Physics · 6 Space physics

6.2.3 The Universe

Before you start

A galaxy is made up of many billions of stars, and the Sun is one star in the galaxy called the Milky Way (6.2.2).
A light-year is the distance travelled by light through (the vacuum of) space in one year, and one light-year is $9.5 \times 10^{15}$ m (6.2.2).
A supernova is the explosion of a star (6.2.2).
The regions of the electromagnetic spectrum in order of wavelength, with microwaves at a much longer wavelength than visible light (3.3).
The wave equation $v = f λ$ , and that electromagnetic waves travel at $3.0 \times 10^{8}$ m/s in a vacuum (3.1, 3.3).
Working in standard form, and converting km to m by multiplying by $1000$ .

Syllabus objectives covered (11)

Know that the Milky Way is one of many billions of galaxies making up the Universe and that the diameter of the Milky Way is approximately 100 000 light-years
Describe redshift as an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies
Know that the light emitted from distant galaxies appears redshifted in comparison with light emitted on the Earth
Know that redshift in the light from distant galaxies is evidence that the Universe is expanding and supports the Big Bang Theory
Know that microwave radiation of a specific frequency is observed at all points in space around us and is known as cosmic microwave background radiation (CMBR)
Explain that the CMBR was produced shortly after the Universe was formed and that this radiation has been expanded into the microwave region of the electromagnetic spectrum as the Universe expanded
Know that the speed v at which a galaxy is moving away from the Earth can be found from the change in wavelength of the galaxy’s starlight due to redshift
Know that the distance d of a far galaxy can be determined using the brightness of a supernova in that galaxy
Define the Hubble constant H0 as the ratio of the speed at which the galaxy is moving away from the Earth to its distance from the Earth; recall and use the equation H0 = v / d
Know that the current estimate for H0 is 2.2 × 10–18 per second
Know that the equation d / v = 1 / H0 represents an estimate for the age of the Universe and that this is evidence for the idea that all the matter in the Universe was present at a single point

Part 1 of 7

The Milky Way: one galaxy among many billions

In 6.2.2 you met galaxies. Each one is made up of many billions of stars, and the Sun is one star in the galaxy called the Milky Way.
The Milky Way is not the whole Universe. It is one of many billions of galaxies, and the Universe is made up of all of them.
The diameter of the Milky Way is approximately 100 000 light-years. The objective asks you to know this number, so learn it.
Recall what a light-year is: the distance light travels through space in one year. So light takes about $100 000$ years to cross the Milky Way from one side to the other.
The trap is to read '100 000 light-years' as a length of time. A light-year is a distance. The word 'light' only tells you what the distance is measured against.
To give the diameter in metres, multiply by the length of one light-year, $9.5 \times 10^{15}$ m.
$100 000 \times 9.5 \times 10^{15} m = 9.5 \times 10^{20} m$
The far galaxies in the calculations later in this lesson are hundreds of millions of light-years away. Expect very large numbers and work in standard form from the start.

Part 2 of 7

Redshift: the observed wavelength gets longer

Start from 3.3. Visible light is one region of the electromagnetic spectrum. Within it, red light has the longest wavelength and violet the shortest.
Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from stars and galaxies that are receding. Receding means moving away from us.
Why does moving away make the wavelength longer? Picture a galaxy giving out crest after crest while it moves away from you. Each crest leaves from a little further away than the one before, so the crests reach you further apart. The distance between crests is the wavelength, so the wavelength you observe is longer than the wavelength that was emitted.
graph diagram — not drawn yet
Two sine waves drawn one above the other against the same horizontal axis labelled 'distance': the upper wave is labelled 'emitted' with one wavelength marked λ between two crests; the lower wave has the same amplitude but a clearly longer wavelength, labelled 'observed from a receding galaxy', with its longer wavelength marked between two crests.
Redshift: the same radiation, observed with a longer wavelength because its source is moving away.
The name comes from visible light. A longer wavelength is a shift towards the red end of the spectrum. But redshift happens to all electromagnetic radiation, not only light. Radio waves and ultraviolet from a receding galaxy are also observed at longer wavelengths.
The speed of the waves does not change: electromagnetic waves travel at $3.0 \times 10^{8}$ m/s in a vacuum. In $v = f λ$ , if $λ$ goes up and $v$ stays the same, $f$ must go down. So redshifted radiation also has a lower observed frequency.
How can anyone tell that a wavelength has changed? You need something to compare it with. The atoms of each element absorb and give out light at their own particular set of wavelengths. These show up as lines in a spectrum, and the pattern of lines identifies the element.
Measure the pattern of lines for an element in a laboratory on Earth. Then find the same pattern in the light from a distant galaxy. The pattern of lines is still recognisable, so you know which element it is, but every line is at a longer wavelength than on Earth.
That is what the objective means by light from distant galaxies appearing redshifted in comparison with light emitted on the Earth. A shift only means something when you know where the line would be for a source that is not moving away, and light emitted on the Earth gives you that.
Notice that a redshifted galaxy does not look red. Its whole spectrum is moved a little towards longer wavelengths. Write about the wavelength increasing, never about the light 'turning red'.

