Astronomy

Dark Matter & Galaxies in Universe Sandbox

You may notice that our new galaxy model (added in Update 23, released on June 25, 2019) no longer includes those bright red dots. The dots were how we represented dark matter in the old galaxy model (pre-Update 23), but we’ve decided not to include dark matter in the new model, for a number of reasons.

Short Explanation

Here’s the TL;DR explanation of why we removed dark matter in our new galaxy model:

Dark matter is a theoretical particle proposed to explain the unexpected motion of stars in galaxies. Due to performance constraints, our simplified galaxy dynamics model can’t simulate these complex orbits, so we’ve decided to remove dark matter from our simulations for now.

If you’re looking for a more in-depth explanation, keep reading!


Left: Spiral galaxy with dark matter (pre-Update 23). Right: Spiral galaxy in Update 23.

What is dark matter?

No one knows for sure what dark matter is, or even if it exists! But a number of different observations of our universe have revealed stars and galaxies moving under the gravitational influence of more mass than we can see. This hints at the presence of some kind of matter that affects stars and other bodies via gravity, but that can’t be observed directly. This proposed “dark matter” doesn’t produce light, but it also doesn’t block it, or we would be able to see it silhouetted against brighter stars and galaxies in the background (like we can see dust in the Milky Way).

We don’t know of a type of particle that has mass but that doesn’t interact with light, but a few ideas have been proposed. It may be a new type of particle that we haven’t discovered yet, and several ongoing experiments are trying to directly detect such a particle. Some scientists argue that dark matter does not exist at all, and that the “missing mass” in astronomical observations simply indicates that our mathematical description of gravity is not yet complete.

What does this have to do with galaxies?

Spiral galaxies were one of the first examples of the missing mass problem. Astronomers discovered the problem while calculating the “rotation curve” for these galaxies: a plot of the velocity of a star orbiting in the galaxy, versus the distance of that star to the center of the galaxy. The speed at which an object orbits in space is related to the mass of everything inside its orbit, and the distance to the center of the orbit. In the Solar System, nearly all of the mass inside a planet’s orbit is made up of the mass of the Sun, so the difference in speeds of planet orbits is due mostly to their distance from the Sun. Thus, the rotation curve of planets in the Solar System starts with the high speed of Mercury’s orbit, and then drops off as you move outwards to Venus, Earth, and the rest of the planets.

But in a galaxy, most of the mass is distributed among the stars that make up the galaxy, so stars farther from the center are orbiting more mass than stars closer in. We can estimate the distribution of mass based on the stars that we see, and predict a slightly more complicated curve: First, the velocities of orbiting stars should increase as you move away from the center, as more and more mass is enclosed by the orbit. But eventually, the extra mass inside the orbit won’t be enough to make up for the increased distance from the center, and the velocities will start to decrease again. The predicted curve has a sort of hump shape, with a long, decreasing tail.

Rotation curve of the galaxy M33. The yellow and blue dots indicate the data, while the dashed line represents the curve you would expect based on the amount of visible mass in the galaxy. Instead, the velocity increases with distance, indicating that more mass is present than we can see. Credit: Mario De Leo

But when astronomers actually measure these velocities and create rotation curves of spiral galaxies, the curves don’t drop off with distance. Instead, the velocities get faster and faster as you move outwards, with stars on the outer edges moving so fast that you would expect them to fly off, pulling the galaxy apart. One explanation for this discrepancy is that some kind of unseen mass (“dark matter”) may be present in spiral galaxies, keeping those stars gravitationally bound to the galaxy despite their high speeds.

Dark matter in Universe Sandbox

Since Universe Sandbox is at its core a gravity simulator, we tried to show the influence of dark matter in our previous galaxy model. For a given galaxy, we would calculate the distribution of dark matter that we would expect based on real observations of galaxy rotation curves. Specifically, we used what’s called the Navarro-Frenk-White (NFW) profile, after the astronomers who identified the distribution. We simulated the dark matter as points of mass scattered through the galaxy, and displayed them as bright red dots (because dark matter is invisible, we wanted to make it clear that we weren’t showing what dark matter “really” looks like!).

