The Universe: Redshift, Big Bang and Hubble's Law

Physics· 6.2.3· 15 min read

1. Redshift (Core)★★☆☆☆⏱ 4 min

📘 Definition

Redshift

An increase in the observed wavelength of electromagnetic radiation emitted from receding (moving away) stars and galaxies.

Example:

Light from a receding galaxy has its spectral lines shifted towards the longer-wavelength (red) end of the spectrum.

The light emitted from distant galaxies appears redshifted compared with the light from the same elements measured on the Earth. Because the observed wavelength is longer than the wavelength measured in a laboratory, the galaxy must be moving away from us. Galaxies that are further away show a larger redshift.

📐 Worked Example

A hydrogen spectral line has a wavelength of 656 nm when measured in a laboratory on Earth. The same line from a distant galaxy is observed at 678 nm. State whether the galaxy is moving towards or away from Earth, and justify your answer.

1
Step 1: Compare the observed wavelength with the rest (laboratory) wavelength.
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$\lambda_{\text{observed}} = 678\text{ nm} > \lambda_{\text{rest}} = 656\text{ nm}$
3
Step 2: The observed wavelength is longer than the rest wavelength, so the light is redshifted.
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Conclusion: redshift means the galaxy is moving away from the Earth.

Exam tip:

Core students are never asked to calculate a redshift, only to interpret a wavelength change: longer observed wavelength = redshift = moving away.

2. The Expanding Universe and the Big Bang (Core)★★☆☆☆⏱ 4 min

The Milky Way is just one of many billions of galaxies that make up the Universe. (The diameter of the Milky Way is about 100 000 light-years.)

📘 Definition

Big Bang theory

The theory that the Universe began from a single, very hot, very dense point and has been expanding ever since.

Example:

The redshift seen in almost all distant galaxies matches a Universe that is expanding in all directions.

The key Core evidence for the Big Bang is redshift. Almost all distant galaxies are redshifted, which shows they are moving away from us, and more distant galaxies are moving away faster. This tells us the whole Universe is expanding. If it is expanding now, then in the past everything must have been much closer together, supporting the idea that the Universe began from a single point (the Big Bang).

📐 Worked Example

Explain how the redshift of distant galaxies provides evidence that the Universe is expanding.

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The light from almost all distant galaxies is redshifted, so those galaxies are moving away from us.
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The further away a galaxy is, the greater its redshift, so more distant galaxies are moving away faster.
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Galaxies moving apart in every direction means the Universe as a whole is expanding, which supports the Big Bang theory.

Exam tip:

When asked how redshift supports the Big Bang, make three points: galaxies are moving away, more distant ones move away faster, so the Universe is expanding.

3. Extended Only: Cosmic Microwave Background Radiation (CMBR)★★★☆☆Extended only⏱ 3 min

📘 Definition

Cosmic microwave background radiation (CMBR)

Microwave radiation of a specific frequency that is observed at all points in space around us.

CMBR was produced shortly after the Universe was formed. As the Universe expanded, this radiation was stretched (its wavelength increased) until it now lies in the microwave region of the electromagnetic spectrum. Because it is detected uniformly from all directions, CMBR is strong evidence that the whole Universe began in a hot, dense state and has expanded, supporting the Big Bang theory.

📐 Worked Example

Explain how cosmic microwave background radiation supports the Big Bang theory.

1
CMBR is radiation that was produced shortly after the Universe formed, when it was very hot and dense.
2
As the Universe expanded, this radiation was stretched into the microwave region of the electromagnetic spectrum.
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It is observed at the same low level from all directions in space, which is exactly what the Big Bang theory predicts.

Exam tip:

State three things about CMBR: it is microwave radiation from all points in space, it was produced just after the Universe formed, and it was stretched into the microwave region as the Universe expanded.

4. Extended Only: The Hubble Constant and the Age of the Universe★★★★☆Extended only⏱ 6 min

To study the expansion of the Universe, astronomers need the speed $v$ at which a galaxy is moving away and its distance $d$ from Earth:

The speed $v$ at which a galaxy is moving away from the Earth is found from the change in wavelength of the galaxy's starlight (its redshift).
The distance $d$ to a far galaxy is found from the brightness of a supernova in that galaxy.

📘 Definition

Hubble constant

H_0 = v/d

The Hubble constant H0 is the ratio of the speed at which a galaxy is moving away from Earth to its distance from Earth. In SI units v is in m/s and d is in m, so H0 has units of per second (s^-1).

H_0 = \frac{v}{d}

The current estimate for the Hubble constant is:

H_0 \approx 2.2 \times 10^{-18}\ \text{s}^{-1}

📐 Worked Example

A galaxy is moving away from Earth at a speed of $v = 2.2 \times 1 0^{6}$ m/s. Its distance from Earth is $d = 1.0 \times 1 0^{24}$ m. Calculate the Hubble constant, and give its unit.

