Archive for simulation

Illustris, Cosmology, and Simulation…

Posted in The Universe and Stuff with tags , , , , , , on May 8, 2014 by telescoper

There’s been quite a lot of news coverage over the last day or two emanating from a paper just out in the journal Nature by Vogelsberger et al. which describes a set of cosmological simulations called Illustris; see for example here and here.

The excitement revolves around the fact that Illustris represents a bit of a landmark, in that it’s the first hydrodynamical simulation with sufficient dynamical range that it is able to fully resolve the formation and evolution of  individual galaxies within the cosmic web of large-scale structure.

The simulations obviously represent a tremendous piece or work; they were run on supercomputers in France, Germany, and the USA; the largest of them was run on no less than 8,192 computer cores and took 19 million CPU hours. A single state-of-the-art desktop computer would require more than 2000 years to perform this calculation!

There’s even a video to accompany it (shame about the music):

The use of the word “simulation” always makes me smile. Being a crossword nut I spend far too much time looking in dictionaries but one often finds quite amusing things there. This is how the Oxford English Dictionary defines SIMULATION:

1.

a. The action or practice of simulating, with intent to deceive; false pretence, deceitful profession.

b. Tendency to assume a form resembling that of something else; unconscious imitation.

2. A false assumption or display, a surface resemblance or imitation, of something.

3. The technique of imitating the behaviour of some situation or process (whether economic, military, mechanical, etc.) by means of a suitably analogous situation or apparatus, esp. for the purpose of study or personnel training.

So it’s only the third entry that gives the meaning intended to be conveyed by the usage in the context of cosmological simulations. This is worth bearing in mind if you prefer old-fashioned analytical theory and want to wind up a simulationist! In football, of course, you can even get sent off for simulation…

Reproducing a reasonable likeness of something in a computer is not the same as understanding it, but that is not to say that these simulations aren’t incredibly useful and powerful, not just for making lovely pictures and videos but for helping to plan large scale survey programmes that can go and map cosmological structures on the same scale. Simulations of this scale are needed to help design observational and data analysis strategies for, e.g., the  forthcoming Euclid mission.

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Cosmic Swirly Straws Feed Galaxy

Posted in The Universe and Stuff with tags , , , , , on June 5, 2013 by telescoper

I came across this video on youtube and was intrigued because the title seemed like a crossword clue (to which I couldn’t figure out the answer). It turns out that it goes with a piece in the Guardian which describes a computer simulation showing the formation of a galaxy during the first 2bn years of the Universe’s evolution. Those of us interested in cosmic structures on a larger scale than galaxies usually show such simulations in co-moving coordinates (i.e. in a box that expands at the same rate as the Universe), but this one is in physical coordinates showing the actual size of the objects therein; the galaxy is seen first to condense out of the expanding distribution of matter, but then grows by accreting matter in a complicated and rather beautiful way.

This calculation includes gravitational and hydrodynamical effects, allowing it to trace the separate behaviour of dark matter and gas (predominantly hydrogen).  You can see that this particular object forms very early on; the current age of the Universe is estimated to be about 13 – 14 billion years. When we look far into space using very big telescopes we see objects from which light has taken billion of years to reach us. We can therefore actually see galaxies as they were forming and can therefore test observationally whether they form as theory (and simulation) suggest.

Simulations and False Assumptions

Posted in The Universe and Stuff with tags , , , , on November 29, 2012 by telescoper

Just time for an afternoon quickie!

I saw this abstract by Smith et al. on the arXiv today:

Future large-scale structure surveys of the Universe will aim to constrain the cosmological model and the true nature of dark energy with unprecedented accuracy. In order for these surveys to achieve their designed goals, they will require predictions for the nonlinear matter power spectrum to sub-percent accuracy. Through the use of a large ensemble of cosmological N-body simulations, we demonstrate that if we do not understand the uncertainties associated with simulating structure formation, i.e. knowledge of the `true’ simulation parameters, and simply seek to marginalize over them, then the constraining power of such future surveys can be significantly reduced. However, for the parameters {n_s, h, Om_b, Om_m}, this effect can be largely mitigated by adding the information from a CMB experiment, like Planck. In contrast, for the amplitude of fluctuations sigma8 and the time-evolving equation of state of dark energy {w_0, w_a}, the mitigation is mild. On marginalizing over the simulation parameters, we find that the dark-energy figure of merit can be degraded by ~2. This is likely an optimistic assessment, since we do not take into account other important simulation parameters. A caveat is our assumption that the Hessian of the likelihood function does not vary significantly when moving from our adopted to the ‘true’ simulation parameter set. This paper therefore provides strong motivation for rigorous convergence testing of N-body codes to meet the future challenges of precision cosmology.