This model would give the “right” distribution of dark matter in a galaxy, but it couldn’t reproduce the most important feature of dark matter in galaxies: the rotation curve. This is because of the way that galaxy simulation works in Universe Sandbox.

How galaxies are simulated in Universe Sandbox

In both the old and the new versions of our galaxy model, we represent the galaxy as a collection of non-attracting particles orbiting a single attracting body, the black hole at the center. Each particle represents a cloud of gas, dust, and stars, which we call a nebula. This means that to our physics engine, the nebulae have zero mass, and the only gravity in the galaxy comes from the black hole.

But wait, earlier we said that the mass in a galaxy is spread out among all the stars in the galaxy, instead of being concentrated in the center like the Solar System. Why don’t we make all the nebulae into attracting particles? This would certainly make the motion of the galaxy more accurate, but in any gravity simulator, the number of attracting particles significantly affects performance. (You can see this for yourself by opening a simulation with a lot of attracting bodies, like Earth & 50 Moons.) To make galaxies look as good as they do, we need to use hundreds or even thousands of nebulae. A simulation with a thousand attracting particles would run extremely slowly even on a very powerful gaming computer. So instead, we used a simplified model of non-attracting nebulae orbiting an attracting black hole.

In the old version of galaxies, nebulae moved on circular orbits around the black hole, and the initial structure of a galaxy, whether it was a spiral or elliptical, would quickly lose its distinctive shape. In our upgraded version, nebulae are given specific orbits to allow the galaxy to hold its shape over time. The presence of another attracting body besides the black hole will pull the galaxy out of shape. (You can watch this happen in any galaxy collision simulation, or just by adding multiple galaxies to one of your own simulations!) During the development of this upgrade, we realized that adding attracting particles to represent dark matter would make it difficult to maintain the shape of spiral and elliptical galaxies for the same reason.

Because we are using a simplified galaxy model, we can’t reproduce the galaxy rotation curves we would expect either with or without dark matter. Instead, the rotation curves for our galaxies look more like the Solar System’s: the velocities of the nebulae drop off quickly as you move outwards from the center. Since this model can’t demonstrate the major effect of dark matter in galaxies, we decided to remove it for now.

We are hoping that a future version of galaxies will use computational methods like Smoothed-Particle Hydrodynamics (SPH) that will allow us to simulate hundreds to thousands of attracting nebulae orbiting the galaxy. This even more accurate model will be able to produce realistic galaxy rotation curves, and at that point, we’ll add dark matter back in so users can see its observable effect. In the meantime, we hope you enjoy our improved, interactive galaxy model!

 


Universe Sandbox at the American Astronomical Society Conference

Super Bowl of Astronomy

In early January we gathered some of our team in Seattle, Washington to show off Universe Sandbox at the 233rd meetup of the American Astronomical Society (AAS).

We’ve attended other conferences before that focus on video games, like PAX, but AAS gave us an opportunity to show Universe Sandbox to a different crowd. If you are a researcher, educator, science journalist, or student in the world of astronomy, then AAS is the go-to conference, what some call the “Super Bowl of Astronomy.” And while the government shutdown meant that hundreds of NASA employees who planned on attending couldn’t go, there was still plenty of folk there who had never heard of Universe Sandbox and wanted to learn more.

 

Come for the Collisions, Stay for the Accurate Mass Loss

Drawing people into our booth was helped a bit by two gigantic TVs showing off some of the usual Universe Sandbox scenarios — you know the ones: Earth melting, stars exploding, moons ripping apart under massive tidal stress.

But what made many attendees stick around and talk to us was the fact that what we were showing not only looked great, but it was also based in science. Universe Sandbox: Come for the fiery collisions, stay for the accurate mass loss when Ceres makes a near pass of a white dwarf!

 

Communicating with Universe Sandbox

In talking to AAS attendees, we hoped to show the potential for using Universe Sandbox for education and visualizations. While most Universe Sandbox players know and appreciate how useful it can be as an educational tool, we want to make sure it gets used in actual classrooms. We believe Universe Sandbox makes it quick and easy to demonstrate astronomy and physics concepts with intuitive and interactive experiments. But don’t take our word for it — here’s astronomy YouTuber Scott Manley with a similar message.

