by Ian O’Neill

Astronomy has traditionally focused on bright celestial objects in our night sky. If you’re lucky with the weather and own a modest telescope, you can observe the bright band of stars that form the disk of the Milky Way. You’ll pick out elegant galaxies, nebulae and binary stars. You’ll also be able to track the planets of the Solar System. But on gazing deep into the sparkling cosmological bounty, it’s worth remembering that all the stuff you can see (and everything on our planet that we can experience) is actually only a tiny fraction of the matter in the known Universe. Indeed, all those billions of stars and galaxies, all those countless planets and clouds of dust and gas only represent 4.9% of the mass-energy of the Cosmos.  Yes, less than five percent of the Universe is visible matter – a.k.a. “baryonic matter.”

So where’s the rest?

26.8% of the Universal mass-energy is holed up in a mysterious substance known only as “dark matter” whereas the remaining 68.3% comes from an even more perplexing force known as “dark energy.” And in the beginning of April -- just as Global Astronomy Month kicked off to primarily celebrate the visible matter in the Universe -- physicists shared a profound discovery that shed light on a dark cosmic underworld.

AMS on ISS 700kAMS on IS. Image courtesy: NASA

On April 3, scientists managing a $2 billion particle detector attached to the exterior of the International Space Station announced the possible discovery of the signature of particles thought to originate from the annihilation of dark matter. Attached to the station by space shuttle Endeavour in May 2011, the Alpha Magnetic Spectrometer (AMS) has been detecting high-energy electrons and positrons (the electron’s antiparticle) from all directions of the sky.

It is hoped that by detecting these particles, measuring their energies very precisely, some of the most profound mysteries of the origins and nature of our Universe may be understood. By measuring the ratio of electrons versus positrons, for example, the international AMS consortium hopes to uncover some clues as to why the Universe is dominated by matter (and not antimatter). What conditions shortly after the Big Bang created a bias toward matter?

But there’s also hope that the AMS could expose the true nature of dark matter, and the first batch of data has been analyzed, giving us a tantalizing clue.

One theory behind dark matter is that it is composed of weakly interacting massive particles (WIMPS for short). These particles are thought to be so weakly interacting that they can fly through our planet without ever colliding with anything. WIMPS are non-baryonic matter and do not interact with electromagnetic radiation – i.e. light – hence the “dark” moniker. But there are two ways that WIMPS may be detected. The first is to measure their large-scale collective gravitational effect on the spin of galaxies or colliding galactic clusters. Another is to measure the baryonic particles WIMPs generate when they collide. This is where the AMS comes in.

When matter collides with antimatter, it annihilates. This annihilation creates energy and a shower of “daughter” particles. WIMPs, however, are their own antiparticles, so when they collide, they annihilate, generating energy plus electrons and positrons. But WIMP annihilation generates positrons with a specific energy signature, a range of energies that the AMS is extremely sensitive to.

As the first batch of AMS data was released, Nobel laureate Samuel Ting, of the Massachusetts Institute of Technology (MIT) and lead scientist of the AMS, said, “As the most precise measurement of the cosmic ray positron flux to date, these results show clearly the power and capabilities of the AMS detector … Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin.”

This positron signature is not quite the “smoking gun” of dark matter annihilation, but the data is certainly behaving in such a way that suggests something significant is going on. Perhaps even more interesting is the fact that this positron flux appears to be coming from all directions, suggesting that dark matter – in the form of WIMPs – pervades the entire Universe, as predicted.

Like any particle physics experiment, however, more time is needed. The more positrons that are detected, the more precise the data becomes – much like exposing a photographic plate to a light source; the longer it’s exposed, the more photons hit the plate and the more defined the photograph becomes. As the AMS will be attached to the space station until it is decommissioned, assuming the instrument continues to operate as planned, AMS scientists are confident they will be able to solve the dark matter riddle within the next few months to years.

So, as you look up on a dark night, remember that you can only see a fraction of the mass of the Universe. The rest is hiding in plain sight, creating a gravitational force that binds the visible Universe together, annihilating and generating a shower of high-energy particles that we are only just beginning to understand.


IanIan O'Neill is the space science producer for Discovery News based in Los Angeles. He holds a Ph.D. in Solar Physics where he specialized in the heating of the lower corona and computational modeling of coronal loops. His favorite topics include the colonization of Mars, manned spaceflight, extraterrestrial speculation and weird physics topics. Aside from Discovery News, he also writes for Al Jazeera English and on a variety of space science topics.


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