The question of whether space has an end is one of humanity’s oldest and most profound inquiries, touching on the fundamental nature of reality. While the terms “space” and “universe” are often used interchangeably, modern cosmology requires a clear distinction between the two. “Space” refers to the three-dimensional fabric or medium that contains everything, while “the universe” is the totality of all matter, energy, and spacetime within it. The scientific answer to the question of an end or edge relies entirely on the principles of modern physics and the observed geometry of this spatial fabric.
The Observable Universe and the Cosmic Horizon
When people look up and consider the vastness of space, they are contemplating the observable universe. This region does have a boundary, known as the cosmic horizon, but it is a limit of knowledge, not a physical wall. This boundary is imposed by two factors: the finite age of the universe and the finite speed of light.
Light travels at a constant, ultimate speed, and the universe has existed for approximately $13.8$ billion years. This means that we can only see objects from which light has had enough time to travel to Earth since the universe began. Although light has traveled for $13.8$ billion years, the expansion of space means those objects are now much farther away.
The current distance to the edge of this observable sphere, often called the particle horizon, is estimated to be about $46.5$ billion light-years in every direction. The observable universe is a spherical region centered on the observer. This limitation is similar to standing on a vast, flat plain at night; in the cosmos, the restriction is a horizon of time.
The Three Possible Geometries of Space
Whether the entire universe has an “end” is determined by its overall geometry, which is defined by the total density of matter and energy within it. This density dictates the curvature of space, and there are three possible shapes for the three-dimensional spatial fabric. These shapes are analogous to different two-dimensional surfaces: a flat sheet, the surface of a sphere, or the surface of a saddle.
The first possibility is a flat universe, which has zero curvature, often referred to as Euclidean geometry. In this case, parallel lines remain parallel forever, and the angles of a triangle always sum to exactly $180$ degrees. A flat universe is typically considered to be infinite in extent.
The second possibility is a closed universe, which possesses positive curvature, like the surface of a sphere. If one were to travel in a straight line in this universe, they would eventually return to their starting point, much like traveling around the surface of the Earth. This geometry describes a universe that is finite in volume but has no edges or boundaries.
The third option is an open universe, which has negative curvature, resembling the shape of a saddle. In this hyperbolic geometry, parallel lines diverge from one another, and the angles of a triangle sum to less than $180$ degrees. An open universe, like a flat one, is infinite in size and extends outward without any boundary.
Expansion, Curvature, and the Absence of an Edge
The constant expansion of space eliminates the idea of a traditional, static “end” to the cosmos. Space itself is not expanding into some pre-existing external void; rather, the fabric of space is growing everywhere simultaneously.
This phenomenon is often illustrated with the raisin bread analogy, where the dough represents space and the raisins represent galaxies. As the dough bakes and expands, the raisins move further apart from each other, carried by the stretching of space between them.
This expansion does not have a center point from which everything is flying away, and there is no edge that the expansion is moving toward. If the entire universe is infinite, as in the flat or open models, then the expansion is simply a scaling up of infinity. If the universe is finite but closed, the expansion is analogous to the surface area of a sphere growing larger, which still has no boundary.
The Current Cosmological View: Finite or Infinite
Modern cosmological observations strongly favor one of the three possible geometries. Data collected from missions like the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite, which mapped the cosmic microwave background (CMB), indicate that the universe is geometrically “flat.” The precise measurements of the CMB’s temperature fluctuations reveal the universe’s curvature to be extremely close to zero.
This finding has profound implications for the question of the universe’s size. The most straightforward interpretation of a flat geometry is that the universe is truly infinite in extent. However, a flat universe can also be finite if it is multiply connected, meaning it loops back on itself in a complex shape, like a three-dimensional torus. Since observations have not revealed any repeating patterns in the CMB, the consensus is that the universe is either infinite or functionally indistinguishable from infinite.
Regardless of its total size, the geometry confirms that space has no boundary or edge in the traditional sense. Even if the universe were finite and closed, traveling in one direction would simply bring an observer back to their starting point without ever encountering a wall or an end. The current cosmological view concludes that while our knowledge is limited by a cosmic horizon, space itself is either boundless or a self-contained, finite volume without an exterior.