THE UNIVERSE AND ITS STARS

1. THE UNIVERSE AND ITS STARS 13.6 Dark Energy & The Expansion of the Universe

3. Dark Matter • 1920’s - Edwin Hubble theorized that the gravity of the galaxies and other matter that existed in the universe should be strong enough to slow down the expansion of the universe, or, even reverse it. • This is known as the oscillating theory • 1970’s – This theory ran into problems when astronomers discovered that clusters and superclusters of galaxies did not produce enough mass to provide the gravity which keeps them together • This meant that galaxy clusters should, technically, not exist and that galaxies should be spread out evenly in the universe. • Where did the mass come from to account for what we ACTUALLY see in the universe?

4. Dark Matter • Dark matter used to account for this mass. • Dark matter is defined as a particle which has not yet been discovered. • Astronomers think that this dark matter is present in the outer fringe of a galaxy • Even thought we are not able to identify it, continuous research supports its existence

5. Dark Energy • Astronomers have observed that the expansion of the universe has actually increased in speed, not decreased. • Energy in empty space which is speeding up, not slowing down, called dark energy • It is not an antigravity force because its strength increases with distance, while the strength of gravity weakens with distance.

6. The best estimate of how all the mass and energy of the universe are distributed is as follows: • 73 % dark energy • 23 % dark matter • about 3.6 % atoms of normal matter; mostly hydrogen gas between the galaxies • 0.4 % visible matter; stars and galaxies The visible universe where we live is less than half a percent of all that exists

7. CYU p444 #2, 3, 6, 10, 11