This expansion can be explained with a model of the universe that includes a term known as a cosmological constant. This constant describes how much energy is contained within a vacuum, in other words, empty space. This energy of a vacuum was originally proposed by Albert Einstein, who was looking for a way to explain why the universe doesn’t collapse on itself due to gravity.

The cosmological constant now appears in a model for the universe called the Lambda Cold Dark Matter model, or Lambda-CDM. In this model, the universe contains a substance called dark matter in addition to the regular matter that we are familiar with in everyday life. This dark matter is thought to not interact with electromagnetic radiation (including visible light, radio waves, x-rays etc.), but does exert gravitational forces. In addition to matter and dark matter, the Lambda-CDM model includes a form of energy called dark energy, which includes the vacuum energy proposed by Einstein.

By including matter, dark matter, and dark energy, the Lambda-CDM model describes a universe in which expansion and cooling of the universe leads to a transition from a universe dominated by effects from radiation to a universe dominated by effects from matter, and finally to a universe dominated by effects from dark energy (which is where we are thought to be currently). In each of these three eras, the expansion of the universe occurs at different rates, and the theory explains the accelerating universe we currently see. However, although the theory can explain many modern astronomical measurements, the experimentally determined cosmological constant is much, much smaller than the value of the constant predicted by theory. Resolving this discrepancy is an area of open research that seeks to better explain the nature of our universe!