P1bL15  The Expanding Universe

Key Words

Big Crunch - when all matter comes back to one point.

Dark Matter - material in the Universe that cannot be seen.

Doppler Effect - the change in wavelength of light from objects moving relative to the Earth.

Red shift - light appears to be redder than what it should be, due to objects moving away from us.

Spectral pattern - a distinctive pattern of colours given out by different elements when very hot.

Grade E

Astronomers can tell that galaxies are moving away from us, because their light is redder than it should be.  This means that the wavelength of the light is longer.  It is called red shift.

This is due to the Doppler effect.  The horn of an approaching train is of higher pitch as it comes towards us.  As it passes, the note is of the correct pitch.  Then as it goes away, the pitch seems to get lower.  This is because the wavelength changes - shorter, normal, then longer.

The astronomer, Edwin Hubble, observed that the further the galaxies were away from us, the greater the red shift.  This meant that the further the galaxies were away, the faster they were travelling.  Therefore the Universe is expanding.

So it seems that they all originated in one point, the Big Bang.

Grade C

When elements get hot, they emit a pattern of light with coloured lines.  This is called a spectral pattern, or a spectrum.  Each spectrum is unique for that element.

Hubble noticed that, when he analysed spectra from distant galaxies, he could see the distinctive patterns of elements, but they were not in the right place.  They had been shifted towards light of a longer wavelength.

In this picture, you can see that the pattern of lines is the same, but look at the blue line on top.  The same line has now become green, showing how the wavelength has increased, because of the Doppler effect.

Grade A

The fate of the Universe is of interest to scientists.  There are three possibilities:

1. The force of gravity may overcome the expansion, which will slow down.  Then it will stop, and all the galaxies will come together again.  This is the opposite to the Big Bang, and is called the Big Crunch.  It's like a cosmic bungee-jump.
2. The force of gravity is not strong enough to overcome the expansion, which will continue.  The Universe will be a cold, lonely, and bleak place.
3. The force of gravity and the expansion will balance out, leaving the Universe in a stable state.

It all depends on how much material there is in the Universe.  We can see a lot of it, but there is thought to be even more that we can't see.  It is called dark matter.  Until scientists know exactly how much there is, which one of the three will be the fate cannot be known for sure.

What is certain is that it will happen eons after we are all long dead, gone, and forgotten.