Observing the Cosmic Microwave Background

1. Penzias and Wilson with their Horn Antenna. Penzias and Wilson were astronomers, from Bell Labs, who built a sensitive antenna to observe microwave wavelength radiation. They found something very strange, however, when they used the telescope: everywhere they looked in the sky, they recorded static. At first, they believed it to be a problem with their telescope. However, after carefully removing and ruling out all possible causes of the noise, they realised that what they had observed was in fact the relic radiation of the early, hot Universe. This was the evidence needed to convince scientists that the Big Bang model – in which the Universe had begun as a compact, superheated fireball before expanding and cooling to become the Universe we see today - was correct. The radiation detected is now known as the Cosmic Microwave Background (CMB).

The two scientists were awarded the Nobel Prize for Physics in 1978 for their discovery of the CMB.

What do observations of the CMB tell us?

• That the age of the Universe is ~13.7 billion years

• That the Universe is flat (angles in a triangle add up to 180°)

• That the Universe is made up of 4% atoms, 23% dark matter and 73% dark energy.

2. Pigeons. One of the possible sources of static that Penzias and Wilson removed was pigeon droppings. A large number of pigeons had taken to roosting in the radio horn and the two scientists thought that the warm droppings could be the source of the static. They went to great pains to catch the pigeons in traps and to remove every last speck of the droppings!

5. Above: The Very Small Array (VSA). This telescope was built by astronomers from Cambridge, Manchester and Tenerife. It observes tiny fluctuations in the CMB.

Left: An image of the CMB made with the VSA. The variations in the signal correspond to temperature fluctuations or less than one 10,000tth of a degree Kelvin in the CMB radiation.

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4. Right: Cosmic Anisotropy Telescope (CAT). The prototype of the Very Small Array (VSA), the CAT proved that useful observations of the CMB could be made from the ground.

Below: An image of the CMB made with the CAT. Compare this to the much higher resolution image made with the VSA.

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6. Left: Planck Surveyor. The European Space Agency is planning to launch the Planck Surveyor in 2007. The spacecraft will observe the CMB radiation with the highest accuracy ever achieved. The spacecraft will orbit at the L2 point, a stable position about 1.5 million kilometres from Earth, where it will be shielded from radiation from the Earth, Moon and Sun.

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3. Left: An image of the CMB made with the Cosmic Background Explorer (COBE). Formed when the Universe was at 3000 K, the CMB radiation has been ‘stretched’ by the expansion of the Universe and now appears in the microwave region of the spectrum, corresponding to a temperature of 2.7 K. The CMB is remarkably uniform. The fluctuations (that appear as blobs of different colours in this image) relate to temperature differences of less than one 10,000th of a degree Kelvin.

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