Starship Asterisk*

dlw wrote:When I was a little boy I looked up at the stars and wondered what all was up there and how far it went. That's why I've been an avid APOD follower for many years. I was (and still am) puzzled by the notion that the super-universe is infinite - literally without edge and not curved so that going far enough would bring you back to where you started.

It is entirely likely that the Universe is not infinite. Most cosmologists would probably bet against it being so. But the details of inflation, and the exact geometry of the Universe make it impossible to know for certain its true extents.

The notion that there is a 13.7 BLY "sphere of observability" from any point in the universe reinforces the concept of an infinite universe.

It shouldn't. It should only suggest the possibility that the Universe itself is larger than the part we can see from here. That certainly doesn't imply it is infinite.

Perhaps then the conceptual mistake is thinking of the "big bang" as occuring at a single infinitely small point. Perhaps a better way to think of it as occuring at a single point in "time" (for some definition of time) but physically everywhere (for some definition of those things).

Really, a single point in spacetime. Thus, a point that is outside the three-dimensional spatial universe we observe.

But then what "meaning" does the 91 BLY have? The notion of "observable universe" versus "sphere of observability" is confusing. I understand the latter but not the former if indeed there is a 13.7 BLY SoO from any point in the OU. What's beyond the OU or is there a 91 BLY OU around every point as well?

I'd say both your terms are the same thing. There is a large universe, presumably many times larger than 91 billion ly. From any point in this universe you have an observable sphere with a diameter of 91 billion ly, a diameter dictated by the speed of light and the expansion rate of space. Beyond any observable universe is just more space, the same as anywhere. Don't get confused by the fact that what we see at the edge of the observable Universe is very young; we're just seeing photons that took a long time to get here. What is really out there "now" isn't younger than what is here. Everything in the Universe is the same age, 13.7 billion years old. It's not like you could travel outwards billions of light years and see your surroundings get younger.

Is there a good reference on this subject that a layman might understand?

As already noted, the Wikipedia articles on cosmology, the Universe, the age of the Universe, and the observable Universe are all good starting points.

Chris Peterson wrote:It is entirely likely that the Universe is not infinite. Most cosmologists would probably bet against it being so.

If the rules can be broken, the universe will be infinite. if not universes.

mark swain wrote:If the rules can be broken, the universe will be infinite. if not universes.

It's not a question of rules. "Rules" as used here means the laws of nature, and by definition they can't be broken. The Universe either is or is not infinite. Current knowledge isn't sufficient to determine the answer to that. Someday, maybe.

Chris Peterson wrote:It's not a question of rules. "Rules" as used here means the laws of nature, and by definition they can't be broken. The Universe either is or is not infinite. Current knowledge isn't sufficient to determine the answer to that. Someday, maybe.

quantum fluctuation, makes another set of rules for you chris. read up .. http://en.wikipedia.org/wiki/Quantum_fluctuation

great read.

mark swain wrote:quantum fluctuation, makes another set of rules for you chris. read up .. http://en.wikipedia.org/wiki/Quantum_fluctuation

Nothing new there; this theory has been around for a long time. And I wouldn't say it makes another set of rules, it's just one part of the rules. It has nothing to do with the large scale structure of the Universe or the geometry of the Universe. It's a fundamental part of quantum mechanics and the nature of the vacuum. Interesting, but not topical in this discussion.

so, back to one of my questions, if i've been following corectly..., if i go 9 billion ly in any direction, then when i get there kepp going 9 billion ly in another direction, and so forth and so forth and so forth say 9 billion times more, then how can we possibly make a statement that dark matter is unaccounted for? Seems like an infinite number so how can anybody be close to being right?

My 2¢.....Chris might be best to answer this - 'course anyone's free to take a crack at it....
My understanding is somewhat contrary to the 'spherical' viewpoints.
I read somewhere long ago that our observable universe is oval-shaped, or perhaps more accurately egg- (thinking 3 dimensionally).
This being because we, ie our own galaxy (et al) is moving at a pretty darn good clip also....for instance our entire Local Group is hurtling toward the center of the Virgo cluster at roughly one million miles per hour - and without counting our movement within that, with regard to blue- & red-shifting we can "see" a bit farther in the direction we are moving (blue), & the effect becomes apparent.

