
[...]

JG Well, that makes sense, but then why couldn't we just define the
JG universe as a scattering of giant clumps of stuff, each arising
JG from a big bang, in which stars have formed? There is no apparent
JG (...) reason why there couldn't be another such blob of gas a
JG quadrillion light years away. This could also explain where God
JG went, for those who believe in such things.

As George Dishman has pointed out, this sounds similar to Guth's
pocket universe idea. Each pocket universe would be the result of a
small section of spacetime undergoing inflation. However, each pocket
universe would be much larger than our observable Universe, so we
could never see into another pocket universe.

JG I don't understand why this universe wouldn't be visible from
JG "outside."
How would one get "outside" our Universe to view it?

JG That would be easy (conceptually) if what we call the universe is
JG just a collection of visible matter in an otherwise empty
JG container. You just pack for a long trip and head out in a
JG direction where there are no galaxies in your way. Keep going
JG until the last one is behind you. If you find yourself in
JG familiar territory, having travelled for billions of years without
JG doing anything to alter the direction of travel and having never
JG found a place where there are no more galaxies ahead, you have
JG some tangible evidence for the antisocial universe..

Yes, but that's not what general relativity predicts. The Universe
did not "explode" into a pre-existing region, even that's always
what's shown in popular descriptions. Rather, in the past everything
was closer together and in the future everything will be farther
apart. Moreover, within GR there's no outside. In order to describe
the Universe, there are only four dimensions, three space and one
time.

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

"John Griffin" wrote in message om...
(George Buyanovsky) wrote
(Matthew F Funke) wrote
John Griffin wrote:

Two tiny, fuzzy points of light are observed. Observations of
each object indicate that it's a globular galaxy ten billion light
years distant. The first one is exactly that, but the second one
happens to be another entire universe. It's 100,000 times as
far away as the galaxy, and it's emiting ten billion times as
much light. Is there anything that would distinguish these
objects from one another?

I wish I had been more specific in my original question.

In the hypothetical situation, the observations I mentioned were that
radiation from the other universe had the same redshift as radiation
from a galaxy ten billion light years from "here." (The cosmologists
have me wondering if the word "here" is just some kind of strange
idea.) The equal redshift would be purely coincidental, since the
other universe's motion would be in no way related to the recession of
galaxies (just some shrapnel from the big bang) in this universe,
which from its perspective is a barely perceptible bang.

Your first problem is that the term Big Bang for people in this
group is associated with expansion of space as well as the contents,
it is definitely _not_ an explosion that happened in a pre-existing
vacuum. That has caused some confusion.

The second problem is that we can see and measure something
called Cosmic Microwave Background Radiation. The CMBR was
emitted about 13.7 billion years ago when the whole of our
universe was filled with hot gas, about 73% hydrogen and
26% helium at about 2975 degrees Kelvin (2700 Celsius).
Note that in the first 200 thousand years, the whole of the
universe was filled with this gas, no spaces!

That gas was opaque so we cannot see beyond that time. That
means we can, in one sense, see a spherical volume about
13.7 billion light years in radius. Of course since the
universe is expanding, it was smaller then. Ned Wright's
tutorial is a good starter for any questions on what
distances mean, it is not a simple subject.

Cosmology generally says our universe is vastly bigger than
that region. 13.7 billion light years is about 10^28cm while
some discussions I have seen in the past suggest our universe
might be have had a scale of the order of 10^3000cm when the
bit we can see was the size of a grapefruit. Sorry that's not
very specific, it comes from my understanding of a number of
disparate sources, but might give you a general feel for the
scale of things.

Also, I can't see any reason why the inverse square law wouldn't hold
between universes,

It doesn't hold in our universe for distant galaxies. Because
they are moving away from us at very high speed, they appear
relativistic effects create additional dimming. The surface
brightness follows an inverse fourth power law. Search for
pages on the "Tolman Test" if you are interested.

so I used it to choose numbers that make the galaxy
and the universe have the same apparent brightness. I'm glad I said it
was hypothetical.

The bottom line is that if we can see it, it is in our observable
universe by definition, and that is a tiny subset of the whole
universe that cosmology studies.

It turns out that the cosmologists have theories and some experimental
data that say the other universe can't exist.

Not true, at least two of the sources give you links to pages
detailing ways in which other universes might exist, they are
just at a scale much larger than you are thinking.

Twenty-first-century
mathematical models don't incorporate an outside of what they call the
universe, but which I think is actually just the current extent of a
blast zone.

Anyway, I've decided that if other universes exist, they can only
exist at points on a hypothetical fixed lattice, so the redshift of
the other universe would be zero. The astronomers would freak out
when they found it. At first, it would look like a weird star, with
the same composition as the universe. After they showed it was an
impossible distance away, the cosmologists would freak out.

George