"In the beginning, there was nothing at all. Earth was not found, nor Heaven above, a Yawning-Gap there was, but grass nowhere."
In this lecture period we discuss:
From the University of Colorado
Electromagnetic Radiation includes visible light, radio waves, microwaves, x-rays, gamma rays, and infra-red (heat) rays. All these forms of radiation are characterized by traveling oscillations of a combined electric and magnetic field. These electromagnetic waves differ by the wavelength of the oscillation, with shorter wavelength radiation carrying more energy than longer wavelength radiation. All possible wavelengths make up the electromagnetic spectrum.
Planck's Law is sometimes called the "black-body" formula works very well for celestial bodies:
Where E (lambda) is the amount of radiant energy emitted at a given wavelength, lambda. T is the temperature of the object, and a and b are constants.
The spectrum of wavelengths emitted by a body at a temperature, T, has a characteristic shape that is strongly dependent on the wavelength (to the inverse fifth power).
This law describes the spectral distribution of radiation emitted by a black body. Very hot bodies (3000 - 20,000 K) like our Sun emit a lot of light at visible wavelengths. The Sun acts like a black body near 6000K, whereas the Earth acts like a black body near 300 K (can you guess where its curve would lie?).
Stefan-Boltzmann Law ("E equals sigma T to the fourth")
where E is the total energy emitted (calculated by adding up the areas under the curves of Figure 1), sigma is a constant, and T is temperature.
The wavelength of the peak radiance [lambda (max)] decreases linearly as the temperature increases, where c is a constant:
Summary of radiation laws
The Doppler Effect
Click here to see how the speed of an object affects wavelength.
The Doppler Effect for Light is calculated by:
Big Bang Theory
The Big Bang theory states that the Universe began when primordial mass exploded. This fireball gradually cooled as it expanded outward, and giant clouds of swirling gas formed the celestial bodies. The Big Bang theory does not explain why the bang occurred, but predicts (with surprising accuracy) what the consequences of the event.
Evidence #1: atomic physics
If the big bang occurred, the initial temperatures must have been so unimaginably high that matter could only have existed in exotic and unstable forms. As the temperature cooled in the first second, free hydrogen nuclei (atomic mass 1) were formed that could undergo fusion reactions to give heavier forms of hydrogen (atomic mass 2, 3) and helium (atomic mass 3 and 4). Thus, the theory predicts a early universe with only a mixture of ~75% hydrogen and 25% helium (by weight) and no heavier species. This ratio is exactly what is observed in stars.
Evidence #2: red shift
Measurements of the red shifts of virtually all galaxies (except a few in our immediate vicinity) show that the visible universe is expanding in all directions. The constant of proportionality between the distance and velocity of recession is known as Hubble's constant.
Evidence #3: microwave radiation
Measurements of low energy microwave radiation show that the visible universe is permeated by "cosmic background" microwave radiation, coming from all directions and similar to what is expected from a black body at 3K. The Big Bang theory predicts that such radiation is the red-shifted remnant of the radiation released when matter and light became decoupled about 1 million years after the Big Bang. The American scientists who first made this measurement in 1965 (Penzias and Wilson) obtained the Nobel Prize.
Measurements from the NASA Cosmic Ray Background Explorer (COBE) mission provide additional support.
COBE satellite results show that the cosmic ray background is not completely uniform in direction (see Figure), but that there is some clumping in preferred directions with differences in effective temperature of only one hundred millionth of a degree. This clumpiness would have been necessary for the big bang to produce galaxies, since a perfectly uniform explosion would not produce localized high densities.
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