Part 3 of 7

Redshift as evidence: an expanding Universe

Light from distant galaxies is redshifted in whichever direction you look. Redshift means receding, so distant galaxies are moving away from us.
A few nearby galaxies are moving towards the Milky Way, and their light is not redshifted. That is why the objectives say distant galaxies. Keep that word in your answers.
If distant galaxies in every direction are moving away, the distances between galaxies are increasing. The Universe is getting bigger: the Universe is expanding. Redshift in the light from distant galaxies is the evidence for this.
More distant galaxies show a larger redshift, so they are moving away faster. You will put a number on this with the Hubble constant later in the lesson.
Expansion does not put the Earth at the centre. The distances between all the far galaxies are increasing, so an observer in any other galaxy would also see the distant galaxies moving away from them.
Now run the expansion backwards in time. If galaxies are moving apart now, they were closer together in the past. Further back still, everything was packed together.
The Big Bang Theory says the Universe began from a single, extremely hot and dense point and has been expanding ever since.
Redshift supports the Big Bang Theory: it is evidence for the theory, and that is the wording of the objective. Write 'supports' or 'is evidence for'. Do not write that redshift 'proves' the Big Bang.
Learn the chain in order, because each link is a separate point: light from distant galaxies is redshifted, so the galaxies are moving away, so the Universe is expanding, so it was smaller in the past, which supports the Big Bang Theory.

Part 4 of 7

Cosmic microwave background radiation

A second piece of evidence arrives as microwaves. Microwave radiation of a specific frequency is observed at all points in space around us, from every direction in the sky.
It does not come from one star or one galaxy. It is a background that fills the whole sky. It is called the cosmic microwave background radiation, or CMBR.
The Big Bang Theory explains where it came from. The CMBR was produced shortly after the Universe was formed, when the whole Universe was extremely hot.
When it was produced, the radiation had a much shorter wavelength than microwaves.
As the Universe expanded, the radiation travelling through it was stretched along with it. Its wavelength increased, and it has now been expanded into the microwave region of the electromagnetic spectrum. This is the same increase in wavelength as redshift.
Why from every direction? The radiation was produced throughout the early Universe, so it filled the whole Universe then. It still fills it now, so it reaches us from all directions.
For an Explain, give both halves. When it was produced: shortly after the Universe was formed. What happened to it: its wavelength was stretched into the microwave region as the Universe expanded. One half alone answers only half the question.
The objective says 'a specific frequency' but does not ask you for its value. Do not learn a number for it.

