Alternative theory of gravity makes a nearly testable prediction

Alternative theory of gravity makes a nearly testable prediction

Enlarge / Galaxy clusters generated by the Universe simulator IllustrisTNG. (credit: TNG Collaboration)

From our current perspective, the Universe seems to be dominated by two things we find frustratingly difficult to understand. One of these is dark matter, which describes the fact that everything from galaxies on up behaves as if it has more mass than we can detect. While that has spawned extensive searches for particles that could account for the visual discrepancy, it’s also triggered the development of alternative theories of gravity, ones that can replace relativity while accounting for the discrepancies in apparent mass.

So far, these proposals have fallen well short of replacing general relativity. And they say nothing about the other big mystery, dark energy, which appears to be accelerating the expansion of the Universe. Instead, researchers have developed an entirely separate class of theories that could modify gravity in a way that eliminates

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LIGO may have spotted a black hole-neutron star merger

LIGO may have spotted a black hole-neutron star merger

Enlarge / Our gravitational wave detectors really are a series of tubes. (credit: Caltech/MIT/LIGO Lab)

On April 1, the teams behind the three gravitational wave detectors started them up for a new observational run, the first with all three operating in parallel for the full run. With the benefit of three detectors and some upgrades that were done during the downtime, we’re seeing a flood of new data. In just one month, LIGO/VIRGO has seen five gravitational wave events. Three of those are from merging black holes, one was the second neutron star merger, and another may have been the first instance of a neutron star-black hole merger.

A new season

The two LIGO detectors have been a work in progress for years, starting with an early version that everyone acknowledged was unlikely to pick up gravitational waves. But each iteration has allowed scientists to understand the sources of

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