On 2/8/15 3:44 PM, The Starmaker wrote:
> Where is the center of the universe?
>
>
Physics FAQ: Where is the centre of the universe?
>
http://math.ucr.edu/home/baez/physics/Relativity/GR/centre.html
> The cosmological principle
>
> The idea that the universe should be uniform (homogeneous and
> isotropic) over very large scales was introduced as the "cosmological
> principle" by Arthur Milne in 1933. Not long before that, it had
> been argued by some astronomers that the universe consisted of just
> our galaxy, and the centre of the Milky Way would have been the
> centre of the universe. Hubble put an end to that debate in 1924
> when he showed that other galaxies exist outside our own. Despite
> the discovery of a great deal of structure in the distribution of the
> galaxies, most cosmologists still hold to the cosmological principle
> either for philosophical reasons or because it is a useful working
> hypothesis that no observation has yet contradicted. Nevertheless,
> our view of the universe is limited by the speed of light and the
> finite time since the Big Bang. The observable part is very large,
> but it is probably very small compared to the whole universe, which
> may even be infinite. We have no way of knowing what the shape of
> the universe is beyond the observable horizon, and no way of knowing
> whether the cosmological principle has any validity on the largest
> distance scales possible.
>
> In 1927 Georges Lemaître found solutions of Einstein's equations of
> general relativity in which space expands. He went on to propose the
> Big Bang theory with those solutions as a model of the expanding
> universe. The best known class of solutions that Lemaître looked at
> were the homogeneous solutions now known as the
> Friedman-Lemaitre-Robertson-Walker (FLRW) models. (Friedmann found
> the solutions first but did not think of them as reasonable physical
> models). It is less well known that Lemaître found a more general
> class of solutions that describe a spherically symmetric expanding
> universe. These solutions, now known as Lemaître-Tolman-Bondi (LTB)
> models, describe possible forms for a universe that could have a
> centre. Since the FLWR models are actually a special limiting case
> of the LTB models, we have no sure way of knowing that the LTB models
> are not correct. The FLWR models may just be good approximations
> that work well within the limits of the observable universe but not
> beyond.
>
> Of course there are many other even less uniform shapes the universe
> could have, with or without an identifiable centre. If it turned out
> to have a centre on some scale beyond the observable universe, such a
> centre might turn out to be just one of many "centres" on much larger
> scales, just as the centre of our galaxy did before.
>
> In other words, although the standard Big Bang models describe an
> expanding universe with no centre, and this is consistent with all
> observations, there is still a possibility that these models are not
> accurate on scales larger than we can observe. We still have no real
> answer to the question "Where is the centre of the universe?".
--
sci.physics is an unmoderated newsgroup dedicated to the discussion
of physics, news from the physics community, and physics-related
social issues.