Part 5 of 7

Measuring a far galaxy's speed and distance

To test the expansion with numbers, astronomers need two measurements for each galaxy: the speed $v$ at which it is moving away from the Earth, and its distance $d$ from the Earth.
Speed from redshift. Measure the wavelength of a known line in the galaxy's starlight. Compare it with the wavelength of the same line measured on Earth. The difference is the change in wavelength due to redshift.
$change in wavelength = λ_{observed} - λ_{on Earth}$
The faster a galaxy is moving away, the bigger the change in wavelength. So the change in wavelength of the galaxy's starlight gives its speed $v$ . The objective asks you to know this method. It does not list an equation for it that you must recall.
Compare like with like: the same line, with the same wavelength on Earth. Then the galaxy with the larger change in wavelength is the one moving away faster.
Distance from a supernova. A supernova is a star exploding. For a short time it can give out more light than all the other stars in its galaxy together, so it can be seen in galaxies far too distant for single ordinary stars to be picked out.
One type of supernova always reaches about the same greatest brightness. So astronomers know how much light such a supernova really gives out.
The further away a source of light is, the dimmer it looks from Earth. Compare how bright the supernova appears with how bright it really is. The dimmer it appears, the further away its galaxy is. That gives the distance $d$ .
Think of identical street lamps along a long straight road at night. They all give out the same light, so the dimmest one you can see is the one furthest from you.
Keep the two methods apart. Redshift gives the speed. The brightness of a supernova gives the distance. An answer that swaps them is wrong physics and does not earn the mark.

Part 6 of 7

The Hubble constant

Plot $v$ against $d$ for many far galaxies and the points lie close to a straight line through the origin. Speed is proportional to distance: a galaxy twice as far away is moving away twice as fast.
graph diagram — not drawn yet
Axes with 'distance d / m' on the horizontal axis and 'speed of recession v / m/s' on the vertical axis, both starting at zero; about ten data points scattered closely about a straight best-fit line through the origin; a gradient triangle on the line labelled 'gradient = v / d = H₀'.
For far galaxies, speed is proportional to distance. The gradient of the line is the Hubble constant.
So the ratio $v / d$ comes out the same for every far galaxy. That ratio is the Hubble constant, $H_{0}$ .
Define it in these words: the Hubble constant is the ratio of the speed at which a galaxy is moving away from the Earth to its distance from the Earth.
$H_{0} = \frac{v}{d}$
You must recall this equation. Paper 4 gives you no formula sheet.
Check the unit. Speed in m/s divided by distance in m leaves 1/s. So $H_{0}$ is measured per second, written /s.
The current estimate is $H_{0} = 2.2 \times 10^{- 18}$ per second. The objective says you must know this value, so learn it with its unit.
What does the number mean? Each extra metre of distance adds $2.2 \times 10^{- 18}$ m/s to the speed. In more useful units, each extra million light-years adds about $21$ km/s.
$1 \times 10^{6} \times 9.5 \times 10^{15} m \times 2.2 \times 10^{- 18} /s \approx 21 000 m/s$
To use the equation, put $v$ in m/s and $d$ in m. If a distance is in light-years, multiply by $9.5 \times 10^{15}$ to get metres. If a speed is in km/s, multiply by $1000$ to get m/s.
Rearrange as you need. To find a speed, use $v = H_{0} d$ . To find a distance, use $d = \frac{v}{H_{0}}$ .
The trap is putting light-years or km/s straight into the equation. If you work out $H_{0}$ from data and your answer is nowhere near $10^{- 18}$ per second, check your units before anything else.

Part 7 of 7

The age of the Universe from 1 / H0

Turn $H_{0} = v / d$ upside down. Both sides flip, so the equation still balances.
$\frac{d}{v} = \frac{1}{H_{0}}$
Distance divided by speed is a time. $d / v$ is the time a galaxy moving at speed $v$ takes to travel a distance $d$ .
Suppose every galaxy started from the same point and has moved away at a steady speed ever since. Then $d / v$ is the time since they all set off. That is the age of the Universe.
Here is the key step. $H_{0}$ is the same for every far galaxy, so $d / v = 1 / H_{0}$ is the same for every far galaxy too. A nearer, slower galaxy and a more distant, faster one give the same time.
The same time for all of them means they were all in the same place at the same moment. This is evidence for the idea that all the matter in the Universe was present at a single point.
Now put in the number. Dividing $1$ by $2.2 \times 10^{- 18}$ per second gives a time in seconds.
$\frac{1}{2.2 \times 10^{- 18} /s} = 4.5 \times 10^{17} s$
One year is $365 \times 24 \times 3600 \approx 3.15 \times 10^{7}$ s. Divide to convert to years.
$\frac{4.5 \times 10^{17} s}{3.15 \times 10^{7} s/year} \approx 1.4 \times 10^{10} years$
So $1 / H_{0}$ gives an age of about $14$ billion years. It is an estimate, not an exact age. It assumes each galaxy has moved at the same speed the whole time, and the speeds may have changed.

