Cosmological perturbations in a generalised axion-like dark energy model
Cosmological perturbations in a generalised axion-like dark energy model
The nature of dark energy remains one of the most profound mysteries in modern cosmology. While the standard ΛCDM model, which includes a cosmological constant (Λ), has been remarkably successful in explaining the observed expansion of the universe, it leaves many theoretical questions unanswered—particularly concerning the fine-tuning and coincidence problems. This has motivated the exploration of dynamic models of dark energy, with axion-like fields emerging as a promising candidate due to their strong theoretical motivation and rich phenomenology.
In our current investigation, we delve into the behavior of cosmological perturbations within a generalized axion-like dark energy model. Unlike standard scalar field models that assume a canonical kinetic term and a simple potential, axion-like models often feature non-standard potentials inspired by high-energy physics (e.g., string theory), as well as modifications to the kinetic structure. These features enable them to act as dynamical fields evolving over cosmological timescales, potentially explaining the late-time acceleration of the universe without invoking an unnaturally small constant.
We consider a class of generalized axion-like models described by a scalar field φ with a periodic or quasi-periodic potential of the form:
V(φ) = Λ⁴ [1 − cos(φ/f)] + additional corrections
Here, Λ is a dynamically generated energy scale, and f is the axion decay constant. The model is extended to include generalized kinetic terms and couplings to curvature or other sectors, allowing us to investigate its full impact on background evolution and perturbative dynamics.
Perturbation Analysis
At the linear level, scalar perturbations in the metric and matter fields play a crucial role in determining the model’s viability. We derive and analyze the perturbed Einstein equations, focusing on gauge-invariant variables such as the Bardeen potentials. The scalar field's fluctuations introduce additional degrees of freedom that interact with matter perturbations and gravitational potentials, modifying the growth of structure compared to the ΛCDM baseline.
One of the key aspects of our study is to track how the effective sound speed and anisotropic stress evolve in the presence of an axion-like dark energy component. These factors directly impact the Integrated Sachs-Wolfe (ISW) effect, the matter power spectrum, and the cosmic microwave background (CMB) anisotropies. Furthermore, we examine the possibility of suppressing power on small scales or enhancing clustering, depending on the chosen parameter space.
Numerical Implementation
To achieve robust predictions, we incorporate our model into a Boltzmann solver such as CLASS or CAMB, adapting the code to include the generalized field dynamics. This allows for comparison with existing CMB data, baryon acoustic oscillations, and Type Ia supernovae measurements.
Interestingly, certain parameter choices lead to distinct observational signatures—such as a time-varying equation of state w(z) crossing the phantom divide (w < –1), or deviations in the gravitational slip parameter, which can be probed through weak lensing and redshift-space distortions.
Conclusion
Generalized axion-like models represent a compelling alternative to the cosmological constant by offering rich phenomenology and theoretical elegance. By studying their impact on cosmological perturbations, we move closer to distinguishing among competing dark energy models through precision cosmological observations. Future surveys like Euclid, LSST, and SKA will provide critical data to test such frameworks.
Global Particle Physics Excellence Awards
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