The origin of how the Universe created its voids and filaments can now be studied within seconds after researchers developed an artificial intelligence tool called Dark Emulator.

Advancements in telescopes have enabled researchers to study the Universe with greater detail and to establish a standard cosmological model that explains various observational facts simultaneously. But there are many things researchers still do not understand. Remarkably, the majority of the Universe is made up of dark matter and dark energy, of which no one has been able to identify its nature. A promising avenue to solve these mysteries is the structure of the Universe. Today’s Universe is made up of filaments where galaxies cluster together and look like threads from far away, and voids where there appears to be nothing (image 1). The discovery of the cosmic microwave background has given researchers a snapshot of what the Universe looked like close to its beginning, and understanding how its structure evolved to what it is today would reveal valuable characteristics about what dark matter and dark energy is. 

A team of researchers, including Kyoto University Yukawa Institute for Theoretical Physics Project Associate Professor Takahiro Nishimichi, and Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Principal Investigator Masahiro Takada, used the world’s fastest astrophysical simulation supercomputers ATERUI and ATERUI II to develop the Dark Emulator. Using the emulator on data recorded by several of the world’s largest observational surveys allows researchers to study possibilities concerning the origin of cosmic structures, and how dark matter distribution could have changed over time.

“We built an extraordinarily large database using a supercomputer, which took us three years to finish, but now we can recreate it on a laptop in a matter of seconds. I feel like there is great potential in data science. Using this result, I hope we can work our way towards uncovering the greatest mystery of modern physics, which is to uncover what dark energy is. I also think this method we’ve developed will be useful in other fields such as natural sciences or social sciences,” says lead author Nishimichi. {module INSIDE STORY}

This tool uses an aspect of artificial intelligence called machine learning. By changing several important characteristics of the Universe, such as those of dark matter and dark energy, ATERUI and ATERUI II have created hundreds of virtual Universes. Dark Emulator learns from the data, and guesses outcomes for new sets of characteristics without having to create entirely new simulations every time. When testing the resulting tool with real-life surveys, it was able to successfully predict weak gravitational lensing effects in the Hyper Suprime-Cam survey, along with the three-dimensional galaxy distribution patterns recorded in the Sloan Digital Sky Survey to within 2 to 3 percent accuracy, in a matter of seconds. In comparison, running simulations individually through a supercomputer without the AI would take several days.

The researchers hope to apply their tool using data from upcoming surveys in the 2020s, enabling deeper studies of the origin of the Universe.

Details of their study were published in the Astrophysical Journal on 8 October 2019.

Researchers at The University of Tokyo have used computational methods and analysis of recent experimental data to demonstrate that water molecules take two distinct structures in the liquid state. The team investigated the scattering of X-ray photons through water samples and showed a bimodal distribution hidden under the first diffraction peak that resulted from tetrahedral and non-tetrahedral arrangements of water molecules. This work may have important implications throughout science, but especially with regard to living systems, like proteins and cell structures, which are strongly affected by their surrounding water molecules.

Given the ubiquity of water on our planet and the central role it plays in all known life, it may be hard to believe that there is anything left to learn about this most familiar fluid. A simple molecule made up of just two hydrogen atoms and one oxygen; water still hides fundamental mysteries that remain to be unraveled. For example, water has unusually high melting and boiling points, and even expands when it freezes (unlike most liquids, which contract). These and other unusual properties make it very different from almost all other liquids but also allow life as we know it to exist. {module INSIDE STORY}

The weirdness of water can be best understood by thinking about the very unique interactions between H2O molecules--the hydrogen bond. Water tends to form four hydrogen bonds with its four neighbors, which leads to tetrahedral arrangements of the neighbors. Such arrangements can be largely distorted under thermal fluctuations. However, whether the distortion leads to the coexistence of distinct tetrahedral and non-tetrahedral arrangements has remained controversial.

Now, scientists at The University of Tokyo have combined supercomputer simulations and the analysis of scattering experimental data to find the "structure factor" of water - the mathematical function that represents the paths of dispersed X-rays when they scatter off the hydrogen and oxygen atoms. The analysis showed two overlapping peaks hiding in the first diffraction peak of the structure factor. One of these peaks corresponded to the distance between oxygen atoms as in ordinary liquids, while the other indicated a longer distance, as in a tetrahedral arrangement. "The combination of new computational methods and analysis of recent X-ray scattering data allowed us to see what was not visible in previous work," first author of the study Rui Shi explains.

This discovery may have huge implications across many scientific fields. Knowing the exact structural ordering of water is critical for a complete understanding of molecular biology, chemistry, and even many industrial applications. "It is very satisfying to be able to unravel the liquid structure of such a fundamental substance," senior author Hajime Tanaka says.