At the South Pole, where the sun does not set below the horizon in summer, the South Pole Telescope (SPT) and BICEP telescopes operate. With their help, scientists study the cosmic microwave background - the oldest light preserved from the first moments of the Universe's existence. This is reported by Phys.org with reference to materials from the US Department of Energy.
According to inflation theory, in the first instant after the Big Bang the Universe expanded faster than the speed of light - from a region smaller than a proton to the size of a marble. This expansion smoothed space and left behind almost nothing but the energy of the field. Then the energy turned into a hot soup of quarks and gluons - the building blocks of matter. Soon quarks and gluons combined into protons and neutrons. All of this fit into the first second, during which the Universe grew to more than 161 trillion kilometers in diameter.
Millennia later, the cosmic microwave background arose. Until that moment the Universe was opaque, like a glass of orange juice: free protons and neutrons scattered light. When hydrogen atoms formed, light could propagate freely. Today this radiation has been stretched into the microwave range by the expansion of the Universe and has a temperature slightly below 3 kelvins. It contains most of the photons in the Universe but is invisible to the eye - only radio telescopes detect it.
The main goal is to detect B-modes: a special pattern in the polarization of the cosmic microwave background (the orientation of light oscillations). If primordial gravitational waves exist, they would have left exactly such a trace. This would be weighty confirmation of inflation theory. For now B-modes have not been found, but scientists are increasingly constraining possible inflation models.
The difficulty is that the signal is extremely weak: B-modes are roughly ten times weaker than E-modes, which in turn are ten times weaker than temperature fluctuations. In addition, Milky Way dust and gravitational lensing - the bending of light by massive structures - distort the picture, turning E-modes into fake B-modes. To separate the true signal, delensing is used - a procedure for removing the lensed contribution using mass distribution maps.
In 2014 the BICEP2 team announced the detection of B-modes from primordial gravitational waves, but it later turned out that the signal was explained by galactic dust. Since then, the SPT and BICEP collaboration has strengthened: BICEP provides high sensitivity on large angular scales, while SPT provides excellent resolution for lensing maps. In 2021 the constraint on the tensor-to-scalar ratio r (a quantity characterizing the amplitude of primordial gravitational waves) fell to 0.036 - the strictest limit to date. The simplest inflation models predicted r from 0.05 to 0.1, so many of them have already been ruled out.
The near-term plans include new cameras SPT-3G+ and BICEP Array+, which will map the sky ten times faster than current ones. Scientists hope that a joint analysis of data collected through 2024 will bring the mystery of the Universe's first moments closer to resolution.