NASA Puts Big Bang to the Test
By Greg Clark

Staff Writer

posted: 01:57 pm ET
15 October 1999

Antimatter galaxies populated with anti-stars producing anti-particles should populate half the known universe, according to a popular interpretation of the Big Bang theory

Antimatter galaxies populated with anti-stars producing anti-particles should populate half the known universe, according to the most popular interpretation of the Big Bang theory.

The Big Bang predicts that equal quantities of matter and antimatter should have been produced when the universe was created -- but nobody has ever detected signs of such huge amounts of antimatter.


The detector arrangement for the Alpha Magnetic Spectrometer (AMS), designed to measure anti-matter. Credit: European Space Agency. Click to enlarge.
That may change once the International Space Station is assembled and in orbit.

NASA is planning to send up a unique charged-particle detector that should be able to measure antimatter -- as well as the elusive dark matter which scientists believe makes up 90 percent of the universe's mass, but has also never been found.

The Alpha Magnetic Spectrometer (AMS) is a 7-ton instrument that astronauts are scheduled to attach to the space station in 2003 for a three-year mission.

The detector, which has been designed and developed by Nobel laureate Samuel Ting and an international team of physicists, will analyze the charge, momentum and velocity of cosmic ray particles. These particles cannot be detected from the ground because they are absorbed by Earth's atmosphere.

Specifically, the instrument will be looking for relatively heavy antimatter -- anti-helium nuclei or particles even more massive than that, such as anti-carbon.

"If there is antimatter out there at all, a three-year stay on the station… should detect it," said Mark Sistilli, the program manager of the Alpha Magnetic Spectrometer experiment at NASA.

"A null result wouldn't close the book on antimatter galaxies, but let's just say it would cast a pretty big question mark as far as the viability of such a theory."

Antimatter isn't quite as odd as it may seem. It isn't anti-massive. Essentially, an anti-helium nucleus looks exactly the same as regular helium nucleus except for one thing: its charge is negative rather than positive, Sistilli explained.

Cosmic ray particles travel with such high energy that their accompanying electrons have long since been ionized away. A helium nucleus has two protons, each with a positive charge, and two neutrons. An anti-helium nucleus has two anti-protons with two negative charges.

"If we detect something that weighs like a helium -- that has two protons and two neutrons, but it has a negative charge or, in this case, two negative charges -- we know that that wasn't a regular helium nuclei that went through the detector, it's got to be an anti-helium," Sistilli said.

Finding massive antimatter particles would be significant, Sistilli said, because normal high-energy particle interactions involving regular matter don't generally produce heavyweight antimatter such as anti-helium or anti-carbon.

There are interactions involving regular matter that do produce lighter particles such as anti-protons or anti-electrons (called positrons), but nothing that makes anti-nuclei.

"An anti-helium or anti-carbon could only be interpreted as being a particle from a source body somewhere out in the universe made of an anti-star, or an anti-galaxy or a cluster of anti-galaxies," Sistilli said.

So by the time the AMS finishes its three-year run in 2006, physicists may have evidence that antimatter galaxies do exist. Either that, Sistilli said, or something is going on with the fundamental physics of the Big Bang that scientists don't understand, namely, that for some reason more matter than antimatter was created in the universe.

Dark matter is another enigma that the Alpha Magnetic Spectrometer may help scientists understand. If dark matter is made of some kind of exotic subatomic particle, the spectrometer might provide indirect evidence of its existence.

"Might," Sistilli said. "We don’t know. Because we don't know what the nature of dark matter is."

The total cost of the spectrometer is estimated at $33 million. That includes the development, and two flights of the instrument. The detector flew aboard space shuttle in 1998, when scientists used it to study the interaction between cosmic ray particles and Earth's magnetic field.

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