The Accelerated Acceleration of the Universe

Abstract

We present a simple mechanism which can mimic dark energy with an equation of state w < -1 as deduced from the supernova data. We imagine that the universe is accelerating under the control of a quintessence field, which is moving up a very gently sloping potential. As a result, the potential energy and hence the acceleration increases at lower redshifts. Fitting this behavior with a dark energy model with constant w would require w<-1. In fact we find that the choice of parameters which improves the fit to the SNe mimics w = -1.4 at low redshifts. Running up the potential in fact provides the best fit to the SN data for a generic quintessence model. However, unlike models with phantoms, our model does not have negative energies or negative norm states. Future searches for supernovae at low redshifts 0.1 < z < 0.5 and at high redshifts z>1 may be a useful probe of our proposal.

Figures (4)

• Figure 1: Sketch of the form of the potential including an example of the possible non-linear behavior before the onset of the linear regime.
• Figure 2: Residual magnitudes for high redshift supernovae data relative to a universe with a best fit cosmological constant. The dashed line thus gives the prediction for $\Omega_\Lambda=0.71$. The green (light) line gives the best fit for a linear potential with $\Omega_{DE,0}= 0.77$ and $z_*=2$, which requires the field to run up the potential. The red (dark) line gives the best fit for a change in $w$ from -0.73 for $z>0.47$ to $w=-1$ for $z<0.47$ with $\Omega_{DE}= 0.80$.
• Figure 3: On the left we show $\Omega_M$ as a function of $z$ with the red (dark) line which dominated for $z>0.5$, $\Omega_{kin}$ with the green (light) line, and $\Omega_{DE}$ with the dashed line for a scalar field with the best fit linear potential for $z<1$ and with a simple toy model for the asymmetric part of the potential for $z>1$ as described in the text. On the right we show in red (dark) the time dependent effective equation of state parameter $w_{eff}$ for the scalar field in the same scenario, while in green (light) we show the full $w_{eff}$ including dark matter.
• Figure 4: Residual magnitudes relative to a flat universe with a best fit cosmological constant. Data points are SN Ia gold data in redshift bins of 0.1. The bottom, green (light) line gives the best fit for a linear potential, with $\Omega_{DE,0}= 0.77$. The top, red (dark) line gives the best fit for a phantom with $w=-1.4$ with $\Omega_{DE}= 0.6$. The blue (darkest) line gives the behavior when the linear potential is matched onto a quadratic potential at $\phi(z_*=1)$, as described in the text.