Worked examples

Example 1. The diameter of the Milky Way is approximately $100 000$ light-years. (a) Calculate the diameter in metres. (b) Use your answer and the speed of light, $3.0 \times 10^{8}$ m/s, to calculate the time for light to cross the Milky Way, in years. Take one year as $3.15 \times 10^{7}$ s.

Show the working

(a) One light-year is $9.5 \times 10^{15}$ m. The diameter is $100 000$ of them, so multiply.
$100 000 \times 9.5 \times 10^{15} m = 9.5 \times 10^{20} m$
Writing $100 000$ as $10^{5}$ makes the powers easy: $10^{5} \times 10^{15} = 10^{20}$ . Multiplying powers of ten means adding the indices.
(b) Time is distance divided by speed. Use metres and m/s so the time comes out in seconds.
$t = \frac{9.5 \times 10^{20} m}{3.0 \times 10^{8} m/s} = 3.2 \times 10^{12} s$
Convert seconds to years by dividing by the number of seconds in one year.
$\frac{3.2 \times 10^{12} s}{3.15 \times 10^{7} s/year} \approx 1.0 \times 10^{5} years$
Check that it makes sense. A light-year is the distance light travels in one year, so $100 000$ light-years takes light $100 000$ years. The calculation agrees.

Answer: (a) $9.5 \times 10^{20}$ m. (b) About $1.0 \times 10^{5}$ years ( $100 000$ years).

Example 2. A particular line in the spectrum of an element has a wavelength of $656$ nm when measured in a laboratory on Earth. In the light from galaxy A, the same line is observed at $663$ nm. In the light from galaxy B, it is observed at $689$ nm. (a) Explain how these values show that both galaxies are receding. (b) Determine which galaxy is moving away from the Earth faster.

Show the working

(a) Compare each observed wavelength with the wavelength on Earth. Both $663$ nm and $689$ nm are longer than $656$ nm.
An increase in the observed wavelength is redshift, and redshift happens when the source is moving away. So both galaxies are receding.
(b) Find the change in wavelength for each galaxy, observed minus Earth value.
$A: 663 - 656 = 7 nm B: 689 - 656 = 33 nm$
The faster a galaxy moves away, the bigger the change in wavelength. B has the bigger change, so B is moving away faster.
You can go one step further. Far galaxies all have the same ratio $v / d$ , so the faster one, B, is also the more distant one.

Answer: (a) Both observed wavelengths are longer than the wavelength measured on Earth, which is redshift, so both galaxies are moving away. (b) Galaxy B: its change in wavelength is $33$ nm compared with $7$ nm for A.

Example 3. The distance to a far galaxy is found from the brightness of a supernova in it to be $5.0 \times 10^{8}$ light-years. Calculate the speed at which the galaxy is moving away from the Earth. Use the current estimate of the Hubble constant.

Show the working

Write down what you have to recall: $H_{0} = v / d$ , $H_{0} = 2.2 \times 10^{- 18}$ per second and one light-year $= 9.5 \times 10^{15}$ m. None of these is given, so all three come from memory.
Convert the distance to metres first, because $H_{0}$ is per second and works with metres and m/s.
$d = 5.0 \times 10^{8} \times 9.5 \times 10^{15} m = 4.75 \times 10^{24} m$
Rearrange $H_{0} = v / d$ for $v$ by multiplying both sides by $d$ .
$v = H_{0} d$
Substitute and calculate.
$v = 2.2 \times 10^{- 18} /s \times 4.75 \times 10^{24} m = 1.0 \times 10^{7} m/s$
Check the unit: per second times metres gives m/s, a speed. Check the size: $1.0 \times 10^{7}$ m/s is $10 000$ km/s, much less than the speed of light, which is sensible.

Answer: $1.0 \times 10^{7}$ m/s (about $10 000$ km/s).

Example 4. Use the current estimate of the Hubble constant to calculate an estimate for the age of the Universe, in years. Take one year as $3.15 \times 10^{7}$ s. Then explain why this calculation is evidence that all the matter in the Universe was once at a single point.

