Monday, August 07, 2006

The chim stuff...

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The chim stuff...


Red-Shift
Observations of distant galaxies and quasars show that these objects are red-shifted, meaning that the light emitted from them has been shifted to longer wavelengths. This is seen by taking a frequency spectrum of the objects and then matching the spectroscopic pattern of emission lines or absorption lines corresponding to atoms of the chemical elements interacting with the light. From this analysis, a red-shift corresponding to a Doppler shift for the radiation can be measured which is explained by a recessional velocity. When the recessional velocities are plotted against the distances to the objects, a linear relationship, known as Hubble's law, is observed.

Doppler Effect
Named after Christian Andreas Doppler, is the apparent change in frequency and wavelength of a wave that is perceived by an observer moving relative to the source of the waves. 1st proposed in 1842.
This hypothesis was tested for sound waves by the Dutch scientist Christoph Hendrik Diederik Buys Ballot in 1845. He confirmed that the sound's pitch was higher as the sound source approached him, and lower as the sound source receded from him.
Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848.
It is important to realize that the frequency of the sounds that the source emits does not actually change. To understand what happens, consider the following analogy. Someone throws one ball every second in a man's direction. Assume that balls travel with constant velocity. If the thrower is stationary, the man will receive one ball every second. However, if the thrower is moving towards the man, he will receive balls more frequently because the balls will be less spaced out. The converse is true if the thrower is moving away from the man. So it is actually the wavelength which is affected; as a consequence, the perceived frequency is also affected.

Cosmic Microwave Background Radiation
Form of electromagnetic radiation discovered in 1965 that fills the entire universe. It has a thermal 2.725 kelvin black body spectrum which peaks in the microwave range at a frequency of 160.4 GHz, corresponding to a wavelength of 1.9 mm.
Black body radiation is the kind of radiation given off from an object which if cold would be perfectly black, that is would absorb radiation of all wavelengths equally well. When heated such a body emits radiations with a well defined distribution over wavelengths.



Big Bang’s Theory on CMBR

In the theory, the early universe was made up of hot plasma of photons, electrons and baryons. The photons were constantly interacting with the plasma through Thomson scattering. As the universe expanded, the cosmological redshift caused the plasma to cool until it became favorable for electrons to combine with protons and form hydrogen atoms. At this point, the photons did not scatter off of the neutral atoms and began to travel freely through space. This process is called recombination or decoupling (referring to electrons combining with nuclei and to the decoupling of matter and radiation respectively).
The photons continued cooling until they reached their present 2.725 K temperature. Accordingly, the radiation from the sky we measure today comes from a spherical surface, called the surface of last scattering, from which the photons that decoupled from interaction with matter in the early universe, 13.7 billion years ago, are just now reaching observers on Earth. The big bang suggests that the cosmic microwave background fills all of observable space, and that most of the radiation energy in the universe is in the cosmic microwave background, which makes up a fraction of roughly 5×10-5 of the total density of the universe.

Thomson Scattering
In physics, Thomson scattering is the scattering of electromagnetic radiation by a charged particle. The electric and magnetic components of the incident wave accelerate the particle. As it accelerates, it in turn emits radiation and thus, the wave is scattered. Thomson scattering is an important phenomenon in plasma physics and was first explained by the physicist J.J. Thomson.