The question of what the universe is expanding into is a natural one, stemming from our everyday experience with expanding objects like an inflating balloon. This intuition suggests a boundary and an external area. However, the expansion of the universe is fundamentally different, challenging this common understanding. Modern cosmology explains that the universe is not enlarging within a pre-existing void. Instead, the fabric of space itself is stretching everywhere simultaneously, a concept known as metric expansion.
Understanding Metric Expansion
The universe’s expansion is not the movement of galaxies flying outward through a fixed space. It is the growth of the space between them. Metric expansion means that the distances between objects not bound by gravity increase because the space itself is getting longer. This mechanism explains why more distant galaxies appear to recede from us at faster rates, a relationship formalized by Hubble’s Law. While local gravitational forces hold structures like our Milky Way galaxy and our solar system together, the space between gravitationally unbound clusters of galaxies continues to expand.
The light traveling from distant galaxies is also stretched by this expanding space. This stretching causes the light’s wavelength to increase, a phenomenon observed as cosmological redshift. This redshift is the primary observational evidence for the ongoing expansion of the universe.
The Illusion of Expanding Into Something
The idea that the universe must expand into a void assumes it has a center and an external edge. Current models suggest this is not the case. The expansion is uniform throughout the entire volume of space. Thinking of the universe as a raisin bread loaf rising in an oven helps illustrate this concept. Every raisin moves away from every other raisin as the dough (space) expands, and there is no single center point to the expansion.
Cosmologists describe the universe as either infinite or as “finite but unbounded.” A two-dimensional analogy for a finite but unbounded space is the surface of a sphere. The sphere’s surface has a finite area but lacks any edge to fall off of. An object traveling in a straight line on this surface would eventually return to its starting point without ever encountering a boundary. The expansion, therefore, is an increase in the scale factor of the universe’s geometry. It is not an encroachment into an external volume.
The Driving Force: Dark Energy
Observations revealed that the expansion of the universe is not slowing down due to gravity, but is actually accelerating. This surprising discovery led to the hypothesis of a mysterious component called Dark Energy. Dark energy is theorized to be a form of energy that permeates all of space. It possesses a strong negative pressure, which acts as a repulsive force against gravity.
Dark energy is thought to be uniformly distributed across space, unlike Dark Matter, which clumps around galaxies. Current cosmological models estimate that dark energy makes up approximately 68% of the total energy density of the present-day universe. Dark Matter contributes about 27%, and ordinary matter makes up just 5%.
The most common explanation for dark energy is that it is the cosmological constant, a concept originally introduced by Albert Einstein. This constant represents the intrinsic energy density of empty space. As space expands, more space is created, and thus more dark energy appears, maintaining a constant energy density.
The Limits of Observation
A well-defined boundary exists for us: the Observable Universe. This limit is not a physical edge of space but a horizon defined by how far light has been able to travel since the Big Bang. Because the speed of light is finite, we can only see objects whose light has had enough time to reach us.
The Observable Universe forms a sphere around any observer. Its current radius is estimated to be about 46.5 billion light-years. This distance is greater than the age of the universe multiplied by the speed of light because the space between us and distant objects has expanded while the light was traveling. The Observable Universe is merely the portion of the entire universe that is causally connected to us. The Total Universe, which encompasses everything that exists, is likely much larger than the observable portion, and may even be infinite.