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The Big Bounce: Why our universe might be eternal

When it comes to theories of the universe, the Big Bang theory is almost accepted as a fact. However, it's still uncertain, and some scientists believe that the universe didn't began with a bang, but a bounce.

Big Bounce
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  • The Big Bang theory is treated as the de facto way the universe began, but it's had some issues.
  • One issue was that it could not describe how the universe became uniform and homogeneous, which is what we observe today.
  • Physicists tweaked Big Bang theory to accommodate this, but the Big Bounce theory can address these issues without too much tweaking.


Most of us are familiar with the standard narrative of how the universe began. There was an infinitely dense point of an infinite temperature with no size called a singularity. This singularity exploded, creating all the space, energy, and matter that we consider to be our universe in an event called the Big Bang. Between 10-36 seconds (that's 0.000000000000000000000000000000000001 seconds) and 10-32 seconds, space expanded exponentially, growing much, much larger in size. After this period, space continued to expand, but at a much slower rate, and eventually we see the universe that we observe today. This is the inflationary Big Bang theory, the most popular and broadly accepted theory of how the universe began. However, we have yet to prove this theory, and some think it doesn't paint an accurate picture.

Why we need inflation

A graph of the expansion of the universe. On the far left of this image, you can see the very brief moment of inflation that many physicists believe counterbalanced the randomizing effects of early quantum fluctuations.

Wikimedia Commons

Among these critics is Princeton physicist Paul Steinhardt, who actually contributed to the development of the theory described above, the idea that there was a moment of massively expanding space referred to as the inflationary epoch that quickly slowed down to the rate of expansion seen today. But including the inflationary epoch seems odd — why would there be this sudden change in the pace of expansion? It's actually something of an invention, a means of patching a troublesome quirk in the vanilla Big Bang theory.

"The Big Bang is not something that we really deeply understand, we have no theory of the Big Bang," says Steinhardt.

"But our notion is that it's some random, highly turbulent, quantum beginning from nothing to something. And so, it would leave a universe which is very random and distorted. Yet we don't observe that in the way the universe looks today. So we need some idea to fix that."

Today, when you zoom out far enough, the universe looks fairly flat and uniform — matter and energy are all fairly evenly distributed, and spacetime doesn't appear to have any curves. Inflation helped bridge the gap between the very random explosion of the singularity and the uniformity we see today — space expanded so rapidly that it smoothed out all the irregularities that would have occurred due to quantum effects during the Big Bang.

Does inflation cause more problems than it solves?

Despite helping to develop it, Steinhardt sees a few issues with the inflation model. For instance, those quantum effects that the inflation theory was supposed to deal with can actually create patches of the universe where inflation goes on forever. "The problem is," said Steinhardt in an interview with Nautilus, "due to the effects of quantum physics these patches are not all the same. The effects of quantum physics, when you include them properly, lead to a situation where some patches are like us, but some patches are not like us; and in fact, every conceivable possible outcome of the universe can occur if you look from patch to patch to patch and there's no particular reason why ours is more likely than any other." What Steinhardt is describing here is a multiverse, an infinite number of different universes with different rules. The one where we exist just so happens to have the right rules.

The trouble with this is that it almost seems like cheating. If inflation produces an infinite number of universes, then of course we would end up with the one we see around us; it doesn't really explain our specific universe. And for this reason, the inflation theory can't be disproved either — it predicts everything, and so makes no testable predictions. What if there were a simpler explanation?

Cue the Big Bounce

Instead of a Big Bang, with its attendant issues requiring the introduction of inflation, Steinhardt and other scientists have been toying with the idea of a Big Bounce. There are a variety of Big Bounce theories, but they essentially boil down to the idea that the universe is caught in a cycle where it expands after the Big Bang, then begins to contract. Some theories say that it contracts to the point of a singularity, where classical physics breaks down, and explodes again into a new Big Bang, while other theories suggest that the universe contracts to a point just above a singularity, where classical physics continue to apply.

