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

> Physics · CIE IGCSE 0625 (2026-2028)
> Source: https://www.owlsprep.com/study/cie-0625-u6-the-universe-redshift-big-bang/

This guide covers redshift and the Big Bang theory (Core), and cosmic microwave background radiation and the Hubble constant in SI units (Extended), aligned to CIE IGCSE Physics 0625 syllabus 6.2.3 for 2026-2028 exams.

**Prerequisites:** [Basic wave properties including wavelength and frequency](https://www.owlsprep.com/study/cie-0625-u3-properties-of-waves/); [The Sun, galaxies and light-years](https://www.owlsprep.com/study/cie-0625-u6-the-sun-and-the-life/)

## Learning objectives

- Describe redshift and explain how the redshift of distant galaxies is evidence that the Universe is expanding, supporting the Big Bang theory (Core)
- State that the Milky Way is one of many billions of galaxies making up the Universe (Core)
- (Extended) Describe cosmic microwave background radiation (CMBR) and explain how it supports the Big Bang theory
- (Extended) Know that a galaxy's speed v is found from redshift and its distance d from the brightness of a supernova
- (Extended) Define and use the Hubble constant H0 = v/d in SI units, and use age = d/v = 1/H0 to estimate the age of the Universe

## Redshift (Core)

**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.
2. $$\lambda_{\text{observed}} = 678\text{ nm} > \lambda_{\text{rest}} = 656\text{ nm}$$
3. Step 2: The observed wavelength is longer than the rest wavelength, so the light is redshifted.
4. 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.

## The Expanding Universe and the Big Bang (Core)

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.)

**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).

> **Extended only**
>
> A second piece of evidence, cosmic microwave background radiation (CMBR), is Supplement content and is covered in the Extended section below. Core answers about Big Bang evidence should focus on redshift.

**Worked example:** Explain how the redshift of distant galaxies provides evidence that the Universe is expanding.

1. The light from almost all distant galaxies is redshifted, so those galaxies are moving away from us.
2. The further away a galaxy is, the greater its redshift, so more distant galaxies are moving away faster.
3. 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.

## Extended Only: Cosmic Microwave Background Radiation (CMBR)

> **Extended Only Content**
>
> CMBR is Supplement content, required only for Extended (Paper 2/4) candidates. Core students can skip this section.

**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.
3. 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.

## Extended Only: The Hubble Constant and the Age of the Universe

> **Extended Only Content**
>
> The Hubble constant and the age-of-the-Universe calculation are Supplement content for Extended (Paper 2/4) candidates only. All quantities are in SI units.

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.

**Hubble constant** — 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).

*Notation:* H_0 = v/d

$$H_0 = \frac{v}{d}$$

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 $v = 2.2 \times 10^{6}$ m/s. Its distance from Earth is $d = 1.0 \times 10^{24}$ m. Calculate the Hubble constant, and give its unit.

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

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 $H_0 = 2.2 \times 10^{-18}$ s^-1, estimate the age of the Universe in seconds.

1. Step 1: The age of the Universe is estimated by 1/H0.
2. $$t = \frac{1}{H_0} = \frac{1}{2.2 \times 10^{-18}}$$
3. Step 2: Calculate.
4. $$t \approx 4.5 \times 10^{17}\ \text{s}$$
5. (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.

## Common pitfalls

- **Wrong:** Confusing redshift with blueshift
  - Why it fails: Redshift is an increase in observed wavelength (moving away); a decrease in wavelength (blueshift) means moving towards the observer.
  - Correct: Longer observed wavelength than the rest wavelength = redshift = the source is moving away.
- **Wrong:** Naming CMBR as the Core evidence for the Big Bang
  - Why it fails: In 0625, the Core evidence you must give is redshift of distant galaxies; CMBR is Supplement (Extended) content.
  - Correct: For Core answers, use redshift of distant galaxies as the evidence that the Universe is expanding.
- **Wrong:** (Extended) Working the Hubble constant in Mpc or km/s per Mpc
  - Why it fails: 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: 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:** (Extended) Taking the age of the Universe as H0
  - Why it fails: H0 has units of per second; the age (a time) is its reciprocal.
  - Correct: Estimate the age of the Universe as 1/H0 (equivalently d/v), giving a value in seconds.
- **Wrong:** Claiming the Milky Way is at the centre of the Universe because all galaxies move away from it
  - Why it fails: The Universe is expanding everywhere, so an observer in any galaxy would see all other galaxies moving away; there is no centre.
  - Correct: Explain that uniform expansion means every galaxy sees the others receding, so no galaxy is the centre.

## 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) |

## 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'.

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