GENEVA, Switzerland — Monster black holes in the early universe may have taken an unusual route to becoming so massive.
Giant gas clouds in some of the first galaxies in our universe may have collapsed under their own gravity. In the process, they could have formed supermassive black holes, suggests Lucio Mayer. He’s a theoretical astrophysicist who works at the University of Zurich in Switzerland. He spoke on December 15, here, at the Texas Symposium on Relativistic Astrophysics. (Despite its name, the meeting was not held in Texas, or even in the United States.)
The process Mayer proposed would offer black holes a major shortcut to reaching monster size. Astronomers have suspected black holes start small and gradually grow by merging with others and gobbling up matter. The new mechanism also doesn’t rely on stars to create black holes in the first place.
Mayer’s proposal still faces some hurdles before other astrophysicists accept it. But if it is confirmed, it would solve the mystery of why astronomers keep spotting gargantuan black holes at a time when the universe was less than a billion years old. (They know that because the light from those very ancient times took so long to cross the universe to us that it’s just arriving now.)
This puzzle over how early black holes might have gotten big so fast boils down to timing. The first stars, some of them with a mass 100 times that of the sun, took shape a few hundred million years after the Big Bang. (The Big Bang is when our universe exploded into existence.) The largest stars soon died in a violent explosion. They left behind black holes that were roughly the same mass that they had been.
But here’s the mystery. Telescopes have recently shown that within about 500 million years later (not very long on cosmic timescales) some black holes had 10 billion times the mass of our sun. No matter how often ancient black holes feasted and combined, they would have had trouble getting 100 million times bigger so quickly.
Mayer has tried to figure out what could have given birth to such jumbo black holes.
His new recipe requires getting huge amounts of matter to fall together until the collective gravity of this mass is strong enough to prevent light from escaping. Galactic gas seems like an ideal ingredient for building black holes. Still, it never seems to reach the necessary ultradense state. Instead, it tends to cool and gather in small clumps that go on to become stars.
But what if two primordial galaxies collided? Perhaps their gas wouldn’t have a chance to build stars, Mayer says. Such a galactic merger would spark turbulent swells. These waves would warm the gas and prevent it from clumping, he suspects.He used a computer to model these interactions. And this computer model showed it was likely that the gas would grow to form a dense disk. What’s more, it appeared immune to breaking into stars. Instead, that gas eventually packed together so tightly that it eventually collapsed into a black hole that was hundreds of millions of times as massive as the sun. Mayer calls this direct progression from gas to shadowy darkness a “dark collapse.”
“There’s no light emitted,” he says. “It’s just a big black hole.”
Mitchell Begelman is an astrophysicist at the University of Colorado Boulder. He likes Mayer’s line of thinking. Still, he worries that what might stop stars from forming — such as gas molecules rotating too quickly — also would prevent the disk of gas from reaching a critical size needed to make a black hole. Indeed, he says, “I’m pretty skeptical you can get a collapse.”
Mayer says he plans to run a more rigorous relativity-based computer model to see if anything halts the gravitational cave-in. Proving black holes were actually born big more than 13 billion years ago will be much harder. However, the formation of such monsters should trigger potentially detectable ripples through space. These are those much-sought gravitational waves.
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astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
black hole A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.
Big Bang The rapid expansion of dense matter that, according to current theory, marked the origin of the universe. It is supported by physics’ current understanding of the composition and structure of the universe.
galaxy (adj. galactic) A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.
general relativity A set of mathematical expressions that define gravity and space over time (also known as spacetime). It was first published by Albert Einstein in November 1915. The field of research that focuses on this is described as relativistic.
gravitational waves Ripples in the fabric of space that are produced when masses undergo sudden acceleration. Some are believed to have been unleashed during the Big Bang, when the universe got its explosive start.
gravity Schools tend to teach that gravity is the force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity. But Einstein’s general theory of relativity redefined it, showing that gravity is not an ordinary force, but instead a property of space-time geometry. Gravity essentially can be viewed as a curve in spacetime, because as a body moves through space, it follows a curved path owing to the far greater mass of one or more objects in its vicinity.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.
primordial An adjective that refers to something that goes back to the beginning of time or to the earliest existence of something.
quasar Short for quasi-stellar light source. This is the brilliant core of some galaxy (massive collections of stars) that contains a super-massive black hole. As mass from the galaxy is pulled into that black hole, a huge quantity of energy is released, giving the quasar its light.
radio waves Waves in a part of the electromagnetic spectrum; they are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals; it is also used in radar.
spacetime A term made essential by Einstein’s theory of relativity, it describes a designation for some spot given in terms of its three-dimensional coordinates in space, along with a fourth coordinate corresponding to time.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
theory (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.
universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).