December 18, 2019
Enjoy these interesting stories from the scientific community.
*-- Underground fiber-optic cables help scientists record thunderquakes --*
Loud thunderclaps can cause the ground to shake. The quakes are too subtle for humans to feel, but new research showed fiber optic cables can detect the thunderquakes.
A new study, published Wednesday in the Journal of Geophysical Research: Atmospheres, suggests underground fiber-optic cables could be used to track thunderstorms.
"Severe weather has strong interactions with the ground, but we haven't had the capability to study the coupling between the atmosphere and the solid Earth," Tieyuan Zhu, lead study author and an assistant professor of geophysics at Pennsylvania State University, said in a news release. "With this new technology, we can utilize existing fiber-optics networks to clearly see how thunderstorm energy passed through campus."
To track the tiny seismic events caused by thunder, scientists deployed a new technology called a distributed acoustic sensing array, or DAS array, which fires a laser down one of the glass fibers in fiber-optic cables. The laser registers tiny changes in pressure, recording measurements every six feet. A few miles of underground cables create a network featuring thousands of sensors.
"If there is any change in the external energy on the ground above, even walking steps, you will have a very small change that's going to stretch or compress the fiber," Zhu said. "The laser is very sensitive and can detect these small changes."
When the thunder-generated acoustic pressure wave hits the ground, it travels outward like a wave in a pond. The DAS records the location and movements of these pressure waves.
When scientists compared the measurements recorded by the newly deployed array, the thunderquake data matched the distribution of lightning recorded by the U.S. National Lightning Detection Network.
In addition, to help scientists track extreme weather, the new technology could be used to study Earth's interior and model earthquake risks. Seismic events are relatively rare on the East Cast, but researchers could potentially use fiber optic cables to measure the movement of pressure waves through Earth's mantle and crust.
"This research is an example of taking an existing technology and using it to serve another purpose," said study co-author David Stensrud, head of the department of meteorology and atmospheric science at Penn State. "Having technologies that are multifunction maximizes the benefits to society."
*-- Dark matter may explain mysterious gamma ray source at center of Milky Way --*
New analysis by astrophysicists at the Massachusetts Institute of Technology suggests dark matter could explain a mysterious source of gamma rays in the center of the Milky Way.
Gamma rays are the the most energetic electromagnetic waves. Throughout the Milky Way, astronomers have traced gamma rays to two sources: supernovae and pulsars. But at the center of the Milky Way, scientists have struggled to account for a glow of gamma rays.
Previously, MIT scientists developed simulations suggesting a collection of pulsars, rapidly rotating neutron stars, were responsible for the stream of gamma rays.
More recently, the same team of astrophysicists reexamined their model. The researchers attempted to trick the simulation by supplying it with a false dark matter signal. The model, unable to distinguish between the pulsars and dark matter, produced the same result as before.
The new research, published this week in the journal Physical Review Letters, suggests dark matter is back in play as a viable explanation for the Milky Way center's gamma ray emissions.
"It's exciting in that we thought we had eliminated the possibility that this is dark matter," researcher Tracy Slatyer, an associate professor of physics at MIT, told MIT News. "But now there's a loophole, a systematic error in the claim we made. It reopens the door for the signal to be coming from dark matter."
Most of the Milk Way's matter is situated on a flat plane. The galaxy is shaped like a giant spiral disk. But the gamma rays emanating from the galaxy's center appear more like a giant sphere, pushing out 5,000 light-years in every direction.
The original model developed by Slatyer and her colleagues was designed to determine whether the sphere of gamma rays in the Milky Way's galactic center appeared "smooth" or "grainy."
Because pulsars are so bright, they should produce a grainy appearance, while gamma rays traced to dark matter should yield a smooth sphere.
The model they came up with was tweaked to produce both a smooth and grainy sphere. When scientists supplied the model with data collected by the Fermi telescope, they found the grainy simulation best matched Fermi's observations.
"We saw it was 100 percent grainy, and so we said, 'oh, dark matter can't do that, so it must be something else,'" Slatyer said. "My hope was that this would be just the first of many studies of the galactic center region using similar techniques. But by 2018, the main cross-checks of the method were still the ones we'd done in 2015, which made me pretty nervous that we might have missed something."
Suspecting there was something amiss with their model, Slatyer and MIT postdoc Rebecca Leane began working to undermine it -- to poke holes in it.
Leane and Slatyer decided to rerun the 2015 simulations using fake Fermi data, a fictional map of the sky featuring a made-up dark matter signal. Despite the dark matter signal, the model still spit out a grainy sphere, once again implicating pulsars as the primary source of the gamma rays in the middle of Milky Way.
The researchers decided to run their model again, this time with a fake dark matter signal hidden in real Fermi data. Scientists ran the simulation several times, each time turning up the volume of the fake dark matter signal. The model failed to distinguish between the dark matter and pulsars.
The scientists had indeed found a "mismodeling effect."
"By that stage, I was pretty excited, because I knew the implications were very big -- it meant that the dark matter explanation was back on the table," Leane said.
As usual, the latest findings, while exciting, mean there is a lot more work to do. Slatyer, Leane and their colleagues are once again working to rid their model of its bias and reexamine Fermi's observations of the mysterious glow of gamma rays.
Maybe it's been dark matter producing those gamma rays all along.
"If it's really dark matter, this would be the first evidence of dark matter interacting with visible matter through forces other than gravity," Leane said. "The nature of dark matter is one of the biggest open questions in physics at the moment. Identifying this signal as dark matter may allow us to finally expose the fundamental identity of dark matter. No matter what the excess turns out to be, we will learn something new about the universe."
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