E4 Cosmology

E.4.1 Describe Newton's model of the universe.

Newton assumed that the universe was INFINITE IN SPACE AND TIME, UNIFORM and STATIC.
This implied that the universe was unchanging, and contained an infinite number of stars spreading out to infinity.

E.4.2 Explain Oblers' paradox.

1823- Heinrich Oblers described an apparent paradox, in that if the Netwonian model of the universe were right, it would imply that there were an infinite number of stationary stars, no matter which direction you look in the night sky, you should see a star. This meant that the sky at night should be bright, whereas it is dark = paradox.

There is also a quantitative explanation for this paradox:
Assume: Stars are evenly distributed in an infinite number of thin shells. Each star has the same luminosity L related it its apparent brightness b and the distance d by the inverse square law:

The image above shows a thin shell of stars with thickness T and distance d. The volume of this thin shell will be the thickness x surface area = 4πd^2T

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

• This means that the total number of stars is directly proportional to d^2.
• If the shell is moved out to a greater distance then the shell will be dimmer according to the inverse square law: 
• Since the number of stars is DIRECTLY proportional to distance squared and the brightness is INVERSELY proportional to distance squared, the amount of light we receive from a shell does not depend on distance.
• If there were a billion stars therefore, total energy = energy received from one shell x BILLION = if the universe was infinite, then the NIGHT sky would be INFINITELY bright! 

E.4.3 Suggest that the red-shift of light from galaxies indicates that the universe is expanding.


1929 Edwin Hubble suggests that universe is expanding (not static) after making detailed observations of many observed galaxies and finding that the absorption lines in their spectra were usually shifted towards the red end of the spectrum. This, as explained by the Doppler effect, meant that these galaxies were moving away from us.


E.4.4. Describe both space and time as originating with the Big Bang.


If galaxies are moving away from eachother, then they must have been much closer in the past. Therefore, at some point of time, everything in the universe must have been located at a single point, called SINGULARITY. This is the basis for the idea that the universe began at certain time in the past - 13.7 billion years ago -with an explosion known as THE BIG BANG. 


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


CMB first observed inadvertently by Penzias and Wilson in 1965. Radiation was acting as a source of excess noise in a radio receiver they were building. Initially, the radiation was thought to be some sort of contamination and they tried to remove it by cleaning the receiver. The cosmic microwave background radiation is a kind of echo of the 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.

Pensizas and Wilson found that the intensity of the radiation that they received, from all directions, had a wavelength in the microwave region. When they plugged this wavelength into Wien's displacement law they found that it gave a temperature of 2.7 K, and we know that 2.7K is the temperature of the ambient universe so CMB radiation provides excellent support for the Big Bang model. The universe has cooled down to this temperature from its extremely hot origin.


E.4.7. Suggest how the Big Bang model provides a resolution to Oblers' paradox.

If the galaxies are moving away from us in all directions then the radiation reaching us from them will be red shifted owing to the Doppler Effect. This explains why the sky appears dark at night - the light from receding stars has been shifted into the infrared region of the electromagnetic spectrum, therefore is no longer visible to us.

Some Images and diagrams from: Heinemenn HL Physics Course Companion