Quintom Cosmology: Theoretical implications and observations (0909.2776v2)

Abstract: We review the paradigm of quintom cosmology. This scenario is motivated by the observational indications that the equation of state of dark energy across the cosmological constant boundary is mildly favored, although the data are still far from being conclusive. As a theoretical setup we introduce a no-go theorem existing in quintom cosmology, and based on it we discuss the conditions for the equation of state of dark energy realizing the quintom scenario. The simplest quintom model can be achieved by introducing two scalar fields with one being quintessence and the other phantom. Based on the double-field quintom model we perform a detailed analysis of dark energy perturbations and we discuss their effects on current observations. This type of scenarios usually suffer from a manifest problem due to the existence of a ghost degree of freedom, and thus we review various alternative realizations of the quintom paradigm. The developments in particle physics and string theory provide potential clues indicating that a quintom scenario may be obtained from scalar systems with higher derivative terms, as well as from non-scalar systems. Additionally, we construct a quintom realization in the framework of braneworld cosmology, where the cosmic acceleration and the phantom divide crossing result from the combined effects of the field evolution on the brane and the competition between four and five dimensional gravity. Finally, we study the outsets and fates of a universe in quintom cosmology. In a scenario with null energy condition violation one may obtain a bouncing solution at early times and therefore avoid the Big Bang singularity. Furthermore, if this occurs periodically, we obtain a realization of an oscillating universe. Lastly, we comment on several open issues in quintom cosmology and their connection to future investigations.

• The paper demonstrates that two-field quintom models enable a stable crossing of the dark energy equation of state from above to below w = -1.
• It employs perturbation theory and dynamic potentials to validate models accommodating bounce scenarios and cyclic cosmologies.
• The study reveals observable imprints on cosmic microwave background and structure growth, suggesting future data may distinguish quintom behavior from ΛCDM.

Overview of Quintom Cosmology: Theoretical Implications and Observations

The field of cosmology has witnessed significant advances over the past few decades, primarily due to improved observational techniques and equipment such as supernovae surveys and cosmic microwave background measurements. These observations have led to the understanding that the universe is undergoing an accelerated expansion, driven by a mysterious component known as dark energy (DE), which constitutes approximately 70% of the universe's energy content.

The concept of quintom cosmology arises from the motivation to comprehensively describe DE, especially given certain observational data suggesting that the equation of state (EoS) of DE might have crossed the cosmological constant boundary — marked by $w = -1$. This hypothesis is mildly favored by data, although definitive conclusions remain elusive.

Theoretical Foundations and the No-Go Theorem

Traditional models such as quintessence and phantom fields are unable to transition through $w = -1$ with a single scalar field due to a prevailing no-go theorem. This theorem constrains scenarios within single-fluid or single-scalar field frameworks under general relativity, where crossing $-1$ poses stability issues at both classical and perturbation levels. As a result, quintom models introduce additional degrees of freedom to overcome these constraints, typically by employing two scalar fields: one "quintessence-like" and one "phantom-like."

Two-Field Quintom Models

The quintessential model within the quintom framework involves two scalar fields, leading to a stable EoS that transitions smoothly across $w = -1$. These two-field models are naturally extended to higher complexity with dynamic potentials, with each field dominating the energy density at different epochs. Perturbation theory reveals these models as stable, provided that the inclusion of such dynamic interactions respects both background and perturbative stability.

Advanced Cosmological Scenarios: Higher Derivative Terms

Quintom scenarios have been expanded by incorporating higher derivative terms in the Lagrangian, motivated by string theory and attempts to address fundamental theoretical challenges. This adds complexity to the model, potentially mirroring ghost condensate mechanisms but still facing challenges with quantum instabilities inherent in phantom-like behaviors. Such constructs also suggest crossover behaviors observable at high-energy scales, consistent with predictions from various string-inspired models.

Applications Across Cosmological Epochs

Quintom models have intriguing implications for both early and late-time cosmology. In particular, they can accommodate a bouncing cosmology paradigm, where the universe transitions from collapse to expansion, thus avoiding singularities. Violation of the null energy condition (NEC) can provide a phenomenological basis for these bounce transitions, offering insights into cosmological scenarios such as cyclic models, potentially resolving the Big Bang singularity issue.

Observational Implications and Challenges

The tangible effect of quintom DE on cosmic structure and evolution echoes through changes in the growth rate of cosmic perturbations, notably those impacting the integrated Sachs-Wolfe effect visible in CMB observations, albeit modestly separated from typical $\Lambda$CDM predictions. Theoretical underpinning and observational tests converge on differentiating quintom cosmology from other DE models through precise measurements of $w$ and its evolutionary trajectory over cosmic history.

Conclusion and Future Prospects

Quintom cosmology, through its versatile treatment of the DE EoS crossing $-1$, remains a compelling paradigm with numerous models tailored to theoretical and observational challenges. Future advancements hinge on tightly coupling theoretical models with increasingly precise observational data, potentially unraveling the dynamical nature of dark energy. Cross-disciplinary interactions between cosmology, particle physics, and advanced gravitational theories are instrumental in shaping a coherent understanding of quintom-like behavior in the cosmic fabric. Continued investigations into the stability of higher derivative models and non-scalar presentations of quintom behavior may yield breakthroughs essential for a holistic cosmological narrative.