Our understanding of the Universe has greatly increased over the past few decades. The current model of how the Universe formed is known as the Big Bang theory. This article discusses the highlights of that theory.
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.
The Cosmic Microwave Background
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.
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.
|Omega (Density Ratio)||Fate of the Universe|
|Less than One||Open; Eternal Expansion, Cold Death|
|One||Flat; Cold Static Death|
|Greater than One||Closed; Big Crunch, Hot Death|
For theoretical reasons, cosmologists believe that omega = 1. Unfortunately, attempts to measure omega yield results of about 0.1.
When astrophysicist Vera Ruban looked at Galaxies, she noticed a curious problem. She expected that the outer parts of a galaxy would move slower than the inner parts. But she found that this is not the case. The rotation curves of galaxies (a graph of the radius of a galaxy versus rotational speed) is flat, meaning that the outer parts move at the same speed as the inner parts. Large amounts of mass would account for the unexpected speed, but we don't see the mass that should be there.
To aid your understanding of this, think of how planets revolve around the Sun in our solar system. Mercury (the closest planet to the Sun) zips around the Sun in 88 days, but it takes the furthest planet, Pluto, 248 years to orbit the Sun. If there were a solid sphere of mass between the Sun and Pluto, than Pluto's orbital period would be the same as Mercury's. No one is suggesting that galaxies are actually solid spheres of matter, but there must be more mass in these galaxies then we can see. Because we can't see it, the mass is called dark matter.
Dark matter may account for up to 90% of the Universe's total mass. Bernard Sadoulet, who leads a search for dark matter at theCenter for Particle Astrophysics has stated that "Not only can we not see what most of the Universe is made of, we aren't even made of what most of the Universe is made of!" What did he mean by this? Scientists tend to categorize everything and matter is no different. The matter you are familiar with, matter composed of neutrons and protons, is called baryonic matter. Non-baryonic matter also exists, but is generally difficult to detect. Professor Sadoulet's experiment is looking for exotic, non-baryonic particles called WIMPs. WIMP stands for Weakly Interacting Massive Particle. There is a great deal of theoretical work which suggests that WIMPs exist and probably account for a large fraction of dark matter.
If you don't believe that WIMPs exist, you aren't alone. But some sort of exotic non-baryonic dark matter is required for omega = 1. Big bang nucleosynthesis limits the total number of baryons to be a fraction of the Universes total mass. And since there are compelling reasons to believe big bang nucleosythesis, and also that omega = 1, one is led to the conclusion that there must be exotic non-baryonic dark matter. Note the use of the word exotic. Neutrinos are another type of non-baryonic particle, but are not considered exotic. Neutrinos do exist, in huge numbers, but all known neutrinos have zero mass. The search continues for neutrinos with mass, but a massive neutrino is unlikely to completely account for the flat nature of galactic rotation curves. Hence, an exotic class of non-baryonic dark matter particle must exist if WIMPs do not.
There are several candidates for baryonic dark matter. MACHOs (Massive Compact Halo Objects) are objects about the size of Jupiter. Jupiter is quite massive, but like all planets, does not emit any light of its own; it only reflects sunlight. Although we can see Jupiter quite well from Earth, chances are that someone looking at our solar system from any far distance would not be able to see Jupiter. So, it is reasonable to assume that there are Jupiter-sized objects in other solar systems that we cannot see. By a technique known as micro-lensing, several MACHOs have already been found. VMOs (Very Massive Objects) are about 100 times more massive than our Sun, which makes them very heavy indeed. They are likely to be found in the form of black holes. By the way, in case you're wondering whether the existence of many Earth-sized objects might account for all the dark matter, bear in mind that Jupiter is roughly 300 times more massive than Earth. Thus you would need so many Earth-sized objects that the galaxy would be littered with them.
These are exciting times for cosmologists. New telescopes, space missions, and experiments are generating data at an awesome rate, and new experiments are going online almost daily. The early part of the twenty-first century promises to be an amazing time for astrophysicists. And hopefully, we'll find answers to some of the big questions.
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