Modern cosmology is built on three theoretical pillars: special relativity, Newtonian gravity, and quantum mechanics. Each is supported by considerable experimental evidence, but each describes the physical world in a way that contradicts the other two.
Quantum theory describes a small. Objects that are driven by electromagnetic forces, both strong and weak. The dim world of atoms and molecules. Newton’s model describes growth. The necks of stars, black holes, and the orbits of planets. A special relationship defines space and time. The nature by which atoms, planets and people move and interact.
These two concepts can be combined into a coherent model. Combine special relativity with gravity, and get a general relativity, which explains how gravity is a distortion of the atmosphere. Connect special relativity to quantum mechanics, and you get quantum field theory. Combine quantum mechanics with Newtonian gravity, and you get weak gravity, which can explain how atoms and molecules behave in a weak magnetic field like Earth.
What we don’t have is a concept that unites all three. One of the biggest problems is the problem of changing conditions. For example, through special relativity, matter can be converted into energy and energy into matter. In quantum theory, particles can appear and disappear at will as particles within the limits of quantum instability. When you combine the two, the visible particles have energy, which creates more real particles. If you try to calculate the total energy of all the particles, it blows up to infinity.
Fortunately, only limited power is important. Through a mathematical process known as renormalization, you can cancel out the existing energies of the quantum particles to get the answer you need. But when you add gravity into the mix, all of this falls apart. The force of visible particles must distort spacetime, and without a stable background of space, you cannot change space.
Many methods of quantum gravity suffer from this problem and cannot be regenerated. But one method, known as quadratic quantum gravity, can be renewed. In fact, this model adds quadratic terms to the Einstein field equations so that it can be reformulated as a quantum field theory. The problem is that it also includes the quantum field of “ghost particles.” These particles do not appear in particle physics experiments, so quadratic quantum gravity is not very popular. It is possible that ghost particles are too large to show up in modern physics experiments, but this makes the theory untested.
Or so we thought.
Comparison of the quadratic gravity model and observations. Credit: Liu, et al.
A new paper in *The Physical Review Letters* argues that quadratic quantum gravity is the reason why the Universe expanded so rapidly in its youth. The authors show that in quadratic quantum gravity, quadratic terms drive the expansion of the universe naturally. Once the universe passes its initial expansion, the shape of space is governed by the general effects of general relativity.
The authors go on to show that the model also predicts the minimum of recent gravity waves created during inflation. These waves are too small to be detected by current platforms, but are within the scope of future space probes such as LISA. So the model will be time tested.
Reference: Liu, Ruolin, Jerome Quintin, and Niayesh Afshordi. “Ultraviolet Completion of Big Bang in Quadratic Gravity.” *Physical Review Letters* 136.11 (2026): 111501.
#Theory #Combines #Early #Cosmic #Inflation #Quantum #Gravity