8.2 - Red-Shift

• The light from very distant galaxies has an increased wavelength compared to light from closer galaxies.
• If light is taken from the sun and passed through a prism, we get a spectrum, however it has dark lines because certain elements in the sun absorb light of specific wavelengths, so those wavelengths appear as dark lines.

  
• If we take light from a distant galaxy and pass it through a prism, the spectrum's dark lines will have shifted slightly towards the red end of the spectrum.

  
• The red shift tells us that the galaxies are moving away from each other. Because the galaxies are moving away, the light waves are stretched i.e. their observed wavelength is increased.
• Galaxies that are further away have a bigger red shift so these galaxies are moving faster than galaxies that are closer.
• The fact that distant galaxies are moving faster than nearby galaxies provides evidence that the universe is expanding.
• Scientists believe that the universe began from a very small region that was extremely hot and dense which then expanded into the universe we see today. This is called the Big Bang Theory.
• Scientists assumed that gravity would cause the expansion of the universe to gradually slow down, however observations of supernovae show that the rate of expansion is actually increasing.
• Dark matter and dark energy refers to matter and energy that we cannot detect/do not know about.
• Any circular motion has an acceleration towards the centre of the circle
• Gravity accelerates the Earth towards the sun. The planet is effectively falling towards the sun but is travelling fast enough so that it constantly misses falling into the sun.
• The force that accelerates an object moving in a circular motion is called the centripetal force. For orbital motion, this force is gravity.
• In 1929, Edwin Hubble announced that almost all galaxies appeared to be moving away from us. In fact, he found that the universe was expanding - with all of the galaxies moving away from each other.