Illuminated

Scientists Are Making Mysterious Dark Matter Less Mysterious But Others Still Fail to See the Light

In the beginning, the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move.
– Douglas Adams, The Restaurant at the End of the Universe


Current accepted scientific theory states that our Universe sprang into being through a sudden expansion known as the Big Bang. As the energy from this intensely hot beginning cooled, it formed matter. The distribution of matter was not uniform, so it began clumping together, eventually forming stars, planets, and everything else we can see throughout the vastness of space. The largest structures to form were galaxies and galaxy clusters. Here, astrophysicists ran into a problem.

Observable matter did not contain sufficient mass – and, thus, sufficient gravity – for galaxies to hold their colossal shapes. They ought to break down fairly quickly. Since they clearly do not, scientist theorized there must be something out there besides visible matter with sufficient mass to provide the necessary gravitational glue binding galaxies together. They called this invisible stuff “dark matter.” What is more, there had to be a lot it – about six times as much as ordinary visible matter – to square with observation.

The dark matter detected in
galaxy cluster Abell 1689 is
shown in light blue

Scientists did not know anything about dark matter. It was more than merely invisible and did not interact with ordinary matter in any way. However, they stubbornly insisted it must exist because factual observation demanded it and scientists regard empiricism as the highest form of human thinking. Some religious people did not think it was fair that they could not make up God to explain the unexplainable while scientists could make up dark matter to explain gravity.

Quite a few practical people – people who believe that common sense, as opposed to empiricism, is the highest form of human thinking – agreed. It proved to them that advanced degrees from universities did not make scientists smarter than they were. The practical people believe they would be better scientists than those trained to do so but they are too busy with practical concerns, such as making money or waging wars, to waste time over pointless matters like how the Universe holds itself together. They just accept that it does so and regard it as part of their natural rights.

The practical people realized scientists were cooking up dark matter in the pot of imagination. Luckily, the practical people were too smart to fall for it. They knew the source of gravity was either the same Supreme Being who can allow an infinite number of angels to dance on the head of a pin or natural, long-term climate shifts. “It stands to reason,” they said. This is something the practical people always say when they want to end a discussion and just get on with things.

Unfortunately, the scientists, some of whom actually did stand to reason whereas other preferred to do their reasoning while sitting down, refused to let go of the problem. They decided, in typically impractical fashion, that if they looked at what appeared to be nothing hard enough and long enough, they might actually see something. This demonstrates no common sense whatsoever and is probably the main reason why it worked. The trick lay in figuring out what not to look for.

The November 10 issue of Astrophysical Journal contains the observations of a group of astronomers, led by Dan Coe of NASA’s Jet Propulsion Lab and Edward Fuselier of West Point. The group pointed the Hubble Space Telescope’s Advanced Camera for Surveys at Abell 1689, a massive galaxy cluster located 2.2 billion light years distant in the constellation Virgo. Abell 1689 contains about a thousand galaxies, representing trillions of stars. However, none of these galaxies interested the astronomers. Instead, they focused on the galaxies behind the cluster.

Abell 1689 is so massive that it acts as a gravitational lens, distorting the light passing by it from the galaxies behind it. The astronomers found that, massive as it is, Abell 1689 did not have enough mass to explain the degree of distortion seen. The difference was the gravitational impact of dark matter. By measuring the distortions in many different places and then translating it as light blue coloration superimposed on the Hubble image, they were able to “see” the dark matter within Abell 1689 and how it was distributed.

They discovered dark matter, much like ordinary matter, distributed irregularly through space, forming massive, dense clumps found at the heart of galaxies as well as close around them. Another group of researchers, led by Meghan Gray of the University of Nottingham and Catherine Heymans of the University of British Columbia got exactly the same results pointing Hubble at the Abell 901/902 galaxy supercluster. They published their findings in the Monthly Notices of the Royal Astronomical Society.

So now that we can see dark matter, what is it exactly? Astrophysicists think the most likely candidates are gauginos, hypothetical particles predicted by gauge theory combined with supersymmetry. Gauginos carry opposite spins from the particles making up ordinary matter (i.e. anti-matter). Like Standard Model particles, some carry a charge while others, called neutralinos, do not.

Neutralinos are likely dark matter candidates because they interact with other particles only through gravity and the weak nuclear force. The lack of electromagnetic and strong nuclear interactions makes them difficult to detect. Calculations demonstrate their possible thermal production in the early hot universe, with approximately the right amount remaining after cooling to account for dark matter.

Neutrinos are the only type of likely dark matter particles detected in the laboratory to date but have almost zero mass. However, the lightest type of neutralino predicted, called the photino, would be both stable and heavy enough to qualify as a WIMP (weakly interactive massive particle) making up dark matter. Interestingly, neutralinos and anti-neutralinos are identical, meaning any two pieces of dark matter colliding would self-destruct like any other interaction of matter and anti-matter.

The result of such explosions would be a stream of particles called positrons, the anti-matter counterpart to electrons. A 2009 article in the journal Nature describes the findings of PAMELA (Payload for Antimatter Matter Exploration and Light nuclei Astrophysics), an Italian satellite designed to measure radiation in space. PAMELA found a much higher number of positrons than expected, suggesting that dark matter collisions, although rare, do sometimes occur.

Such explosions would also produce gamma rays. Yet another team of researchers, led by cosmologist Dan Hooper from the University of Chicago, used the Fermi space telescope, NASA’s gamma ray observatory, and found an abundance of gamma rays emanating from the core of our own Milky Way galaxy. We now know galactic cores are clumping points for dark matter.

One problem is that the gamma radiation measured by Hooper’s team points to WIMPs only eight to nine times heavier than protons. This is lighter than expected for dark matter particles. On the other hand, the astronomical surveys of the Abell clusters found a slightly higher amount of dark matter present than expected, so perhaps it all evens out.

The practical people are not going to like this dark matter/anti-matter connection. Anti-matter is scary stuff. The annihilations it produces are so devastating they may well be the only force in the Universe powerful enough to get Bristol Palin voted off Dancing with the Stars. The practical people have no time for annihilation. After all, what is the point of permanently ending/extending the Bush tax cuts or repealing/saving healthcare reform when two foreign, possibly Muslim, particles could meet at any point and time, wiping out everything within parsecs of the event?

On the other hand, it is estimated that if one could somehow bottle anti-matter and sell it, a price of about $62.5 trillion a gram (i.e. $1.75 quadrillion an ounce) would be fair market value, although I am talking auction estimate here as opposed to retail. This would be enough to pay off the entire U.S. national debt. Luckily, scientists have dedicated themselves to continue looking at nothing. A project called CLASH (Cluster Lensing And Supernova survey with Hubble) plans to survey twenty-five galaxy clusters over the next three years.

This might be enough for the practical people finally to start seeing the light. They are not so good actually coming up with ideas, preferring to outsource this to the scientists they otherwise disdain, but they are highly skilled at exploiting useful ones. Once there is money to make or power to gain from it, dark matter will suddenly start making a whole lot of common sense to them.

It stands to reason.


The fact that we live at the bottom of a deep gravity well, on the surface of a gas covered planet going around a nuclear fireball 90 million miles away and think this to be normal is obviously some indication of how skewed our perspective tends to be.
– Douglas Adams