Cosmology (CIE A Level Physics)

Explain how cosmologists use observations of emission spectra from stars in distant galaxies to determine that the Universe is expanding.

Explain how Hubble’s law and the idea of the expanding Universe lead to the Big Bang theory of the origin of the Universe.

Measurements of type 1a supernovae are commonly used to determine a value for the Hubble constant.

The distance from Earth is known for many type 1a supernovae.

Fig. 1.1 shows the emission spectrum of some wavelengths of light that were observed using a source in a laboratory.

Fig. 1.1

The same wavelengths of light were observed on an emission spectrum from a distant galaxy but were found to be redshifted.

An astronomer observes that light from a galaxy has been shifted towards the blue end of an emission spectrum.

Hydrogen is an element of particular interest to cosmologists, as it can emit visible light waves.

A lot of useful information can be obtained by measuring the wavelength of the emitted light from the hydrogen in distant galaxies and comparing them to a laboratory sample. Some sample data is shown in Table 1.2.

Table 1.2

Compare the motions of galaxy 1a and galaxy 1b relative to Earth.

Use data from Table 1.2 to calculate the speed of galaxy 1b relative to Earth.

An astronomer analyses the light from a distant galaxy.

One of the observed spectral lines of hydrogen has a wavelength of 722 nm. The same spectral line has a wavelength of 656 nm when measured in the laboratory.

Explain how comparing the values of cosmological redshift for distant galaxies provides evidence for the Big Bang theory.

Cosmologists commonly quote the recessional velocity v of astronomical objects in relation to the speed of light c. This ratio is known as redshift z:

The Andromeda galaxy is the closest galaxy to our own Milky Way. Andromeda has a redshift value of z = −0.001.

Table 1.1 contains information about two galaxies.

Table 1.1

Fig. 1.1a shows the variation of recession velocity v for a number of galaxies with their approximate distance d from Earth.

Fig. 1.1a

Radiation from a star in a distant galaxy is analysed.

Fig. 1.1b shows the spectral lines of the radiation from the star at different wavelengths compared to the same spectral lines produced by a source in the laboratory.

Fig 1.1b

Using the information in Fig. 1.1a and 1.1b, determine the distance of this galaxy from Earth.

Astronomers use the cosmic scale factor R to describe the scaling up of physical distances in the Universe.

As the universe expands, the wavelength of the detected radiation at a time t is larger than it was at a time t₀ in the past by a factor of

This expression represents the expansion rate of the universe.

Where R represents the separation of two galaxies at time t, R₀ represents the value of R at the time t₀ the radiation was emitted, and z represents the redshift, which is given by

Historically, there has been considerable debate over the true value of the Hubble constant.

Fig. 1.2 shows Hubble's original plot of velocity against distance for 46 galaxies.

Fig. 1.2

Two stars, A and B, in a binary system move in an anti-clockwise direction around a common centre of mass, as shown in Fig. 1.1.

Fig. 1.1

On Fig. 1.2a and Fig. 1.2b below, mark the positions of stars A and B corresponding to their spectra as observed on Earth.

Fig. 1.2a                                                      Fig. 1.2b          

When measured in the laboratory, one of the spectral lines of hydrogen has a wavelength of 4.861 × 10⁻⁷ m. 

The same hydrogen line in the spectrum of one of the stars fluctuates from its laboratory wavelength by ± 0.420 × 10⁻⁷ m and the line in the spectrum of the other star fluctuates by ± 0.865 × 10⁻⁷ m. 

Calculate the velocity of each star in the binary system and identify the faster star and the slower star.

The orbital period of the binary system about the common centre of mass is 21 days.

Show that the orbital radius of star B is about twice the orbital radius of star A.

Newton assumed that the universe was infinite, uniform and static. The Big Bang model suggests space and time originated at one point around 14 billion years ago. At this time the temperature was very high.

In 1965, Penzias and Wilson discovered cosmic microwave background (CMB) radiation with a wavelength of around 1 mm.

Determine the current temperature of the CMB.

Surface temperature of the Sun = 5800 K

Peak wavelength emission of the Sun = 500 nm

Fig. 1.1 shows the map of microwave background radiation from the COBE satellite. It shows the variation in the mean temperature of the CMB in different parts of the sky.

Fig. 1.1

This radiation was first emitted around 300 000 years after the Big Bang, during a stage of the universe's history called the 'era of recombination'.

At this stage, the radiation was in the visible region of the spectrum and had a temperature of around 3000 K.

Determine the factor by which the universe has expanded since the universe was at a temperature of 3000 K.

Using Fig. 1.1, your answers to (a) and (b), and your knowledge of redshift and cosmology