Chapter 1: Understanding the Oscillating Universe

In the vastness of what we call "the bulk," three-dimensional branes—each representing its own universe—can coexist. This concept allows for the existence of multiple universes in a shared space.

Picture a scorching day at a crowded beach, where a shark washes ashore. The sun blazes overhead, illuminating footprints and melting ice cream in colorful buckets. Through a throng of people, I catch fleeting glimpses of the shark—its pale underside and lifeless jaws, punctuated by hints of blood. It appears so well-preserved, as if it might still blink at the bustling Florida shore. Instead, its gaze seems fixed on me, a lifeless yet watchful presence beneath the shade of a distant umbrella.

This encounter now feels distant for many reasons. The shark's remains may have decomposed, been buried, or consumed by other creatures. There exists an inherent separation across species, where the shark cannot grasp my fears or desires, just as I cannot comprehend its instincts. Furthermore, this memory belongs to the past, making it inaccessible. Just as we perceive ourselves as separated by spatial distances, we should also view time similarly. The past and future are merely points along the continuum of spacetime.

When contemplating the multiverse and the challenges of interworld travel, we often assume the distances are insurmountable. To reach the next universe in the multiverse, we would require superluminal spacecraft, or perhaps even more exotic technology. However, the cyclical model of the multiverse encourages us to think beyond mere distance and to consider time in our explorations.

Section 1.1: The Cyclical Multiverse Concept

The cyclical multiverse, or oscillating universe, posits that our universe is just one of many three-dimensional branes navigating through hyperspace. Each brane represents a unique universe, and the gaps separating them could be as minuscule as the nucleus of an atom. The real intrigue arises when two branes collide.

When vacuum energy draws two universes together, the ensuing collision is akin to a "Big Bang." This cataclysmic event obliterates existing structures, releasing a torrent of particles and radiation that catalyze a new expansion phase. This moment embodies both destruction and creation, eventually cooling to allow for the formation of new matter, galaxies, and star systems.

In this framework, the Big Bang is not the universe's genesis but rather a recurring event within a broader cycle. Unlike the inflationary multiverse, which suggests that each parallel world emerges from its own distinct Big Bang, the cyclical model proposes that these worlds are temporally separated.

Chapter 2: The Implications of Cyclical Cosmology

Prominent physicists like Neil Turok and Paul Steinhardt argue that the random occurrences in the inflationary multiverse lack predictability. They contend that the complex models of inflation might be unnecessary, as the universe may be simpler than previously thought. Their cyclical model aligns closely with satellite measurements, offering precise predictions comparable to those of inflation.

So, what does a cyclical multiverse's beginning look like? The enduring nature of these collisions suggests that there need not be a definitive start or end. However, the concept of entropy complicates matters. Entropy, often viewed as disorder, is inherently linked to the passage of time. According to the second law of thermodynamics, entropy consistently increases, implying it decreases as one looks back in time.

As the universe contracts, the density of entropy increases, leading to a stronger rebound during the next cycle. Turok and Steinhardt propose that while entropy increases, it fills an ever-expanding space, which means that its density decreases. As each cycle concludes, the density of entropy approaches zero, suggesting that time may not require a beginning or an end.

Though the cyclical multiverse model is not without its flaws—such as free parameters that may lead to inconsistency—it presents a credible alternative to inflation. It challenges the notion of a singular beginning, suggesting instead that what we perceive as the start may not be so.

The strongest challenge to cyclical cosmology comes from the potential detection of primordial gravitational waves. Unlike the inflationary model, which predicts gravitational waves from brane collisions, cyclical cosmology lacks strong markers in the cosmic microwave background (CMB). Detecting specific polarization patterns in the CMB would provide compelling evidence for inflation, potentially disproving cyclical models. Future detectors like LISA and LIGO aim to enhance their sensitivity and contribute to this crucial line of inquiry.

In contemplating other universes—each possibly teeming with life and experiences akin to our own—one is struck by the bittersweet realization of their unreachability. Yet, this knowledge can be comforting, reminding us that we are part of a vast cosmos. I reflect on that day at the beach, the shark that washed ashore, and the shared gaze—reminders that even across time and space, connections linger.