Show the working

Recall that $d / v = 1 / H_{0}$ estimates the age of the Universe. Recall $H_{0} = 2.2 \times 10^{- 18}$ per second.
Calculate $1 / H_{0}$ . Dividing by 'per second' gives seconds.
$\frac{1}{H_{0}} = \frac{1}{2.2 \times 10^{- 18} /s} = 4.5 \times 10^{17} s$
Convert to years by dividing by the number of seconds in one year.
$\frac{4.5 \times 10^{17} s}{3.15 \times 10^{7} s/year} = 1.4 \times 10^{10} years$
Now the explanation. $d / v$ is the time a galaxy has taken to reach its present distance, travelling at its present speed.
$H_{0}$ is the same for every far galaxy, so $d / v$ is the same for every far galaxy. All of them took the same time, so all of them started from the same place at the same moment: all the matter was at a single point.

Answer: About $1.4 \times 10^{10}$ years (14 billion years). The time $d / v$ is the same for every galaxy, so all galaxies set off from the same place at the same time, which is evidence that all the matter in the Universe was at a single point.

Where people go wrong

Mistake: Writing that redshift means the galaxy's light 'turns red' or that the galaxy looks red.

Instead: Redshift is an increase in the observed wavelength. Every line in the spectrum moves towards longer wavelengths.

The 'red' in the name only describes the direction of the shift in visible light. Redshift happens to all electromagnetic radiation, including radio waves and ultraviolet, which have no colour at all.

Mistake: Putting a distance in light-years, or a speed in km/s, straight into $H_{0} = v / d$ .

Instead: Convert distances to metres (multiply light-years by $9.5 \times 10^{15}$ ) and speeds to m/s (multiply km/s by $1000$ ) before using the equation.

The value $2.2 \times 10^{- 18}$ per second only works with metres and m/s. With other units the answer is wrong by a huge factor, and the final mark is lost.

Mistake: Saying that the CMBR was produced as microwaves, or that it comes from stars in the Milky Way.

Instead: The CMBR was produced shortly after the Universe was formed, with a much shorter wavelength. The expansion of the Universe stretched it into the microwave region. It arrives from every direction, not from any one object.

The whole point of the CMBR as evidence is that its wavelength has been increased by the expansion. An answer that leaves out the stretching leaves out the physics the objective asks you to explain.

Mistake: Concluding that the Earth is at the centre of the Universe because all distant galaxies move away from us.

Instead: The distances between all far galaxies are increasing. An observer in any galaxy would see distant galaxies moving away.

Expansion means every gap is growing, not that everything is fleeing one place. Redshift is evidence that the Universe is expanding, not that we are at its centre.

Mistake: Mixing up the two measurements: saying redshift gives a galaxy's distance, or a supernova gives its speed.

Instead: The change in wavelength (redshift) gives the speed $v$ . The brightness of a supernova gives the distance $d$ .

Each method measures one quantity. Redshift depends on how fast the source moves away; apparent brightness depends on how far away it is. Swapping them contradicts the correct method.

Check yourself

State two facts about the Milky Way that you need to know for this subtopic.
Answer
It is one of many billions of galaxies making up the Universe. Its diameter is approximately $100 000$ light-years.
Describe what is meant by redshift.
Answer
An increase in the observed wavelength of electromagnetic radiation emitted from a star or galaxy that is moving away (receding).
Light from distant galaxies appears redshifted in comparison with light emitted on the Earth. State what this tells you about the distant galaxies, and what it is evidence for.
Answer
The distant galaxies are moving away from us. This is evidence that the Universe is expanding, and it supports the Big Bang Theory.
The CMBR was produced with a much shorter wavelength than microwaves. Explain why it is observed as microwaves today.
Answer
As the Universe expanded, the radiation was stretched with it. Its wavelength increased until it was in the microwave region of the electromagnetic spectrum.
State what astronomers compare in order to find the speed at which a galaxy is moving away from the Earth.
Answer
The wavelength of a line in the galaxy's starlight with the wavelength of the same line measured on Earth. The change in wavelength due to redshift gives the speed: the bigger the change, the faster the galaxy is moving away.
Two supernovae of the same type are seen in two different far galaxies. The supernova in galaxy P appears much dimmer than the one in galaxy Q. Deduce which galaxy is further from the Earth, and explain your answer.
Answer
Galaxy P. Supernovae of this type reach about the same brightness, and a source looks dimmer the further away it is, so the dimmer one is further away.
A far galaxy is $3.8 \times 10^{24}$ m from the Earth and is moving away at $8.0 \times 10^{6}$ m/s. Calculate the Hubble constant from these data.
Answer
$H_{0} = v / d = (8.0 \times 10^{6}) \div (3.8 \times 10^{24}) = 2.1 \times 10^{- 18}$ per second. This is close to the current estimate of $2.2 \times 10^{- 18}$ per second.
State the current estimate for the Hubble constant, with its unit, and state what the quantity $1 / H_{0}$ represents.
Answer
$2.2 \times 10^{- 18}$ per second. $1 / H_{0}$ , which equals $d / v$ , is an estimate for the age of the Universe.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. Astronomers study the light from stars and galaxies. A particular line in the spectrum of an element has a wavelength of $486$ nm when it is measured in a laboratory on Earth.