This paper asks an important question which I could paraphrase as “Do we trust N-body simulations too much?”.  The use of numerical codes in cosmology is widespread and there’s no question that they have driven the subject forward in many ways, not least because they can generate “mock” galaxy catalogues in order to help plan survey strategies. However, I’ve always worried that there is a tendency to trust these calculations too much. On the one hand there’s the question of small-scale resolution and on the other there’s the finite size of the computational volume. And there are other complications in between too. In other words, simulations are approximate. To some extent our ability to extract information from surveys will therefore be limited by the inaccuracy of our calculation of  the theoretical predictions.

Anyway,  the paper gives us quite a few things to think about and I think it might provoke a bit of discussion, which is why I mentioned it here – i.e. to encourage folk to read and give their opinions.

The use of the word “simulation” always makes me smile. Being a crossword nut I spend far too much time looking in dictionaries but one often finds quite amusing things there. This is how the Oxford English Dictionary defines SIMULATION:

1.

a. The action or practice of simulating, with intent to deceive; false pretence, deceitful profession.

b. Tendency to assume a form resembling that of something else; unconscious imitation.

2. A false assumption or display, a surface resemblance or imitation, of something.

3. The technique of imitating the behaviour of some situation or process (whether economic, military, mechanical, etc.) by means of a suitably analogous situation or apparatus, esp. for the purpose of study or personnel training.

So it’s only the third entry that gives the intended meaning. This is worth bearing in mind if you prefer old-fashioned analytical theory!

In football, of course, you can even get sent off for simulation…

Stargazing (virtually) Live

Posted in Television, The Universe and Stuff with tags , , , , , , on January 18, 2012 by telescoper

I hope you’ve all been tuning in to the BBC’s astronomy jamboree Stargazing Live. There have been two episodes so far, with one last one to follow tonight, plus a huge range of activities across the country (including Wales) giving members of the public the chance to look at the sky through telescopes. The programmes and other activities have been getting an excellent response, especially from the younger generation, which is excellent news for the future of astronomy.

Working in a School of Physics & Astronomy makes one realise just how much public interest there is in astronomy, not just among schoolkids but in the numerous amateur astronomical societies, the members of which actually know the night sky better than many professionals! Most of us astronomers and astrophysicists are regularly asked to give public lectures and Cardiff in particular runs a  host of other outreach activities related to our astronomy research. Our colleagues in mainstream physics subjects such as condensed matter physics don’t get the same level of direct public interest – I don’t think there are any amateur semiconductor physics  clubs in the UK! – but many students attracted into universities by astronomy do turn to other branches of physics when they get here, because something else catches their imagination.

But important though that role is, let’s not forget that astronomy isn’t just about outreach. It’s actually real science, making real discoveries about the way our universe works. It’s worth doing in its own right as well as being good for other branches of physics.

Anyway, being a theoretical astrophysicist I usually feel a bit left out of these stargazing actitivies because I don’t really know one end of a telescope from the other. The other day I jokingly  asked whether Stargazing Live was ever going to include a theory component…

Last night’s episode actually did, in the form of a discussion of a numerical simulation of galaxy formation between the presenters and young Dr Andrew Pontzen from Oxford University. He even made a little video about the simulation, sort of virtual reality rendition of the formation of the Milky Way, as shown on the telly:

Apparently, making this required 300,000 CPU hours on 300 processors and it is based on 16 Terabytes of raw data. Phew!

It’s a very impressive simulation, but the use of the word simulation in this context always makes me smile. Being a crossword nut I spend far too much time looking in dictionaries but one often finds quite amusing things there. This is how the Oxford English Dictionary defines SIMULATION:

1.

a. The action or practice of simulating, with intent to deceive; false pretence, deceitful profession.

b. Tendency to assume a form resembling that of something else; unconscious imitation.

2. A false assumption or display, a surface resemblance or imitation, of something.

3. The technique of imitating the behaviour of some situation or process (whether economic, military, mechanical, etc.) by means of a suitably analogous situation or apparatus, esp. for the purpose of study or personnel training.

It’s only the third entry that gives the intended meaning. This is worth bearing in mind if you prefer old-fashioned analytical theory!

In football, of course, you can get sent off for simulation…