Physical Cosmology, Part II,
Lead: Eberhard
Möbius
2/7/2000
Summary by Aaron Clegg:
Hubble's redshift discovery led to a realization that the universe is expanding. More specifically, no matter in which direction we look, everything seems to be moving away from us, and the farther objects are moving away faster than the closer objects. This can lead to the naive conclusion that the earth is at the center of the universe, as before the Copernican paradigm shift.
However, there is another interpretation of Hubble's findings: the Cosmological Principle. In this model, the whole universe, stars, space, and all, is expanding, just like a raisin cake baking in the oven. As a cake rises and expands, everything moves away from every other thing. No matter which raisin the observer is "sitting on," all the other raisins are moving away, and the farthest ones are moving away fastest. It is important to note that the dough between the raisins also expands, as does the space in our universe, according to this model.
A group of researchers have mapped out a portion of our universe, with special attention to the number density of galaxies. Interestingly, the pattern is not uniform -- the galaxies are gathered together in strands that resemble a spider's web or a sink full of soap bubbles.
Will our universe continue to expand? Will it eventually collapse again? The outcome depends on many parameters.
A projectile fired from a massive object such as the earth must be travelling at a minimum speed, the "escape velocity," in order to escape. If its speed is less than the escape velocity, it will reverse its course and fall back toward the massive object. Projectiles fired from more massive objects, such as Jupiter or the sun, require faster escape velocities. If we fired a projectile at 11.2 km/s from the earth, it would barely escape earth's gravity; but if we fired the object at the same speed from Jupiter, it would not escape, because Jupiter's mass is too large. Where does that leave us in the expanding universe? If the mass of the universe is below some critical value, the universe will continue to expand forever; if it is above that value, the universe will begin to "fall together," or contract.
There are three possible geometries that the universe can have: spherical, flat (Euclidian), and saddle-shaped. Where we are in the universe, everything appears to be flat and Euclidian, just as the earth seems flat to a local observer. But just as farther-reaching observations showed the earth to be spherical, a farther-reaching analysis of the universe could show the universe to have some other fundamental geometry. A spherical universe, in which diverging lines will eventually meet again (such as meridians on a globe meeting at the north and south poles), will eventually collapse. A saddle-shaped universe, in which diverging lines will more sharply diverge, will continue expanding forever. A flat geometry, in which parallel lines will never meet, serves as the critical boundary between the two: the expansion will barely continue, ever slower.
A Berkeley group has formulated a theory that there is a force that pushes things apart at large distances, and this force is responsible for the expansion of our universe. While still controversial, this theory is supported by data of supernovae.
A quasar is a bright, dense object that gets its energy from black holes. While its luminosity is greater than that of an entire galaxy, it is smaller than our solar system.
Because of the travel time of light, looking farther away means looking deeper into the past. This can give us clues about the evolution of the universe. Since quasars are only observed very far away from us, we infer that they only existed in the distant past. On the other hand, galaxies are primarily observed in the closer regions of space; those galaxies that we see farther away (up to 10 billion light years) are different from the ones that are nearer. This indicates that galaxies probably go through an evolutionary process. It has been proposed that galaxies may have evolved out of quasars.
According to the laws of thermodynamics, the universe had to be very hot at the beginning, cooling as it expanded. At an age of about 300,000 years, the universe was at a temperature of 3000 K, the temperature of the surface of the sun. The radiation emitted then serves now as an opaque curtain, obscuring our view of the universe before that time. All of our knowledge of what went on previously must come from inference based on observations of events after that time.
In the beginning, according to the big bang theory, everything in the universe existed in the form of radiation. If the big bang theory is correct, then some remnant of that early cosmic background radiation should still remain. In the 60's, this radiation was observed by accident at the Bell Labs. More recently, NASA launched the COBE (Cosmic Background Explorer) satellite to gather data about the radiation emitted in the early universe.
The Big Bang model has a few problems that could mean it is an incorrect or incomplete theory.
A small change in some parameters of the universe (such as total mass and rate of expansion) could drastically alter the universe's fate, from expansion to collapse or vice versa. In the present state of the universe, changing these parameters by one order of magnitude (a factor of 10) would alter the outcome. This is not too troubling. However, when the universe was just three minutes old, a change of 10^-15 (changing the 15th decimal place by one digit) would have decided between eternal expansion and eventual collapse. The Big Bang model cannot explain how our universe landed within this extremely narrow tolerance.
Let us pick an arbitrary direction in the universe, and call it "left," then call its opposite "right." If we look as far as we can to the left, and do the same for the right, we find that the universe is fundamentally the same in both places. Yet the two regions we are looking at are sufficiently far removed from each other that no communication is possible between them, as governed by the speed of light. In fact, no communication has been possible since the time they evolved into their present form. How can this be? This is known as the Horizon Problem.
According to special relativity, matter and energy (including radiation) are different forms of the same thing. The Big Bang theory says that in the beginning, the entire universe consisted of radiation, which is pure, massless energy. Light decomposes into matter through a process called "pair production," in which equal amounts of matter and antimatter are produced: not approximately equal amounts; exactly equal. Since this is a perfectly symmetrical process, how is it that there is matter in the universe today? The equal amounts of matter and antimatter would have annihilated each other and turned back into radiation.
One possible solution to the "Matter Problem" is the theory that pair production is not completely symmetrical. Perhaps one in a billion photons decays into matter without producing antimatter. Known as "inflation," this theory requires that some latent heat be used up in the transition from symmetry to asymmetry, in much the same way as latent heat is used up when ice turns into water.
Why is the universe such that we are in it? The universe started out before inflation with just the right physics to allow life to develop. Why? The answer lies in a philosophical idea known as the Anthropic Principle.
1) The Strong Anthropic Principle -- that the universe was designed to support intelligent life.
2) The Weak Anthropic Principle -- that our universe is one of many, and we are in this one because we are only capable of existing in one that supports life.
John Archibald Wheeler envisioned the universe looking back at the big bang through our eyes, somewhat like a serpent contemplating its own tail. We are a part of the universe which we observe, and our science is confined to probe within it. Both facts prove to be profound limitations and suggest that answers to many of our questions must be found outside the realm of science.
February 28, 2000