What is a singularity? How do singularities form?
A singularity is a point in space-time where density, gravity, and space-time curvature are infinite. A black hole singularity is formed when a massive star depletes its reserves of nuclear fuel and starts to contract, eventually collapsing into an infinitely small point of infinite gravity and density.
Learn about the concept of singularity, explained in simple terms.
The Birth of a Singularity
A singularity is born when a star exhausts its supply of hydrogen and nuclear fuel. At this point, the star begins to cool, causing it to contract. Stars that are more than one and a half times the mass of our sun collapse into a singularity of infinite density—a black hole. A black hole has infinite gravity, such that not even light can escape.
So can we actually observe singularities? Because nothing can escape from them to reach us, it would seem that we could only observe their effects. In 1967, scientists at Cambridge first detected radio waves coming from a distant neutron star. This was the first absolute evidence that neutron stars were real, instead of just being predicted mathematically.
Neutron stars are objects of extremely high density, almost as dense as black holes. The exclusion principle is the only thing that prevents them from becoming a black hole singularity. Once their existence was proven, the notion of black hole singularities suddenly wasn’t so far-fetched.
Black holes can’t be seen, but they still exert gravitational influence over other objects. This brings us to the strange case of the Cygnus X-1 system. This system consists of a star orbiting around an unseen object. Based on the star’s orbit, we know that the unseen object must have great density. The mass of the star is six times greater than the sun— too large for the unseen object it is orbiting to be anything other than a black hole.
Other likely black hole singularities have been identified throughout the observable universe, including one possibly at the center of our own galaxy. Because of the age of the universe, there are probably more black holes than observable stars, as many stars must have undergone collapse since the Big Bang.
Low-mass stars could also form black holes, though not through internal gravitational collapse. Instead, massive external pressures could cause their compression. Such high-energy conditions would only have been possible in the early universe, the period immediately following the Big Bang. Identifying such primordial black holes could provide a window into the early universe.
The Big Bang and Singularities
Before the Big Bang, both the density of the universe and the curvature of space-time would have been infinite. It is impossible to know anything about the time before this, because mathematical theory depends upon a flat or nearly flat space-time. Predictability breaks down under conditions of infinite curvature. Events “before” the Big Bang are inconsequential because they are, by definition, unobservable and unable to affect anything that came “after” the Big Bang. Indeed, the very concept of time itself can be said to have come into existence at the Big Bang.
This notion was troubling to many in the scientific community, because it seemed too much like the Judeo-Christian creation story, that time had a finite beginning. And it’s true that the Big Bang is consistent with (or at least does not preclude) a deity who may have set it in motion. The Catholic Church even pronounced the Big Bang as being in accordance with the Bible in 1951.
But did general relativity require a Big Bang event? The British mathematician and physicist Roger Penrose sought to answer this in 1965. Reasoning from general relativity and the principle that gravity is always attractive, Penrose theorized that when a star died and collapsed under the weight of its own massive gravity, it would be compressed to a space of zero surface and volume. This would be a singularity—a point in space-time of infinite density and curvature, much like conditions before the Big Bang. This singularity is called a black hole. As Penrose explained, singularity would form any time a massive body undergoes a gravitational collapse.
The great insight of Stephen Hawking, a colleague of Penrose’s, was to put Penrose’s theorem in reverse: If all stars ended up as singularities when they collapsed, then an expanding universe must have begun with a singularity.
In 1970, Hawking and Penrose jointly published a paper proving that general relativity necessitated the occurrence of a Big Bang event.
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