
#37
Jan1912, 03:12 AM

Sci Advisor
P: 4,721

Anyway, the main point here is that a center is a point of symmetry. It's a unique point in the space where stuff is the same in basically every direction from it. The Sun, for example, is a reasonable center for our solar system because if we choose it as our center, the rest of the solar system moves around it. If you're near the Earth, then the center of the Earth makes for a good center for most things. If you're talking about our galaxy, well, that's got a center too: Sagittarius A*. What makes all of these important is they are points of symmetry: the galaxy, or the solar system, or the Earth all look very similar if you rotate those respective systems around those centers. The universe as a whole has no such unique point of symmetry. Instead, you can rotate the entire universe all you like around any point and it will, more or less, look the same. There simply isn't any unique point of symmetry for the whole universe. 



#38
Jan1912, 03:19 AM

P: 75





#39
Jan1912, 03:27 AM

P: 53

But our universe is at least 4d object. And I am talking about time.




#40
Jan1912, 03:42 AM

Sci Advisor
P: 4,721





#41
Jan1912, 11:48 AM

P: 150





#42
Jan1912, 12:38 PM

P: 15,325





#43
Jan1912, 12:44 PM

P: 71

It seems that my sticking point is going from the 'common sense' idea of the universe being a big loaf of baked bread to only a surface of a sphere or a flat or near flat surface. If I just accept the surface idea then I have no problem understanding the other points. Maybe someone can help me to understand that model. Or better stated, at one point decades ago the big 3 dimensional loaf of bread idea must have been the popular idea. Then, at some point, scientists said 'no, even though that seems common sensical, here is some evidence that points to the idea that the universe is curved...or flat...or whatever' and then that seemed correct. Please help me understand that transition and maybe it will help me see all of this.
tex 



#44
Jan1912, 01:13 PM

Astronomy
Sci Advisor
PF Gold
P: 22,800

In cosmology when people say "flat" they mean zero curvature. This can be with 3D space as well as the more familier 2D examples.
What would be an example of UNflat in the 3D case? Well you would know that you were living in an Unflat region of 3D space if when you measured the angles of a triangle you kept getting different from 180 degrees. In a NONflat 3D region, the bigger the triangle the more noticeable the discrepancy. The bread analogy came into use as a picture of expansion. Popular science writers would talk about RAISIN BREAD DOUGH RISING. The raisins were the galaxies and they were getting farther apart. An important part of that picture was that the dough was infinitely big. The loaf had no boundary. If you didn't get that detail then you missed an important part of the story. The rising unbaked breaddough analogy is still used in popular accounts. We sometimes use it here. You seem to have gotten the notion that things have changed because you used to hear the bread dough analogy and now you hear the word "flat". That doesn't represent a changethe messages are consistent. The raisin breaddough analogy actually depicts the case we call "flat". That is infinite volume 3D space with zero curvature where, for example, the angles of a triangle always add up to 180. 



#45
Jan1912, 01:36 PM

P: 38

If a 2D balloon surface makes sense for a 3D thing, wouldn't make an explosion make even more sense ?




#46
Jan1912, 01:48 PM

Sci Advisor
P: 4,721

http://www.youtube.com/watch?v=78l4cU5F2YE By contrast, the early universe was extraordinarily smooth and uniform, only becoming more clumpy much later on due to the gravitational attraction of matter. 



#47
Jan1912, 01:52 PM

P: 38

No need to tell me about explosions see my site




#48
Jan1912, 02:49 PM

P: 75

Can someone post a picture of a geometric shape that they believe the universe most likely resembles. I heard some cosmologist say that they think the universe is like a soccer ball, and it's looped in on its self.




#49
Jan1912, 03:06 PM

Sci Advisor
P: 4,721

Imagine the soccer ball, for example. If our universe happens to be of that shape, then the part of it that we can see is only a teeny tiny fraction of one of the little sections of the ball. So even if the overall shape is rather like a soccer ball, the fact that we can only see this itsy bitsy bit of it means that it would be extremely hard, if not impossible, to ever actually show that it genuinely is that shape. Our best bet of finding the overall shape, if it is possible at all, is to demonstrate that the start of our universe had to happen in a certain way. It may be (but it is absolutely not guaranteed!) that this kind of start tends to occur with a particular shape. So we may be able to indirectly deduce that our universe is probably of that shape, though we're not anywhere close to that kind of indirect deduction yet. Even then, finding direct evidence will always be hard if not impossible. 



#50
Jan1912, 03:18 PM

P: 75





#51
Jan1912, 03:38 PM

P: 150





#52
Jan1912, 03:55 PM

P: 38





#53
Jan1912, 05:47 PM

P: 15,325

There's a fair bit of reading, but you could start with the Cosmological principle and the Principle of Mediocrity, though they are not compelling. 


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