The question of whether the universe is “never ending” forces cosmologists to consider two distinct concepts: space and time. Spatially, it addresses the universe’s ultimate size and whether it possesses a boundary or edge. Temporally, it explores the ultimate future, asking if the cosmos will continue to exist indefinitely or if it faces a definitive end. Modern cosmology, informed by decades of observation, suggests the universe is likely without spatial limits, while its temporal existence will follow a predictable, though immensely distant, path toward a final state.
Defining the Observable Universe
Humans are constrained in their view of the cosmos by the speed of light, which establishes a boundary called the cosmic horizon. This horizon defines the observable universe, representing the region from which light has had enough time to reach Earth since the universe began. Since the Big Bang occurred approximately 13.8 billion years ago, one might assume the boundary is 13.8 billion light-years away.
However, the universe has been continuously expanding while light has been traveling toward us, meaning objects that emitted light 13.8 billion years ago are now much farther away than their original distance. Current calculations based on the expansion rate suggest that the edge of the observable universe is approximately 46.5 billion light-years away in every direction.
The resulting spherical volume that we can theoretically observe has a diameter of about 93 billion light-years. This measurement is a physical limit based on light travel time, not a reflection of the universe’s total size or shape. The total universe beyond this horizon remains inaccessible to direct observation.
Is the Universe Spatially Infinite?
The total size of the universe is not known, but its geometry offers strong clues about whether it is spatially infinite. The structure of the cosmos can be classified into three possible geometries based on its curvature: closed, open, or flat. A closed universe, which has positive curvature like the surface of a sphere, is finite and would eventually collapse. An open universe has negative curvature, resembling a saddle shape, and is infinite.
The third possibility is a flat universe, which has zero curvature and is also spatially infinite, similar to a vast, boundless plane. The universe’s geometry is directly related to its total density of matter and energy compared to the critical density. If the density is equal to the critical density, the universe is flat.
Cosmologists have measured the universe’s curvature by analyzing the Cosmic Microwave Background (CMB), the faint radiation left over from the Big Bang. Missions such as WMAP and the Planck satellite provided precise maps of the temperature fluctuations in the CMB. These fluctuations correspond to the seeds of structure formation in the early universe.
By examining the angular size of these fluctuations, scientists can determine the geometry of the space through which the light traveled. The data from both WMAP and Planck overwhelmingly indicate that the universe is spatially flat. The measured value for the curvature parameter is consistent with zero, with an error margin of less than one percent.
A flat geometry strongly implies that the universe is spatially infinite. However, a flat universe could still be finite if its overall topology is complex, such as a three-dimensional torus, similar to a video game map that wraps around. Even if it is finite with a flat geometry, its size is still vastly larger than the observable portion.
The Universe’s Temporal Fate
Addressing the universe’s temporal fate requires understanding the dominant force shaping its future: Dark Energy. Scientists discovered in the late 1990s that the expansion of the universe is not slowing down due to gravity, but is actually accelerating. This acceleration is attributed to Dark Energy, a mysterious component that makes up about 68% of the total mass-energy density of the cosmos.
The continued, accelerating expansion driven by Dark Energy leads to the currently accepted ultimate fate known as the Big Freeze, or Heat Death. This scenario is a direct consequence of the second law of thermodynamics, which states that entropy, or disorder, in an isolated system must always increase. For the universe, this means all energy will eventually be spread out evenly.
In the Big Freeze, the universe will continue to expand, causing galaxies beyond our local group to recede faster than light, eventually becoming invisible to us. Star formation will cease entirely after about 100 trillion years as the gas clouds needed to create new stars are used up or dispersed. Over immense timescales, all existing stars will burn out, leaving behind remnants like white dwarfs, neutron stars, and black holes.
Ultimately, the universe will reach a state of thermodynamic equilibrium, where all matter and energy are uniformly distributed at a temperature just above absolute zero. No usable energy will remain to power any process or motion, leading to a cold, dark, and inactive cosmos. While alternative fates like the Big Crunch or the Big Rip have been considered, the evidence for Dark Energy strongly supports the Big Freeze as the universe’s likely temporal end.