In August, for the first time ever, scientists witnessed the electromagnetic lightning and gravitational gusts from the stormy collision of two neutron stars in a distant galaxy. The cosmic cataclysm created a “kilonova” — a phenomenon that had never been seen before — and the observations by both traditional telescopes and gravitational wave detectors heralded a new era for science. In the years to come, astrophysicists will use two “messengers” to understand the universe: electromagnetism and gravity.
The effort to capture the event's fleeting signals involved three gravitational wave detectors, more than five dozen telescopes on every continent including Antarctica, seven space-based observatories, and, according to one estimate, 15 percent of the world's astronomers. It yielded 20 scientific papers published in three separate journals and answered a broad array of questions about the cosmos: What happens when neutron stars collide? How are precious elements like gold produced? Where do some bursts of high-energy gamma rays originate?
Those discoveries are just the beginning: “This is opening a new brand of research and science,” Eleonora Troja, an astrophysicist at NASA's Goddard Space Flight Center and the University of Maryland, said Tuesday.
Here are just two of the ways the kilonova's detection will likely shift the course of astronomy.
The sensational science of colliding neutron stars
Mergers of neutron stars — the dense husks of stars that collapsed in on themselves after running out of fuel for nuclear fusion — have been theorized about for decades. No one had witnessed such a cataclysm until the gravitational and light signals from an event 130 million light-years away reached Earth on Aug. 17.
As the neutron stars spiraled into each other, they flung off gravitational waves like spray from a whirlpool. Their final collision produced two intense, narrow jets of electromagnetic radiation, as well as a cloudburst of energy and debris that emanated the radioactive glow of the kilonova. Astronomers all over the world dropped what they were doing to observe the event in every wavelength of the electromagnetic spectrum — from high-energy gamma rays through the visible light spectrum all the way to long, low radio waves.
Just 1.7 seconds after the LIGO and Virgo gravitational wave detectors felt the first pulse of the collision, NASA's Fermi space telescope caught a faint, short burst of gamma rays streaming from the same spot in the sky. Those rays were the leading edge of one of the powerful radiation jets.
It stood to reason the jet would also contain X-rays, another high-energy form of radiation. Yet when Troja and her colleagues focused their telescopes on the event, they saw nothing.
For nine days, Troja waited, mystified. Finally, her instruments began to pick up a faint signal, which grew stronger as the days wore on. Right now the signal is obscured by the sun, but Troja expects to keep seeing it months from now.
Something similar happened in radio wavelengths: The first radio signal from the jet didn't arrive on Earth until 16 days after the gravitational wave detection, according to Texas Tech University astronomer Alessandra Corsi, and it could linger in the sky for years.
The reason for the delays? The initial jet of radiation was so powerful and moving so quickly that it formed a concentrated beam of light that was only visible if you looked at it straight on, much the way you can't see the light of a laser pointer unless it's aimed straight at you. As the jet interacted with the interstellar medium — the sparse, cold matter that fills the void between stars — it fanned out, becoming more like the wide beam of a flashlight.
Telescopes on Earth, it turned out, were not facing the jet straight on. Based on the sensitivity of the instrument and the type of radiation involved, scientists had to wait for the jet to spread out before they could see it in their chosen wavelength.
Our off-kilter glimpse of this jet was actually a boon for astronomers attempting to study the neutron stars's collision. If they'd been better aligned, the intensity of the jet's light would have obscured the kilonova's radioactive blaze. The event would have looked just like any of the hundreds of other gamma-ray bursts scientists see every year. They would have missed the opportunity to analyze the light and infrared glow of the kilonova, which is how they've already gained insight into the processes that unfolded in the wake of the neutron star collision, including creation of precious elements like silver and gold.
The only reason anyone was even watching this event, several scientists acknowledged at a news conference Monday, was because LIGO had tipped them off.
Now astronomers who work with electromagnetic radiation know what to look for. They don't necessarily need another LIGO detection to find future neutron star mergers, because this one has given them a road map for locating these events on their own: an extra-short gamma ray burst followed by delayed X-ray and radio emission.
“We are going to plan our mission and our strategy in a different way,” Troja said.
The goal is to see as many neutron star mergers as possible, because the cataclysms involve some of the most extreme physics scientists have ever seen. The more data they can gather about these mergers, the better they can test their theories about general relativity, nuclear physics and the dynamics of the cosmos.
Meanwhile, astronomers are not done studying their first detection. The delayed emissions from the merger are still streaming in, carrying yet more information about what happens in the collision. This ongoing radiation may hold clues about the energy and mass involved in the explosion, as well as the life cycles of stars.
Clues to dark energy and other cosmological mysteries
The dawn of “multi-messenger astrophysics,” which pairs telescope observations with gravitational wave detections to deepen scientists' understanding of cosmic events, also promises answers to some of the most persistent questions about the universe.
For years, scientists have puzzled over the nature of dark energy, the mysterious force that accounts for the accelerating expansion of the universe. To measure its effect, they need “standard candles” — objects with known distance and brightness that can be tracked as they are carried away from us on dark-energy-driven currents. Calculations based on these values help cosmologists arrive at the “Hubble constant,” the rate at which the universe is expanding.
Scientists have traditionally used supernovas and certain stars as their standards. It's a fraught, error-prone process to measure the exact distance of very far-off objects. Right now, astronomers use something called the “cosmic distance ladder,” cobbling together what they know about various nearby stars to estimate the distances of objects further away, then folding that data into their calculations of the universe's expansion rate. The resulting measurements are imperfect, to say the least. They also conflict with the results of a parallel effort to measure the universe's expansion based on the cosmic microwave background, the afterglow of the Big Bang.
Neutron star mergers, witnessed via both gravitational waves and light, “could be the tiebreaker,” said Brandeis University astrophysicist Marcelle Soares-Santos.
Gravitational waves eliminate the challenge of calculating distance, because the amplitude of the wave encodes exactly how far away their source was. Observations with optical telescopes can then reveal the velocity at which the kilonova is moving away.
In an article in the journal Nature, members of the LIGO collaboration worked with dark-energy experts to take their first stab at calculating the expansion rate of the universe. They came up with a value for the Hubble constant that was more or less consistent with other measures, though with just one merger to analyze, the calculation was still laden with uncertainty.
The LIGO and Virgo detectors are getting upgraded, and still more detectors are being built. In years to come, astronomers say, we may detect a neutron-star merger every few weeks. Getting a better value for the Hubble constant — and by extension, a better understanding of dark energy — could be right on the horizon.