Hubble’s Law

• Our goals for learning

• What is Hubble’s Law?

• How do distance measurements tell us the age of the universe?

• How does the universe’s expansion affect our distance measurements?

We measure speeds with the Doppler shift.

All galaxies except the nearest have a redshift

i.e. They’re all moving away from us

By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way

Hubble’s Law: velocity = H0 x distance

the further away a galaxy is, the faster it is travelling

Redshift of a galaxy tells us its distance through Hubble’s Law:

distance = velocity H0

Distances of farthest galaxies are measured from redshiftsThis is the furthest distance-finding technique on the 'Cosmological Distance Ladder'Some techniques use standard candles and some do not.

How do distance measurements tell us the age of the universe?

The expansion rate appears to be the same everywhere in space

The universe has no center and no edge (as far as we can tell)

One example of something that expands but has no center or edge is the surface of a balloon

All observers see the same view.

All see other galaxies moving away, with the ones farther away moving more quickly.

The universe is expanding uniformly.

Cosmological Principle

The universe looks about the same no matter where you are within it

• Matter is evenly distributed on very large scales in the universe

• No center & no edges• Not proved but consistent with all

observations to date

Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies

Age =

~ 1 / H0

Distance

Velocity

Calculating 1 / H0 gives an age for the universe of 13.75 billion years old!

What have we learned?

• What is Hubble’s Law?– The faster a galaxy is moving away from us, the

greater its distance: velocity = H0 x distance

• How do distance measurements tell us the age of the universe?– The measurements let us calculate the expansion

rate of the universe, which lets us calculate how long the universe took to expand.

Thought Question

What do we mean by the expansion of the universe?

A. Galaxies are moving apart through space.

B. Spacetime itself is expanding.

C. Everything is expanding, including Earth, our bodies, etc.

Evidence for the Big Bang

• Our goals for learning• How do we observe the radiation left over

from the Big Bang?• How do the abundances of elements

support the Big Bang theory?

How do we observe the radiation left over from the Big

Bang?

The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965

Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR

Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves

Background has perfect thermal radiation spectrum at temperature 2.73 K

WMAP gives us detailed baby pictures of structure in the universe

How do the abundances of elements support the Big Bang theory?

Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old

Big Bang theory prediction: 75% H, 25% He (by mass)

Matches observations of nearly primordial gases

What have we learned?

• How do we observe the radiation left over from the Big Bang?– Radiation left over from the Big Bang is now in

the form of microwaves—the cosmic microwave background—which we can observe with a radio telescope.

• How do the abundances of elements support the Big Bang theory?– Observations of helium and other light elements

agree with the predictions for fusion in the Big Bang theory

Thought QuestionHow do we know the universe is expanding?

A. the big bang theory

B. the motion of the Andromeda Galaxy

C. the redshift measured in many galaxies

D. inflation

Thought QuestionAt which wavelength is the Universe the brightest?

A. UV

B. Visible

C. Far Infrared

D. Microwaves

What is the history of the universe according to the Big

Bang theory?

Planck Era

Before Planck time (~10-43 sec)

No theory of quantum gravity

Four known forces in universe:

Strong Force Electromagnetism

Weak Force

Gravity

•We think at moment of the big bang the 4 forces were unified at super high temperatures.

• At the first instant the forces separated out.

•First gravity from the other 3 sub-atomic forces.

•Then the strong force

•Then finally electromagnetism separates from the weak force.

Inflation• As the forces separate, the fabric of the

universe expands out like a balloon

• This extreme rapid expansion is called “inflation”, a critical moment in the Big Bang

• Occurs after gravity separation, but before electromagnetism separation

Particle Era

The inflating

universe starts to cool, energy starts being stored as matter

E = mc2

Photons converted into both matter and anti-matter as pairs of particles and antiparticles

E = mc2

Early universe was full of particles and radiation because of its high temperature

Particle Era

Amounts of matter and antimatter nearly equal

(Roughly 1 extra proton for every 109 proton-antiproton pairs!)

NOTE –everything so far: Force separation, Inflation, Fundamental matter formation,

has taken place within 1 second of the universe’s existence!

Era of Nucleo-synthesis

Nuclei begin to fuse

Protons and neutrons will start to stick together

(takes ~ 3 minutes)

Era of Nuclei

Helium nuclei form

Universe has become too cool to blast helium apart

Era of Atoms

Atoms form at age ~ 380,000 years

Background radiation released

•So far every photon of light released from a particle has been absorbed by another particle.

• Now the universe is sufficiently large (and the particles sufficiently clumped) that light escapes absorption and roams the universe.

•This is the Cosmic Background Radiation

Era of Galaxies

Galaxies form at age ~ 1 billion years

Thought QuestionHow does the motion of the photons change

after the universe has cooled to below 3000 K and atoms form?

A. They move more slowly.

B. They can move further because there are less things to bump into

C. They move more quickly

D. They can't move very far without bumping into things.

What have we learned?• What were conditions like in the early universe?

– The early universe was so hot and so dense that radiation was constantly producing particle-antiparticle pairs and vice versa

• What is the history of the universe according to the Big Bang theory?– As the universe cooled, particle production stopped,

leaving matter instead of antimatter– Fusion turned remaining neutrons into helium– Radiation traveled freely after formation of atoms