The expanding universe

• Red-shift is the change in wavelength of light from a distant star moving away from Earth.
• The Big Bang Theory is a theory about the development of the Universe based on astronomical observations and ideas such as red-shift and dark energy.
• Red-shift is observed in virtually all galaxies, it is a result of the space between the Earth and the galaxies expanding, increasing the wavelength of the light from these galaxies, shifting them towards the red end of the spectrum.
• The more red-shifted the light from a galaxy is, the faster the galaxy is moving away from Earth.
• The Big Bang theory describes the early development of the Universe, stating that about 13.8 billion years ago the whole Universe was a very small, extremely hot and dense region.
• From this tiny point, the whole Universe expanded outwards to what exists today.
• Astronomers have discovered that, in general, the further away a galaxy is, the more red-shifted its light is, providing evidence that the Universe, including space itself, is expanding.
• The cosmic microwave background radiation (CMBR), which comes from all directions in space and has a temperature of about -270 °C, is the remains of the thermal energy from the Big Bang, spread thinly across the whole Universe.
• Theories about the development of the Universe, such as the Big Bang theory, are based on astronomical observations and ideas such as red-shift and dark energy.
• More distant galaxies should move away faster according to the Big Bang theory, and this is supported by evidence from red-shift.
• For many years, scientists have tried to work out the density of the Universe.
• The initial Big Bang heat should now be thinly spread across the whole Universe, as predicted by the Big Bang theory, and this is supported by evidence from the CMBR.
• The answer to this would give them an idea of whether the Universe is going to expand forever, or if the gravitational attraction between all objects will eventually slow to a stop, attracting everything back together in a 'Big Crunch'.
• Astronomical observations of supernova have suggested that distant galaxies are moving away increasingly faster.
• The expansion of the Universe appears to be accelerating.
• Scientists do not entirely understand how this could happen, but they have come up with an idea called dark energy.
• The nature of dark energy is still a complete mystery, but it is thought to cause the Universe to expand faster all the time.
• Astronomers have calculated that to make the Universe accelerate as observed, dark energy must account for 68 per cent of the Universe.
• Another recently discovered anomaly is that galaxies seem to rotate too quickly for the mass of their stars.
• This suggests that there is mass in the Universe that is invisible to the instruments used by scientists.
• Scientists only know it is there because it has gravity that affects objects nearby.
• This unknown material has been called dark matter.
• Astronomers have calculated that 27 per cent of the Universe must be made of this dark matter.
• There is much about the Universe that is not understood, for example dark energy and dark matter.
• Only five per cent of the Universe is made of the matter (stars, galaxies, people, planets and so on) that is currently understood.