How did the universe begin? Did we start with a big bang or was there a rebound? Could the cosmos evolve in a boom and bust cycle, again and again for all eternity? Now, in two papers, researchers have punched holes in different models of a so-called bouncing universesuggesting that the universe we see around us is likely a unique proposition.
Proponents of the bouncing universe argue that our cosmos did not emerge on its own from nothing. Instead, proponents claim, a previous universe shrunk back on itself and then rejoined the one we live in. It may have happened once or, according to some theories, an infinite number of times.
So which scenario is correct? The most widely accepted explanation for the history of the universe has it begin with a big bang, followed by a period of rapid expansion known as cosmic inflation. According to this model, the glow left behind by the universe’s time when it was hot and young, called the cosmic microwave background (CMB), should be about the same no matter which direction you face. But data from the Planck space observatory, which mapped the CMB from 2009 to 2013, showed unexpected variations in microwave radiation. They could be statistically insignificant fluctuations in the temperature of the universe, or they could be signs of something interesting going on.
One possibility is that the CMB anomalies imply that the universe did not come out of thin air. Instead, it happened after an earlier universe collapsed and rebounded to create the space and time we live in today.
Bouncing universe models can explain these CMB models as well as accommodate persistent quibbles about the standard description of the origin and evolution of the universe. In particular, the big bang model of the universe begins with a singularity – a point that appeared out of thin air and contained the precursors to everything in the universe in a region so small it had virtually no size. at all. The idea is that the universe grew out of the singularity and, after inflation, settled into the more gradually expanding universe we see today. But singularities are problematic because physics, and math itself, doesn’t make sense when everything is lumped together into an infinitely small point. Many physicists prefer to avoid singularities.
A bouncing pattern that avoids singularities and makes CMB anomalies a bit less anomalous is known as loop quantum cosmology (LQC). It builds on a bridge between classical physics and quantum mechanics known as loop quantum gravity, which posits that the force of gravity depletes at very small distances rather than increasing to infinity. “Cosmological models inspired by loop quantum gravity can solve certain problems”, explains cosmologist from the University of Geneva Ruth Durrer, “in particular the problem of the singularity”. Durrer is co-author of one of two new studies on bouncing universes. In it, she and her colleagues searched for astronomical objects signs of such patterns.
In an LQC model, a precursor to our universe could have contracted under the force of gravity until it became extremely compact. Eventually, quantum mechanics would have taken over. Instead of collapsing into a singularity, the universe would have started expanding again and even gone through a phase of inflation, as many cosmologists believe ours did.
If that happened, says physicist Ivan Agullo of Louisiana State University, it should have left a mark on the universe. Agullo, who was not affiliated with any of the recent reviews, proposed that the brand would be appear in a feature of CMB data known as “bispectrum”, a measure of how different parts of the universe would have interacted in a bouncing storyline. The bispectrum would not be apparent in a CMB image, but it would appear in analyzes of old CMB microwave frequencies.
“If observed,” says Agullo, the bispectrum “would serve as compelling evidence for the existence of a bounce instead of a bang.” Agullo’s group before calculate the bispectrum as it would have appeared 400,000 years after a cosmic rebound. Durrer and his colleagues took the calculation further, but when they compared it to current Planck CMB data, there was no significant sign of a bispectral fingerprint.
Although many other bouncing cosmos models may still be viable, the failure to find a significant bispectrum means that models that rely on LQC to deal with CMB anomalies may be ruled out. It’s a sad outcome for Agullo, who had high hopes of finding concrete evidence of a bouncing universe. But Paola Delgado, who holds a doctorate in cosmology. candidate at Jagiellonian University in Poland, who worked on the new analysis co-authored by Durrer, says there is potential upside. “I have long heard that [attempts to merge quantum physics and cosmology] cannot be tested,” Delgado says. “I think it was really nice to see that for certain classes of models you still have some contact with observations.”
The exclusion of signs of an LQC-induced cosmic bounce in the Planck data means that the CMB anomalies remain unexplained. But an even larger cosmic problem persists: did the universe have a beginning? As far as big bang supporters go, it is. But that leaves us with the inscrutable singularity that started it all.
Alternatively, according to the theories of so-called cyclic cosmologies, the universe is immortal and undergoes endless rebounds. Although a bouncing universe may experience one or more cycles, a truly cyclical universe has no beginning or end. It consists of a series of bounces that go up for an infinite number of cycles and will continue for an infinite number more. And because such a universe has no beginning, there is no big bang or singularity.
The study co-authored by Durrer and Delgado does not rule out immortal cyclic cosmologies. Many theories describe such a bouncing universe in ways that would be difficult, if not impossible, to distinguish from the “big bang plus inflation” pattern by looking at Planck CMB data.
But a critical flaw lurks in the idea of an eternally cycling universe, according to University at Buffalo physicist William Kinney, co-author of the second recent analysis. That flaw is entropy, which builds up as the universe bounces around. Often thought of as the amount of disorder in a system, entropy is related to the amount of useful energy the system has: the higher the entropy, the less energy there is available. If the universe increases in entropy and disorder with each bounce, the amount of usable energy available decreases each time. In this case, the cosmos would have had greater amounts of useful energy in earlier times. If you extrapolate far enough back, this implies a big bang-like beginning with an infinitesimally small amount of entropy, even for a universe that then goes through cyclical bounces. (If you’re wondering how this scenario doesn’t violate the law of conservation of energy, we’re talking about available energy. Although the total amount of energy in the cosmos remains static, the amount that can do useful work decreases with increasing entropy.)
New cyclical models get around the problem, Kinney and one of his colleagues found, by requiring the universe to expand a lot with each cycle. Expanding allows the universe to smooth out, dissipating entropy before collapsing again. Although this explanation solves the entropy problem, the researchers calculated in their recent paper that the solution itself ensures that the the universe is not immortal. “I feel like we’ve demonstrated something fundamental about the universe,” Kinney says, “that it probably had a beginning.” This implies that a big bang happened at some point, even though this event happened many bouncing universes ago, which in turn suggests that it took a singularity to make it all happen in the first place.
Kinney’s paper is the latest in the debate over cyclic universes, but proponents of a universe with no beginning or end have yet to respond in the scientific literature. Two leading proponents of a cyclic universe, astrophysicists Paul Steinhardt of Princeton University and Anna Ijjas of New York University, declined to comment for this article. If the the history of the debate is an indicationhowever, we may soon hear of a workaround to counter Kinney’s analysis.
Cosmologist Nelson Pinto-Neto of the Brazilian Center for Physics Research, who has studied rebound and other cyclic patterns, agrees that Planck’s data probably exclude a bounce under loop quantum cosmology, but he is more optimistic on the question of a cyclic universe. “Existence is a fact. We are all here and now. Non-existence is an abstraction of the human mind,” Nelson says. “That’s why I think a [cyclic universe], which has always existed, is simpler than the one that was created. However, as a scientist, I have to be open to both possibilities.