Could the quantum origins of gravity explain dark energy?

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For decades, cosmologists have wondered about the nature of dark energy, the proposed antigravity force behind the accelerated expansion of the universe. Astronomers have observed since the 1990s that the universe is not only expanding, it is also increasing its rate of expansion. This is very strange because the collective pull of all the “stuff” in the universe is expected to eventually reverse, or at least slow down, cosmic expansion. Instead, a mysterious force – the aforementioned “dark energy” – like a ball gently thrown above us suddenly pushes up into the sky, distant, galaxy-filled regions of space away from us at ever greater speed. No known physics has fully explained this phenomenon; it remains a cosmic mystery, and its true, as yet unknown, nature will profoundly shape the ultimate fate of our universe.

Now, however, a new theoretical study has been submitted for publication at the Journal of Cosmology and Astroparticle Physics, suggests that the apparent antigravity properties of dark energy may be the natural, inevitable consequence of how gravity works in the first place on the most basic quantum scales in the universe. If finally confirmed by further cosmological evidence, the idea would mark a major breakthrough in the long quest to overcome the split between the two most popular theories of physicists: quantum mechanics, which describes the microscopic world of particles and fields, and the general Theory of relativity, the. describes the macroscopic cosmos of planets, stars and galaxies. General relativity postulates that gravity is an emergent property of curves and curvatures in spacetime – the fabric of reality itself – but the theory is losing its predictive power on quantum scales; Conversely, quantum mechanics takes into account all other known fundamental forces with the exception of gravitation, which does not fit into the theory. Therefore, many physicists suspect that a quantum theory of gravity is the only way to combine these two opposing approaches.

According to Daniele Oriti, a co-author of the new article, the core idea of ​​any theory of quantum gravity is that gravity arises from a multitude of tiny, discrete quantum objects that form a kind of hidden underworld, a deeper substructure beneath the familiar dimensions of space and time. “These quantum objects, which are difficult to imagine,” says Oriti, “are essentially the building blocks of space itself. They do not exist in space, but are themselves the stuff of which space is made. If they exist at all, they are absolutely tiny and have a microscopic scale that even the most powerful microscopes cannot see. “

In the study, Oriti and his co-author Xiankai Pang, both at the University of Munich in Germany, initially focused on developing a new quantum gravity model by trying to better understand the properties of force at the microscopic level. “After we constructed our new model,” says Oriti, “we decided to trace it through time from the beginning of our modeled universe to see what would happen as its expansion developed. We were definitely surprised when we saw something very similar to dark energy. The model generated an acceleration of the expansion of the universe in the stage that corresponds to our time, which agrees very well with the current observation results. “

“That’s a pretty neat result,” says Abhay Ashtekar, a prominent theorist at Penn State who works on modern theories of quantum gravity and was not involved in the new study. “Because the new approach starts with a general framework for quantum gravity at the subspace level and then applies it to the cosmological scale, while other methods limit oneself to the cosmological context from the start, the new idea starts from a more” basic perspective than so far, and that is an advantage. “

Oriti explains that the acceleration of the expansion of the universe by the model during the current stage is caused by interactions between the quantum objects of the subspace, which in theory constitute gravity. After the expanding universe reaches a critical volume, these quantum objects begin to interact with each other in new ways. It’s 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, at which point the conditions are just right to make them take action, resulting in a quick expansion. In the quantum gravity model, this causes the emergence of the dark energy-like phenomenon, which is characterized by an acceleration of the spatial volume growth.

“In the model, the quantum objects that make up space interact during the early universe, when the volume is small, in a way that makes them subdominant compared to their large-scale long-term development,” says Oriti. “But as the universe continues to expand over time, these interactions eventually become relevant and begin to affect the evolution of the universe – the dynamics of the universe – in a significant way, leading to an acceleration of the expansion. At this stage, the interactions between the quantum objects that make up space create an acceleration similar in description and magnitude to that observed by dark energy cosmologists. “

“It is very interesting to have such a phenomenological effect of dark energy in a quantum gravity model,” says Ana Alonso Serrano, physicist at the Max Planck Institute for Gravitational Physics in Germany, who was also not involved in the study. “I think it’s important that we examine our quantum gravity models this way to see if they can make predictions about cosmology and compare them to observations.”

“The next step will be to build on your theory and model to make further predictions that can be compared to real cosmological observations,” she says. “But I think there is still a long way to go before we really get a good understanding of the quantum nature of gravity and actually have a solid relationship with dark energy.”

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