10.2 Red-shift and the Big Bang — Practise with help

1. Red light has the longer wavelength.
2. $v = f\lambda = 5.0 \times 10^{14} \times 6.0 \times 10^{-7} = 3.0 \times 10^{8}\,\text{m/s}$.

If question 1 went wrong, revisit the electromagnetic spectrum in Chapter 5. If question 2 went wrong, revisit the wave equation in Chapter 4.

Compare the light from the galaxy with the same light measured in a laboratory.

Say which way the wavelength changes, then say which part of the spectrum the dark lines move towards.

The wavelength of the light from the galaxy is longer than the wavelength of the same light measured in a laboratory. The dark lines in the spectrum of the galaxy are moved towards the red end, where the wavelengths are longer. This happens because the galaxy is moving away from the Earth.

Compare the two wavelengths. 550 nm is longer than 500 nm.

Say that the wavelength has increased, so the light has been red-shifted. Then link the red-shift to the direction of motion.

The wavelength in the galaxy light is 550 nm, which is longer than the laboratory wavelength of 500 nm. The light is red-shifted. Red-shift is caused by a source moving away, so the galaxy is moving away from the Earth.

Start with the motion of one galaxy. Then add the pattern that appears when you compare many galaxies at different distances.

Say that the galaxies are moving away from us, and that the further away a galaxy is, the faster it moves away. Then say what that pattern means for space itself.

The red-shift shows that most galaxies are moving away from us. The further away a galaxy is, the faster it is moving away. This pattern is what we expect if space itself is expanding. As space grows, it carries the galaxies apart, so the universe is expanding.

State what the model says about the start of the universe.

Say what the model predicts about the universe since it began, then link the prediction to what the red-shift shows.

The Big Bang model says that the universe began from a very small region that was extremely hot and dense. The model predicts that the universe has been expanding ever since. Red-shift shows that the galaxies are moving apart, so the universe is expanding. This matches the prediction, so red-shift supports the model.

It is a signal, not a substance. Say where in space it comes from and which part of the electromagnetic spectrum it belongs to.

Say that it comes from every direction. Then say what it is left over from, and which model that supports.

The cosmic microwave background radiation is a faint microwave signal that comes from every direction in space. It is left over from the hot, dense early universe. This matches a prediction of the Big Bang model, so it is evidence that supports the model.

Use the same equation as for any other journey: time is distance divided by speed.

Work out the time in seconds first. Then divide that time by the number of seconds in a year.

The light has taken 2.5 million years to reach the Earth.

Think about the journey the light has made, and how long that journey took.

Say when the light that we see now set out from the galaxy. Then say which version of the galaxy we see.

Light takes a very long time to travel from a very distant galaxy, because the distance is enormous. The light that we see now set out from the galaxy billions of years ago. So we see the galaxy as it was then, not as it is now.

Both models can explain red-shift. Look for the evidence that only one of them can explain.

Say what both models explain, then say which piece of evidence the Steady State model does not explain.

Both the Steady State model and the Big Bang model can explain the red-shift of the galaxies. The Steady State model does not explain the cosmic microwave background radiation. That radiation fits a hot, dense beginning, so more evidence supports the Big Bang model. It is the accepted model today.

Think about whether the cause of the speeding up is known.

Describe what the supernova observations show, then name the two things that are not understood.

The distant supernovae are further away than expected, so the expansion of the universe seems to be getting faster. The cause of the speeding up is not known. It is called dark energy. Dark matter is also not understood, so there is still much about the universe that is not known.