Essay

MAGNETIC FIELDS

Adapted from P. Coles, 1999, The Routledge Critical Dictionary of the New Cosmology, Routledge Inc., New York. Reprinted with the author's permission. To order this book click here: http://www.routledge-ny.com/books.cfm?isbn=0415923549

Fields associated with the electromagnetic interaction. Although the strongest fundamental interaction on the large scales relevant to cosmology is gravity, there are situations in which magnetic fields play an important role. The Earth has a significant magnetic field, as does the Sun. Galaxies like the Milky Way also possess a large-scale magnetic field (with a strength of a few microgauss) and there is evidence for magnetic fields in the intergalactic medium, particularly inside rich clusters of galaxies (see large-scale structure). Active galaxies also show evidence of strong magnetic effects: radio galaxies, for example, produce synchrotron radiation as electrons spiral around the magnetic field lines.

The magnetic fields in galaxies are thought to arise from a dynamo effect: a small initial field, generated perhaps by turbulence, becomes amplified and ordered as it is wound up by the rotation of the galactic disk. Although this is a plausible model for the generation of the fields observed, there are some problems with it. For example, it is not clear whether typical spiral galaxies have experienced enough rotation in the age of the Universe for the fields to have been sufficiently amplified. Moreover, some objects at very high redshifts, such as the damped Lyman-alpha systems seen in quasar spectra, appear also to possess magnetic fields strong enough to produce a characteristic Faraday rotation of the polarisation of electromagnetic radiation passing through them. The detailed mechanism by which these astrophysical magnetic fields may have been generated has yet to be elucidated in a completely satisfactory fashion.

It has also been speculated that there might be a cosmological magnetic field pervading the Universe that could have been generated early on in the thermal history of the Universe as a result of primordial phase transitions. If such a field exists, it must be very weak. Since magnetic fields are vector fields, they possess direction as well as strength. The resulting pressure forces would have caused the Universe to expand more quickly in some directions than in others, so a large-scale cosmological field would produce an anisotropic cosmological model. This is one of the few situations where physically realistic exact solutions of the Einstein equations of general relativity can be obtained that do not invoke the cosmological principle. However, the observed near-isotropy of the cosmic microwave background radiation means that a large-scale coherent magnetic field would have to be very weak.

A cosmological magnetic field need not, however, be uniform: it might be tangled up on a relatively small scale so that the effects of its anisotropy are not evident on large scales. Even this kind of field would have to be very weak. A tangled web of field lines acts as a source of pressure which behaves in a very similar way to the pressure of radiation. Any such field present at the time of nucleosynthesis, for example, would alter the rate of expansion of the Universe at that epoch, and the observed light element abundances would no longer agree with our theoretical calculations. We can argue, though, that the observed agreement requires the cosmological magnetic field to contribute no more than one part in a million to the total energy density of the Universe.

Although there are thus tight limits on the strengths of galactic or cosmic magnetic fields, they could, in principle at least, oblige us to modify any models based on the assumption that gravity alone is relevant. For example, we usually estimate the amount of dark matter ingalaxies by using the virial theorem. If galactic magnetic fields were sufficiently strong, they could significantly alter the form of the equilibrium configuration of galaxies by introducing a pressure force independent of the gas pressure and gravity.