By Don Wood

Are pulsars, those strange stars found just three decades ago, holding the first—and last—pieces of the Big Bang puzzle?BY DON WOOD

Little green men.

The unwelcome thought wouldn’t go away. But how else could she explain those tiny squiggles coming from her chart recorder?

It was July 1967. Jocelyn Bell was a 24-year-old graduate student working for astronomer Antony Hewish in Cambridge, England. She and Hewish had just finished building a new radio telescope, covering four and a half acres, to track quasars—mysterious, deep space objects that constantly boil off vast amounts of energy, especially in the form of X-rays and radio waves.

Bell, in charge of the telescope, was making observations toward her Ph.D.

And suddenly, here were these curious signals—which she called “bits of scruff”—showing up among her data. What to make of them?

Bell decided she needed more detail on the odd blips, so she installed a faster chart recorder (this was before the age of ubiquitous computers) to get a clearer picture.

For two months nothing showed up. Then the mysterious signal reappeared. The new recorder showed that the radio pulses were coming at an unvarying interval of 1.33 seconds.

Exactly every one point three three seconds, with not a fraction of variance.

At first, Hewish thought the signals had to be artificial—almost nothing in nature shows that degree of regularity. But his student wasn’t so sure. Bell carefully followed the signals over a period of weeks and proved they always came from the same point in the sky, thus ruling out an artificial source such as a satellite. Another member of the team calculated the distance to the source—it was a thousand light years away.

Logic dictated that if the signals were indeed artificial, they must be coming from intelligent aliens. Little green men.

Bell dubbed the source LGM-1 (the first “little green men” signal) and continued her observations while she, Hewish and the others debated how to announce their discovery. They didn’t really believe the signals were coming from aliens, but they were at a loss for any natural explanation.

And then Bell found another source, and soon another, and another, all in different parts of the sky and all showing the same regularity (but with slightly different periods). It seemed very unlikely that four groups of aliens were all broadcasting at the same frequency. Bell and Hewish quickly dropped the LGM hypothesis.

Radical Neutrons

Publication of their findings in 1968 generated a flurry of excitement among astronomers and physicists trying to comprehend the peculiar phenomena, which Hewish (or the press—the record’s not clear) had dubbed pulsars (“pulsating stars”). Within a few months, the late American-based astronomer Thomas Gold (he died in June 2004) came up with the answer. Bell’s “scruff” turned out to be the telltale “voice” of one of the most bizarre objects in the universe, born in a cataclysmic stellar explosion.

Gold realized that Bell had discovered neutron stars—strange objects that had been speculated about in the 1930s, but never found. These super-dense objects—one spoonful would weigh billions of tons—were thought to be the remains of a dying star that finally explodes into a supernova, one of the most dazzling feats in the cosmos.

Gold showed that a neutron star would spin rapidly and generate an unbelievably powerful magnetic field (about a million times more powerful than can be created at the Tallahassee-based National High Magnetic Field Laboratory). This vast magnetic field, Gold said, channels charged particles to the star’s magnetic poles, where they create an intense, directed stream of radio waves. Since (as on Earth) the magnetic pole does not line up with the axis of rotation, this immensely powerful beam of radiation sweeps around space like a lighthouse beacon. If the beam happens to sweep past Earth, we detect a pulse—one pulse for each rotation of the neutron star. Thus Bell’s LGM-1 was rotating once each 1.33 seconds. Scientists soon accepted Gold’s explanation.

Over the years since, astronomers have discovered hundreds of neutron stars, but the objects still remain profoundly mysterious. Measuring their properties is difficult to impossible, so no one can say for sure what the stars are made of.

Lack of hard data hasn’t stopped theoretical physicists, of course. They calculate that at least in their crusts, neutron stars contain neutrons packed together at roughly the same density as they’re found in an atomic nucleus, which is the densest speck of matter on Earth.

But deep in the interior of these weird objects, conditions are so extreme that they can’t be replicated in earthly laboratories. Some physicists say that the cores of neutron stars are so dense—up to 10 times denser than their neutron-packed surface—that extreme pressures might create new forms of matter not seen since the first fraction of a second after the Big Bang created the universe.

Does this mysterious stuff really exist? No one knows, but one FSU astrophysicist thinks he may have found the best way to find out.

Clues in a Fish Sinker

Jorge Piekarewicz, a theoretical physicist, and his collaborator Charles Horowitz of Indiana University, say they may have found a way to cut through the thicket of speculations and tell—with a few good measurements—whether or not the cores of neutron stars contain the kind of exotic material physicists have speculated about. They say that the key to their discovery lies in the common earthly element lead, the stuff of car batteries and fishing sinkers.

To know whether a particular neutron star contains exotic matter, Piekarewicz and Horowitz realized, you first have to know what the star would look like if it didn’t contain exotic matter. But neutron stars are so far beyond our earthly experience that scientists can’t say for sure what their properties should be even if (as traditional ideas say) they consist almost entirely of neutrons, with just a smattering of protons, electrons and other particles thrown in.

