How to Detect Black Hole Explosions

by Chris Shallue
figures by Fiona Qu

There’s an equation engraved on Stephen Hawking’s gravestone in Westminster Abbey:

This unassuming equation, discovered by Hawking in 1975, has extraordinary implications. It suggests that black holes, previously thought to be eternally stable, actually decay over time. This process begins slowly, but Hawking predicted it would eventually accelerate, culminating in a cataclysmic “explosion.” Yet, almost 50 years later, no black hole explosions have been observed. Why is this, and how might we go about detecting them?

What are black holes?

All objects exert invisible gravitational forces on all other objects. The greater an object’s mass, the stronger the gravitational force it exerts. Most everyday objects aren’t massive enough for us to notice their gravity, with one major exception: the Earth. Its gravity pulls all terrestrial objects towards its surface, preventing them from flying off into space. Yet, it’s possible to escape the Earth’s gravity by riding a rocket into outer space.

A black hole is a region of space with such a strong gravitational pull that no object that enters can ever escape, even with the most powerful rocket. Any object that falls into a black hole will be dragged unavoidably towards the center, from which there is no return.

Black holes can form through a process called gravitational collapse. Consider a star like the Sun, which is a giant ball of hot gas. Gas particles at the outer boundary experience an inward pull from the star’s gravity, but the star doesn’t collapse because fusion reactions in its core produce energy to oppose the inward pull of gravity. However, eventually the core will run out of fuel, causing the star to collapse. If the star is massive enough, nothing will be able to stop its contraction and the entire star will shrink towards a single point. The region surrounding this point will have such highly concentrated gravity that it forms a black hole. This process was first predicted by Oppenheimer and Synder in 1939, though the term “black hole” was not popularized until decades later. (Incidentally, our Sun is not massive enough to collapse into a black hole; it will end its life as a white dwarf.)

Black holes that form from collapsing stars are called stellar black holes. Astronomers have discovered numerous stellar black holes in our galaxy, for example by detecting the gravitational ripples emitted when they collide. There are also supermassive black holes, which live at the centers of galaxies and are thought to have formed from smaller black holes merging over cosmic history. Finally, there are so-called intermediate-mass black holes, which fall in between stellar and supermassive black holes.

What is the temperature of a black hole?

Ordinary objects are made up of particles. These internal particles are constantly in motion, jiggling about and vibrating. This motion generates light, making objects glow. An object’s temperature measures the amount of internal motion of its particles—its thermal energy. All objects with thermal energy emit light, and the higher the temperature, the higher the average energy of emitted light. The surface of the Sun has a temperature of about 5,500°C (10,000 F) and mostly emits visible light. The human body has a temperature of around 30°C (90 F), so it emits lower-energy infrared light, which is invisible to our eyes but can be detected by infrared cameras.

When a black hole forms from gravitational collapse, all particles that made up the original object become concentrated at a central point. After the collapse, there is no room for those particles to move around. Therefore, physicists originally thought black holes should have zero temperature. Without internal particle motions to generate light, black holes were thought to be truly “black.”

Considering this, it might come as a surprise to learn that black holes have nonzero temperatures! This was Hawking’s groundbreaking discovery in 1975: the equation engraved on his gravestone defines the “Hawking temperature” of a black hole. According to Hawking, black holes aren’t completely black—they emit light as if they possess thermal energy. How can this be, if there are no internal particle motions? The mechanism that generates this light, which is known as Hawking radiation, is still not fully understood. Scientists believe the full picture requires a deeper theory of physics, known as “quantum gravity,” that will unify the theories of quantum mechanics and general relativity. Hawking’s prediction of black hole radiation is not the full story, rather it’s a signpost on the way to a new theory of physics.

Why don’t we see black holes glowing?

In the past few years, scientists have captured the first images of black holes. These images depict a bright ring of hot dust and gas orbiting a dark central region: the black hole, cloaked in shadow. If Hawking’s theory is correct, why don’t black holes visibly glow?

It turns out that the light emitted by these black holes is very, very faint. Hawking’s formula for the black hole temperature says that the more massive a black hole, the lower its temperature and the dimmer it is (Figure 1). Of the three known categories of black hole, stellar black holes are the lightest and therefore expected to be the brightest. But an entire stellar black hole radiates less than one million-trillion-trillionth the energy of a 100W light bulb!

Do black holes explode?!

Even if black holes only emit tiny amounts of light, that energy must come from somewhere. Where does it come from?

Einstein’s famous formula E=mc² says that energy and mass are interchangeable. Physicists believe that the energy emitted as Hawking radiation must come from the mass of the black hole. Therefore, as a black hole radiates, it loses mass, and shrinks. This process is known as black hole evaporation, akin to a puddle shrinking as it evaporates in the sunlight. Although this process starts slowly, it will eventually accelerate because the Hawking temperature increases as the black hole loses mass, which in turn increases the rate of radiation. The result is a runaway process that Hawking predicted would end in a cataclysmic “explosion”! However, an explosion is just one of the possibilities. Physicists are uncertain how black hole evaporation ends, so perhaps it halts before a dramatic explosion occurs. The only way to know for sure is to observe black holes evaporating in nature.