And beyond the classroom, it’s just as quick and easy to use Universe Sandbox for creating visualizations for research, lectures, and articles. There are more sophisticated tools for gathering data with the accuracy needed for research, but there’s nothing quite as convenient as Universe Sandbox for then using the data to create a visual representation, as shown here with the discovery of exoplanets around our nearby star Wolf 1061.

If you’re an educator, a researcher, or are otherwise curious how you can use Universe Sandbox for science communication, please get in touch!

 


New Year, New Limits of our Solar System for New Horizons

Happy New Year!

While we celebrate one more trip of our beautiful planet around the Sun, the spacecraft New Horizons sets a record for traveling to the most distant object in our Solar System ever visited, 2014 MU69, nicknamed “Ultima Thule.” This object is currently 1 billion miles beyond Pluto, or more than 43 AU from the Sun, which means it is more than 43 times the distance between the Earth and the Sun. New Horizons is expected to make its closest approach to Ultima Thule shortly after midnight EST January 1, 2019.

Check out the flyby in Universe Sandbox:

Home > Open > New Horizons Ultima Thule Encounter in 2019

 

New Limits for New Horizons

After the record-setting 2015 flyby of Pluto and its moons, the New Horizons spacecraft continued its journey through the outer reaches of the Solar System. In that same year, NASA selected a new target for New Horizons to observe: a Kuiper Belt object discovered by the Hubble Space Telescope the year earlier, known as 2014 MU69. Unofficially named Ultima Thule in 2018 based on a public vote, this object will be the most distant ever visited by a human spacecraft (breaking the record New Horizons itself set when it flew past Pluto).

The team says it hopes to set a new target for New Horizons once it passes Ultima Thule. With plenty of remaining fuel and equipment and instruments that remain in good condition, New Horizons is all set to head toward another distant object in the Kuiper Belt, arriving sometime in the 2020s, the team said.

 

Simulation Limitations

Simulations in Universe Sandbox are not perfect representations of reality. Rather, they’re meant to provide a visual — and as a result, a more intuitive understanding — of what is happening farther away than we can see or even imagine. With that in mind, there are a couple of limitations currently in this simulation:

1 –  Trajectory

The trajectory shown is according to the NASA Jet Propulsion Laboratory’s orbital predictions as of September 2018. Additional maneuvering with thruster burns is expected, which would change the final trajectory. New Horizons will make an approach much closer than is represented in the simulation: it should pass about 3,500 km from 2014 MU69. Once actual trajectories have been recorded, we will update the simulation.

2 – Shape

Previous observations show that 2014 MU69 is likely not spherical, but rather cigar-shaped. Researchers suspect that Ultima Thule may even be two separate bodies that are either orbiting very closely as a binary or actually touching each other, which is called a contact binary. We should know more once New Horizons sends back data from its flyby! Right now, Ultima Thule is represented in Universe Sandbox as just a single, spherical body.

 

Other Far Out Objects

Update 22.1 of Universe Sandbox added three other simulations that feature very distant objects in our Solar System.

 

1 – Voyagers 1 & 2 in Interstellar Space

In November 2018, more than 40 years after its launch, and long since trips past Jupiter, Saturn, Uranus, and Neptune, the Voyager 2 probe entered interstellar space. It now joins its twin, Voyager 1, in exploring beyond our Solar System. They are expected to continue to send back data until they run out of power in 2025.

Home > Open > Voyagers 1 & 2 Start 2019 Outside the Solar System

 

2 – 2018 VG18, “Farout”

On December 17, 2018, astronomers announced the discovery of the most distant known object in the Solar System, 2018 VG18. Nicknamed “Farout” (can you guess why they chose that name?), the trans-Neptunian object is currently around 120 AU (1 AU is the distance from the Sun to the Earth) from the Sun. While this object is the most distant ever observed, there are other known objects, like Sedna and the Goblin (see below), that have orbits that take them much farther from the Sun.

Farout’s orbit shown in this simulation is a preliminary estimate; its distance means it will take years of observation before its precise orbit is known.

Home > Open > 2018 VG18: The Most Distant Object in the Solar System

 

3 – 2015 TG387, “The Goblin”

On October 1, 2018, astronomers announced the discovery of the trans-Neptunian object 2015 TG387, which they nicknamed “The Goblin.” It was observed at about 80 AU from the Sun, but because of its extremely elongated orbit, it likely travels to a distance of more than 2300 AU at its farthest point.

