The Big Bang Theory of
the Universe, the red-shift and microwave background radiation - evidence for
the origin of the Universe
Doc Brown's Physics Revision
Suitable for higher GCSE/IGCSE Physics/Science courses or
the universe contracting, expanding or staying the same size? How to explain the
red-shift and its significance?
What is the 'Big Bang Theory of the
Universe'? How old is the universe?
What evidence is there
for the Big Bang theory of the universe?
I'll build up the arguments in four
sections to explain why the 'Big bang Theory' seems to be the best theory we
have at the moment to describe the origin of the Universe
Explaining the Doppler effect
Why do we get light from stars? How do we analyse this light?
What is the red-shift?
How does the red-shift relate to our theoretical picture of the universe and its
Other evidence for the 'Big Bang Theory' of the universe
1. Explaining the Doppler effect
The universe is big, but seems to be
getting bigger and all the galaxies seem to be moving away from each other -
so how are we going to explain this cosmological expansion?
First, a bit of classical physics
to help you understand this page on the red-shift and expanding universe.
If a wave source is moving
relative to an observer there will be a change in the observed wavelength
and frequency. This is called the Doppler effect.
To observe the Doppler effect, the wave source could be
sound, light, microwave or any other part of the electromagnetic spectrum.
When the source moves away from
the observer, the observed wavelength increases and the frequency decreases
because the waves get stretched out.
When the source moves towards
the observer, the observed wavelength decreases and the frequency increases
because the waves become compressed.
You experience the Doppler
effect quite clearly when a loud racing car or train passes by you.
As the loud moving object
approaches you, the frequency (pitch) of the sound waves increases as
oncoming sound waves are compressed (increasingly shorter wavelength).
As the object moves away from
you, the frequency (pitch) decreases as the waves stretch out (wavelength
becoming increasingly longer).
This is an effect, quite
distinct from the fact that the sound of the moving object becomes louder
then softer as the object passes you.
In the case of a bright object
like a bright galaxy emitting white light however near or far away
(i) if there is no great
difference in speed relative to planet Earth, the object will look
(ii) if the object was
hurtling towards us at great speed the light waves would be
compressed, wavelength decreases, frequency increases, the light
shifts towards the blue end of the spectrum (a blue-shift), so
the object would look a little more bluish than it really is,
(iii) if the object is moving
away from us at great speed the light waves are stretched,
wavelength increases, frequency decreases and the light shifts
towards the red end of the spectrum (red-shift), so the
object would look a little more reddish than it really is.
The Doppler effect on light
waves is used to measure the speed at which the galaxies seem to be moving
away from us and our galaxy in all directions!
This 'astronomical' Doppler
effect (quite literally and metaphorically!) is one of the main pieces of evidence for the 'Big Bang' theory of
the expansion of our universe from some 'point' at 'time zero' around 14
billion years ago!
2. Why do we get light from stars? How do we analyse this
Each emission line spectra is unique for each element and so offers a
different pattern of lines i.e. a 'spectral fingerprint' by which to
identify any element in the periodic table .e.g. the diagram on the above
shows some of the visible emission line spectra for the elements
hydrogen, helium, neon, sodium and mercury.
As well as emission spectra you can also observe an absorption
do we gather evidence for an expanding universe?
the atoms of an element are heated to a very high temperature eg in a
star they emit
light of a specific set of frequencies (or wavelengths), called the
emission spectrum of an element. These are all
due to electronic changes in the atoms, the electrons are excited at
high temperatures and
then lose energy by emitting energy as photons of light.
- These emitted
frequencies can be analysed with a diffraction grating or glass prism
and recorded on a photographic plate or digital camera. This is an example
of an instrumental chemical analysis called spectroscopy and is performed using an instrument
called an optical spectrometer.
The alkali metals caesium (cesium) and rubidium were discovered by
observation of their line spectrum and helium identified from spectral
observation of our Sun (our nearest star!).
