It took quite a bit more than seven days to create the universe as we know it today. SPACE.com looks at the mysteries of the heavens in our eight-part series: The history & future of the cosmos. This is Part 5 in that series.
Our universe was born about 13.7 billion years ago in a massive expansion that blew space up like a gigantic balloon.
That, in a nutshell, is the Big Bang theory, which virtually all cosmologists and theoretical physicists endorse. The evidence supporting the idea is extensive and convincing. We know, for example, that the universe is still expanding even now, at an ever-accelerating rate.
Scientists have also discovered a predicted thermal imprint of the Big Bang, the universe-pervading cosmic microwave background radiation. And we don't see any objects obviously older than 13.7 billion years, suggesting that our universe came into being around that time.
"All of these things put the Big Bang on an extremely solid foundation," said astrophysicist Alex Filippenko of the University of California, Berkeley. "The Big Bang is an enormously successful theory."
So what does this theory teach us? What really happened at the birth of our universe, and how did it take the shape we observe today?
Traditional Big Bang theory posits that our universe began with a singularity — a point of infinite density and temperature whose nature is difficult for our minds to grasp. However, this may not accurately reflect reality, researchers say, because the singularity idea is based on Einstein's theory of general relativity.
"The problem is, there's no reason whatsoever to believe general relativity in that regime," said Sean Carroll, a theoretical physicist at Caltech. "It's going to be wrong, because it doesn't take into account quantum mechanics. And quantum mechanics is certainly going to be important once you get to that place in the history of the universe."
So the very beginning of the universe remains pretty murky. Scientists think they can pick the story up at about 10 to the minus 36 seconds — one trillionth of a trillionth of a trillionth of a second — after the Big Bang.
At that point, they believe, the universe underwent an extremely brief and dramatic period of inflation, expanding faster than the speed of light. It doubled in size perhaps 100 times or more, all within the span of a few tiny fractions of a second.
(Inflation may seem to violate the theory of special relativity, but that's not the case, scientists say. Special relativity holds that no information or matter can be carried between two points in space faster than the speed of light. But inflation was an expansion of space itself.)
"Inflation was the 'bang' of the Big Bang," Filippenko told SPACE.com "Before inflation, there was just a little bit of stuff, quite possibly, expanding just a little bit. We needed something like inflation to make the universe big."
This rapidly expanding universe was pretty much empty of matter, but it harbored huge amounts of dark energy, the theory goes. Dark energy is the mysterious force that scientists think is driving the universe's current accelerating expansion.
During inflation, dark energy made the universe smooth out and accelerate. But it didn't stick around for long.
"It was just temporary dark energy," Carroll told SPACE.com. "It converted into ordinary matter and radiation through a process called reheating. The universe went from being cold during inflation to being hot again when all the dark energy went away."
Scientists don't know what might have spurred inflation. That remains one of the key questions in Big Bang cosmology, Filippenko said.
Most cosmologists regard inflation as the leading theory for explaining the universe's characteristics — specifically, why it's relatively flat and homogeneous, with roughly the same amount of stuff spread out equally in all directions.
Various lines of evidence point toward inflation being a reality, said theoretical physicist Andy Albrecht of the University of California, Davis.
"They all hang together pretty nicely with the inflationary picture," said Albrecht, one of the architects of inflation theory. "Inflation has done incredibly well."
However, inflation is not the only idea out there that tries to explain the universe's structure. Theorists have come up with another one, called the cyclic model, which is based on an earlier concept called the ekpyrotic universe.
This idea holds that our universe didn't emerge from a single point, or anything like it. Rather, it bounced into expansion — at a much more sedate pace than the inflation theory predicts — from a pre-existing universe that had been contracting. If this theory is correct, our universe has likely undergone an endless succession of bangs and crunches.
"The beginning of our universe would have been nice and finite," said Burt Ovrut of the University of Pennsylvania, one of the originators of ekpyrotic theory.
The cyclic model posits that our universe consists of 11 dimensions, only four of which we can observe (three of space and one of time). Our four-dimensional part of the universe is called a brane (short for membrane).
There could be other branes lurking out there in 11-dimensional space, the idea goes. A collision between two branes could have jolted the universe from contraction to expansion, spurring the Big Bang we see evidence of today.
The universe we know takes shape
But first, how did our universe spring up out of nothing? Cosmologists suspect that the four forces that rule the universe — gravity, electromagnetism and the weak and strong nuclear forces — were unified into a single force at the universe's birth, squashed together because of the extreme temperatures and densities involved.
But things changed as the universe expanded and cooled. Around the time of inflation, the strong force likely separated out. And by about 10 trillionths of a second after the Big Bang, the electromagnetic and weak forces became distinct, too.
Just after inflation, the universe was likely filled with a hot, dense plasma. But by around 1 microsecond (10 to the minus 6 seconds) or so, it had cooled enough to allow the first protons and neutrons to form, researchers think.
In the first three minutes after the Big Bang, these protons and neutrons began fusing together, forming deuterium (also known as heavy hydrogen). Deuterium atoms then joined up with each other, forming helium-4.
Recombination: The universe becomes transparent
These newly created atoms were all positively charged, as the universe was still too hot to favor the capture of electrons.
