DESI Constraints on Exponential Quintessence

DESI Constraints on Exponential Quintessence

May 30, 2024 | Omar F. Ramadan, Jeremy Sakstein, and David Rubin
The DESI collaboration analyzed their first year of data and found a preference for thawing dark energy scenarios when using parameterized equations of state for dark energy. However, when analyzing the data within the context of the exponential quintessence model, no preference for this model over ΛCDM was found. Both models were poorer fits to the data than the Chevallier-Polarski-Linder $ w_{0}-w_{a} $ parameterization. The worse fit for exponential quintessence is due to a lack of sharp features in the potential, resulting in a slowly-evolving dark energy equation of state that cannot simultaneously fit the combination of supernovae, DESI, and cosmic microwave background data. The origin of the present-day acceleration of the cosmic expansion, dark energy (DE), remains a mystery. Previously, all observations were compatible with dark energy driven by a cosmological constant Λ, but this has recently been challenged by the DESI first year data release, which shows a preference for thawing dark energy at the level of 2.5σ, 3.5σ, and 3.9σ. In this scenario, the equation of state (EOS) of dark energy $ w(z) $ was frozen at a constant value in the past but recently began to evolve away from this, in contrast to Λ which has constant $ w(z) = -1 $. In quintessence models, dark energy is driven by a scalar field $ \phi $ with mass m that is initially frozen at its initial condition by Hubble friction so that $ w = -1 $ but begins to roll sometime in the recent past when $ H \sim m $. This rolling causes the EOS to deviate from -1 with $ w \geq -1 $. The specific action considered is $ S = \int\mathrm{d}^{4}x\sqrt{-g}\left[\frac{M_{\mathrm{Pl}}^{2}}{2}R(g)-\frac{1}{2}\partial_{\mu}\phi\partial^{\mu}\phi-V(\phi)\right] $. The evolution of the scalar is determined by the Klein-Gordon equation $ \ddot{\phi}+3H\dot{\phi}+\frac{\mathrm{d}V}{\mathrm{d}\phi}=0 $. The phenomenology of quintessence DE depends upon the choice of potential. In this work, the exponential quintessence model $ V(\phi)=V_{0}e^{-\lambda\frac{\phi}{M_{\mathrm{Pl}}}} $ is studied. This model is well-understood because the equations can be written in an autonomous form, implying that dynamical systems methods can be used to identify the steady-state solutions. The thawing DE scenario can be realized within this potential as follows. At early times, the field is frozen such that $ w_{\phiThe DESI collaboration analyzed their first year of data and found a preference for thawing dark energy scenarios when using parameterized equations of state for dark energy. However, when analyzing the data within the context of the exponential quintessence model, no preference for this model over ΛCDM was found. Both models were poorer fits to the data than the Chevallier-Polarski-Linder $ w_{0}-w_{a} $ parameterization. The worse fit for exponential quintessence is due to a lack of sharp features in the potential, resulting in a slowly-evolving dark energy equation of state that cannot simultaneously fit the combination of supernovae, DESI, and cosmic microwave background data. The origin of the present-day acceleration of the cosmic expansion, dark energy (DE), remains a mystery. Previously, all observations were compatible with dark energy driven by a cosmological constant Λ, but this has recently been challenged by the DESI first year data release, which shows a preference for thawing dark energy at the level of 2.5σ, 3.5σ, and 3.9σ. In this scenario, the equation of state (EOS) of dark energy $ w(z) $ was frozen at a constant value in the past but recently began to evolve away from this, in contrast to Λ which has constant $ w(z) = -1 $. In quintessence models, dark energy is driven by a scalar field $ \phi $ with mass m that is initially frozen at its initial condition by Hubble friction so that $ w = -1 $ but begins to roll sometime in the recent past when $ H \sim m $. This rolling causes the EOS to deviate from -1 with $ w \geq -1 $. The specific action considered is $ S = \int\mathrm{d}^{4}x\sqrt{-g}\left[\frac{M_{\mathrm{Pl}}^{2}}{2}R(g)-\frac{1}{2}\partial_{\mu}\phi\partial^{\mu}\phi-V(\phi)\right] $. The evolution of the scalar is determined by the Klein-Gordon equation $ \ddot{\phi}+3H\dot{\phi}+\frac{\mathrm{d}V}{\mathrm{d}\phi}=0 $. The phenomenology of quintessence DE depends upon the choice of potential. In this work, the exponential quintessence model $ V(\phi)=V_{0}e^{-\lambda\frac{\phi}{M_{\mathrm{Pl}}}} $ is studied. This model is well-understood because the equations can be written in an autonomous form, implying that dynamical systems methods can be used to identify the steady-state solutions. The thawing DE scenario can be realized within this potential as follows. At early times, the field is frozen such that $ w_{\phi
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