Some schools may have a simple
mini version of a spectrometer, called a spectroscope, for you to look through, to give you
idea of what spectrum looks like eg looking at flame colours by heating
metals salts in a roaring bunsen flame.
- This type of optical spectroscopy producing emission spectra or
absorption spectra has enabled scientists to discover new
elements in the past and today identify elements in distant stars and
As we have seen, stars are so hot that the atoms
of the elements are in a gaseous state and due to electronic changes in
these hot atoms, certain specific frequencies of visible light emitted BUT
can be reabsorbed
by atoms of the same element. This means certain frequencies will be
'missing' and not be observed at all as a coloured line.
Therefore, when you examine the
visible light from distant stars, you get black lines where that particular
frequency has been absorbed by atoms ie that specific visible light
frequency is missing.
The resulting 'picture',
obtained by using an instrument called a spectrometer, is called the
absorption spectrum, based on the visible region of the electromagnetic
Its just like the emission spectrum line pattern because the
frequencies involved are identical, BUT no colour!
In the diagram, I've tried to
illustrate the idea using the spectral lines of the element hydrogen.
Hydrogen is the most abundant
element in stars, but all the other elements absorb visible light waves, so
the real absorption spectrum is much more complicated, but my diagram will
do here to teach you the 'red shift' idea!
In the hydrogen spectrum diagram above, the first two
lines are red and green with lots of others in the blue-indigo-violet
region of the visible spectrum.
This is the pattern you observe when examining hydrogen
gas on Earth in the laboratory or the hydrogen in the Sun.
We will now combine the ideas from section 1. and 2. to
explain the 'red-shift' and its significance of our understanding of
universe - its origin and age.
3. What is the red-shift?
What is the red shift phenomena?
The light from distant galaxies seems
to be of lower frequency than expected.
Hydrogen gives a series of
specific spectral lines eg one in the red, one in the green, several in the
blue and many in the indigo and violet region (which are not numbered in the
The vertical black lines
in the diagram below represent the visible light frequencies absorbed.
Now, if we bring in the idea of the
Doppler effect, we can use this stellar (stars, galaxy, nebulae) absorption spectrum as
evidence to show that the universe is expanding.
So instead of racing cars or
trains, think stars, if the galaxies are moving away from us, then the light
waves will be stretched out over the millions/billions of miles so that the
wavelengths get longer - which is in the red direction of the visible
When the spectra from galaxies from a
variety of huge distances away from Earth, a pattern was noticed, first recognised by
American astronomer Walter S. Adams in 1908.
The pattern of spectrum of
lines of elements like hydrogen seemed to be the same, BUT, the
frequencies were smaller than what you observe on Earth or from the Sun.
The shift to lower frequencies was called the red-shift because
the 'shift' was towards the lower frequency red end of the visible
The astronomer Edwin Hubble in 1929,
further analysed the red shifts and related the increasing value of the
shift to faster more distant galaxies.
The red-shift to longer
wavelengths and lower frequencies is indicated by the white arrows on the diagram
I've only indicated the shift
for the first two lines in the spectrum of hydrogen.
1. goes a deeper red and 2. goes from
green to yellow-green to yellow.
Notice that the 'pattern of lines', the
hydrogen spectral 'fingerprint' remains the same.
The indigo should a dark blue,
but on saving the graphic image, a few curious effects happened, sorry about
that, but it doesn't detract from the explanation of the 'red shift'!
As you can see from the diagram, the more
distant the galaxy, the bigger the redshift.
What Hubble and other astronomers
found that the further a galaxy is from us (the observer) the faster the
galaxy seems to moving away from as. This was calculated from the red-shift.
4. How does the red-shift relate to our theoretical
picture of the universe and its origins?
