Define astronomical unit (AU) .

The mean distance from the centre of the Earth to the centre of the Sun (1 AU ≈ 1.5 × 10 11 m).

The distance travelled by light in one year (1 ly ≈ 9.5 × 10 15 m).

A unit of distance that gives a parallax angle of 1 arcsecond , using the radius of the Earth's orbit (1 AU) as the baseline of a right-angled triangle (1 pc ≈ 3.1 × 10 16 m).

Why are the parsec and light-year more useful than the astronomical unit for measuring interstellar distances?

They are much greater in size than the AU, making them better suited to the huge distances between stars or neighbouring galaxies, whereas the AU is suited to distances on the scale of the solar system .

1 degree is equal to .......... arcseconds.

1 degree is equal to 3600 arcseconds.

True or False? The astronomical unit (AU) is larger than the parsec.

False. The parsec (≈ 3.1 × 10 16 m) is much larger than the astronomical unit (≈ 1.5 × 10 11 m).

Define the parallax principle .

The principle that the apparent position of an object changes depending on the position from which it is observed (e.g. viewing the meniscus in a measuring cylinder from different angles).

Define stellar parallax .

The apparent shifting in position of a nearby star against a background of distant stars, when viewed from different positions of the Earth during its orbit about the Sun.

Why does the background of distant stars appear fixed when measuring stellar parallax, while the nearby star's position appears to shift?

The distant stars are so far away that their position does not observably change with time, so only the apparent movement of the nearby star is seen.

The parallax equation p = 1/ d is only accurate for measuring distances up to .......... pc.

The parallax equation p = 1/ d is only accurate for measuring distances up to 100 pc.

True or False? Stellar parallax can be used to accurately measure the distance to stars more than 100 pc away.

False. Beyond 100 pc , the parallax angles involved are so small that they are hard to measure accurately .

Define the cosmological principle .

The universe is isotropic , homogeneous , and the laws of physics are universal.

Define isotropic (in cosmology).

The universe is the same in all directions to every observer.

Define homogeneous (in cosmology).

Matter is uniformly distributed throughout the universe, giving it a uniform density overall.

Of the three model universes (isotropic-only, homogeneous-only, and both), which one demonstrates the cosmological principle , and why?

The model that is both isotropic and homogeneous : the universe is the same in all directions to every observer and has a uniform distribution of matter throughout.

A model universe that is isotropic but has matter clumped unevenly in different regions is not described as .......... .

A model universe that is isotropic but has matter clumped unevenly in different regions is not described as homogeneous .

True or False? The cosmological principle states that the laws of physics are different in different parts of the universe.

False. The cosmological principle states that the laws of physics are universal — the same laws and models apply everywhere in the universe as they do on Earth.

Define the Doppler effect .

The apparent shift in wavelength occurring when the source of the waves is moving .

What happens to the wavelength of waves in front of a moving source (i.e. the side the source is moving towards)?

The wavelength decreases (waves become squashed together) and the frequency increases .

What happens to the wavelength of waves behind a moving source (i.e. the side the source is moving away from)?

The wavelength increases (waves become stretched out) and the frequency decreases .

Redshift observed in the light from distant galaxies provides evidence that the .......... is expanding.

Redshift observed in the light from distant galaxies provides evidence that the universe is expanding.

If the observer is assumed to be stationary ( v o = 0), what does the relative speed Δ v between source and observer simplify to?

Δ v = v s , the velocity of the source relative to the observer.

True or False? The Doppler shift equation Δ λ / λ ≈ v / c is valid for any relative speed between source and observer, however large.

False. The equation only holds when the relative speed , Δ v , is small compared with the wave speed, c .

Define Hubble's law .

The recessional velocity , v , of a galaxy is proportional to its distance , d , from Earth.

What did Hubble observe about the light spectra of more distant galaxies compared to closer ones, and what did he conclude from it?

Light from more distant galaxies was shifted further towards the red end of the spectrum, so he concluded that more distant galaxies are moving away faster than closer ones.

