I guess this is almost a philosophical question.

Spacetime is expanding yes? So if we took a snapshot of the universe as it is now its "not infinite" but its continually expanding right? So its um... "potentially infinite?"

If its not currently infinite, what is at the "edges"? Nothing? So if you somehow fly in a spaceship to the edge of the current universe.. what happens? You hit a wall of "nothing" not even empty space?

I guess this is almost a philosophical question.

Spacetime is expanding yes? So if we took a snapshot of the universe as it is now its "not infinite" but its continually expanding right? So its um... "potentially infinite?"

If its not currently infinite, what is at the "edges"? Nothing? So if you somehow fly in a spaceship to the edge of the current universe.. what happens? You hit a wall of "nothing" not even empty space?

If you travel on the surface of a sphere you can go on forever without finding any boundary. And the radius of the sphere might be expanding.
Tullio

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<<<<<<<<<<
I guess this is almost a philosophical question.

Spacetime is expanding yes? So if we took a snapshot of the universe as it is now its "not infinite" but its continually expanding right? So its um... "potentially infinite?"

If its not currently infinite, what is at the "edges"? Nothing? So if you somehow fly in a spaceship to the edge of the current universe.. what happens? You hit a wall of "nothing" not even empty space?

______

If you travel on the surface of a sphere you can go on forever without finding any boundary. And the radius of the sphere might be expanding.
Tullio
>>>>>>>

If you travel on the surface of a sphere, you are always accelerating to remain on that surface. Whether due to gravitational attraction toward the interior masses, dark matter or whatever, it is still needed. Using internal mass to provide that thrust will not work for long.

duke

If you travel on the surface of a sphere, you are always accelerating to remain on that surface. Whether due to gravitational attraction toward the interior masses, dark matter or whatever, it is still needed. Using internal mass to provide that thrust will not work for long.

duke

The path might be a geodetic line in a spacetime continuum, according to Einstein's general relativity.

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I guess this is almost a philosophical question.

Spacetime is expanding yes? So if we took a snapshot of the universe as it is now its "not infinite" but its continually expanding right? So its um... "potentially infinite?"

If its not currently infinite, what is at the "edges"? Nothing? So if you somehow fly in a spaceship to the edge of the current universe.. what happens? You hit a wall of "nothing" not even empty space?

If you travel on the surface of a sphere you can go on forever without finding any boundary. And the radius of the sphere might be expanding.
Tullio

Philosophy was never my strong point, but if I may enquire?
Do you refer to the interior or exterior surface of a sphere?
Are you within, or without (outside) the sphere whose surface you travel?

Andrew

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AndrewM

I guess this is almost a philosophical question.

Spacetime is expanding yes? So if we took a snapshot of the universe as it is now its "not infinite" but its continually expanding right? So its um... "potentially infinite?"

If its not currently infinite, what is at the "edges"? Nothing? So if you somehow fly in a spaceship to the edge of the current universe.. what happens? You hit a wall of "nothing" not even empty space?

If you travel on the surface of a sphere you can go on forever without finding any boundary. And the radius of the sphere might be expanding.
Tullio

Philosophy was never my strong point, but if I may enquire?
Do you refer to the interior or exterior surface of a sphere?
Are you within, or without (outside) the sphere whose surface you travel?

Andrew

I would be outside, but the surface I am thinking of is three dimensional, while the sphere is four dimensional (three spatial dimension and one temporal dimension). So the sphere is like an expanding bubble. There is a very interesting article in the NYTimes on modern cosmological theories that look even more far-fetched than this, so I am in good company.
Tullio

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I guess this is almost a philosophical question.

Spacetime is expanding yes? So if we took a snapshot of the universe as it is now its "not infinite" but its continually expanding right? So its um... "potentially infinite?"

If its not currently infinite, what is at the "edges"?

The easiest model for my silly brain to understand is curved, finite, unbounded, and expanding into 5D space.

The sphere analogy....

I guess the definition of "infinite" would be in question in that scenario, at least from a layman perspective if you can "go on forever and never find and end" i guess that's "virtually(subjectively?) infinite" if its not , lets say "objectively infinite".