Home > Open > 2015 TG387: A Goblin at the Edge of the Solar System

 


The Extremes of Our Solar System | Update 21.3

Run Steam to download Update 21.3, or buy Universe Sandbox ² via our website or the Steam Store.

This is a small update that features a new Parker Solar Probe model and new simulations exploring extremes in our Solar System:

Skim past the Sun with the Parker Solar Probe. The probe was launched in August and now has 24 trips around the Sun planned for its 7-year mission. Each year its orbit will take it closer to the Sun as its instruments capture data that will help us better understand our resident star. Its closest approach will bring it within 8.86 solar radii, or 3.83 million miles, of the Sun’s surface, more than 7 times closer than any previous spacecraft.

Home > Open > The Parker Solar Probe
Home > Open > The Parker Solar Probe’s Closest Approach to the Sun
Add > Objects > Parker Solar Probe

And ride along with New Horizons as it continues through the far reaches of our Solar System past Ultima Thule. After the probe’s flyby of Pluto and its moons in 2015, NASA selected the Kuiper Belt object Ultima Thule as its next target. When New Horizons makes its closest approach on January 1, 2019, Ultima Thule will become the farthest object ever visited by a spacecraft.  

Home > Open > New Horizons Ultima Thule Encounter in 2019

 

Plus: what if our Sun was replaced with one of the largest known stars in the universe, the red supergiant Betelgeuse?

Home > Open > Solar System with Betelgeuse Instead of the Sun

This update also includes an improvement to the accuracy of positions for moons and other objects in the Solar System Now & Real Time simulation, plus a few other smaller improvements and bug fixes.

 

Check out a full list of What’s New in Update 21.3

Please report any issues on our forums (local forum | Steam forum) or in-game via Home > Send Feedback.


See Asteroid 2012 TC4’s Trip Past Earth


 
On October 12, 2017, asteroid 2012 TC4 passed Earth early in the morning, flying above Antarctica at around 1:42am EDT.
 
See its close approach in Universe Sandbox ²:
Home > Open > Historical > 2012 TC4 Passes Earth on October 12, 2017
 
The roughly house-sized asteroid passed just a bit beyond the orbits of communications satellites, well within the Moon’s orbit. This wasn’t close enough to pose any threat, but its orbit was slightly changed by Earth’s gravity, as you can see in the GIF above from Universe Sandbox ². Learn more on NASA’s website
 
For the latest Universe Sandbox ² news, follow us on Twitter and Facebook.
 

Cassini’s Limitations in Universe Sandbox ²

It’s been two weeks now since we said goodbye to the Cassini spacecraft. After its 20-year-long mission in space, it was running low on fuel, so NASA directed it toward Saturn in order to eliminate the risk of it contaminating the moons. On September 15, 2017,  Cassini sent its final image back to Earth before disintegrating in Saturn’s atmosphere.

To commemorate this event, we released Ciao, Cassini | Update 20.2 which featured a simulation of Cassini’s final hours and collision with Saturn.

See Cassini’s final hours in Universe Sandbox ²:

Home > Open > Core/Historical > Cassini collision with Saturn on September 15, 2017
Home > Tutorials > Science > What Is Cassini’s Grand Finale?

Simulation Limitations

We had initially been wary of including a simulation of this event, as there are several limitations in Universe Sandbox ² that reduce the realism of this particular simulation. In the end, however, we decided it was still an informative and interesting visualization of an event that is very important to science and the world.

But instead of brushing these limitations under the rug, we’d like to take a closer look at them.

1 – Atmospheric Entry

We love collisions in Universe Sandbox ². But just because an object is on a collision course with a planet, that doesn’t mean it should actually collide with that planet in every scenario. If it’s small enough, then it should instead disintegrate in the planet’s atmosphere. This is what happened to Cassini as it entered Saturn’s atmosphere.

Universe Sandbox ² doesn’t yet simulate the forces associated with atmospheric entry, like atmospheric drag and aerodynamic heating. So instead, small objects will simply collide with planets. We’re very interested in simulating these effects in Universe Sandbox ², not just for objects like Cassini, but also for meteoroids that should burn up in an atmosphere, creating meteors or “shooting stars.”