Which made/makes me wonder, are there objects that we cannot now see that we once could have, presuming we'd been technologically capable at that point in time - in effect, objects we've lost sight of because of our movement? Or, because even as rapid as our movement might be, we're still not moving faster than light - hence any object once seen will "forever" remain seeable, & not disappear out the short 'back' end of our 'egg'? Which summarily lends that new discoveries will lie in the direction of our movement.

Your thoughts?
~S*H

Star*Hopper wrote:My understanding is somewhat contrary to the 'spherical' viewpoints.
I read somewhere long ago that our observable universe is oval-shaped, or perhaps more accurately egg- (thinking 3 dimensionally).
This being because we, ie our own galaxy (et al) is moving at a pretty darn good clip also....for instance our entire Local Group is hurtling toward the center of the Virgo cluster at roughly one million miles per hour - and without counting our movement within that, with regard to blue- & red-shifting we can "see" a bit farther in the direction we are moving (blue), & the effect becomes apparent.

Which made/makes me wonder, are there objects that we cannot now see that we once could have, presuming we'd been technologically capable at that point in time - in effect, objects we've lost sight of because of our movement? Or, because even as rapid as our movement might be, we're still not moving faster than light - hence any object once seen will "forever" remain seeable, & not disappear out the short 'back' end of our 'egg'? Which summarily lends that new discoveries will lie in the direction of our movement.

Your thoughts?

One observes exactly the same photons whether one is standing still or one is moving.

However, these photons will have higher energy in one direction and lower energy in the other:
http://apod.nasa.gov/apod/ap090906.html

Of course the center of our current observable spherical universe
differs from the center of our previous observable spherical universes
so that the set of all our previous observable spherical universes
might be considered "egg shaped."

Star*Hopper wrote:I read somewhere long ago that our observable universe is oval-shaped, or perhaps more accurately egg- (thinking 3 dimensionally).
This being because we, ie our own galaxy (et al) is moving at a pretty darn good clip also....for instance our entire Local Group is hurtling toward the center of the Virgo cluster at roughly one million miles per hour - and without counting our movement within that, with regard to blue- & red-shifting we can "see" a bit farther in the direction we are moving (blue), & the effect becomes apparent.

That argument doesn't make sense to me. The oldest photons we see are from the period of recombination, about 377,000 years after the Big Bang. And if we develop technology to detect gravity waves, we should be able to see right back to the beginning. I don't see how those fixed times- recombination or the BB- could be different distances because of our motion. They would show different redshifts, of course, but that's a different matter.

It is possible that the observable Universe isn't spherical because space itself is distorted, for instance by some huge mass outside the observable Universe, as has been under discussion elsewhere here.

Which made/makes me wonder, are there objects that we cannot now see that we once could have, presuming we'd been technologically capable at that point in time - in effect, objects we've lost sight of because of our movement?

Definitely. There was certainly material around us before the inflationary period that was causally connected to us then, and isn't now.

Or, because even as rapid as our movement might be, we're still not moving faster than light - hence any object once seen will "forever" remain seeable, & not disappear out the short 'back' end of our 'egg'? Which summarily lends that new discoveries will lie in the direction of our movement.

But we are moving faster than light- with respect to objects outside our observable volume of the Universe. That's what defines that volume. However, the observable Universe itself is getting larger as time goes on. We can see more, not less.

Chris Peterson wrote:

Star*Hopper wrote:I read somewhere long ago that our observable universe is oval-shaped, or perhaps more accurately egg- (thinking 3 dimensionally).
This being because we, ie our own galaxy (et al) is moving at a pretty darn good clip also....for instance our entire Local Group is hurtling toward the center of the Virgo cluster at roughly one million miles per hour - and without counting our movement within that, with regard to blue- & red-shifting we can "see" a bit farther in the direction we are moving (blue), & the effect becomes apparent.

That argument doesn't make sense to me. The oldest photons we see are from the period of recombination, about 377,000 years after the Big Bang. And if we develop technology to detect gravity waves, we should be able to see right back to the beginning. I don't see how those fixed times- recombination or the BB- could be different distances because of our motion. They would show different redshifts, of course, but that's a different matter.

It is possible that the observable Universe isn't spherical because space itself is distorted, for instance by some huge mass outside the observable Universe, as has been under discussion elsewhere here.

I would assume that the relative motion of earth vis-a-vis the observable universe
means that the observable universe is an oblate spheroid due to Fitzgerald contraction.