Monthly Archives: August 2016

The invisible chaos of superluminous supernovae

Sightings of a rare breed of superluminous supernovae — stellar explosions that shine 10 to 100 times brighter than normal — are perplexing astronomers. First spotted only in last decade, scientists are confounded by the extraordinary brightness of these events and their explosion mechanisms.

To better understand the physical conditions that create superluminious supernova, astrophysicists are running two-dimensional (2D) simulations of these events using supercomputers at the Department of Energy’s National Energy Research Scientific Computing Center (NERSC) and the Lawrence Berkeley National Laboratory (Berkeley Lab) developed CASTRO code.

“This is the first time that anyone has simulated superluminous supernovae in 2D; previous studies have only modeled these events in 1D,” says Ken Chen, an astrophysicist at the National Astronomical Observatory of Japan. “By modeling the star in 2D we can capture detailed information about fluid instability and mixing that you don’t get in 1D simulations. These details are important to accurately depict the mechanisms that cause the event to be superluminous and explain their corresponding observational signatures such as light curves and spectra.”

Chen is the lead author of an Astrophysical Journal paper published in December 2016. He notes that one of the leading theories in astronomy posits that superluminous supernovae are powered by highly magnetized neutron stars, called magnetars.

How a star lives and dies depends on its mass — the more massive a star, the more gravity it wields. All stars begin their lives fusing hydrogen into helium; the energy released by this process supports the star against the crushing weight of its gravity. If a star is particularly massive it will continue to fuse helium into heavier elements like oxygen and carbon, and so on, until its core turns to nickel and iron. At this point fusion no longer releases energy and electron degeneracy pressure kicks-in and supports the star against gravitational collapse. When the core of the star exceeds its Chandrasekhar mass — approximately 1.5 solar masses — electron degeneracy no longer supports the star. At this point, the core collapses, producing neutrinos that blow up the star and create a supernova.

This iron core-collapse occurs with such extreme force that it breaks apart nickel and iron atoms, leaving behind a chaotic stew of charged particles. In this frenzied environment negatively charged electrons are shoved into positively charged positrons to create neutral neutrons. Because neutrons now make up the bulk of this core, it’s called a neutron star. A magnetar is essentially a type of neutron star with an extremely powerful magnetic field.

In addition to being insanely dense — a sugar-cube-sized amount of material from a neutron star would weigh more than 1 billion tons — it is also spinning up to a few hundred times per second. The combination of this rapid rotation, density and complicated physics in the core creates some extreme magnetic fields. The magnetic field can take out the rotational energy of a neutron star and turn this energy into energetic radiation. Some researchers believe this radiation can power a superluminous supernova. These are precisely the conditions that Chen and his colleagues are trying to understand with their simulations.

“By doing a more realistic 2D simulation of superluminous supernovae powered by magnetars, we are hoping to get a more quantitative understanding about its properties,” says Chen. “So far, astronomers have spotted less than 10 of these events; as we find more we’ll be able to see if they have consistent properties. If they do and we understand why, we’ll be able to use them as standard candles to measure distance in the Universe.”

He also notes that because stars this massive may easily form in the early cosmos, they could provide some insights into the conditions of the distant Universe.

The potential of metal grids for electronic components

Nanometer-scale magnetic perforated grids could create new possibilities for Computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole (“antidot”) three magnetic states can be configured. The results have been published in the journal “Scientific Reports.”

Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in order to program its magnetic properties. His colleagues from the National University in Singapore produced the grid using a photolithographic process similar to that currently used in chip manufacture. Approximately 250 nanometers sized holes, so-called antidots, were created at regular intervals — with interspaces of only 150 nanometers — in the cobalt layer. In order to be able to stably program it, the Singapore experts followed the Dresden design, which specified a metal layer thickness of approximately 50 nanometers.

At these dimensions the cobalt antidot grid displayed interesting properties: Dr. Bali’s team discovered that with the aid of an externally applied magnetic field three distinct magnetic states around each hole could be configured. The scientists called these states “G,” “C” and “Q.” Dr. Bali: “Antidots are now in the international research spotlight. By optimizing the antidot geometry we were able to show that the spins, or the magnetic moments of the electrons, could be reliably programmed around the holes.”

Building blocks for future logic

Since the individually programmable holes are situated in a magnetic metal layer, the grid geometry has potential use in computers that would work with spin-waves instead of electric current. “Spin-waves are similar to the so-called Mexican waves you see in a football stadium. The wave propagates through the stadium, but the individual fans, in our case the electrons, stay seated,” explains Dr. Bali. Logic chips utilizing such spin-waves would use far less power than today’s processors, because no electrical current is involved.

Predicted to win Super Bowl

“Increasingly, we are seeing NFL coaches and executives embracing analytics to improve their overall knowledge of the game and give them data-driven competitive advantages over their opponents. I believe this study is yet another step in that direction,” said Konstantinos Pelechrinis, an associate professor in Pitt’s School of Information Sciences.

Pelechrinis’s study, published in PLOS, analyzed 1,869 regular and postseason games from 2009 to 2015. Through in-depth analysis, he identified key in-game factors — turnover differential and penalty yardage, among others — that directly correlate with winning probability. The analysis found that committing one fewer turnover than the opposition presented a 20 percent gain in winning probability. A 10-yard advantage in penalty yardage correlated to a 5 percent difference.

He then used a probability model to create a Football Prediction Matchup (FPM) engine to compare teams. Pelechrinis compared the Patriots’ and the Falcons’ performances in those key in-game factors during the 2016 regular season. Finally, Pelechrinis ran 10,000 simulations of the game in order to draw his conclusion: The Atlanta Falcons have a 54 percent probability of prevailing in Super Bowl 51.

“I believe both die-hard football fans and casual viewers will be in for an exciting game this Sunday. The Patriots and the Falcons are two dynamic, high-scoring football teams that perform extraordinarily well in the key areas of the game that most impact winning,” said Pelechrinis. “However, we are confident that it will be the Atlanta Falcons walking away with that franchise’s first Vince Lombardi Trophy.”

When Pelechrinis ran his model on the 2017 NFL Playoffs, the FPM had an accuracy rate of 90 percent. Pelechrinis said the system can reliably foretell the outcomes of all NFL games with an accuracy of 63 percent. This rate is comparable to existing state-of-the-art prediction systems and outperforms expert NFL analysts more than 60 percent of the time.

In addition to predicting upcoming game matchups, an expanded version of the study explored strategic on-field decision-making. Most notably, it found that coaches are overly conservative in key situations such as fourth-down conversions and point-after-touchdown options, which reduces the team’s winning probability percentage. Pelechrinis points to fourth-down conversion attempts when deep within an opponent’s territory as a prime example of coaches being too cautious with their in-game decisions.

“When faced with, let’s say, a fourth-and-1 from an opponent’s 25-yard line, conventional football wisdom says a field-goal attempt — potentially resulting in three points — would be a coach’s best option. The research shows that continuing to pursue a touchdown — eventually resulting in six to eight points — would be best for maximizing this scoring opportunity as well as the overall goal of winning the game,” Pelechrinis said.