Meteoroid meteor meteorite.gif
An animation from Wikipedia of a meteoroid losing material as it enters Earth’s atmosphere

 

2 – Rocket Engines & Spacecraft Flight

Cassini didn’t stay in orbit around Saturn for 13 years, weaving through its rings and doing close flybys of its moons, all by the grace of gravity. No, its trajectory was carefully planned by engineers and frequently adjusted with its rocket engines. But while Universe Sandbox ² has models of various spacecraft, it doesn’t yet include any way to simulate or control propellant. You can manually change an object’s velocity, but you can’t apply a force to it in the same way that a rocket engine would.

We’re still a while away from controlling thrust on spacecraft and flying them through simulations in Universe Sandbox ², but we’re taking steps in that direction with our work on rigid body physics. This new tech will better simulate the physics of smaller-scale objects, such as spacecraft, allowing for precise collisions and parts that can be attached with breakable joints and other constraints.

 

3 – Minimum Size for Impact Visuals

When the Shoemaker–Levy 9 fragments collided with Jupiter, observers from Earth could see marks on Jupiter that were more easily visible than the famous Great Red Spot. Some of these fragments were larger than one kilometer in diameter. The Cassini spacecraft, on the other hand, was only about 6.8 meters high and 4 meters wide. Its disintegration in Saturn’s atmosphere should not be easily visible, and even if it were to collide with Saturn instead of disintegrating, it should not leave a large impact mark. [Our Cassini update added a quick fix for this problem to disable an impact mark for its collision.]

Unfortunately, in Universe Sandbox ², impact marks have a minimum visual size. This minimum size is much larger than it should be for small asteroids and human-sized objects. The alternative is to not show any impact mark at all, but that can also be unsatisfactory. There are technical reasons for this limitation, but let’s not worry about those, because there’s good news…

We are currently working on a new system which will allow for impact marks and craters of any size, no more minimum. And on top of that, the new system will be a lot faster, too, and scale to the system resolution, which should help lower-end hardware a lot.


Work-in-progress screenshot of the new impact visuals in Universe Sandbox ²/span>

Opportunities, Not Limitations

The first rule of self-criticism (which we didn’t follow above) is that it’s necessary to throw things in a positive light. Drawbacks, weaknesses, limitations — they’re all opportunities for improvement! Often times these simulations of historical events are great tests of the simulation. When we put in the known properties and velocities for a set of bodies, does the simulation play out similarly to how it did in observed reality? If something is different, how can we improve the simulation in order to achieve the same results?  But if instead it’s remarkably similar, as it sometimes is, then we can give ourselves a nice pat on the back and sit back and relax… Until we load a different simulation and notice another opportunity for improvement. We often say that building a universe simulator is never a completed job. A big chunk of that lies within our commitment to continuously notice where we can improve, and then take the necessary steps to make these improvements.

The above items are all on our to-do list, though we can’t promise when they will be added to Universe Sandbox ². Learn more about what we’re working on in our 2017 Roadmap.

For the latest Universe Sandbox ² news, follow us on Twitter and Facebook.

Ciao, Cassini | Update 20.2

Run Steam to download Update 20.2, or buy Universe Sandbox ² via our website or the Steam Store.

November 3: Updates 20.2.3 and 20.2.4 are small updates which add two sims from the latest Vsauce video as well as a few improvements and bug fixes.

November 2: Update 20.2.2 is a minor update which includes a number of improvements and bug fixes.

October 5: Update 20.2.1 a minor update which adds a Cassini spacecraft model, Quicksave & Quickload (F5 & F9), and a number of smaller improvements and fixes.

 

On September 15, 2017, the world says goodbye to the Cassini spacecraft as it ends its historic mission with a final plunge into Saturn.

During its 13 years orbiting Saturn, Cassini made a number of invaluable discoveries about the planet, its rings, and its moons.

We now know that massive geysers covering the south polar region of the moon Enceladus shoot icy particles into space, forming most of Saturn’s E-ring and hinting at a massive, subsurface ocean. And we now know that the surface of Titan, Saturn’s biggest moon, shares a surprising number of characteristics with Earth, including dunes, mountain ranges, rivers, lakes, and seas.

