9702 · 25.3
Hubble's law and the Big Bang theory flashcards
Revision flashcards for Cambridge 9702 Hubble's law and the Big Bang theory (syllabus 25.3). Flip, recall, then mark a real past-paper question.
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What is the fundamental concept behind the Big Bang Theory?
The universe originated from an initial, extremely hot and dense 'singularity' that rapidly expanded and cooled, creating space and time.
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State Hubble's Law in words and as a formula.
A galaxy's recessional speed (v) is directly proportional to its distance (d) from us. Formula: v = H₀d.
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How does redshift provide evidence for the expanding universe?
Redshift, the observed increase in wavelength of light from distant galaxies, indicates they are moving away from us, directly supporting the idea of an expanding universe.
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What is a 'standard candle' and why is it important for cosmology?
A standard candle is an astronomical object with a known intrinsic luminosity, used to measure distances to remote galaxies, which is crucial for determining Hubble's constant.
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How is recessional velocity linked to redshift?
For galaxies at non-relativistic speeds, recessional velocity (v) is approximately proportional to redshift (Z). The relationship is Z ≈ v/c, where c is the speed of light.
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What does Hubble's constant (H₀) represent?
Hubble's constant (H₀) represents the rate at which the universe is expanding at the present time. Its units are typically km s⁻¹ Mpc⁻¹.
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Explain the 'Doppler effect' in the context of galactic light.
The Doppler effect describes the change in observed wavelength (or frequency) of light due to the relative motion between the source and observer. For receding galaxies, this causes light to redshift.
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How is distance to a standard candle calculated using flux and luminosity?
Distance (d) is calculated using the inverse square law: d = \sqrt{\frac{L}{4\pi F}}, where L is luminosity and F is observed flux.
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What does it mean that there is 'no central point' of expansion in the universe?
Every observer in any galaxy would see other galaxies receding from them, implying the expansion is uniform throughout space, rather than expanding *from* a specific centre.
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What is a 'singularity' in the Big Bang context?
A singularity is an initial, infinitely hot and dense point from which the universe is theorised to have begun its expansion, as described by the Big Bang Theory.
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What is the Cosmic Microwave Background (CMB) Radiation?
The CMB is faint, uniform thermal radiation filling all of space, with a temperature of ~2.7K. It is considered the 'afterglow' or remnant heat from the Big Bang.
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How can the age of the universe be estimated from Hubble's constant (H₀)?
The age of the universe can be estimated as the reciprocal of Hubble's constant (t ≈ 1/H₀). This is known as the Hubble time. H₀ must be converted to SI units (s⁻¹) for the calculation.
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What is the difference between Doppler redshift and cosmological redshift?
Doppler redshift is caused by an object's motion *through* space. Cosmological redshift is caused by the expansion *of* space itself, which stretches the wavelength of light as it travels.