Cosmic Inflation and CMB

The inflationary Universe theory predicts various phenomena, such as the almost flat geometry of the Universe and the production of gravitational waves during the sudden expansion of the Universe that caused density variations in the gas. The cosmic microwave background (CMB) is the key to prove these.

CMB and Inflation

To prove that the Universe is flat, we should look at great look-back times and measure the curvature of space-time to compare the angular and linear sizes of things in the past. This is where CMB comes in, which is the background radiation of the Universe coming from the Universe’s last scattering surface. Recall that one of the main properties of CMB is that it is almost isotropic, meaning that it appears the same whichever direction you look at. As you remember, some minor irregularities are evident in CMB maps: some of the spots were a little brighter or weaker than the others. These variations were actually caused by sound waves.

CMB and Soundwaves

Explosions in space do not really produce any sound, although most science-fiction works portray that sound can travel even through a vacuum. The Universe, during the time it was still young, was very dense; so dense that sound could travel through gas, so Big Bang may have produced a noise (but not so loud, more of like a “deafening hiss” as described by theorists). The sound waves produced by this noise produced the minor irregularities observed in CMB radiation.

In addition, the angular sizes of these variations are also predicted by inflationary Universe theory, which is around 1 degree, should the Universe be flat. An angular size smaller than 1 degree would indicate a negatively curved Universe, while an angular size larger than 1 degree would indicate a positively curved Universe. Observations of CMB show that the spots are actually around 1 degree in diameter, further supporting the fact that the Universe is flat. In fact, the irregular spots of CMB produced by a flat Universe will measure at around 1 degree in diameter (but it will be hard for you to measure it by just looking at your monitor).

CMB and the Big Bang

Gravitational waves are predicted by inflationary Universe theory to be also produced during the sudden expansion of the Universe. These waves are believed to cause density variations in the gas produced during Big Bang, causing a “twist” in the CMB radiation photons that scattered the electrons during the time of recombination. This “twist” is what we should observe as the polarization of CMB.

In 2014, a team of international researchers showed that this was the case. They observed a “curl” called the “B-mode pattern” in the CMB’s polarization. This was eventually called the B-mode polarization, and is the main evidence for inflation.