With its array of sophisticated instruments, Cassini watched as storms raged on Saturn and seasons changed. It discovered eight new moons, provided insight into the behavior of its famous rings, and completely changed our understanding of its magnetosphere. The mission has been extended twice and now Cassini has been in orbit nine years longer than originally intended. Its fuel is nearly gone and so, to prevent possible contamination of any of the moons, its course has intentionally been set for disintegration in Saturn’s atmosphere.

So now we say ciao, Cassini. Thanks for all of your work.

And thanks to all of the hard-working scientists from NASA, the European Space Agency, and the Italian Space Agency who made the Cassini mission possible.

See Cassini’s final hours in Universe Sandbox ²

Home > Open > Core/Historical > Cassini collision with Saturn on September 15, 2017
Home > Tutorials > Science > What Is Cassini’s Grand Finale?

This update also includes a number of smaller improvements and bug fixes.

Check out the full list of What’s New.

 

For the latest Universe Sandbox ² news, follow us on Twitter and Facebook.

Science Works. Science Helps. Science Matters.


Observation. Hypothesis. Prediction. Experiment. Refine. Begin again.

Science is neither truth nor faith. Science is the process by which we reject or refine testable theories. These theories explain and predict the rules and processes that govern the behavior of the natural universe. Science doesn’t find universal objective truth; it narrows the error bars of our understanding. By its very definition, the scientific method works: if it is not reproducible, if it is not predictive, or if evidence rules it out, then it is rejected by science. And if it isn’t testable, then it isn’t in the realm of science (instead I would argue it is– and should remain– personal).

Science solves problems, and it solves them efficiently. Science makes us healthier, safer, more comfortable, and better at solving the problems of our daily lives. Applying the rigor of science to any decisions or areas of understanding that affect the lives of others can only serve to benefit lives and minds (for major decisions, not for when a friend wants you to choose the restaurant). Observation, hypothesis, prediction, experiment, analysis, adjust, rethink, repeat. We use the scientific method to make better meals, we can trust it to pick our diets, we can use the method to choose the best products, or to determine the best route to work.

Science and skepticism go hand in hand. We build our understanding of the world based on our observations of it, but also by the input of others. We can understand logical fallacies, accept new data, and test our assumptions against that data. In doing so, we can constantly refine and adapt our worldviews, and we can grow as people.

Science is not a political issue. The beauty of science is that it has to be reproducible and predictive. That means you don’t just have to believe what you are told. You can check for yourself! Some things might require expensive labs to verify, but if you, say, thought the world was flat, well you can check that!

Universe Sandbox ² is a live simulation that takes our understanding of the motion of objects and uses it to decide where each body will move as we step through simulation time. This matches closely to reality (at reasonable time steps, for non-relativistic situations) because science is reproductive and predictive. Society’s current understanding of physics allows us to send missions like Rosetta, or Juno, or New Horizons, billions of miles away to planned locations with an error equivalent to “throwing an object from New York and having it hit a particular key on a keyboard in San Francisco.”

Because this is how science works. Ignoring actionable, well-established scientific predictions is unconscionable. It’s plugging your ears and going “la la la” when someone tells you there’s an atrocity happening right behind your back, an atrocity that you have the power to stop. Not only can you turn your head and easily verify that the person is speaking the truth, but you can even do something to help, and instead you choose not to. Our choice cannot be to ignore this. Our choice matters. So today, I march for Science.

To very loosely quote Hank Green:
Science increases the Awesome and decreases the Suck in our world, and for that reason, I will always love it.

A note: I don’t want people confusing scientific institutions and cultures with the method itself. It is important to acknowledge the biases and failings of our scientific institutions historically and at present, especially with regard to equality and intersectionality, but let us not convolute science with academia or STEM institutions.

Celebrating Our Rare and Special Planet on Earth Day

Universe-Sandbox-²-Earth Day

I hear a lot of people say that looking at our universe from a solar system, galactic, or cosmological scale makes them feel small and insignificant. Insignificant to who and what? I look at our universe and I see the incredible complexity that has arisen from a few forces and a few fundamental laws. You see it in the structure of the cosmic background, you see it in the shapes of galaxies, you see it in the heavy elements formed in our stars. Nowhere do you see that complexity more than when you look at life. It was a glorious accident that allowed us to exist, and we exist in an incredibly delicate balance.

