The Universe is Expanding – Galactic Redshift

• Recall from waves, that the frequency of a wave can be distorted by relative motion – The Doppler Effect.

Light from distant galaxies also exhibit the Doppler Effect

Recall the structure of the Atom

Recall the structure of the Atom

Recall the structure of the Atom

Recall the structure of the Atom

Absorption and Emission Spectra

n=1 (Ground State)

n=3 (2nd excited state)

n=2 (1st excited state)

n=4n=5

n=3262 52 42

Larger Jump = More Energy = Bluer Wavelength

n=6

n=1 (Ground State)

n=3 (2nd excited state)

n=2 (1st excited state)

n=4n=5

Photons of all other energies (wavelengths) are ignored and pass on by unabsorbed.

n=2326 25 24n=6

Hydrogen

Helium

Oxygen

Neon

Iron

The Suns Absorption Spectrum

The Suns Absorption Spectrum from 420 – 430 nm. (TOP)The emission spectrum of Iron (Bottom)

Galactic Redshift

• When looking at the spectral lines of distant galaxies, we see a shift in the spectral lines.

• example

Earth Lab

Galaxy

R O Y G B I V

The Universe is Expanding – Galactic Redshift

• Recall from waves, that the frequency of a wave can be distorted by relative motion – The Doppler Effect.

Light from distant galaxies also exhibit the Doppler Effect

Galactic Redshift• Using the Doppler Effect – we can state

Earth Lab

Galaxy

R O Y G B I V

'

1

1

vcvc

v w here recessiona l velocity

Galactic Redshift• If v <<< c then …

Earth Lab

Galaxy

R O Y G B I V

'

1

1

vcvc

v

c becom es approxim ate ly

PSRT

Teacher Note: The derivation can been seen on page 638 IB Book by Paul Ruth

• Example Problem 1– The wavelength of a particular spectral line in the star

Megrez in the Big Dipper (Ursa Major, aka the Great Bear) is 486.115nm. Laboratory measurements demonstrate that the normal wavelength is 486.113nm. What speed is this star moving relative to the Earth?

Earth Lab

Galaxy

R O Y G B I V

vcv c

v m s

( )( )

, /

1 234

• Example Problem 2– As measured in the laboratory on earth a spectral line of

Hydrogen has a wavelength = 656.285 nm. But in the spectrum of the star Vega this line has a wavelength of 656.255nm. Calculate the speed at which Vega is moving relative to Earth.

– Is Vega moving towards or away from Earth?

Earth Lab

Galaxy

R O Y G B I V

vc

v c

v m s

( )( )

, . /

13 713 554

m oving tow ards earth

Classwork

• Do #7 (speeds only – for now) & 9ab

• page 541 of Tsokos