Particle physics laboratory

BICEP3 tightens the limits of cosmic inflation


Physicists looking for signs of primordial gravitational waves by sifting through the first light in the cosmos – the cosmic diffuse background (CMB) – have reported their findings: still nothing.

But far from a failure, the latest results of the BICEP3 experiment at the South Pole have tightened the limits of models of cosmic inflation, a process which theoretically explains several puzzling characteristics of our universe and which should have produced gravitational waves. shortly after the universe started.

“Formerly promising inflation models are now ruled out,” said Chao-Lin Kuo, a BICEP3 principal investigator and physicist at Stanford University and the Department of Energy’s SLAC National Acceleration Laboratory.

The results were published on October 4 in Physical examination letters.

Explode the universe

Cosmic inflation is the idea that very early in the history of the universe, the amount of space in the universe exploded from about the size of a hydrogen atom to about a light year, about the time it would take for light to travel one -billionth of the passage of the same atom.

Inflation can explain a lot of things, including why the universe seems to be quite smooth and the same in all directions, why space is flat, and why there are no magnetic monopoles. Yet physicists failed to pinpoint the exact details, and they found many different ways that inflation could have happened.

One way to determine which of these inflationary models is correct, if any, is to look for gravitational waves that would have been produced during the expansion of space and the movement of matter and the energy within it. . In particular, these waves should leave an imprint on the polarization of light in the cosmic diffuse background.

Polarizing gravitational waves

This polarized light has two components: B-modes, which swirl in the sky, and E-modes, which are arranged in neater lines. Although the details depend on the correct inflation model, the primordial gravitational waves should appear as particular models of B and E modes.

Beginning in the mid-2000s, researchers began studying B-mode polarization in the CMB, looking for evidence of primordial gravitational waves. Over time, the details of the experiments changed dramatically, says SLAC lead scientist Zeeshan Ahmed, who worked on a few embodiments of the BICEP experiment at the South Pole.

The first BICEP experiment deployed around 50 machined metal horns that detect tiny differences in microwave radiation, each fitted with thermal sensors and polarizing grids to measure polarization. The next generation, BICEP2, required a technological leap – new superconducting detectors that could be denser in the same area as previous telescopes. The successor Keck Array was essentially several BICEP2 telescopes in one.

To take it to the next level, BICEP3, “we had to invent things along the way,” explains Ahmed.

With the support of a research and development grant led by a laboratory at SLAC, Kuo, Ahmed and other scientists at SLAC developed a number of new systems and materials. These include more modular and easier to replace detector components, as well as lenses and filters that are more microwave transparent while further blocking infrared light, which helps keep microwave detectors superconducting. temperature sensitive to cool.

These advances, according to Ahmed, combined with data from previous experiments such as BICEP2, Keck, WMAP and Planck, allowed researchers to set the most stringent limits to date on the types of primordial gravitational waves that might exist – and therefore the most stringent limits to date on models. of cosmic inflation.

Research continues

“Experimenters are doing a heroic job,” says Eva Silverstein, a theoretical physicist at Stanford who studies cosmic inflation. “It’s a big step forward.”

The results exclude a number of inflation models, including some popular older models and some versions of newer models driven by string theory, says Silverstein. The results suggest that the correct model will be slightly more complicated than those that were excluded, although there is still a wide range of viable alternatives. “It’s not like we’re going back to the drawing board,” says Silverstein, but the results “help us focus”.

As more data arrives from BICEP3 and its immediate successor, the BICEP Array, along with other projects, physicists will begin to obtain clues that will help them focus their search for better models even more. inflation. Still, Ahmed said, they may have to wait for CMB-S4, a project currently under review at the Energy Ministry, to get clearer answers. CMB-S4 will deploy the equivalent of 18 BICEP3 experiments – or more, according to Ahmed – and draw heavily on researchers and expertise from the Department of Energy’s laboratory, including ideas developed for BICEP3. “It will take a decade to build this thing,” he says, “but it’s starting to take shape.”

The BICEP project is supported by grants from the National Science Foundation, Keck Foundation, NASA Jet Propulsion Laboratory, NASA, Gordon and Betty Moore Foundation, Canada Foundation for Innovation, UK Science and Technology Facilities Council and the US Department of Energy Bureau of Science.

Reference: PAR Ade et al., Physical examination letters, October 4, 2021 (10.1103 / PhysRevLett.127.151301)

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