The current best estimate of the Hubble constant , based on Planck satellite data, is .......... km s -1 Mpc -1 .

The current best estimate of the Hubble constant , based on Planck satellite data, is 67.4 km s -1 Mpc -1 .

How can the Hubble constant be found graphically from a plot of recessional velocity, v , against distance, d ?

H 0 is equal to the gradient of the v - d graph.

True or False? The value of the Hubble constant is known with complete certainty.

False. It is difficult to be certain of its exact value due to random and systematic errors involved in measuring the distance to a galaxy or star.

Observed when spectral lines from a distant galaxy shift towards the red end of the spectrum, because the light waves are stretched by the expansion of the universe (wavelength increases), showing the galaxy is receding .

What relationship exists between a galaxy's distance from Earth and the amount of redshift observed in its spectrum?

The greater the distance to a galaxy, the greater the redshift, meaning the galaxy is receding faster .

What three conclusions did astronomers draw from Hubble's galactic redshift data?

All galaxies are moving away from the Earth; galaxies are moving away from each other ; more distant galaxies are receding faster than closer ones.

Tracing the expansion of the universe back in time suggests it began from a single point in an event known as the .......... .

Tracing the expansion of the universe back in time suggests it began from a single point in an event known as the Big Bang .

Using the explosion analogy for the expanding universe, why do the fastest-moving fragments of matter end up furthest from the origin?

Lighter matter travels at a greater speed and moves further from the source of the explosion, while heavier matter travels slower and stays closer.

True or False? Galaxies closer to Earth are receding faster than more distant galaxies.

False. It is the opposite — the further away a galaxy is, the faster it is receding from Earth.

Define hot singularity .

A hot singularity is a single point that is infinitely dense , hot and small, from which the universe began at the Big Bang .

What are the two main pieces of evidence supporting the Big Bang theory?

Hubble's law — redshift of light from distant galaxies shows the universe is expanding Microwave background radiation — shows the universe expanded from a single point and has cooled significantly

Why do all galaxies appear to be moving away from each other?

Space itself is expanding (space–time) and carries the galaxies along with it — the galaxies are not moving through space themselves.

Microwave background radiation has a uniform temperature of .......... K.

Microwave background radiation has a uniform temperature of 2.73 K.

True or False? Galaxies move through space to create the expansion of the universe.

False. Space itself expands, carrying galaxies along with it — the galaxies are not moving through space.

Define cosmological principle .

The cosmological principle states that the universe is homogeneous (matter is uniformly distributed) and isotropic (the same in all directions to every observer).

Why was microwave background radiation not detected until 1964?

Microwaves are absorbed by the atmosphere , so detection required telescopes in orbit above it, made possible by the development of space flight.

Why has the wavelength of the microwave background radiation increased from gamma rays to microwaves since the Big Bang?

As the universe has expanded over 13.7 billion years , the wavelength of the original high-energy radiation has been stretched (redshifted) into the microwave region of the spectrum.

What assumption is made when using Hubble's law to estimate the age of the universe?

That the recessional speed of each galaxy has been constant throughout the history of the universe, and that all matter started at the same point.

Using Hubble's law, astronomers estimate that the universe has been expanding for around .......... billion years.

Using Hubble's law, astronomers estimate that the universe has been expanding for around 13.7 billion years.

What happens to the temperature of the universe as time since the Big Bang increases?

The temperature decreases as the universe expands and cools.

During .......... , the universe expanded rapidly, between 10 -43 s and 10 -35 s after the Big Bang.

During inflation , the universe expanded rapidly, between 10 -43 s and 10 -35 s after the Big Bang.

Decoupling is the point at which electrons combine with nuclei to form atoms, allowing radiation and matter to separate so that photons can travel freely through space.

Why did the early universe become matter-dominated rather than containing equal matter and antimatter?

There was slightly more matter than antimatter in the early universe; as matter and antimatter annihilated , only the small excess of matter remained.

