PHYSICS 24 LECTURE NOTES
Student web page authors: Dean Chen and Chris Mayor
The hot Big Bang model is the currently accepted description of the universe's origins. This theory hypothesizes that the universe as we know it was born in an explosion of tremendous proportions. The three main observations supporting the big bang model are the Hubble expansion, the cosmic microwave background, and the relative primordial abundances of light elements (Helium 3 and 4, Deuterium, Lithium). Following are brief explanations of these observations and how they support the notion of the grand explosion called the Big Bang.
1) The Hubble Expansion:
A familiar law to astronomers is that the apparent brightness of an object decreases with increasing distance (as the inverse square of the distance). The farther away an object is, the darker is it. This relation, along with more sophisticated techniques described in the second lecture "Expansion of the Universe", is used to determine relative distances of similar objects. Distance, however, is only part of the picture. It is also important to know the velocity of an object. To determine this, astrophysicists rely on a phenomenon known as the Doppler effect. As most individuals have experienced, sound waves emitted from an approaching object have an increased apparent frequency to stationary observers. One common example is that of a rapidly approaching train--its sound is much higher in pitch on its approach than on its departure. Although the sonic Doppler effect is caused by compression of sonic wave fronts, the effect can be generalized to all wavelike phenomena in particular electromagnetic radiation (this involves more complicated considerations such as relativity, a subject too complex to be summarized adequately). The magnitude of an object's Doppler shift is a function of its radial velocity relative to the observer; by taking advantage of this fact, astronomers have tabulated velocities of various objects (stars, galaxies, etc.) in our vicinity. Surprisingly, when distances for various galaxies were plotted versus their velocities, a nearly straight line with positive slope was obtained. This implies that objects that are farther away have greater velocities! This lends support to the Big Bang theory--if the universe does indeed expand in a manner consistent with the Big Bang model, then two objects that are close to each other should have smaller relative velocities than in comparison with distant objects.
2) Cosmic Microwave Background. Another of the factors mentioned above, the Cosmic Microwave Background, lends support as a result of theorized energetics of the Big Bang.
In 1965 Penzias and Wilson, researchers at AT&T's Bell Laboratory, discovered that there is microwave radiation emanating from all directions in our observable locality of the universe. This has been termed cosmic microwave background (CMB). It had been predicted approximately 20 years earlier by Gamov in connection with the early synthesis of matter. The CMB is viewed as a relic of the high energy, early universe. The isotropy (uniformity) of the CMB is in stark contrast to the relative anisotropy (disparity) of matter distribution. Another interesting aspect of the CMB is a distribution of wavelengths in the centimeter and millimeter wavelengths that is characteristic of black body radiation at 2.7 K. A black body radiator is an idealized object that absorbs and emits all wavelengths of radiation, the extent of absorption and emission of a given wavelength depending on the blackbody's temperature. The Big Bang model predicts that at high temperature the universe is filled with a plasma, a blackbody radiator. This would occur as a result of an ambient temperature of about 3000K, at which hydrogen (the main component of the universe) can only exist as a plasma.
3) Primordial abundance of light elements. The final main support for the Big Bang theory is the observed abundance of elements in the universe.
By examining the spectra of various objects astronomers have made the approximation that
About 23% of the observable mass in the universe is helium. This figure is far too large to be accounted for by stellar fusion. The abundance of lighter nuclei such as Li is also difficult to explain by stellar nucleosynthesis; however, the Big Bang model theorizes that these nuclei were created during the violent explosion at the beginning of the universe. Although the mechanics of the theory are too complex to be outlined here, it can be said that it accounts nicely for the current observed abundances of most elements in space.
With advances in observational technology (the Cosmic Background Explorer COBE and the Hubble Space Telescope, there has been a torrent of new data uncovered about the universe. Although there have been some inconsistencies uncovered in the Big Bang theory, most of these are quantitative and do not contradict it directly. It seems that for now, the Big Bang theory is the most plausible description of the birth of our universe.
1.The Doppler effect is considered to be so fundamental that a recording of its effect was put on the gold record mounted on Voyager I, to describe our culture in case the spacecraft is found by another civilization. This spacecraft was launched in 1977 and recently left the solar system, one of two potential envoys to other life forms in our universe.
2. An oft used explanation of this is the "Plum-Pudding" universe. Imagine that the plum pudding is space (an admittedly abstract concept in astrophysical discussion) and raisins in the plum pudding are solar systems. When the plum pudding is stretched out, raisins near each other will move with like speeds, as opposed to raisins that are farther apart. Try It! A little Silly-Putty and a few M&Ms should do the trick.
3. Although this may seem like an outrageous figure (300K is temperature comparable to the temperature at the surface of the sun), keep in mind that temperatures at the Big Bang are estimated to be in excess of 3 billion K at the time of helium formation! Before it was even hotter!
4. The primary component of the universe is Hydrogen. The only known mechanism for conversion of Hydrogen into other elements is nuclear fusion, the process which and in the mammoth thermonuclear reaction cores of stars. However, given the current estimations for the age of the universe, there is far too much matter in the universe to be accounted for by the existence of stars. Instead, the Big Bang model theorizes that the universe began its life as a conglomeration of pure energy, with matter being formed in various proportions as the universe cooled down.
Books for the General Public
S. Weinberg: The First Three Minutes
More Advanced Books
J. Silk: The Big Bang (1992)