I assume there has got to be standard definitions for these concepts somewhere. The stuff i read is all 'general reader" stuff (Brain Greene , Carl Sagan etc) and they tend to use analogies , rather than the actual terms i guess.

The sphere analogy....

I guess the definition of "infinite" would be in question in that scenario, at least from a layman perspective if you can "go on forever and never find and end" i guess that's "virtually(subjectively?) infinite" if its not , lets say "objectively infinite".

I assume there has got to be standard definitions for these concepts somewhere. The stuff i read is all 'general reader" stuff (Brain Greene , Carl Sagan etc) and they tend to use analogies , rather than the actual terms i guess.

There is a solution to Einstein's general relativity equations in which the Universe returns, after a long time, to its initial state, that is its evolution is cyclic. It was found by the great logician Kurt Goedel but of course there is no way to verify or falsify it. It seems to me that the sphere analogy is patterned along this idea, for whatever it may be worth.
Tullio

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If you are really interested, I've read two great books on the subject:

"The View from the Center of the Universe" - by Joel R. Primack and Nancy Ellen Abrams.

and "Cosmic Jackpot" by Paul Davies.

I think this books complememnt each other and they are very well written.

Most likely the best way to get your mind around this is to think about Dimensions. Lets take away one spatial dimension and think that we are in a two dimensional space (plus time). Assume that our universe is shaped like the surface of a sphere and we cannot see the third dimension --In other words we are now living right on the surface--no meaning to the inside of the sphere and no height above this surface. So we are just like a very flat bug crawling along on a basketball.

The surface of a sphere is curved back upon itself . It is said to be finite but there is no boundary in the two dimensions. If the sphere were sufficiently big the bug would think that his universe was flat. He would also see that Euclid's Geometry would hold over regions that were relatively small when compared to the size of the bug's total universe.

If the two dimensional universe were expanding then it would be expanding into the third dimension of which the bug has no knowledge. The measurable size of the two dimensional surface would, of course, be increasing

So if we extrapolate this thinking to three dimensions then we would be in a finite, unbounded universe that would appear to be flat. It would be increasing in size. I don't know what it means (or if it is true) that it is expanding into the fourth spatial dimension.

Mathematically, adding another dimension and then differentiating Einstein's equations of general relativity in three dimensions yields Maxwell's equations. It was really Maxwell who showed that the speed of light is not dependent on the speed of the observer. Does this mean that there really is a fourth spatial dimension (or 10 or 25) that we can't see? I would think, "yes" but Math doesn't always yield reality all of the time--just most of the time.

Regards,

DADDIO

Hi again,

I'm definetly a layman when it comes to all of this. (I.e., I haven't gone far beyond just watching Sagan's Cosmos series and reading some of Brian Greene's Elegant Universe.)

Previously, I posted:

"The easiest model for my silly brain to understand is curved, finite, unbounded, and expanding into 5D space."

Or, as William posted, expanding into a 4D spatial dimension.

Anyway, while reading something, my layman brain came up with an idea. I thought it may have been a unique idea, until I discovered that it was asked a century ago. LOL

I wouldn't even know how to begin doing the math on this, but it just seemed to "make sense".

Here is the idea: The universe is curved, finite, unbounded, expanding into a dimension we can't experience, etc, but also spinning.

Would it make sense if the entire universe was spinning? I.e., could a spinning universe account for so-called "dark matter"? "Dark matter" simply being the centrifugal force?

The "makes sense" part comes when I look at galaxies. Galaxies look almost identical to hurricanes. The spinning of the Earth causes hurricanes' unique design, so couldn't the spinning of the entire universe cause the similiar design of galaxies?

Just drawing straws here, I know...

Hi again,

I'm definetly a layman when it comes to all of this. (I.e., I haven't gone far beyond just watching Sagan's Cosmos series and reading some of Brian Greene's Elegant Universe.)

Previously, I posted:

"The easiest model for my silly brain to understand is curved, finite, unbounded, and expanding into 5D space."

Or, as William posted, expanding into a 4D spatial dimension.

Anyway, while reading something, my layman brain came up with an idea. I thought it may have been a unique idea, until I discovered that it was asked a century ago. LOL

I wouldn't even know how to begin doing the math on this, but it just seemed to "make sense".

