well known question about the universe and its size

[Pierebean]

when i see this i am quiet disturbed: http://www.anzwers.org/free/universe/universe.html
here the picture:



if our artificial eyes are able to see several billion of light years away, it's clear that we can see bearly the entire universe. Indeed the universe is about 13E9 years old. though, the universe do not have any border either a centre(because it is not an "euclidian geometric structure" blablabla).

so how it is, i mean what geometric shape?
-not a sphere,i guess
-maybe a donut?

please tell me stargazer communauty.

[t00fri]

when i see this i am quiet disturbed: http://www.anzwers.org/free/universe/universe.html
here the picture:



if our artificial eyes are able to see several billion of light years away, it's clear that we can see bearly the entire universe. Indeed the universe is about 13E9 years old. though, the universe do not have any border either a centre(because it is not an "euclidian geometric structure" blablabla).

so how it is, i mean what geometric shape?
-not a sphere,i guess
-maybe a donut?

please tell me stargazer

Pierebean,

the spectacular and highly accurate results from the recent Cosmic Microwave Background (CMB) experiments (BOOMERANG, WMAP) have told us that the geometry of the Universe is flat to good accuracy.

This means the usual rules of Euclidean geometry taught in schools are observed in the Cosmos: straight lines can be extended to infinity, the angles of a triangle add up to 180 degrees and the circumference of a circle is equal to 2pi times the radius, etc.

The precise geometry has been the subject of much debate since Albert Einstein suggested that the Universe might actually be "curved". Some cosmologists have championed spherical and even hyperbolic (saddle-like) models.

Bye Fridger

[Michael Kilderry]

What made the universe so flat exactly? Sounds a little unnatural.

Michael Kilderry

[Pierebean]

yes, what do you mean by flat?
afterall, the earth might be flat as well, if we consider it as an "closed space" . when you walk straight fowards you come back at the same point after a long and boring treck. Is it the same for the universe? when you fly straight fowards after a long and endless trip you come back at the same point (considering a closed space with 4 dimension). But i think this is a clich?©.
i'm sorry, i don't get this topologic stuff at all.

[t00fri]

Please, you must read my mail carefully. I have given the /precise/
definition of a flat (Euclidean) geometry above. In a spherical
geometry, for example, the angular sum in a triangle is NOT equal to
180 degrees and parallel lines intersect
at finite distance, etc...(amateur astronomers should at least have
heard about "spherical trigonometry")

A 2 dimensional spherical (hence NON-flat) geometry is what we have
on the surface of a sphere of finite radius. Just go and measure the
angles of a triangle you have drawn on an apple, for example.
3-dimensional non-flat geometries are harder to visualize, but
mathematically there is nothing special.

Please note that a flat Universe, of course, does not mean that the
Universe has a pancake shape! The statement is about the /type of
geometry/ that holds in space (-time).

A flat geometry of the Universe is all but unnatural. It perfectly fits
into our favorite picture of "Inflation" of the early Universe. It would
take me too much time to recapitulate "Inflation" in case this is not yet
familiar. But there is plenty of pedagogical literature (also in the net)
about it.

Bye Fridger