Hypothetical astrophysics question

"John Griffin" wrote in message
...

"Matthew F Funke" wrote in message
...


To be honest, I took the OP's "another entire universe" to be a
poetic description of its scale. If it were, in fact, another universe,
we wouldn't be able to see it as a fuzzy point of light -- or anything
else, for that matter. The entire question would be irrelevant;
information (including light information) doesn't go between universes.

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

That's the sticking point. Our universe is not a
collection of stuff exploding out into a pre-existing
space. It's an expansion of space itself. It is
self-contained (even if infinite in extent, if you can
wrap your head around that).


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

Light cannot leave space (the universe) as it follows
spacetime geodesics. Again, the universe is not a bunch
of stuff rushing through some fixed, pre-existing space.
It's all of space and everything in it, all expanding.


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

There is no outside from the point of view of inside. There
is no direction that you can point that is in the direction
of "outside". Consider the oft-used balloon analogy. The
surface of the balloon is a 2D representation of the universe.
The flatlanders that live on the surface have absolutely no
conception of the third dimension: they're strictly 2D beings
who've never heard of "up" or "down". They are incapable of
moving "up" or "down", or seeing anything that's "upwards" or
"downwards". Light, in their universe, traverses the balloon
surface (geodesics) and does not leave it. Another balloon
across the room would be absolutely unknown, and non-
interactive.


Not to mention that spacetime is a function of being within a
universe (assuming it's one like ours); speaking of universes moving
"toward or away from each other" would be nonsensical.

If photons don't stop at "the edge of the universe," what would
prevent them from entering another one when they encountered
it?

There's no edge. Like the surface of the balloon.


Can you refer me to a website or some literature that addresses
those questions?

I think that Ned Wright's Cosmology Tutorial might be just
the ticket for you.

(Jeff Root) wrote in sci.astro:


But that only applies to light originating from some object
which took part in OUR Big Bang. It doesn't apply to any
other universe.


An intriguing idea. However current theory also say that the Big Bang
didn't happen _in space_, but created our space and time, and nothing else.
For us it's all there is. So that other universe couldn't possible be
(visible) in "our" space-time.

Even if there was a way for the light of another universe to penetrate our
universe, it would have to travel even longer than the object _in_ our own
universe behind our observational horizon, thus not being visible to us.

--
CeeBee


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


Google CeeBee @ www.geocities.com/ceebee_2

"John Griffin" wrote in sci.astro:


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

I guess that's the difficult point. Our universe isn't space in some
other space, space was created with the Big Bang. It's self-contained,
there's no space outside. There's no need for our universe to be "in"
something.


It seems to me that you're saying light emitted by a universe
stops or vanishes at some kind of boundary. (A universal
event horizon?)


No, it is contained in space and stays there. There's no boundary for it
to encounter. Even if it conflicts with our 3D view of spaces we are in,
the universe can both be limited in total space yet have no end or
beginning.

A circle has a limited lenght yet the line it's made has neither a
beginning nor an end.


I don't know if that's a consequence of the
big bang theory, common sense, or just a conjecture. If
it has to do with the curvature of space, can the path of a
photon that doesn't collide with another object in the galaxy
be described?

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


It is well possible that our universe is one of many universes, but
these universes do not necessarily need some "space" to be in. They
created it themselves, and all in it can not get "out", as there's
nothing "out" to cater the transition of information.


http://www.astro.ucla.edu/~wright/cosmology_faq.html
gives some basic ideas about cosmology.

All the best,

--
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 replied to John Griffin:

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

This universe isn't just "in" space. Space was created during the
Big Bang.

How do you know that? What observations support that idea?

Space doesn't "surround" the universe; space is *part of* the
universe.

How do you know that? What observations support that idea?

It seems to me that you're saying light emitted by a universe
stops or vanishes at some kind of boundary. (A universal
event horizon?) I don't know if that's a consequence of the
big bang theory, common sense, or just a conjecture.

AIUI, it's common sense. If the Big Bang created space, then
outside the universe, there's no space for light to travel in.

What gave you the idea that the Big Bang created space?
What observations support that idea?

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

What would the light travel through?

Space.

What allows the light to perpetuate outside the universe?

There's nothing to stop it from doing so, is there?

There's also the problem that universes can have radically
different natural laws,

Possibly. But only possibly.

-- Jeff, in Minneapolis

..

"Jeff Root" wrote in message
m...
Greg Neill replied to John Griffin:

That's the sticking point. Our universe is not a
collection of stuff exploding out into a pre-existing
space. It's an expansion of space itself. It is
self-contained (even if infinite in extent, if you can
wrap your head around that).

How do you know that?

How do you know that space (whatever "space" is) did not exist
before (whatever "before" means) the Big Bang?

How do you know that space itself is expanding? It is clear
that galaxies which are separated by distances of tens of
millions of light-years or more are usually moving apart from
each other, and that the greater the separation, the greater
the speed at which they move apart, on average. But why do
you say that space itself is expanding? Such an expansion of
space itself has certainly never been observed.

How do you know that the Universe is "self-contained"?

What do you mean by "self-contained"?