But that changed about 380,000 years after the Big Bang. In an epoch known as recombination, hydrogen and helium ions began snagging electrons, forming electrically neutral atoms. Light scatters significantly off free electrons and protons, but much less so off neutral atoms. So photons were now much more free to cruise through the universe.
Recombination dramatically changed the look of the universe; it had been an opaque fog, and now it became transparent. The cosmic microwave background radiation we observe today dates from this era.
But still, the universe was pretty dark for a long time after recombination, only truly lighting up when the first stars began shining about 300 million years after the Big Bang. They helped undo much of what recombination had accomplished. These early stars — and perhaps some other mystery sources — threw off enough radiation to split most of the universe's hydrogen back into its constituent protons and electrons.
This process, known as reionization, seems to have run its course by around 1 billion years after the Big Bang. The universe is not opaque today, as it was before recombination, because it has expanded so much. The universe's matter is very dilute, and photon scattering interactions are thus relatively rare, scientists say.
Over time, stars gravitated together to form galaxies, leading to more and more large-scale structure in the universe. Planets coalesced around some newly forming stars, including our own sun. And 3.8 billion years ago, life took root on Earth.
Before the Big Bang?
While much about the universe's first few moments remains speculative, the question of what preceded the Big Bang is even more mysterious and hard to tackle.
For starters, the question itself may be nonsensical. If the universe came from nothing, as some theorists believe, the Big Bang marks the instant when time itself began. In that case, there would be no such thing as "before," Carroll said.
But some conceptions of the universe's birth can propose possible answers. The cyclic model, for example, suggests that a contracting universe preceded our expanding one. Carroll, as well, can imagine something existing before the Big Bang.
"It could just be empty space that existed before our Big Bang happened, then some quantum fluctuation gave birth to a universe like ours," he said. "You can imagine a little bubble of space pinching off through a fluctuation and being filled with just a little tiny dollop of energy, which can then grow into the universe that we see through inflation."
Filippenko also suspects something along those lines might be true.
"I think time in our universe started with the Big Bang, but I think we were a fluctuation from a predecessor, a mother universe," Filippenko said.
Will we ever know?
The European Space Agency's Planck mission, which orbited Earth from 2009 to 2013, helped cosmologists fine-tune their ideas about the nature of our universe and its origins. The detailed map of the cosmic microwave background the spacecraft generated revealed that our universe, even if it may have sprung up from a predecessor, is not likely to contract again in the future, astrophysicist Dave Clements of Imperial College London, told Space.com.
"Planck can't exclude the bouncing universe concept altogether, but given the current values of the cosmological parameters, our universe is not going to recollapse," Clements said. "The dark energy component, which is accelerating the expansion of the universe at the moment, would have to change to reverse that expansion and drive a big crunch."
Using Planck's data, scientists were able to fine-tune their estimates of the universe's age as well as of the amount of visible matter, dark matter and dark energy in it. The mission, Clements said, didn't deliver any surprises and mostly confirmed existing theories.
"It shows that this is the maximally boring universe," Clements said.
Still, a few new questions emerged from its results. For example, the Hubble constant, which describes the rate of the universe's expansion, appears marginally different as measured by Planck in the distant universe, compared to its value given by the Hubble Space Telescope based on measurements in the near universe, said Clements.
All these bits of information help cosmologists better model the universe's evolution and get closer to answering the big questions about the origins of everything. The upcoming European Space Agency mission called Euclid (opens in new tab), which is scheduled for launch in 2023, is expected to make further steps in that direction.
The Euclid mission will look at how clusters and galaxies are scattered in the universe on the large scale to help astronomers better understand the effects of dark energy. It will also study what astronomers call weak gravitational lensing, the bending of light caused by the gravitational pull of very massive objects. Since over 80% of matter in the universe is invisible, the strength of the lensing could give astronomers hints about the distribution of dark matter.
"What Euclid will be able to do is measure this over much, much larger scales over maybe nearly half the the extragalactic sky or more," said Clements.
Further pieces of this cosmic jigsaw puzzle may come from the study of gravitational waves, the ripples in space time generated in collisions of supermassive objects such as black holes and neutron stars.
Gravitational waves, Clements said, must have been produced during inflation, the period of rapid expansion in the first moments of the universe's existence. Detecting those early gravitational waves and decoding their properties may therefore provide unprecedented insights into the universe's birth.
"This will tell us something about the physics that drove the early, very rapid expansion of the universe," said Clements. "We're really getting back to the very, earliest moments and if we understand inflation better, we will hopefully be able to understand better whether the Big Bang was a singular event or whether this bouncing idea might be correct."
For more information about the study of primordial gravitational waves and how they can help unlock the mysteries of the universe's birth, read this MIT article.
Muia, F., Big bang: how we are trying to ‘listen’ to it – and the new physics it could unveil, The Conversation, July 15, 2021
Castelvecchi, D., How gravitational waves could solve some of the Universe’s deepest mysteries, Nature, April 11, 2018
https://www.nature.com/articles/d41586-018-04157-6 (opens in new tab)
This reference article, originally posted on Oct. 21,2011, was updated on Feb. 4, 2022.