Cosmology

Looking at the pictures: what
do you think the title of the

lesson is going to be
Challlenge can you come up
with three success criteria for
the lesson.
Title: Cosmology Date:
November 25, 2024
Learning objective: To determine what redshift is and the
significance it has on the structure of the universe.
Do
now
Recall the life cycle of a Success criteria:
massive star I can recall what what the Doppler Effect is.
Compare the life cycle of a I can use the Doppler equation to calculate z shift
massive star to a sun like
I can evaluate using the doppler equation the
star.
Gravity has a value of 9.81 recessional velocities of galaxies and link this to the
structure of the universe.
on Earth. Analyse how life
could be different if the
value suddenly changed
to:
i) Half g ii) Double g
Title: Cosmology Date:
November 25, 2024
Learning objective: To determine what redshift is and the
significance it has on the structure of the universe.
Do Same up until the main sequence (other than
now
Gas+Dust  Protostar  mass). Then Sun-like stars turn into red giant
Main Sequence  Red and then white dwarf as the mass is too small
Super Giant  for supernova (no fusion after producing
carbon/oxygen).
Supernova  Black Hole
Massive stars fuse up until iron and then
or Neutron Star produce a supernova, then black hole or neutron
Higher ‘g’ star (depending on mass)
Maximum height and weight would be less
Bone density would have to be greater
Muscle strength would have to be greater
Maximum jump would be smaller
Opposite for half ‘g’.
Doppler
and red
shift
Make a note of these equations
What causes the Doppler Effect?
Can you write a few sentences, supported by a diagram, to state what the Doppler
effect is, with respect to sound, and how it happens?
3-5 minutes
Green Pen!
What causes the Doppler Effect?
Can you write a few sentences, supported by a diagram, to state what the Doppler
effect is, with respect to sound, and how it happens?
• An object moving towards you will sound higher-pitched, and object moving away
will sound lower pitched.
• The moving object always emits sound waves at the same speed in all directions,
but because the object is moving, the next sound waves emitted will be from
closer to one wavefront than the other.
• The wavefronts on one side are closer together (higher frequency) and the
wavefronts on the other side are further apart (lower frequency).
3-5 minutes
Green Pen!
Doppler and red shift
Doppler
and red
Notice! Notice!
shift
As Wavelength increases: As Wavelength decreases:
Waves move towards infraRED part of Waves move towards ultraVIOLET part
spectrum of spectrum
They have Lower energy & Low They have high energy & high
Frequency Frequency
Red Shift – moving away from Blue shift- Moving towards observer
obsserver
Doppler Observed with Light
The importance of the Doppler effect is that it is seen with light waves and radio
waves.
This chart compares the IR light intensity at different wavelengths from the Sun and
from Arcturus.
What does it tell us? 3 marks
Doppler Observed with Light
The importance of the Doppler effect is that it is seen with light waves and radio
waves.
This chart compares the IR light intensity at different wavelengths from the Sun and
from Arcturus.
What does it tell us? 3 marks
• The data from

Arcturus is shifted
to longer
wavelengths.
• That equates to
lower frequencies.
• This means it is
moving away from
us (redshifted)
Looking at the light spectrum from a star is useful to
identify movement of stars and galaxies
If you pass light through a

gas some wavelengths of
light will be absorbed by the
gas, forming an ‘absorption
spectrum’.
Our Sun contains helium. We know this because there are black
lines in the spectrum of the light from the Sun where helium has
absorbed light. These lines form the absorption spectrum for
helium.
So how are these useful for us to tell
how a star/galaxy are moving?
If the light source is moving

away, the absorption spectra
looks a bit different. Spectrum of a distant Star
The faster a light source

moves the further its light
What way have the absorption
will be ‘shifted’ and the lines shifted?
further away it is Towards the red end of the spectrum, red-
shift
Light spectra from different stars and from the edge of the universe shows ‘red-shift’. This
suggests that everything in the universe is moving away from a single point.
OBSERVABLE EVIDENCE FOR THE BIG BANG THEORY
Complete the worksheets
on the Doppler effect and
red shift.
Make sure these are
completed in your book.
Challenge: Who is
Edwin Hubble and what
is his link to the big
bang/red shift?
Equation - Red Shift
For any object that is moving with a speed
much less than that of light, it can be shown
that the change in frequency is given by:
f v
∆f - change in frequency (Hz)
f - original frequency (Hz) 
v - speed of object (m/s) f c
c - speed of light (m/s)
for v  c only!
Equation – Red Shift
The equation in terms of wavelength is? …
 v

