Everything we can observe in the Universe takes place in four dimensions—the three dimensions of space and the dimension of time. This basic system, known as spacetime, can distort in the presence of massive astronomical objects, bending light and even affecting time.
Gravitational Waves and Fluctuations in Spacetime
Based on Albert Einstein’s 1915 General Theory of Relativity, massive objects bend the fabric of spacetime, giving rise to what we know as gravity. But when these objects accelerate, like when two black holes are orbiting each other, they cause tiny disturbances in spacetime, called gravitational waves, that propagate throughout the Universe at the speed of light.
Gravitational waves are extremely small, roughly one billionth the width of a single atom. As they travel uninterrupted throughout the Universe, they slightly compress and stretch spacetime. We are periodically distorted by gravitational waves, although we cannot sense it. Though the search for gravitational waves has taken decades, technology has only recently advanced to the point where we can directly detect them.
The first directly observed gravitational waves reached Earth on September 14, 2015 after traveling more than a billion light years. By analyzing the signal, astronomers were able to deduce that two black holes were locked in a binary orbit and as they spiraled into each other, they released energy in the form of gravitational waves.
Several more examples of gravitational waves were observed, demonstrating that these events are not that rare in the Universe. Then, on August 17, 2017, a different kind of signal was recorded that corresponded to the merger of two neutron stars, the super dense compact objects created by supernovae. A call went out to the astronomical community, and within hours, an electromagnetic counterpart was discovered.
What We’re Learning
In the few years since the direct detection of gravitational waves, these barely perceptible bends in spacetime have taught us a lot about our Universe.
- How Old is the Universe? In the case of the neutron star collision, by measuring the strength of gravitational waves, we’re able to compute a distance to the event and its host galaxy, NGC 4993. We know that the further a galaxy is, the faster it moves away. When we measure how the the light from NGC 4993 is stretched, or redshifted, we know how fast it is moving. With these values, we can work backward and calculate the age of the Universe. This novel way of dating the Universe agrees with the currently accepted age of 13.8 billion years.
- Where Gamma-Ray Bursts Come From. Since the late 60’s, scientists have observed short bursts of high-energy gamma-ray radiation but could not pinpoint their origin. After detecting a gamma-ray burst and the gravitational wave event almost simultaneously and in the same area of the sky, it was determined that neutron star mergers must be the source.
- Origin of Heavy Elements. Heavy elements like gold and platinum were thought to be created in hot radioactive events, like supernovae explosions. But the amount of these elements observed in supernova remnants was less than sufficient to explain the abundance we see in the Universe. After the 2017 neutron star merger, astronomers saw the radioactive aftermath suggesting that neutron star collisions are the perfect factories for heavy elements. That one collision alone formed several Earth masses of gold and platinum. We now know that these events are responsible for most of the heavy elements in the Universe.
- Test of Dark Matter. Some theories have attempted to explain the peculiar motion of galaxies and clusters of galaxies without invoking dark matter, the invisible material that makes up 80% of the matter in the Universe. This involved altering the current model of gravity to fit the observations. While the theory of general relativity says that light and gravity travel at the same speed, many of these adjusted models require them to be different. But after traveling 130 million light years, the 2017 gravitational wave arrived 1.7 seconds before the corresponding electromagnetic radiation. This means that the speeds couldn’t differ by more than 1 in 1,000,000,000,000,000. In other words, they’re pretty much equal.
Of course, we’re not done learning from gravitational waves. By continuing to study these flickers in spacetime, we may be rewarded with the discovery of new particles, new models for what happens to matter at extreme densities, and a deeper understanding of gravity itself. Gravitational waves have opened a new realm of astronomy.
1×10-21 meters, Size of the average gravitational wave
The detection of gravitational waves was a testament to incredible engineering and the power of the theory of general relativity. But it also showed what was possible when the astronomical community banded together. Within hours of the August 17, 2017 gravitational wave event, many major observatories were looking for the optical counterpart, with over 70 eventually participating. Since the event occured in the Southern Hemisphere, only certain telescopes could observe that region of the sky.
A few hours after the gravitational wave detection, as night set in Chile, CFA astronomers used the powerful Dark Energy Camera on the Blanco telescope to search the region of sky from which the gravitational waves emanated. In less than an hour they located a new source of visible light in the galaxy NGC 4993.
NASA’s Chandra X-ray Observatory tried observing the optical counterpart two days after gravitational waves detected but with no luck. Undeterred, Chandra observed again after another week and discovered X-rays right where they should be.
But the delay was curious. CFA scientists determined, using radio observations with the Very Large Array in New Mexico, that the collision blasted a narrow jet of high energy radiation about 30 degrees away from us. It was only when this energy heated the surrounding medium, around nine days after the collision, that Chandra was able to detect X-rays.
The discovery of a electromagnetic event coupled with a gravitational wave event is a first for “multi-messenger” astronomy. By combining the two messengers, we are learning more about the source than we ever would separately.
Featured image: An artist’s conception of the collision between two neutron stars. These collisions produce phenomenal amounts of energy in the form of gravitational waves, as observed using the LIGO and Virgo gravitational wave observatories. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
Provided by CFA Harvard