CMBR Dipole: Speeding Through the Universe; Credit: DMR, COBE, NASA, Four-Year Sky Map
WMAP Resolves the Universe; Credit: WMAP Science Team, NASA
The discovery that the universe was expanding led naturally to the question, What did it expand from? Although some suggested that the universe had been continually expanding, others suggested that the universe might have had a beginning, when everything was compressed into a point. This idea led George Gamow, Ralph Alpher and Robert Herman in 1948 to predict that the universe should be suffused with left-over energy from the time when it was born, and that this energy should be detectable as radiation (light) at microwave wavelengths.
This prediction gained little attention for the next 20 years, until Arno Penzias and Robert Woodrow Wilson at Bell Telephone Laboratories developed radio telescopes (based on ideas of Robert Dicke and colleagues at Princeton) that detected this radiation. This discovery established the Big Bang model as the predominant theory for how the universe formed. The temperature of this radiation was 3.5 degrees Kelvin, which provided the first insight into the conditions in the first few moments (100,000 years?) of the universe. This light is now known as the cosmic microwave background.
The Big Bang theory states that the universe began as a point, filled with matter of an uncertain form with temperatures unimaginably high, and radiation (light) that is inevitably produced by hot matter. As the universe expanded, it cooled, and the initial cosmic soup condensed into particles that we are now familiar with, protons, electrons, and eventually neutrons. As the soup cooled further, the universe became transparent to light. It is this point at which the the cosmic microwave background formed.
Later, hydrogen, helium, and lithium formed, which served the as the basis for the first stars. One of the biggest triumphs of the Big Bang theory is that it correctly explains the relative amount of hydrogen and helium in the Universe.
Eventually, the universe cooled enough for gas to cool, and collapse to form the first stars. The first stars and galaxies were seeded by slight fluctuations in the density of matter after the Big Bang. For years, these fluctuations were considered to be too slight to be detectable. However, with the launch of the Cosmic Microwave Background Explorer (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP), and a number of balloon-borne experiments, those slight fluctuations have been measured to exquisite detail. An image of those fluctuations from WMAP is shown above.
The fluctuations have been used to determine the geometry of space (it is flat, meaning that the sum of a triangle's angles would be 180 degrees), that the amount of energy in the universe is equal to that needed to counteract the force of gravity that would pull it back together. However, it appears that only 25% of this energy is matter, and only 20% of that is the protons, electrons, and neutrons that we are familiar with. The rest of the 75% of the energy of the universe is a mysterious dark energy, which observations of supernova suggest is pushing the universe apart.
The puzzles do not end there; there is a lot what we do not understand about this process. We do not know what form matter took in the first few moments, before it condensed into protons and neutrons. There is also a puzzle as to the amplitude of the fluctuations. They are actually much smaller than would be predicted by a simple analysis of the quantum fluctuations that should have developed in the first few moments of the universe. As a result, theoretical physicists suspect that the universe underwent a period of extremely rapid expansion, called "inflation" before protons and electrons had a chance to form. What caused this inflation is unknown, but it might be related to the aforementioned dark energy.