The steady state theory of cosmology claims that the Universe simply exists without changing with time. This theory presents many physical as well as philosophical difficulties. Evidence suggests that the Universe is expanding. While there are ways to explain expansion in a steady state universe, few astrophysicists believe this theory, because there is little evidence to support it. As the first widely held theory about the Universe it is included here for historical completeness. The big bang theory states that at some time in the distant past there was nothing.
A process known as vacuum fluctuation created what astrophysicists call a singularity. From that singularity, which was about the size of a dime, our Universe was born. It is hard to imagine the very beginning of the Universe. Physical laws as we know them did not exist due to the presence of incredibly large amounts of energy, in the form of photons. Some of the photons became quarks, and then the quarks formed neutrons and protons. Eventually huge numbers of Hydrogen, Helium and Lithium nuclei formed. The process of forming all these nuclei is called big bang nucleosynthesis.
Theoretical predictions about the amounts and types of elements formed during the big bang have been made and seem to agree with observation. Furthermore, the cosmic microwave background (CMB), a theoretical prediction about photons left over from the big bang, was discovered in the 1960’s and mapped out by a team at Berkeley in the early 1990’s. After some period of time following the big bang, gravity condensed clumps of matter together. The clumps were gravitationally pulled towards other clumps and eventually formed galaxies.
It is extremely difficult to model how this clumping may have occurred, but most models agree that it occurred faster than it should have. A possible explanation is that right after the big bang the Universe began a period of exaggerated outward expansion, with particles flying outward faster than the current speed of light. This explanation is known as inflation theory, and has widespread advocacy within the astrophysics community because it reconciles theory with observation. It should be noted, however, that inflation theory is not directly verifiable.
Whether you believe inflation theory or not, galaxies did form. And since they formed from matter that was moving rapidly, they also move rapidly. Due to a phenomenon called doppler shifting, the wavelength emitted by something moving away from us is shifted to a lower frequency, and the wavelength of something moving towards us is shifted to a higher frequency. A good example of this is the sound of a fire truck siren as it drives by; the pitch of the siren is higher as the fire truck moves towards you, and lower as it moves away from you.
Although this example illustrates the effect for sound waves, the same effect occurs for all wavelengths (incuding light), the result being that visible wavelengths emitted by objects moving away from us are shifted towards the red part of the visible spectrum, or redshifted. And the faster they move away from us, the more they are redshifted. Thus, redshift is a reasonable way to measure the speed of an object (this, by the way, is the principal by which radar guns measure the speed of a car or baseball). Here’s the point: When we observe the redshift of galaxies outside our local group, every galaxy appears to be moving away from us.
We are therefore lead to the conclusion that our Universe is expanding. This is called hubble expansion, after Edwin Hubble, who discovered the phenomenon in 1929. Here’s a subtle point that you may have wondered about: If we look out into the Universe and every galaxy we see is moving away from us, doesn’t that mean that we are at the center of the Universe? The obvious answer seems to be ‘yes’, but actually the answer is ‘no’. Hopefully the following analogy will explain why. Image a loaf of 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 cosmology 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 the critical density is given by the Greek letter omega. If omega is less than one the Universe will continue expanding until it is so large that it dies a cold death. If omega equals one the Universe will eventually stop expanding but will not collapse. In this case the Universe will also die a cold static death. But, if omega is greater than one, then the Universe is doomed to collapse under it’s own gravitational mass, and will die a hot, fiery death in a big crunch. But don’t worry, the ultimate fate of the Universe is atleast ten billion years away.