The universe began with a bang, not a bounce, according to new studies
How did the universe begin? Did we start with a big bang or was there a rebound? Could the cosmos evolve in a cycle of expansion and collapse, over and over again for all eternity? Now, in two papers, the researchers have drilled holes in different models of the so-called bouncing universe, suggesting that the universe we see around us is likely a unique proposition.
Proponents of the bouncing universe argue that our cosmos did not just arise out of nothing. Instead, advocates claim, an earlier universe contracted in on itself and then grew back into the one we live in. This 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 begins with a Big Bang, followed by a period of rapid expansion known as cosmic inflation. According to that model, the glow left behind when the universe was young and hot, called the cosmic microwave background (CMB), should look pretty much the same no matter which way you look. But data from the Planck space observatory, which mapped the CMB from 2009 to 2013, showed unexpected variations in microwave radiation. They could be meaningless statistical fluctuations in the temperature of the universe, or they could be signs that something interesting is going on.
One possibility is that the CMB anomalies imply that the universe did not come from nothing. Instead, it came about after an earlier universe had collapsed and recovered to create the space and time we live in today.
Models of bouncing universes can explain these CMB patterns, as well as account for lingering subtleties 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 nowhere and contained the precursors of everything in the universe in a region so small that it was essentially sizeless. 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 mathematics itself, doesn’t make sense when everything is packed into a point that is infinitely small. Many physicists prefer to avoid singularities.
A bounce model that avoids singularities and makes CMB anomalies slightly less anomalous is known as loop quantum cosmology (LQC). It is based on a bridge between classical physics and quantum mechanics known as loop quantum gravity, which postulates that the force of gravity vanishes over very small distances instead of increasing to infinity. “Cosmological models inspired by loop quantum gravity can solve some problems,” says University of Geneva cosmologist Ruth Durrer, “especially the singularity problem.” Durrer is co-author of one of two new studies on bouncing universes. In it, she and her colleagues searched for astronomical signs of such patterns.
In an LQC model, a precursor to our universe could have contracted under the force of gravity to become extremely compact. Eventually, quantum mechanics would have taken over. Instead of collapsing into a singularity, the universe would have started to expand again and might even have gone through an inflationary phase, as many cosmologists believe it 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 analyses, proposed that the mark would appear in a feature of the CMB data known as a “bispectrum,” a measure of how different parts of the universe would have interacted in a bounce. script. The bispectrum would not be evident in an image of the CMB, but it would show up in analyzes of the microwave frequencies of the old CMB.
“If observed,” says Agullo, the bispectrum “would serve as irrefutable proof of the existence of a rebound instead of an explosion.” Agullo’s group previously calculated the bispectrum as it would have appeared 400,000 years after a cosmic bounce. Durrer and his colleagues took the calculation further, but when compared to current Planck CMB data, there were no significant signs of a bispectrum signature.
Although many other models of the bouncing cosmos may still be viable, the failure to find a significant bispectrum means that models that rely on LQC to deal with anomalies in the CMB can be ruled out. It’s a sad result for Agullo, who had high hopes of finding concrete evidence of a bouncing universe. But Paola Delgado, a doctor in cosmology. The candidate from Jagiellonian University in Poland, who worked on the new analysis Durrer co-authored, says there is a potential upside. “I heard for a long time that [attempts to merge quantum physics and cosmology] it cannot be proven”, says Delgado. “I think it was really nice to see that for some model classes, you still have some contact with the observations.”
Ruling out signs of an LQC-driven cosmic bounce in the Planck data means that the CMB anomalies remain unexplained. But an even bigger cosmic problem remains: Did the universe have a beginning? As far as big bang proponents go, it did. But that leaves us with the inscrutable singularity that started it all.
Alternatively, according to the theories of so-called cyclical cosmologies, the universe is immortal and is going through 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 back for an infinite number of cycles and continue for an infinite number more. And because such a universe has no beginning, there is no big bang and no singularity.
The study co-authored by Durrer and Delgado does not rule out immortal cyclical cosmologies. Many theories describe a universe so bouncing in ways that it would be difficult or impossible to distinguish from the “big bang plus inflation” model by looking at Planck’s CMB data.
But a critical flaw lurks in the idea of an eternally cyclical universe, according to University at Buffalo physicist William Kinney, co-author of the second recent analysis. That flaw is entropy, which accumulates when a universe bounces around. Often thought of as the amount of disorder in a system, entropy is related to the amount of useful energy in the system: the higher the entropy, the less energy available. If the universe increases in entropy and disorder with each bounce, the amount of usable energy available decreases each time. In that case, the cosmos would have had larger amounts of useful energy in earlier times. If you extrapolate far enough back, that implies a start similar to the Big Bang with an infinitely small amount of entropy, even for a universe that subsequently goes through cyclical rebounds. (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.)
Kinney and one of his colleagues found that the new cyclical models get around the problem by requiring the universe to expand greatly with each cycle. The expansion 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 universe is not immortal. “I feel like we’ve shown something fundamental about the universe,” says Kinney, “which is that it probably had a beginning.” That implies that a big bang happened at some point, even if that event happened many bouncing universes ago, which in turn suggests that it took a singularity to make it all work in the first place.
Kinney’s paper is the latest in the debate over cyclic universes, but proponents of a universe without beginning or end have yet to respond in the scientific literature. Two leading proponents of a cyclical universe, astrophysicists Paul Steinhardt of Princeton University and Anna Ijjas of New York University, declined to comment for this article. However, if the history of the debate is any indication, we may soon hear of a workaround to counter Kinney’s analysis.
Cosmologist Nelson Pinto-Neto from the Brazilian Center for Physical Research, who has studied rebound and other cyclical models, agrees that the Planck data probably rules out a bounce under loop quantum cosmology, but is more optimistic on the question of a cyclical universe. “Existence is a fact. We are all here and now. Nonexistence is an abstraction of the human mind,” says Nelson. “This is the reason why I believe that a [cyclic universe], which has always existed, is simpler than one that has been created. However, as a scientist, I must be open to both possibilities.