How Might Dark Matter Interact With Ordinary Matter?

An international team of scientists that includes University of California, Riverside, physicist Hai-Bo Yu has imposed conditions on how dark matter may interact with ordinary matter — constraints that can help identify the elusive dark matter particle and detect it on Earth.

Dark matter — nonluminous material in space — is understood to constitute 85 percent of the matter in the universe. Unlike normal matter, it does not absorb, reflect, or emit light, making it difficult to detect.

Physicists are certain dark matter exists, having inferred this existence from the gravitational effect dark matter has on visible matter. What they are less certain of is how dark matter interacts with ordinary matter — or even if it does.

In the search for direct detection of dark matter, the experimental focus has been on WIMPs, or weakly interacting massive particles, the hypothetical particles thought to make up dark matter.

But Yu’s international research team invokes a different theory to challenge the WIMP paradigm: the self-interacting dark matter model, or SIDM, a well-motivated framework that can explain the full range of diversity observed in the galactic rotation curves. First proposed in 2000 by a pair of eminent astrophysicists, SIDM has regained popularity in both the particle physics and the astrophysics communities since around 2009, aided, in part, by work Yu and his collaborators did.

Yu, a theorist in the Department of Physics and Astronomy at UCR, and Yong Yang, an experimentalist at Shanghai Jiaotong University in China, co-led the team analyzing and interpreting the latest data collected in 2016 and 2017 at PandaX-II, a xenon-based dark matter direct detection experiment in China (PandaX refers to Particle and Astrophysical Xenon Detector; PandaX-II refers to the experiment). Should a dark matter particle collide with PandaX-II’s liquefied xenon, the result would be two simultaneous signals: one of photons and the other of electrons.

Yu explained that PandaX-II assumes dark matter “talks to” normal matter — that is, interacts with protons and neutrons — by means other than gravitational interaction (just gravitational interaction is not enough). The researchers then search for a signal that identifies this interaction. In addition, the PandaX-II collaboration assumes the “mediator particle,” mediating interactions between dark matter and normal matter, has far less mass than the mediator particle in the WIMP paradigm.

“The WIMP paradigm assumes this mediator particle is very heavy — 100 to 1000 times the mass of a proton — or about the mass of the dark matter particle,” Yu said. “This paradigm has dominated the field for more than 30 years. In astrophysical observations, we don’t, however, see all its predictions. The SIDM model, on the other hand, assumes the mediator particle is about 0.001 times the mass of the dark matter particle, inferred from astrophysical observations from dwarf galaxies to galaxy clusters. The presence of such a light mediator could lead to smoking-gun signatures of SIDM in dark matter direct detection, as we suggested in an earlier theory paper. Now, we believe PandaX-II, one of the world’s most sensitive direct detection experiments, is poised to validate the SIDM model when a dark matter particle is detected.”

The international team of researchers reports July 12 in Physical Review Letters the strongest limit on the interaction strength between dark matter and visible matter with a light mediator. The journal has selected the research paper as a highlight, a significant honor.

“This is a particle physics constraint on a theory that has been used to understand astrophysical properties of dark matter,” said Flip Tanedo, a dark matter expert at UCR, who was not involved in the research. “The study highlights the complementary ways in which very different experiments are needed to search for dark matter. It also shows why theoretical physics plays a critical role to translate between these different kinds of searches. The study by Hai-Bo Yu and his colleagues interprets new experimental data in terms of a framework that makes it easy to connect to other types of experiments, especially astrophysical observations, and a much broader range of theories.”

PandaX-II is located at the China Jinping Underground Laboratory, Sichuan Province, where pandas are abundant. The laboratory is the deepest underground laboratory in the world. PandaX-II had generated the largest dataset for dark matter detection when the analysis was performed. One of only three xenon-based dark matter direct detection experiments in the world, PandaX-II is one of the frontier facilities to search for extremely rare events where scientists hope to observe a dark matter particle interacting with ordinary matter and thus better understand the fundamental particle properties of dark matter.

Particle physicists’ attempts to understand dark matter have yet to yield definitive evidence for dark matter in the lab.

“The discovery of a dark matter particle interacting with ordinary matter is one of the holy grails of modern physics and represents the best hope to understand the fundamental, particle properties of dark matter,” Tanedo said.

For the past decade, Yu, a world expert on SIDM, has led an effort to bridge particle physics and cosmology by looking for ways to understand dark matter’s particle properties from astrophysical data. He and his collaborators have discovered a class of dark matter theories with a new dark force that may explain unexpected features seen in the systems across a wide range, from dwarf galaxies to galaxy clusters. More importantly, this new SIDM framework serves as a crutch for particle physicists to convert astronomical data into particle physics parameters of dark matter models. In this way, the SIDM framework is a translator for two different scientific communities to understand each other’s results.

