Doppler effect lecture slides

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Doppler effect lecture slides

The Doppler Effect in

Astronomy and Cosmology


The Doppler Effect

A passing train

A passing ambulance

Doppler Effect in Cosmology 2


The Doppler Effect for Sound

The radio emits a series of “beeps,”

with period T. If the Flash is

stationary, he will hear the beeps

with a period T’ = T. There is no

Doppler shift.

Doppler Effect in Cosmology 3


The Doppler Effect for Sound

Now the Flash starts running away from the radio.

The Flash is running away, and so the beeps take longer

to reach him, and so he hears a period T’, which is not

equal to T, but is bigger.

Doppler Effect in Cosmology 4


The Doppler Effect for Sound

Now the Flash starts running away from the radio.

Suppose the Flash is running at a constant velocity, v, and

the sound is traveling at a speed v S. They start an initial

distance d apart. Flash hears the first beep at time

t 1 =

d

v s " v

Doppler Effect in Cosmology 5


The Doppler Effect for Sound

Now the Flash starts running away from the radio.

The radio waits a time T to send out the next beep, while

the Flash is running the whole time. So, he hears the

next beep at time

t 2 = d + v s T

v s " v

Doppler Effect in Cosmology 6


The Doppler Effect for Sound

Now the Flash starts running away from the radio.

So, the Flash hears a period T’=t 2 - t 1.

T " = vs vs # v

T =

T

1# v v S

Doppler Effect in Cosmology 7


The Doppler Effect for Sound

Now the Flash starts running away from the radio.

Since the frequency is f =1/T’, then we can write

f " = 1# v $

&

%

v S

'

) f

(

Doppler Effect in Cosmology 8


The Doppler Effect for Sound

What if both are moving?

If the speed of the source is v 0, the speed of sound is v S,

and the speed of the receiver is v R, then

f " = v #

S m v &

R

% ( f

$ vS ± v0 '

Doppler Effect in Cosmology 9


!

The Doppler Effect for Light

Because light behaves

like a wave, it still has a

Doppler effect.

However, the equations

are different!

"

f =

c ± v

c m v f

The Special Theory of

Relativity.

Doppler Effect in Cosmology 10


The Doppler Effect for Light

A source of light moving

at v = 0.7c.

Redshift.

Blueshift.

Doppler Effect in Cosmology 11


Spectral Lines

When electrons in an atom

make a transition from a

bigger orbit to a lower one,

they emit light.

Every atom has a characteristic spectrum.

Doppler Effect in Cosmology 12


Spectrum of a Star

The Sun emits a continuous

spectrum like a blackbody with

a temperature of 5800 K.

The spectrum of the Sun

shows absorption lines.

By looking at

the spectrum,

we can

determine the

chemical

composition of

a star.

Doppler Effect in Cosmology 13


Spectrum of Stars in a Distant Galaxy

The stars (of a certain type) have

the same spectrum, no matter

where they are.

The spectrum from stars

shows same absorption lines.

The lines are

shifted!

Doppler Effect in Cosmology 14


Edwin Hubble

The Universe is

expanding!!!

v = H 0 d

Doppler Effect in Cosmology 15


Doppler Effect in Cosmology 16


The Evolution of the Universe

Doppler Effect in Cosmology 17


The Cosmic Microwave Background

The Universe

started off

extremely hot and

dense, with LOTS of

high-energy photons

around.

As the Universe expands,

the UV photons can start

freestreaming from the

Surface of Last

Scattering at z ~ 1100.

Doppler Effect in Cosmology 18


The Cosmic Microwave Background

The photon wavelength grows with the expansion!

The Universe has expanded by a factor of z ~ 1100

since the LSS, and so the photons are in the

microwave region.

If the Big Bang picture is correct, there should be a

Cosmic Microwave Background!

Doppler Effect in Cosmology 19


Penzias and Wilson

The CMB was discovered

(accidentally) in 1964, and

has a perfect blackbody

spectrum with tiny

deviations!

Doppler Effect in Cosmology 20


Tiny Deviations Lead to Structure in the Universe

COBE

WMAP

This is an area of active

research today, and is based

very much on the Doppler

effect!

Doppler Effect in Cosmology 21

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