Recreating Our Galaxy in a Supercomputer

http://www.caltech.edu/news/recreating-our-galaxy-supercomputer-51995

by chmaynard • 10 years ago

173 66 10 years ago

jessriedel• 10 years ago

For context, this question of whether vanilla dark matter models are in conflict with galactic observations has been raging for many years. (It includes both the dwarf galaxy problem mentioned in the article, and things like the "core-cusp" problem concerning the radial dark matter density distribution.) These numerical simulations are fiendishly complicated, and there's always a risk that you just keep adding new tweaks or effects until you get the results you're looking for. I'd be interested in reading a skeptical assessment from the folks on the other side of this question.

starmole• 10 years ago

Can somebody who knows about this tell me how much of the novelty in this is CS and how much physics? For example in graphics all the physics of light is well understood - it's just infeasible to compute - so all advances are in CS and how to sample better. Is this the case here? Or is it adding new models?

sillysaurus36• 10 years ago

For example in graphics all the physics of light is well understood - it's just infeasible to compute - so all advances are in CS and how to sample better.

Nitpick: The physics of light is understood. Human vision is also understood. But critically, human vision is completely overlooked in most graphical applications. It's not accurate to say that "the path toward making a computer-generated video indistinguishable from a camcorder recording is just a matter of sampling better." There will need to be a fundamental shift in the industry's thinking before realism will be achieved.

It's a fascinating topic that occupied many years of my life, though this is a bit off-topic.

Le_SDT• 10 years ago

I'm curious. What do you mean by "Human vision is completely overlooked"? Are you talking about color perception? 3d rendering?

Thanks :)

dekhn5• 10 years ago

There are scenes in Pixar's movies, rendered entirely by Renderman, that are effectively indistinguishable from a high resolution recording of a real instance of the subject.

That really was all about algorithms and sampling.

privong5• 10 years ago

Can somebody who knows about this tell me how much of the novelty in this is CS and how much physics?

It's a little bit of both. Over the past 5–8 years, some new numerical techniques ("moving mesh") have been developed (or at least first applied in astronomy) for hydrodynamic simulations[0,1]. These new techniques do a much better job of handling fluid instabilities, which can be important for properly simulating the gas in galaxies (which can have a huge range in densities and temperatures).

Also, people have developed what they call "cosmological zoom" simulations. In these simulations, you run a low-resolution (sometimes dark-matter only) simulation of a fairly large volume of the universe. Then you identify a particular dark matter halo at the end, that you think matches the type of galaxy you're interested in (say, a Milky Way type galaxy). Then you go back to the initial conditions, re-initialize it at very high resolution and include "normal matter" (baryons) and re-simulate the whole thing to study the formation of that particular dark matter halo and its associated galaxy.

Finally, the general increase in computing speed has allowed people to add in more physics to the simulations and run higher-resolution simulations. In this case, it is the addition of a physically-motivated model for energy injection into the gas in galaxies, from the ongoing star formation. This means energy from supernovae explosions and associated shocks, stellar winds, and momentum from scattering/absorption of photons by dust grains[2,3]. This more realistic set of models manages to reproduce the overall efficiency of star formation (what fraction of the gas gets turned into stars over some period of time) _and_ create a realistic temperature-density structure for the gas.

Now that people seem confident that these models can reproduce the overall star formation properties in galaxies, they are starting to see what this type of star formation / feedback model implies for the properties of galaxies. Of course, there's still lots of work to be done in making detailed tests of these models. While they reproduce the overall efficiency of star formation, we aren't yet sure if they can reproduce the shapes and sizes of galaxies. But people are working on that and we should know before too long...

[0] https://arxiv.org/abs/0901.4107

[1] https://arxiv.org/abs/1409.7395

[2] https://arxiv.org/abs/1311.2073

[3] https://arxiv.org/abs/1307.5639

Zardoz84• 10 years ago

And if these image comes from the simulation... Where is the stars bar around the core ??

adekok• 10 years ago

Can somebody who knows about this tell me how much of the novelty in this is CS and how much physics?

It's not so much a novelty as it is pushing the envelope.

The main problem with these kind of simulations is that they're O(N^2). You can't get away with modelling each star, you have to model the interaction of each star with every other star.

And then add in interstellar dust, which has non-neglible mass. And then run the simulation for 8 billion years.

There are naturally some optimizations and short-cuts to be made. :)

privong5• 10 years ago

The main problem with these kind of simulations is that they're O(N^2). You can't get away with modelling each star, you have to model the interaction of each star with every other star.

Summary of what's below: The type of simulations described in the article do not scale like O(N^2) but more like O(N log N).

A small correction here.. The gravitational force calculation for N-body simulations are only O(N^2) if they're done using direct summation and force calculation. That's only necessary if you care about the motions of individual stars. The largest simulations I'm aware of that do that use a few 10s or 100s of millions of stars. They're typically used for studying individual star clusters, the regions around supermassive black holes, and perhaps dwarf galaxies. But the simulation in this article/press release has a few 10s of billions of particles, I believe. And those particles are all much more massive than individual stars. So while there are "star particles" in the simulations, it's more accurate to think of them as a set of particles which constitute a monte carlo sampling of the stellar mass distribution.

