Astronomers of the past went to absurd lengths to justify placing the Earth at the center of their models of the cosmos. Belief in God, and the assumption that He would give us pride of place, were so entrenched that the mark of a viable astronomical theory was its ability to explain away any counter-evidence.
We now know the Sun rather than the Earth lies at the center of our Solar System. (The Catholic Church was the last to come around; they didn't admit that Galileo was right until 1992.) However, even though the Earth isn't at the center of the Solar System, it is at the center of the Universe.
Because everything is! Every point in the Universe can be thought of as its center - the point from which it expands outward.
If you've heard this before but have a hard time getting to grips with it, that's probably because there are too many dimensions involved. I have a hard time contemplating three dimensional expansion, but thinking about a 2-D Universe expanding is easy:
Furthermore, traveling in a straight line along the surface of the balloon for long enough would bring you back to where you started.
The actual Universe is the 3-D analogue of the surface of a sphere. Each point in 3-D space acts as its center; traveling in a straight line eventually brings you back to your starting point. Unfortunately there is no way to draw a 3-D "surface" embedded in four dimensions, so we must content ourselves with the inklings of understanding we get from analogies.
I thought this was an excellent and very informative post! However, the last part of your balloon analogy is a little misleading - it's a good way to understand an expanding space, but it doesn't seem to be true that our universe has positive spatial curvature like the surface of a balloon.
ReplyDeleteThis is because the the current "best guess" at the large-scale physical parameters of the universe, which is got by combining measurements of the cosmic microwave background with other measurements, doesn't agree with positive spatial curvature (see e.g. Table 2 in reference below). Instead it favors a curvature that is close to zero - so if you want to think about it as a balloon, then our universe would be a small patch on a really huge balloon, so huge that it seems flat.
http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/cosmology/wmap_7yr_cosmology.pdf
Thank you Fritz. I'll change the wording in a couple of places in case not everyone reads the comments. I like the idea of thinking of the Universe as a small section of a very large balloon.
ReplyDeletewhen i was a kid i always thought of the universe as a basketball which enormous giants were bouncing around
ReplyDeleteThis wrong idea that the earth was the center of the universe wasn't all bad. Lots of natural philosophers worked really hard on explaining the peculiar motion of the stars. They eventually said that the stars rotated around bodies rotating around other bodies (...) rotating around the earth. While this is not what is physically happening, a lot of very useful math was developed in the process.
ReplyDelete