cosmological expansion
Quick Summary
The cosmological expansion refers to the observation that the universe is growing larger over time. This phenomenon was discovered by astronomers like Vesto Slipher and Edwin Hubble in the 1920s, who noticed that galaxies are moving away from us, and the farther away they are, the faster they recede. This relationship is known as the Hubble law.123
This expansion is not like an explosion through space, but rather an expansion of space itself. The current understanding is that the universe began about 13.8 billion years ago in a very dense and hot state, often referred to as the big bang. Evidence for this includes the cosmic microwave background radiation, which is thought to be a remnant of that initial event.32
A significant aspect of this expansion is the role of dark energy, a mysterious force that opposes gravity and is causing the universe's expansion to accelerate. Dark energy makes up about 69.4 percent of the universe's total mass-energy budget.45
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