2. Objectives Summarize the big bang theory.
List evidence for the big bang theory.
Explain how Hubble’s discoveries lead to an understanding that the universe is expanding.
Describe the three main types of galaxies.
Explain how a quasar differs from a typical galaxy.
Describe characteristics of the universe in terms of distance, and organization

3. The Big Bang Theory Astronomy: study of the universe
Cosmology: the study of the origin and evolution of the universe
Big bang theory: the theory that all matter and energy in the universe was once compressed into a small volume, called a Singularity, at an extremely high temperature
Exploded approximately 15 billion years ago
Began expanding in all directions and cooling
All matter and space in the universe resulted from the Big Bang
By the mid-20th century, almost all astronomers and cosmologists accepted the big bang theory.
The Big Bang

5. Timeline of the Big Bang

6. After the Big Bang Gravity pulls matter into small clusters
Clusters of matter become huge clouds of dust and gas
Clouds become solar systems and galaxies
We will discuss how these form in more detail later in the course

7. Raisin Bread Analogy of an Expanding Universe
Think loaf of unbaked raisin bread. The raisins are galaxies and the dough is space

8. Expanding Universe
Every object in the universe is moving away from the center.
Think of waves spreading out from where you drop a pebble into a pond.
Just like the waves furthest from the center….

9. Expanding Universe
….objects near the very edge of the universe are the oldest
The most distant known objects in the universe are over 12 billion light years from the Earth!
Wait a minute….this all sounds great, but how do we KNOW these galaxies and other objects are moving away from the center?

10. Big Bang Evidence #1
We KNOW the universe is expanding thanks to our ability to detect electromagnetic radiation.
The types of radiation emitted by galaxies moving towards Earth are nearest the blue light end of the spectrum (shorter wavelength)
The types of radiation emitted by galaxies moving away from Earth are nearest the red light end of the spectrum (longer wavelength)
We know which are moving toward and away thanks to the DOPPLER EFFECT

11. Doppler Effect
Change in the wavelength of light emitted by an object due to its motion
Movement toward “squeezes” the wavelength
Shorter wavelength
Light shifted toward the blue end of the spectrum
Movement away stretches the wavelength
Longer wavelength
Light shifted toward the red end of spectrum

12. Big Bang Evidence #1: Red Shift
In 1929, Edwin Hubble found that the most distant galaxies consistently showed the greatest red shift and thus were moving away from Earth the fastest.

13. Big Bang Evidence #2: Cosmic Background Radiation
Cosmic Background Radiation: background radiation left behind after the Big Bang, which has now cooled to approximately 270°C below zero.
Maps of cosmic background radiation over the whole sky show ripples.
Like the waves which spread out from dropping a pebble into water, these cosmic background radiation ripples indicate the history of the formation of the universe.

14. What Happens Next?
Big Bang theory leads to two possible futures for the Universe:
Open Universe
Closed Universe

15. Open Universe: The End The universe will continue to expand.
All stars will die as the last of their energy is released.
Eventually, all matter will be reduced to single atoms moving away from each other. There will be nothing left.
The end.

16. Closed Universe: Collapse
Gravitational attraction between objects will cause the expansion of the universe to slow and, eventually, come to a halt.
Gravity will then begin to pull everything back to the center of the universe.
Eventually, all matter and energy will again condense into a single point.
Another Big Bang will occur and the formation of the universe will begin all over again.

17. Galaxies
Galaxy: a collection of stars, dust, and gas bound together by gravity
Galaxies are the major building blocks of the universe. Astronomers estimate that the universe contains hundreds of billions of galaxies.
That’s more than 100,000,000,000 galaxies!
A typical galaxy, such as the Milky Way (that’s our galaxy), has a diameter of approximately 100,000 light-years and may contain more than 200 billion stars.

18. Looking Back in Time Remember it takes time for light to reach us
travels at 300,000 km/s
So we see things “as they were” some time ago
The farther away, the further back in time we are looking
1 billion LY means looking 1 billion years back in time
So, when we see the stars, we are actually looking at light which was emitted long ago.

19. Spiral Galaxies
Galaxies are classified by shape into three main types.
Spiral galaxy : has a nucleus of bright stars and flattened arms, which spiral around the nucleus.

20. Elliptical Galaxies
Elliptical galaxy: has various shapes, are extremely bright in the center, and do not have spiral arms.

21. Irregular Galaxies
Irregular galaxy: has no particular shape, and is fairly rich in dust and gas.

22. The Milky Way
The galaxy in which we live, the Milky Way, is a spiral galaxy in which the sun is one of hundreds of billions of stars.
Two irregular galaxies, the Large Magellanic Cloud and Small Magellanic Cloud, are our closest neighbors.
These three galaxies are called the Local Group.
Galaxies

23. Features of the Milky Way
Large, spiral galaxy
The distance across is about 100,000 light years (ly)
The thickness at the nucleus (bulge) is about 10,000 light years
Sun is in outer edge of a spiral arm & more than 26,000 ly away from the center of the galaxy
Contains more than 1 trillion stars