I thought I would quote his summary of this chapter from p. 301 and then add some "expansionary" comments:
1. "Early on, the energy of the universe was carried by the inflaton field, which was perched away from its minimum energy state."
The current majority opinion is that the "bang" of the big bang did not happen immediately but a small fraction of a fraction of a fraction of a second after zero (let's call it creation). Measurements indicate that the universe is expanding. And yet the temperature of the universe as a whole is more or less the same.
Alan Guth and Henry Tye realized in 1979 that if the Higgs field paused for the briefest of fractions at a not quite minimum value ("supercooled"), a special situation would be set up where gravity didn't attract but actually would repel at a tremendous value. This could explain how the universe could go from a speck to its massive size in a fraction of a fraction of a second.
2. "Because of its negative pressure, the inflaton field drove an enormous burst of inflationary expansion. Then, some 10-35 seconds later, as the inflaton field slid down its potential energy bowl, the burst of expansion drew to a close and the inflaton released its pent-up energy to the production of ordinary matter and radiation."
So this would explain the "horizon problem," the fact that microwave background radiation is basically the same wherever you look in the universe. All the parts of space we see were once touching each other. Another problem it eventually helped solve is the "flatness problem."
The density of the universe would seem to be just right for a flat universe--at least as far as we can see. Nebraska seems flat when you're standing in it. But in the light of the earth, it isn't. But the matter/energy density of the universe seems just right, a "critical density." This is quite astounding at first glance. If it were a little more or a little less, we would think we would observe a vast difference today from what we do.
But repulsive gravity apparently pushes the value of the matter/energy density toward its critical value, so that space in our neck of the woods at least looks flat. Also, repulsive gravity seems to have gone "slower" at the beginning of the fraction of a fraction of a fraction of a second so that the temperature could even out (cf. Andrei Linde, Paul Steinhardt, and Andreas Albrecht).
3. "For many billions of years, these familiar constituents of the universe exerted an ordinary attractive gravitational pull that slowed the spatial expansion."
There is a bit of a puzzle in that, while the universe seems to fit with the critical density being the case, we can only observe 5% of matter and energy toward what that amount should be. Starting in the 1930s, there was a suggestion that some sort of matter we cannot see must be out there, keeping the stars of the galaxies we see from flinging out of them.
Thus the idea that there is some sort of "dark matter" out there has been around a long time. And it seems to account for another 25% of the matter/energy we would need for the critical density number that fits with observation to be realized.
4. "But as the universe grew and thinned out, the gravitational pull diminished. About 7 billion years ago, ordinary gravitational attraction became weak enough for the gravitational repulsion of the universe's cosmological constant to become dominant, and since then the rate of spatial expansion has been continually increasing."
By "cosmological constant," Greene refers to a debate that goes back to Einstein. Einstein didn't like the idea of an expanding universe, so he suggested a constant in his equations that held the universe static. When Hubble observed in 1929 that the universe was expanding, Einstein was embarrassed for letting his sense of how the universe should be interfere with his equations.
Years later, though, something like Einstein's constant seems to be in play. In particular, careful measurements in the 1990s suggest that the universe suddenly began to speed up big time at some point. If something else was functioning something like the constant Einstein had speculated about, it would explain it. In fact, if the remaining missing 70% of the matter/energy density were some kind of "dark energy" we can't see, that would do it.
And so there is the current suggestion. The observable universe is only 5% of the matter/energy that exists. Another 25% is a kind of matter we cannot observe ("dark matter") that holds galaxies together. The remaining 70% is a kind of energy we cannot observe ("dark energy") which is responsible for the massive acceleration that started about half-way through time. It will eventually result in a cosmic rip where the universe looks very dark and empty indeed.
Reality's Arena
1. Overview
2. Spinning Space Buckets
3. Relativity and the Absolute
4. Particles Separated at Birth
Time and Experience
5. Does time flow?
6. Does time have an arrow?
7. Quantum crazy
Spacetime and Cosmology
8. Universal symmetry
9. The Higgs Ocean
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