The big bang was not a bang in the traditional sense, but it was nevertheless the beginning of important things: on the one hand, space; another, once. Third, it set in motion the conditions and processes that ultimately resulted in the creation of us humans who can sit here and wonder about space and time. The Big Bang was actually the beginning of the universe. According to the logic of the human brain, it seems that there must have been something before the big bangeven if “before” is not the right word because there was no time before after.
The good news for us is that physicists have ways of thinking about, and even empirically studying, the origins of the origin of the universe. As counterintuitive and impossible as it may seem, cosmologists are even making progress in determining what crazy ideas might lift the veil on this early era, even if it remains inaccessible to telescopes.
For millennia, what happened before and at the beginning of the universe was not a question that scientists could even address. Cosmological questions were the preserve of philosophers, says Jenann Ismael, herself a philosopher of physics at Johns Hopkins University. The most fundamental question, of course, is where we come from – a question as popular among philosophers as it is among us. Other questions, says Ismael, include questions such as “What is space and time? Does time have a beginning? Does space have limits?”
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Even after cosmology became a hard science, the field remained a bit vague, Ismael says. “Science was only one and a half facts,” she adds. This feeling, she says, is generally attributed to the physicist James Jeans. But this has changed over the past century, as philosophers’ thoughts have expanded into the realm of theory, experiment, and data. “These old conceptual questions arise in new angles, a new twist and a new framework,” continues Ismael.
It is unclear whether science as a discipline – and scientists as people – I will never be able to answer some questions definitely. After all, no one can “see” before the Big Bang, and no one ever will – at least not directly. But researchers are learning that the current and future universe may hold clues to the distant past.
And as scientists push the boundaries of what can be known, they test their theories about the past – the only way to get closer to the potential truth. “I’m happy to listen to any framework, but I only start taking it seriously when it produces a clean observational target that a real instrument can track,” says Brian Keating, a cosmologist at the University of California, San Diego. “If there is no discriminant you can measure, you are doing metaphysics with equations.”
Here are three ideas that he and other scientists take seriously about the ultimate origins of the cosmos.
The proposal without borders
Quantum mechanics is the physics of the extremely small, governed by statistics and uncertainty. This is also what may have shaped the primitive universe. To understand the quantum cosmos, scientists calculate the probability of a given output given a certain input.
In cosmology, the “result” is the universe as it appears today. “The question is: what should the contribution be? says Jean-Luc Lehners, former head of the theoretical cosmology group at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Germany.
Physicists can divide the problem into output and input chunks. If they view the modern universe as an output, they can try to determine what input could have produced it. Then they can go back, taking that input as a new output, and determine what conditions might have produced earlier in the universe. that state, and so on. They can theoretically (if they have a lot of free time) do this forever, taking steps to get to the front – and even before that.
This infinite regress, however, made no sense to physicists Stephen Hawking and James Hartle, who worked together on the issue in the 1980s. They decided to eliminate the ultimate contribution of the universe: its “beginning”. Instead, they formed a model of the universe called the Boundaryless Proposition. They suggested that time and space form a closed, rounded surface: a four-dimensional hemisphere of space-time.
Doesn’t that make sense? Try this: imagine the universe as the globe. The big bang is the North Pole. There is no “before,” just as there is no north of the north. Before becomes irrelevant as a concept. “It’s almost a Zen idea,” Lehners says. And it’s the one he’s playing with in his calculations to see if he can recreate the universe we see today from a round place with no north or north.
“The boundaryless proposal has decent support, or at least interest, within the physics community,” says Sean Carroll, a professor of natural philosophy at Johns Hopkins University. He notes that some scientists are concerned about the accuracy of this idea, but he believes it is a “natural starting point” given what we know about quantum gravity.
A bouncing and cyclical cosmos
Paul Steinhardt, a physicist at Princeton University, has another idea of what happened before the creation of the universe as we know it. This opposes an idea he helped shape: this concept suggests that after the Big Bang, space-time expanded very rapidly for a very short period of time called inflation. THE inflation scenario aims to explain why the universe appears flat and similar everywhere our telescopes can look.
However, after helping to establish the theory of inflation, Steinhardt began to doubt the idea, in part because it required constant adjustments to keep it consistent with our measurements of the cosmos. “It’s really hard to think of a historical example where this actually led to what turned out to be the right answer,” Steinhardt says. “Almost always, it is a sign that the Titanic is sinking.
