twice as fast

9/28/2011 2:26:42 PM

I don't think I've ever seen the universe depicted as flat. Expanding cones, balloons, etc, yeah, but it is assumed to be infinite in all directions. Thus the gravity from all shit to the right for infinity is cancelled by all the gravityfrom the shit to the left... you only need to look at local gravitational influences, since in an infinite universe, there are so many galaxies randomly dispersed that it is considered homogeneous.

[Edited on September 28, 2011 at 5:05 PM. Reason : .]

9/28/2011 5:04:35 PM

^galaxies aren't randomly distributed. There are groups and then clusters and superclusters of galaxies.


(example)

[Edited on September 28, 2011 at 5:25 PM. Reason : ]

9/28/2011 5:24:29 PM

Locally, you are correct. Over infinite distance, the clusters approximate out to random homogeneous distributions.

[Edited on September 28, 2011 at 5:32 PM. Reason : actually i shouldnt say approximate... it IS a random homogeneous distribution over inf distance]

9/28/2011 5:30:29 PM

if you are talking about infinite distance, then the all of the matter in the universe would resemble a single point in the center of an infinite expanse of nothingness.

9/28/2011 5:34:37 PM

Not quite... when calculating gravitational influences, if one were to mark a center of an object, all matter outside of a given radius (assuming uniform spherical distribution) has no net gravitational impact. For this reason, the vast majority of matter in the universe can be ignored, as it is an infinite sphere with infinite matter.

All matter inside of that radius has a net gravitation attraction, though. Which can be represented a a sum of the mass acting from the center of mass on said chosen body.

Feel free to argue with hawking on the issue, if it troubles you, iirc he discusses it in some detail in one of his books.

It's pretty widely accepted that we can ignore all the shit we can't see, as a result.

Then theres the argument that if it's infinite, any one point can be chosen as the center of mass, since the universe extends infinitely in all directions from this point.

[Edited on September 28, 2011 at 6:09 PM. Reason : .]

[Edited on September 28, 2011 at 6:11 PM. Reason : .]

[Edited on September 28, 2011 at 6:14 PM. Reason : nd if im doing shitty job explining, its my birthdy and im drunk ]

9/28/2011 6:06:34 PM

happy birthday.
i've read hawking's pop stuff too.



and our observable universe (a sphere with a radius of ~14 billion light years) would be the only thing that matters when calculating gravitational stuff (since gravity moves at the speed of light)

[Edited on September 28, 2011 at 7:45 PM. Reason : ]

9/28/2011 7:43:16 PM

Also correct. But that's to us. An object 14 billion ly away sees quite a different universe. It has a different 14 Gly bubble.

9/28/2011 8:53:06 PM

this isnt really a big deal.

taco bell and/or white chocolate breaks the speed of light through my innards.

9/28/2011 8:57:53 PM

Quote :

"random homogeneous"

Aren't these things almost opposites?

The odds that a random distribution would turn out even remotely homogenous are really low.

Clusters would appear in any random distribution. Distribution without clumps would be evidence against randomness.

But, then again, I know nothing of astronomy.

9/28/2011 9:35:30 PM

But you left infinity out of that equation. Look at clusters by putting pennies spaced some distance apart... the more you repeat the random dispersion, and the further out you go, its close enough to a uniform distribution. As you approach infinity, it becomes uniform.

/as taught by a prof who was an astrophysicist

9/28/2011 9:41:38 PM

ha, yeah, I realized that about 10 seconds after I posted.

9/28/2011 9:58:43 PM

Sometimes I wish I had grown up to be an Astrophysicist. Then I come here and read through discussions like this one and I'm very glad I'm not. My brain would have exploded long ago.

9/29/2011 7:20:59 AM

but there is not infinite mass in the universe.

9/29/2011 7:21:27 AM

if the universe were on a treadmill would it take off?

9/29/2011 1:54:48 PM

if we never talked about the universe again, would it exist?

