Dark Matter: Expert Q&A

On July 1, 2008, astronomer Richard Massey answered questions about dark matter and other otherworldly enigmas.

Q: Particularly concerning the Large Hadron Collider (LHC), what experiments are planned to detect dark matter?

Richard Massey (RM): The main experiment to find dark matter within the LHC is called ATLAS. Like other colliders, the LHC will create new particles via Einstein's famous equation E = mc2. This says that mass m can be converted into energy <>E (as in a nuclear bomb), or energy E can be converted into mass m (to create new particles). In colliders, particles are accelerated to give them more energy, then smashed into each other to concentrate that energy into a small space, from where new particles emerge. Most known particles were discovered when they emerged from collisions like this, although the dark matter particle is predicted to be much more massive than anything yet created. I hope that the faster speeds and higher energies in ATLAS will produce some dark matter particles-although, interestingly, they will still not be seen directly. Dark matter is still invisible, and it does not interact with other particles. The breakthrough detection will be when more energy goes in than particles are seen coming out. The missing particles, presumably, would be the dark matter.

Experiments are also ongoing in astronomy-this is one of those exciting areas where scientists in different fields can tackle the same problem from different approaches. For example, the GLAST satellite launched two weeks ago. Although dark matter may give off light only very, very rarely, GLAST will try to see that light by looking at the enormous cloud of dark matter surrounding the Milky Way. Also, the SNAP satellite will map the distribution of dark matter using gravitational lensing, so we can study its behavior on large scales. Astronomical objects like the Bullet cluster are natural examples of particle collisions on scales bigger than those we can ever hope to replicate on Earth. [For more on the Bullet cluster, see The Dark Matter Mystery. Here's just hoping that the particle physicists and the astronomers meet somewhere in the middle!

Q: What is the difference between dark matter and dark energy, and are they consistent throughout the cosmos?

RM: Dark matter is odd stuff, but it does have some familiar properties. For example, some regions of space have more dark matter than others: there are large amounts of dark matter around galaxies and clusters of galaxies, but very little in the spaces between. This is because dark matter, although invisible, is subject to the same force of gravity as ordinary matter. An apple falls towards the Earth because of gravity, and "lumps" of dark matter fall toward each other, or toward lumps of ordinary matter. Gravity does not discriminate! Current theories suggest that dark matter is the same, simple material everywhere in the universe; however, that understanding may spring from a lack of data and oversimplistic theories.

On the other hand, dark energy appears to be something completely unfamiliar to us in our everyday lives. We're not really sure, but it could be an intrinsic property of space itself-in which case, there would be the same amount of it in every direction. Furthermore, it effectively has the opposite effect of gravity on ordinary or dark matter. If you threw an apple into the air, and dark energy had its way, the apple would not come back down, but accelerate upwards, racing off into space. Fortunately for our lunch, the effect is really, really tiny, and gravity wins the battle!

Q: When using current gravitation math on our own solar system, how is dark matter worked into the equation? If it is not worked into the equation then wouldn't any result be incorrect?

Q: If dark matter exists in space, shouldn't there be evidence of it on Earth, and if so, why can't we find it?

RM: There is dark matter in our solar system, and a large cloud of it envelops the entire Milky Way. Indeed, dark matter is passing through us on Earth all the time. Not to worry, though, it doesn't do any harm or seem to interact with us at all-which is precisely why it's so difficult to find! The experiments in underground mines are looking carefully for dark matter, but it interacts so little with ordinary matter that they still haven't managed to detect a single particle.

Nor do lumps of dark matter whiz through the solar system, affecting the orbits of planets. Dark matter is spread evenly throughout our solar system. Since the big bang, gravity has had time to work dark matter into very large structures, coalescing around galaxies and clusters of galaxies. For ordinary matter, electromagnetism (as well as the weak and strong nuclear forces) then took over to help create smaller objects like planets and stars. But dark matter doesn't take part in these forces, so it is still spread out. On the scales that we are used to, there is the same amount of dark matter in every direction, and its net effect is to do nothing.

Q: When I first heard the term "dark matter" I thought, "Oh, this is about black holes.'" As I learned more, I kept being struck by how similar dark matter-a material that does not interact with light-sounds, compared to black holes. So my question is: What is the observable difference between a field of dark matter and a group of small black holes?

RM: Even though a large black hole lives at the center of most galaxies (including the Milky Way), these black holes are not dark matter. From the motion of stars, we know that dark matter must instead be scattered throughout galaxies. One early candidate to explain dark matter was multiple miniature black holes: one of a class of candidates known as MACHOs, or MAssive Compact Halo Objects. However, unlike a uniform field of dark matter, individual black holes would occasionally pass in front of a star, making the star's light flicker. This is an effect known as gravitational microlensing. Surveys designed to detect this flickering, such as MACHO and OGLE, saw no flickering-so miniature black holes have now been ruled out.

Q: If dark matter provides extra gravity that keeps some galaxies intact, how was it involved during the expansion phase of the big bang?

Q: Is the amount of dark matter and dark energy included in the most recent big bang models of the universe? If not, how can they be correct? If they are included, how do scientists know what properties to give dark matter and dark energy in the model since they don't know what either are?

RM: Dark matter has held intact galaxies, clusters of galaxies, and even the universe itself! The initial expansion of the universe was so rapid that, without dark matter, it would have quickly become too large, and its contents spread too thin, for stars and planets to form. By now, the universe would have become so big it would have ripped itself apart. The extra gravity of the dark matter slowed the expansion and held the whole universe together!

