Primordial Black Holes
In September 2015, the LIGO observatory detected gravitational waves from two black holes, each about 30 times the mass of our sun, spiraling together and merging. The event stunned astronomers -- stellar black holes that large are hard to explain through the ordinary death of a single star. Could some black holes have formed a completely different way -- not from a dying star at all, but from the extreme density of the universe in the first fraction of a second after the Big Bang? Physicists call these hypothetical objects primordial black holes.
What You'll Learn
By the end of this lesson, you will be able to: - Explain how primordial black holes are thought to differ from stellar black holes in how they form - Describe the theory first proposed by Stephen Hawking and Bernard Carr in 1971 - Explain why primordial black holes are a candidate for dark matter - Describe Hawking radiation and how it causes black holes to slowly evaporate
Born From Density, Not From a Star
Ordinary stellar black holes form when a massive star -- roughly 20 or more times the mass of our sun -- runs out of nuclear fuel and collapses under its own gravity at the end of its life. Primordial black holes (PBHs) are hypothesized to have formed a completely different way: in the first fraction of a second after the Big Bang, when tiny regions of the infant universe were so extraordinarily dense that they collapsed directly into black holes without ever becoming a star first. Because they don't depend on stellar collapse, theorists predict they could exist across an enormous range of masses -- from smaller than an atom to thousands of times the mass of our sun.
Hawking and Carr's 1971 Proposal
The idea was first proposed in a 1971 paper by physicists Stephen Hawking and Bernard Carr. They argued that tiny quantum density fluctuations present in the extremely early universe could have collapsed into black holes of nearly any mass, depending on exactly when in that first fraction of a second they formed.
In 1974, Stephen Hawking proposed that black holes are not perfectly black -- quantum effects near the event horizon cause them to slowly radiate energy and lose mass over time, a process now called Hawking radiation. Smaller black holes evaporate faster than larger ones. A primordial black hole with roughly the mass of a mountain, formed at the Big Bang, would theoretically be finishing its evaporation right around now -- a process some physicists have proposed could explain certain unexplained gamma-ray bursts.
A Candidate for Dark Matter
Dark matter makes up roughly 27 percent of the universe's total mass-energy content, yet it doesn't emit, absorb, or reflect light, so it can only be detected through its gravity. Primordial black holes are appealing as a dark matter candidate because, unlike most other proposals, they don't require inventing any new type of particle -- just gravity and extreme early-universe density. Most mass ranges for PBHs have now been ruled out by observations, but a narrow window -- roughly asteroid-sized masses -- remains difficult to rule out. Astronomers have searched for them using gravitational microlensing, monitoring millions of background stars (including surveys of the Andromeda galaxy using Japan's Subaru telescope) for the brief brightening caused by an invisible massive object passing in front of a star.
No primordial black hole has ever been directly confirmed. The theory remains an active, debated area of research -- it is a candidate explanation, not an established fact, and most of the possible mass ranges have already been ruled out by decades of observation.
Flashcards — click each card to reveal the answer
How does the proposed formation of a primordial black hole differ from that of an ordinary stellar black hole?
Why are primordial black holes considered an appealing candidate to help explain dark matter?
Model the Mass Range
Primordial black holes are theorized to span masses from smaller than an atom to thousands of times the mass of the sun. Create a labeled number-line poster comparing at least five reference masses on a logarithmic scale -- for example: a grain of sand, a mountain, Earth, the sun, and a large stellar black hole. Mark where the 'still possible' asteroid-mass window for primordial black holes falls on your scale, and write two sentences explaining why that narrow window is so hard for astronomers to rule out.
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