9/29/2011 2:13:07 PM

When I said flat I wasn't referring to shape, but curvature. http://en.wikipedia.org/wiki/Shape_of_the_Universe

I don't fully understand it (who the hell does) but the bottom line is you ain't looping around from one side to another by getting to the edge (if that was even possible).

Spacially, it's most likely spheroid in shape like you'd expect.

Quote :

"but there is not infinite mass in the universe."

He said "as you approach infinity" for a reason.

Quote :

"if the universe were on a treadmill would it take off?"

If it's anything like a plane, then yes.

[Edited on September 29, 2011 at 2:45 PM. Reason : .]

9/29/2011 2:42:31 PM

An old astrophysicist joke:

If i'm sitting on a plane, and someone asks me what i do for a living... if I want to have a conversation, I say i'm an astronomer. if i dont, I say i'm a physicist.

--
I did some astrophysics research under a professor my freshman year. It was cool shit, I like thinking about it, and talking about it. But it got to a point where I wasn't sure I could continue doing the classes to get a PhD in physics.. quantum was too hard. I think I have the discipline now, but frankly I'd rather be an engineer and design and build shit on these massive experiments, like the lhc, etc.

/cool story bro

9/29/2011 6:48:29 PM

^ I had a similar experience - did a really cool study program in radio astronomy at the NRO in Green Bank, WV under some big name guys from UNC. I really enjoyed it, but the thought of getting into quantum and all that mess was much less appealing than mechanical engineering, and as you said, getting to design and build the cool machines sounded much better than analyzing their data.

[Edited on September 29, 2011 at 7:31 PM. Reason : ]

9/29/2011 7:29:48 PM

Quote :

"if the universe were on a treadmill would it take off?"

Quote :

"Spacially, it's most likely spheroid in shape like you'd expect."

You don't know that, you don't even have a good reason to say that.

The classic "scenarios" are just like the suppositions for the ultimate fate of the universe. Cosmology has long sought to answer the question of whether it will collapse, even out to a given size, or expand forever. We know the answer to that, not for the reason we expected, but due to the presence of dark energy. The universe will expand forever, rip itself apart, and do so faster than we were ever able to theorize before taking measurements that told us this.

The idea that the universe is likely spheroid because that would be theoretically easier is just another example of the hubris of humans. As far as we've been able to measure (and lord knows astronomers have tried), space appears to be completely flat on the largest scale. This doesn't mean we should stop measuring, because even the smallest negative or positive curvature would have profound implications. In the case that space is hyperbolic, then we have no choice other than to accept space as being infinite. If the curvature is positive, then it's like a 4-D beach ball and everything makes perfect sense as a finite universe.

The only thing we really know is that we haven't figured it out.

9/29/2011 7:49:37 PM

Quote :

"He said "as you approach infinity" for a reason."

yes, but the amount of space approaches infinity a lot faster than the amount of matter.

from all the data we have available, the universe is definitely organized into superclusters and whatnot, and to say "oh well if you zoom out far enough it won't" isn't accurate... if you zoomed out far enough, the matter would be clustered in the "center" of the universe and empty space would spread out away from the matter (assuming that the space expanded faster than c during and after the big bang and normal matter did not spread faster than c.)

I'm not physicist, and admittedly I like watching stuff with Michio Kaku, et al. (he shares my birthday by the way)... not claiming to be "right" as I would love to meet someone who truly does understand the universe completely (and i'm willing to bet nobody on earth does)

10/1/2011 12:17:28 AM

man i came to check to see if anyone had eaten their boxer shorts on live television, but you dorks are just conjecturing on tangentially-relevant shit

10/1/2011 8:58:36 PM

YEEEEEP!

10/1/2011 10:29:46 PM

Quote :

"We know the answer to that, not for the reason we expected, but due to the presence of dark energy."

you left out the word "supposed" before the word "presence". At this point, the postulation of DE is essentially a giant fudge factor to make the observations fit with our expectations.

