2. HOW DID THE UNIVERSE COME TO BE??? The Big Bang Theory Cosmologists believe that 14 billion years ago, the universe was extremely compact, small, and unimaginably dense. According to the Big Bang Theory, this hot, dense state suddenly & violently exploded causing a rapid expansion of matter & energy The Big Bang Theory was proposed by George Gamow & Ralph Alpher The universe continues to expand (Hubble’s theory) This moment marks the beginning of the universe and time

3. WHAT IS THE EVIDENCE FOR THE THEORY? There are 2 key pieces of evidence for Big Bang theory. Red Shift Cosmic Microwave Background Radiation.

4. RED SHIFT You may have noticed that when an ambulance or police car goes past, its siren is high-pitched as it comes towards you, then becomes low-pitched as it goes away. This effect, where there is a change in frequency and wavelength, is called the Doppler effect. This happens with light too. Our sun contains helium. There are black lines in the spectrum of the light from the sun, where helium has absorbed light. These lines form the absorption spectrum for helium.

5. When we look at the spectrum of a distant star, the pattern of lines has moved towards the red end of the spectrum. RED SHIFT (continued … ) Spectrum of the Sun Spectrum of a Distant Star This is called RED SHIFT. It is a change in frequency of the position of the lines.

7. COSMIC BACKGROUND RADIATION The universe was initially filled with gamma rays – rays that have a very short wavelength As the universe expanded, the wavelengths stretched into other parts of the electromagnetic spectrum. The radiation left over from the Big Bang has been travelling through space ever since

8. BLUE SHIFT Red Shift = wavelengths are stretching out and galaxies/stars are moving away Blue Shift = wavelengths are shortening and galaxies/stars are moving closer together

9. As Hubble was observing the galaxies, he made measurements of the spectral shift in the lines All of the galaxies were shifted toward the red end of the spectrum, indicating that the planets were moving away from each other If planets were moving towards each other, they would shift to the blue end of the spectrum THE RED SHIFT IS EVIDENCE THAT THE UNIVERSE IS EXPANDING! SPECTRAL SHIFTING