What must happen to the universe before the first hadrons (protons and neutrons) can form from quarks?

The universe must cool sufficiently — before this, temperatures were too high for quarks to combine into hadrons.

True or False? Atoms formed before nuclei in the early universe.

False. Nuclei formed first, during nucleosynthesis (10 2 s to 10 5 s), followed by atoms once electrons combined with nuclei (10 5 s to 10 8 s).

A LevelPhysicsOCRFlashcards5. Newtonian World & AstrophysicsCosmology

Cosmology (OCR A Level Physics): Flashcards

Exam code: H556

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Define astronomical unit (AU).

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Cards in this collection (67)

Define astronomical unit (AU).
The mean distance from the centre of the Earth to the centre of the Sun (1 AU ≈ 1.5 × 10¹¹ m).
Define light-year.
The distance travelled by light in one year (1 ly ≈ 9.5 × 10¹⁵ m).
Define parsec.
A unit of distance that gives a parallax angle of 1 arcsecond, using the radius of the Earth's orbit (1 AU) as the baseline of a right-angled triangle (1 pc ≈ 3.1 × 10¹⁶ m).
Why are the parsec and light-year more useful than the astronomical unit for measuring interstellar distances?
They are much greater in size than the AU, making them better suited to the huge distances between stars or neighbouring galaxies, whereas the AU is suited to distances on the scale of the solar system.
1 degree is equal to .......... arcseconds.
1 degree is equal to 3600 arcseconds.
True or False?
The astronomical unit (AU) is larger than the parsec.
False.
The parsec (≈ 3.1 × 10¹⁶ m) is much larger than the astronomical unit (≈ 1.5 × 10¹¹ m).
Define the parallax principle.
The principle that the apparent position of an object changes depending on the position from which it is observed (e.g. viewing the meniscus in a measuring cylinder from different angles).
Define stellar parallax.
The apparent shifting in position of a nearby star against a background of distant stars, when viewed from different positions of the Earth during its orbit about the Sun.
Why does the background of distant stars appear fixed when measuring stellar parallax, while the nearby star's position appears to shift?
The distant stars are so far away that their position does not observably change with time, so only the apparent movement of the nearby star is seen.
State the parallax equation linking the parallax angle, p (in arcseconds), to the distance to a star, d (in parsecs).
$p = \frac{1}{d}$
The parallax equation p = 1/d is only accurate for measuring distances up to .......... pc.
The parallax equation p = 1/d is only accurate for measuring distances up to 100 pc.
True or False?
Stellar parallax can be used to accurately measure the distance to stars more than 100 pc away.
False.
Beyond 100 pc, the parallax angles involved are so small that they are hard to measure accurately.
Define the cosmological principle.
The universe is isotropic, homogeneous, and the laws of physics are universal.
Define isotropic (in cosmology).
The universe is the same in all directions to every observer.
Define homogeneous (in cosmology).
Matter is uniformly distributed throughout the universe, giving it a uniform density overall.
Of the three model universes (isotropic-only, homogeneous-only, and both), which one demonstrates the cosmological principle, and why?
The model that is both isotropic and homogeneous: the universe is the same in all directions to every observer and has a uniform distribution of matter throughout.
A model universe that is isotropic but has matter clumped unevenly in different regions is not described as ...........
A model universe that is isotropic but has matter clumped unevenly in different regions is not described as homogeneous.
True or False?
The cosmological principle states that the laws of physics are different in different parts of the universe.
False.
The cosmological principle states that the laws of physics are universal — the same laws and models apply everywhere in the universe as they do on Earth.
Define the Doppler effect.
The apparent shift in wavelength occurring when the source of the waves is moving.
What happens to the wavelength of waves in front of a moving source (i.e. the side the source is moving towards)?
The wavelength decreases (waves become squashed together) and the frequency increases.
What happens to the wavelength of waves behind a moving source (i.e. the side the source is moving away from)?
The wavelength increases (waves become stretched out) and the frequency decreases.