10/1/2011 11:34:17 PM

http://www.sciencedaily.com/releases/2011/09/110929144645.htm

10/2/2011 9:27:05 PM

http://youtu.be/0Tqdc6x7sZY

10/3/2011 10:52:24 AM

Quote :

"from all the data we have available, the universe is definitely organized into superclusters and whatnot, and to say "oh well if you zoom out far enough it won't" isn't accurate... if you zoomed out far enough, the matter would be clustered in the "center" of the universe and empty space would spread out away from the matter (assuming that the space expanded faster than c during and after the big bang and normal matter did not spread faster than c.)"

This is simply not correct. The idea of something expanding faster than c is muddling up the concepts of general relativity. There are two different values for c, and locally light always travels at c in every direction. However, when separated by a large gravitational potential and experiencing a large gravitational field, the apparent value of the speed of light will be different. Something can travel faster than the speed of light and something can be prohibited from traveling in certain directions altogether (like in a black hole), but these are only from the "perspective" of a different observer and is a result of the warping of space.

So to directly address your misunderstanding, yes, matter is traveling away from us faster than the speed of light, but the the laws of physics are still consistent for us as well as that matter. In general, distance, speed, and all that does not have the conventional properties you're used to in the presence of an event horizon. Matter traveling away faster than the speed of light is beyond an event horizon of a sort imposed by the acceleration of space (dark energy).

Quote :

"I'm not physicist, and admittedly I like watching stuff with Michio Kaku, et al. (he shares my birthday by the way)... not claiming to be "right" as I would love to meet someone who truly does understand the universe completely (and i'm willing to bet nobody on earth does)"

Michio Kaku is one of the worst promulgators of bad science out there. It's not that he doesn't understand things like quantum field theory, but the mediums through which he communicates leave some of the most confused and distorted pictures of physics out there. He's too wrapped up in the desire of people to believe so he goes on anti-intellectual diatribes trying to justify people's interest in things like light sabers. It's embarrassing.

10/3/2011 11:18:59 AM

my point was not that the universe is actually configured like that... my point was that it IF the universe had infinite space (as you seem to be saying), then that might be what it would look like since there is definitely NOT infinite mass.

and every image of our observable universe shows a definite structure of the universe, and there is no evidence to suggest that this is a fluke, and therefore matter is NOT scattered at random like you attest.

10/3/2011 11:29:21 AM

Observable <<< infinity

10/3/2011 11:34:36 AM

durr

10/3/2011 11:56:37 AM

^^^ It's preposterous to think of a universe with infinite space and finite mass. Infinite mass follows with infinite space. It's the mass density that's constant over large scales. An infinite universe is still a possibility that no one can mange to disprove (believe me, I would love to rule it out, I hate the idea, but I can't). Such an infinite universe would entail infinite mass.

10/3/2011 1:19:22 PM

Quote :

"It's the mass density that's constant over large scales."

every model of the universe that I have ever seen refutes this statement.

10/3/2011 4:15:31 PM

What would be the implications if we found out the universe really did extend to infinity in all directions?

10/3/2011 5:30:34 PM

Quote :

"every model of the universe that I have ever seen refutes this statement."

...

Would you like to describe to me a universe that is heterogeneous on large scales?

Quote :

"What would be the implications if we found out the universe really did extend to infinity in all directions?"

Nothing really if the expansion of the universe prevents us from ever reaching beyond a certain a certain galaxy.

10/3/2011 5:44:09 PM

Quote :

"Nothing really if the expansion of the universe prevents us from ever reaching beyond a certain a certain galaxy."

doubt it.

Given any point you pick beyond a certain galaxy, we extend passed that certain galaxy if you were to be standing at that point looking backwards.

10/3/2011 6:02:31 PM

is there a finite # of galaxies that a photon emitted from Earth, today, could potentially reach until the end of the universe?

10/3/2011 6:54:33 PM

If you know the date the universe ends, then yes.


Otherwise, time is infinite

10/3/2011 7:10:23 PM

ok, good. at least I know what you're claiming.

