For decades cosmologists have wondered about the nature of dark energy, the proposed antigravitational force behind the accelerating expansion of the universe. Since the 1990s astronomers have observed that the universe is not only expanding, but also increasing its expansion rate. This is very strange, because the collective gravitational pull of all the “stuff” in the universe would be expected to eventually reverse cosmic expansion, or at least slow it down. Instead, just like a ball gently tossed overhead suddenly soaring off into the heavens, some mysterious force—the aforementioned “dark energy”—is pushing far-distant, galaxy-filled regions of space away from us at ever-greater speeds. No known physics has fully explained this phenomenon; it remains a cosmic enigma, and its true, as-yet-unknown nature will profoundly shape the ultimate fate of our universe.
Now, however, a new theoretical study, submitted for publication at the Journal for Cosmology and Astroparticle Physics, suggests dark energy’s apparent antigravitational properties may be the natural, inevitable consequence of how gravity works in the first place, at the universe’s most fundamental quantum scales. If eventually verified by further cosmological evidence, the idea would represent a major breakthrough in the long quest to mend the schism between physicists’ two most cherished theories: quantum mechanics, which describes the microscopic world of particles and fields, and general relativity, which describes the macroscopic cosmos of planets, stars and galaxies. General relativity posits that gravity is an emergent property of curves and warps in spacetime—the fabric of reality itself—but the theory loses its predictive power at quantum scales; conversely, quantum mechanics accurately incorporates all other known fundamental forces save for gravity, which fails to fit into the theory. Thus, many physicists suspect a quantum theory of gravity is the only way to unify these two opposing approaches..
According to Daniele Oriti, a co-author of the new paper, the core idea behind any theory of quantum gravity is that gravitation arises from a myriad of tiny, discrete, quantum objects that form a sort of hidden underworld, a deeper substructure beneath the familiar dimensions of space and time. “These quantum objects, which are very difficult to imagine,” Oriti says, “are essentially the building blocks of space itself. They do not exist in space, but are themselves the very stuff out of which space is made. If they exist at all, they are absolutely tiny in their size, and are at a microscopic scale which even the most powerful microscopes cannot see.”
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In the study Oriti and his co-author Xiankai Pang, both at the University of Munich in Germany, focused first on developing a new quantum gravity model by trying to better understand the force’s properties at the microscopic level. “Once having constructed our new model,” Oriti says, “we decided to track it through time from the beginning of our modeled universe, to see what would happen during the evolution of its expansion. We were definitely surprised when we saw something closely resembling dark energy. The model produced an acceleration of the expansion of the universe at the stage corresponding to the time we are at today, which matches very closely with current observational evidence.”
“This is quite an elegant result,” says Abhay Ashtekar, an eminent theorist at Penn State who works on modern theories of quantum gravity and who was not involved in the new study. “Because the new approach begins with a general framework for quantum gravity at the subspace level, and then applies it to the cosmological scale, while in other methods one restricts oneself to the cosmological context right from start, the new idea is beginning from a more fundamental perspective than we have done before, and that is an advantage.”
Oriti explains that the model's acceleration of the expansion of the universe, during the stage corresponding to today, is caused by interactions between the subspace quantum objects that make up gravity in the theory. After the expanding universe reaches a critical volume, these quantum objects begin to interact with each other in new ways. It is a bit like baking a cake. Imagine a cake where the yeast—in this case the subspace quantum objects—is not so important until a critical temperature—in this case the volume of the universe—is reached, whereafter conditions are just right to kick it into action, causing a rapid expansion. In the quantum gravity model, this is what causes the emergence of the dark energy–like phenomenon, which is characterized by an acceleration of the growth in volume of space.
“In the model, during the early universe, when the volume is small, the quantum objects out of which space emerges, interact in a manner which makes them subdominant compared to their large-scale long-term evolution,” says Oriti. “But then, because the universe keeps expanding through time, at some point these interactions become relevant and they start affecting the evolution of the universe—the dynamics of the universe—in a considerable manner, causing an acceleration of the expansion. So, at that stage, the interactions between the quantum objects which make up space produce an acceleration which is similar in description and magnitude to the dark energy cosmologists observe.”
“Having a dark energy phenomenological effect like this from a quantum gravity model is very interesting,” says Ana Alonso Serrano, a physicist at the Max Planck Institute for Gravitational Physics in Germany, who was also not involved in the study. “I think it is important that we explore our quantum gravity models in this kind of way, so to see if they can make predictions about cosmology and compare them to observations.”
“The next step will be to build on their theory, and their model, so to make further predictions which can be compared against real cosmological observations,” she says. “But I think there is still a long road ahead before we really establish a good understanding about the quantum nature of gravity, and indeed if there is a firm relationship with dark energy.”