I think that the best thing I can do here is refer you
to Ned Wright's Cosmology Tutorial. All these things
are basic to the Standard Theory, which so far seems to
agree with all empirical data.


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

Light cannot leave space (the universe) as it follows
spacetime geodesics.

Why not? What stops it?

Lack of space. Light travels in space. Space has properties
that are required for light propagation. I refer you to
Maxwell's Theory, and the constants (properties of space)
therein.


Again, the universe is not a bunch of stuff rushing through
some fixed, pre-existing space.

How do you know that?

Standard BB theory.


It's all of space and everything in it, all expanding.

Distances between widely-separated galaxies is increasing.
Who says that space itself is expanding? Who has observed it?
What does it mean to say "space is expanding"?

Standard BB theory. The evidence supports it.


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

There is no outside from the point of view of inside.

But from another point of view, there may be? You are saying
that whether an outside exists depends on whether you are
inside or outside? Is it also true that there is no inside
from the point of view of outside?

Perhaps the outside is not visible from the inside, but that
is not the same as asserting that it doesn't exist.

You would have to postulate some extra-dimensional being
with a "god's-eye-view". There is no outside in our
three dimensions.


There is no direction that you can point that is in the
direction of "outside".

How do you know that?

Standard theory. I admit that I don't "know" from personal
evidence.


Consider the oft-used balloon analogy. The surface of the
balloon is a 2D representation of the universe.
The flatlanders that live on the surface have absolutely no
conception of the third dimension: they're strictly 2D beings
who've never heard of "up" or "down". They are incapable of
moving "up" or "down", or seeing anything that's "upwards" or
"downwards". Light, in their universe, traverses the balloon
surface (geodesics) and does not leave it. Another balloon
across the room would be absolutely unknown, and non-
interactive.

In order for the balloon analogy to work, the surface of the
balloon has to be curved and closed.

Are you asserting that the Universe is curved and closed,
or are you assuming that it is curved and closed?

The balloon analogy is limited in this respect. It is
possible for space to have other geometries other than spherical
and closed. For example, recent data indicates that it is
flat (euclidean) or very nearly so. Even so, whether or not
it is open or closed remains an open issue.

Matthew F Funke replied to Jeff Root:

Matthew F Funke replied to John Griffin:

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.

Why would it necessarily be redshifted at all? Since it's a
different universe, it didn't take part in the Big Bang. It
may have had its own Big Bang a quadrillion years or so ago,
in which case its near side could be moving toward us at high
speed. The two universes could even be moving toward or away
from each other overall.

To be honest, I took the OP's "another entire universe" to be
a poetic description of its scale. If it were, in fact, another
universe, we wouldn't be able to see it as a fuzzy point of light --
or anything else, for that matter. The entire question would be
irrelevant; information (including light information) doesn't go
between universes.

Could you cite a few examples?

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

Not to mention that spacetime is a function of being within a
universe (assuming it's one like ours); speaking of universes
moving "toward or away from each other" would be nonsensical.

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?

-- Jeff, in Minneapolis

..

(Jeff Root) wrote in sci.astro:


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


It's not as much a matter of "knowing", as well 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.

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. 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.

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

--
CeeBee


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


Google CeeBee @ www.geocities.com/ceebee_2

"Jeff Root" wrote in message
om...
Greg Neill replied to Jeff Root:

Not to mention the fact that if you're seeing it, it's *in* our
universe and not a separate universe.

Under *some* definitions of "universe". Not all.

Clearly, this imaginary thing had no part in our Big Bang, and
existed some time before our Big Bang occurred. I think that
calling it a "universe" is entirely reasonable, and I think
that such other universes could very well exist-- perhaps
infinitely many of them, if space is infinite. Whether we
would be able to see such a distant universe is a question
I'm not sure anyone can answer until we understand the nature
of our own Universe a bit better.

The usual definition applied to our universe is that it
contains everything.

Obviously that definition doesn't apply to the scenario that
John Griffin suggested. "Universe" is a reasonable term to
apply to an entity which resembles our Universe, but which
had a separate and independant origin. Not a new idea.

A separate and independent origin implies no connection.
Embedded means its part of our universe.


If the BB scenario is taken to describe it, space itself was
created and began evolving at the moment of the BB. If so,
there is no space for this other universe to pre-exist in,
in our frame of reference.

But in some other frame of reference there *could* be space
for this other universe to exist in? So whether such space
exists or not depends on where you are? And whether the
other universe exists or not depends on where you are?

We can know nothing about something that does not exist
inside our own universe and amenable to measurement.
It must remain pure speculation.


Certainly it could not exist in our universe and be a
universe itself.

It depends on what you consider a universe to be. The thing
that John Griffin suggested is clearly something which had no
involvement in the Big Bang which began our Universe. It is
also just as big and as well-populated as our Universe. I'd
say that qualifies it as a separate universe, even if there
is some interaction (perhaps only one way) between the two
universes.

I see no profit in arguing semantics. When I say "universe"
I intend it to mean all that is contained and connected by
space and time in which we are embedded. It would seem that
you are wishing to take the more old-fashioned view of the
early astronomers who first recognized other galaxies for
what they were, and referred to them as "island universes".


