If a star is receding from Earth, its light is .......... and z is .......... .

If a star is receding from Earth, its light is redshifted and z is positive .

What happens to the observed wavelength and frequency of light from a source moving towards Earth?

Wavelength decreases (blueshift) Frequency increases

True or False? A blueshifted star has a positive value of redshift z .

False. Blueshift occurs when a source approaches, giving a negative value of z ; only a receding source, showing redshift, has a positive z .

Define an eclipsing binary star system .

A system where two stars orbit a common centre of mass with their orbital plane in the Earth's line of sight, so they appear to periodically eclipse one another when observed from Earth.

How can eclipsing binary star systems be identified?

By their characteristic light curves , which show periodic dips in intensity of two different sizes.

What causes the primary (larger) minima in an eclipsing binary light curve?

The cooler star passing in front of the hotter star , blocking more light and producing deeper minima.

The secondary minima are .......... than the primary minima because the .......... star blocks less light when eclipsed.

The secondary minima are shallower than the primary minima because the cooler star blocks less light when eclipsed.

True or False? The primary minima occur when the hotter star passes in front of the cooler star.

False. The primary minima occur when the cooler star passes in front of the hotter star , blocking more light and producing the larger dip.

Define galactic redshift .

The shift of spectral lines from a distant galaxy towards a longer wavelength (the red end of the spectrum), compared to a laboratory or nearby reference source.

What does comparing distant-galaxy spectra to a nearby source, such as the Sun or a laboratory sample, allow astronomers to observe?

Whether the galaxy's spectral lines are shifted towards longer wavelengths ( redshift ), which indicates the galaxy is receding.

How does the balloon analogy explain the expansion of the Universe?

As a balloon inflates, dots on its surface move apart because the rubber between them stretches — the dots themselves do not move. Similarly, space stretches between galaxies; the galaxies themselves do not move through space.

True or False? The greatest redshifts are observed in the galaxies closest to Earth.

False. The greatest redshifts are observed in the most distant galaxies, which are also receding the fastest.

On a graph used to determine the Hubble constant, what is plotted on each axis, and what does the gradient represent?

Recessional velocity, v (km s -1 ), on the y-axis Distance, d (Mpc), on the x-axis The gradient of the graph equals the Hubble constant , H

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

The current best estimate of the Hubble constant, based on Planck satellite CMB observations, is 67.4 ± 0.5 km s -1 Mpc -1 .

What two assumptions are made when using Hubble's law to estimate the age of the Universe?

All points in the Universe were initially together The recessional speed of a galaxy is, and has always been, constant

Define Cosmic Microwave Background Radiation (CMBR) .

Uniform radiation detected in all parts of the Universe, with a black-body spectrum that peaks in the microwave region, corresponding to a temperature of 2.7 K .

What are the three key pieces of evidence for the Big Bang theory?

Galactic redshift and Hubble's law Cosmic Microwave Background Radiation (CMBR) The relative abundance of hydrogen and helium

The predicted relative abundance of hydrogen to helium nuclei in the early Universe is .......... .

The predicted relative abundance of hydrogen to helium nuclei in the early Universe is 3:1 .

How was CMBR produced, according to the Big Bang theory?

It was emitted as high-energy gamma radiation when protons and electrons combined into neutral atoms ('decoupling'). This radiation has since been redshifted into the microwave region as the Universe expanded.

True or False? CMBR is evidence that the early Universe was cool and sparse.

False. CMBR is evidence that the Universe had a hot, dense beginning; its originally short wavelength has been redshifted into the microwave region over billions of years.

What does the relative abundance of hydrogen and helium suggest about the early Universe?

The Universe was once far hotter and denser than it is now, allowing a brief period of hydrogen fusion into helium before it cooled too much for fusion to continue.

A hypothesised mysterious energy source suggested to cause the observed accelerating expansion of the Universe.

What is the 'Hubble tension'?

The discrepancy between different measured values of the Hubble constant, for example 68 km s -1 Mpc -1 from CMB observations versus 73 km s -1 Mpc -1 from type 1a supernovae.

What evidence suggests the expansion of the Universe is accelerating?

Measurements of distant type 1a supernovae give a higher value of the Hubble constant than closer sources, suggesting the rate of expansion is increasing.

