raisin bread baking in the oven. As the bread bakes it gets bigger, and every raisin moves away from every other raisin. Now imagine that you are sitting on one of the raisins (ignore the heat of the oven). All the other raisins are moving away from you, so you might conclude that you are at the center of the loaf of bread. But if you were on a different raisin you would also see every raisin moving away from you and would also conclude that you are at the center of the loaf. The same thing is happening in the Universe. No matter where you are in the Universe, every galaxy you see is moving away from you. That's why astrophysicists say that you shouldn't talk about the center of the Universe; there really is no center of the Universe.The oscillatory Universe model claims that the Universe started with a big bang, and that it is currently expanding. Eventually, however, the expansion will slow, stop, and then the Universe will begin to contract. The contraction will continue until all of the mass of the Universe is contained in a singularity, a process known as the big crunch. The singularity then undergoes a big bang, and the process begins afresh. Although we shall discuss reasons why this is probably not the case, it does explain what happened before the big bang.Top three reasons to believe big bang cosmologyBig Bang Nucleosynthesis Cosmic Microwave Background Hubble Expansion Current Big Questions in CosmologyIs the Universe Closed?The question of whether the Universe will collapse in a big crunch or continue expanding forever hinges on knowing the density of the Universe. Density is defined as mass divided by volume. One can measure the density of the universe by observing the local group of galaxies and assuming that the Universe is all the same. One can also calculate the density required such that the Universe will eventually stop expanding. That density is called the critical density, and the ratio of the observed density to t...
