Re: Ten

If we assume a hamster is of the order of 10cm long, add one to each number and you're sorted.

Re: Ten

At least ten shitloads

Re: Noodly appendages

No, no , no. It's obvious. Those are highways.

Re: Noodly appendages

They are waveguides for a tachyon based communication and transport system. (The little green people who live in my garden shed told me that.)

To some degree, they are

but it simply isn't enough. There are two main observations I can think of off the top of my head that demonstrate this:

1) If all the dark matter were in the form of MACHOs they'd be absolutely everywhere. And while by definition we struggle to see MACHOs, they still have gravity, and therefore act as gravitational lenses. So we've run surveys looking for so-called "microlensing" events, when a MACHO passes directly in front of a background source and we see the source brighten and then dip again. There are certainly microlensing events out there, and there are certainly MACHOs, but they are nothing like enough to even begin to alleviate the dark matter problem.

2) We know for as sure as anything in cosmology that "baryonic" (ie standard model) matter can only possibly account for 5% of the matter in the universe. When we look out there we can't even make up 5% in stars, gas, estimates of the number of planets and brown dwarfs, white dwarfs, black holes and the like, which is the "missing baryon" problem. (Missing baryonic matter crops up relatively frequently, though.) But we also know that about 30% of the universe has to be in the form of stuff that gravitates, or else cosmology simply doesn't work. So there's not only some missing baryonic matter but an enormous raft of missing gravitating matter. That discrepency is known as "dark matter", and it may or may not have the blindest thing to do with the "dark matter" evidently needed in galaxies and galaxy clusters. (It probably does, but that's not necessary.) The final 70% is made up of something that doesn't cluster and in some manner of speaking produces "anti-gravity".

@HFG

Good answer. You're right that it no longer appears likely that dark matter can be accounted for by MACHOs. But there's a problem with the alternative of Weakly Interacting Massive Particles (WIMPs), which is - where the hell are they? They haven't shown up at CERN, in cosmic rays or in neutrino detectors, which seems odd given that it's proposed that they make up the great majority of gravitating matter.

The other possibility is that we don't understand gravitation as well as we think we do. A minority of cosmologists and physicists are working on Modified Newtonian Dynamics (MoND), which could account for the anomalous rotation of galaxies and the other problems for which dark matter is put forward as a solution.

I had the opportunity of an informal discussion of these issues with a leading British astronomer and cosmologist at a conference in Oxford earlier this year. His answer was along the lines of: I'd give the experimentalists another few years to find evidence of dark matter, failing which the theoreticians will need to start looking seriously for alternative explanations.

FWIW 'Dark energy' is even easier to explain away. It may well be that our understanding of type 1a supernovae and/or the transmission of their light across a substantial fraction of the observable universe is incomplete.

@Chris Miller

Very interesting post, and one I agree with practically everything on. Whoever your leading astronomer was, I'd agree with him, too, absolutely. The LHC may be able to turn up evidence on any supersymmetry in nature. If nature is supersymmetric, then there exists a lightest supersymmetric particle, into which eventually all other supersymmetric particles will tend to decay. The LSP itself can't decay easily, since the only possible route is into normal standard model particles. If there's an LSP, then, that *is* a dark matter. It may not be abundant enough to solve the dark matter problem either - personally I suspect that an LSP does exist and it won't be abundant enough to be the entire solution - but it's certainly part of the answer.

For the record, neutrinos are technically a dark matter, too, but they definitely cannot be "the" dark matter -- they don't cluster anything like enough to give us the observed distribution of galaxy clusters.

Otherwise, I'd agree with most of what you've said. I'm not at all convinced by MOND itself, since it's an ad-hoc, unmotivated modification of one of Newton's laws - and Milgrom himself is very happy to admit that it's nothing other than phenomenology. It cannot be applied to cosmology and even applying it on cluster scales it dies. However, it fits galaxy rotation curves so well that personally I think it's a sign that there's something interesting going on - even if it turns out to be (chiefly) particulate in origin and happens to be describable by the MOND equations. MOND is not the only modification to gravity being actively studied; a relativistic version of MOND can be described as a "scalar/vector/tensor" theory of gravity (a scalar field - in effect similar in a way to the Higg's, actually - a vector field similar in a way to an electromagnetic field, and a tensor field similar to the gravitational field of general relativity). There are other SVT theories kicking around, and enormous numbers of scalar/tensor theories starting with Brans-Dicke theories back in the 60s. There are also things like "Einstein-aether theories" which attempt to keep practically every allowed symmetry, bimetric theories which are related to SVT theories (or vice-versa, depending which you prefer) and so on. This is definitely an active field of research.

Along those lines, it's also worth noting that galaxies do not live in the Minkowski spacetime of special relativity. Instead, if we assume there is no dark matter, they live in some sort of cylindrical spacetime while if we assume there *is* dark matter they live in a complicated mix of cylindrical and spherical. While the gravitational potentials on the outskirts of a galaxy are certainly small, it's not entirely obvious that they're being defined with respect to the right background spacetime -- we almost universally assume that to be Minkowski. It's still controversial whether this has any impact. Probably not, but in principle we definitely are applying gravity wrongly.

