1. FORMATION OF THE UNIVERSE Chapter 15 – 4 Part 2

2. Gravity and the Universe After the big bang, gravitational attraction caused the matter to form into galaxies. Because gravity acts over great distances, gravity controls the size and shape of the universe.

3. A Cosmic Repetition Every object in the universe is part of a larger system, and gravity acts within each system.

4. Galaxy Clusters Each galaxy cluster can contain up to several thousand galaxies.

5. Galaxies Galaxies are composed of stars and planets. Galaxies such as the Milky Way include planetary systems such as our solar system.

6. How Old Is The Universe? Astronomers can estimate the age of the universe by studying white dwarfs, the oldest stars in the Milky Way. The oldest white dwarfs are 12 billion to 13 billion years old. It took 1 billion years for the first to appear so the universe is 14 billion years old.

7. A Forever-Expanding Universe? The matter that makes up the universe is only 4%. Another 23% of the universe is dark matter, which has no light but does have gravity. The remainder is made of dark energy.

8. Dark Energy Dark energy seems to be speeding up the expansion of the universe, and counteracting the effects of gravity. The discovery of dark energy is so recent that scientists don’t know the properties of it yet.

9. The Future? The universe could keep expanding. If that happens, stars will age and die, and the universe will become dark and cold. Even after that, the universe will keep expanding forever.

11. THE END!