Theory from the Planck Experiment

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The Cosmic Microwave Background (CMB) provides compelling evidence for the hot big-bang model. It is now generally agreed amongst cosmologists that the Universe began around 13 billion years ago in a hot, dense state which we call the big-bang. The Universe then expanded and cooled, in the process producing subatomic particles such as protons, neutrons and electrons. The protons and neutrons combined a few minutes after the big-bang to form light elements. The electrons, however, would take many thousands of years before they would combine. During this time the temperature was high enough so that the nuclei and electrons formed a matter-radiation plasma (with radiation in the form of photons). As the Universe continued to expand and cool there came a time when (around 300,000 years after the big-bang) the photons decoupled from matter. This radiation then cooled further, and today we measure these photons as microwaves with a temperature of around 2.7 K. Cosmologists soon learnt, however, that the temperature of the CMB is not uniform over the entire sky. The first thing they found was that the CMB is hotter in one direction of the sky and cooler in the opposite direction. The amplitude of this effect is around 0.1% of the average temperature, and is due to our motion with respect to the background radiation (from this one can show our solar system is moving at around 370 km/s with respect to the CMB). However, it wasn't until 1992 that the Cosmic Microwave Background Explorer (COBE) satellite made the first detection of temperature anisotropy on smaller scales (the amplitude for which is only around 0.001% of the average temperature). The results from COBE were used to show that the big-bang was immediately proceeded by a short period of rapid expansion called 'inflation'. It is this period of inflation which generated fluctuations in temperature. These fluctuations eventually grew by gravitational collapse into structures such as galaxies which we see today. The second generation CMB satellite WMAP was launched in 2001 and provided detailed full sky maps of the temperature fluctuations. These results were used to place tight limits on the geometry of the Universe, the amount of matter and which models of inflation were compatible with data. The third generation satellite Planck was launched in 2009. Planck will map the sky with much higher sensitivity and angular resolution than WMAP. It will also provide accurate measurements of the polarization of the CMB. The CMB is polarized (at the level of around 0.0001% of the average temperature!) due to the scattering of photons off electrons during the period of decoupling. The polarized signal is extremely weak and difficult to measure. Furthermore, there are two types of polarized signal in the CMB, which cosmologists separate into so-called E and B-modes. The latter is even weaker still, but a detection would be very exciting as this signal directly links to the physics which caused inflation.