Expanding Universe and Big Bang

The Expanding Universe

Hubble made two important observations:

  • The light from all the distant galaxies is red-shifted.
  • The further away the galaxy the bigger the red-shift.

This means:

  • All the distant galaxies are moving away from us.
  • The further away the galaxy, the faster it is moving away.

We would not see these patterns in the red-shifts just because we, or the galaxies, are moving through space, but it is what we would see if space was expanding.

This is why scientists think we live in an expanding Universe. The Universe is everything that exists. There is nothing outside the Universe – not even empty space.


All of the space in the Universe is expanding, but we do not notice this in the Solar System. This is because gravity is an attractive force that stops mass spreading out when space expands.

Stars and the nearest galaxies may show redshifts or blue-shifts because they are moving through space and these are sometimes called Doppler shifts. Distant galaxies show red-shifts because space is expanding, so these are sometimes called cosmological red-shifts.


The Big Bang

The fact that the Universe is expanding is strong evidence for the fact that it started as a small point. Starting from today and working backwards scientists calculate that the Universe began with a ‘Big Bang’ about 14 thousand million years ago. This is called the Big Bang Theory.

The Big Bang Theory started as a hypothesis – a suggested explanation thought up creatively to account for the data. Scientists then used it to make a prediction. They said that the Big Bang would have produced radiation that, by now, would be found in the microwave region of the spectrum. It would come from all parts of the Universe. Scientists called this the cosmic microwave background radiation (CMBR).

Questions that can’t be answered

Some questions like ‘What happened before the Big Bang?’ scientists may never be able to answer because they can’t collect data from before the Big Bang.

Some questions scientists can’t answer because of difficulties in collecting the data they need. For example, whether the Universe will continue to expand, or whether it will slow down and stop, or whether it will start to contract, depends on the mass of the Universe and the distance to the edge of the Universe. These are very large and difficult to measure, so scientists cannot yet answer the question, ‘What will be the ultimate fate of the Universe?’

The Steady State Theory

The Steady State Theory is an alternative theory that suggests the Universe is expanding, but has always looked the same. It will continue to look the same, because new matter is being created at various places. There were already some problems with data that could not be explained by the Steady State Theory and the discovery of the CMBR led to the general acceptance of the Big Bang Theory.


Scientists search for extra terrestrial intelligence (SETI)

Radio Signals

  • SETI looks for radio wavelength signals being sent to Earth
  • These signals can only be sent by a transmitter
  • SETI has been running for 40 years…
  • not found anything
  • Now searching for laser signals too!

Robots Collect Photos and Samples

  • Spacecraft are sent to planets to look for microorganisms
  • Collect samples and take photos
  • Scientists analyse samples and look for fossils etc.
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