99.99999999…% of the universe is empty space. 99.9999999…% of that which is not empty space is lifeless. Why does it matter if somewhere light years away something doesn’t care about your individual existence?  We have this amazing planet and it might not be unique in the universe in its complexity, but it is incredibly rare and special, and I feel very significant — overwhelmed with a sense of purpose and duty, in fact — because it is up to us to try as hard as we can to preserve this rare and special place and all the life on it.

The structure of the universe doesn’t make me feel insignificant. It makes me feel incredibly lucky. By some chance I exist in this rare place. Thus it is my duty to try to keep life existing here, and to try to make that life as pleasant as possible for the other rare and lucky creatures who share it with me.

– Jenn Seiler, astrophysicist and Universe Sandbox ² developer.

 

Learn how we simulate Earth’s climate and how you can explore it in Universe Sandbox ²:

 

Gravitational Waves & Universe Sandbox ²

A black hole in Universe Sandbox ². Researchers have concluded the gravitational waves they detected were the result of two black holes colliding.

A black hole in Universe Sandbox ². Researchers concluded the detected gravitational waves resulted from two black holes colliding.

What’s the significance of discovering gravitational waves?

This announcement is a huge deal.  It is on par with the discovery of the Higgs Boson particle which provided the missing evidence for a prediction of the Standard Model of particle physics. Gravitational waves are a century-old (almost exactly) prediction now confirmed by a huge number of relentless, and brilliant people after many years of hard work. It is the first direct confirmation of the prediction from Einstein’s General Relativity that matter and energy determine the motion of bodies by warping the fabric of spacetime itself, and in so doing, emanate ripples when massive bodies are accelerated through that space.

It is not only confirmation of general relativity, though. It is also the first of many future observations that will look at the universe in a completely new way. Up until now we’ve used only photons (telescopes all along the electromagnetic spectrum) and sometimes neutrinos. Now we can add listening to the fabric of space to our list of tools. This will allow us to see the dark and the obscured parts of the universe: the early universe, centers of galaxies, things blocked by dust clouds, and so on, by listening for changes in space itself. It is the start of a new age in astronomy.

In addition to this detection being the first direct proof that the predictions of general relativity that matter and energy warp space time are true, and some of the strongest evidence for the reality of black holes, this is also a new kind of astronomy.  Though gravity is the weakest force and gravitational waves are very hard to detect, they do have a few advantages over observations of photons.

  • First, gravitational waves are practically impervious to matter in their path. This means we can see into regions of space that are blocked to optical observatories, such as inside dense clouds of dust, the centers of galaxies, behind large or close bodies.
  • Second, this is an observation of the warping of space itself, meaning we can detect things that have mass but might not produce observable light, such as black holes, dense sources of dark matter (if such were to exist), cosmic string breaks, etc. 
  • Third, gravitational waves fall off in amplitude much more slowly than light. This means that we can receive signals from very far away that we might not notice optically.
  •  And fourth, because gravitational waves also travel at the speed of light and don’t have to bounce off intervening matter, and begin to be potentially detectable from bodies getting close rather than just after the moment of collision, this means that we can work with other telescopes and tell them “Look over there! You’re probably going to see something exciting!”

This all of means that this detection means the beginning of a new kind of observational astronomy, as well as a better understanding of of of the fundamental forces of the universe, gravity.

 

What role did Jenn, astrophysicist and Universe Sandbox ² developer, play in the discovery?

While I was in the field I ran super-computer simulations to make predictions about the gravitational wave signals that would be produced by binary black hole mergers. Those waveforms are used as templates in the detector pipeline. The detector matches the template banks against the incoming data to find real signals amidst the noise of the detector, while also doing searches for large burst signals (how this one was found). Those waveforms are then used again to determine where the signal came from, what it was (two black holes, a neutron star and a black hole, two neutron stars, etc), and the properties of the bodies that created the signal (spins, masses, separation, etc.). I also worked on developing the analytical formulas to determine those spins and masses from those signals.

