The question of what lies past the universe moves from the limits of human observation to the farthest reaches of theoretical physics. The term “universe” holds two distinct meanings. Scientifically, it is defined as all of space, time, and its contents, encompassing everything that exists. However, in cosmology, the term often describes only the portion of reality we can observe, a distinction that shapes our understanding of cosmic boundaries.
The Observable Limit
The primary boundary we encounter when looking out into the cosmos is not a physical wall but a horizon set by the finite speed of light. Because light travels at a constant speed, we can only see objects whose light has had enough time to reach us since the universe began approximately 13.8 billion years ago. This region we can observe is known as the Observable Universe.
The edge of this observable region is much farther than 13.8 billion light-years because space has been expanding since the Big Bang. Objects that emitted light billions of years ago have been carried farther away by this expansion. Although the light traveled for 13.8 billion years, the objects themselves are currently estimated to be about 46.5 billion light-years away. This radius defines the cosmic horizon, the maximum extent of our sensory contact with the cosmos.
Looking to the farthest limits of the observable sphere, we encounter the Cosmic Microwave Background (CMB) radiation. The CMB is faint heat left over from when the universe was only about 380,000 years old and cooled enough for electrons and protons to form neutral atoms. Before this time, the universe was an extremely hot, dense plasma that was opaque to light. This “wall” of light constitutes the furthest back in time we can see using electromagnetic radiation, marking a limit to our direct observation.
The Unobservable Universe
The space immediately outside the cosmic horizon is not a void, but simply a region of the universe too distant for its light to have reached us yet. This unobservable part is still considered the same universe we inhabit, governed by the same physical laws and consisting of the same matter and energy. Cosmologists operate under the Cosmological Principle, the foundational assumption that the universe is homogenous and isotropic on the largest scales.
Homogeneity means that any large volume of space looks the same as any other, while isotropy means the universe looks the same in all directions. This principle, strongly supported by the uniformity of the CMB, implies that if we reached the edge of our observable limit, we would find the same galaxies, physical constants, and cosmic structures extending beyond that new vantage point. The unobservable region is simply more of our universe that we are causally disconnected from due to light travel time.
The theory of cosmic inflation, which describes a period of ultra-rapid expansion in the early universe, strongly suggests the entire universe is vastly larger than the part we can see. Inflation would have expanded the universe so much that our current observable bubble is only a tiny fraction of the total volume. While the exact size of this total universe is unknown, it could be spatially infinite, or it could be finite but so large that its boundary is effectively unreachable.
Exploring Multiverse Hypotheses and Higher Dimensions
The most speculative answers to what lies “past the universe” involve structures existing outside our cosmos, known as the Multiverse. These ideas move beyond the spatial extension of our universe and propose the existence of other, separate universes. One prominent theoretical model arises from eternal inflation, an extension of the theory describing our universe’s early growth.
In this scenario, inflation never completely stops everywhere; instead, it continues eternally in most regions, creating an infinite number of isolated “bubble” or “pocket” universes. Our universe is one such bubble, where inflation ceased and the energy was converted into matter. These other Level II multiverses would be completely unreachable, and they could have fundamentally different properties, such as distinct physical constants or different particle masses.
Another line of thought, arising from String Theory, is the Brane Multiverse. String theory mathematically requires additional spatial dimensions beyond the three we experience, often totaling ten or eleven. Our entire four-dimensional spacetime (three spatial dimensions plus time) is hypothesized to be a “brane” or membrane.
This brane is thought to be floating within a higher-dimensional space, sometimes called “The Bulk.” Other universes could exist on separate branes in this Bulk, parallel to ours. These brane universes might occasionally interact, perhaps causing a “Big Bang” event in our cosmos through collision or proximity. These multiverse and higher-dimension concepts are highly theoretical frameworks, not observations, representing the current frontier of physics.