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No black holes after all?

Aug. 11, 2006
Courtesy Harvard-Smithsonian 
Center for Astrophysics
and World Science staff

One of the brightest and furthest known objects in the universe might not be a black hole as traditionally believed, but rather an exotic new type of object, a new study suggests. 

An artist's image of a black hole at the center of a galaxy. The black hole itself would be hidden at the center of the disc of swirling gas, with two jets spurting in opposite directions. In new research, some astronomers claim the central object is not a black hole, but another type of object that shrinks itself at an imperceptibly slow rate, forever. (Image Courtesy NASA Goddard Space Flight Center, A. Kamajian)

And the researchers say this raises doubts as to whether other so-called black holes are really that, either.

The astronomers are elbowing aside the time-honored concept of the black hole: a large object that compacts itself, under its own gravity, to an infinitely dense point with such gravitational strength that nothing nearby can escape its grip.

Instead, the researchers are picturing a body with a definite size, and a surprising property: it gradually crams itself into a smaller space forever, but never achieves a black hole’s infinitely small size.

In the study, Rudy Schild of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. and colleagues scrutinized an object of a stupendously bright type known as a quasar. 

Quasars, most astronomers agree, are the centers of far-off galaxies.

Scientists traditionally picture a quasar’s core as a disc of gas spiraling into a “supermassive” black hole, which sucks it in. The brilliance comes from the gas, which heats up as it races inward. Some of it also shoots out in two oppositely-directed jets.

Quasars appear only in the furthest reaches of the known cosmos. Astronomers reason that this is because they existed only long ago. The furthest areas are those where we see the universe as it was long ago, because it takes so long for light from those places to reach us.

Quasar-like structures also exist in the more recent, and thus nearby, universe. They persist as the “black holes” also believed to lie at the center of most galaxies. But these are dimmer than quasars. Scientists think this is because they’ve consumed much of the available gas.

Theorists have struggled to understand the workings of quasars’ jets and discs, called accretion discs. It has also been hard for observers to see the hearts of quasars, because the regions are so compact and distant.

Schild’s group studied a quasar designated Q0957+561, about 9 billion light-years away in the direction of the constellation Ursa Major. A light-year is the distance light travels in a year. 

The quasar holds a central compact object weighing the equivalent of 3 to 4 billion Suns. Most scientists would call it a black hole, but Schild said his findings suggest otherwise: surprisingly, it’s magnetic, unlike a black hole.

Double vision

The researchers chose Q0957+561 because it’s associated with a so-called cosmic lens. Einstein’s Theory of Relativity holds that a galaxy’s gravity bends space nearby it. It thus also acts as a sort of lens, bending light. This results in two images of the distant quasar and magnifies its light. Stars and planets within the nearby galaxy also affect the quasar’s light, a related phenomenon called microlensing.

“With microlensing, we can discern more detail from this so-called ‘black hole’ two-thirds of the way to the edge of the visible universe than we can from the black hole at the center of the Milky Way,” our galaxy, said Schild. Schild monitored the quasar’s brightness for 20 years, along with an international consortium of observers at 14 telescopes. 

The team studied the quasar’s core, pinpointing a proposed location where the jets form—something that 60 years of past research have failed to explain, Schild said.

His team calculated that the jets come from two regions each about 25 times wider than the distance between the sun and Pluto. These lie directly above the poles of the central compact object, at about 200 times the Sun-Pluto distance.

Just one proposed scenario can easily explain these locations, Schild said. The central object is magnetic, and interacts with the disc through its surrounding magnetic field. As it spins, the field winds up like a spool. Eventually it winds so tightly that it “breaks” explosively before re-forming itself in a more relaxed configuration. The breakages release energy that powers the jets.

But a black hole in an accretion disc can’t have its own magnetic field, Schild added. This is because normally, a spinning object can be magnetic only if it carries an electric charge. A black hole can’t sustain such charge, because any charged hole will immediately suck in enough oppositely-charged material to cancel out its own charge. (There are two types of electric charge, positive and negative).

Forever shrinking

The problem vanishes, Schild and colleagues argue, with the new type of compact object that they propose, called a Magnetospheric Eternally Collapsing Object, or MECO.

This body, a variant of an object whose existence was first proposed by the Indian physicist Abhas Mitra in the late 1990s, is one that not unlike a black hole, continually shrinks into an ever-smaller space. 

But it never becomes a black hole. Instead, its shrinkage slows down until it becomes imperceptible, but goes on steadily—so slowly, it could go on for many times the lifetime of the universe. Unlike a black hole, a MECO also has definite size. Moreover, objects sucked in can theoretically go back out, albeit with extreme difficulty.

A MECO, essentially a dense ball of plasma, continually generates magnetic fields through surface currents, explaining the magnetism, Schild said. His team’s research appeared in the July issue of The Astronomical Journal.

It won’t be easy for the MECO theory to gain wide acceptance among scientists, astronomers say, given that black holes have been the accepted scenario since Einstein. But Mitra and a few other theorists claim black holes don’t exist at all—only Eternally Collapsing Objects.

A stringent test might soon be available to resolve the dispute. Within 10 years, astronomers say, technology will let them observe the signature feature of black holes, the “event horizon.” This is the area surrounding a black hole, within which no infalling object can ever come out. 

Such observations could either confirm black holes’ existence, or raise new questions over it, if no event horizons show up where they’re expected.

For now, Schild said he’s not disputing the existence of all black holes. He’s just focusing on Q0957+561. His team wants to avoid “inflated claims,” he wrote in an email, as some critics might use this to “discredit the entire body of work.”

* * *

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References
R. E. Schild, D.J. Leiter, S. L. Robertson, 2006. Observations Supporting the Existence of an Intrinsic Magnetic Moment Inside the Central Compact Object Within the Quasar Q0957+561. E-AJ 132, 430-432.

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