Now with the PandaX-II experimental collaboration, Yu has shown how self-interacting dark matter theories may be distinguished at the PandaX-II experiment.

“Prior to this line of work, these types of laboratory-based dark matter experiments primarily focused on dark matter candidates that did not have self-interactions,” Tanedo said. “This work has shown how dark forces affect the laboratory signals of dark matter.”

Yu noted that this is the first direct detection result for SIDM reported by an experimental collaboration.

“With more data, we will continue to probe the dark matter interactions with a light mediator and the self-interacting nature of dark matter,” he said.

Breakthrough In The Search For Cosmic Particle Accelerators

Using an internationally organised astronomical dragnet, scientist have for the first time located a source of high-energy cosmic neutrinos, ghostly elementary particles that travel billions of light years through the universe, flying unaffected through stars, planets and entire galaxies. The joint observation campaign was triggered by a single neutrino that had been recorded by the IceCube neutrino telescope at the South Pole, on 22 September 2017. Telescopes on earth and in space were able to determine that the exotic particle had originated in a galaxy over three billion light years away, in the constellation of Orion, where a gigantic black hole serves as a natural particle accelerator. Scientists from the 18 different observatories involved are presenting their findings in the journal Science. Furthermore, a second analysis, also published in Science, shows that other neutrinos previously recorded by IceCube came from the same source.

The observation campaign, in which research scientists from Germany played a key role, is a decisive step towards solving a riddle that has been puzzling scientists for over 100 years, namely that of the precise origins of so-called cosmic rays, high-energy subatomic particles that are constantly bombarding Earth’s atmosphere. “This is a milestone for the budding field of neutrino astronomy. We are opening a new window into the high-energy universe,” says Marek Kowalski, the head of Neutrino Astronomy at DESY, a research centre of the Helmholtz Association, and a researcher at the Humboldt University in Berlin. “The concerted observational campaign using instruments located all over the globe is also a significant achievement for the field of multi-messenger astronomy, that is the investigation of cosmic objects using different messengers, such as electromagnetic radiation, gravitational waves and neutrinos.”

Messengers from the high-energy universe

One way in which scientists expect energetic neutrinos to be created is as a sort of by-product of cosmic rays, that are expected to be produced in cosmic particle accelerators, such as the vortex of matter created by supermassive black holes or exploding stars. However, unlike the electrically charged particles of cosmic rays, neutrinos are electrically neutral and therefore not deflected by cosmic magnetic fields as they travel through space, meaning that the direction from which they arrive points straight back at their actual source. Also, neutrinos are scarcely absorbed. “Observing cosmic neutrinos gives us a glimpse of processes that are opaque to electromagnetic radiation,” says Klaus Helbing from the Bergische University of Wuppertal, spokesperson for the German IceCube network.””Cosmic neutrinos are messengers from the high-energy universe.”

Demonstrating the presence of neutrinos is extremely complicated, however, because most of the ghostly particles travel right through the entire Earth without leaving a trace. Only on very rare occasions does a neutrino interact with its surroundings. It therefore takes huge detectors in order to capture at least a few of these rare reactions. For the IceCube detector, an international consortium of scientists headed by the University of Wisconsin in Madison (USA) drilled 86 holes into the Antarctic ice, each 2500 metres deep. Into these holes they lowered 5160 light sensors, spread out over a total volume of one cubic kilometre. The sensors register the tiny flashes of light that are produced during the rare neutrino interactions in the transparent ice.

Five years ago, IceCube furnished the first evidence of high-energy neutrinos from the depths of outer space. However, these neutrinos appeared to be arriving from random directions across the sky. “Up to this day, we didn’t know where they originated,” says Elisa Resconi from the Technical University of Munich, whose group contributed crucially to the findings. “Through the neutrino recorded on 22 September, we have now managed to identify a first source.”

From radio waves to gamma radiation

The energy of the neutrino in question was around 300 tera-electronvolts, more than 40 times that of the protons produced in the world’s largest particle accelerator, the Large Hadron Collider at the European accelerator facility CERN outside Geneva. Within minutes of recording the neutrino, the IceCube detector automatically alerted numerous other astronomical observatories. A large number of these then scrutinised the region in which the high-energy neutrino had originated, scanning the entire electromagnetic spectrum: from high-energy gamma- and X-rays, through visible light, to radio waves. Sure enough, they were able for the first time to assign a celestial object to the direction from which a high-energy cosmic neutrino had arrived.

