Colliding Black Holes Provide Another way to Measure Distance in the Universe

A simulation of two merging black holes. Credit: Simulating eXtreme Spacetimes (SXS) Project

We know the universe is expanding, and we have a pretty good idea of how fast it’s expanding, but we don’t know the rate exactly. That’s because of the different methods we have to measure the rate of cosmic expansion keep giving us slightly different results. It’s a nagging problem that bugs astronomers, so while they have worked to ensure current methods are accurate, they have also looked to new ways to measure cosmic expansion. One of these new ways involves gravitational waves.

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Gravitational Waves Will Give Astronomers a new way to Look Inside Neutron Stars

Illustration showing the merger of two neutron stars. Credit: NASA's Goddard Space Flight Center/CI Lab

It’s difficult to study neutron stars. They are light years away and only about 20 kilometers across. They are also made of the most dense material in the universe. So dense that atomic nuclei merge together to become a complex fluid. For years our understanding of the interiors was based on complex physical models and what little data we could gather from optical telescopes. But that’s starting to change.

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A Gravitational Wave Observatory on the Moon Could "Hear" 70% of the Observable Universe

Concept for a Gravitational-wave Lunar Observatory for Cosmology (GLOC). Credit: Jani, et al

Gravitational-wave astronomy is set to revolutionize our understanding of the cosmos. In only a few years it has significantly enhanced our understanding of black holes, but it is still a scientific field in its youth. That means there are still serious limitations to what can be observed.

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Astronomers Detected a Black Hole-Neutron Star Merger, and Then Another Just 10 Days Later

An artistic image inspired by a black hole-neutron star merger event. Credit: Carl Knox, OzGrav/Swinburne

The interior of a neutron star is perhaps the strangest state of matter in the universe. The material is squeezed so tightly that atoms collapse into a sea of nuclear material. We still aren’t sure whether nucleons maintain their integrity in this state, or whether they dissolve into quark matter. To really understand neutron star matter we need to pull it apart to see how it works and to do that takes a black hole. This is why astronomers are excited about the recent discovery of not one, but two mergers between a neutron star and a black hole.

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Astronomers see a Hint of the Gravitational Wave Background to the Universe

Artist view of orbiting black holes. Credit: Caltech/R. Hurt (IPAC)

Gravitational-wave astronomy is still in its infancy. LIGO and other observatories have opened a new window on the universe, but their gravitational view of the cosmos is limited. To widen our view, we have the North American Nanohertz Observatory for Gravitational Waves (NANOGrav).

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Next Generation Gravitational Wave Detectors Should be Able to see the Primordial Waves From the Big Bang

An artist view of primordial gravitational waves. Credit: Carl Knox, OzGrav/Swinburne University of Technology

Gravitational-wave astronomy is still in its youth. Because of this, the gravitational waves we can observe come from powerful cataclysmic events. Black holes consuming each other in a violent chirp of spacetime, or neutron stars colliding in a tremendous explosion. Soon we might be able to observe the gravitational waves of supernovae, or supermassive black holes merging billions of light-years away. But underneath the cacophony is a very different gravitational wave. But if we can detect them, they will help us solve one of the deepest cosmological mysteries.

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Merging Black Holes and Neutron Stars. All the Gravitational Wave Events Seen So Far in One Picture

The mergers of compact objects discovered so far by LIGO and Virgo (in O1, O2 and O3a). The diagram shows black holes (blue), neutron stars (orange) and compact objects of unknown nature (grey), which were detected by their gravitational-wave emission. Each merger of a binary system corresponds to three compact objects shown: the two merging objects and the result of the merger. A selection of black holes (violet) and neutron stars (yellow) discovered by electromagnetic observations is shown for comparison. Image Credit: LIGO Virgo Collaboration / Frank Elavsky, Aaron Geller / Northwestern

The Theory of Relativity predicted the existence of black holes and neutron stars. Einstein gets the credit for the theory because of his paper published in 1915, even though other scientists’ work helped it along. But regardless of the minds behind it, the theory predicted black holes, neutron stars, and the gravitational waves from their mergers.

It took about one hundred years, but scientists finally observed these mergers and their gravitational waves in 2015. Since then, the LIGO/Virgo collaboration has detected many of them. The collaboration has released a new catalogue of discoveries, along with a new infographic. The new infographic displays the black holes, neutron stars, mergers, and the other uncertain compact objects behind some of them.

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14% of all the Massive Stars in the Universe are Destined to Collide as Black Holes

This illustration shows the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other. The black holes—which represent those detected by LIGO on Dec. 26, 2015—were 14 and 8 times the mass of the sun, until they merged, forming a single black hole 21 times the mass of the sun. In reality, the area near the black holes would appear highly warped, and the gravitational waves would be difficult to see directly. Credit: LIGO/T. Pyle

Einstein’s Theory of General Relativity predicted that black holes would form and eventually collide. It also predicted the creation of gravitational waves from the collision. But how often does this happen, and can we calculate how many stars this will happen to?

A new study from a physicist at Vanderbilt University sought to answer these questions.

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LIGO Will Squeeze Light To Overcome The Quantum Noise Of Empty Space

The LIGO Hanford Observatory in Washington State. Credit: LIGO Observatory
The LIGO Hanford Observatory in Washington State. Credit: LIGO Observatory

When two black holes merge, they release a tremendous amount of energy. When LIGO detected the first black hole merger in 2015, we found that three solar masses worth of energy was released as gravitational waves. But gravitational waves don’t interact strongly with matter. The effects of gravitational waves are so small that you’d need to be extremely close to a merger to feel them. So how can we possibly observe the gravitational waves of merging black holes across millions of light-years?

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Hubble Has Looked at the 2017 Kilonova Explosion Almost a Dozen Times, Watching it Slowly Fade Away

An artistic rendering of two neutron stars merging. Credit: NSF/LIGO/Sonoma State/A. Simonnet

In 2017, LIGO (Laser-Interferometer Gravitational Wave Observatory) and Virgo detected gravitational waves coming from the merger of two neutron stars. They named that signal GW170817. Two seconds after detecting it, NASA’s Fermi satellite detected a gamma ray burst (GRB) that was named GRB170817A. Within minutes, telescopes and observatories around the world honed in on the event.

The Hubble Space Telescope played a role in this historic detection of two neutron stars merging. Starting in December 2017, Hubble detected the visible light from this merger, and in the next year and a half it turned its powerful mirror on the same location over 10 times. The result?

The deepest image of the afterglow of this event, and one chock-full of scientific detail.

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