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REDSHIFT

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The Doppler Effect

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Light is an electromagnetic wave

o Light consists of fluctuations in the electromagnetic field.

o The wavelength (or distance from one wave crest to the next wavecrest) of light is extremely small for visible light it ranges from

four to seven ten millionths of a metre (400nm 700nm).

o The frequency is very high measured in millions of Hertz.

o Red light has a low frequency compared to blue light.

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The _ _ _ _ _ _ _ Effect

D O P P L E

R

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Imagine a source of light at a constant distance from us emittingwaves of light at a constant wavelength.o The wavelength of the waves we receive will be the same

constant wavelength at which they are emitted by the source.

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Suppose now that the source starts moving directly away from us.o When the source emits the next wave crest, it will be further

from us, so the distance we will see between the two wave

crests arriving will appear to be larger than when the star was

stationary.

o This means that the wavelength of the waves we receive will

be longer (or shifted toward the red end of the spectrum) thanwhen the source was not moving.

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All stars show redshift

Therefore, they are moving( towards / away from )

the point of observation

away from

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Hubbles Law

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= 0

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Deduce a formula to calculate the distance to

a star

Edwin Hubble Christian Doppler

Hint:

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Deduce a formula to calculate the distance to

a star

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0 =

0

=

0

0

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The Big Bang

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Balloon Demo

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o Light from celestial objects is red-shifted.

o Most of them are moving away from us

o This is not true of all celestial objects - the Andromeda galaxy is blue-shifted

o It is moving towards us due to the gravitational attraction of the Milky Way

o Some galaxies are partly red-shifted, and partly blue-shifted.

o This is due to their rotation - some parts of the galaxy are rotating towards us, while others

are rotating away from us.

o The majority of celestial objects are moving away from us though. If we

extrapolate backwards, we find that the universe must have started at a

single point.

o HOWEVER we are assuming that:o the universe has always expanded

o movement alone can cause redshift.

Evidence for the Big Bang

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o In fact, other factors can cause redshift:

o Quantum Doppler Redshiftresults from the relative motion of the light

emitting object and the observer at speeds comparable to the speed of

light. A relativistic Doppler formula is required when v c.

o Cosmological Redshift is caused by the expansion of space. The

wavelength of light increases as it traverses the expanding universe

between its point of emission and its point of detection by the same

amount that space has expanded during the crossing time.

o Gravitational Redshiftis a shift in the frequency of a photon to lower

energy as it climbs out of a gravitational field.

Evidence for the Big Bang

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Red shift provides evidence for a Big Bangbut does notprove it.

Evidence for the Big Bang

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1. Measure wavelength of light (from the galaxy).

2. Compare it to the wavelength for a source on the Earth.

3. Reference to spectral line or line spectrum.

4. Reference to the Doppler effect / redshift.

5.