Dark energy is considered controversial because it cannot be .......... , and no mechanism currently explains how it drives accelerated expansion.

Dark energy is considered controversial because it cannot be detected directly , and no mechanism currently explains how it drives accelerated expansion.

If dark energy is not the correct explanation for accelerating expansion, what alternatives might need to be reconsidered?

Newton's second law and law of gravitation Einstein's Theory of General Relativity The cosmic distance ladder The Big Bang Theory

True or False? Dark energy and dark matter refer to the same phenomenon.

False. Dark matter relates to rotational motion in galaxies and gravitational lensing of starlight, whereas dark energy is linked to the accelerating expansion of the Universe.

A quasar is an extremely luminous, star-like source of radiation with a very high redshift.

What features led to the discovery of quasars in the 1960s?

Strong radio emissions High luminosities Extremely large redshifts Small size

A quasar is a type of active galactic nucleus — a supermassive .......... surrounded by an accretion disc of matter, found at the centre of extremely distant galaxies.

A quasar is a type of active galactic nucleus — a supermassive black hole surrounded by an accretion disc of matter, found at the centre of extremely distant galaxies.

What happens as matter falls into a quasar's black hole?

Jets of radiation are emitted from the poles, and the gravitational potential energy of the infalling matter is transferred to electromagnetic radiation .

How much matter can fall into a quasar in one year?

The equivalent of 100 solar masses of matter can fall into a quasar each year.

True or False? Quasars are strong emitters of radio waves only.

False. Quasars are now known to be strong emitters of all wavelengths , not just radio waves — radio emission is simply how they were first discovered.

Why are the large redshifts of quasars significant to astronomers?

They show that quasars are among the most distant measurable objects in the universe, allowing astronomers to see far back to an early Universe , not long after the Big Bang.

An exoplanet is a planet found outside our Solar System, in orbit around another star.

Why are exoplanets difficult to detect directly?

Light from the host star is much brighter than the reflected light from the planet Exoplanets subtend extremely small angles compared to the resolution of telescopes

Astronomers use two main indirect detection techniques for exoplanets: the .......... method and the radial velocity method.

Astronomers use two main indirect detection techniques for exoplanets: the transit method and the radial velocity method.

How does the transit method detect an exoplanet?

As the planet passes in front of its star, it obscures some of the star's light, reducing the intensity received at Earth. Measuring this over time produces a light curve .

What can the transit method's light curve reveal about an exoplanet?

The depth of the dip in brightness gives the planet's size The duration of the dip gives the planet's orbital period

What are the limitations of the transit method?

Accuracy is reduced if the Earth, planet and star are not aligned in the same plane Only planets with a short orbital period can be detected

How does the radial velocity method detect an exoplanet?

A planet and its star orbit a common centre of mass . As the star wobbles towards and away from Earth, its line spectrum shows blueshift then redshift , producing periodic shifts in wavelength.

True or False? The radial velocity method can easily detect low-mass, Earth-like exoplanets.

False. Low-mass, Earth-like planets cause less 'wobble' than high-mass planets, since high-mass planets have a greater gravitational pull on the star, making them harder to detect with this method.

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Cosmology (AQA A Level Physics): Flashcards

Exam code: 7408

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FrontThe Doppler Effect of Light
Define redshift, z.

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Cards in this collection (50)

