1. Empty space. 2. Two particles suddenly appear. 3. Particles ram together and annihilate each other. 4. They leave ripples of energy through space. [Tim Jones]
A concept known as vacuum energy or the cosmological constant suggests that space itself produces energy, which is “pushing” the universe outward. Albert Einstein invented the cosmological constant as part of his theory of gravity, known as General Relativity. Einstein’s equations showed that the gravity of all the matter in the universe would exert a strong pull, pulling all the stars and galaxies toward each other and eventually causing the universe to collapse.

At the time, though, astronomers believed that the universe was static – that it was neither expanding nor contracting. To counteract this problem, Einstein added another term to his equations, called the cosmological constant, to balance the inward pull of gravity.

Once astronomer Edwin Hubble discovered that the universe is expanding, Einstein discarded the cosmological constant, calling it his greatest scientific blunder. Today, physicists explain the cosmological constant as the vacuum energy of space.

In essence, this says that pairs of particles are constantly popping into existence throughout the universe. These “virtual pairs” consist of one particle with a negative charge and one with a positive charge. They exist for only a tiny fraction of a second before they collide and annihilate each other in a tiny burst of energy. This energy may be pushing outward on space itself, causing the universe to accelerate faster.

One of the appealing elements of vacuum energy is that it could explain why the acceleration has only started fairly recently on the cosmic timescale.

One of the appealing elements of vacuum energy is that it could explain why the acceleration has only started fairly recently on the cosmic timescale.
In the early universe, all the matter was packed much more densely than today. In other words, there was less space between galaxies. With everything so close together, gravity was the dominant force, slowing down the acceleration of the universe that was imparted in the Big Bang. In addition, since there was less space in the universe, and the vacuum energy comes from space itself, it played a much smaller role in the early universe.

Now – 13.7 billion years after the Big Bang – the universe has grown much larger, so the galaxies are not packed so close together. Their gravitational pull on each other is weakened, allowing the vacuum energy to play a more dominant role.

Vacuum energy has its own set of problems, though. It should be far too weak to account for the acceleration seen in the present-day universe, for example — by a factor of at least 1057 (a one followed by 57 zeroes), and perhaps as much as 10120 (a one followed by 120 zeroes). Yet it is the most complete scenario to date, so it leads the pack of dark-energy contenders.

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