P1.5.4 Red-shift

AQA GCSE Science A

There are two main pieces of evidence for the Big Bang:

1. The expansion of the universe

2. Cosmic microwave background radiation

There are two main pieces of evidence for the Big Bang

1. The expansion of the universe

It seems that our universe is expanding.

The evidence is based on what is called red shift. This red shift is the stretching of light waves from a source that is moving away from us. We can hear a similar effect by listening to a car engine. As it comes towards you it has a higher frequency than when it is moving away. This is known as the Doppler effect.

When seen through a spectroscope most stars and galaxies show this red shift effect, indicating that they are all moving away from us.

In 1929 an American astronomer Edwin Hubble discovered a relationship between the size of the red-shift and the distance of the galaxies.

If we know how fast a galaxy is travelling and how far away it is from us, then we can work back and find out how long it took for the universe to expand from a small hot lump. It is as simple as

time = velocity/ distance!

The Hubble Constant H = Speed of recession/ Distance from us

The Hubble constant thus enables us to calculate the age of the earth. Most estimates give an answer of around 15 thousand million years.

There are two main pieces of evidence for the Big Bang:

1. The expansion of the universe

2. Cosmic microwave background radiation (CMBR)

2. Background radiation (CMBR) If the Big Bang model is correct then we would expect that when the small hot solid exploded there was a tremendous amount of heat energy generated. Over time the temperature of the universe would have cooled down.

1948 George Gamov predicted that if the Big Bang model was correct then there should be radiation in the universe coming from all directions at around - 263 °C.

1965 Penzias and Wilson, were using a radio telescope to detect signals from space. They kept getting an annoying hiss which was present wherever they pointed their telescope.

This annoying hiss was the remains of the Big Bang fireball.

More recently this background radiation has been studied in more detail. Scientists now think that the radiation is not quite as smooth as was originally thought. This evidence fitted in even better with the Big Bang model. The discovery of these variations in the temperature background help explain how the universe is "clumpy" (that is made up of planets, stars etc ) and not all the same throughout.

Problems with the Big Bang model

Not everyone agrees with the Big Bang model. Some from religious reasons others from scientific reasons and some from a mixture of both!

1. Redshift can be explained in other waysAs light leaves a star it is possible that the star's gravity may lengthen the wavelength. A light source moving at right-angles to an observer will also appear red-shifted; this could mean that the universe is in circular motion instead of expansion.

2. The Hubble constant is always not accurateSome estimates of the Hubble constant have meant that the universe is younger than the age of some of the star clusters in it!

3. Background radiationThe results of variations in the background radiation may be a result of gravitational waves or even noise from the instruments, rather than evidence of the clumping of matter in the early universe.

4. There is no single theory of how galaxies are formed. It is not known whether large clusters formed first and then broke up into galaxies, or galaxies were formed first and then collected into clusters.

Problems with the Big Bang model (contd)

To ponder?

Does the following equation work:

Matter + time + chance + energy = life, the universe and everything?

P1.5.4 Red-shift

b) There is an observed increase in the wavelength of light from most distant galaxies. The further away the galaxies are, the faster they are moving, and the bigger the observed increase in wavelength. Thiseffect is called red-shift.

a) If a wave source is moving relative to an observer there will be a change in the observed wavelength and frequency. This is known as the Doppler effect.

e) The ‘Big Bang’ theory is currently the only theory that can explain the existence of CMBR.

d) Cosmic microwave background radiation (CMBR) is a form of electromagnetic radiation filling the universe. It comes from radiation that was present shortly after the beginning of the universe.

c) How the observed red-shift provides evidence that the universe is expanding and supports the ‘Big Bang’ theory (that the universe began from a very small initial point).

The visible part of the electromagnetic spectrum from a star includes a dark line. This line is at a specific wavelength.The diagram shows the position of the dark line in the spectrum from the Sun and in the spectrum from a distant galaxy.

Explain how the spectrum .shift. of the dark line supports the theory that the Universe began from a very small initial point.

Name one other piece of evidence that supports the theory that the Universe began from a very small initial point.