Define redshift, z.
Redshift is the fractional change in wavelength of light from a receding source:
$z = \frac{Δ λ}{λ} = \frac{v}{c}$
It is positive for objects moving away from the observer.
What is the Doppler shift equation relating frequency, wavelength and speed for a non-relativistic (v << c) light source?
$\frac{Δ f}{f} = \frac{Δ λ}{λ} \approx \frac{v}{c}$
In the Doppler shift equation, what assumption is normally made about the velocity of the observer, $v_{o}$ ?
The observer (on Earth) is assumed to be stationary, so $v_{o} = 0$ , meaning the relative speed $Δ v$ becomes simply the speed of the source, v.
If a star is receding from Earth, its light is .......... and z is ...........
If a star is receding from Earth, its light is redshifted and z is positive.
What happens to the observed wavelength and frequency of light from a source moving towards Earth?
- Wavelength decreases (blueshift)
- Frequency increases
True or False?
A blueshifted star has a positive value of redshift z.
False.
Blueshift occurs when a source approaches, giving a negative value of z; only a receding source, showing redshift, has a positive z.
Define Δλ in the Doppler shift equation.
$Δ λ = λ' - λ$
Where λ' is the observed wavelength and λ is the reference (source) wavelength.
Define an eclipsing binary star system.
A system where two stars orbit a common centre of mass with their orbital plane in the Earth's line of sight, so they appear to periodically eclipse one another when observed from Earth.
How can eclipsing binary star systems be identified?
By their characteristic light curves, which show periodic dips in intensity of two different sizes.
What causes the primary (larger) minima in an eclipsing binary light curve?
The cooler star passing in front of the hotter star, blocking more light and producing deeper minima.
The secondary minima are .......... than the primary minima because the .......... star blocks less light when eclipsed.
The secondary minima are shallower than the primary minima because the cooler star blocks less light when eclipsed.
True or False?
The primary minima occur when the hotter star passes in front of the cooler star.
False.
The primary minima occur when the cooler star passes in front of the hotter star, blocking more light and producing the larger dip.
Define galactic redshift.
The shift of spectral lines from a distant galaxy towards a longer wavelength (the red end of the spectrum), compared to a laboratory or nearby reference source.
What does comparing distant-galaxy spectra to a nearby source, such as the Sun or a laboratory sample, allow astronomers to observe?
Whether the galaxy's spectral lines are shifted towards longer wavelengths (redshift), which indicates the galaxy is receding.
How does the balloon analogy explain the expansion of the Universe?
As a balloon inflates, dots on its surface move apart because the rubber between them stretches — the dots themselves do not move. Similarly, space stretches between galaxies; the galaxies themselves do not move through space.
In the Doppler redshift equation, $\frac{Δ f}{f} = \frac{v}{c}$ , a negative value of $Δ f$ indicates the source is .......... the observer.
In the Doppler redshift equation, $\frac{Δ f}{f} = \frac{v}{c}$ , a negative value of $Δ f$ indicates the source is moving away from the observer.
True or False?
The greatest redshifts are observed in the galaxies closest to Earth.
False.
The greatest redshifts are observed in the most distant galaxies, which are also receding the fastest.
Define Hubble's law.
The recessional velocity of a galaxy is proportional to its distance from Earth:
$v = H d$
On a graph used to determine the Hubble constant, what is plotted on each axis, and what does the gradient represent?
- Recessional velocity, v (km s^-1), on the y-axis
- Distance, d (Mpc), on the x-axis
- The gradient of the graph equals the Hubble constant, H
The current best estimate of the Hubble constant, based on Planck satellite CMB observations, is .......... km s^-1 Mpc^-1.
The current best estimate of the Hubble constant, based on Planck satellite CMB observations, is 67.4 ± 0.5 km s^-1 Mpc^-1.
What two assumptions are made when using Hubble's law to estimate the age of the Universe?
- All points in the Universe were initially together
- The recessional speed of a galaxy is, and has always been, constant
Define the age of the Universe in terms of the Hubble constant.
$t = \frac{1}{H}$
The reciprocal of the Hubble constant, giving a current best estimate of around 13.7 billion years.
True or False?
The age of the Universe, calculated from Hubble's law, is equal to t = H.
False.
The age of the Universe is equal to the reciprocal of the Hubble constant, $t = \frac{1}{H}$ .
Define Cosmic Microwave Background Radiation (CMBR).
Uniform radiation detected in all parts of the Universe, with a black-body spectrum that peaks in the microwave region, corresponding to a temperature of 2.7 K.
What are the three key pieces of evidence for the Big Bang theory?
- Galactic redshift and Hubble's law
- Cosmic Microwave Background Radiation (CMBR)
- The relative abundance of hydrogen and helium
The predicted relative abundance of hydrogen to helium nuclei in the early Universe is ...........
The predicted relative abundance of hydrogen to helium nuclei in the early Universe is 3:1.