Since they couldn’t bring a neutron star into the lab, the two collaborators found a surrogate instead. They realized that the nucleus of lead is enveloped by a “skin” of neutrons, a phenomenon that closely resembles the crust of a neutron star. Both entities consist mostly of neutrons at similar densities and are thus subject to the same forces.

“What we have proposed is that you can understand a lot of the behavior of neutron stars by studying very carefully the nucleus of lead,” Piekarewicz says.

They chose lead because it has a large excess of neutrons (126 neutrons to 82 protons) and also because it is one of the most studied nuclei in all of physics. Nuclear forces within the lead nucleus tend to squeeze the extra neutrons to the nucleus’ surface where they form the neutron-rich “skin” surrounding the core.“The pressure that is pushing neutrons away from the core in the lead nucleus has to be the same thing that is pushing neutrons in the neutron star,” Piekarewicz says.The connection between lead and neutron stars is real, he believes, and other physicists agree. “There is definitely a relationship there,” neutron star expert James Lattimer of the State University of New York at Stony Brook said in a comment on the research to the American Physical Society’s News Online.

Trouble is, there aren’t any reliable measurements of exactly how much pressure is pushing neutrons from the core of the lead nucleus. That pressure manifests itself as the radius of the nucleus—the more pressure there is, the greater the radius. But the radius is difficult to measure. The values that exist are plagued by uncertainties in the assumptions underlying the measurements.

Nonetheless, Piekarewicz and Horowitz plugged a range of possible values into their equations and saw that, once the pressure is known, out will pop a number of hitherto unknown properties of neutron stars.

“We saw a perfect correlation between how the nucleus of lead reacts to the different equations of state and how the pressure affects the properties of the neutron star,” Piekarewicz says.

For example, if the pressure is greater, the radius of the lead nucleus will be a little bigger, and the radius of the neutron star will be a little bigger. If the pressure is smaller, both radii will be smaller, Piekarewicz explains.

What’s needed now are better numbers from physicists who do elaborate experiments on such things in national labs, says Piekarewicz, a theorist rather than experimentalist.

“If we can make a very precise measurement of the neutron skin in lead, we will learn a lot of the properties of the neutron star.

”Better numbers may not be long in coming. An experiment using a new technique to measure the neutron skin in lead has been accepted at the Thomas Jefferson National Accelerator Facility in Virginia. The technique (with the jawbreaker name “parity violating electron scattering”) promises a clean, clear measurement, without underlying assumptions, accurate to within one percent of the true value. Results are expected by next year.

Once the experimentalists nail down a value for the radius of the lead nucleus, Piekarewicz believes his equations can be used to predict with great confidence what neutron stars truly are made of.

Are they impossibly dense balls of nothing but neutrons, or do they contain other, extraordinary kinds of matter? Piekarewicz is hoping for the latter, because from a physics standpoint, it’s simply a lot more exciting.

Quark Soup, Anyone?

If indeed Piekarewicz gets good measurements plugged into his equations, and the math shows there’s more to neutron stars than mere neutrons, what could that stuff be?

There’s only one leading contender for what it might be, he says. It involves quarks, a family of sub-subatomic particles that make up protons and neutrons. These point-like particles, discovered in the 1960s, have truly bizarre properties. For one thing, quarks are always found in pairs and triplets, trapped inside larger particles such as protons and neutrons, and they are never found in isolation. For another, the force holding quarks together actually grows weaker the closer the quarks get to each other, one of nature’s oddest phenomena.

At least in theory, the Big Bang that created the universe produced a “soup” of freely moving, independent quarks. Within a fraction of a second, as the newborn universe rapidly cooled, the quarks came together to form protons and neutrons. The “quark soup” disappeared, never to be appear again.

But some physicists think that the unfathomable pressures at the heart of neutron stars could be high enough to recreate the earliest stages of the universe. Theory predicts that if you can push neutrons or protons close enough to each other, the quarks themselves get squeezed close enough to break the bonds holding them together. As a result, protons and neutrons dissolve, freeing quarks to fly around as individual particles, ergo, quark soup.

If true, then neutron stars could be the only place in the universe where quark soup exists. Outside rarified experiments in particle accelerators on Earth, physicists know of no other place where conditions are extreme enough for such material to form.

Whatever the case may be, getting a final answer won’t be possible until astronomers collect more reliable measurements of neutron stars. Space-based telescopes such as Hubble and Chandra have provided a wealth of new data, but so far there’s not been a single reliable measurement taken on both the mass and radius of a neutron star, Piekarewicz says.

“We need to know the mass and radius for many, many different neutron stars to help decide among the different models. Those measurements are hard, but they can be done. And I think the next generation of telescopes might be able to achieve them.

”Piekarewicz hopes the data will show there is more to a neutron star than neutrons. If that proves to be the case, then studying neutron stars—both theoretically and observationally—may well produce new insights into the fundamental particles and forces of the universe and into the Big Bang that started it all. The prospect of such new worlds to conquer makes Piekarewicz’s eyes light up.

“If exotic matter is proven to exist,” Piekarewicz says, “it will probably start a new field of study, using neutron stars for the study of exotic states of matter.

“But if there’s nothing exotic there—even though we can then say we understand most of the physics of neutron stars—from a scientific point of view, it would be disappointing.”