Our universe is greatest described by the LCDM mannequin. That is an increasing universe stuffed with darkish vitality (Lambda), and dense clumps of chilly darkish matter (CDM). It can also be sprinkled with common matter that makes up planets, stars, and us, however that solely makes up about 4% of the cosmos. While we don’t know what darkish matter and darkish vitality are, we all know how they behave, so the ?CDM mannequin works exceptionally nicely. There’s only one small drawback.
Different measures of the universe give barely totally different values for darkish matter, and darkish vitality. Credit: ESOThe LCDM is outlined by a number of parameters, equivalent to the worth of the Hubble Constant, which determines how rapidly the universe is increasing, or the baryon density parameter, which describes the scale at which galaxies cluster collectively. Several impartial experiments have measured these parameters, and so they get outcomes that barely disagree. For instance, observations of distant galaxies give a Hubble Constant that’s increased than the worth discovered from the Cosmic Microwave Background (CMB). These disagreements are referred to as a pressure in the LCDM mannequin. It is maybe the greatest drawback in trendy cosmology.
One approach to deal with this drawback is to look to new experiments, equivalent to these involving gravitational wave astronomy. But thus far, these haven’t made issues higher. Another path is to look towards new physics, particularly, theories that reach the commonplace mannequin of particle physics.
Like the LCDM mannequin of cosmology, the commonplace mannequin of particle physics works extraordinarily nicely. But there are some hints round the edges that there might be one thing extra. There isn’t any particle in the commonplace mannequin that may account for darkish matter, for instance. So physicists have developed fashions to transcend the commonplace mannequin. The hottest of those is a category of fashions referred to as string principle. There is, nonetheless, a much less common mannequin referred to as Loop Quantum Gravity (LQG).
Diagram exhibiting evolution of the Universe in keeping with the paradigm of Loop Quantum Origins. Credit: APS/A. Stonebraker
In LQG fashions, house and time have a elementary granularity to them at teeny tiny scales. We would by no means discover this in our on a regular basis lives, and even in the sorts of excessive-vitality experiments we do in trendy particle accelerators. However, at the most intense areas of the universe, equivalent to the interiors of black holes or the huge bang’s first moments, this granularity would matter considerably.
Recently, a workforce checked out how Loop Quantum Gravity may have interacted with vitality and matter throughout the huge bang interval. They discovered that the construction of LQG in the early universe can be magnified by cosmic growth to shift the noticed cosmic parameters. In different phrases, the pressure we see in the LCDM mannequin may outcome from a cosmic dance between the very tiny and the very giant.
That’s all nice and good, however simply because a principle can work doesn’t imply it’s the principle that does work. So the workforce additionally seemed for a method their mannequin might be distinguished from different options. They discovered that LQG would additionally depart its mark on the Cosmic Microwave Background. If their mannequin is appropriate, the CMB ought to have clusters of small fluctuations that aren’t statistically random. The granular construction of house and time ought to depart a detectable imprint.
These fluctuations can be too small for satellites equivalent to Planck to look at, however future missions equivalent to the Cosmic Origins Explorer ought to detect them. If the workforce is true, we would not solely clear up the thriller of cosmic pressure, we would additionally take our first step into a brand new realm of physics.
Reference: Ashtekar, Abhay, et al. “Alleviating the tension in the cosmic microwave background using Planck-scale physics.” Physical Review Letters 125 (2020) 051302.
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