Wednesday, July 19, 2017

Scientists Reveal New Connections Between Small Particles and the Vast Universe

The cosmos can be considered as a collider for human to access the results of particle physics experiments at ultimate high energies. Credit: Department of Physics, HKUST

Our observable universe is the largest object that physicists study: It spans a diameter of almost 100 billion light years. The density correlations in our universe, for example, correlations between numbers of galaxies at different parts of the universe, indicate that our vast universe has originated from a stage of cosmic inflation.

On the other hand, elementary particles are the smallest object that physicists study. A particle physics Standard Model (SM) was established 50 years ago, describing all known particles and their interactions.

Are density distributions of the vast universe and the nature of smallest particles related? In a recent research, scientists from the Hong Kong University of Science and Technology (HKUST) and Harvard University revealed the connection between those two aspects, and argued that our universe could be used as a particle physics "collider" to study the high energy particle physics. Their findings mark the first step of cosmological collider phenomenology and pave the way for future discovery of new physics unknown yet to mankind.

The research was published in the journal Physical Review Letters on June 29, 2017 (doi:10.1103/PhysRevLett.118.261302) and the preprint is available online (

"Ongoing observations of cosmological microwave background and large scale structures have achieved impressive precision, from which valuable information about primordial density perturbations can be extracted, " said Yi Wang, a co-author of the paper and an assistant professor at HKUST's department of physics. "A careful study of this SM background would be the prerequisite for using the cosmological collider to explore any new physics, and any observational signal that deviates from this background would then be a sign of physics beyond the SM."

The team carried out a two-step task to work out the background of the SM model. The first step was to work out the SM spectrum during inflation, which turned out to be dramatically different from that obtained from the particle physics calculation in flat space. The second one was to figure out how the SM fields entered the cosmological density correlation functions.

"Just like the line pattern of the light you see when observing a mercury lamp through a spectrometer, the mass distribution of the fundamental particles in SM also presents a special pattern, or a 'mass spectrum', which can be viewed as the fingerprint of SM," explained Zhong-Zhi Xianyu, a co-author and physicist at Center for Mathematical Sciences and Applications in Harvard University, "However, this fingerprint is subject to change if we change the ambient conditions. Just like the light spectrum changes when applying strong magnetic field to the lamp, the spectrum of the SM particles turns out to be very different at the time of inflation from it is now due to the inflationary background." The team carefully examined all effects from inflation and showed how the mass spectrum of SM would look like for different inflation models.

"Through inflation, the spectrum of elementary particles is encoded in the statistics of the distribution of the contents of the universe, such as the galaxies and cosmic microwave background, that we observe today", explains Xingang Chen, a co-author and scientist in the Harvard-Smithsonian Center for Astrophysics. "This is the connection between the smallest and largest."

Many problems along this direction remain to be explored. "In our minimal setup, the Standard Model particles interact with the inflaton (the driving force of inflation) rather weakly. But if some new particles can mediate stronger interactions between these two sectors, we would expect to observe a stronger signal of new physics," said Wang. "The cosmological collider is an ideal arena for new physics beyond SM."