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03/19/2020 - The Universe's Darkest Secrets

By Sally Johnson

a photo of a telescope on a hillsideSubaru Prime Focus Spectrograph's instruments being transported up to the Subaru Telescope, shown on the right.

Theoretical physics – a branch of physics devoted to predicting and explaining all manner of natural phenomena using theoretical methods – is helping reveal the secrets of dark energy, which makes up the majority of the universe’s energy, and dark matter. Experimental and observational projects are also currently underway at the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) at the University of Tokyo to probe the universe for more clues about dark energy and dark matter.

During the past few decades, thanks to increasingly precise measurements of the universe, physicists discovered that it’s made of things that we haven’t observed on Earth. While the standard model of elementary particles has been extremely good at explaining most of the phenomena we observe on Earth, much more exists within the universe that can’t be explained by the model.

What is dark energy? It’s one of the universe’s biggest mysteries: more remains unknown than known about dark energy. It affects the universe’s expansion, so physicists are able to infer that dark energy makes up roughly 68% of the universe and it appears to be somehow tied to the vacuum of space.

Dark matter is another largely unknown component of the universe, and its name stems from the fact that it doesn’t absorb, emit, or reflect light. Although invisible, dark matter’s gravitational effects on visible matter allow physicists to infer its existence. They estimate that dark matter makes up approximately 27% of the universe.

For perspective, “ordinary” matter that the stars and galaxies are made of, which we can detect, composes a mere 5% of the universe.

In the case of dark energy, researchers are observing how the universe has evolved and expanded since the Big Bang, when the universe began. From that, they’re inferring properties of dark energy and the distribution of dark matter within the universe and how it has changed. By looking at the light from distant galaxies, for example, we can infer the distribution of dark matter between us and the galaxies.

“Dark energy has a strange feature: when you try to compress it the pressure is negative,” says Hirosi Ooguri, a theoretical physicist and director of Kavli IPMU. “Things normally try to bounce back, since the pressure is positive. We’re trying to understand it theoretically and also through observations of dark matter and dark energy. In the case of dark matter, there are many ongoing efforts by experimenters to detect it directly or to study its properties by observing the universe.”

One of the projects Kavli IPMU is working on includes wide-field imaging surveys of galaxies at the Subaru Telescope on the summit of Hawaii’s Mauna Kea using the Hyper-Subprime-Cam (HSC) Camera.  

Another is the Subaru Prime Focus Spectrograph (PFS) project, which will start measuring spectra of about 2,400 astronomical objects simultaneously on a large portion of the sky.

For both HSC and PFS, Kavli IPMU is a leading institute of these international collaborations. The big data they provide will help answer fundamental questions about the composition of the universe – particularly whether dark energy and dark matter interact with stars and galaxies.

“In one shot, we can take a photo of many stars and galaxies and study them simultaneously,” Ooguri explains. “We can also use their measured spectra to determine the distance to each galaxy, which will teach us a lot.” 

This type of experimental and observational project will help answer big and small questions. “The various areas of science aren’t in isolation; a discovery within one area can open up others,” Ooguri says. “Fundamental discoveries are so important because of their universal value. This is one of the things I love most about theoretical physics, and I’m very fortunate to have this career.”

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