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Shrinking the incredible universal magnet.

Shrinking the incredible universal magnet

In the Beginning, did a magnetic field permeate the expanding universe? And if so, how did it affect the universe we see today?

Cosmologists agree that the strength of that ancient field, if it existed, determined whether magnetism helped gravity form the structures taht fill the present universe -- from individual galaxies to vast galactic clusters -- and whether the magnetic fields of those structures derived from a primordial field.

Radio astronomer jacques P. Vallee of the Herzberg Institute of Astrophysics in Ottawa, Ontario, has conducted the most extensive search to date for traces of a uniform, universal magnetic field. His analysis of the magnetic fields of more than 300 distant galaxies and quasars reduces by five times the lowest prior estimate of the maximum strength possible for a universal field. To the limits of current measurements, "the universe as a whole lacks magnetism," Vallee reports in the Sept. 1 ASTROPHYSICAL JOURNAL.

He reaches his conclusion despite recent discoveries of magnetic fields on far grander scales than ever before -- spanning entire galaxy clusters--and last year's report of a magnetic "bridge" between two clusters. Vallee concedes that a universal magnetic field might yet exist, but at a strength too weak for detection by current techniques. As observers find additional cosmic magnetic fields, he says, the degree of precision for measuring a universal field will improve. But he adds that it "will take perhaps 100 years" to collect data on enough fields to reveal a universal field.

Vallee and others say the revised limit constrains theories of galaxy formation and clustering, ruling out the possibility that any existing universal field participates in the creation of new galaxies or clusters. And "it's debatable whether [such a weak universal field] could have had any effect at all [on cosmic structure] in the early universe," Vallee says.

The limit further weakens the ailing superconducting-cosmic-string theory, which requires a stronger magnetic field than Vallee's calculations allow, says astrophysicist Abraham Loeb of the Institute for Advanced Study in Princeton, N.J. That theory suffered a major blow last spring when the Cosmic Background Explorer satellite confirmed the near-perfect smoothness of the cosmic background radiation, Loeb says (SN:3/24/90, p.184; 4/21/90, p.245).

Vallee's finding also affects theories of how galactic and intergalactic magnetic fields formed. It reduces the probability that they condensed out of a more vast, universal field, Loeb says, and supports the idea that they originated with locally grown "seed" fields, amplified to current strengths by galactic rotations.

Astronomers detect cosmic magnetic fields by measuring the rotation of polarized radio waves emitted from distant galaxies and quasars. This rotation, caused by intervening magnetic fields, correlates with field strength and direction. Researchers then use statistical techniques to distinguish regional influences, such as the field of our own galaxy, from the universal field.

But that approach appears flawed, contends Philipp P. Kronberg of the University of Toronto, a co-discoverer of the magnetic "bridge" between clusters. Earth-bound observers looking across vast expanses of space can only detect an average magnetic field, he says. If intergalactic magnetic fields change direciton every few million light-years or so, as Kronberg suspects, their opposite twists on radio waves reaching Earth cancel each other out to some degree.

"If you were to take a probe and walk around the universe," Kronberg asserts, "you could find that the field is a good bit higher" than Vallee has calculated. Moreover, he argues that Vallee's limit depends strongly on an assumed value for the density of matter in the universe -- a number, he says, that astronomers "pick out of a hat."
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Author:Weiss, P.L.
Publication:Science News
Date:Sep 15, 1990
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