The Beginning and the End of Everything by Paul Parsons

Author:Paul Parsons
Language: eng
Format: epub, pdf
Publisher: Michael O'Mara

In 2015, analysis of the data returned by ESA’s Planck spacecraft determined that 69.1 per cent of the critical density of the universe is dark energy.

If dark energy is the same thing as Einstein’s cosmological constant then it’ll probably be made up of vacuum energy. This is energy locked away in the structure of empty space. As we saw when looking at quantum theory in the last chapter, vacuum energy arises because of virtual particles that are allowed to pop in and out of existence over very short timescales. This constant buzz of matter coming and going lifts the energy of empty space, the energy of the vacuum, up above zero. And this alters the overall mass of the universe, which is what determines its ultimate fate. Of course the density of vacuum energy is tiny. If the observations are correct then the density is approximately 10 to the power -27 (or 0.000000000000000000000000001) of a gram – which is a little less than the mass of a single hydrogen atom – per cubic metre. However, that’s every cubic metre in the entire universe, which is how this dark energy adds up to dominate the mass of everything else.

Dark energy behaves weirdly as the universe expands. Imagine a cube of expanding space, representing a chunk of our universe. If the cube was full of matter then the density of the matter decreases as the universe expands and the cube gets bigger. This is because density is just mass divided by volume, and so if the mass stays constant and the volume increases then the density must get smaller.

If the cube is filled with radiation rather than matter then the density decreases even faster as the universe expands. As before, the volume of the cube increases but, in addition, the radiation gets stretched out and redshifted (the effect we met back in Chapter 1) by the expansion.

But here’s where things get strange. The density of vacuum energy inside the box remains constant as the box gets bigger. This happens because vacuum energy is produced by the vacuum itself – so the more vacuum there is, the more vacuum energy there will be. This makes the density of vacuum energy constant as the universe expands; and this is why vacuum energy is usually identified with the cosmological constant (as already outlined, this was introduced as a term representing a constant energy density).

Vacuum fluctuations cause the expansion of the universe to accelerate because the dark energy that they create actually has negative pressure within it. And the gravity of all this negative-pressure material has a weird effect on space. In Newton’s theory of gravity, mass is the only thing that can create a gravitational field and because mass is always positive – or, at least, it always was back in Newton’s day – then the gravitational force was always attractive. But in general relativity, mass, energy and pressure are all valid sources of gravity. And, as you might expect, the gravitational force produced by

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