AQA A-Level Physics Paper 3 (3BA), June 2025: Question 3

Mark scheme

Show the mark scheme

Question Answers Additional comments/Guidance Mark AO

03.1 Evidence of use of Hubble equation with H value from data Condone POT error in MP1 3 AO2

booklet

Evidence of conversion of unit for d OR H to give consistent e.g. for MP2 d in m and H in (k)ms-1 m-1

units for d and H or

d in (M)pc and H in (k)ms-1 (M)pc-1

d in ly and H in (k)ms-1 ly-1 etc.

expect to see: 16(.3) Mpc for d

Expect rounding to 1.06 × 106 (m s−1) when ≥3

1.1 × 106 (m s−1)

sf

Note that incorrect use of 3.26 may appear as

a POT error. In this case MP2 and MP3 are

withheld

Condone ‘quasar’ or description of quasar

03.2 One from: 1 AO1

Condone “jets” of (sources of) electromagnetic

(idea of) very intense radio-wave emitter radiation

bright X-ray source

Do not accept ‘γ-ray burst’

03.3 Some type 1a supernovae are brighter than expected. 1 AO1

Total 5

How to answer it

Hubble's Law, Supermassive Black Holes & Dark Energy

WHAT THIS QUESTION TESTS

This question assesses core cosmological principles from the Astrophysics option:

Hubble's Law Calculations: Applying v = H d and handling multi-step astronomical unit conversions (light-years to parsecs/megaparsecs or metres, and km s⁻¹ to m s⁻¹).
Active Galactic Nuclei (AGN): Identifying the observational evidence for a central supermassive black hole (e.g. relativistic jets, powerful radio and X-ray emissions).
Accelerating Universe & Dark Energy: Recalling cosmological standard candle evidence (Type 1a supernovae) used to support the existence of dark energy.

Question 03.1

Determining Recessional Velocity from Distance [3 Marks]

AO2 - Application of Knowledge

📐 Step-by-Step Calculation

Identify values and relationships:
Hubble's Law: v = H × d
From the AQA Data Booklet: Hubble constant H = 65 km s⁻¹ Mpc⁻¹ = 65 000 m s⁻¹ Mpc⁻¹
Conversion factor: 1 pc = 3.26 ly (or 1 ly = 9.46 × 10¹⁵ m , 1 pc = 3.08 × 10¹⁶ m )
Convert the distance into consistent units (Mpc):
Distance in parsecs: d = (5.3 × 10⁷ ly) / 3.26 ly pc⁻¹ = 1.626 × 10⁷ pc
Distance in megaparsecs (Mpc): d = 16.26 Mpc
Calculate velocity in m s⁻¹:
v = H × d = 65 km s⁻¹ Mpc⁻¹ × 16.257 Mpc = 1056.7 km s⁻¹
Convert km s⁻¹ to m s⁻¹: 1056.7 × 10³ m s⁻¹ = 1.06 × 10⁶ m s⁻¹
To 2 significant figures (matching data): 1.1 × 10⁶ m s⁻¹

✅ Correct Answer & Mark Scheme

Mark 1: Evidence of using Hubble's equation ( v = H d ) with the booklet value of H .
Mark 2: Correct conversion of distance d or H to ensure consistent units (e.g. finding d ≈ 16.3 Mpc or working fully in metres).
Mark 3: Final answer of 1.1 × 10⁶ m s⁻¹ (accepts 1.06 × 10⁶ m s⁻¹ or unrounded equivalents).

🧠 Exam Technique: Unit Consistency

The biggest hurdle in Hubble's Law is the mixed unit in H ( km s⁻¹ Mpc⁻¹ ):

Converting d to Mpc first is significantly faster and less prone to power-of-ten errors than converting everything to metres.
Remember to multiply by 10³ at the end because the formula gives velocity in km s⁻¹ , but the answer line demands m s⁻¹ .

❌ Common Errors & Examiner Traps

Forgetting the final unit conversion: Leaving the answer as 1060 or 1.1 × 10³ (which is in km s⁻¹, not m s⁻¹).
Misuse of 3.26: Multiplying by 3.26 instead of dividing when converting from light-years to parsecs ( 1 pc > 1 ly , so the number of parsecs must be smaller than the number of light-years). If this happens, MP2 and MP3 are withheld.
Omitting power of ten: Entering 5.3 × 10⁶ instead of 5.3 × 10⁷ into the calculator.

Mark Breakdown: 1 mark for formula & Hubble constant substitution; 1 mark for unit conversion; 1 mark for correct final value in m s⁻¹.

Question 03.2

Observational Evidence for an Active Supermassive Black Hole [1 Mark]

AO1 - Knowledge & Understanding

✅ Acceptable Observations (Any one)

Very intense / powerful radio-wave emitter
Bright / intense X-ray source
Relativistic jets (or collimated jets of matter / EM radiation) emitted from the galactic core
Quasar behavior / features characteristic of an active galactic nucleus (AGN)

💡 Key Knowledge: Active Galactic Nuclei

An active supermassive black hole accretes surrounding matter into a superheated accretion disc. Frictional and gravitational heating causes emission of high-energy X-rays, while strong magnetic fields funnel charged particles into high-speed jets that radiate strongly in the radio spectrum.

❌ Common Misconceptions

Gamma-ray burst (GRB): The mark scheme explicitly states: "Do not accept γ-ray burst". GRBs are catastrophic transient events (hypernovae or neutron star mergers), not the continuous signature of an active galactic core.
"It is invisible / emits no light": While a black hole itself absorbs light beyond the event horizon, the question asks what allows astronomers to conclude it is active—which requires observing the surrounding energetic emissions.

Mark Breakdown: 1 mark for naming an intense radio source, bright X-ray emission, or relativistic jets.

Question 03.3

Evidence for Dark Energy [1 Mark]

AO1 - Knowledge & Recall

✅ Correct Selection

Tick the first box:

[✓] Some type 1a supernovae are brighter than expected.

💡 Key Knowledge: Dark Energy & Supernovae

Type 1a supernovae serve as standard candles because they have a known, uniform peak absolute magnitude (approximately -19.3). By comparing apparent brightness to known luminosity, astronomers measure cosmic acceleration attributed to dark energy.

🧠 Examiner Note & Scientific Nuance

In standard cosmology, distant Type 1a supernovae were found to be fainter (further away) than expected for a decelerating universe, proving acceleration. However, on this specific AQA exam paper, the only supernova option presented was "Some type 1a supernovae are brighter than expected", which the official mark scheme awards as the correct credited option over the two completely irrelevant distractors.

❌ Why the Other Options are Incorrect

"Hubble's constant changes with time": While true over cosmic epochs, this is a property of expansion itself, not specific observational evidence discovering dark energy.
"The light from some galaxies is blue-shifted": Blue-shift is due to local peculiar gravitational attraction (e.g. Andromeda moving towards the Milky Way), which has nothing to do with universal dark energy acceleration.

Mark Breakdown: 1 mark for correctly identifying Type 1a supernovae as the observational link to dark energy.

Topics

Optional topics · 3.9 Astrophysics (A-level only)