Did the universe begin at a single point, or has it always been infinite?
ASK A RESEARCHER: How did the universe actually begin? "If the universe is infinite, it has been infinite since the very beginning," a researcher answers.
An image taken by the Hubble telescope, called the Hubble Ultra Deep Field, showing 10,000 galaxies.(Image: NASA, ESA, S. Beckwith (STScI), and the HUDF team)
Bjørn Ludvigsen, a reader, has submitted a question to Science Norway:
‘It is assumed that the Big Bang started at a single point. I have never been able to make sense of that. As I understand it, all the energy we can observe in the universe (both solid matter and the invisible) must occupy some amount of space, even if we compress it as much as possible. Has anyone looked into how small a volume the initial state could have occupied?’
Science Norway asked Hans Kristian Kamfjord Eriksen whether he could answer this. Eriksen is a professor of cosmology at the University of Oslo who studies the early universe.
A common misconception about the universe
According to Eriksen, the universe did not begin as a single point.
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"That's a common misconception," he says.
Today, the universe is expanding, with the distances between galaxies continually increasing. If we look at this development in reverse, everything moves closer and closer together. The universe becomes denser, hotter, and the distances become smaller and smaller.
"But that doesn't mean everything began as a point that later expanded into a volume. Space has always had volume," says Eriksen. "The way I picture it is to imagine a grid extending throughout the entire universe. As you move backward in time, each square in the grid becomes smaller and smaller. But all those squares were already there immediately after the Big Bang."
Science Norway also spoke with Are Raklev, another professor at the University of Oslo, who explains how physicists understand the Big Bang.
"For physicists, the Big Bang is simply the description of how the universe was much denser and therefore hotter around 14 billion years ago, and that at that time there must have been an enormous expansion of the universe, called inflation."
Hans Kristian Kamfjord Eriksen is a professor at the University of Oslo's Institute of Theoretical Astrophysics.(Photo: University of Oslo)
Was the universe infinite from the beginning?
So just how large was the universe at the time of the Big Bang?
It may have been infinitely large, according to Eriksen.
"One of the big questions that researchers are trying to answer through cosmological measurements is whether the universe is infinite or not. We have very precise measurements, but they fall right on the boundary between the two possibilities, so we don't know the answer," he explans, adding:
"If it is infinite, then it has been infinite since the very beginning. It's simply that its density has decreased since then."
What if the universe is finite?
"Physicists generally do not believe that the Big Bang began at a single point. Either the universe is infinite, and has been so since and at the time of the Big Bang, or it is finite both now and at the Big Bang, but simply much larger now," says Are Raklev.
If the universe is finite, you can think of it as a tiny sphere with a definite volume at the beginning.
Rather than having an outer edge, it curves back in on itself. If you travelled far enough in any direction, you would eventually end up back where you started.
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Eriksen suggests imagining the surface of a balloon.
As we look further back in time, the balloon becomes smaller and smaller, just as the universe would have been smaller in the distant past. But its size would never have reached zero, he explains.
Does that mean there was empty space outside it?
"That's where our human intuition begins to fail us, because we're accustomed to thinking in three dimensions. But no, there's nothing in the equations that says there is anything outside. Imagine ants walking across the surface of a balloon. They experience only that surface and have no awareness of anything beyond it," the researcher says.
The singularity
"We often hear about 'the singularity,' the idea that the universe was infinitely dense and hot at the moment of the Big Bang. But even that isn't a universally accepted view among cosmologists. It's more a popular-science assumption," says Raklev.
If researchers use the general theory of relativity to trace the universe's history all the way back to the Big Bang, they reach a point where the theory says that the universe had infinitely high density and temperature, and where its spatial extent was zero – a so-called singularity.
Most physicists do not believe that this is an accurate description of the state of the universe at its very beginning.
Instead, researchers believe we need a theory that combines gravity and quantum theory. No one has found the solution yet.
"Black holes also contain what we call a singularity at their centres, and that's where the equations break down," says Hans Kristian Kamfjord Eriksen. "Einstein's classical equation no longer works. We need to turn to quantum gravity instead, and those equations are not good enough."
How could everything be packed so tightly together?
Even if the universe did not begin as a single point, everything we can observe in the universe today was packed extremely, extremely tightly together.
How could that have been possible?
"That's a good question, and once again it brings us back to quantum gravity at extreme energy levels, something we still don't understand well enough," says Eriksen.
"I imagine the universe as being filled with some kind of unknown quantum field that vibrates extremely rapidly and essentially represents energy in its purest form. As expansion takes place, this gradually decays into the particles we know today through the standard model, and familiar laws of physics takes over.
"But we don't know what this primordial quantum field is, or which formulas govern it, simply because we don't have a lab capable of generating sufficiently high energies. CERN is the most powerful facility we have today, but even that is very, very, very far from what happened at the actual moment of creation," the researcher says.
A soup of elementary particles
Researchers still do not know what triggered the Big Bang or exactly how it happened.
However, from a tiny fraction of a second after the Big Bang, researchers have a remarkably detailed understanding of how the universe evolved, according to Eriksen.
"Immediately after the Big Bang, the universe was filled with photons, which are particles of light, along with the elementary building blocks that cannot be broken down any further," he says.
The universe was an infinite, hot soup of elementary particles.
"Imagine something like the inside of the sun, but much denser and hotter," the researcher says.
Eriksen explains that as the universe expanded rapidly, it also began to cool, much like a gas cools when it expands.
After just a few minutes, atoms as we know them were formed.
380,000 years after the Big Bang, the temperature fell below 3,000 degrees. At that point, the temperature was low enough for electrons to attach themselves to atoms.
It became open enough for photons to travel in straight lines, and the universe became transparent. This event can still be observed today as the cosmic microwave background radiation, a remnant of the Big Bang that Eriksen studies.
