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COSMOLOGY
Evidence
and theories for
the origin of the Universe
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Doc Brown's school physics revision notes: GCSE
physics, IGCSE physics, O level physics, ~US grades 8, 9 and 10
school science courses or equivalent for ~14-16 year old students of
physics
The
Big Bang Theory of the Universe -
evidence from observations of the
'red-shift', cosmic microwave background radiation, cosmology and the Hubble
equation
Sub-index for this page
(a)
An introduction
to cosmological models
(b)
INDEX of the technical sections
on this page concerning the 'Big Bang' theory of the universe
1.
Explaining the
Doppler effect
2.
Why do we get
light from stars? How do we analyse this light?
3.
What is the
red-shift?
4.
How does the
red-shift relate to our theoretical picture of the universe and its
origins? Hubble equation
5.
Other evidence
for the 'Big Bang Theory' of the universe
See also
Astronomy -
solar system and satellites
and the
Life Cycle of Stars for more detailed
notes on stars
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(a) An introduction
to cosmological models
What is cosmology? Is
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
Cosmology
Cosmology
is
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 and has always looked that way.
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
a singularity).
This tiny space or volume of material would be incredibly dense and
hot.
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
and frequency.
 (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.
TOP OF PAGE
and sub-index
(b) INDEX of the technical sections on this page concerning the 'Big Bang' theory of
the universe
1.
Explaining the Doppler effect
2.
Why do we get light from stars? How do we analyse this light?
3.
What is the red-shift?
4.
How does the red-shift relate to our theoretical picture of the universe and its
origins? Hubble equation
5.
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
bang’.
-
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 closer together (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
white,
-
(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!
-
This piece of evidence for the big
bang theory is called the red shift and is explained in section 3. after
you have a learned little spectroscopy of how elements are detected
using emitted light from the very hot atoms of stars.
TOP OF PAGE
and sub-index
2. Why do we receive light from stars? How do we analyse this
light?
-
 How
do we gather evidence for an expanding universe?
-
If
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
an
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
galaxies.
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!).
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
right
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
spectrum - this provides better evidence for the expanding universe
theory than emission spectra.
-
-
The vertical black lines are where the light from hot
hydrogen atoms has been absorbed by the gases of the 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, but, certain specific frequencies of visible light emitted
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
spectrum.
-
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.
-
You can analyse the spectral data for
helium, its more complex, BUT, shows the same red shift pattern i.e.
characteristic absorption spectrum lines are shifted to lower
frequencies, longer wavelengths.
-
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.
TOP OF PAGE
and sub-index
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
diagram below).
-
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
spectrum!
-
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
below.
-
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.
-
Also note that the more distant the
galaxy, the greater the red shift in frequencies/wavelengths.
-
-
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 red shift - 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.
TOP OF PAGE
and sub-index
4. How does the red-shift relate to our theoretical
picture of the universe and its origins?
and the Hubble equation
-
How do we know the universe is
expanding?
-
-
Measurements of the red-shift seem to show that
distance galaxies are receding away from us in all directions at incredible
speeds
-
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 red shift,
-
and Hubble discovered an approximate
relationship between the red shifts 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!
-
The observed
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
Universe expands.
-
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.
-
Unanswered questions !!!
-
The Hubble
Equation
-
Despite the fact that neither dark matter
or dark energy is understood, there is one equation that fits in with a 'big
bang' theory and experimental data (at least at the moment).
-
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.
TOP OF PAGE
and sub-index
5. Other evidence for the 'Big Bang Theory' of the
universe
-
As well as the 'red-shift' evidence,
there is other evidence for the Big Bang theory of the origin of the
universe, and not the steady state theory.
-
Cosmic microwave background
radiation (CMBR or CMB radiation)
-
Apparently 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, BUT, 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).
-
Microwave radiation is part of the
electromagnetic spectrum, but the CMBR is of lower frequency than in
your microwave cooker!
-
This microwave radiation was
accidently discovered, and still studied, using a radio telescope.
-
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'!
See also
Astronomy -
solar system and satellites and the
Life Cycle of Stars for more detailed
notes
TOP OF PAGE
and sub-index
WAVES - electromagnetic radiation, sound, optics-lenses, light and astronomy revision notes index
General
introduction to the types and properties of waves, ripple tank expts, how to do
wave calculations
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
gcse physics
Electromagnetic spectrum,
sources, types, properties, uses (including medical) and dangers gcse physics
The absorption and emission of radiation by
materials - temperature & surface factors including global warming
See also
Global warming, climate change,
reducing our carbon footprint from fossil fuel burning gcse
chemistry
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
analysis)
gcse notes
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
physics
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