Cosmology

What is cosmology?

The study of the origin, evolution, structure and future of the universe.

What is the Doppler effect of light?

The apparent change in wavelength or frequency of light due to relative motion between the source and observer.

What is redshift?

When light is shifted to longer wavelengths because the source is moving away.

What is blueshift?

When light is shifted to shorter wavelengths because the source is moving towards the observer.

What is radial velocity?

The component of velocity along the line joining the source and observer.

Why is radial velocity important?

Only radial velocity causes Doppler shifts.

What is the Doppler shift equation?

What is the redshift equation?

How are spectral lines used to measure redshift?

Compare observed wavelengths with laboratory wavelengths.

The shift gives the redshift.

What is galactic redshift?

The redshift observed in light from distant galaxies.

It indicates galaxies are moving away from Earth.

What is recessional velocity?

The velocity at which a galaxy is moving away from Earth.

What is a binary star system?

Two stars orbiting a common centre of mass.

What is an eclipsing binary?

A binary system where one star periodically passes in front of the other.

Light curve of an eclipsing binary

What does the light curve of an eclipsing binary show?

Regular dips in brightness caused by eclipses.

What is the primary eclipse?

The larger drop in brightness when the brighter star is eclipsed.

What is the secondary eclipse?

The smaller drop in brightness when the dimmer star is eclipsed.

What is a spectroscopic binary?

A binary system detected using alternating redshifts and blueshifts in spectral lines.

What is meant by an expanding universe?

The distance between galaxies increases with time.

What did Hubble discover?

More distant galaxies have greater recessional velocities.

What is Hubble's Law?

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

What is Hubble's Law equation?

v = Hd

What does a graph of velocity against distance show?

A straight line through the origin.

The gradient is the Hubble constant.

What is the Hubble constant?

The rate at which the universe is expanding.

What is the unit of Hubble's constant?

km s⁻¹ Mpc⁻¹

How can the age of the universe be estimated?

1/H

Why is 1/H₀ only an estimate?

The expansion rate of the universe has changed over time.

What is the Big Bang theory?

The universe began in an extremely hot, dense state and has expanded ever since.

What are the three main pieces of evidence for the Big Bang?

• Galactic redshift

• Cosmic microwave background radiation

• Relative abundance of hydrogen and helium

How does redshift support the Big Bang?

Most galaxies are moving away, showing the universe is expanding.

What is the CMBR?

Microwave radiation left over from the early universe.

Temperature ≈ 2.7 K.

How was the CMBR produced?

When electrons combined with nuclei to form atoms, radiation was able to travel freely through space.

Why is the CMBR observed as microwaves today?

Expansion of the universe stretched its wavelength.

Why is the CMBR evidence for the Big Bang?

It is the cooled remnant radiation predicted by the Big Bang theory.

Why is the CMBR black-body radiation?

It has the exact spectrum expected from a hot object that has cooled as the universe expanded.

What is Big Bang nucleosynthesis?

Formation of light nuclei in the first few minutes after the Big Bang.

Which elements were produced in Big Bang nucleosynthesis?

• Hydrogen

• Helium

• Small amounts of lithium

What is the observed abundance of hydrogen and helium?

Approximately:

• 75% hydrogen

• 25% helium

(by mass)

Why does hydrogen and helium abundance support the Big Bang?

Observed abundances closely match theoretical predictions.

What is the cosmological principle?

The universe is homogeneous and isotropic on very large scales.

What is dark energy?

An unknown form of energy believed to cause the accelerated expansion of the universe.

What is accelerating expansion?

The rate at which the universe expands is increasing.

What is the evidence for dark energy?

Observations of distant Type Ia supernovae.

Why is dark energy controversial?

It has never been directly detected.

What is the cosmic distance ladder?

A sequence of methods used to measure increasingly large distances.

What are the stages of the cosmic distance ladder?

• Parallax

• Cepheid variables

• Type Ia supernovae

• Hubble's Law

What is a quasar?

An extremely luminous active galactic nucleus powered by a supermassive black hole.

How do quasars form?

Matter falling into a supermassive black hole releases huge amounts of energy.

Why do quasars have large redshifts?

They are extremely distant and moving away rapidly due to the expansion of the universe.

Why are quasars useful?

They allow astronomers to study the early universe.

What is an exoplanet?

A planet orbiting a star other than the Sun.

What is the transit method?

Detecting a planet when it passes in front of its star and blocks some light.

What does a transit light curve show?

Regular dips in brightness caused by a planet crossing the star.

What does the depth of a transit tell us?

The size (radius) of the planet.

What does the time between transits tell us?

The orbital period of the planet.

What is the radial velocity method?

Detecting planets by observing Doppler shifts in a star's spectrum.

Why does a star wobble?

The star and planet orbit a common centre of mass.

What Doppler shifts are observed in exoplanet detection?

• Blueshift when the star moves towards Earth

• Redshift when the star moves away

What can the radial velocity method determine?

The planet's minimum mass and orbital period.

What is an advantage of the transit method?

It can determine the size of a planet.

What is an advantage of the radial velocity method?

It can determine the planet's mass.

What is a limitation of the transit method?

The orbit must be aligned with Earth.

What is a limitation of the radial velocity method?

Small planets produce very small Doppler shifts.