PHYSICS 24 LECTURE NOTES
Topic: The Big Bang Model
January 23, 1997
LECTURER: Professor Bernard Sadoulet
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
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 (see note 1). Although the sonic Doppler effect is caused
by compression of sonic wavefronts, 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 which 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 (note 3), 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
about 23% of the observable mass in the universe is helium. This
figure is far too large to be accounted for by stellar fusion (note 4).
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 current advances in observational technology (the
Background Explorer and the
Hubble Space Telescope
just to name two), 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. So, it seems
that for now, the Big Bang theory is the most plausible description of the
birth of our universe (note5 ).
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
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
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.
5:Want more? Check out arguably the finest Physics, Astrophysics, and Cosmology
institute in the world at
Books for the Public at Large
- S. Weinberg: The First Three Minutes
Brief History of Time
- G. Smoot, K. Davidson: Wrinkles in Time
- H. Reeves: Atoms of Silence
- M. Bartusiak: Through a Universe Darkly
More Advanced Books
- J. Silk: The Big Bang (1992)
- F. Shu: The Physical Universe University Science Books (1982)
- D. Goldsmith: The Evolving Universe (1985)
- M. Harwit: Astrophysical Concepts (1973)
- M. Zeilik, E. V. Smith: Introductory Astronomy and Astrophysics (1987)
- E. Chaisson: Universe, an evolutionary approach to astronomy (1988)
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