Negative cosmological constant supports cyclic universe models

What the study found: The study reports that two simplified non-singular cyclic universe models can have physically acceptable evolution with positive energy density when they include a negative time-varying cosmological constant. The authors also describe a sign flip in cosmic pressure in a quintessence-dominated universe and a crossing of the phantom divide line near the bounce in a matter-bounce scenario.
Why the authors say this matters: The authors say the negative time-varying cosmological constant represents a non-conventional mechanism that is expected to address late-time cosmic acceleration. The findings indicate that such models may support cyclic cosmological behavior under the conditions described.
What the researchers tested: The researchers investigated two simplified cyclic cosmological models with a negative time-varying cosmological constant. One model was quintessence-dominated, where quintessence refers to a form of dark energy, and the other was a matter-bounce scenario.
What worked and what didn't: Positive energy density was realized in the models with the negative cosmological constant, while negative energy density dominated in cases with a positive or zero cosmological constant. In the first model, cosmic pressure changed sign without violating the null energy condition, and in the second model the phantom divide line was crossed near the bounce. The authors also report positive kinetic term and scalar potential, but the sum of scalar and quantum potentials was negative.
What to keep in mind: The abstract describes simplified models only, so the scope is limited to the cases studied. No additional limitations are stated in the available summary.

Key points

• Two simplified non-singular cyclic universe models were studied with a negative time-varying cosmological constant.
• Positive energy density was reported for the negative cosmological constant case.
• Negative energy density dominated when the cosmological constant was positive or zero.
• In the quintessence-dominated model, cosmic pressure flipped sign without violating the null energy condition.
• In the matter-bounce model, the phantom divide line was crossed near the bounce.