There can be no connection betwixt or between, so no path
for light to follow.

Why not? What observations support that idea?

By my definition of universe, there can be no connection,
no path between, or else they would be connected by the
same space, thus all the same universe.


The plasma haze you refer to encompassed all of space
existing at the time, and before it things just get worse.
Even if anything were "outside" and shining in, our
universe is likely immensely larger than our teensy
drop-in-the-ocean cosmic horizon.

That idea is almost completely derived from the idea that
an enormous inflation took place during the first fraction
of a second of our Universe's history. As I understand it,
inflation was invented to explain the overall uniformity of
the Universe despite the fact that there was insufficient
time for light to travel to all parts of the Universe, to
give it that uniformity. This appears to have a serious
(and I'd say very obvious) logical flaw which I have never
seen raised:

Why did all parts of the expanding Universe share in the
expansion even before inflation had begun? Had some kind
of information been conveyed to all parts of the Universe
even before inflation started? Yes, obviously so.

If I'm missing something, please tell me what it is.

As the theory goes, the universe (all of space, time, and
its contents) were created at once in a state of immense
density and energy, and began expanding at once. It was
of substantial size before inflation kicked in, and there
would have been initial anisotropies, fluctuations or
wrinkles, if you will, in the uniformity of things at the
time. These anisotropies would have carried through to
present day in the form of immense structures in the
clumping of matter and in the background radiation had
expansion persisted at moderate expansion rates.

Instead, it is thought that inflation kicked in, like
some sort of phase change across the entire existant
universe. This vastly enlarged space and dropped the
density of matter and energy to such an extent that
the anisotropies were lost across local horizons.


Light entering our universe after the decoupling of matter and
light would have unimaginable numbers of light years to travel
to get to us, all of it in space that is moving away from
us at more that the speed of light outside the horizon;
it simply could not get here.

So you think that the speed of light depends on how far away
the light is from the observer? Light coming toward us from a
galaxy nearly at our horizon is moving toward us very slowly?
And light from a galaxy beyond our horizon is actually moving
away from us?

Notice that careful scientists always refer to the "local speed
of light". General relativity places no constraints on the
speed at which space itself may move relative to another patch
of space at great distances. It is just that the speed of
light, as measured by a local observer in any given patch, will
always measure the "local speed of light" to be c.

And, if you think about it, it's not so much that light is
moving more slowly towards us from distant source, but that
the expansion of space creates more space in between the the
source and the observer as the light travels. A series of
observers stationed along the trajectory and sampling the
light stream would always measure the light's speed to be c.

As the light traverses the intevening and expanding space,
the wavelength gets stretched. This accounts for a non-doppler
red-shift component of light from distant sources.

Light heading to us from the very edge of our cosmic horizon
is essentially red-shifted down to infinite wavelength, or
equivalently, zero energy. Light from beyond the horizon
is being emitted by sources travelling away from us at
relative velocities greater than c. The light travels fine
and at c in the local space, but that space is receding
from us at greater than c. The light will never reach us,
as there will forever be increasing distance between the
light and us per unit time than light can cover.

Jeff Root wrote:
Matthew F Funke replied to John Griffin:

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

This universe isn't just "in" space. Space was created during the
Big Bang.

How do you know that? What observations support that idea?

Space is currently expanding and "being created" all the time
*within* the space that we see. (Expansion of the universe doesn't only
happen at the "edges", as it were.) Rewind the tape, so to speak, and
it's not too difficult to visualize the space in one point.

Space doesn't "surround" the universe; space is *part of* the
universe.

How do you know that? What observations support that idea?

Because anything beyond space would be non-universe.

Consider asking an ant what lies beyond the land. All he could
answer definitively would be "non-land", which would somehow be
qualitatively *different* from the land (so that you could *tell* you were
somewhere different).

It seems to me that you're saying light emitted by a universe
stops or vanishes at some kind of boundary. (A universal
event horizon?) I don't know if that's a consequence of the
big bang theory, common sense, or just a conjecture.

AIUI, it's common sense. If the Big Bang created space, then
outside the universe, there's no space for light to travel in.

What gave you the idea that the Big Bang created space?
What observations support that idea?

See above. It's more inference than observation, but there are other
observations (e.g., the 3K background radiation) that fit predictions of
the universe coming into being from a singularity.

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

What would the light travel through?

Space.

Then how do we know we're outside the universe?

What allows the light to perpetuate outside the universe?

There's nothing to stop it from doing so, is there?

Different natural laws. Light follows the simplest possible path in
spacetime; once we're in something *different from* spacetime, and
therefore know that we're outside the universe, light might or might not
perpetuate.

There's also the problem that universes can have radically
different natural laws,

Possibly. But only possibly.

Since we only have one data point, it's difficult to tell with
certainty (hence the helping verb "can" -- and not "must" or "would" --
above). However, it's travelling between universes, where spacetime as we
understand it does not exist, that would especially give problems.
--
-- With Best Regards,
Matthew Funke )

"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.

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?

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