Plasma Cosmology - Astrophysical Plasma

Astrophysical Plasma

In contrast to plasma cosmology, plasma physics is accepted uncontroversially as having great influence on many astrophysical phenomena. The majority of ordinary matter in the universe is in the form of plasma, and plasma is a good conductor of electricity. An important figure in this field was Hannes Alfvén, who devoted much of his professional career to investigating plasmas and was awarded the 1970 Nobel Prize in Physics for his work on magnetohydrodynamics (MHD).

Alfvén's view was that plasma played an important role in the universe. He asserted that electromagnetic forces are far more important than gravity when acting on interplanetary and interstellar charged particles. Alfvén's worked to scale plasma theory from the laboratory to the magnetosphere. Alfvén wrote a paper in 1939 supporting the theory of Kristian Birkeland, who had written in 1913 that what is now called the Solar wind generated currents in space that caused the aurora. Birkeland's theory was disputed at the time and Alfvén's work in turn was disputed for many years by the British geophysicist and mathematician Sydney Chapman, a senior figure in space physics, who argued the mainstream view that currents could not cross the vacuum of space and therefore the currents had to be generated by the Earth. But eventually in 1967 Birkeland's then fringe theory was proved to be correct after a probe was sent into space, and these magnetic field aligned currents are now named Birkeland currents in his honour. The crucial results were obtained from U.S. Navy satellite 1963-38C, launched in 1963 and carrying a magnetometer above the ionosphere.

Plasma effects being vital in slowing down a protostar's spin in stellar formation is accepted as mainstream science today (although the actual mechanism is not so clearcut). One proposed mechanism to remove angular momentum and allow a protostar to contract is magnetic braking. Other things in the Solar System that are beyond the Earth's magnetosphere in which plasma plays a central role are the heliospheric current sheet and the interplanetary medium. Theories in astrophysical plasma in the Solar system are a fundamental part of plasma cosmology.

On a larger scale, galaxy groups and clusters have a lower plasma density by several orders of magnitude, and magnetic fields are not strong enough to significantly affect virializing processes. Standard astrophysical structure formation models, at the level of galaxy formation, depend on the mass distribution of the simulated system rather than its electrodynamic interactions. Such models do however have to assume the existence of dark matter to account for observed galaxy rotation curves. Plasma cosmologists propose that plasma effects explain galaxy rotation curves without the need for dark matter.

Alfvén hypothesized that Birkeland currents (here meaning currents in space plasmas which are aligned with magnetic field lines) were responsible for many filamentary structures and that a galactic magnetic field and associated current sheet, with an estimated galactic current of 1017 to 1019 amperes, might promote the contraction of interstellar clouds and may even constitute the main mechanism for contraction, initiating star formation. This is in opposition to the standard view that magnetic fields can hinder collapse. However large-scale Birkeland currents have not been observed and the length scale for charge neutrality is predicted to be far smaller than the relevant cosmological scales.

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