A-Level Physics: Astrophysics – Cosmology and the Expanding Universe

How do we know the universe is expanding, how old it is, and what evidence supports the Big Bang? This material covers the cosmology part of A level astrophysics: the Doppler shift of light and what it reveals about galaxies, binary stars and planets around other stars, and the observations on which our picture of the universe rests. It follows on from the telescopes and stars material in this category.

The quiz (12 questions) starts with red shift. You calculate z from a shifted spectral line, then the recession speed and the distance of a galaxy using Hubble's law, and a distance directly from a given recession velocity. You convert the Hubble constant into s⁻¹ and estimate the age of the universe as 1/H₀ in years, a calculation where unit conversions decide the answer. Doppler shifts also reveal spectroscopic binaries: one question finds the orbital period and the star's speed from a periodically shifting hydrogen line, and another checks what such systems are. The evidence questions cover the cosmic microwave background (including its peak wavelength from Wien's law), the hydrogen–helium abundance, the properties of quasars, why type Ia supernovae are standard candles and what distant ones revealed about the accelerating expansion, and the transit method for finding exoplanets.

The flashcards (12 cards) give the red-shift and Hubble equations, the age estimate, Wien's law, type Ia supernovae, the radial velocity and transit methods, quasars, dark energy, the temperature of the cosmic microwave background, spectroscopic binaries and the speed of light.

The written work (8 questions) asks you to carry a spectral measurement through to a galaxy's distance, convert the Hubble constant and justify 1/H₀ as an age with its assumption, explain the cosmic microwave background as evidence for the Big Bang, set out the three main pieces of evidence for the Big Bang, explain standard candles and the discovery of the accelerating expansion, describe how we know quasars are distant and powerful, compare the radial velocity and transit methods for exoplanets with their limitations, and find the orbital speed and radius of a star in a spectroscopic binary.

The content is based on the astrophysics and cosmology content of A level Physics specifications in England (for example the AQA Astrophysics option, section 3.9.3 cosmology, and OCR A module 5.5). Values of constants such as the Hubble constant are given in each question, because different boards and data sheets quote slightly different values.

• Calculate red shift and recession speed using z = Δλ/λ = v/c for v ≪ c
• Use Hubble's law to find distances, convert H₀ to s⁻¹ and estimate the age of the universe
• Explain the evidence for the Big Bang: red shift, the cosmic microwave background and light-element abundances
• Describe quasars and the use of type Ia supernovae as standard candles, including the accelerating expansion
• Explain how spectroscopic binaries and exoplanets are detected from Doppler shifts and transits

Practice material written by Zestly, based on the astrophysics content of A level Physics specifications in England (for example the AQA 7408 Astrophysics option, section 3.9.3 cosmology, and OCR A module 5.5).