The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [81]
As we will see in the pages that follow, the limited view on offer for future astronomers is all the more striking when compared with the enormity of the cosmic expanse to which our generation has been led in attempting to explain the accelerated expansion.
The Cosmological Constant
If you saw a ball’s speed increase after someone threw it upward, you’d conclude that something was pushing it away from the earth’s surface. The supernova researchers similarly concluded that the unexpected speeding up of the cosmic exodus required something to push outward, something to overwhelm the inward pull of attractive gravity. As we’re now amply familiar, this is the very job description which makes the cosmological constant, and the repulsive gravity to which it gives rise, the ideal candidate. The supernova observations thus ushered the cosmological constant back into the limelight, not through the “bad judgment of conviction” to which Einstein had alluded in his letter decades earlier, but through the raw power of data.
The data also allowed the researchers to fix the numerical value of the cosmological constant—the amount of dark energy suffusing space. Expressing the result in terms of an equivalent amount of mass, as is conventional among physicists (using E = mc2 in the less familiar form, m = E/c2), the researchers showed that the supernova data required a cosmological constant of just under 10–29 grams in every cubic centimeter.8 The outward push of such a small cosmological constant would have been trumped for the first 7 billion years by the inward pull of ordinary matter and energy, in keeping with the observational data. But the expansion of space would have diluted ordinary matter and energy, ultimately allowing the cosmological constant to gain the upper hand. Remember, the cosmological constant does not dilute; the repulsive gravity supplied by a cosmological constant is an intrinsic feature of space—every cubic meter of space contributes the same outward push, dictated by the cosmological constant’s value. And so the more space there is between any two objects, arising from cosmic expansion, the stronger the force driving them apart. By about the 7-billion-year mark, the cosmological contant’s repulsive gravity would have carried the day; the universe’s expansion has been speeding up ever since, just as the data in Figure 6.2 attest.
To conform more fully to convention, I should re-express the cosmological constant’s value in the units physicists more typically use. Much as it would be strange to ask a grocer for 1015 picograms of potatoes (instead, you’d ask for 1 kilogram, an equivalent measure in more sensible units), or tell a waiting friend that you’ll be with her in 109 nanoseconds (instead, you’d say 1 second, an equivalent measure in more sensible units); it is similarly odd for a physicist to quote the energy of the cosmological constant in grams per cubic centimeter. Instead, for reasons that will shortly become apparent, the natural choice is to express the cosmological constant’s value as a multiple of the so-called Planck mass (about 10–5 grams) per cubic Planck length (a cube that measures about 10–33 centimeters on each side and so has a volume of 10–99 cubic centimeters). In these units, the cosmological constant’s measured value is about 10–123, the tiny number that opened this chapter.9
How sure are we of this result? The data establishing accelerated expansion have only become more conclusive in the years since the