The cosmos presents a fascinating reality: it is continuously expanding. Observations show that distant galaxies are receding from us, with the speed of recession increasing for those further away. Although certain celestial bodies and systems are gravitationally bound—like stars, planets, and galaxies—the space that separates them has been expanding for the entirety of the universe's 13.8 billion-year history and is set to continue this trend.

Yet, is this expansion an absolute phenomenon? Are there instances where contraction is possible? This inquiry arises from Brent Minder, a supporter on Patreon, who poses the following question:

“Must the universe's current status be strictly expanding or collapsing? Is there a theory suggesting a simultaneous mid-state of both? Could the universe have perpetually expanded and collapsed, with black holes being remnants of past expansions? Might the central regions collapse while the outer regions are moving away rapidly?”

To address these questions, we should first examine theoretical perspectives on the universe's potential for expansion or contraction.

When Einstein formulated General Relativity, he identified a significant challenge: the universe appeared uniformly filled with light in all directions, suggesting a homogenous and isotropic distribution. However, upon solving the Einstein field equations for such a universe, he discovered it would inevitably collapse into a singularity, such as a black hole. This posed a contradiction, as the universe clearly was not collapsing.

To remedy this, Einstein considered the introduction of a new form of energy, a cosmological constant, that could provide an outward pressure to counteract gravitational attraction and stabilize the universe.

Balancing matter, radiation, and the cosmological constant proved delicate. A slight expansion reduces the matter density while the cosmological constant remains constant, leading to accelerated expansion. Conversely, a minor contraction increases matter density, resulting in further contraction. The only viable scenario for a stable universe involves:

• An initial expansion,
• Gravity from matter slowing this expansion,
• A near-perfect match between matter-and-energy density and the initial expansion rate.

These theoretical insights led to the concept of an expanding universe and the hot Big Bang model shortly after General Relativity was proposed. Long before Edwin Hubble's observations revealed the extragalactic nature of celestial bodies, the foundations of modern cosmology were already laid.

While the universe is not perfectly uniform, it approximates uniformity on the largest scales, with small density variations present:

• Regions of higher-than-average density,
• Areas of average density,
• Regions of below-average density.

These fluctuations have been observed on all scales, from vast cosmic distances to smaller fractions of the sky. Over time, gravity causes denser regions to attract matter from their surroundings, forming a complex cosmic web.

One might wonder how the expansion is balanced against regions of higher density that are growing under gravity. Below-average density areas will expand more rapidly, while above-average density regions will expand more slowly, eventually reaching a point where the matter density becomes sufficient to halt expansion. This transition leads to gravitational binding, creating areas that can contract.

To counteract cosmic expansion effectively, a region must possess a sufficient energy density of matter, dark matter, and radiation to overcome the initial expansion's momentum and dark energy's outward push.

Quantitatively, a region needs to be about 68% denser than average to begin collapsing. Below this threshold, it will expand slowly. In an expanding universe, matter density decreases as volume increases, while radiation density diminishes even faster due to wavelength stretching.

Once regions exceed the critical density threshold, they experience rapid growth, entering a phase of nonlinear growth. As these regions become gravitationally bound, they begin to contract, leading to the formation of structures like stars and galaxies.

Evidence from cosmic microwave background (CMB) observations, galaxy clustering statistics, and inflationary fluctuations supports this narrative. Initial fluctuations from the hot Big Bang evolved over time, becoming significantly pronounced, particularly at an angular scale of about one degree.

By the time the universe was only a few hundred thousand years old, fluctuations had grown substantially. Regions that were initially slightly overdense began to collapse under gravity, paving the way for the formation of the first stars and potentially supermassive black holes.

However, these early structures cannot be remnants of a previous collapsing phase, as the CMB evidence does not support the existence of such significant overdensities.

You may also ponder the possibility of larger fluctuations beyond the observable universe. However, data suggests that these superhorizon fluctuations are minimal, consistent with the small-scale variations predicted by inflation, indicating that they cannot lead to a collapse.

Ultimately, the expansion versus collapse debate is nuanced and scale-dependent. Unless regions of space achieve significant density above the cosmic average, they will continue to expand. As dark energy has come to dominate the universe, the potential for structural growth diminishes.

If a region's density surpasses the average by around 68%, it becomes gravitationally bound, ceasing expansion and potentially leading to collapse. The challenge lies in how quickly a region can achieve this density threshold.

The absence of large overdensities on cosmic scales aligns with our understanding of the universe's structure, which features vast regions of expansion between the bound structures. Thus, cosmic expansion is not an absolute phenomenon; it varies based on scale and density.

For further inquiries or to submit your questions, contact Ethan at startswithabang at gmail dot com!

Starts With A Bang is authored by Ethan Siegel, Ph.D., who has written "Beyond The Galaxy," "Treknology," and "The Littlest Girl Goes Inside An Atom." Additional works, including the "Encyclopaedia Cosmologica," are forthcoming!