Now, given the following:

- "dark-energy" exists, meaning that all galaxies (aside from the nearby ones) are accelerating away from us
- according to general relativity, when ( acceleration x distance ) is greater than a certain critical value, an event horizon is formed

These two things imply that anything on this Earth today is fundamentally physically limited in terms of the furthest galaxy it could ever reach. In fact, there exists a solar system somewhere that the photons emitted from Earth today can reach, but the photons we emit tomorrow can not reach.

Every second there are countless worlds slipping out of our maximum field of reachability. Every day another star falls out the edge of the universe. For Baryonic matter like us, the furthest extent of what we may ever practically reach is much more limited.

You seem to disagree. Now, there are plenty of avenues to do so. The nature of dark energy remains fairly elusive. It could just cut off in another billion years. We don't know. We don't have a theory that sufficiently explains dark energy.

10/3/2011 8:29:25 PM

Quote :

"Would you like to describe to me a universe that is heterogeneous on large scales?"

superclusters are pretty damn large scale.

10/3/2011 8:46:07 PM

Concept of infinity is grossly misunderstood itt

10/3/2011 9:01:01 PM

http://www.nature.com/nature/journal/v319/n6056/abs/319751a0.html

"Large-scale homogeneity of the Universe measured by the microwave background"

basically, this paper states that the universe is measured to have homogeneous distribution at scales on order of 100 million light years.

Pretty huge, but on the scale of infinity, it's really small. Regardless of how big your superclusters are, infinity is still bigger.

10/3/2011 9:10:00 PM

of course infinity is bigger, however the universe is not infinite.

10/4/2011 10:33:59 AM

and the background microwave radiation came from a time when the universe was undergoing the big bang and was a LOT different.


This image is about a billion light years across and the mass is definitely NOT homogeneous:

10/4/2011 10:36:31 AM

Quote :

"Not quite... when calculating gravitational influences, if one were to mark a center of an object, all matter outside of a given radius (assuming uniform spherical distribution) has no net gravitational impact."

Is this really a controversial statement?

10/4/2011 11:06:30 AM

^No, it was never intended to be. Its fact... its background information..

^^You keep trying to apply smaller scales to objects than are even being talked about. That image, if you broke it into pixels 100 million ly across, and measured the mass, it would be homogeneous. If I wanted to apply a scale of 20cm to it, it's OBVIOUSLY not homogeneous. i give up, you're reasonably intellegent, but there's no getting through to you.

[Edited on October 4, 2011 at 12:25 PM. Reason : .]

10/4/2011 12:19:39 PM

Boy, I sure am glad the universe is not homogenous on the scale of a human!

10/4/2011 2:34:35 PM

Quote :

"and every image of our observable universe shows a definite structure of the universe, and there is no evidence to suggest that this is a fluke, and therefore matter is NOT scattered at random like you attest."

Can someone explain/elaborate this?

Sounds interesting and counter-intuitive. If I dump marbles on the floor, there will be marbles clumped together randomly but I wouldn't expect a "structure" either.

10/4/2011 3:22:10 PM

gravity.

[Edited on October 4, 2011 at 3:26 PM. Reason : its structured, but randomly structured... and is homogeneous over scale > 100 Mly]

10/4/2011 3:25:17 PM

Well, I figured that much. Same would go for galaxies, solar systems, stars, and planets, no?

But Smath74 seems to be implying a non random distribution.

10/4/2011 3:56:20 PM

Quote :

"You keep trying to apply smaller scales to objects than are even being talked about. That image, if you broke it into pixels 100 million ly across, and measured the mass, it would be homogeneous. If I wanted to apply a scale of 20cm to it, it's OBVIOUSLY not homogeneous. i give up, you're reasonably intellegent, but there's no getting through to you. "

don't get upset, this is all really interesting

I think I might be learning something here. My up the minute on cosmology and GR, in general, isn't that strong.

...

why does puck_it think the universe is infinite again?

I'm confused a bit.

...

or more importantly, are we saying the big bang DID NOT create spacetime?

[Edited on October 4, 2011 at 6:02 PM. Reason : .]

10/4/2011 5:59:51 PM