and theories for
the origin of the Universe
Big Bang Theory of the Universe - evidence from the observations of the
'red-shift' and cosmic microwave background radiation
Doc Brown's Physics Revision
Suitable for higher GCSE/IGCSE Physics/Science courses or
solar system and satellites
Life Cycle of Stars for more detailed
What is cosmology?
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
a branch of astronomy that involves observing and studying the origin
and evolution of the universe, from the Big Bang at the start, its
current state and what is it's future?
According to NASA, the definition of cosmology is
"the scientific study of the large scale properties of the universe
as a whole."
At the moment the evidence suggests the Universe is getting bigger -
some sort of expansion is taking place.
Two models have been put forward to explain this expansion and the
creation of the Universe itself.
The Steady State model
The Steady State theory states that the Universe has always existed
in its current state and will remain so in the future.
The model is based on the idea that the Universe looks roughly the
same in which ever direction you look.
In order for this to be so, the expansion is explained (but not
proved) by new matter being created all the time ie more matter is
created to cause the expansion.
In the Steady State model of the Universe, there is no beginning and
no end - an interesting philosophical concept!
The Big Bang model
The Big Bang theory relies on the idea that all the matter of the
Universe started out by occupying a very small space (referred to as
This tiny space or volume of material would be incredibly dense and
It then 'explodes' and expands to create space distributing the
matter in it.
This expansion is considered to be continuous - see evidence for
this cosmological expansions in later sections of this page.
Measurements of the rate of this expansion can give you an estimate
of the finite age of the Universe - now calculated to be about 13.8
billion years (Our Sun is estimated to be 4.5 billion years old).
Of the two theories, the Big Bang model more is the accepted one
because it fits the ever increasing experimental data better than
the Steady State model - the latter just does not fit in with
several important observations.
NOT for GCSE, BUT at a
'deeper' level - The initial singularity was of apparently infinite
density and considered to have contained all of the mass and
space-time of the Universe that exists. Eventually some sort of
quantum fluctuations caused it to rapidly 'explode' in a 'Big Bang'
resulting in massive expansion, creating the present-day Universe as
we know it - the mind boggles! (I wish I understood more, but
my old grey cells are not up to it these days, and where did the
singularity come from in the first place? - mind boggling again!)
Some learning objectives for this page
- not an easy topic!
(a) Know and understand that if a wave source is moving
relative to an observer there will be a change in the observed wavelength
(b) You should know that there is an observed increase
in the wavelength of light from most distant galaxies.
(c) Appreciate how the observed
red-shift provides evidence that the universe is expanding and supports the
‘Big Bang’ theory (that the universe began from a very small initial point).
(d) Know that cosmic microwave background
radiation (CMBR) is a form of electromagnetic radiation filling the universe.
(e) Appreciate that the ‘Big Bang’ theory is currently the only theory that can explain the existence of CMBR.
INDEX of the technical sections on this page concerning the 'Big Bang' theory of
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
Know that current evidence suggests that
the universe is expanding and that matter and space expanded violently and
rapidly from a very small initial ‘point’, ie the universe began with a ‘big
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!
TOP OF PAGE
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.
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
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!).
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
spectrum. It is an example of the Doppler Shift described in section 1.
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 - the more the waves are
stretched out with a longer wavelength and lower frequency.
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.
These speeds are calculated from the red-shift.
Cosmologists have therefore concluded
that the whole of the universe is expanding.
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.
A good analogy is blowing up a balloon
with spots marked all over the surface.
The spots move apart in ALL directions
as the balloon expands.
The spots represent galaxies spread
throughout the Universe.
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 an 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.
Its as if all the matter and energy of
the Universe was compressed into some incredibly small volume of space (of
extraordinary high density) and
then 'something' triggered this great cosmological explosion.
The result we
see around us in the night sky!
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!
Apparently dark matter doesn't emit
electromagnetic radiation - so, unlike most material in the Universe, its a
bit difficult to detect - very frustrating! and difficult to test the
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 ~14 billion years.
5. Other evidence for the 'Big Bang Theory' of the
Cosmic microwave background
radiation (CMBR or CMB radiation)
Although both the Steady State model and
Big Bang model both explain the observed red-shifts and expansion of a
universe from our observations that objects are moving away from us in all
directions, the Steady State model cannot explain CMBR.
Radio-astronomers have detected low
frequency microwave radiation coming from all directions from all parts of
our universe - known as cosmic background microwave radiation (CMBR).
It is believed that this radiation comes from radiation that was present shortly after the beginning of the universe,
soon after the 'Big Bang' started.
CMBR is associated with low
temperatures as the young universe was cooling down and the background
radiation reduces in frequencies.
The ‘Big Bang’ theory is currently the only theory that can explain the existence of CMBR.
If other evidence comes to light, the
theory may have to be modified - 'watch this space'!
solar system and satellites and the
Life Cycle of Stars for more detailed
WAVES - electromagnetic radiation, sound, optics-lenses, light and astronomy revision notes index
introduction to the types and properties of waves, ripple tank expts, how to do
Illuminated & self-luminous objects, reflection visible light,
ray box experiments, ray diagrams, mirror uses
Refraction and diffraction, the visible light
spectrum, prism investigations, ray diagrams explained
sources, types, properties, uses (including medical) and dangers gcse physics
The absorption and emission of radiation by
materials - temperature & surface factors including global warming
Global warming, climate change,
reducing our carbon footprint from fossil fuel burning gcse
Optics - types of lenses (convex, concave, uses),
experiments and ray
diagrams, correction of eye defects
The visible spectrum of colour, light filters and
explaining the colour of objects gcse physics revision notes
Sound waves, properties explained, speed measure,
uses of sound, ultrasound, infrasound, earthquake waves
The Structure of the Earth, crust, mantle, core and earthquake waves (seismic wave
Astronomy - solar system, stars, galaxies and
use of telescopes and satellites gcse physics revision notes
The life cycle of stars - mainly worked out from emitted
electromagnetic radiation gcse physics revision notes
Cosmology - the
Big Bang Theory of the Universe, the red-shift & microwave background radiation gcse
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