In search of the echo of the Big Bang with the Simons Observatory

The Simons Observatory, which has just completed its main construction phase on the heights of the Atacama Desert in Chile, will now be able to start collecting data that will lead to the most precise measurements ever made of the oldest light in the Universe. This light, known as the Cosmic Microwave Background, was emitted around 380,000 years after the Big Bang and holds the secrets of the birth of the cosmos.

Scientists predict that a period of rapid expansion in the very early universe, known as inflation, may have generated gravitational waves in the fabric of space-time. These waves affect the polarisation properties of the light in the cosmic microwave background by imprinting a particular pattern on it, which cosmologists call "polarisation B modes". "Their discovery would provide an unprecedented window into how the Universe came into being and offer confirmation of the theory of inflation", explains Mark Devlin, co-director of the observatory at the University of Pennsylvania. "The amplitude of the primordial B modes will tell us about the state of the Universe in the first moments after its birth.

"The question of the origin of the Universe has always fascinated humans," says Brian Keating, principal investigator of the observatory at the University of California, San Diego. "With the Simons Observatory, we are on the verge of discovering answers rooted not in mere speculation, but in the most precise data ever collected by the world's most advanced telescopes."

"We are taking research into the primordial Universe to a new level," says Suzanne Staggs, co-director of the Simons Observatory at Princeton University. "The sensitivity of our instruments is opening up new perspectives for the field."

The quest

One of the main scientific aims of the Simons Observatory is to help elucidate what happened in the first decillionth of a second after the Big Bang (i.e. one trillionth of a trillionth of a billionth of a second). In that brief instant, scientists believe that the Universe multiplied in size by a factor of 100 trillion trillion. This would be comparable to a bacterium growing to the size of a galaxy. Quantum fluctuations in the primordial Universe would have generated the first inhomogeneities in the cosmos, which subsequently evolved to create the distribution of matter that we observe in the modern Universe. These same fluctuations also generated ripples in space-time known as primordial gravitational waves.

Although this inflationary period was a crucial moment in the history of the Universe, we cannot observe it directly. The early Universe was too hot and dense for light to propagate freely. It was only after 380,000 years of evolution, and the cooling of the plasma that made up the primordial Universe, that light could begin to travel unhindered. This is the cosmic microwave background that we observe today.

Like light passing through a pair of polarised sunglasses, the light in the cosmic microwave background can have a preferred orientation, or 'polarisation'. Inflationary gravitational waves would have left subtle patterns called B-modes in the polarisation of the cosmic microwave background. Detecting these B-modes would provide unprecedented information about the first instants of the Universe.

"We are on the trail of a signal generated during the first billionth of a trillionth of a trillionth of a second after the Big Bang," says Arthur Kosowsky, spokesman for the Simons Observatory collaboration at the University of Pittsburgh. "No one knows whether this signal is still large enough to be seen today. Seeing it would be like winning the physics lottery - the scientific impact would be immense."

Mapping the cosmic microwave background

The Simons Observatory comprises three 0.4-metre small aperture telescopes (SATs) and a 6-metre large aperture telescope (LAT), which together will achieve unprecedented sensitivity for measuring the polarisation of the cosmic microwave background. Since April 2024, two of the SATs have been calibrated and are now in the observation phase, while the third SAT should be operational in the coming months and the LAT early next year.

The Simons Observatory's size and innovative use of new technologies enable it to create detailed maps of the cosmic microwave background at a rate several times faster than the previous generation of telescopes. Together, the observatory's four telescopes will have 60,000 detectors collecting data, more than all the other projects combined. The observatory's superconducting detectors operate at temperatures 0.1 degrees above absolute zero, using cooling technology similar to that used for quantum computers. "I'm impressed that our instruments are working so well," says Jeff McMahon, a founding member of the Simons Observatory at the University of Chicago. "I'm even more excited by the scientific data these telescopes are starting to produce."

Together, the three SATs will study an area covering 20% of the southern hemisphere sky, while the LAT will map 40% of the sky at a finer resolution. By combining the sensitivity of the telescopes with innovative data analysis techniques, the Simons Observatory team is maximising its chances of spotting the B-modes it is looking for.

The future of the observatory

After about four years of operation, the observatory will benefit from the addition of 30,000 detectors thanks to a grant from the National Science Foundation (USA). The total observation period for the telescopes will be around 10 years. "Ten years may seem a long time, but if you use the capabilities of today's telescopes, it would take 60 years to reach our sensitivity," explains Mark Devlin. Additional telescopes funded by Japan and the UK are also due to come on stream in 2026, doubling the number of SATs.

French participation in the observatory

Teams from IN2P3 are taking part in the Simons project, with the participation of the APC and IJCLab laboratories. "Detecting polarisation B modes is like finding a needle in a haystack," explains Josquin Errard, winner of a European Research Council (ERC) grant on the subject and co-responsible for measuring primordial B modes within the collaboration. "Observations of the cosmic microwave background are affected by all sorts of astrophysical and environmental emissions that contaminate the signal, in particular emissions from our own galaxy, the Milky Way. We are working on developing new data analysis methods that will enable us to separate the different contributions.

In parallel, a possible French instrumental contribution to the observatory is under discussion, led by the LPSC, in partnership with CNRS Physique and CNRS Terre & Univers. The aim is to add a new SAT focusing on the characterisation and subtraction of galactic dust emissions that contaminate the cosmological signal. This new telescope would make it possible to fully exploit the observatory's sensitivity to primordial gravitational waves.

"With the success of the Planck satellite mission, France has positioned itself as a leader in the science of the primordial Universe. Greater participation by our community in the observatory would enable us to make the most of all our expertise, whether in instruments or data analysis," adds Thibaut Louis, a researcher at the IJCLab and head of the 'Simons Observatory' master project at IN2P3.