How do we know our

Universe is expanding?

• Throughout our universe, light is bursting from stars, bouncing off planets, diving into black holes, wandering into nebulae, and generally going every which way.

• Meanwhile, a little bit of it actually shows up here on earth.

• The light that does arrive here all seems to bear the same message: the universe is expanding.

Each type of atom gives off a unique set of colors. The colored lines (or Spectral Lines) are a kind of "signature" for the atoms.

Red Shift• How can light from the night sky tell us

that the universe is growing in size?

The main clue comes from something called redshift.

• Redshift is light's version of a phenomenon we experience all the time with sound.

• Have you ever noticed how the pitch of a police siren seems to drop suddenly as the car zooms by you?

As the siren approaches you, the waves of sound are squeezed together, and you hear them as being higher-pitched. After the car passes by, sound waves from the receding siren are stretched apart. You hear these stretched waves as being lower-pitched.

• Just like a cosmic police car, a star zooming toward you has its light waves squeezed together.

• You see these light waves as having a higher frequency than normal.

• Since blue is at the high-frequency end of the visible spectrum, we say the light from an approaching star is shifted toward blue, or blueshifted.

• Likewise, if a star is zooming away from you, any light it emits gets stretched.

• You see these stretched-out light waves as having a lower frequency.

• Since red is at the low-frequency end of the visible spectrum, we say that light from a receding star is shifted toward red, or redshifted.

Red Shift• Observers looking

at an object that is moving away from them see light that has a longer wavelength than it had when it was emitted-- a redshift.

Blue Shift• Observers looking

at an approaching source see light that is shifted to shorter wavelength-- a blueshift.

• The amount of the shift depends on the speed of the star, relative to you.

• For a moving object to create an appreciable redshift or blueshift requires some pretty serious speeds.

• To get just a 1% change in the frequency of light, a star has to be moving 1,864 miles per second.

• For a blue lightbulb to look red, it would have to be flying away from you at 3/4 of the speed of light.

• Studying light from galaxies throughout our universe, astronomers have noticed something surprising: almost all of it is redshifted.

• In fact, not only is it redshifted, galaxies that are farther away are more redshifted than closer ones.

• So it seems that not only are all the galaxies in the universe moving away from us, the farther ones are moving away from us the fastest.

Red Shift• Observers looking

at an object that is moving away from them see light that has a longer wavelength than it had when it was emitted-- a redshift.

Blue Shift• Observers looking

at an approaching source see light that is shifted to shorter wavelength-- a blueshift.

Birth of the Big Bang• Edwin Hubble noticed:

– the further away a galaxy is, the faster it is moving away from us.

– galaxies are all moving, and are all moving away from each other

• Interpretation: Hubble said that the Universe was expanding.

Birth of the Big Bang

• This implied that the galaxies were once much closer together.

• This must mean that the whole Universe was once a single point, which exploded into existence!

• The idea of the Big Bang was formed.

Big Bang and Universe Expanding

• Big Bang- The universe was formed billions of years ago in an enormous explosion. The universe has been expanding ever since.

• We see evidence for this big bang because astronomers looking at stars and galaxies in the sky see that over time, they are gradually moving apart.

http://www.youtube.com/watch?v=oLMrybV-6ys