Agenda

Read Unit 8 Distance to galaxies (review) Redshift Hubble’s Law The Big Bang

Calculating distances to other galaxies

Two standard candles:

Cepheid variable stars Type I supernovae

NASA

10 billion ly away!

Cepheid variables

Gene Smith, UCSD/CASS

Cepheids are a standard candle.

Discovered by Henrietta Leavitt. (photo: AAVSO)

Harvard

Supernovasin 4 galaxies

How do we determine whether a galaxy is approaching or receding?

The Doppler Effect.

Next step: velocities

Demonstration: doppler buzzer

How does the buzzer sound change as it moves away?

A. Louder

B. Softer

C. Higher pitch

D. Lower pitch

Source moving away from you

The light is shifted toward the red: longer wavelength, lower pitch (frequency)

Source moving toward you

The light is shifted toward the blue:shorter wavelength, higher pitch (frequency)

What about galaxies?

Galaxies have specific spectral lines that can be identified.

Measure wavelength. Compare to the “rest” wavelength. The difference is the doppler shift. From doppler shift we get speed.

Hubble’s distance velocity relation

(Distance from us)(Spe

ed a

t w

hich

gal

axy

rece

des

from

us)

Two years later (note scale)

Hubble’s law

The more distant a galaxy is, the faster it’s receding

This means it has a larger redshift!! The universe is expanding!

The universe:

Is expanding (Hubble’s law) Has galaxies evenly distributed

throughout (homogeneous) Contents evolve with time (H fuses to

He, etc)

Any theory must be consistent with these facts!

Space is getting bigger!

Wikipedia

The universe has no center

Consider only the surface of the balloon

The big bang

An explosion of space, not in space A consequence of relativity

Some heat is left over from this Cosmic background radiation Seen everywhere in the universe

How old is the universe?

How can we estimate? Precisely map the expansion seen now from

Earth Run the physics “backwards”

Many ways to check. Examples: Age of oldest stars (globular clusters) WMAP measurements => 13.7 Billion years

(plus or minus 1%)

Boomerang team

Tiny differences in temperatureseen in every direction!

The universe is 2.73 K +/- 0.0001

WMAP

The whole sky is filled with radiation left over from the Big Bang

What if this heat comes from galaxies and clusters?

How would the heat be distributed in the night sky?

What is the ultimate fate of the universe?

Possibilities to consider:

Big Crunch Growth will slow down Growth will remain constant Growth will continue to speed up

NASAEvidence suggests this is happening

Summary

The universe is expanding Actually, it’s speeding up! Heat from the Big Bang is seen The matter in the universe in changing

with time (because of stars)

What is the universe made of?

Dark matter 25%

Free H and He 4%

Stars 0.5%

Neutrinos 0.3%

Heavy elements 0.03%

Dark energy 70% Pie chartFrom Wikipedia

Group activity #10

Hubble’s law