31 replies to this topic

[StarMountainKid]

I don't like the two dimensional balloon analogy because it's difficult to transfer it to three dimensions. Also, if we picture the universe as a three dimensional expanding globe filled with galaxies, there is still a center of that globe from which everything is expanding away from.

If we imagine the universe as containing 1000 galaxies, and we stand on each one, one after another, and look around, from every galaxy we stand on we see all the other 999 galaxies are moving away from us. In this scenario, where is the center galaxy?

If the universe had a center, the universe would look different than it does. In a universe with a center, in the above thought experiment, as we stood on each galaxy and looked around, we would see some galaxies moving away from us, and in a specific direction some galaxies 'following' us, the galaxies closer to the center. The difference would be that as we looked in a certain direction the universe would look different.  In that direction the galaxies would be seen to be moving away from a specific point. This I think is easy to picture in the mind.

But in reality, we never see this different view, this special direction of expansion. From everywhere we look the expansion looks the same.

[sepulchrave]

Pyridium, on 08 January 2013 - 02:42 AM, said:

WMAP shows a nice round radiation background.  Why does WMAP look like a balloon?

Because WMAP really is a 2D surface. The microwave background permeates all of (3D) space, but we have only one perspective (namely that from Earth) of this background. Since the background is continuous, we can't really make any judgment of ``depth'', so we show the entire microwave background as a 2D projection on the sky... it is basically the reverse of a map projection of the Earth.

The only reason why WMAP looks like a ``balloon'' is because NASA chose to use the Mollweide projection to display the data.

Pyridium, on 08 January 2013 - 02:42 AM, said:

I thiink one of the most important discoveries came just a few months ago when after mapping many points of light over many years, we now know that the universe if flat.  This puts to rest the idea that space and our universe is curved.  That is just one old tired theory that went out the window with just a few years measuring the angles of triangles of stars as observation points.  If the universe was curved, the angles would have added up to more than 180 degrees.  After years of looking, the angles remain a constant 180 degrees, therefore the universe is flat, not curved.

We now know the Universe is flat if it is accurately described by the FLRW metric, and if the laws of physics are static in space and time.