THE SEARCH FOR DARK MATTER, Part One

This is Dennis “The Wizard” Turner’s initial Science column for To The Point. In addition to his weekly column on computers, we hope this is the first of many. —JW

Dark matter is usually thought of as something ‘out there.’ But we will never truly understand it unless we can bring down to earth.

If we could see dark matter, the Milky Way galaxy would look like a much different place. The familiar spiral disk, where most of the stars reside, would be shrouded by a dense haze of dark matter particles. Astronomers think the dark haze is 10 times as massive as the disk and nearly 10 times as big in diameter.

(Note: I am neither an astronomer nor a particle physicist. It’s been years since I’ve been a practicing mathematician first at IBM and then on Wall Street. However I’ve kept an intense interest in science, particularly cosmology, the possibility of extraterrestrial intelligent life, and evolutionary theory.)

This article is the result of keeping up to date by reading, attending seminars at Hebrew University, and using Quintura (see my Taking Directions column this week) on the web.

I computer program for Dr. Gerald Schroeder, a well known nuclear physicist who earned his PhD at MIT, and for Dr. Steven Wiesner, a quantum physicist. Steve is known as the father of quantum cryptology. Both have worked on projects that I can’t mention here. Both have helped me on this article, usually at Café Aroma after synagogue.

The universe around us is not what it appears to be. The stars make up less than 1 percent of its mass; all the loose gas and forms of ordinary matter, less than 5 percent.

The motions of this visible material reveal that it is mere flotsam on an unseen sea of unknown material. We know little about that sea. The terms we use to describe its components, “dark matter” and “dark energy,” serve mainly as expressions of our ignorance.

For 70 years, astronomers have steadily gathered circumstantial evidence for the existence of dark matter, and nearly everyone accepts that it is real.

But circumstantial evidence is unsatisfying. It cannot conclusively rule out alternatives, such as modified laws of physics. Nor does it reveal much about the properties of the supposed material.

Essentially, all we know is that dark matter clumps together, providing a gravitation anchor for galaxies and larger structures such as galaxy clusters. It almost certainly consists of a hitherto undiscovered type of elementary particle.

Dark energy, despite its confusingly similar name, is a separate substance that entered the picture only in 1998. It is spread uniformly through space, exerts a negative pressure and causes the expansion of the universe to accelerate.

Ultimately the details of these dark components will have to filled in not by astronomy but by particle physics. Over the last ten years the two disciplines have pooled their resources, coming together at meetings such as the Symposia on Sources and Detection of Dark Matter and Dark Energy in the Universe.

Another was held in February 2004 in Marina del Rey, California (where I once lived as a teenager).

The goal has been to find ways to detect and study dark matter using the same techniques that have been successful for analyzing particles such as positrons and neutrinos. Rather than inferring its presence by looking at distant objects, scientists would seek the dark matter here on Earth.

The search for dark matter particles is among the most difficult experiments ever attempted in physics. The search for particles of dark energy is even less tractable and has been put aside, at least for the time being.

At the first symposium, in February 1994, participants expressed a nearly total lack of confidence that a particle detector in an Earth-based lab could ever register dark matter.

The sensitivity of even the best instruments was a factor of 1,000 too low to pick hypothesized types of dark particles. However, since then, detector sensitivity has improved more than 1,000 fold, and instrument builders expect soon to wring out another factor of 1,000.

More than 15 years of research and development on detector methods are finally bearing fruit.

We may soon know what the universe is really like. Either dark matter will proved to be real, or else the theories that underlie modern physics will have to fall on their swords.

What kind of particle could dark matter be made of? Astronomical observation and theory provide some general clues. It cannot be protons, neutrons, or anything that was once made of protons, or neutrons, such as massive stars that became black holes.

According to calculations of particle synthesis during the big bang, such particles are simply too few in numbers to make up the dark matter.

Those calculations have been corroborated by measurements of primordial hydrogen, helium and lithium in the universe.

Nor can more than a small fraction of the dark matter be neutrinos, a lightweight breed of particle that zips through space and is unattached to any atom. Neutrinos were once a prominent possibility for dark matter, and their role remains a matter of discussion, but experiments have found that they are probably too lightweight.

Moreover, they are ‘hot’ – that is, far more energetic. They will hit a nucleus far more often and when they do the heat and light they give off are at least 10,000 times more detectable than the slow, sluggish particles that supposedly make up dark matter.

Current estimates of the contribution to the mass of the universe of neutrinos are only 0.3%.

More next time. We’ll talk about why particle physicists have a single supersymmetrical particle in mind. Either they find the interaction in the next, say 10 years, or the laws of physics are deficient.

Dennis Turner