Dark matter also played a second crucial role during the early universe. Immediately after the big bang, the universe was a hot, swirling maelstrom-and its contents were almost uniformly spread out. Since dark matter is not affected by other material, it started condensing out of this primaeval soup early. Gravity brought together small clumps of dark matter to make bigger clumps, and bigger clumps to make large structures. Gradually, a crisscrossing network of dark matter filaments formed. Later, gravity pulled the ordinary matter inside that, and built it into the stars and galaxies we see today. Dark matter is the invisible scaffolding of the universe.

Both dark matter and dark energy are included in the latest cosmological models. They have a peculiar status in current science because we know roughly what properties they must have (to explain other observations), but we don't yet know what they are made of. It seems odd, but this status isn't uncommon in science. Phenomena are first noticed, then placeholder names are given to the suspects, before the real culprits are finally pinned down.

Q: Does dark matter contain the antiparticles for all existing particles in nature?

Q: Can dark matter be a new type of particle that has more antiquarks than quarks?

RM: No, not antiparticles in the sense of electrons vs. positrons and quarks vs. antiquarks. Antiparticles interact with ordinary matter through the electromagnetic, weak and strong forces; if an antiparticle hit a particle, both would disappear in a flash of light. Dark matter does not emit light, so it must be made of something even more mysterious! However, one of the leading suspects for dark matter is the "supersymmetric" partner of an ordinary particle. Supersymmetry is another mirror in nature, and a corresponding set of superparticles or "sparticles" are speculated to exist for each ordinary particle: electrons vs. selectrons and quarks vs. squarks. One of these might well account for dark matter.

Q: What is "gravitational lensing"?

RM: Gravitational lensing is the bending of rays of light by gravity. Light rays get bent all the time: whenever you look through spectacles (as we say in the U.K.), or see a drinking straw in a glass of water, light reaches your eyes via a detour. What you're looking at through spectacles gets less blurry or bigger, and the straw looks broken! In those cases, light rays are bent because light travels at different speeds in air, water, and glass. We could even calculate how much water was in the glass by measuring the shape of the straw.

The physics behind gravitational lensing is very different: anything heavy warps space (and time). This bends everything in it, including light rays. However, the end result is the same: galaxies behind a heavy object appear larger and distorted. By measuring the amount of distortion, we can map out massive material like dark matter, even if it is invisible.

[Editor's Note: For more information about gravitational lensing, see NOVA scienceNOW's profile with Arlie Petters.]

Q: How can you be certain that dark matter and dark energy even exist since you are studying events billions of light years away?

Q: Is it possible that the postulated existence of dark matter turns out to be superfluous much like the hypothesis of the luminous aether of the 19th century?

Q: Has the existence of dark matter been proven and accepted by the vast majority of scientists or it is still an unproven, powerful hypothesis? Isn't it possible that we do not yet fully understand gravitational physics?

RM: These are always good questions in astronomy and cosmology, which are unusual sciences. We have the biggest laboratories in the universe, but we can only observe experiments; we lack the hands-on control of other disciplines. (For the clumsy undergraduates among us, that can turn out to be a good thing!) However, the scientific method still applies: we come up with theories, make predictions, and compare our theories to observations. Occasionally, observation tries to sneak ahead of theory, although we consciously try to avoid it.

Astronomers are a conservative bunch and have not been convinced easily of the existence of this bizarre new, invisible particle. The evidence for dark matter has been accumulating from many different observations since the 1930s, and it is only generally accepted today because of that diversity. Nobody would believe a crazy-sounding idea that explains only one observation. To list a few of the mysteries dark matter solves: the (fast) rotation of galaxies; the (fast) motion of galaxies in clusters; the (large) masses of clusters, which can separate from gas during collisions; the (large) number of clusters; the (long) lifetime of the universe; and the (flat) geometry of spacetime, given the small amount of ordinary matter that we know is present from its behavior immediately after the big bang.

Yet I think all astronomers applaud the search for alternative explanations. Astronomy's (long) history reminds us that a revolution usually is just around the corner, and those who claim cosmology is nearly solved are always wrong. Most alternative theories do involve modifications to the laws of gravity, although none have yet been successful, and some invoke even more complications than "simple" dark matter and dark energy.

Q: If we are not sure exactly what dark matter is, how are we sure the laws of gravity apply the same way as they do to normal matter?

RM: We aren't exactly sure! But physicists prefer theories that keep laws universal and assign different properties to the particles. For example, we could formulate a different law of gravity for each planet in our solar system, but it feels more elegant to have a single equivalent law that just needs to know each planet's mass. With dark matter, we know that it is not normal matter. So we prefer a theory that introduces a mysterious new particle rather than theories that introduce a mysterious new particle and simultaneously alter the behavior of gravity.

Q: Could there be more than one type of dark matter?

RM: Maybe...we don't know yet! My favorite theory holds that all dark matter particles are the same: although many dark particles existed initially, they tended to decay or relax into a single "lightest supersymmetric particle," which is now trapped and unable to interact with everything else. It's a bit like raindrops falling on a tiled roof: they all land in different places, but eventually cascade down to the same gutter. Furthermore, the gutter is blocked, so they pool there, unable to meet up with the raindrops that land on the garden, and which we feel. But that's only one theory. It's proving very tricky to investigate an invisible, non-interacting substance! This is a very exciting time in physics, with a lot of interesting questions and only tentative answers.