Computer simulations of expanding space

[nearc]

i'm looking for computer simulations of expanding space. i made a couple of very very very simple models and my results were no where near our current model of space. hence, I'm looking for work that has already been done on the subject.

 

[twofish-quant]

i'm looking for computer simulations of expanding space. i made a couple of very very very simple models and my results were no where near our current model of space. hence, I'm looking for work that has already been done on the subject.

If you just want to calculate the expansion of space, then that's easy

http://www.astro.ucla.edu/~wright/CosmoCalc.html

If you want to calculate structure formation that's a bit harder, but there are lots of bits of software

http://gyudon.as.utexas.edu/~komatsu/links.html

 

[Chronos]

Even using our most powerful supercomputers, it is incredibly difficult to model simple turbulence at present. Until we master the intricacies of that task, modeling structure formation in the universe will remain a formidible challenge.

 

[nearc]

Even using our most powerful supercomputers, it is incredibly difficult to model simple turbulence at present. Until we master the intricacies of that task, modeling structure formation in the universe will remain a formidible challenge.

quite, i figure that building an accurate model of the universe would be daunting. as a first step just a simple simulation to represent the current model of the universe. first let's be clear I'm talking about a simulation not a demonstration; the two are often confused. the balloon/penny and bread/raisin are good examples of demonstrations: visual aids to help you see the concept. however those demonstrations could be seen more as stretching than expanding.

back to the point, a simple model following simple rules laid out by our understanding of the universe should yield a simulation that mimics the general behavior of the universe? let's use a fair number of bodies [e.g. galaxies] spread randomly over an arbitrary space. then let the space in the voids between the bodies expand. we can only allow the sections of space not dominated by gravity to expand. this is where the demonstrations break down. if you have a local group of pennies/raisins then gravity should keep them from spreading apart, but with the balloon/bread they keep spreading. the expanding of space is not uniform, different sections of space are influenced by various amounts of gravity and given different frames of reference at different times. hence space does not expand all at once, so a simple mechanism to model that would be to pick a random point in space that is not too close to anybody then move all bodies directly away from it by some small amount, then repeat, repeat, repeat, etc...

so what i would like feed back on is how well you think the following model would work and if you have any suggestions:

1. random scatter of bodies [e.g. galaxies] in arbitrary space
2. pick random spot not to close to any body, move all bodies away from that point
3. repeat step #2 over and over.

 

[twofish-quant]

back to the point, a simple model following simple rules laid out by our understanding of the universe should yield a simulation that mimics the general behavior of the universe?

What people do is to use perturbation theory. First they calculate the "average" condition of the universe, and that turns out to be a really simple equation. They then write equations that describe "deviations from average", and solve those equations. As long as the deviations aren't large, you end up with results that look sensible.

There's a very large debate over whether this is the correct approach, and there are people that are convinced that when you calculate deviations that you will get "back-reaction" which will modify the averaged equation. This matters because some people think that back-reaction can explain dark energy.

1. random scatter of bodies [e.g. galaxies] in arbitrary space

You start with a homogenous gas. You'll find that gas will clump up and if you assume (and note that this is an assumption) that galaxies form in places where the density is thicker then you end up with what people see.

Also people generally don't model individual galaxies but rather model sound waves. You have gas, gas carries sound, and you model frequency responses.

If you take a watermelon in the supermarket and hit it, you can tell whether it's good or not by the thump you hear. If it doesn't make a good thump that means that there is something in it that is absorbing sound and so it's probably not a good watermelon.

That's one reason we think that dark matter exists. You thump the universe and something appears to eat the noise.

http://cmb.as.arizona.edu/~eisenste/acousticpeak/acoustic_physics.html

 

[nearc]

What people do is to use perturbation theory...You start with a homogenous gas. You'll find that gas will clump up and if you assume (and note that this is an assumption) that galaxies form in places where the density is thicker then you end up with what people see.

interesting stuff, but i am not attempting simulate the formation of the universe from scratch. rather a generalized model of present day. i just want to see if i can mimic the expansion of space. this should be a simple thing to do?