(a) State the approximate diameter of the Milky Way. Give the unit. 1 mark No calculator

Mark scheme

B1 100 000 light-years; unit required; accept 9.5 × 10²⁰ m or 1 × 10⁵ light-years

(b) Describe what is meant by redshift. 2 marks No calculator

Mark scheme

B1 an increase in the (observed) wavelength (of light / electromagnetic radiation); accept a decrease in the (observed) frequency; 'the light becomes red' is insufficient
B1 from a star or galaxy that is moving away / receding

(c) In the light from a distant galaxy, the same line is observed at a wavelength of $510$ nm. Deduce whether the galaxy is moving towards the Earth or away from it. Give a reason for your answer. 2 marks No calculator

Mark scheme

B1 moving away from the Earth
B1 observed wavelength (510 nm) is longer / greater than the wavelength on Earth (486 nm), so the light is redshifted

(d) The light from distant galaxies is redshifted. Explain how this supports the Big Bang Theory. 2 marks No calculator

Mark scheme

B1 (distant) galaxies are moving away, so the Universe is expanding
B1 so in the past the Universe was smaller / everything was closer together / it began from a single point, owtte

Question 2. A supernova is observed in a far galaxy. Using the brightness of the supernova, astronomers find that the galaxy is $1.2 \times 10^{9}$ light-years from the Earth.

(a) Explain how the brightness of the supernova can be used to determine the distance of the galaxy. 2 marks No calculator

Mark scheme

B1 this type of supernova has a known (greatest) brightness / gives out a known amount of light
B1 compare with how bright it appears from Earth: the dimmer it appears, the further away the galaxy, owtte

(b) Astronomers can also find the speed at which this galaxy is moving away from the Earth by studying its starlight. State what they measure to find this speed. 1 mark No calculator

Mark scheme

B1 the change in wavelength (due to redshift) of a line in the galaxy's starlight, compared with the wavelength of the same line measured on Earth, owtte; accept 'the redshift of the starlight'; 'the brightness' or 'the supernova' scores 0

(c) State the current estimate for the Hubble constant $H_{0}$ . Give the unit. 1 mark No calculator

Mark scheme

B1 2.2 × 10⁻¹⁸ per second / 2.2 × 10⁻¹⁸ /s; unit required

(d) Calculate the speed at which the galaxy is moving away from the Earth. 3 marks

Mark scheme

C1 d = 1.2 × 10⁹ × 9.5 × 10¹⁵ = 1.14 × 10²⁵ m seen
C1 v = H₀ × d, or 2.2 × 10⁻¹⁸ × 1.14 × 10²⁵ seen
A1 2.5 × 10⁷ m/s (accept 2.51 × 10⁷ m/s or 2.5 × 10⁴ km/s); unit required

(e) The equation $d / v = 1 / H_{0}$ gives an estimate for the age of the Universe. Calculate this estimate in years. Take one year as $3.15 \times 10^{7}$ s. 2 marks

Mark scheme

C1 1 / (2.2 × 10⁻¹⁸) = 4.5 × 10¹⁷ s seen
A1 1.4 × 10¹⁰ years (accept 1.44 × 10¹⁰ years or 14 billion years); unit required

(f) Explain why the value of $d / v$ is evidence that all the matter in the Universe was once present at a single point. 2 marks No calculator

Mark scheme

B1 d/v is the time for a galaxy to travel to its present distance (at its present speed)
B1 d/v (= 1/H₀) is the same for all galaxies, so all galaxies / all matter started from the same place at the same time

Question 3. Microwave radiation of a specific frequency reaches the Earth from space.