Unfortunately, all black holes we see today will survive far too long to observe their demise. For example, a stellar black hole is expected to live at least 10⁶⁵ years before it potentially explodes—much longer than the current age of the universe, which is about 14 billion years.

To observe black hole deaths in nature, we must turn to a hypothetical fourth category of black holes: primordial black holes, which are suspected to have formed in the earliest moments of the universe. We don’t know the mass of these black holes, but they could have been much lighter than stellar black holes—possibly as light as one gram! These black holes would have much shorter lifetimes than the black holes we observe today, with their Hawking radiation shining brighter than the Sun and their deaths coming in less than a second (Figure 2). If such black holes existed, they would have disappeared long ago—but perhaps their explosive ends left traces that are detectable today.

How can we observe primordial black hole explosions?

If primordial black holes formed and evaporated in the early universe, they would have released enormous amounts of light—much more than even the brightest supernovae of exploding stars. Unfortunately, it’s impossible to detect the light from these primordial fireballs because it would have been dispersed by the dense plasma that filled the early universe.

Rather than looking for light emitted from primordial black holes, we can look for other kinds of emission instead. Hawking’s theory of black hole radiation predicts that black holes emit all elementary particles, not just photons (particles of light). This could include new, undiscovered types of elementary particles that contribute to the mysterious dark matter! Unlike particles in the Standard Model of particle physics, these “dark-sector” particles would not necessarily interact with the plasma that filled the early universe. If such particles were emitted by evaporating primordial black holes, they might still be streaming through the universe undisturbed since they were created billions of years ago (Figure 3).

But if dark-sector particles don’t interact with ordinary matter, how can we detect them? Remember that all objects, including dark sector particles, exert gravitational forces. According to the standard cosmological theory, dark matter played a leading role in shaping the universe through its gravitational influence. In the early universe, matter was distributed nearly uniformly throughout space, with only minor differences in density between different regions. Over time, the denser regions pulled in surrounding matter through their stronger gravitational attraction, eventually gathering enough material to form stars and galaxies. The presence and behavior of all types of matter, including dark matter, contributed to this process, helping craft the intricate cosmic web of galaxies that fills the universe today.

In a recent study, scientists explored how the standard picture of cosmic history would change if primordial black holes formed in the early universe and evaporated explosively, releasing new types of dark-sector particles. They found that these particles would be ejected with such high energy that they would still be moving rapidly today, 14 billion years later! Although these particles would only account for a small fraction of the total dark matter, their large velocities would have impacted the formation of galaxies during cosmic evolution. Regions with strong gravitational forces tend to develop more galaxies than regions with weaker gravitational attraction. The fast-moving dark-sector particles would have escaped the weaker gravitational pull of smaller regions, reducing their ability to attract matter and limiting the number of galaxies that formed there. By modeling the universe’s evolution with and without particles from evaporating black holes, scientists can precisely calculate differences in the distribution of galaxies throughout space in each scenario, providing a measurable prediction to test for primordial black hole explosions.

Could scientists soon detect the signature of primordial black hole explosions? It’s possible! To do so, they need a very precise map of the positions of galaxies in the local universe, particularly smaller, fainter galaxies that are difficult to detect. Current and upcoming galaxy surveys, such as DESI and HETDEX, are designed to map the universe in unprecedented detail with the primary goal of studying dark energy. The data collected by precise surveys could also reveal the telltale evidence of exploding primordial black holes!

Stephen Hawking’s prediction that black holes evaporate, and perhaps even explode, is one of his most groundbreaking scientific results. Nature has not made it easy to observe this phenomenon, but the evidence could be out there—and it might just be encoded in the cosmic web of galaxies millions of light years from Earth.

Chris Shallue is a PhD candidate in astrophysics at Harvard. His research focuses on cosmology, black holes, and machine learning.

Fiona Qu is a PhD candidate in the Systems, Synthetic, and Quantitative Biology program at Harvard.

Cover image by AlexAntropov86 on pixabay.

For more information:

• The New York Times obituary of Stephen Hawking describes his discovery of the Hawking temperature and prediction of black hole explosions
• On this webpage, NASA describes the different types of black holes, including how they formed and how they can be detected
• On this webpage, NASA describes the three building blocks of the universe: normal matter, dark matter, and dark energy. Dark-sector particles emitted from exploding primordial black holes might contribute to the dark matter.
• In this research paper, scientists modeled the evolution of the universe if primordial black holes formed in the early universe and evaporated explosively, releasing new types of dark-sector particles.