Studying cosmic expansion using methods from many-body physics

 

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down. More recently, the expansion has begun to speed up again. Credit: NASA's Goddard Space Flight Center

In cosmological calculations, it is virtually generally believed that matter is distributed evenly throughout the cosmos. This is because including the positions of every single star would make the computations far too difficult. The universe is not uniform in reality: some locations have stars and planets, while others have nothing but nothingness. Physicists Michael te Vrugt and Prof. Raphael Wittkowski from the University of Münster's Institute of Theoretical Physics and the Center for Soft Nanoscience (SoN) have developed a new model for this problem with physicist Dr. Sabine Hossenfelder from the Frankfurt Institute for Advanced Studies (FIAS). The Mori-Zwanzig formalism, a method for expressing systems with a large number of particles and a small number of measurands, was their starting point.

Background: Albert Einstein's general theory of relativity is one of the most effective theories in modern physics. It has been associated with two of the last five Nobel Prizes in Physics: in 2017 for the detection of gravitational waves, and in 2020 for the finding of a black hole at the heart of the Milky Way. One of the theory's most important uses is in understanding the universe's cosmic expansion since the Big Bang. The amount of energy in the universe determines the rate of expansion. Dark matter and dark energy, in addition to visible matter, play a role here—at least, according to the Lambda-CDM model, which is currently utilised in cosmology.

"Including the mean value of the universe's energy density in the equations of general relativity is theoretically incorrect," Sabine Hossenfelder explains. The question now is how "serious" this blunder is. Some specialists dismiss it as inconsequential, while others regard it as the key to unlocking the mystery of dark energy, whose physical form remains unknown. The speed of cosmic expansion may be affected by the uneven distribution of mass in the cosmos.

"Because the Mori-Zwanzig formalism is already being used successfully in many domains of research, from biology to particle physics," Raphael Wittkowski adds, "it also offers a potential approach to this astrophysical problem." The researchers modified this formalism such that it could be used to general relativity, and as a result, they were able to establish a model for cosmic expansion that took into account the universe's unequal distribution of matter.

The influence of these so-called inhomogeneities on the speed of the universe's expansion is predicted in detail by the model. This forecast differs slightly from that of the Lambda-CDM model, providing an opportunity to put the new model to the test. ""At the moment, astronomical data aren't precise enough to measure this variation," Michael te Vrugt adds, "but considerable progress has been made—for example, in the measurement of gravitational waves—giving us cause to hope that this will change." In addition, the novel edition of the Mori-Zwanzig formalism may be used to solve other astrophysical problems, hence the research is not limited to cosmology."

Originally Published Here.

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