Causality and the speed of sound for general scalar field models, including w < -1, tachyonic, phantom, k-essence and curvature corrections

Details Causality and the speed of sound for general scalar field models, including w < -1, tachyonic, phantom, k-essence and curvature corrections Causality and the speed of sound for general scalar field models, including w < -1, tachyonic, phantom, k-essence and curvature corrections

Sep 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004jcap...09..012d&link_type=abstract

Journal of Cosmology and Astroparticle Physics, Issue 09, pp. 012 (2004).

Physics

7

Scientific paper

The results from the SN1a projects suggest that the dark energy can be represented by a fluid with w < -1. However, it is commonly argued that a fluid with |w| > 1 contradicts causality.
Here, we will show that a fluid with |w| > 1 does not contradict causality if w is not constant. Scalar fields are the most promising candidates for describing the dark energy and they do not have a constant equation of state parameter w. Scalar potentials may lead to regions where w is larger or smaller than one and even regions where the group velocity dp/drgr diverges.
We study the evolution of scalar field perturbations and we show that the 'speed of sound' is always smaller than the speed of light, independently of the value of w = p/rgr or dp/drgr. In general, it is neither the phase velocity nor the group velocity that gives the 'speed of sound'. In the analysis we include the special cases of |w| > 1, tachyonic, phantom, k-essence scalar fields and curvature corrections. Our results show that scalar fields do not contradict causality as long as there is dispersion.

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