How was CMBR produced, according to the Big Bang theory?
It was emitted as high-energy gamma radiation when protons and electrons combined into neutral atoms ('decoupling'). This radiation has since been redshifted into the microwave region as the Universe expanded.
True or False?
CMBR is evidence that the early Universe was cool and sparse.
False.
CMBR is evidence that the Universe had a hot, dense beginning; its originally short wavelength has been redshifted into the microwave region over billions of years.
What does the relative abundance of hydrogen and helium suggest about the early Universe?
The Universe was once far hotter and denser than it is now, allowing a brief period of hydrogen fusion into helium before it cooled too much for fusion to continue.
Define dark energy.
A hypothesised mysterious energy source suggested to cause the observed accelerating expansion of the Universe.
What is the 'Hubble tension'?
The discrepancy between different measured values of the Hubble constant, for example 68 km s^-1 Mpc^-1 from CMB observations versus 73 km s^-1 Mpc^-1 from type 1a supernovae.
What evidence suggests the expansion of the Universe is accelerating?
Measurements of distant type 1a supernovae give a higher value of the Hubble constant than closer sources, suggesting the rate of expansion is increasing.
Dark energy is considered controversial because it cannot be .........., and no mechanism currently explains how it drives accelerated expansion.
Dark energy is considered controversial because it cannot be detected directly, and no mechanism currently explains how it drives accelerated expansion.
If dark energy is not the correct explanation for accelerating expansion, what alternatives might need to be reconsidered?
- Newton's second law and law of gravitation
- Einstein's Theory of General Relativity
- The cosmic distance ladder
- The Big Bang Theory
True or False?
Dark energy and dark matter refer to the same phenomenon.
False.
Dark matter relates to rotational motion in galaxies and gravitational lensing of starlight, whereas dark energy is linked to the accelerating expansion of the Universe.
Define quasar.
A quasar is an extremely luminous, star-like source of radiation with a very high redshift.
What features led to the discovery of quasars in the 1960s?
- Strong radio emissions
- High luminosities
- Extremely large redshifts
- Small size
A quasar is a type of active galactic nucleus — a supermassive .......... surrounded by an accretion disc of matter, found at the centre of extremely distant galaxies.
A quasar is a type of active galactic nucleus — a supermassive black hole surrounded by an accretion disc of matter, found at the centre of extremely distant galaxies.
What happens as matter falls into a quasar's black hole?
Jets of radiation are emitted from the poles, and the gravitational potential energy of the infalling matter is transferred to electromagnetic radiation.
How much matter can fall into a quasar in one year?
The equivalent of 100 solar masses of matter can fall into a quasar each year.
True or False?
Quasars are strong emitters of radio waves only.
False.
Quasars are now known to be strong emitters of all wavelengths, not just radio waves — radio emission is simply how they were first discovered.
Why are the large redshifts of quasars significant to astronomers?
They show that quasars are among the most distant measurable objects in the universe, allowing astronomers to see far back to an early Universe, not long after the Big Bang.
Define exoplanet.
An exoplanet is a planet found outside our Solar System, in orbit around another star.
Why are exoplanets difficult to detect directly?
- Light from the host star is much brighter than the reflected light from the planet
- Exoplanets subtend extremely small angles compared to the resolution of telescopes
Astronomers use two main indirect detection techniques for exoplanets: the .......... method and the radial velocity method.
Astronomers use two main indirect detection techniques for exoplanets: the transit method and the radial velocity method.
How does the transit method detect an exoplanet?
As the planet passes in front of its star, it obscures some of the star's light, reducing the intensity received at Earth. Measuring this over time produces a light curve.
What can the transit method's light curve reveal about an exoplanet?
- The depth of the dip in brightness gives the planet's size
- The duration of the dip gives the planet's orbital period
What are the limitations of the transit method?
- Accuracy is reduced if the Earth, planet and star are not aligned in the same plane
- Only planets with a short orbital period can be detected
How does the radial velocity method detect an exoplanet?
A planet and its star orbit a common centre of mass. As the star wobbles towards and away from Earth, its line spectrum shows blueshift then redshift, producing periodic shifts in wavelength.
True or False?
The radial velocity method can easily detect low-mass, Earth-like exoplanets.
False.
Low-mass, Earth-like planets cause less 'wobble' than high-mass planets, since high-mass planets have a greater gravitational pull on the star, making them harder to detect with this method.

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