Cosmology is the study of the origin and evolution of the universe. In the last half of the twentieth century, astronomers made enormous progress in understanding cosmology. The fact that we appear to live in a universe which began at a specific point in time and has undergone evolution ever since is one of the most revolutionary discoveries in science.

The Universe began in what astronmers dubbed "The Big Bang" -- an initial event, after which the Universe began to expand. During the first seconds after the Big Bang, the universe was extremely hot and dense. The physics that we need to understand the Universe in these early stages is very speculative, since it is impossible to recreate these conditions in an experiment today to check the preditions of theory. Before 10^-44 seconds after the Big Bang, the four fundamental forces of nature -- gravity, the electromagnetic force and the strong and weak nuclear forces -- were unified into a single force. At 10^-44 seconds, gravity separated from the others; at 10^-34 seconds the strong force became separate and at 10^-11 seconds, the weak force separated from the electromagnetic force. During this period the Universe began a sudden burst of exponential expansion -- faster than the speed of light. This expansion is called "inflation" and explains why the Universe we observe today is so uniform. Temperatures were so hot (10^27 K) before inflation that the familiar particles which make up atoms today (protons and neutrons) were not stable --the universe was a hot soup of quarks (particles which are hypothesized to make up baryons), leptons (electrons and neutrinos), photons and other exotic particles.

As the universe expanded after inflation it continued to cool. For the first 3 minutes conditions everywhere were similar to those at the center of stars today, and fusion of protons into deuterium, helium and lithium took place. Most of the helium we see today in stars is believed to have been produced during these early minutes. The universe was an extremely opaque plasma, and photons dominated the mass density and dynamical evolution of the Universe. When the universe cooled sufficiently to allow the free electrons to recombine with the hydrogen and helium nuclei, suddenly the opacity dropped, and the photons were free to stream through space unimpeded. These photons are seen today as the Cosmic Microwave Background, a bath of 2.7 degree light which we see in all directions today. The detection of the cosmic microwave background experimentally was one of the great triumphs of the Big Bang theory. Recombination and the subsequent production of the cosmic microwave background occured about 180,000 years after the Big Bang.

At this point, the matter distribution of the Universe was still fairly uniform, with only small density fluctuations from place to place. As the Universe expanded, the slightly overdense regions began to collapse. Sheets and filaments in the gas formed, which drained into dense clumps where star-formation began. Eventually, these protogalactic fragments merged and galaxies and quasars formed. The Universe began to look like it does today.

The future of the universe can be predicted by cosmologists as well. Whether the Universe will expand forever, or eventually slow, turn-around and recollapse depends on how fast the galaxies are moving apart today, and how much gravity there is to counter the expansion -- quantities which in principle can be measured. The modern theory of gravity, General Relativity, was invented by Albert Einstein. He used the idea that space could be curved to reformulate Isaac Newton's theory of gravity. In general relativity, the mass of an object curves the space around it, and parallel lines no longer go on forever without intersecting. In many textbooks the curvature of space is represented by a sphere or a saddle shape -- but in reality, space is three-dimensional, and the "curvature" is not in a particular direction. Einstein wrote down what we call "field equations" which described how the curvature of space can be calculated from mass and energy. When he solved the equations he realized that even if the universe is infinite, isotropic (the same in all directions) and homogeneous (the same density everywhere), it would not be static. Depending on the geometry, it would expand or contract. Hubble had not yet discovered that the Universe expands, so in 1917, Einstein added a term to the field equations and a "parameter" lambda, called the cosmological constant. Later, when Hubble showed that the Universe is expanding, and that there was no need to add a cosmological constant to the field equations, Einstein called the cosmological constant "the biggest blunder of my life".

Were Einstein alive to day, he would be amazed to learn about recent observations that suggest that in fact the cosmological constant is not zero, and the expansion is accelerating. In this case, the curvature of space is not so easily related to the dynamical evolution of the Universe. At the time of this writing, theorists had not come up with a theory for the origin of a non-zero lambda which has testable predictions. Certainly, more observations are called for, to confirm or refute this result.

Nonetheless, the conditions in the Universe in the distant future can be described, given the physics that we understand today. If the Universe is CLOSED, then the Hubble expansion will eventually stop, and the Universe will then collapse. If the density of the Universe is, say for the sake of arguement, about twice the critical density for closing the Universe, then the expansion stops about 50 billion years after the Big Bang. At about 85 billion years after the Big Bang, the density of the Universe will again be about what it is today. At this point, the nearby galaxies will appear blue-shifted, more distant galaxies will be standing still, and the very distant galaxies will be red-shifted. Eventually, the galaxies will all touch, and the Universe will continue to contract and heat. Soon the stars are cooler than the Universe as a whole, so radiation can't flow out of them, and they explode. One hundred billion years after the big bang will be the Big Crunch. At this point the Universe may become a black hole -- or it may bounce, and cycle again.

If the Universe is OPEN or FLAT, the Hubble expansion goes on forever. Physical processes which take such a long time that they are irrelevant in today's Universe, will eventually have time to occur. After 1 trillion years, star-formation will have used up all the gas, and no new-stars will be forming. Stellar remnants such as white dwarfs, neutron stars, and black holes will remain. After a million trillion years, galaxies evaporate -- their stars disperse into space. After a billion trillion trillion trillion years, protons and neutrons decay into positrons and electrons. After that, only black holes exist. These eventually evaporate by Hawking Radiation. At 10^100 years after the big bang, all the black holes, even the supermassive ones in quasars will be gone. The Universe will be very black and cold indeed.

What came before the Big Bang? Cosmologists have no shortage of answers to this question, but we may never know from direct observation what the true story is. Perhaps spacetime had such a peculiar topology that it curved around on itself -- and so asking what came before the Big Bang is like asking was is south of the South Pole.

The questions asked by cosmologists are some of the most simple and yet most profound questions intelligent creatures can ask. What is the origin of this beautiful and complex Universe we live in, and what is its ultimate fate? Amazing progress was made over the last hundred years in cosmology, but clearly many important parts of of the story are yet to be discovered.

--Prof. Jill Bechtold, University of Arizona

Bibliography -- Books

Guth, Alan H. and Alan P. Lightman, The Inflationary Universe: The Quest for a New Theory of Cosmic Origins, Perseus Press, 1998

Hogan, Craig J. and Rees, Martin, The Little Book of the Big Bang: A Cosmic Primer, Copernicus Books, 1998

Livio, Mario and Allan Sandage, The Accelerating Universe: Infinite Expansion, the Cosmological Constant, and the Beauty of the Cosmos, NY: John Wiley and Sons, 2000.

Rees, Martin, J. Just Six Numbers: The Deep Forces that Shape the Universe, NY: Basic Books, 2001