Re: Cosmic simulations that once needed supercomputers now run on a laptop

It would be wise to take notice when it is able to match our best human compute power to within an order of magnitude. Silicon already can for more esoteric problem solving in very specific domains. Machine vision is finally beginning to work effectively and cheaply.

They will soon have to find an alternative to Captcha!

How long does it take to verify that the sims are accurate?

About 13.6 billion years.

This does assumes that the complex cosmological models are accurate.

There's a Brazilian physicist who claims that standard cosmological models don't model Lorenz contraction accurately enough, and has run his own simulations (including detailed Lorentz correction corrections) that get correct galactic orbital velocity versus orbital radius results without invoking any dark matter.

The simulations have to run a lot longer than standard simulations because you can't lump big chunks of the galaxy into a single averaged-out blob.

Or what currently passes for reliability

Garbage in still gives you garbage out.

It works both ways. The rest of the universe looks shorter to moving object, and the gravitational attraction of the stationary universe is thus higher.

It's what converts static electric attraction into magnetic attraction when charged particles are moving, even when they are only moving at walking speed.

We infer the orbital velocity of stars in different parts of of our galaxy (and other adjacent galaxies) from the Doppler shift in the frequency of their line emissions.

We've been doing this since the 1920s, and Fritz Zwicky worked out that it pointed to a missing mass problem in 1933.

The problem is that this involves a lot of stars, that tempts people to over-simplify the calculations.

I'm on it. Universe.asc.

Seems to work well with a 5 ps max time step. I skipped the initial conditions solution and started all the power supplies at zero.

Since nobody has a clue about dark matter or dark energy or early galactic clustering or a bunch of other things, we have people burning up compute power on - and publishing - nonsense.

Reminds me of the climate business.

Gravitational waves show that some very fast stuff is going on out there.

The effects of g-waves all across the universe must of course be included in any meaningful model.

The climate business is real, and the climate models have been getting progressively more accurate over the past twenty years, as anthropogenic global warming has progressed from a barely detectable trend buried in the random noise to a clear trend that sticks out of the random fluctuations (which are starting to look less random). The atmosphere is still a very complicated structure, but we know quite a bit more about it now, and its a lot easier to observe than the early universe.

Nobody knows anything about dark energy or dark matter - they are just hypotheses that have been invented as possible explanations of stuff we can see going on but can't easily explain in terms of the stuff we can see.

This is a very different kind of problem from anthropogeneic global warming. You don't seem to know very much at all about either problem so they may in fact look the same to you, but it isn't wise to admit it.

What's the same about those systems is that they are both radically unstable chaotic systems where we don't even understand the basics.

Which makes modeling, umm, inexact.

The ludicrous failures of past climate predictions are always explained by "but we have better models now."

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It's an old idea - the fluid was called "the ether" and it led to the Michelson-Morely ether drift measurements, which didn't find any, leading Lorenz to invent Lorenz contraction, which Einstein generalised to special relativity and - a bit later - general relativity.

Twaddle.

Fora bubble-head like you. It doesn't explain enough to qualify as a useful theory, but if you can't construct explanations worthy of the name - and you clearly can't - this isn't going to matter to you.

Every black hole could look like a big bang to any creature trapped inside one. Our universe could be a black hole in a higher universe.

Not all that convincingly.

We certainly understand the basics of our climate. Weather is chaotic, but climate isn't.

Not as inexact as you like to think. For the last couple of million years the earth's climate has been flipping between ice ages and interglacials every hundred thousand years or so, and we now know how and why. The current interglacial was always going to be a long one, and anthropogenic global warming is now big enough to block a transition to another ice age until atmospheric C02 levels drop back close to the interglacial norm.

There aren't any "ludicrous failures of past climate predictions". The climate change denial propaganda machine mines the ludicrous misquotes of inept reporters for "climate predictions" - peer-reviewed published reports have always been much too careful to serve as misleading propaganda.

Gullible suckers like you fall for that kind of trick.

There might be but that doesn't make him right. I find it extremely unlikely that any perturbations from the Kerr metric on a galactic scale would be even remotely in the right ball park to affect galactic rotation curves to the extent that is observed. The velocities of normal stars are relatively low when compared to the speed of light.

Dark matter originally included *all* non-luminous material including but not limited to dust, gas, planet, lost biros and old furniture. It was later when it was observationally constrained by deep infrared images that it became impossible to hide missing mass as normal matter.

Cold dark matter then took on the meaning of the hidden material that is only detectable by its gravitational influence and not influenced at all by electromagnetism. I'm not wedded to it actually existing but since neutrinos do have a slight mass I'm not inclined to rule it out either.

Something must have allowed the original primordial plasma to clump sufficiently to form galaxy clusters, galaxies and stars and in simulations CDM seems to do the trick incredibly well.

I have less affinity for dark energy which is well after my time. But the constant was there in the original field equations (put there by Einstein to make a steady state universe solution work). It is ironic that it now seems that the universe is destined to tear itself apart because that particular term is very slightly non-zero.

TBH I'd prefer there to be something wrong with the early supernovae that make them different to the ones used to calibrate standard candles.

Modelling large clusters of stars in a galaxy was just about possible on

Does this mean that reducing global CO2 levels will make the next ice-age happen sooner - so that we shall have to burn even more fuel to keep warm? Shouldn't we just keep on burning fuel at the present rate and continue to stave off the ice-age?

Nothing rhetorical about any of that. And my wife and I didn't "flee" the Netherlands in 2011. We owned a flat in Nijmegen for years after we left in 2011 and came back for a month or so most years until the Covid pandemic put a stop to that in 2020.

If atmospheric CO2 levels started falling below the normal interglacial level of 270ppm, and we started getting year-round snow cover well below the Arctic circle, it might be handy to have at least some fossil carbon left to burn. The switch over from an interglacial to an ice age takes a few thousand years. and only happens in the right phase of the Milankovitch cycles

We wouldn't have to do it forever - continental drift will eventually move the land masses out of the positions where an ice age is accessible. They've only been in more or less the right places for the past two million years, and they haven't stopped moving.

The sun is also getting progressively larger as it burns hydrogen to helium, and in a billion years or so we wouldn't be able to get ice ages by moving the continents around.

It does seem unlikely that gravito-magnetism could create the right effect, but it's worth remembering that magnetism originates from Lorentz distortion of the electric field between moving charges

You do have to do the calculations very carefully to make sure that you are taking everything into account. It may be that he screwed up and got bigger effects than he should have, but the peer-review process should have caught that. People are less picky about calculations that give them the answer that they expect.

It's certainly a plausible answer, but until we find something we can observe, Occams Razor wants to cut it out.

Early supernova had a lot less metallic content than most of the ones we see today. Of course gravitational waves could be a plausible proxy for dark energy. If fusing stellar mass black hole radiate a couple of star masses worth of energy in the gravitational waves they radiate, there will be more around now than there was earlier.

But how careful were they to get the relativistic effects right? Gravito-magnetism may be well understood by a least some people today, but it isn't much talked about.

Legacy code always works, but it may not work perfectly correctly. The persistent enthusiasm for the 555 timer gives a worrying perspective on how some people operate.

CO2 is great stuff. Something like 1000 PPM would be ideal.

Maybe we live in a multiverse where the only coupling between universes is gravity.