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
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!
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