But crucially, this process of contraction gives the universe time to become uniform throughout. When the bounce occurs, all matter is fairly uniform, becoming disordered over time. We currently live in a time when the universe is orderly, but it will become disorderly as time goes on. Once it begins contracting, the universe becomes increasingly orderly again. As it contracts further and further, matter and energy become more evenly distributed throughout the universe. Things flatten out and become more homogeneous as the next bounce approached. It could be that our universe has no definite beginning and will have no definite end — it could just bounce eternally.


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Image source: carlos castilla/Shutterstock
  • Quantum particles can tunnel through seemingly impassable barriers, popping up on the other side.
  • Quantum tunneling is not a new discovery, but there's a lot that's unknown about it.
  • By super-cooling rubidium particles, researchers use their spinning as a magnetic timer.

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Exactly why or even how quantum tunneling happens is unknown: Do particles just pop over to the other side instantaneously in the same way entangled particles interact? Or do they progressively tunnel through? Previous research has been conflicting.

That quantum tunneling occurs has not been a matter of debate since it was discovered in the 1920s. When IBM famously wrote their name on a nickel substrate using 35 xenon atoms, they used a scanning tunneling microscope to see what they were doing. And tunnel diodes are fast-switching semiconductors that derive their negative resistance from quantum tunneling.

Nonetheless, "Quantum tunneling is one of the most puzzling of quantum phenomena," says Aephraim Steinberg of the Quantum Information Science Program at Canadian Institute for Advanced Research in Toronto to Live Science. Speaking with Scientific American he explains, "It's as though the particle dug a tunnel under the hill and appeared on the other."

Steinberg is a co-author of a study just published in the journal Nature that presents a series of clever experiments that allowed researchers to measure the amount of time it takes tunneling particles to find their way through a barrier. "And it is fantastic that we're now able to actually study it in this way."

Frozen rubidium atoms

Image source: Viktoriia Debopre/Shutterstock/Big Think

One of the difficulties in ascertaining the time it takes for tunneling to occur is knowing precisely when it's begun and when it's finished. The authors of the new study solved this by devising a system based on particles' precession.

Subatomic particles all have magnetic qualities, and they spin, or "precess," like a top when they encounter an external magnetic field. With this in mind, the authors of the study decided to construct a barrier with a magnetic field, causing any particles passing through it to precess as they did so. They wouldn't precess before entering the field or after, so by observing and timing the duration of the particles' precession, the researchers could definitively identify the length of time it took them to tunnel through the barrier.

To construct their barrier, the scientists cooled about 8,000 rubidium atoms to a billionth of a degree above absolute zero. In this state, they form a Bose-Einstein condensate, AKA the fifth-known form of matter. When in this state, atoms slow down and can be clumped together rather than flying around independently at high speeds. (We've written before about a Bose-Einstein experiment in space.)

Using a laser, the researchers pusehd about 2,000 rubidium atoms together in a barrier about 1.3 micrometers thick, endowing it with a pseudo-magnetic field. Compared to a single rubidium atom, this is a very thick wall, comparable to a half a mile deep if you yourself were a foot thick.

With the wall prepared, a second laser nudged individual rubidium atoms toward it. Most of the atoms simply bounced off the barrier, but about 3% of them went right through as hoped. Precise measurement of their precession produced the result: It took them 0.61 milliseconds to get through.

Reactions to the study

Scientists not involved in the research find its results compelling.

"This is a beautiful experiment," according to Igor Litvinyuk of Griffith University in Australia. "Just to do it is a heroic effort." Drew Alton of Augustana University, in South Dakota tells Live Science, "The experiment is a breathtaking technical achievement."

What makes the researchers' results so exceptional is their unambiguity. Says Chad Orzel at Union College in New York, "Their experiment is ingeniously constructed to make it difficult to interpret as anything other than what they say." He calls the research, "one of the best examples you'll see of a thought experiment made real." Litvinyuk agrees: "I see no holes in this."

As for the researchers themselves, enhancements to their experimental apparatus are underway to help them learn more. "We're working on a new measurement where we make the barrier thicker," Steinberg said. In addition, there's also the interesting question of whether or not that 0.61-millisecond trip occurs at a steady rate: "It will be very interesting to see if the atoms' speed is constant or not."

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