(a) This radiation is known as the cosmic microwave background radiation (CMBR). State where in space the CMBR is observed. 1 mark No calculator

Mark scheme

B1 at all points in space around us / from every direction; 'from the Sun' or 'from one galaxy' scores 0

(b) Explain how the CMBR was produced and why it is now in the microwave region of the electromagnetic spectrum. 2 marks No calculator

Mark scheme

B1 produced shortly after the Universe was formed / shortly after the Big Bang
B1 as the Universe expanded, the radiation was expanded / stretched / redshifted, so its wavelength increased into the microwave region, owtte

How this is examined

Using $H_{0} = v / d$ to find a speed or a distance, with the units converted first: a three- or four-mark Calculate in which the distance is given in light-years or the speed in km/s The distance converted to metres with $9.5 \times 10^{15}$ m per light-year (or the speed converted to m/s); $H_{0} = v / d$ rearranged correctly; $2.2 \times 10^{- 18}$ per second used for $H_{0}$ ; a final answer with its unit, m/s for a speed or m for a distance.
How redshift shows the Universe is expanding and supports the Big Bang: a Describe of redshift followed by a two-mark Explain of what it is evidence for For redshift: 'an increase in the observed wavelength' and 'from a source that is moving away / receding'. For the evidence: 'distant galaxies are moving away, so the Universe is expanding' and 'so it was smaller in the past / began from a single point, which supports the Big Bang Theory'.
Estimating the age of the Universe from $1 / H_{0}$ , and why it points to a single starting point: a two-mark Calculate of $1 / H_{0}$ in seconds or years, followed by a two-mark Explain $1 / (2.2 \times 10^{- 18}) = 4.5 \times 10^{17}$ s, or $1.4 \times 10^{10}$ years with the unit. For the Explain: ' $d / v$ is the time for each galaxy to reach its present distance' and 'it is the same for all galaxies, so all the matter started from a single point at the same time'.
Where the CMBR came from and why it is now microwaves: a two-mark State of what CMBR is, or a two-mark Explain of its origin What it is: 'microwave radiation' and 'observed from all directions / at all points in space around us'. Its origin: 'produced shortly after the Universe was formed' and 'its wavelength has been stretched / increased into the microwave region as the Universe expanded'.
Which measurement gives the speed and which gives the distance: a one- or two-mark State or Explain about how $v$ or $d$ is found for a far galaxy Speed: 'from the change in wavelength (redshift) of the galaxy's starlight, compared with the wavelength on Earth'. Distance: 'a supernova of known brightness; the dimmer it appears, the further away the galaxy'.
The Milky Way's place and size in the Universe: a one-mark State 'One of many billions of galaxies' or 'diameter approximately $100 000$ light-years', with the unit light-years.

Words to know

Universe: Everything that exists, made up of many billions of galaxies and the space between them.
Galaxy: A group of many billions of stars.
Milky Way: The galaxy that contains the Sun. Its diameter is approximately 100 000 light-years.
Light-year: The distance travelled by light through (the vacuum of) space in one year, equal to $9.5 \times 10^{15}$ m.
Receding: Moving away from the observer.
Redshift: An increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies.
Big Bang Theory: The theory that the Universe began from a single, extremely hot and dense point and has been expanding ever since.
Cosmic microwave background radiation (CMBR): Microwave radiation of a specific frequency observed at all points in space around us. It was produced shortly after the Universe was formed and has been stretched into the microwave region by the expansion of the Universe.
Supernova: The explosion of a star. One type always reaches about the same greatest brightness, so its apparent brightness can be used to find the distance of its galaxy.
Hubble constant, $H_{0}$: The ratio of the speed at which a galaxy is moving away from the Earth to its distance from the Earth, $H_{0} = v / d$ . The current estimate is $2.2 \times 10^{- 18}$ per second.

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