What Went Before?
The Cosmic Microwave Background (CMB) is the furthest we can see back directly. However, we can infer what may have happened before by examining the CMB and other evidence in greater detail.
The currently favoured theory of the early Universe involves a concept called "inflation". This describes a very brief, but very important moment in the first tiny fraction of the Universe's existence. Theories of inflation describe a hugely rapid exponential expansion of the Universe by a factor of around 1050 (that's a 100 million million million million million million million million times!) within the first 10-30 seconds (that's a hundred million million million million millionth of a second!). These number are not precisely known at all, and could be wrong by factors of millions. Inflation does not describe a single theory, but a whole collection of them, with many variables having many values. Only gathering further data will allow us to work out which one theory (if any!) is correct. Inflation has solved various conundrums which existed with previous theories, and the theories can be twisted and morphed to explain the vast majority of observations to date. However, we are still not much closer to narrowing down on which of the many theories is correct.
The image below shows the expansion of the Universe as currently understood, which is supported by existing measurements such as those from the WMAP mission.
Image credit: Rhys Taylor, Cardiff University
A good scientific theory should make testable predictions which can be proved by experiment. We obviously cannot recreate Inflation in the lab, but can only look at what it left behind. There are various subtle imprints on the CMB which are predicted to be present by many theories of Inflation, but which have not been detected yet. This does not mean that they are not there, but simply that our measurements are not yet sensitive enough. One such imprint is the polarisation of the CMB radiation, which describes which way the electromagnetic wave is oscillating. The polarised imprint on the CMB from Inflation is extremely small, predicted to be seen as temperature fluctuations of less than a millionth of a degree. Measurement of this faint imprint would enable us to isolate the valid Inflation theories. Planck will attempt to do this, but it may well take future generations of experiments to achieve this goal, if this imprint is even there.
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