The Universe, Redshift and the Big Bang

This lesson explains The Universe, Redshift and the Big Bang for Cambridge IGCSE Physics 0625. It separates the Core requirements from the additional Supplement work for Extended candidates. Focus on the cause-and-effect explanation and the exact quantities being compared. Read the explanation once, then attempt the worked method from a blank page.

Core: what is the Universe made of?

The Milky Way is one of many billions of galaxies in the Universe. The syllabus gives the approximate diameter of the Milky Way as 100 000 light-years.

A light-year is a unit of distance, not time. It is the distance travelled by light through a vacuum in one year.

Core: what is redshift?

Redshift is an increase in the observed wavelength of electromagnetic radiation emitted by a receding star or galaxy.

Light from distant galaxies appears redshifted when it is compared with light produced by the same elements on Earth. The important chain is:

increased observed wavelength → galaxy is receding → distant galaxies are moving apart → the Universe is expanding

The redshift of light from distant galaxies is evidence that the Universe is expanding. This evidence supports the Big Bang Theory.

Do not write only that “light turns red”. The assessed idea is an increase in observed wavelength from a receding source.

Core original question

Light from a distant galaxy contains a spectral line at a longer wavelength than the corresponding laboratory line measured on Earth.

(a) Name this effect. [1]

Redshift.

(b) State what it shows about the galaxy. [1]

The galaxy is moving away, or receding, from Earth.

(c) Explain how observations of many distant galaxies support the Big Bang Theory. [2]

Their light is redshifted, showing that the galaxies are receding. This is evidence that the Universe is expanding, so in the past the matter in the Universe was closer together.

Original marking guidance

• one mark for redshift
• one mark for recession
• one mark for evidence of expansion
• one mark for the backward inference that matter was closer together

Supplement: cosmic microwave background radiation

Cosmic microwave background radiation, or CMBR, is microwave radiation observed throughout space around us.

The syllabus explanation is that CMBR was produced shortly after the Universe formed. As the Universe expanded, the radiation’s wavelength increased into the microwave region of the electromagnetic spectrum.

CMBR provides an additional line of evidence for the Big Bang model.

Supplement: galaxy speed, distance and the Hubble constant

The Hubble constant is defined by:

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

where:

Quantity	Symbol	Unit
Hubble constant	$H_0$	per second, $\text{s}^{-1}$
recession speed	$v$	m/s
distance from Earth	$d$	m

The current syllabus value is:

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

The syllabus also requires students to know that:

• a galaxy’s recession speed can be found from the change in wavelength caused by redshift
• the distance of a far galaxy can be estimated using the brightness of a supernova in that galaxy
• $d/v = 1/H_0$ gives an estimate for the age of the Universe

The age estimate supports the idea that matter in the Universe was once concentrated much more closely together.

Supplement worked question

A galaxy is receding at $2.0 \times 10^6\text{ m/s}$. Use $H_0 = 2.2 \times 10^{-18}\text{ s}^{-1}$ to calculate its distance from Earth. [3]

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

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

$d = \frac{2.0 \times 10^6}{2.2 \times 10^{-18}} = 9.1 \times 10^{23}\text{ m}$

Original marking guidance

• one mark for selecting $H_0 = v/d$
• one mark for correct rearrangement and substitution
• one mark for $9.1 \times 10^{23}$ m

Common mistakes

• Telling Core candidates to skip redshift and the Big Bang. The basic redshift-expansion link is Core.
• Calling a light-year a time. It is a distance.
• Writing that light becomes physically red. State that its observed wavelength increases.
• Saying redshift proves Earth is the centre of the Universe. The observation supports expansion; it does not give Earth a special central position.
• Using CMBR or Hubble calculations as Core requirements. These are Supplement.
• Multiplying by $H_0$ when solving for distance. From $H_0 = v/d$, distance is $v/H_0$.
• Omitting units or standard-form powers. A galaxy distance in metres should be extremely large.

Exam technique

For a Core explanation, use four linked ideas where the marks allow:

1. the observed wavelength is longer
2. the galaxy is receding
3. many galaxies receding is evidence of expansion
4. expansion supports the Big Bang Theory

For Supplement calculations, write the equation, rearrange before substituting, keep the power of ten visible and finish with the unit.

How this is examined

Core Papers 1 and 3 can test the Milky Way, light-years, the definition of redshift and the expansion-Big Bang evidence chain. Extended Papers 2 and 4 can additionally test CMBR, the Hubble constant, galaxy speed and distance, and the age estimate $1/H_0$. Practical papers are not centred on astronomy experiments, but graph and data skills may be assessed in an astronomical context.