Can the dark energy equation-of-state parameter<i>w</i>be less than<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>−</mml:mi><mml:mn>1</mml:mn><mml:mi>?</mml:mi></mml:math>

Abstract

Models of dark energy are conveniently characterized by the equation-of-state\nparameter w=p/\\rho, where \\rho is the energy density and p is the pressure.\nImposing the Dominant Energy Condition, which guarantees stability of the\ntheory, implies that w \\geq -1. Nevertheless, it is conceivable that a\nwell-defined model could (perhaps temporarily) have w&lt;-1, and indeed such\nmodels have been proposed. We study the stability of dynamical models\nexhibiting w&lt;-1 by virtue of a negative kinetic term. Although naively\nunstable, we explore the possibility that these models might be\nphenomenologically viable if thought of as effective field theories valid only\nup to a certain momentum cutoff. Under our most optimistic assumptions, we\nargue that the instability timescale can be greater than the age of the\nuniverse, but only if the cutoff is at or below 100 MeV. We conclude that it is\ndifficult, although not necessarily impossible, to construct viable models of\ndark energy with w&lt;-1; observers should keep an open mind, but the burden is on\ntheorists to demonstrate that any proposed new models are not ruled out by\nrapid vacuum decay.\n

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Affiliated Institutions

• University of Chicago US

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