Time to get in a lifeboat, he thought. So he imagined a cyclical universe: one that increases in size considerably, as ours seems to do now, then shrinks a little and begins to expand again. “When people think of the universes contracting, they usually think of things coming to a critical point,” says Steinhardt: the cosmos collapsing again to an infinitesimal point. That’s not what Steinhardt is talking about: he thinks the universe may be slowly contracting – to a smaller fraction of its size, but not to nothing. This shrinkage softens things in a way that inflation can’t explain, he says, while producing a cosmos that looks flat and the same in all directions.
Steinhardt adds that what look at as a big bang is actually not: the universe expands, then slowly contracts, and quickly begins to expand again. The rapid transition between contraction and expansion is not a bang but a “big bounce.”
Steinhardt hopes to test this idea not only by examining the past, but also by taking data from the present and carefully observing the future. “This makes an obvious prediction that the current phase of accelerated expansion cannot last forever,” says Steinhardt. “This must end.” This idea in turn raises a new question: “Could this already be ending now?” » he asks.
Our measurements of the expansion of the universe come from relatively distant objects that emitted their light long ago. Things could have changed, and we may not know yet because the effects would be difficult to measure. “You would have to look at very close objects to detect it,” says Steinhardt. This is not the forte of cosmologists, and they should develop new techniques and instruments to look for such effects at close range.
Even more intriguing, Steinhardt claims that because “nothing bad happens to space” during the contraction and rebound, information – even objects such as black holes – can pass from before the rebound to after. “There may be things in our observable universe that date back to before that,” he says. Keep an eye open.
Mirror universe
Another big idea ahead interests Latham Boyle, a researcher at the Higgs Center for Theoretical Physics at the University of Edinburgh, who was once Steinhardt’s graduate student. Much like the concept of the Great Bounce, Boyle’s preferred proposal is quite conceptually simple – and it also avoids inflation. “There’s the post-big bang universe and the pre-big bang universe,” he says, “and they’re sort of mirror copies of each other.”
Imagine it, Boyle says, as if the tips of two ice cream cones were touching, their contact representing the big bang. “Time moves away from the big bang in both directions,” he says. On our side, things are moving forward; on the mirror side, it goes backwards. What happened before the Big Bang is the opposite of what happened after. And that doesn’t just include time: here, there is matter; there is antimatter there. Here, the left is the left; there, left is right.
Boyle has ideas for observations that could support (or disprove) his theory, called a CPT (charge-parity-time-symmetric) symmetric universe. On the one hand, a CPT symmetric universe would not have sent scintillating gravitational waves through space since the beginning of the universe, as classical theories of cosmology predict. Astronomers look for such signals. If these waves were finally detected, it would rule out this idea.
Boyle’s hypothesis also predicts that dark matter could be explained by a particular type of neutrino. He hopes that cosmological instruments will soon reveal more information about neutrinos. The model’s connection to particle physics, among other aspects, makes this idea intriguing, Carroll says.
“What I like about it is the economics,” Keating says, “and the fact that it’s stretched thin,” focusing on the kind of specific physics predictions that experimentalists like him need.
The test of time
Each of these scientists is attached to their own idea. But Lehners, interviewed late last year, isn’t sure any of them will stand the test of time, no matter the time. “I think it’s completely absurd that in 2025 we can understand the beginning of the universe,” he says. “Why not in the year 2,000,025 or something?”
And even if researchers think they’re getting closer, they might be approaching a false summit: that frustrating place that looks, when you’re hiking, like the summit of e mountain but is actually just a bump blocking your view of the true summit – or your view of what you think is the true summit but is actually just another bump. “In general, I think it’s extremely plausible that there was something before the big bang,” Carroll says, “but it’s also very plausible that the big bang truly was the beginning. There are too many things that we’re just unsure of, and I’m a little skeptical that the state of the art is good enough to allow us to draw firm experimental or observational conclusions from any of these models.”
But cosmologists don’t study ultimate origins because they think the mystery will be solved in their lifetime. Lehner imagines himself part of an intergenerational project helping humanity move ever closer to a truth we may never find.
The study of such a physically and philosophically inaccessible subject is fundamentally different from other types of science – these quests exist at least in our plane of space and time. It almost seems like the question isn’t really a scientific one. But science often involves exploring things we don’t have access to, at least at first, explains physics philosopher Ismael. Scientists predicted atoms before we could see them, and black holes and dark matter still lie beyond our ability to detect them directly. Yet studying them is clearly scientific. here. “I think the benchmark for what counts as science has changed,” she says. And it will continue – including, perhaps, returning to a before that may not be a before.