One aspect of that is you now no longer care about the detailed trajectories of individual particles[0], instead caring about the overall representation of the mass distribution. So you don't need as high of accuracy in the force calculation. Going back to the 1980's, people have been using numerical tricks to reduce the scaling of the simulations with the particle number. This includes algorithms such as treecodes, particle-mesh codes, etc., which reduce the scaling to O(N log N) or even O(N). These are sufficient to run simulations of describing the overall gravitational field.

The scaling for hydro codes is probably more complicated, particularly for simulations which use a moving mesh.

There's a nice summary of numerical techniques for this type of work here: http://arxiv.org/abs/1412.5187

[0] One can determine if interactions between individual stars is important to a system's evolution by comparing the 2-body relaxation time to the age of the system. This is discussed on pages 8–9 of the preprint linked above.

fiatjaf• 10 years ago

Nice, you can't model a worm, but you can model a galaxy.

misja111• 10 years ago

http://www.openworm.org/

FeepingCreature• 10 years ago

Still don't have a working integrated model.

JabavuAdams2• 10 years ago

Molecules are tricky. They're obviously machines, but you're dealing with Quantum Mechanics if you go to first principles.

dekhn5• 10 years ago

Which is why, for nearly all simulations, you use Born-Oppenheimer approximation and molecular dynamics using force fields derived from QM, but without the full explicit QM treatment.

This is fine because with only rare examples (as far as modern science knows), QM treatment is not necessary for most molecular machines.

idlewords• 10 years ago

It is considerably easier to model a galaxy than a worm. Even though galaxies contain many worms.

throwaway_yy2Di• 10 years ago

Apropos, the Gaia space telescope's first data release is next week. It's an extremely large dataset of the kinematics (3d position + velocity) of Milky Way stars.

http://www.cosmos.esa.int/web/gaia/dr1

hossbeast2• 10 years ago

I'm always a bit uncomfortable when I read about findings from these simulations. To say they have "answered questions" about galactic evolution seems to strong a statement. How about, "created a more accurate model"?

distrill• 10 years ago

The article does not mention answering questions about anything. It pretty much just says that they created a more accurate model.

hossbeast2• 10 years ago

"The simulation solves a decades-old mystery"

Zenst• 10 years ago

Indeed, and with that it will hopefully expose details in models missing, after all how accurate are weather forecasting models and how quickly do those nuances propagate and yield week ahead forecasts that are totally different.

Though if they can run a model and end up billions of year later approximating aspects of today then they must be getting more right than wrong.

dredmorbius• 10 years ago

There's little difference between the two phrases you're using.

More accurate models offer stronger explanatory or predictive capabilities. They answer questions earlier models couldn't.

a_throwaway38383• 10 years ago

It might seem small, but I'd argue that the difference in phrases is pretty stark.

Saying we "have a better model of"-something leaves room for further exploration and interpretation. It's admission that there's room for different possibilities, and its not taking an /absolute/ position on something.

But saying something "has answered questions" comes off as absolute and final. Is 1+1=2? Yes. Answered. Case closed. If you're going to say that "The simulation solves a decades-old mystery", then you'd better be damn sure that your simulation is correct, your interpretations are correct, your simulation correctly reflects physical reality, etc.

VikingCoder• 10 years ago

"answered questions"

Yes. Like, "Given everything we understand, how could it be that in the physical world, there aren't more dwarf galaxies?"

And there was no answer to that question, until now.

Now they come up with a new model, and run it, and they end up with a number of dwarf galaxies that much more closely matches the physical world. No one had done that before, so we didn't know how it could be.

This has answered the question, "HOW COULD IT BE?"

The answer is, "If reality matches this new model."

a_throwaway38383• 10 years ago

Saying "it has been answered" is a strong definitive statement, though. It's rather arrogant to say "Yes, thanks to this model, we absolutely know <xyz>." Are we absolutely certain that this model is correct? Are we absolutely certain that what we can extrapolate from this model accurately applies to the physical world?

Granted, yes, questions were answered... I guess. But saying that "The simulation solves a decades-old mystery" is uncomfortably confident and counter to how scientific process works.

It's definitely catchy and clickbaity, though

mikekij• 10 years ago

It'd be pretty cool for us to discover that there are tiny virtual humans in that simulation, complete with consciousness, contemplating their own existence.

BurningFrog• 10 years ago

But what if they are wicked?

krylon• 10 years ago

That would just show how realistic the model is...

treehau5• 10 years ago

Define wicked

nsxwolf• 10 years ago

I thought the Milky Way was a barred spiral.

Sir_Cmpwn• 10 years ago

I wonder if any aliens out there have taken pictures of the real deal yet.

MarlonPro• 10 years ago

Title should be: "Creating a Model of the Known Galaxy in a Supercomputer"

DigitalPhysics• 10 years ago

I think Stephen Wolfram is right; Physics is going through a paradigm shift, albeit one that might last decades. Digital Physics is here!

https://vimeo.com/ondemand/digitalphysics/

a_throwaway38383• 10 years ago

shireboy• 10 years ago

See, this is how universes get created. https://en.wikipedia.org/wiki/Simulation_hypothesis

Recreating Our Galaxy in a Supercomputer

Related Stories