T. Davis, P. Davies, and C. Lineweaver. (2003)cite arxiv:astro-ph/0305121Comment: 14 pages, including 6 figures, to appear in Classical and Quantum Gravity, reference added.
L. Alabidi, and J. Lidsey. (2008)cite arxiv:0807.2181
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match the version that has been accepted to Phys Rev D.
W. Kinney, and K. Tzirakis. (2007)cite arxiv:0712.2043
Comment: 10 pages, LaTeX; V2: version submitted to PRD. References added,
minor error in text corrected.