The theories of Leonhard Euler (1707-1783) and Albert Einstein (1879-1955) revolutionised our understanding of the Universe. With the famous equation that bears his name, Euler gave scientists a powerful tool for calculating the movements of galaxies in the Universe. With his theory of general relativity, Einstein demonstrated that the Universe is not a static framework: it can be distorted by star clusters and galaxies.

Physicists have tested these equations in all sorts of ways, which have so far proved successful. However, two discoveries continue to put these models to the test: the acceleration of the Universe’s expansion and the existence of invisible dark matter, which is thought to account for 85% of all matter in the cosmos. Do these mysterious phenomena still obey the equations of Einstein and Euler? Researchers are still unable to answer this question.

The missing ingredient

“The problem is that current cosmological data do not allow us to differentiate between a theory that breaks Einstein’s equations and one that breaks Euler’s equation. This is what we demonstrate in our study. We also present a mathematical method for solving this problem. This is the culmination of ten years of research,” explains Camille Bonvin, associate professor in the Department of Theoretical Physics in the UNIGE Faculty of Science and first author of the study.

Researchers were unable to differentiate between the validity of these two equations at the very edge of the Universe because they were missing an “ingredient”: the measurement of time distortion. “Until then, we only knew how to measure the speed of celestial objects and the sum of the distortion of time and space. We have developed a method for accessing this additional measurement, and it’s a first,” says Camille Bonvin.

If the time distortion is not equal to the sum of time and space — i.e. the result produced by the theory of general relativity — this means that Einstein’s model does not work. If the time distortion does not correspond to the speed of the galaxies calculated with the Euler equation, this means that the latter is not valid. “This will allow us to discover whether new forces or matter, which violate these two theories, exist in the Universe,” explains Levon Pogosian, professor in the Department of Physics at Simon Fraser University, in Canada, and co-author of the study.

Reality check

These results will make a crucial contribution to several missions whose aim is to determine the origin of the accelerated expansion of the Universe and the nature of dark matter. These include the EUCLID space telescope, which will be launched in July 2023 by the European Space Agency (ESA), in collaboration with the UNIGE, and the Dark Energy Spectroscopic Instrument (DESI), which began its 5-year mission in 2021 in Arizona. There is also the international SKA (Square Kilometre Array) giant radio telescope project in South Africa and Australia, which will begin observations in 2028/29.

“Our method will be integrated into these different missions. This is already the case for DESI, whom we have become external collaborators thanks to this research,” Camille Bonvin enthuses. The research team has successfully tested its model on synthetic catalogues of galaxies. The next stage will involve testing it using the first data supplied by DESI, as well as identifying the obstacles and minimising the systematic features that could hamper its application.

The findings, published June 7, 2023, in the journal Nature Geoscience, can be used to adjust earthquake models and help scientists unravel the mechanisms that drive earthquakes.

The research was led by Nathan Bangs, a senior research scientist at the University of Texas Institute for Geophysics. In 2018, Bangs led an ocean seismic survey that resulted in the first ever 3D scan of a large subducting seamount. Known as the Pāpaku Seamount, the long extinct volcano lies some three miles under the seafloor inside the Hikurangi subduction zone off the coast of New Zealand.

Images from the scan show the seamount colliding with the subduction zone and the pattern of stresses, fluids and sediments surrounding it. Previous models suggested sediments are pushed down the subduction zone ahead of the seamount, but the scan revealed something different: a massive sediment trail in Pāpaku’s wake.

In another surprise, the scientists spotted the fading trail of a much larger seamount that had long since sunk beneath New Zealand’s North Island.

According to Bangs, the discovery suggests that sinking seamounts drag down enough water-rich sediment to create conditions in the crust suitable for slow slip earthquakes, at least in New Zealand.

“That older one seems to be very much linked to an uplifted ridge that’s really in the bullseye of where recent slow slip activity has been,” Bangs said. “There could be other places like Cascadia (in the U.S. Pacific Northwest) that have subducting seamounts and a lot of sediment, but because the subducting crust there typically has less water than Hikurangi, they may be less likely to have the same kind of shallow slow slip activity.”

Slow slip earthquakes are slow motion versions of large earthquakes, releasing similar levels of pent-up tectonic energy but in a harmless creeping fashion that can take days or weeks to unfold. Scientists believe that the make-up of the crust is a major factor in how tectonic energy is released, with softer, wetter rocks allowing plates to slip slowly, while drier, brittle rocks store energy until they fail in violent and deadly megaquakes.

The new findings reveal how those conditions sometimes come about and importantly, said Bangs, tell scientists what to look for at the world’s other subduction zones.

The research and seismic survey were funded by the National Science Foundation and similar scientific agencies in New Zealand, the United Kingdom and Japan. The University of Texas Institute for Geophysics is a research unit of the Jackson School of Geosciences.

Primitive meteorites preserve information about the conditions at the birth of the Sun and planets. The meteorite components show an inhomogeneous concentration of a radioactive isotope of aluminum. This variation suggests that an additional amount of the radioactive aluminum was introduced shortly after the Solar System started forming. A nearby supernova explosion is the best candidate for this injection of new radioactive isotopes. But a supernova that was close enough to deliver the amount of isotopes seen in meteorites would have also created a blast wave strong enough to rip the nascent Solar System apart.

A team led by Doris Arzoumanian at the National Astronomical Observatory of Japan proposed a new explanation of how the Solar System acquired the amount of isotopes measured in meteorites while surviving the supernova shock. Stars form in large groups called clusters inside giant clouds of molecular gas. These molecular clouds are filamentary. Small stars like the Sun usually form along the filaments and large stars, which will explode in a supernova, usually form at the hubs where multiple filaments cross.

Assuming that the Sun formed along a dense molecular gas filament, and a supernova exploded at a nearby filament hub, the team’s calculation showed that it would take at least 300,000 years for the blast wave to break up the dense filament around the forming Solar System. The components of meteorites enriched in radioactive isotopes formed in approximately the first 100,000 years of Solar System formation inside the dense filament. The parent filament may have acted as a buffer to protect the young Sun and helped catch the radioactive isotopes from the supernova blast wave and channel them into the still forming Solar System.