A black hole is a region of spacetime where gravity is so intense that nothing, not even light, can escape its pull. This extreme gravitational environment is created when a vast amount of matter is compressed into an incredibly small space, often at the end of a massive star’s life. While black holes are often imagined as static, they are dynamic objects whose size is constantly changing. The core question for astrophysicists is whether these colossal entities primarily grow or shrink over cosmic time. This dynamic nature is governed by the two primary mechanisms of expansion and one theoretical process of mass loss.
Defining the Black Hole’s Size
The physical size of a black hole is not measured by a solid surface, but by its boundary, known as the event horizon. This boundary is the point of no return; anything that crosses it is destined to fall toward the singularity at the center. For a non-rotating black hole, the radius of this event horizon is called the Schwarzschild radius. The size of the event horizon is proportional to the black hole’s mass: more mass means a larger event horizon, and less mass means a smaller one. This relationship between mass and size is the fundamental context for understanding how black holes expand or contract.
Expansion through Accretion
The most common way a black hole grows is through accretion, which is the gradual pulling in of surrounding matter, such as interstellar gas, dust, or entire stars. As this material falls toward the black hole, it does not spiral directly in, but instead flattens into a rapidly spinning structure called an accretion disk. Friction and turbulence within this disk cause the material to lose energy and angular momentum, heating it to millions of degrees and causing it to emit copious amounts of X-rays and other radiation. This intensely hot material slowly spirals inward until it crosses the event horizon, adding its mass to the black hole. Every particle of mass that crosses this boundary increases the total mass of the black hole, which in turn expands the size of its event horizon, making this the primary engine of growth for supermassive black holes at the centers of galaxies.
Expansion through Mergers
Black holes can also experience catastrophic, instantaneous growth through the violent process of merging with another black hole. This occurs when two black holes, often orbiting each other in a binary system, gradually spiral closer together. As they orbit, the two objects radiate tremendous amounts of energy in the form of gravitational waves, which are ripples in the fabric of spacetime. This energy loss causes their orbits to shrink until the two event horizons touch and coalesce to form a single, larger black hole. The resulting black hole’s mass is slightly less than the sum of the two original masses, because a significant portion of the total mass-energy is released as a final burst of gravitational waves during the merger. Although this method of growth is less frequent than accretion, it is a highly effective way to create the most massive black holes observed in the universe.
The Process of Shrinkage
While accretion and mergers cause expansion, there is a theoretical mechanism by which a black hole can shrink and ultimately vanish, a process known as Hawking radiation. This effect is the result of quantum mechanics near the event horizon, where the black hole slowly emits thermal radiation. The physics is complex, but the result is a gradual loss of mass over eons. The rate at which a black hole shrinks is inversely proportional to its mass, meaning smaller black holes evaporate much faster than larger ones. For a stellar-mass black hole, the timescale for complete evaporation is an immense period, estimated to be more than 10^64 years, which is vastly longer than the current age of the universe. Therefore, this shrinkage is only a significant factor for tiny, theoretical primordial black holes, while all observed stellar and supermassive black holes are effectively stable against this mass loss.