What is cosmic inflation?

The discovery of gravitational waves in the echo of the Big Bang lends support to the inflation model of cosmology.

The polarisation of the cosmic microwave background radiation – the “afterglow” of the big bang – suggests the presence of gravitational waves, according to the team analysing data from the BICEP2 telescope.

Gravitational waves are ripples in spacetime that propagate as waves, produced when large masses interact, and are the last unverified prediction of Einstein’s general theory of relativity.

The discovery, if confirmed, will also lend support to the inflation model in cosmology – the hypothesis that the universe underwent a brief period of exponential expansion immediately after the big bang – since only inflation can amplify gravitational waves from the beginning of the universe to a point at which they become detectable.

The inflation model was proposed by Alan Guth in 1980 as one way to explain two problems in cosmology – the horizon problem and the flatness problem.

The horizon problem is that areas of the universe separated by vast distances have identical physical properties such as temperature, but, since the exchange of information is limited by the speed of light, this should be impossible, as regions on opposites sides of the universe have not been in causal contact with one another during the universe's lifetime.

The flatness problem is a similar issue relating to the lack of an explanation as to why the universe appears to be nearly flat on a large scale – i.e. why the density of matter and energy appear to be precisely fine-tuned so that the universe neither rapidly collapsed back in on itself shortly after the big bang nor expands so quickly that galaxies are unable to form.

In the inflationary model, the extremely rapid expansion of the very early universe flattens out any large-scale inhomogeneities in temperature and density, quickly creating a large cosmos out of a much smaller one that was previously causally connected.

Guth’s model also explains the origins of the large-scale structure of the universe – before inflation there are quantum fluctuations in the very early universe, which then get magnified to macroscopic size and become the sites where galaxies, and clusters of galaxies, form.

No physical mechanism capable driving inflation is currently known.

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