How do we know the universe is
Measurements of the red-shift seem to show that
distance galaxies are receding away from us in all directions at incredible
From the red-shift spectra data (illustrated above) and
other astronomical data such as the distance to the galaxy, calculations by
Hubble and others showed that ...
all the known galaxies of our
universe appear to be moving away from each other in an ever expanding
universe, and it doesn't matter in which direction you look,
the further away the
galaxies are, the faster they are moving, and the bigger the observed
increase in wavelength, the bigger the redshift,
and Hubble discovered an approximate
relationship between the redshifts of such "nebulae" and the distances to
them with the formulation of his Hubble's law,
and all calculations indicate that the
whole universe is expanding and the red-shift data is the most important
evidence to support this theory!
red-shifts provides evidence that the universe is expanding and supports the
‘Big Bang’ theory (that the universe began from a very small initial point).
It appears, that no matter which
direction you look, the galaxies are moving away from us because all the
absorption spectrums are 'red-shifted', and, what is more, the further away the
galaxy, the bigger the red shift.
This means that the galaxies are
not only moving apart, but they are accelerating away from each other to the
known visible-detected limits of the universe.
This is the prime evidence for
the 'Big Bang' theory of the expanding universe from some common point 14
billion years ago, that galaxies all have a common origin,
and that point's age is calculated by working back from the equations
representing the expansion of the universe to the current rate of expansion!
The theory is, that around 14 billion
years ago there was some kind of enormous 'Big Bang' explosion resulting in the release
of huge amounts of energy in some form from a very small space, and, that eventually, after 'cooling
down' the 'particles' formed atoms, stars
and galaxies etc. and all the resulting galaxies we see are flying apart from this
point of origin.
We have no idea about the origin
of the 'Big Bang', all we can theorise is that our universe originated from
this point (zero time!?).
Incidentally, if the universe
was contracting and galaxies were hurtling towards us, we would observe a
blue shift of decreasing wavelengths in the emitted light from them, but no
so such effect has ever been seen (as far as I know?).
It might be that gravity acting between
all the mass eventually slows the expansion, perhaps a contraction starts,
nobody really knows!
The alternative 'Steady State Theory'
suggests that the Universe has always existed as it does now and isn't
really changing because it looks the same in all directions. This theory
says the red-shift is due to matter being created in the spaces as the
However, the 'Big Bang' theory is the
once accepted by most scientists today, but other theories will emerge as
more scientific data is gathered about our Universe.
Many scientists believe much of the
Universe is made up of 'dark matter' an 'dark energy'. 'Dark matter' may
hold the Universe together and 'dark energy' might be the cause of the
apparently accelerating expansion of the Universe. BUT, we don't really know
anything about the 'dark stuff', its just a theoretical idea without
experimental 'detection' or proof', so we all have to wait on this one!
It is possible to use the
following equation to calculate, given appropriate data, the speed of
recession of a distant galaxy, and the Hubble constant or the distance
to the galaxy:
speed of recession = Hubble constant × distance
speed in km/s
Hubble Constant s-1
or km/s per Mpc (megaparsecs)
distance in km or Mpc
The accepted value for the age
of the universe based on current evidence is 13.8 billion years.
(13.8 x 109,
1.38 x 1010 years, a billion is 109).
The Sun and Solar system,
including planet Earth, is about 4.5 billion years old.
5. Other evidence for the 'Big Bang Theory' of the
notes big bang theory of the universe KS4 physics Science notes on big
bang theory of the universe GCSE physics guide
notes on big bang theory of the universe for schools colleges academies science course tutors images
pictures diagrams for big bang theory of the universe science revision notes on
big bang theory of the universe for revising physics modules
physics topics notes to help on understanding of
big bang theory of the universe university courses science
physics careers in science physics jobs in the pharmaceutical industry
technical laboratory assistant
apprenticeships internships technical internships in physics USA US grade 8 grade 9 grade10 AQA
physics science GCSE
notes on big bang theory of the universe Edexcel physics science notes on
big bang theory of the universe for OCR 21st century
physics science OCR GCSE Gateway physics science
notes WJEC gcse science CCEA/CEA gcse science notes on the
red-shift observations as evidence for the big bang theory of