As for dark energy, since I've worked myself on novel ways of dealing with it - in my case, chiefly from the fact that cosmology has gone about things totally the wrong way, assuming the universe to be flat and then adding ripples on, whereas the real universe is actually lumpy and we should instead reconstruct the flat background; the complicated nature of GR means that these approaches, which are equivalent in Newtonian theory, bear no resemblance to one another - I'd balk at saying it's "easier to explain away". Certainly the foundations for declaring it are relatively weak... except that observational support for the standard cosmological paradigm, assuming a dark energy and dark matter, whatever they may be, is staggeringly overwhelming. And attempts to do work like I have leads you either to make approximations that aren't quite valid (as I've done) and find nothing that acts like dark energy, or into a problem that's so complicated as to be practically intractable.

Still, you're totally right and everything absolutely must be considered - and it is being considered. My hunch is that "dark matter" is a combination of a lot of things, including that gravity does not behave as GR on large scales (probably not even on galactic scales, more likely not not on cluster scales, and probably not on cosmological scales), that we're applying it wrongly anyway, that there is at least one supersymmetric particle cluttering up the universe, that there may even be sterile neutrinos, and so forth. Problem is that at the minute the data we have is more than good enough to pin down "dark matter" but as soon as we split that in two or three parts the errors bars go shooting through the roof and you constrain practically nothing.

Re: @HFG

No problem, it's always nice to see people interested and informed. Professionalism is over-rated.

Re: @Chris Miller, HFG

"it's also worth noting that galaxies do not live in the Minkowski spacetime of special relativity."

I shall drop that into casual conversation at the earliest opportunity.

Re: @HolyFreakinGhost

I'd put more money on a sterile neutrino (and the seesaw mechanism) than an LSP. Mind you, I was betting against the Higgs, so my record is poor.

Re: @HolyFreakinGhost

To be honest I was also betting against the Higgs. Well, to some degree; I was betting against the LHC seeing it. Now I'm reliant on it being a composite Higgs to recover anything from my wager.

I'm not convinced by either sterile neutrinos or an LSP anymore - I'd prefer a sterile neutrino, but ultimately I'll sit this one out and not put my money on the line :)

Re: @Chris Miller, HFG

That's well worth doing. It might also be worth saying something like "Technically a galaxy embedded in a halo of dark matter should be modelled around a Kerr-Newman solution. However, practically, the magnetic fields are weak enough that a simple Kerr solution would be a perfectly reasonable approximation, and then it's hard to imagine a galaxy spinning rapidly enough for the angular momentum to become relativistically significant, and a Schwarzschild background should suffice."

Then force a small chuckle and say "Of course, exactly the same statements apply to the solar system!"

You'll be respected and loathed in equal measure.

Re: @Chris Miller

What do you think of the idea that dark matter might be cumulative gravity spilling over from the branes that make up quantum parallel worlds?

Re: @Chris Miller

In case you're asking me (or even if you're not ;) ), it's not a theory I'd put very much faith in - the people studying it are using it phenomenologically too. That kind of thing is based on braneworld theory, which is a classical attempt to get a rough idea of what might happen in a quantum gravitational system. In M theories, if you can read that phrase without pain, there is a class of solution called a "brane", which is a multidimensional object to which open strings (ie normal particles) are attached. Since gravity is made of closed strings it can leak off a brane. The idea is then that we live on a 3-brane -- a three-dimensional surface suspended in the 10- or 11-dimensional spacetime. So far so good, but so far as I know there is as yet no good, general way of getting from a genuine M theoretical setup down to something resembling our universe. The problem is currently just far, far too complicated even for the likes of Ed Witten. If nothing else, getting down to non-flat background spacetimes would be very tough.

So we do the next best thing: braneworld theory, where you set up a 3-dimensional brane in a world with 4 spatial dimensions (and add on a time dimension of course). There are various ways of doing this but the usual idea is to have more than one brane hanging there. Each would then be its own universe, or that would be the idea, and an idea is then that the dark matter in our universe is actually normal matter on a neighbouring brane. We can't see it because photons are bound to the brane, but we can feel the gravitational effects because the gravity can leak between the branes. As a bonus (actually, typically as the main motivator) the separation between the branes can be tuned to act like a scalar field living on the brane, and *that* can be tuned to act as a dark energy.

All nice and neat, but the problem is it's also totally ad-hoc. The setups are very artificial, the branes have to hang totally perpendicular to one another, down to some ripples on them (or else at some point they meet, and it gets... interesting where two branes meet), and the worst thing is you've only got one additional dimension.

Still, the motivation is there and genuinely of interest, and while people studying it may know it's phenomenology, it's an attempt to find some of the phenomena that a genuinely 11-dimensional universe should expect to see. As such it's even now quite an active field, although in quite a different form to ten years back when I was mainly exposed to it, and it's certainly of interest if you've no issue with adding in extra spatial dimensions.

