# Gravitational lensing and the indirect observation of dark matter

Can gravitational lensing be explained in laymen’s terms? The Wikipedia article has a disclaimer that the topic is in need of an expert, and so I don’t know whether I can trust it fully. What I’m trying to understand is exactly how gravitational lensing was used to draw inferences about dark matter while observing the Bullet Cluster.

My (possibly inaccurate) understanding is that the light matter passing through was affected by other light matter, while the dark matter passed through unaffected. But affected by what, gravity? I thought that dark matter was just as affected by gravity as light matter, so why didn’t the dark matter behave just like the light matter?

Also, from the animations of this event, it doesn’t look to me like the dark matter is that much more volume than the light matter. But isn’t there supposed to be much more dark than light?

OK, I’ll give it a try to explain lensing in layman’s terms.

Essentially, light wants to be able to travel the shortest point between two points. So, for a flat sheet of paper this is a straight line; for the surface of a sphere its an arc of a circle whose centre is co-incident with the centre of the sphere (this is known as a great circle of a sphere).

One way to think about space is that its like a big stiff rubber sheet that light is confined to the surface of, and that gets deformed by masses being placed on it. If there were no masses between two points A and B, light emitted from A would travel in a straight line to B.

Now, if you stick a mass down somewhere between A and B, the shortest route between A and B is no longer a straight line, but a curve. Now, if the mass that you’d put down was a perfect sphere, then light travelling on either side of the mass would have the same travel time and you’d just see a magnified version of the light emitted from A. However, mass distributions of the size required to cause lensing rarely are spherically symmetric but tend to be distorted; thus light travelling around one side of the mass takes a different amount of time to get to us than light travelling a different route around the mass. So, we end up with distorted images of the original light source.

Now for the Bullet Cluster. A galaxy cluster consists of three main components; dark matter, galaxies and hot gas. The dark matter cannot be observed directly as it doesn’t interact in any way other than gravity, but provides the ‘potential’ that the gas and galaxies move around in. Kind of like the way the Earth provides gravity to keep the atmosphere here and to keep us on the ground. The dark matter also makes up most of the mass of a cluster.

The hot gas and galaxies, whilst they are part of the cluster, are much freer to be disturbed by other objects etc – its easier to push these things around rather than the dark matter because there is much less of it.

So, with the Bullet Cluster, what we believe has happened is that two clusters have collided, one being less massive than the other. When the clusters collided, the gas from the two clusters basically interacted with each other, heated up and essentially was greatly disturbed by the collision, because its lighter. The dark matter of the two clusters on the other hand ended up passing through each other, distorting each other gravitationally but other than that not doing much else to each other.

How does gravitational lensing fit into this then? Well, we can observe the distorted images of galaxies that are behind the bullet cluster, and using the positions of the images and the time delays between different images (i.e. looking at one image and seeing something happen and seeing how long it takes before it happens in another image), we can reconstruct where the mass that’s causing the lensing actually is. Now, because lensing is essentially a gravitational process, this shows us where the mass, i.e. the dark matter is (see above about how most of the mass in a cluster comes from the dark matter). Then, looking at the X-ray, we can see where the hot gas is, and use everything together to make inferences about the cluster.

Does this help clear things up? I can write more later, but right now, I’m supposed to be in a galaxy clusters workshop so I have to dash.

On lensing… When light passes through a lens, say a magnifying glass held by a mean kid burning an ant, light has its path bent so the previously parallel beam converges.

Light is affected by gravity, when light passes by something with mass, it will have its path bent in a similar fashion, only a tiny amount that is best viewed across vast distances.

Imagine it’s a solar eclipse, you can see the stars, a star that would normally just barely be concealed by the edge of the sun will be visible because the light would be bent inward, curving around the sun.

Angua, nice explanation.

About 20 years ago someone had a paper in American Journal of Physics where they tried to make an optical analogue to a gravitational lens. The result was interesting – it looked like the stem and base of a wineglass, rather than like a conventional lens. They made one out of glass and used it to create three “images” of a distant source, just as the real gravitational lens seemed to be doing.

This seems to be it: Gravitational Lens by J. Higbie

American Journal of Physics – July 1981 – Volume 49, Issue 7, pp. 652-655

Somewhat. The explanation of gravitational lensing itself was very good. But I’m still flummoxed by this:

The hot gas and galaxies, whilst they are part of the cluster, are much freer to be disturbed by other objects etc – its easier to push these things around rather than the dark matter because there is much less of it.Doesn’t that imply that dark matter is fundamentally different from light matter? What is it exactly, neutrons without electron clouds? And even if it is different compositively, wouldn’t it follow the same laws of gravity? In other words, why isn’t it just a part of the general mush? Why does it stay to itself?

Until Angua comes back…

Nobody knows what dark matter is. It is the source of much speculation and controversy.

Whatever it is, it doesn’t clump. It has no electromagnetic attraction to other particles, which is what helps matter stay together. It’s why you can’t put your hand through the top of a table.

That’s why one hot topic for speculation is massed neutrinos. Neutrinos are neutral particles and don’t interact very well with regular matter. They, in the famous phrase, could pass through a sheet of lead a light year thick without stopping. But any candidate for dark matter must have such properties.

Just a stub, so to speak, until others fill in the details.

I’m not sure how much more detail anyone knows, at the moment. Observationally, we can say how much dark matter there is and how it’s distributed, and theoretically, we can make a lot of wild guesses about what it might be, but there are about as many of those guesses as there are particle physicists, and none of them are all that good.

