We've Come a Long Way

In 2003 the WMAP craft measured the very small fluctuations – about one part in 100,000 – in the temperature of the cosmic background radiation (coloured regions). These fluctuations, which are in excellent agreement with the predictions of Big Bang theory, originated during inflation and evolved under the influence of both gravity and the pressure of the matter–radiation plasma before particles in the plasma recombined to form hydrogen atoms. Buried in this pattern might also be fluctuations from primordial gravitational waves, but to tease out their signature researchers have to map in detail the polarization of the photons as well as their temperature (white lines represent the electric polarization vector). Since gravitational waves produce a quadrupolar anisotropy and therefore induce polarization without an associated temperature fluctuation, they (and only they) are able to generate a polarization pattern that cannot be expressed as the gradient of a scalar. Source: NASA. See: Sounding out the Big Bang