Astronomers depend on simulations to study the Universe. From relatively straightforward orbital simulations to vast simulations that try to recreate the large scale structure of the Universe from the Big Bang. Today we’re going to talk about some of those simulations, as well as tools you can use simulate the Universe.
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When a star encounters a black hole, tidal forces stretch the star into an elongated blob before tearing it apart, as seen in these images from a computer simulation by James Guillochon of Harvard University.
A telescope peers into the blackness of deep space. Suddenly – a brilliant flash of light appears that wasn’t there before. What could it be? A supernova? Two massively dense stars fusing together? Perhaps a gamma ray burst?
Five years ago, researchers using the ROTSE IIIb telescope at McDonald Observatory noticed just such an event. But far from being your run-of-the-mill stellar explosion or neutron star merger, the astronomers believe that this tiny flare was, in fact, evidence of a supermassive black hole at the center of a distant galaxy, tearing a star to shreds.
Astronomers at McDonald had been using the telescope to scan the skies for such nascent flashes for years, as part of the ROTSE Supernova Verification Project (SNVP). And at first blush, the event seen in early 2009, which the researches nicknamed “Dougie,” looked just like many of the other supernovae they had discovered over the course of the project. With a blazing – 22.5-magnitude absolute brightness, the event fit squarely within the class of superluminous supernovae that the researchers were already familiar with.
But as time went on and more data on Dougie rolled in, the astronomers began to change their minds. X-ray observations made by the orbiting Swift satellite and optical spectra taken by McDonald’s Hobby-Eberly Telescope revealed an evolving light curve and chemical makeup that didn’t fit with computer simulations of superluminous supernovae. Likewise, Dougie didn’t appear to be a neutron star merger, which would have reached peak luminosity far more quickly than was observed, or a gamma ray burst, which, even at an angle, would have appeared far brighter in x-ray light.
That left only one option: a so-called “tidal disruption event,” or the carnage and spaghettification that occurs when a star wanders too close to a black hole’s horizon. J. Craig Wheeler, head of the supernova group at The University of Texas at Austin and a member of the team that discovered Dougie, explained that at short distances, a black hole’s gravity exerts a much stronger pull on the side of the star nearest to it than it does on the star’s opposite side. He explained, “These especially large tides can be strong enough that you pull the star out into a noodle.”
The team refined their models of the event and came to a surprising conclusion: having drawn in Dougie’s stellar material a bit faster than it could handle, the black hole was now “choking” on its latest meal. This is due to an astrophysical principle called the Eddington Limit, which states that a black hole of a given size can only handle so much infalling material. After this limit has been reached, any additional intake of matter exerts more outward pressure than the black hole’s gravity can compensate for. This pressure increase has a kind of rebound effect, throwing off material from the black hole’s accretion disk along with heat and light. Such a burst of energy accounts for at least part of Dougie’s brightness, but also indicates that the original dying star – a star not unlike our own Sun – wasn’t going down without a fight.
Combining these observations with the mathematics of the Eddington Limit, the researchers estimated the black hole’s size to be about 1 million solar masses – a rather small black hole, at the center of a rather small galaxy, three billion light years away. Discoveries like these not only allow astronomers to better understand the physics of black holes, but also properties of their often unassuming home galaxies. After all, mused Wheeler, “Who knew this little guy had a black hole?”
To get a simulated glimpse of Dougie for yourself, check out the amazing animation below, courtesy of team member James Guillochon:
The research is published in this month’s issue of The Astrophysical Journal. A pre-print of the paper is available here.
Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. The left panel displays the continuous distribution of dark matter particles, showing the typical wispy structure of the cosmic web, with a network of sheets and filaments, while the right panel highlights the dark matter halos representing the most efficient cosmic sites for the formation of star-bursting galaxies with a minimum dark matter halo mass of 300 billion times that of the Sun. Credit: VIRGO Consortium/Alexandre Amblard/ESA
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One of the successes of the ΛCDM model of the universe is the ability for models to create structures of with scales and distributions similar to those we view in the universe today. Or, at least that’s what astronomers tell us. While computer simulations can recreate numerical universes in a box, interpreting these mathematical approximations is a challenge in and of itself. To identify the components of the simulated space, astronomers have had to develop tools to search for structure. The results has been nearly 30 independent computer programs since 1974. Each promises to reveal the forming structure in the universe by finding regions in which dark matter halos form. To test these algorithms out, a conference was arranged in Madrid, Spain during the May of 2010 entitled “Haloes going MAD” in which 18 of these codes were put to the test to see how well they stacked up.
Numerical simulations for universes, like the famous Millennium Simulation begin with nothing more than “particles”. While these were undoubtedly small on a cosmological scale, such particles represent blobs of dark matter with millions or billions solar masses. As time is run forwards, they are allowed to interact with one another following rules that coincident with our best understanding of physics and the nature of such matter. This leads to an evolving universe from which astronomers must use the complicated codes to locate the conglomerations of dark matter inside which galaxies would form.
One of the main methods such programs use is to search for small overdensities and then grow a spherical shell around it until the density falls off to a negligible factor. Most will then prune the particles within the volume that are not gravitationally bound to make sure that the detection mechanism didn’t just seize on a brief, transient clustering that will fall apart in time. Other techniques involve searching other phase spaces for particles with similar velocities all nearby (a sign that they have become bound).
To compare how each of the algorithms fared, they were put through two tests. The first, involved a series of intentionally created dark matter halos with embedded sub-halos. Since the particle distribution was intentionally placed, the output from the programs should correctly find the center and size of the halos. The second test was a full fledged universe simulation. In this, the actual distribution wouldn’t be known, but the sheer size would allow different programs to be compared on the same data set to see how similarly they interpreted a common source.
In both tests, all the finders generally performed well. In the first test, there were some discrepancies based on how different programs defined the location of the halos. Some defined it as the peak in density, while others defined it as a center of mass. When searching for sub-halos, ones that used the phase space approach seemed to be able to more reliably detect smaller formations, yet did not always detect which particles in the clump were actually bound. For the full simulation, all algorithms agreed exceptionally well. Due to the nature of the simulation, small scales weren’t well represented so the understanding of how each detect these structures was limited.
The combination of these tests did not favor one particular algorithm or method over any other. It revealed that each generally functions well with regard to one another. The ability for so many independent codes, with independent methods means that the findings are extremely robust. The knowledge they pass on about how our understanding of the universe evolves allows astronomers to make fundamental comparisons to the observable universe in order to test the such models and theories.
The results of this test have been compiled into a paper that is slated for publication in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.
The all-student crew 99 at the Mars Desert Research Station. Credit: MDRS
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Headline from the future? Actually, it’s happening now, although not quite on Mars, but about as close as humans can currently get. Six college students are the latest crew to embark on a two-week stint at the Mars Desert Research Station, a simulated Mars habitat set up by the Mars Society located in the San Rafael Swell of Utah. Looking across the very Mars-like red, rocky, panoramic vistas outside the habitat, participants might think they are on the Red Planet. And this latest crew, the 99th for MDRS, will be testing a microbial detection system and an EVA optimization method using an iPad.
The MDRS Campus in Utah, with the Habitat, observatory and greenhab.
The students — all graduate students or about to be – are from different colleges but came together in the summer of 2010 at the NASA Academy at the Ames Research Center in California, a 10-week immersive research internship.
“At the NASA Academy, we worked on a group project called LAMBDA – the Life and Microbial Detection Apparatus,” participant Max Fagin, from Dartmouth University, told Universe Today. “We wanted to do some follow-up work, in looking at microbial fuel cells, which run off the metabolic activity of bacteria — technology that could be applied to sewage reclamation plants in order to generate power.”