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Step 1: Write the equation for the Hubble constant.
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$H_0 = \frac{v}{d}$
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Step 2: Substitute the SI values.
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$H_0 = \frac{2.2 \times 10^{6}}{1.0 \times 10^{24}}$
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Step 3: Calculate. The units are (m/s) / m = 1/s = s^-1.
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$H_0 = 2.2 \times 10^{-18}\ \text{s}^{-1}$

The Hubble constant also gives an estimate of the age of the Universe. Because $H_{0} = v / d$ , its reciprocal is:

\text{age of the Universe} \approx \frac{d}{v} = \frac{1}{H_0}

This works because, if a galaxy has been moving away at a steady speed $v$ ever since the Universe began, the time it has taken to reach distance $d$ is $d / v$ . Getting the same value of $d / v = 1/ H_{0}$ for all galaxies is evidence that all the matter in the Universe was once at a single point.

📐 Worked Example

Using $H_{0} = 2.2 \times 1 0^{- 18}$ s^-1, estimate the age of the Universe in seconds.

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Step 1: The age of the Universe is estimated by 1/H0.
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$t = \frac{1}{H_0} = \frac{1}{2.2 \times 10^{-18}}$
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Step 2: Calculate.
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$t \approx 4.5 \times 10^{17}\ \text{s}$
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(This is about 14 billion years.)

Exam tip:

Keep everything in SI: v in m/s, d in m, H0 in s^-1. The age of the Universe is 1/H0 (or d/v), not H0 itself.

5. Common Pitfalls

Wrong move:

Confusing redshift with blueshift

Why:

Redshift is an increase in observed wavelength (moving away); a decrease in wavelength (blueshift) means moving towards the observer.

Correct move:

Longer observed wavelength than the rest wavelength = redshift = the source is moving away.

Wrong move:

Naming CMBR as the Core evidence for the Big Bang

Why:

In 0625, the Core evidence you must give is redshift of distant galaxies; CMBR is Supplement (Extended) content.

Correct move:

For Core answers, use redshift of distant galaxies as the evidence that the Universe is expanding.

Wrong move:

(Extended) Working the Hubble constant in Mpc or km/s per Mpc

Why:

0625 uses SI units only: v in m/s and d in m, so H0 comes out in per second (s^-1). Megaparsecs are not part of this syllabus.

Correct move:

Substitute v in m/s and d in m into H0 = v/d to get H0 in s^-1 (current estimate 2.2 x 10^-18 s^-1).

Wrong move:

(Extended) Taking the age of the Universe as H0

Why:

H0 has units of per second; the age (a time) is its reciprocal.

Correct move:

Estimate the age of the Universe as 1/H0 (equivalently d/v), giving a value in seconds.

Wrong move:

Claiming the Milky Way is at the centre of the Universe because all galaxies move away from it

Why:

The Universe is expanding everywhere, so an observer in any galaxy would see all other galaxies moving away; there is no centre.

Correct move:

Explain that uniform expansion means every galaxy sees the others receding, so no galaxy is the centre.

6. Quick Reference Cheatsheet

Concept	Tier	What you must know
Scale of the Universe	Core	The Milky Way is one of many billions of galaxies; its diameter is about 100 000 light-years
Redshift	Core	Increase in observed wavelength of light from receding galaxies; larger redshift = faster recession
Big Bang evidence	Core	Redshift of distant galaxies shows the Universe is expanding, supporting the Big Bang
CMBR	Extended	Microwave radiation from all points in space, produced just after the Universe formed and stretched to microwaves as it expanded
Finding v and d	Extended	v from the change in wavelength (redshift); d from the brightness of a supernova
Hubble constant	Extended	H0 = v/d; v in m/s, d in m, H0 in s^-1; current estimate 2.2 x 10^-18 s^-1
Age of the Universe	Extended	age = d/v = 1/H0, about 4.5 x 10^17 s (roughly 14 billion years)

7. Frequently Asked

Do Core students need to do Hubble constant calculations?

No. The Hubble constant H0 = v/d and the age-of-the-Universe calculation (age = 1/H0) are Extended (Paper 2/4) only. Core students need the qualitative link: redshift of distant galaxies shows the Universe is expanding, which supports the Big Bang theory.

What evidence for the Big Bang do I need for the Core tier?

For Core, the evidence you must give is the redshift of distant galaxies, which shows the Universe is expanding. Cosmic microwave background radiation (CMBR) is also evidence for the Big Bang, but it is Supplement (Extended) content.

What units are used for Hubble's Law in 0625?

0625 uses SI units only: v in metres per second (m/s), d in metres (m), and H0 in per second (s^-1), with a current estimate of 2.2 x 10^-18 s^-1. Megaparsecs (Mpc) and km/s per Mpc are not used in this syllabus.

Going deeper

syllabusCIE IGCSE Physics 0625 2026-2028 SyllabusRefer to section 6.2.3 for official topic requirements

What's Next

You have now covered redshift and the Big Bang (Core), and CMBR and the Hubble constant in SI units (Extended). This completes Unit 6 Space Physics. Core candidates should be able to interpret redshift and explain how it supports an expanding Universe; Extended candidates should practise using H0 = v/d in SI units and estimating the age of the Universe as 1/H0. Revise the whole unit alongside past-paper structured questions, paying attention to command words such as 'state', 'describe' and 'explain'.