“In our case, we saw an active galaxy, which is a large galaxy containing a gigantic black hole at its centre,” explains Kowalski. Huge “jets” shoot out into space at right angles to the massive vortex that sucks matter into the black hole. Astrophysicists have long suspected that these jets generate a substantial proportion of cosmic particle radiation. “Now we have found key evidence supporting this assumption,” Resconi emphasises.

The active galaxy that has now been identified is a so-called blazar, an active galaxy whose jet points precisely in our direction. Using software developed by DESY researchers, the gamma-ray satellite Fermi, operated by the US space agency NASA, had already registered a dramatic increase in the activity of this blazar, whose catalogue number is TXS 0506+056, around 22 September. Now, an earthbound gamma-ray telescope also recorded a signal from it. “In the follow-up observation of the neutrino, we were able to observe the blazar in the range of very high-energy gamma radiation too, using the MAGIC telescope system on the Canary Island La Palma,” says DESY’s Elisa Bernardini, who coordinates the MAGIC observations. “The gamma-rays are closest in energy to neutrinos and therefore play a crucial role in determining the mechanism by which the neutrinos are created.” The programme for the efficient follow-up observation of neutrinos using gamma-ray telescopes was developed by Bernardini’s group.

The NASA X-ray satellites Swift and NuSTAR also registered the eruption of the blazar, and the gamma-ray telescopes H.E.S.S., HAWC and VERITAS as well as the gamma-ray and X-ray satellites AGILE, belonging to the Italian Space Agency ASI, and Integral, belonging to the European Space Agency ESA, all took part in the follow-up observations. All in all, seven optical observatories (the ASAS-SN, Liverpool, Kanata, Kiso Schmidt, SALT and Subaru telescopes, as well as the Very Large Telescope VLT of the European Southern Observatory, ESO) observed the active galaxy, and the Karl G. Jansky Very Large Array (VLA) studied its activity in the radio spectrum. This led to a comprehensive picture of the radiation emitted by this blazar, all the way from radio waves to gamma-rays carrying up to 100 billion times as much energy.

Search in archives reveals further neutrinos

A worldwide team of scientists from all the groups involved worked flat out, conducting a complicated statistical analysis to determine whether the correlation between the neutrino and the gamma-ray observations was perhaps just a coincidence. “We calculated that the probability of it being a mere coincidence was around 1 in 1000,” explains DESY’s Anna Franckowiak, who was in charge of the statistical analysis of the various different data sets. This may not sound very large, but it is not small enough to quell the professional scepticism of physicists.

A second line of investigation rectified this. The IceCube researchers searched through their data from the past years for possible previous measurements of neutrinos coming from the direction of the blazar that had now been identified. And they did indeed find a distinct surplus of more than a dozen of the ghost particles arriving from the direction of TXS 0506+056 during the time between September 2014 and March 2015, as they are reporting in a second paper published in the same edition of Science. The likelihood of this excess being a mere statistical outlier is estimated at 1 in 5000, “a number that makes you prick up your ears,” says Christopher Wiebusch from RWTH Aachen, whose group had already noted the hint of excess neutrinos from the direction of TXS 0506+056 in an earlier analysis. “The data also allows us to make a first estimate of the neutrino flux from this source.” Together with the single event of September 2017, the IceCube data now provides the best experimental evidence to date that active galaxies are in fact sources of high-energy cosmic neutrinos.

“We now have a better understanding of what we should be looking for. This means that we can in future track down such sources more specifically,” says Elisa Resconi. And Marek Kowalski adds, “Since neutrinos are a sort of by-product of the charged particles in cosmic rays, our observation implies that active galaxies are also accelerators of cosmic ray particles. More than a century after the discovery of cosmic rays by Victor Hess in 1912, the IceCube findings have therefore for the first time located a concrete extragalactic source of these high-energy particles.”

Krakatau Volcano (Indonesia):Continuous Intense Explosions Heard 42 Km Away, Off Scale Seismicity

Our Indonesian volcano expedition leader Andi has sent us the below video of the current seismicity of Anak Krakatau: “Krakatau is going crazy …100 times explosion a day … very loud could be heard untill Carita 42 km away.

The explosive activity of the volcano continues and seems to be increasing with strombolian to vulcanian-style explosions from the summit crater.

Ash plumes can be seen on satellite imagery now, but so far have been low and do not affect air traffic significantly.

We will be visiting the island and camp 3 nights during our next Volcano Special tour from 10-15 August.

