The vacuum is not empty. After removing all forms of matter and radiation, what remains could still carry a constant energy per unit volume. This constant can be ignored in all quantum calculations which care about energy differences, but must be considered in the context of gravity, which is sourced by all forms of energy. Albert Einstein’s equations of General Relativity imply that the vacuum energy density induces repulsive gravity, owing to its negative pressure. Hence, when the cosmic budget is dominated by the vacuum, the expansion of the Universe is expected to accelerate.
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