Redshift observed in the light from distant galaxies provides evidence that the .......... is expanding.
Redshift observed in the light from distant galaxies provides evidence that the universe is expanding.
State the Doppler shift equation relating the fractional change in wavelength, Δλ/λ, the fractional change in frequency, Δf/f, and the relative speed of source and observer, v, compared to the wave speed, c.
$\frac{Δ λ}{λ} \approx \frac{Δ f}{f} \approx \frac{v}{c}$
If the observer is assumed to be stationary (v_o = 0), what does the relative speed Δv between source and observer simplify to?
Δv = v_s, the velocity of the source relative to the observer.
True or False?
The Doppler shift equation Δλ/λ ≈ v/c is valid for any relative speed between source and observer, however large.
False.
The equation only holds when the relative speed, Δv, is small compared with the wave speed, c.
Define Hubble's law.
The recessional velocity, v, of a galaxy is proportional to its distance, d, from Earth.
State the equation for Hubble's law.
$v \approx H_{0} d$
What did Hubble observe about the light spectra of more distant galaxies compared to closer ones, and what did he conclude from it?
Light from more distant galaxies was shifted further towards the red end of the spectrum, so he concluded that more distant galaxies are moving away faster than closer ones.
The current best estimate of the Hubble constant, based on Planck satellite data, is .......... km s^-1 Mpc^-1.
The current best estimate of the Hubble constant, based on Planck satellite data, is 67.4 km s^-1 Mpc^-1.
How can the Hubble constant be found graphically from a plot of recessional velocity, v, against distance, d?
H₀ is equal to the gradient of the v-d graph.
State the unit of the Hubble constant when recessional velocity is measured in km s^-1 and distance in km.
s^-1
True or False?
The value of the Hubble constant is known with complete certainty.
False.
It is difficult to be certain of its exact value due to random and systematic errors involved in measuring the distance to a galaxy or star.
Define redshift.
Observed when spectral lines from a distant galaxy shift towards the red end of the spectrum, because the light waves are stretched by the expansion of the universe (wavelength increases), showing the galaxy is receding.
What relationship exists between a galaxy's distance from Earth and the amount of redshift observed in its spectrum?
The greater the distance to a galaxy, the greater the redshift, meaning the galaxy is receding faster.
What three conclusions did astronomers draw from Hubble's galactic redshift data?
All galaxies are moving away from the Earth; galaxies are moving away from each other; more distant galaxies are receding faster than closer ones.
Tracing the expansion of the universe back in time suggests it began from a single point in an event known as the ...........
Tracing the expansion of the universe back in time suggests it began from a single point in an event known as the Big Bang.
Using the explosion analogy for the expanding universe, why do the fastest-moving fragments of matter end up furthest from the origin?
Lighter matter travels at a greater speed and moves further from the source of the explosion, while heavier matter travels slower and stays closer.
True or False?
Galaxies closer to Earth are receding faster than more distant galaxies.
False.
It is the opposite — the further away a galaxy is, the faster it is receding from Earth.
Define hot singularity.
A hot singularity is a single point that is infinitely dense, hot and small, from which the universe began at the Big Bang.
What are the two main pieces of evidence supporting the Big Bang theory?
- Hubble's law — redshift of light from distant galaxies shows the universe is expanding
- Microwave background radiation — shows the universe expanded from a single point and has cooled significantly
Why do all galaxies appear to be moving away from each other?
Space itself is expanding (space–time) and carries the galaxies along with it — the galaxies are not moving through space themselves.
Microwave background radiation has a uniform temperature of .......... K.
Microwave background radiation has a uniform temperature of 2.73 K.
True or False?
Galaxies move through space to create the expansion of the universe.
False.
Space itself expands, carrying galaxies along with it — the galaxies are not moving through space.
Define cosmological principle.
The cosmological principle states that the universe is homogeneous (matter is uniformly distributed) and isotropic (the same in all directions to every observer).
