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Originally posted by Merriman Weir
I'm not entirely sure about the 'centre of the universe' issue. However, if that's the case, then maybe it goes some way to explaining why our planet appears to a hotspot for alien visitation. Surely scientifically advanced alien races would be incredibly curious about probing or visiting the 'centre of the universe'?
The balloon analogy is very good but needs to be understood properly--otherwise it can cause more confusion. As Hoyle said, "There are several important respects in which it is definitely misleading." It is important to appreciate that three-dimensional space is to be compared with the two-dimensional surface of the balloon. The surface is homogeneous with no point that should be picked out as the centre. The centre of the balloon itself is not on the surface, and should not be thought of as the centre of the universe. If it helps, you can think of the radial direction in the balloon as time. This was what Hoyle suggested, but it can also be confusing. It is better to regard points off the surface as not being part of the universe at all. As Gauss discovered at the beginning of the 19th century, properties of space such as curvature can be described in terms of intrinsic quantities that can be measured without needing to think about what it is curving in. So space can be curved without there being any other dimensions "outside". Gauss even tried to determine the curvature of space by measuring the angles of a large triangle between three hill tops.
When thinking about the balloon analogy you must remember that. . .
The 2-dimensional surface of the balloon is analogous to the 3 dimensions of space.
The 3-dimensional space in which the balloon is embedded is not analogous to any higher dimensional physical space.
The centre of the balloon does not correspond to anything physical.
The universe may be finite in size and growing like the surface of an expanding balloon, but it could also be infinite.
Galaxies move apart like points on the expanding balloon, but the galaxies themselves do not expand because they are gravitationally bound.
In a conventional explosion, material expands out from a central point. A short moment after the explosion starts, the centre will be the hottest point. Later there will be a spherical shell of material expanding away from the centre until gravity brings it back down to Earth. The Big Bang--as far as we understand it--was not an explosion like that at all. It was an explosion of space, not an explosion in space. According to the standard models there was no space and time before the Big Bang. There was not even a "before" to speak of. So, the Big Bang was very different from any explosion we are accustomed to and it does not need to have a central point.
If the Big Bang were an ordinary explosion in an already existing space, we would be able to look out and see the expanding edge of the explosion with empty space beyond. Instead, we see back towards the Big Bang itself and detect a faint background glow from the hot primordial gases of the early universe. This "cosmic microwave background radiation" is uniform in all directions. This tells us that it is not matter that is expanding outwards from a point, but rather it is space itself that expands evenly.
It is important to stress that other observations support the view that there is no centre to the universe, at least insofar as observations can reach. The fact that the universe is expanding uniformly would not rule out the possibility that there is some denser, hotter place that might be called the centre, but careful studies of the distribution and motion of galaxies confirm that it is homogeneous on the largest scales we can see, with no sign of a special point to call the centre.
The detailed, all-sky picture of the infant universe from three years of WMAP data. The image reveals 13.7 billion year old temperature fluctuations (shown as color differences) that correspond to the seeds that grew to become the galaxies. The signal from the our Galaxy was subtracted using the multi-frequency data. This image shows a temperature range of ± 200 microKelvin.