Cyclic Cosmology Model Utilises Instant Folded Strings to Avoid Zero Coupling

The quest to understand the universe’s origins and ultimate fate drives ongoing research into cyclic cosmological models, and a new theory proposes a universe that continuously bounces between expansion and contraction, avoiding the problematic singularity of the Big Bang. Nissan Itzhaki and Uri Peleg, both from Tel Aviv University, alongside Paul J. Steinhardt from Princeton University, present a self-consistent model built on string theory, incorporating unusual objects called instant folded strings. These strings, far lighter and longer than conventional strings, violate established energy conditions and play a dual role in triggering both the universe’s bounces and periods of dark-energy dominance. This research offers a compelling alternative to inflationary cosmology, naturally predicting time-varying dark energy and, crucially, the absence of primordial B-mode polarization. This feature has eluded detection in current observations of the cosmic microwave background.

Cyclic Universes and Alternatives to Inflation

Researchers are actively exploring alternatives to the standard inflationary model of the early universe, focusing on cyclic and bouncing universe models that avoid the initial singularity predicted by classical general relativity. These models propose that the universe undergoes cycles of expansion and contraction, offering a different perspective on cosmic origins. Several mechanisms are being investigated, including the use of entropy fluctuations, specific field configurations like those found in ekpyrotic and Galileon models, and fluctuations in the curvature of spacetime itself. Modified gravity theories, such as Horndeski gravity, are also being explored to achieve non-singular bounces and generate these crucial perturbations.

This research often draws upon concepts from string theory and quantum gravity, attempting to resolve singularities and understand the universe at its earliest moments. Innovative ideas like Instant Cosmology, which proposes a rapid emergence from a pre-existing state, are also gaining attention, alongside investigations into the role of fermion condensates in driving bounces and generating perturbations. This body of work represents a significant effort to develop viable alternatives to inflation and understand the very early universe.

Instant Folded Strings and Cyclic Cosmology

Researchers have developed a novel cosmological model that avoids the problematic big bang singularity by proposing a cyclic universe governed by a smooth transition between contraction and expansion phases. This approach, known as bouncing cosmology, addresses several issues inherent in inflationary models. Instead of an initial rapid expansion, the universe undergoes a period of slow contraction before “bouncing” into its current expanding state. Central to this model are Instant Folded Strings (IFSs), unconventional string-like objects with unusual properties; they are significantly lighter than typical strings yet extend far beyond their usual length.

These IFSs emerge naturally within the framework of string theory and play a crucial role in mediating the bounce itself, effectively violating the conventional null energy condition required for such a transition. The team demonstrates how these IFSs can be incorporated into a cyclic model alongside established concepts like dilaton-gravity and an enhanced symmetry point, creating a self-consistent framework. This innovative approach allows for a detailed description within established string theory principles, incorporating a sequence of phases, decelerated expansion, accelerated expansion driven by dark energy, slow contraction, and the bounce, that flow naturally from one to the next. This carefully constructed sequence results in a cyclic cosmic engine capable of indefinite operation, offering a compelling alternative to traditional big bang cosmology and providing a framework for testing predictions about the evolution of dark energy.

Cyclic Universe Model Using Folded Strings

Researchers have developed a new cosmological model that offers a compelling alternative to standard Big Bang theory, successfully addressing several long-standing problems and predicting unique features for the universe’s evolution. This model proposes a cyclic universe, endlessly bouncing between expansion and contraction, eliminating the need for a singular beginning and avoiding the problematic initial conditions of inflation. The key to this cyclical behavior lies in the properties of “instant folded strings,” hypothetical fundamental strings with unusual characteristics that allow them to mediate a smooth transition between cosmic phases. The model elegantly explains how the universe avoids a catastrophic collapse or expansion by leveraging the unique properties of these strings, which violate a fundamental principle known as the Null Energy Condition.

This violation generates a repulsive force during contraction, effectively “bouncing” the universe back into an expanding phase without requiring extreme energy densities or fine-tuned parameters. Crucially, the bounce occurs without significant curvature, meaning it’s governed by well-behaved equations and remains under perturbative control. Perhaps most surprisingly, this model predicts a dynamic form of dark energy, responsible for the current accelerated expansion of the universe, that is intrinsically linked to the behavior of these instant folded strings. Unlike the constant dark energy predicted by many models, this dark energy density increases for a period before eventually decreasing, offering a potential explanation for the observed cosmic acceleration and providing a testable prediction for future observations. The model also predicts the absence of primordial B-mode polarization in the cosmic microwave background, a feature that distinguishes it from inflationary models and offers another avenue for observational verification. Furthermore, the model incorporates a natural mechanism for smoothing out initial anisotropies and inhomogeneities in the universe during the contracting phase, driven by the dilaton potential, effectively flattening the universe and preparing it for the next cycle of expansion.

Cyclic Universe From String Theory and Instantons

This research presents a self-consistent cyclic cosmology built upon string theory, offering an alternative to standard big bang inflationary models. The model incorporates a dilaton field, an enhanced symmetry point where a second field becomes massless, and the introduction of instant folded strings, unusual string-like objects that become significant when the string coupling grows. These components work together to create a universe that undergoes repeated cycles of expansion and contraction, avoiding the initial singularity typically associated with the big bang. The cosmology predicts a universe where each cycle begins with a contracting phase, transitions through a period dominated by instant folded strings and dark energy, and then enters a slow contraction before bouncing into a new expanding phase.

A key feature is the avoidance of primordial B-mode polarization in the cosmic microwave background, a prediction that distinguishes this model from some inflationary scenarios. The authors acknowledge that the model relies on specific parameter choices and potential adjustments to ensure a stable and bounded potential for the dilaton field. Future research directions involve refining the understanding of instant folded string dynamics and exploring the implications of this cyclic model for the evolution of the universe over many cycles. The authors also note the need for further investigation into the precise parameter constraints required for a stable cosmology, and how these parameters might influence observable features of the universe.