Why was microwave background radiation not detected until 1964?
Microwaves are absorbed by the atmosphere, so detection required telescopes in orbit above it, made possible by the development of space flight.
Why has the wavelength of the microwave background radiation increased from gamma rays to microwaves since the Big Bang?
As the universe has expanded over 13.7 billion years, the wavelength of the original high-energy radiation has been stretched (redshifted) into the microwave region of the spectrum.
Define Hubble's law.
Hubble's law states that the recessional speed v of a galaxy is directly proportional to its distance d from Earth: $v = H_{0} d$ , where $H_{0}$ is the Hubble constant.
What assumption is made when using Hubble's law to estimate the age of the universe?
That the recessional speed of each galaxy has been constant throughout the history of the universe, and that all matter started at the same point.
How is the age of the universe calculated from the Hubble constant?
$t = \frac{1}{H_{0}}$ , assuming all matter began at the same point at t = 0.
Using Hubble's law, astronomers estimate that the universe has been expanding for around .......... billion years.
Using Hubble's law, astronomers estimate that the universe has been expanding for around 13.7 billion years.
True or False?
A larger value of the Hubble constant gives a larger estimated age of the universe.
False.
Since $t = \frac{1}{H_{0}}$ , the age is inversely proportional to $H_{0}$ — a larger Hubble constant gives a smaller estimated age.
In the worked calculation for the age of the universe, why must H₀ be converted from km s^-1 Mpc^-1 to s^-1?
So that the units of distance and speed cancel correctly, leaving $H_{0}$ in s^-1, which gives the age t directly in seconds.
What happens to the temperature of the universe as time since the Big Bang increases?
The temperature decreases as the universe expands and cools.
During .........., the universe expanded rapidly, between 10^-43 s and 10^-35 s after the Big Bang.
During inflation, the universe expanded rapidly, between 10^-43 s and 10^-35 s after the Big Bang.
Define decoupling.
Decoupling is the point at which electrons combine with nuclei to form atoms, allowing radiation and matter to separate so that photons can travel freely through space.
Why did the early universe become matter-dominated rather than containing equal matter and antimatter?
There was slightly more matter than antimatter in the early universe; as matter and antimatter annihilated, only the small excess of matter remained.
What must happen to the universe before the first hadrons (protons and neutrons) can form from quarks?
The universe must cool sufficiently — before this, temperatures were too high for quarks to combine into hadrons.
True or False?
Atoms formed before nuclei in the early universe.
False.
Nuclei formed first, during nucleosynthesis (10² s to 10⁵ s), followed by atoms once electrons combined with nuclei (10⁵ s to 10⁸ s).
What is the final temperature of the universe today, according to the Big Bang model?
2.7 K
Roughly how long after the Big Bang did the first stars form?
About 30 million years after the Big Bang.
Define dark energy.
Dark energy is a type of energy that permeates the whole universe and opposes the attractive gravitational force between galaxies by exerting a negative pressure.
Define dark matter.
Dark matter is matter that cannot be seen and does not emit or absorb electromagnetic radiation.
Why do astronomers believe dark matter exists in galaxies?
In galaxies, orbital velocities do not decrease with distance from the centre as expected from weakening gravity (unlike in the solar system), yet observable mass is concentrated in the centre — this mismatch implies unseen mass (dark matter) must be spread throughout the galaxy.
How is dark matter detected, given that it cannot be observed directly?
Through its gravitational effects — on the rotation of galaxies and via the gravitational lensing of starlight.
Dark energy is thought to make up approximately .......... % of the total energy in the universe.
Dark energy is thought to make up approximately 68 % of the total energy in the universe.
What proportion of the universe's mass is estimated to be dark matter?
Approximately 27%
True or False?
Dark matter can be detected directly using telescopes.
False.
Dark matter does not emit or absorb electromagnetic radiation, so it cannot be detected directly — only inferred from its gravitational effects.

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