How to test stringy dark energy?

Abstract

It is suggested, by using a covariant lagrangian formalism to estimate the equation of state w = p/ρ, that stringy dark energy predicts w < −1, a negative pressure larger in magnitude than that for a cosmological constant or quintessence. This would lead to a later transition from decelerating to accelerating cosmological expansion; w = −4/3 is briefly considered as one illustrative example.The last few years have witnessed a revolution in our knowledge of the universe. Paramount among recent observational discoveries the phenomenon of dark energy, probably the most surprising discovery in physics or astronomy since parity violation almost a half a century ago. As in that case, the explanation of dark energy will surely impact a wide range of disciplines.It was recently proposed that the cosmological dark energy which causes the present accelerated expansion of the universe may arise from a stringy origin [1]. In particular, in a toroidal closed string universe the correlation between winding and momentum modes of closed strings leads in the phase transition at the Hagedorn temperature to a condensate phenomenon.For stringy dark energy, the pressure can certainly be negative because for exponentially falling ω(κ) the group velocity v g ∝ dω(κ)/dκ is negative and can dominate the pressure expression. The detailed calculation of the value of w(Z) depends on the Hagedorn phase transition where the closed strings are strongly interacting and hence impracticable to compute. Still we can hope to test the approach already by some estimates. The idea of an exponentially decrease of ω(κ) at large transplanckian κ providing a candidate for dark energy was proposed in [2].We discuss the pressure estimate first in general terms followed by a more specific calculation. Finally, the resultant red-shift for the transition from decelerated to accelerated cosmological expansion is briefly dicussed.General dispersion relation. Consider a spatially flat FRW universe with the spacetime line element,Let φ be a field satisfying a wave equation that, either exactly or in the adiabatic approximation, has plane wave solutions,where k ≡ | k|. If the wave equation were Lorentz covariant in the flat spacetime limit, then one would have ω(k/a(t )) = (k 2 /a 2 +m 2 ) 1/2 . We will suppose that ω has this form when the momentum k/a(t) is small with respect to the Planck mass, m * . For larger momenta, we will suppose that ω(c) is a decreasing function of c ≡ k/a(t).The group velocity, v g , of a wave packet,formed from these plane waves over a narrow range of k's near the value k, is. (4) It follows that the group velocity is negative if k is larger than m * . This means that the wave packet moves in the direction opposite to the momentum, k/a(t). Consider a set of such wave packets contained at time t within a given comoving volume. When one of the wave packets passes out of the comoving volume it transfers momentum into the volume. This means that the pressure exerted by the energy density inside a comoving volume on the bounding surface of...

Dual interacting cosmologies and late accelerated expansion

“…This type of fields may arise in string theory, see [12] and references therein. We write its pressure and energy density admitting both signs for kinetic energy term, see e.g.…”

Preventing eternality in phantom inflation

“…However, one could consider that the phantom fields arise through an effective description of a nonphantom fundamental (probably higher-dimensional) underlying theory, consistently with the basic requirements of quantum field theory [27]. Indeed actions with phantomlike behavior may arise in supergravity [28], scalar tensor gravity [29], higher derivative gravity [30], braneworld [31], kfield [32], stringy [33] and others scenarios [34,35]. The peculiar nature of phantom fields requires the inflation paradigm to be suitably redesigned.…”

Can we be tricked into thinking thatwis less than−1?

“…Nevertheless, such energy components have been known for some time [25,26,27] and their role as possible dark energy candidates was raised by Caldwell [28], who referred to DEC-violating sources as "phantom" components. The implications of these have since been investigated by several authors (for some examples see [16,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44]. Typically, DEC-violating sources with w < −1 are subject to violent instabilities, although these may conceivably be cured in models with higher-derivative kinetic terms [34,43,44].…”