A black hole’s temperature is a complex topic that requires incorporating the effects of the quantum world alongside gravity. A black hole is defined as a region of spacetime where gravity is so intense that nothing, not even light, can escape. This boundary is known as the event horizon. The physics governing the temperature of this cosmic object must account for both immense gravitational forces and the subtle behaviors of subatomic particles.
The Classical Physics Perspective
When considering temperature through the lens of classical general relativity, the perspective is straightforward and cold. A black hole is understood as a perfect absorber, meaning any matter or energy that crosses the event horizon is trapped forever. Since temperature relies on an object radiating energy, a perfect absorber that emits nothing must have a temperature of absolute zero, or 0 Kelvin.
According to this classical view, a black hole is thermodynamically inert and cannot participate in the exchange of heat with its surroundings. This perspective served as the baseline understanding for decades, suggesting that the black hole itself was the coldest object in the universe.
The Extreme Heat of the Accretion Disk
The common perception of black holes as being immensely hot comes not from the black hole itself, but from the material surrounding it. Material that spirals toward the event horizon forms a swirling structure known as an accretion disk. The intense gravitational field compresses and accelerates this material, causing tremendous friction.
This friction heats the gas to extreme temperatures, often reaching tens of millions of Kelvin, far hotter than the surface of any star. The superheated material emits powerful electromagnetic radiation, including visible light, X-rays, and gamma rays. This energetic radiation is the primary reason black holes are observed as highly luminous objects, even though the black hole’s interior remains entirely dark.
Hawking Radiation and Black Hole Temperature
The strictly cold classical view was overturned by the introduction of quantum mechanics near the event horizon. In the 1970s, physicist Stephen Hawking demonstrated that black holes are not perfect absorbers and must emit thermal radiation, a phenomenon now called Hawking radiation. This theory connects gravity, quantum mechanics, and thermodynamics, revealing that a black hole does possess a measurable temperature.
The mechanism relies on virtual particle-antiparticle pairs that constantly pop into and out of existence near the event horizon. If one particle of the pair falls into the black hole, the other may escape, carrying energy away. This escaping particle appears to an outside observer as thermal radiation emanating from the black hole itself.
Since this process causes the black hole to lose energy and mass, it confirms the black hole is a true thermal object. This temperature, however, is inversely proportional to the black hole’s mass. Smaller black holes have a more extreme spacetime curvature, leading to more energetic particle separation and thus a higher temperature. Conversely, more massive black holes have a much lower temperature.
The True Temperature of Cosmic Black Holes
The calculated temperatures of typical black holes are remarkably low, confirming the inverse relationship between mass and heat. A stellar-mass black hole, roughly the mass of our sun, has a theoretical Hawking temperature of about 60 nanokelvin (60 billionths of a degree above absolute zero). Supermassive black holes, like the one at the center of the Milky Way, are even colder, approaching a few picokelvin.
The Cosmic Microwave Background (CMB), the leftover radiation from the Big Bang, bathes all of space in a uniform temperature of 2.7 Kelvin. Because all known stellar-mass and supermassive black holes are significantly colder than the CMB, they absorb more energy from the universe than they emit via Hawking radiation. This means cosmic black holes are currently growing larger and colder over time.
The theoretical evaporation of a black hole can only begin once its temperature exceeds that of the surrounding universe. This process is expected to take an immense amount of time, with a solar-mass black hole requiring over \(10^{67}\) years to fully evaporate. Therefore, while black holes are technically hot, their temperature is so vanishingly small that they are, for all practical purposes, the coldest known objects in the cosmos.