Here’s one of the scientific papers on the process of determining the properties of the source of the signal, with three papers cited on which Jenn Seiler was an author:

https://dcc.ligo.org/public/0122/P1500218/012/GW150914_parameter_estimation_v13.pdf

The Einstein equations for general relativity are ten highly non-linear partial differential equations. This means that it is only possible to obtain exact solutions for astrophysical situations for some very idealized conditions (such as spherical symmetry and a single body). In order to predict the gravitational waveforms produced by compact multi-body systems, or stellar collapse, it is necessary to solve the equations numerically (computationally). This means formulating initial data for spacetimes of interest (such as two in-spiralling black holes of various spins and mass ratios) and evolving them by integrating the solutions of the Einstein equations stepping forward in time by discrete steps. To prove that these computer simulations approximate reality more than just by equations on paper we would run these simulations at multiple resolutions for our discrete spacetimes and show that our solutions converged to a single solution as we approach infinite resolution (that would represent real continuous space) at the rate we expect for the method we were using. 

There were many obstacles in creating these simulations: vast amounts of computational power required for accuracy; the fact that we needed to run tons of these large, slow, computationally intensive simulations in order to cover the parameter space (spins, masses, orientations, etc) of potential sources of gravitational waves; and so on. For black holes, one major challenge was the fact that they contain a singularity. A singularity means an infinity, and computers don’t like to simulate infinities. Numerical relativity researchers had to find a way to simulate black holes without having the singularity point in the slicing of the spacetime integrated in the simulation. The first successful simulation of this kind didn’t happen until 2005 (http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.95.121101).

Once we had working simulations, groups around the world set to work on simulating the gamut of major potential gravitational wave signal sources. These simulation results were not just useful to the detectors to help identify signals, but also to the theorists to help formulate predictions about the results of such astrophysical events. Predictions such as: the resulting velocity of merged black holes from binaries of various spins, the amount of energy released by black hole mergers, the effect that black hole spins have on the spins and orbits of other bodies, etc.

When will you add gravitational waves into Universe Sandbox ²?

We really can’t do gravitational waves in an n-body simulation, which is the method Universe Sandbox ² uses to simulate gravity. N-body simulations look at the effect that each body has on each other body in a system at small discrete time steps.

General relativity requires simulating the spacetime itself. That is, taking your simulation space, discretizing it to a hi-res 3-D grid and checking the effect that each and every point in that grid has on all neighboring points at every timestep. Instead of simulating N number of bodies, you are simulating a huge number of points. You start with some initial data of the shape of your spacetime and then see how it evolves according to the Einstein equations, which are 10 highly non-linear partial differential equations. Accurate general relativity simulations require supercomputers.

There are some effects and features related to relativity that would be possible to add to Universe Sandbox ², however. Here are a few we are discussing:

  • Gravity travelling at the speed of light.

    • Currently if you delete a body in a simulation, the paths of all other bodies instantly respond to the change. The reality is that it would not be instantaneous; it would take time for that information about the altered gravitational landscape to reach a distant object.

  • Spinning black holes.

    • Most black holes are very highly spinning. If you imagine a spinning star collapsing it is easy to understand why. This is the same effect as when a spinning figure skater pulls in their arms; because of conservation of angular momentum, they spin faster. A consequence of this spin is that, while the event horizon would remain spherical, there would be an oblate spheroid (squished ball) around the black hole called an ergosphere. This ergosphere twists up the spacetime contained within it and accelerates bodies that enter this region (as well as affecting their spins). Because it is outside of the event horizon, this means one can slingshot away from this region and even steal energy from the rotation of that black hole.

  • Corrections to the motions of bodies to approximate general relativity.

    • Loss of momentum due to the emission of gravitational waves causes close massive bodies to inspiral. With this you could recreate the decaying orbits of binary pulsars.

    • Spins of close bodies affect each other’s motion and spins (see above). This would give you things like spun up accretion disks around black holes.

    • These corrections would be made by adding post-newtonian corrections to body velocities.

 

Learn more

Bad Astronomy article: LIGO Sees First Ever Gravitational Waves as Two Black Holes Eat Each Other

Video and Comic Explaining Gravitational Waves

Reddit AMA (Ask Me Anything) by LIGO Scientists

Paper by LIGO Researchers: Observation of Gravitational Waves from a Binary Black Hole Merger