Hawaii Kilauea Volcano Update: School, Park Destroyed By Lava, Collapse Events Continue

The Kilauea volcano in Hawaii continues to cause disruptions in daily life for the people on the island. Near-daily collapse events that feel like earthquakes are rocking the island where some people have lost homes since the volcano started erupting in early May.

Lava is continuously flowing from Fissure 8 and a channel, or river, of lava has formed that leads from the fissure to the ocean. That ocean entry point filled an entire bay with lava. A map from the United States Geological Survey shows where the coastline was before it was covered with lava.

That map, which is up to date as of Thursday, also shows that while there were two entry points where the lava was entering the ocean earlier this week, there is now only one.

The channel of lava that formed from Fissure 8 was experiencing some blockages and consequent overflows earlier this week, which have since stopped. On Thursday afternoon, the caldera of the volcano, or the area above the magma pit that’s been feeding the fissures, experienced a collapse event. These events have been fairly common at Kilauea, but each one causes what feels like an earthquake on the island.

Thursday’s collapse event caused an increase in activity from Fissure 8, but the lava didn’t significantly overflow the edges of the channel due to the collapse, according to the USGS. Typically after each collapse, the seismic activity around the summit decreases for a few hours before ramping back up until the next collapse event. Hawaii Civil Defense recommends that residents “check their utility connections of electricity, water and gas after earthquakes.”

One of the most recent island locations to be overrun with lava was Ahalanui Beach Park and the Kua O Ka La Public Charter School along Highway 137, the Hawaii Tribune-Herald reported.

In addition to the danger hot flowing lava presents, there are also gases being released into the air from the lava and the summit of the volcano, as well as where the lava is meeting the ocean.

The ocean-entry point created a lava haze, or laze, plume that is made up of steam and hydrochloric acid and which can cause lung damage to anyone who gets too close. The reaction of the cold water meeting the hot lava can also produce explosive episodes that are potentially dangerous.

The volcano itself was releasing sulfur dioxide and volcanic ash and glass, which can cause irritation to the eyes, skin and lungs.

Could Gravitational Waves Reveal How Fast Our Universe Is Expanding?

Since it first exploded into existence 13.8 billion years ago, the universe has been expanding, dragging along with it hundreds of billions of galaxies and stars, much like raisins in a rapidly rising dough.

Astronomers have pointed telescopes to certain stars and other cosmic sources to measure their distance from Earth and how fast they are moving away from us — two parameters that are essential to estimating the Hubble constant, a unit of measurement that describes the rate at which the universe is expanding.

But to date, the most precise efforts have landed on very different values of the Hubble constant, offering no definitive resolution to exactly how fast the universe is growing. This information, scientists believe, could shed light on the universe’s origins, as well as its fate, and whether the cosmos will expand indefinitely or ultimately collapse.

Now scientists from MIT and Harvard University have proposed a more accurate and independent way to measure the Hubble constant, using gravitational waves emitted by a relatively rare system: a black hole-neutron star binary, a hugely energetic pairing of a spiraling black hole and a neutron star. As these objects circle in toward each other, they should produce space-shaking gravitational waves and a flash of light when they ultimately collide.

In a paper to be published July 12th in Physical Review Letters, the researchers report that the flash of light would give scientists an estimate of the system’s velocity, or how fast it is moving away from the Earth. The emitted gravitational waves, if detected on Earth, should provide an independent and precise measurement of the system’s distance. Even though black hole-neutron star binaries are incredibly rare, the researchers calculate that detecting even a few should yield the most accurate value yet for the Hubble constant and the rate of the expanding universe.

“Black hole-neutron star binaries are very complicated systems, which we know very little about,” says Salvatore Vitale, assistant professor of physics at MIT and lead author of the paper. “If we detect one, the prize is that they can potentially give a dramatic contribution to our understanding of the universe.”

Vitale’s co-author is Hsin-Yu Chen of Harvard.

Competing constants

Two independent measurements of the Hubble constant were made recently, one using NASA’s Hubble Space Telescope and another using the European Space Agency’s Planck satellite. The Hubble Space Telescope’s measurement is based on observations of a type of star known as a Cepheid variable, as well as on observations of supernovae. Both of these objects are considered “standard candles,” for their predictable pattern of brightness, which scientists can use to estimate the star’s distance and velocity.

The other type of estimate is based on observations of the fluctuations in the cosmic microwave background — the electromagnetic radiation that was left over in the immediate aftermath of the Big Bang, when the universe was still in its infancy. While the observations by both probes are extremely precise, their estimates of the Hubble constant disagree significantly.

“That’s where LIGO comes into the game,” Vitale says.

