Superstructures

While de Sitter space provides a model for an expanding universe, the question of the "largest possible space" in reality has a few different answers, depending on whether we're talking about what we can observe or what we theorize exists.

The Largest Observed Structures: Superstructures like Quipu

If we define "largest" by the most massive, coherent structures astronomers have actually detected, the current title-holder is a superstructure named Quipu. Discovered in 2025 by a team led by the Max Planck Institute, Quipu is a cosmic filament—a long, thread-like structure that is part of the universe's vast web [citation:2][citation:10]. It contains 68 galaxy clusters and has a mind-boggling mass equivalent to about 200 quadrillion suns [citation:2][citation:5]. Its length stretches for approximately 1.3 to 1.4 billion light-years, making it the largest known structure to be reliably characterized [citation:5][citation:10]. These superstructures are so massive that they actually affect cosmological measurements, like the expansion rate of the universe (the Hubble constant) and the cosmic microwave background [citation:5][citation:8].

A Potential Rival: The Hercules-Corona Borealis Great Wall

There is another structure that may be significantly larger than Quipu, but its existence is more debated. The Hercules-Corona Borealis Great Wall is a vast concentration of galaxies, mapped by detecting gamma-ray bursts (immense explosions from dying stars). Recent analysis suggests this structure could be an astonishing 15 billion light-years across [citation:1][citation:4]. If confirmed, it would be nearly 11 times larger than Quipu. However, because its detection relies on a less direct method, some scientists are more cautious about confirming it as a definitive structure [citation:1].

The Ultimate Limit: The Observable Universe

These enormous structures, as vast as they are, exist within a much larger sphere: the observable universe. This is not a physical object but a horizon—the maximum volume of space from which light has had time to reach us since the Big Bang [citation:3][citation:9]. Because the universe is expanding, the distance to this edge is not simply the age of the universe (13.8 billion years) times the speed of light. Instead, the current diameter of the observable universe is estimated to be about 93 billion light-years [citation:3][citation:6]. This is the absolute limit of our vision; we cannot, even in principle, see anything beyond this spherical boundary [citation:3].

The "Largest Possible Space": The Entire Universe

Finally, we arrive at the concept of the entire, unobservable universe. This is the truest answer to your question of the "largest possible space." The observable universe is just the tiny fraction we can see from our vantage point. According to the theory of cosmic inflation, the entire universe is staggeringly larger. Some estimates, based on simple inflationary models, suggest the whole universe could be at least 1.5 × 10³⁴ light-years across—that's 3 followed by 23 zeros times larger than the part we can see [citation:3]. Crucially, even this immense figure is a minimum estimate. The universe may very well be infinite in extent [citation:3][citation:6].

In summary, the largest known discrete structures are around 1.4 billion light-years wide, the observable universe is 93 billion light-years wide, and the entire universe may be infinite. This is what makes the question of a "largest possible space" so fascinating—it may not exist at all.

The Planck Length and De Sitter Space

The impossibly small to the unimaginably large. The concepts are related, but they don't represent "the smallest and largest" in a directly comparable way.

Let's break down why.

The Lower Limit: Planck Length

The Planck length (about 1.6 x 10⁻³⁵ meters) is indeed the smallest length that has any physical meaning in our current theories. It's not that things are this size, but rather that trying to measure anything smaller becomes nonsensical.

Here's why it's considered a fundamental limit:

Where Physics Breaks Down: At the Planck scale, the two pillars of modern physics—general relativity (gravity) and quantum mechanics—become inseparably important, and our current understanding of space and time breaks down. The very concepts of "distance" and "time" are thought to lose their meaning.

A Thought Experiment: Imagine trying to measure the position of an object with extreme precision. You'd need a photon with a very short wavelength (high energy). If that energy is high enough (at the Planck scale), it would create a tiny black hole that would swallow the information you were trying to get, making the measurement impossible. This suggests there's a fundamental limit to how finely we can probe space.

So, the Planck length represents a limit to our classical notion of geometry. Below it, we need a new theory of "quantum gravity" (like string theory or loop quantum gravity) to describe what space-time even is.

The Cosmic Scale: de Sitter Space

De Sitter (dS) space is a solution to Einstein's equations of general relativity for an empty universe with a positive cosmological constant (Λ). This constant acts as a repulsive force, causing the universe to expand exponentially.

Think of it this way:

Our Universe's Future: Our universe is currently in a phase of accelerated expansion, seemingly driven by dark energy. If this continues, it will asymptotically approach a de Sitter state.

A Model for Inflation: The very early universe is also thought to have gone through a period of incredibly rapid expansion called "inflation," which is also well-modeled by de Sitter space.

Defined by Curvature: de Sitter space has a constant positive curvature. Its geometry is characterized by a length scale often denoted as ℓ (the curvature radius), which is related to the cosmological constant by the formula Λ = 3/ℓ².

A Cosmological Horizon: Just like a black hole has an event horizon you can't see beyond, an observer in de Sitter space is surrounded by a cosmological horizon. This horizon marks the boundary of the observable universe—regions beyond it are receding from us faster than light due to the expansion of space.

Are They the "Smallest and Largest"?

This is where the comparison gets tricky. They are both fundamental, but in different ways.

The Planck length (ℓₚ) is a fundamental unit of length, a limit to measurement. It marks the scale where quantum gravity effects dominate. In terms of being the "smallest," it is the smallest length with physical meaning.

De Sitter Space (Radius ℓ) is a geometric solution for an expanding universe. It describes a universe (or phase of it) with a positive cosmological constant. However, it is not the "largest space." It is a specific type of space. Our observable universe is a finite patch within a potentially much larger de Sitter space, bounded by a horizon.

The key difference is that the Planck length is a universal constant derived from fundamental constants of nature. It defines the scale at which our classical picture of geometry dissolves. De Sitter space, on the other hand, has a size defined by the cosmological constant, which is a parameter of our universe. If the cosmological constant were different, the "size" (curvature radius) of the de Sitter space would be different. It is not a fundamental limit like the Planck length.

In short: The Planck length is the smallest possible meaningful measurement of space. De Sitter space is a mathematical description of a universe that expands forever, and our universe may be evolving into one.

I hope this clarifies the fascinating relationship between these two concepts. Would you be interested in learning more about the theories that try to unify them, like string theory?