Fagin said the technology has been around a while, but they are trying to adapt it to detect microbes in soil samples, similar to what the Viking mission did in the 1970’s.
“We put a sample into the device and based on the power that is generated you can determine whether that power is coming from microbial activity or organic activity,” Fagin said.
They finished the summer internship with a good theoretical analysis and a non-working prototype, but wanted to field test their research, as well as continue work on other individual projects.
Crew patch for Crew LAMBDA.
Donna Viola, a senior undergraduate at the University of Maryland, Baltimore County, had been on two crew rotations on the MDRS previously and suggested to her fellow NASA Academy team that they apply as a group to the MDRS where they could test LAMBDA in actual conditions, with actual soil samples in the field where there may be potentially extremophile forms of life to find.
The team was accepted and began their crew rotation at MDRS on January 29. They will be there until February 12, all the while in complete Mars simulation. Crew members must wear a space suit when going outside the Habitat; they eat only space-travel type food (along with vegetables grown on-site in a greenhouse); power is provided by batteries or a power generation system; and there is also a water recycling system.
Viola is the Commander, Heidi Beemer is the team geologist and Executive Commander, Kevin Newman is the Engineer, Andie Gilkey is the team scientist and Health and Safety Officer, Chief Biologist, Sukrit Ranjan is the team astronomer and Fagin is the EVA Engineer.
See the crew biographies.
14 students total were part of the NASA Ames Academy, and even though only 6 are at the MDRS, the rest are serving as ground and mission support.
The last six weeks the team has been updating the LAMBDA device and making it field worthy, integrating it with the control system, and testing it.
While at MDRS, the crew has a few other projects, such as working on a proposed combination EVA planner and EVA monitor that runs on an iPad. “It monitors the astronauts’ health, vital signs, how much energy they are consuming, whether they should speed up or slow down – it’s basically an EVA optimizer,” Fagin said.
The Musk Observatory located at the Mars Desert Research Station
They will also fly a payload on a high altitude balloon that tests the feasibility of using balloon borne payloads on Mars. “There are no FAA regulations on Mars, so on Mars you could build a weather station on a balloon – such as on a 10 km tether and reel it in and out to get very nice vertical cuts of the atmospheric profiles of wind velocity and direction and dust profiles,” Fagin explained. “And also you could do astronomy by launching a small telescope. But we can’t do the tether part because they are here on Earth so we’ll be using a balloon and have to retrieve it.” They will also be flying a generic meteorological payloads and doing astronomical projects at the observatory on site, the Musk Observatory, which has a 14-inch telescope.
During their stay, the crew is required to send daily reports and dispatches from the commander, engineers, crew scientists, and journalists through the MDRS website which provides updates on the status of science experiments, updates on crew health and morale, and on the habitat and how it is faring. There is also a live webcam of different parts of the station.
MDRS is the second research station to be built by the Mars Society. The first was the Arctic station (FMARS) on Devon Island, built in 2000. Stations to be built in Europe (European Mars Analog Research Station / Euro MARS) and Australia (Australia Mars Analog Research Station / MARS Oz) are currently in the planning stages.
The goal of these analog research stations is to develop key knowledge, field tactics and equipment needed to prepare for the human exploration of Mars, testing habitat design features and tools, and to assess crew selection protocols. Utah is much warmer than Mars, the desert location is optimal because of its Mars-like terrain and appearance.
The first dispatches from the LAMBDA crew report how they are getting acclimated to the habitat and the equipment, as well as preparing for doing their actual science research.
Fagin said without the NASA Academy at Ames, this group of students wouldn’t be together at the MDRS today.
“This grew out of everything we did at the NASA Academy,” he said. “Without those experiences we would have no idea how to approach the situation, wouldn’t understand the science or engineering that needs to go into such a project, and certainly wouldn’t have the team-working abilities to do this if we hadn’t developed them while we were at the NASA Academy.”