Here is the idea: The universe is curved, finite, unbounded, expanding into a dimension we can't experience, etc, but also spinning.

Would it make sense if the entire universe was spinning? I.e., could a spinning universe account for so-called "dark matter"? "Dark matter" simply being the centrifugal force?

The "makes sense" part comes when I look at galaxies. Galaxies look almost identical to hurricanes. The spinning of the Earth causes hurricanes' unique design, so couldn't the spinning of the entire universe cause the similiar design of galaxies?

Just drawing straws here, I know...

You know what is really weird--not more than 30 minutes ago i was watching Fillipenko's webcast on ASTRONOMY from the Berkley free on-line courses. I was reminded of an observation I made while at a car dealer's coffee bar. I was spilling a little powdered creamer into a cup of black coffee that I was stirring with a swizzle stick. The powder quickly spun out into the edge of the cup and formed a perfect image of a spiral galaxy .

While professor Fillipenko was explaining the rotational speed of the matter in a spiral galaxy and attributing it to dark matter--I asked myself the same question --what if space itself were rotating ! Would that provide the same effect as the missing mass--sort of like the force which appears when we turn the corner in an automobile.

Fascinating to think about.

Hi again,

I'm definetly a layman when it comes to all of this. (I.e., I haven't gone far beyond just watching Sagan's Cosmos series and reading some of Brian Greene's Elegant Universe.)

Previously, I posted:

"The easiest model for my silly brain to understand is curved, finite, unbounded, and expanding into 5D space."

Or, as William posted, expanding into a 4D spatial dimension.

Anyway, while reading something, my layman brain came up with an idea. I thought it may have been a unique idea, until I discovered that it was asked a century ago. LOL

I wouldn't even know how to begin doing the math on this, but it just seemed to "make sense".

Here is the idea: The universe is curved, finite, unbounded, expanding into a dimension we can't experience, etc, but also spinning.

Would it make sense if the entire universe was spinning? I.e., could a spinning universe account for so-called "dark matter"? "Dark matter" simply being the centrifugal force?

The "makes sense" part comes when I look at galaxies. Galaxies look almost identical to hurricanes. The spinning of the Earth causes hurricanes' unique design, so couldn't the spinning of the entire universe cause the similiar design of galaxies?

Just drawing straws here, I know...

You know what is really weird--not more than 30 minutes ago i was watching Fillipenko's webcast on ASTRONOMY from the Berkley free on-line courses. I was reminded of an observation I made while at a car dealer's coffee bar. I was spilling a little powdered creamer into a cup of black coffee that I was stirring with a swizzle stick. The powder quickly spun out into the edge of the cup and formed a perfect image of a spiral galaxy .

While professor Fillipenko was explaining the rotational speed of the matter in a spiral galaxy and attributing it to dark matter--I asked myself the same question --what if space itself were rotating ! Would that provide the same effect as the missing mass--sort of like the force which appears when we turn the corner in an automobile.

Fascinating to think about.

This is a very good and fascinating question. It is a great subject to ponder apon.

---

Would it make sense if the entire universe was spinning? I.e., could a spinning universe account for so-called "dark matter"? "Dark matter" simply being the centrifugal force?

My initial reaction to this was impossible! I mean, if the universe was spinning, then that would mean there has to be a center/axis for it to spin around and we don't see evidence of that. Also, the speed of light would make it impossible for the outer edges of the universe to keep up with the inner portions and again we'd see evidence of spirals in the larger structure of the universe.

Then I realized that I was thinking 3 dimensionally. When a sphere spins in 3 dimensions, the '2D' inhabitants on its surface would not see the sort of evidence above, but it would still spin. If the universe was spinning in 4D space (or 5D if you include time as a dimension), then just maybe we would see these effects (spiral galaxies/expanding space) without the universe wrapping itself into a knot.

[edit]BTW - it's "Dark Energy" that's blamed for the universe expanding, not "Dark Matter". [/edit]

It's time that DADDIO weighed in on this matter (pun intended)

DADDIO, you are wacky! =)

Ok, let's take it down about 50 light years so that I can help understand the
'edge' of the universe theory.

