E4- Cosmology

E4.1: Describe Newton's model of the universe.

Newton believed that the universe was:
-INFINITE in space and time
-UNIFORM
-STATIC

This implied that they universe was unchanging and contained an infinite number of stars spreading out to infinity.

E4.2: Explain Olber's paradox.

In 1823, Heinrich Olber described a
paradox- ARGUED AGAINST Newton's model.
If Newton's model was right, and there were an infinite number of stationary stars, the nights sky should be BRIGHT as there would be stars no matter which direction you looked in.

Quantitative explanation:
Assumption- the stars are evenly distributed in an infinite number of thin shells, spreading out in layers.
Each star has the same luminosity (L) and it is related to the apparent brightness (b) and the distance (d) by the inverse square law:

So if we imagine a thin shell of stars, with a thickness (T) and at a distance (d), then the volume of the shell would be:
Surface area x Thickness

If there are n stars per unit volume in the shell, then the total number of stars in the shell N will be given by:

This suggests that the total number of stars in the shell is directly proportional to d^2.If we move out to a shell at a greater distance, the stars in that shell will be dimmer according to the inverse square law:

In the more distant shell, there will be more stars . Since the number of stars is directly proportional to the distance squared, and the brightness is inversely proportional to it, the amount of light we receive from the shell should not depend upon the distance.

If there were a billion shells, we would multiply the energy received from one shell by a billion to find the total energy. So if the universe were infinite, the night sky should be infinitely bright.

E4.3: Suggest that the red-shift of light from galaxies indicates that the universe is expanding.


In 1929, Edwin Hubble proposed a law that basically states that the universe is expanding. He examined the absorption spectra of different galaxies, and found that the absorption lines were usually shifted to the red end of the spectrum.

The red-shift of light can be explained by the Doppler effect- the increase in the wavelengths of the spectral lines (red light has a longer wavelength) meant that the galaxies were moving away. Light from almost all galaxies show red-shifts- almost all of them are moving away from us; the universe is expanding.


E4.4: Describe both space and time as originating with the Big Bang.

If the universe is currently expanding, at some time in the past all the galaxies would have been closer together. At some point, all the matter in the observable universe would have been together at the SAME point, approximately 15 billion years ago. This point is known as the Big Bang; on average, the temperature and density of the universe have been decreasing. The rate of expansion has been decreasing because of the gravitational attraction between all the masses in the Universe.

The best way to imagine the expansion is to think of the expansion of space itself, rather than the galaxies expanding into a void. BIG BANG= creation of SPACE and TIME.

E 4.5: Describe the discovery of cosmic microwave background (CMB) radiation by Penzias and Wilson.

Cosmic Microwave Background(CMB) radiation provides evidence for the Big Bang model. It was first observed in 1965 by Penzias and Wilson; they build a radiometer that they intended to use for radio astronomy and satellite communication experiments. However, they found that some radiation was acting as a source of excess noise in the radio receiver; initially the radiation was though to be some sort of contamination, and they tried to remove it by cleaning the receiver.
Penzias and Wilson found that the intensity of the radiation they received, from all directions, had a wavelength in the microwave region. When they plugged this wavelength into the Wien's displacement law equation, it gave a temperature of 2.7 K. The cosmic microwave background radiation is a kind of echo of the original Big Bang still resonating around the universe.

E 4.6: Explain how cosmic radiation in the microwave region is consistent with the Big Bang model.

We know that 2.7K is the ambient temperature of the universe, so CMB radiation provides excellent support of the Big Bang model. The universe has cooled down to this temperature from its extremely hot origin.

E4.7: Suggest how the Big Bang model provides a resolution to Olbers' paradox.

The Big Bang model resolves Olbers' paradox- if galaxies are moving away in all directions, then the radiation from them will be RED-SHIFTED owing to the Doppler effect. This explains why the sky is dark at night, as the light from the receding stars has been shifted into the INFRA-RED region of the electromagnetic spectrum (increased wavelength) and is thus no longer visible to us.