Re: @Chris Miller

Thank you! I'm intrigued to hear that the idea also helps with dark energy as well - I fully appreciate what you say about it being insanely complicated :-)

The idea I am exploring is whether this artificial looking setup actually comes good if you consider each of the branes involved to be the "universes" in the many worlds interpretation of quantum physics. There is a paper by Page and Wootters (http://prd.aps.org/abstract/PRD/v27/i12/p2885_1) where they show that it's possible for what we consider the past and the future to be special cases of those universes, and the limitations of our brains would mean that we perceive the world as a "flicker-book" of these branes.

Also, the idea leads to a prediction - since the number of parallel worlds would be increasing all the time, dark matter (and I presume dark energy) should increase over time. I'm guessing it is possible to measure the dark matter from galaxies at different distances (i.e. times) from our own and see if there is any variation.

Since, @HolyFreakinGhost you are one of the few people I've ever had contact with who knows about this stuff, I would be grateful for any more insights...

Re: @Chris Miller

That's so far out of my field of research I'm little more than an educated layman myself. My gut feeling is that it doesn't work physically - though it's hard to explain why (well, obviously, or it would be more than a "gut feeling"). I'd query where a directionality of time comes from here, and how it might connect to the second law of thermodynamics.

Where it comes to constantly splitting branes to make new ones, I then feel that it looks like a gross, and constant, violation of every conservation law formulated, from energy to baryon number to momentum to lepton number. On a quantum level that's not necessarily important, and on a larger scale maybe they can argue it's not (they must do, to some degree; they're not idiots and they know vastly more about this than I do), but we're talking about objects on coherent scales. I don't know, it just feels wrong to me.

Yes, assuming that the model of cosmology we use is right (and, at worst, it's the best we've got at the minute and it fits observation remarkably well) we can track the amount of dark matter. It doesn't seem to be changing; at least, one of the main anchors is that at the time of nucleosynthesis when the universe was a second old or so, it was about flat, and composed of about 5% "baryons", with only trace radiation. Then at the formation of the CMB (and to track the angular scale of the ripples on the CMB up to the present day) you don't need to vary the ratio between dark matter and "baryons" through time; a constant ratio where the universe is about 25% dark matter does it fine. We've got an increasing number of galaxy surveys, such as WiggleZ and BOSS/CMASS that will hopefully be able to pin down the imprint of those same ripples on the galaxy distribution, which will let us track the abundance of dark energy/dark matter very precisely. (And then Euclid, and then SKA, will come along and blow everything out of the water.)

Possible variation will certainly be looked at.

Re: @Chris Miller

This is excellent stuff. I suddenly realised I hadn't explained about the time part of it. The idea of the Page and Wootters paper is that the whole thing (all the many worlds) is "pre-existing" (if you can call it that) and the passage of time is an artefact of the way our brains work. Like when spacetime was discovered, people worried that if everything was this pre-existing block of 4D stuff, everything would be predetermined.

With many worlds, it becomes more like a railway shunting yard that we travel through, changing points when our choices decohere. The whole railway yard already exists, but we cannot see it all at once.

I've realised now that we would need to have a way of modelling this astoundingly complicated picture before working out whether we can prove or disprove it...

I can understand why our current picture would be fine with a constant amount of dark matter, so I shall be very interested to see the results from actual measurements from the projects you mention.

Re: Higg's jokes

Popbitch have asked for their joke back...

http://www.popbitch.com/home/2012/07/05/harry-potter-and-the-beanie-baby/

Re: "Boffo"?

It's been used (albeit infrequently) for many decades to mean roughly "very good indeed"

There's also a Wodehouse character called Boffo but I don't know if that has any direct relevance.

Re: If I can inject a serious question?

It doesn't seem likely. My disclaimer is that I'm not a particle physicist and my particle physics is relatively poor, but I don't think you can get the Higgs to act in that kind of way at all. The Higgs field *can* be used to drive inflation in the early universe, and indeed Guth's first inflation models did so although to make it actually work you have to extend the standard model in a variety of ugly ways, but I think getting the Higgs (either the field or the boson) to act as a dark matter would be much trickier.

Re: If I can inject a serious question?

Correction: dark energy suspected for inflation, dark MATTER suspect for mass, excuse the confusion.

Re: If I can inject a serious question?

Basically, yes, though unless you want crazy models dark matter isn't typically a suspect for the mass of normal matter. That would be the Higg's boson.

'Normal' models of dark energy -- single-field quintessence - are basically nothing other than inflationary models with different and almost equally arbitrary potentials. There are also interesting models that make the quintessence field (and sometimes dark matter, too) be a leftover of a particularly odd inflaton. I quite like that kind of thing.

Sorry, felt like muddying the waters ;)

No,

You're just slightly more restricted in your choice of lightsabre colours.