Cosmic Magnetic Fields Exert Negative Pressure and Act as a Cosmological Constant

Recently there has been mounting evidence for the existence of a cosmological constant (CC). Here we show that cosmic magnetic fields act as a CC, having negative pressure and tending to cause cosmological acceleration. However, if present-day estimates of the intensity of these fields are correct, they would be too weak to be the cause of the acceleration. It is pointed out that large-scale coherent Yang-Mills field structures can also act as a CC. For a few years now empirical evidence from different astrophysical sources has been accumulating that suggests that either the Universe is open or there is a nonzero cosmological constant (CC). Recently the issue has been greatly clarified by groups studying type Ia supernovae. There is strong evidence that the cosmic expansion is accelerating and that there is a positive CC. Here we point out that cosmic magnetic fields act as a CC and are responsible for a part of the experimentally observed acceleration. This is surprising since there is a tendency to think that something that can act as a CC would have to be a very exotic object, but it is actually straightforward to show from the diagonal space components of the stress tensor of cosmic magnetohydrodynamic configurations that these magnetic fields exert a negative pressure and tend to accelerate the expansion of the Universe. The reason that we know so accurately the value of the CC is that, fortunately, the combination of the nonrelativistic mass density ΩM and CC density ΩΛ as measured using supernovae is almost orthogonal to the combination probed by the cosmic microwave background anisotropy spectrum. It is then possible to estimate the values of these two quantities with precision, and it turns out that ΩM ≈ 0.3 and ΩΛ ≈ 0.7. The equations of general relativity for a spatially flat Robertson-Walker (homogeneous and isotropic) universe are the expansion rate equation