We can at least say that it’s not neutrons, since we do have a fairly good understanding of those, and free neutrons (i.e., not part of a nucleus) have a half-life of only about 15 minutes or so. Of known, experimentally-verified particles, neutrinos are the only real contender: Everything else is either unstable (so it wouldn’t last the lifetime of the Universe), massless, or charged, so it’d interact with light.

If dark matter doesn’t “clump”, then are the illustrations and animations of the Bullet Cluster sort of just exaggerations for effect? They clearly show the dark matter clumping to the far side of the regular matter in both directions.

And does it sort of hang around regular matter? Or is empty space filled with it? If the former, why? Is it the mutal gravity of dark and regular matter?

Finally (for now) is it possible that it isn’t matter at all? How is matter defined for cosmological physics?

I’ll pass over the “clumping”, since that’s an informal term, and I’m not sure precisely how it’s being used, here. But dark matter does gravitationally interact with normal matter (and with other dark matter), which will cause it to “hang around” regular matter. Like ordinary matter, it is very sparse far from galaxies. In most current models, it’s actually the dark matter which attracts the normal matter to it to form the galaxies, not the other way around.

And cosmologically, “matter” is anything which has a positive energy density and pressure, or more specifically, anything which has energy density much greater than its pressure (which meaning is meant depends on context). Dark matter definitely meets the first criterion, and according to current observations, probably meets the second one, too, so it’s definitely matter (not that that tells us much). For comparison, light (or extremely hot matter) has positive pressure comparable to its energy density, and dark energy (whatever that is; we’re even more clueless on that score than on dark matter) has negative pressure comparable in magnitude to its energy density.

But the problem with neutrinos is that first of all, they’re very light, and therefore there has to be a hell of a lot of them, yet we don’t detect this massive flux of particles (which are, admittedly, very difficult to detect since they interact so weakly with normal matter), and second that neutrinos formed by any known reaction are moving a very high speeds and would escape “small” structures like galaxies and the like; in fact, we wouldn’t expect to see much clumping of neutrino matter on any scale smaller than massive structures like the Great Wall. It’s supersymmetry twin, the vastly more massive neutralinos are a good candidate for hypothetical dark matter, but physicists have yet to detect such a particle and indeed SUSY theory is errant speculation at this point, though it would potentially solve a lot of existing nasty problems with the current Standard Model.

To the o.p.: as far as what we know of dark matter (which is practically nothing) it doesn’t clump in the way that normal baryonic matter does. The reason for this is complicated (and I’d glurge it up if I tried to parse it into English) but it behaves more like leptonic matter; that is, it doesn’t experience the strong interactions (strong nuclear force) that causes nuclei to form of protons and neutrons (or potentially more exotic baryons). It is drawn together by gravity, but this is a very weak, long range force, and if the particles already have a signficant kinetic energy they’ll orbit about like a big cloud rather than condensing (“clumping”) as normal matter does. Thus, we’d expect something like this to create large scale structures like galaxies but be essentially invisible on the everyday scale, even in structures the size of the Solar System. If dark matter exists, it makes up the bulk of the mass of the universe. Normal matter would better be described as being embedded in it, rather than coupled to it.

Does it exist? shrug Uhduhno. Invoking it relieves us of some significant problems with the formation and maintenance of large scale structures, but pointing to an invisible, undetectable medium/force carrier/whatever harkens back to the luminiferous aether of the 19th Century, when everybody assumed that the aether had to exist in order for light “waves” to propagate, and that was the standing wisdom until a few wiseacres from Europe (well, Shankland was American) started bellying on about Minkowski spacetimes and light being both a particle and a wave and whathaveyou. “Dark matter” could easily turn out to be something similar; a previously unknown wide scale manifestation of the Higgs field, or influence of nearby branes on fine structure, or who knows what, and any reputable cosmologist will admit that we really don’t have a handle on what’s going on out there.

Stranger

I really appreciate all the frank candor in this thread. Even a “we don’t know” helps a lot toward understanding (keeps it all cleaner). And don’t worry about glurging the topic. The metaphors actually help. As Niels Bohr said, “When it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images.”

Like I said, none of the guesses are all that good. But you could get low-energy neutrinos by extreme redshift of high energy ones, comparble to the Cosmic Microwave Background. There’s still probably not enough of them, given the known limits on their mass, but they could at least be a nonneglible contribution. And they do have the big advantage over other candidate particles that we’ve actually detected them, unlike axions or SUSYs or microscopic black holes or quark nuggets or…

You can have my microscopic black holes when you pry them away from my cold dark matter. Quark nuggets, on the other hand, that’s just crazy talk. Playing around with deconfined quarks is a good way to lose a few fingers.

Stranger

Here’s a nice new dark matter ring produced by colliding galaxy clusters.

You can see something funny is going on with the gravitational lensing even without the computed dark matter haze.
Now some poor grad student is going to have to try to compare orbital velocity curves for stars in galaxies from various parts of the cluster. :smack:

Thanks. Unfortunately I was unable to get back to this thread yesterday as I was roped into interminable meetings etc (all on the subject of clusters in fact) at this workshop I’m at. (In a convent…)

I think everyone else has covered things adequately, but I will re-iterate that we simply do not know what dark matter is; only that it interacts gravitationally with “normal” matter and makes up a large fraction of the mass of a galaxy cluster.