ordinary gravitational attraction tends to brake the expansion. Some new effect was showing up. The simplest possibility is a universal negative pressure, which encourages expansion.
The term “dark energy” became a shorthand for both these discoveries—both the additional mass and the accelerating expansion. It was meant to be agnostic about the relative values of density and pressure. If we simply called both of them the cosmological term, we’d be prejudging their relative magnitudes. But apparently we’d be right. The two very different quantities, cosmic mass density and cosmic pressure, observed in very different ways, do seem to be related by ρ = -p/c2.
Thus weighty evidence pushes us to conclude that space weighs and pushes.
Besides making it more lively, as we’ve already discussed, quantum theory challenges traditional concepts of space in ways that beggar the speculations of philosophy, mathematics, and science fiction. Of course, after the fact a few philosophers and science fiction writers, and many mathematicians, have built quantum theory into their speculations. But no such free-thinkers anticipated its peculiar weirdnesses, even remotely, before the fact.
The nub of the issue is this. In quantum mechanics, fundamentally, we predict probabilities. Consider, for example, the problem of describing the positions of two quantum-mechanical particles. Quantum theory provides probability distributions P1(x) for finding particle 1 at position x, if you look for it there, and P2(y) for finding particle 2 at position y, if you look for it there. We can use three coordinates to define x (or y). So those probability-functions, which are (almost*) the most basic ingredients of the quantum mechanical description, depend on three numbers. They live in ordinary space. But if we ask for the probability of finding particle 1 at x and particle 2 at y, when we look for both, then we need a function P12(x, y) that depends on six numbers: three for x, three for y. (This probability is usually not simply the product of the separate probabilities P12(x, y) ≠ P1(x) P2(y). We say the probabilities are “entangled.”) So P12 does not live in ordinary three-dimensional space, but rather in a six-dimensional configuration space. Similarly, the wave function for positions of three particles lives in a nine-dimensional space, and so forth.
(If you like to stretch your mind, you might enjoy thinking about the probability function for a field φ(x). P(φ(x)) is a function of functions of space; so it lives in a space of infinite dimensions!)
We can boil these considerations down further. When I was a wee lad I liked to put together, and take apart, plastic model rockets. These models couldn’t put up satellites, let alone take anyone to the Moon. But they were things I could hold in my hands and play with, and they were aids to imagination. They were built to scale, and there was also a little plastic man on the same scale, so I got a sense of the sizes involved, the difference between an interceptor and a launch vehicle, and
* Actually the most basic ingredients are wave functions. The value of the probability function is the absolute square of the value of the wave function. This complication is beside the point here.
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