 c
∆λ - change in wavelength (m)
λ - original wavelength (m)
v - speed of object (m/s)
c - speed of light (m/s) for v  c only!
For These Equations
• Objects moving towards the observer have a
positive speed; moving away from the
observer the speed is negative.
• If the object is moving away, the frequency is

lower so that ∆f is negative. The wavelength
will be longer.
• If the object is coming towards the observer,
the frequency is higher, so ∆f will be positive.
The wavelength will be shorter.
Worked Example
The wavelength of a pale blue line in the
hydrogen spectrum is 486.27 nm as measured in
the lab.
When the same spectral line is looked at in a star,
the wavelength is now 486.94 nm.
What is the speed of recession?

Answer
Find out the change in wavelength:
∆λ= 486.94 - 486.27 = 0.67 nm = 0.67 × 10 m.
-9
Use:
 v

 c
Substitute into the equation:
0.67 × 10 m ÷ 486.27 × 10 m = -v ÷ 3 × 10 m/s
-9 -9 8
v = -4.1 × 10 m/s
5
Question:
A star is moving away from the Earth at 5000 km/s.
A certain wavelength has been detected in its spectrum
which corresponds to a line of wavelength 350 nm as
measured in a laboratory.
What is the wavelength of this line?

Answer
Use:  v

 c
∆λ = -(-5000 × 10 m/s) × 350 × 10 m ÷ 3 × 10 m/s

3 -9 8
∆λ = 5.83 × 10 m
-9
As the star is receding we add the difference to the

wavelength:
New wavelength = 355.83 nm
Applications of Doppler Effect
We can see that the pattern is the same, but the colours are
different.
The Doppler effect is used in other ways:

• looking at the rotational period of stars
• rotational periods of planets.
• orbital period of binary stars.
The Doppler Effect Summary:
We can measure the wavelengths at which each element absorbs
light in a lab, here on Earth. The element calcium, for example,
absorbs light of wavelengths:
393.3 nanometers -- the K line

396.8 nanometers -- the H line
Now, it turns out that if the material absorbing light is moving

towards or away from us with some radial velocity, we see shifts
in the location of the absorption lines:
material moves towards us: shift to shorter wavelengths (blue)
material moves away from us: shift to longer wavelengths (red)
The same principle as in the question is used to determine the
rotation of a planet or star.
Explain how red-shift provides evidence for the Big Bang theory
What do I need to do?
Explain in detail how red-shift provides evidence for the Big Bang theory. Keywords
to use: red-shift, galaxies, universe, expanding, initial point. Use BBC bitesize and
the grading descriptor below to help.
Grading
Grade 1-3 - you understand that the universe is
expanding.
Grade 4-7 - you have briefly explained how the red-
shift provides evidence for the Big Bang theory.
Grade 7-9 - you have given a detailed explanation of
how the red-shift provides evidence for the Big Bang
theory.
KEY POINTS
• light from (most) galaxies has been observed to have red-shifted.
• the further away the galaxy the bigger the red-shift.
• the furthest galaxies are moving fastest.
• (this shows that) the universe is expanding.
• if the universe has always been expanding.
• it must have once been very small.
Grading
Grade 1-3 - you understand that the universe is
expanding.
Grade 4-7 - you have briefly explained how the red-
shift provides evidence for the Big Bang theory.
Grade 7-9 - you have given a detailed explanation of
how the red-shift provides evidence for the Big Bang
theory.
Complete exam
questions on
phoenix
The theory predicted the presence of ‘Cosmic Microwave Background
Radiation’ (CMBR).
The relatively uniform background radiation is the remains of energy
created just after the Big Bang
This has been detected coming from every direction in space.
Have you met all the Success criteria?
1) I can recal what What the Doppler Effect is.

2) I can use the dopplar equation to calculate z shift
3) I can evaluate using the dopplar equation the

recessional velocities of galaxies

Cosmology

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Looking at the pictures: what

do you think the title of the

• The data from

If you pass light through a

If the light source is moving

The faster a light source

• If the object is moving away, the frequency is

What is the speed of recession?

What is the wavelength of this line?

∆λ = -(-5000 × 10 m/s) × 350 × 10 m ÷ 3 × 10 m/s

As the star is receding we add the difference to the

The Doppler effect is used in other ways:

393.3 nanometers -- the K line

Now, it turns out that if the material absorbing light is moving

1) I can recal what What the Doppler Effect is.

3) I can evaluate using the dopplar equation the

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