Rapidly Descending Dark Energy and the End of Cosmic Expansion (2201.07704v2)

Abstract: If dark energy is a form of quintessence driven by a scalar field $\phi$ evolving down a monotonically decreasing potential $V(\phi)$ that passes sufficiently below zero, the universe is destined to undergo a series of smooth transitions. The currently observed accelerated expansion will cease; soon thereafter, expansion will come to end altogether; and the universe will pass into a phase of slow contraction. In this paper, we consider how short the remaining period of expansion can be given current observational constraints on dark energy. We also discuss how this scenario fits naturally with cyclic cosmologies and recent conjectures about quantum gravity.

• The paper proposes a quintessence dark energy model (Q-SC-CDM) where dark energy density can become negative, leading to a future cessation of cosmic expansion and transition to contraction.
• Numerical analysis suggests these cosmic transitions, from acceleration to deceleration and then contraction, could occur in less than a single Hubble time, possibly within billions of years.
• This dynamic dark energy model challenges the standard cosmological constant and aligns with certain quantum gravity conjectures, emphasizing the need for refined observational methods to test its predictions.

Rapidly Descending Dark Energy and the End of Cosmic Expansion: An Expert Analysis

The paper, authored by Andrei, Ijjas, and Steinhardt, presents a compelling exploration of the role of dark energy in the future of our universe's expansion, proposing a model where the dark energy is characterized by quintessence. This model significantly deviates from the standard $\Lambda$CDM paradigm by suggesting that the universe's accelerated expansion may cease, followed by an end to expansion itself, and transition into a phase of slow contraction.

Core Proposition and Implications

The authors investigate the implications of dark energy described by a scalar field $\phi$ descending along a potential $V(\phi)$ that passes to negative values. This scenario suggests a future cosmological evolution wherein the universe transitions from its current phase of accelerated expansion to decelerated expansion, culminating in contraction. This model, identified as Q-SC-CDM (Quintessence-driven, Slow-Contraction, Cold Dark Matter), contrasts sharply with the $\Lambda$CDM model which predicts eternal acceleration fueled by a positive cosmological constant.

The implications of such a model are profound both theoretically and observationally. Cosmologically, this model aligns with the cyclic models of the universe, offering a natural mechanism for the universe to transition through different phases without encountering singularities or requiring exotic physics beyond damped potentials. The scenario fits recent conjectures in quantum gravity that challenge the conventional understanding of dark energy.

Numerical Findings

The paper provides a quantitative analysis of this proposition under current observational constraints, specifically examining how soon these cosmic transitions could commence. The results are striking; they estimate the transition from acceleration to deceleration and the eventual beginning of contraction may occur considerably sooner than typically expected—in cosmological terms, within less than a single Hubble time.

The authors utilize potentials of form $V(\phi) = V_0 e^{-\phi/M} - V_1 e^{\phi/m}$, revealing that minimal timescales for transitions, where $M$ and $m$ determine the steepness of the potential, could initiate as soon as a few billion years, depending on the parameter set. This analysis leads to predictions that these transitions could be aligned temporally more closely with geological time frames rather than distant cosmic timescales.

Theoretical and Observational Relevance

From a theoretical standpoint, this model challenges the persistent belief in the static nature of dark energy as a cosmological constant or metastable vacuum. By allowing $V(\phi)$ to evolve dynamically through negative regions, the Q-SC-CDM model expands the discourse on viable models in cosmology.

Observationally, the challenge of detecting these transitions lies in the timing; existing measurements rely heavily on past cosmic phenomena, such as CMB and BAO, which may not be sensitive enough to detect these shifts until they become immediate. Enhanced precision in measuring the total cosmic equation-of-state $\varepsilon_{\rm TOT}$ and its time variation might provide insight. The authors call for a more detailed investigation into scalar field couplings that could yield observable markers.

Concluding Remarks and Future Directions

The paper offers a robust conceptual framework that integrates modern theoretical developments with cosmic observations. Its alignment with the swampland conjectures offers potential synchronicity with quantum gravity insights, proposing that accelerated expansion may lapse in the relatively near cosmic future. As such, it encourages broader consideration and re-evaluation of dark energy's nature in understanding both the history and fate of our universe.

Further research will undoubtedly focus on refining observational tools to test these predictions and explore a broader class of fields and couplings that might offer observable implications of a contracting future. The trajectory of this research could redefine our understanding of cosmological dynamics, providing fertile ground for theoretical and observational advancements.