1. Let's remove all galaxies, stars and debris in space.
2. Let's add 1 pineapple hand grenade pin not pulled floating 'stationary' in space.

Ok, let's say that pin suddenly disappears and the hand grenade explodes
(big-bang). Now, all the fragments of this grenade are traveling outward at a
good rate of speed.

*Big questions I hear that don't make sense.*

- "The universe/hand grenade fragments are expanding faster and faster."
I don't get this. If you measured the speed of the fragments from the center of
the explosion, wouldn't that object in motion continue at that rate of speed?
(being that the explosion is done with in the first 1/4 second)

- "There's an edge of the universe"
Let's say I had a craft that travels faster than the speed of the grenade
fragments. So if I started my craft at the center of the explosion and headed
in any direction in a straight line, I would eventually pass the radius of the
fragments. Is the radius of the fragments the edge of the universe? Cause I
don't know about you, but my sweet space craft has tons of magic fuel left and
can keep going for trillions of light years past the fragment radius but the 1
flaw is, I can't turn it. So it continues to go straight until *what happens?*
nothing right? I just keep going into blackness further and fuhrer away from
the fragments.

- Gloves Q

- "The universe/hand grenade fragments are expanding faster and faster."
I don't get this. If you measured the speed of the fragments from the center of
the explosion, wouldn't that object in motion continue at that rate of speed?
(being that the explosion is done with in the first 1/4 second)

Not that I can argue effectively against this, but 'In My [Humble] Opinion':

Newton's First Law of Motion:
I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

One of the 'external forces' is friction.
Wouldn't "the fabric of space" provide one potential source of friction?

- "There's an edge of the universe"
Let's say I had a craft that travels faster than the speed of the grenade
fragments. So if I started my craft at the center of the explosion and headed
in any direction in a straight line, I would eventually pass the radius of the
fragments. Is the radius of the fragments the edge of the universe? Cause I
don't know about you, but my sweet space craft has tons of magic fuel left and
can keep going for trillions of light years past the fragment radius but the 1
flaw is, I can't turn it. So it continues to go straight until *what happens?*
nothing right? I just keep going into blackness further and fuhrer away from
the fragments.

If you were to graph r(theta)=(3/5)cos(theta) (In 'Polar Graph' mode) on a graphing calculator (especially a TI-85 [or TI-92+, which I own]), the 'line' tends to circle around a central point. No matter how far you move (as represented by [theta]), you wouldn't travel far from any point contained on the/a graph of your path. This would be a good visual representation of traveling in a "Closed Universe" or 'Curved Space'.

---

More on the issue.

In theory the mass (energy ) of the initial universe would eventually slow the expansion due to the gravitational attraction--just like when you throw a ball in the air it slows down and stops at the top and then falls towards the earth. The same should happen to the expanding Universe. The expansion should be slowing due to gravity. It should eventually just stop and then maybe begin contracting depending on the total mass and size and their gravitational effect. Instead it appears to be accelerating in its expansion-due to some unknown force that seems to emanate from the vacuum itself. And since there is more and more space there is an increasingly larger amount of vacuum and then more of the energy which seems to spring from the vacuum.

Since we don't perceive the rotation of the earth directly we may not perceive rotation and conservation of momentum in an unseen dimension. All of this is just idle speculation--perhaps someone could add some math to see if this makes sense.

In the beginning your spaceship would circle around the closed universe--just as the bug would walk on an expanding basketball--if the bug moved faster than the expansion then he might come back to places he had been before. A two dimensional bug would not perceive rotation of the basketball which would expand and rotate in dimensions that he could not see. He would see the expanded two-dimensional world however.The universe apparently is expanding and has expanded faster than even the speed of light. There is apparently a horizon beyond which we cannot see since the light has not yet reached us and some of it never will due to space expanding faster than light travels.

Actually we can perform experiments to verify that our earth is rotating. Foucault's Pendulum is one example and the Coriolis effect is another. Artillery calculations have to take into account the fact that the earth rotates under the artillery shell.

Can we devise an experiment that shows if space is rotating relative to some fixed inertial plane in our 3-space universe or rather rotating in another unseen dimension.

Fascinates me to think about it.

DADDIO