LIGO, or the Laser Interferometry Gravitational-Wave Observatory, detects gravitational waves — ripples in the Jell-O of space-time, produced by cataclysmic astrophysical phenomena.

“Gravitational waves provide a very direct and easy way of measuring the distances of their sources,” Vitale says. “What we detect with LIGO is a direct imprint of the distance to the source, without any extra analysis.”

In 2017, scientists got their first chance at estimating the Hubble constant from a gravitational-wave source, when LIGO and its Italian counterpart Virgo detected a pair of colliding neutron stars for the first time. The collision released a huge amount of gravitational waves, which researchers measured to determine the distance of the system from Earth. The merger also released a flash of light, which astronomers focused on with ground and space telescopes to determine the system’s velocity.

With both measurements, scientists calculated a new value for the Hubble constant. However, the estimate came with a relatively large uncertainty of 14 percent, much more uncertain than the values calculated using the Hubble Space Telescope and the Planck satellite.

Vitale says much of the uncertainty stems from the fact that it can be challenging to interpret a neutron star binary’s distance from Earth using the gravitational waves that this particular system gives off.

“We measure distance by looking at how ‘loud’ the gravitational wave is, meaning how clear it is in our data,” Vitale says. “If it’s very clear, you can see how loud it is, and that gives the distance. But that’s only partially true for neutron star binaries.”

That’s because these systems, which create a whirling disc of energy as two neutron stars spiral in toward each other, emit gravitational waves in an uneven fashion. The majority of gravitational waves shoot straight out from the center of the disc, while a much smaller fraction escapes out the edges. If scientists detect a “loud” gravitational wave signal, it could indicate one of two scenarios: the detected waves stemmed from the edge of a system that is very close to Earth, or the waves emanated from the center of a much further system.

“With neutron star binaries, it’s very hard to distinguish between these two situations,” Vitale says.

A new wave

In 2014, before LIGO made the first detection of gravitational waves, Vitale and his colleagues observed that a binary system composed of a black hole and a neutron star could give a more accurate distance measurement, compared with neutron star binaries. The team was investigating how accurately one could measure a black hole’s spin, given that the objects are known to spin on their axes, similarly to Earth but much more quickly.

The researchers simulated a variety of systems with black holes, including black hole-neutron star binaries and neutron star binaries. As a byproduct of this effort, the team noticed that they were able to more accurately determine the distance of black hole-neutron star binaries, compared to neutron star binaries. Vitale says this is due to the spin of the black hole around the neutron star, which can help scientists better pinpoint from where in the system the gravitational waves are emanating.

“Because of this better distance measurement, I thought that black hole-neutron star binaries could be a competitive probe for measuring the Hubble constant,” Vitale says. “Since then, a lot has happened with LIGO and the discovery of gravitational waves, and all this was put on the back burner.”

Vitale recently circled back to his original observation, and in this new paper, he set out to answer a theoretical question:

“Is the fact that every black hole-neutron star binary will give me a better distance going to compensate for the fact that potentially, there are far fewer of them in the universe than neutron star binaries?” Vitale says.

To answer this question, the team ran simulations to predict the occurrence of both types of binary systems in the universe, as well as the accuracy of their distance measurements. From their calculations, they concluded that, even if neutron binary systems outnumbered black hole-neutron star systems by 50-1, the latter would yield a Hubble constant similar in accuracy to the former.

More optimistically, if black hole-neutron star binaries were slightly more common, but still rarer than neutron star binaries, the former would produce a Hubble constant that is four times as accurate.

“So far, people have focused on binary neutron stars as a way of measuring the Hubble constant with gravitational waves,” Vitale says. “We’ve shown there is another type of gravitational wave source which so far has not been exploited as much: black holes and neutron stars spiraling together,” Vitale says. “LIGO will start taking data again in January 2019, and it will be much more sensitive, meaning we’ll be able to see objects farther away. So LIGO should see at least one black hole-neutron star binary, and as many as 25, which will help resolve the existing tension in the measurement of the Hubble constant, hopefully in the next few years.”

This research was supported, in part, by the National Science Foundation and the LIGO Laboratory.

Chris Downgraded To Tropical Storm, Still Moving Away From NC Coast

Chris has been downgraded from a hurricane to a tropical storm, the National Weather Service said Thursday.

The storm became a hurricane Tuesday as it moved further away from the coast of the Carolinas.

Even though Chris is on the move, it is still causing high rip currents at the Outer Banks.

No coastal watches or warnings are in effect.

As of 6 a.m. Thursday, the storm was 245 miles off the coast of Novia Scotia.