On Sun, Dec 23, 2018 at 5:38 PM Bruce Kellett <[email protected]> wrote:
*> Flatness is explained if the unknown parameter k in the FRW solution is > set to zero. The the universe is always flat, no need to fine tune. Setting > k = 1 or k = -1 is just as fine-tuned or not as k=0.* > There are an infinite number of ways space could have been curved but you picked one particular way (no curvature at all) for your initial conditions and did so for no particular reason other than to make the theory fit the facts that you already knew. Inflation explains why spacetime curvature could have any finite value whatsoever when the universe first came into existence and it would still look flat today even with our most sensitive instruments. It didn't have to start out with spacetime being zero or anything close to it, and that doesn't sound fined-tuned to me. And the same thing is true of temperature, why are things at the same temperature when there was no time for them to come into thermal equilibrium? Inflation explains why, your explanation is they just did. Inflation says that 10^-35 seconds after the start of the universe and it had doubled in size about a hundred times (and 10^35 seconds is a long long time compared to the Planck Time of 10^-43 seconds) the difference in temperature in our part of the universe would be almost zero but not precisely zero due to random quantum variations, and quantum theory allows you to calculate the intensity and size of what those temperature variations should have been. And you can also calculate what those temperature variations would evolve into after the universe has been expanding for 380,000 years, and what we calculate and what we see are the same. That's also how we know that at the very largest scale the universe is in general flat. They did this by looking at the oldest thing we can see, the Cosmic Microwave Background Radiation (CMBR) formed just 380,000 years after the Big Bang. So if we look at a map of that background radiation the largest structure we could see on it would be 380,000 light years across, spots larger than that wouldn't have had enough time to form because nothing, not even gravity can move faster than light, a larger lump wouldn't even have enough time to know it was a lump. So how large would an object 13.8 billion light years away appear to us if it's size was 380,000 light years across? The answer is one degree of arc, but ONLY if the universe is flat. If it's not flat and parallel lines converge or diverge then the image of the largest structures we can see in the CMBR could appear to be larger or smaller than one degree depending on how the image was distorted, and that would depend on if the universe is positively or negatively curved. But we see no distortion at all, in this way the WMAP and Planck satellite proved that the universe is in general flat, or at least isn't curved much, over a distance of 13.8 billion light years if the universe curves at all it is less than one part in 100,000. > >> It would seem to me that if two theories can explain observations then >> the one with the simpler initial conditions is the superior. >> > > *> The trouble is that inflation is not a simple theory. Where does the > inflation potential come from?* > >From the same place gravitational potential does I suppose, but inflation would be simpler, in General Relativity gravity needs a tensor field but inflation only needs a scalar field. > > *Why don't we see the inflaton?* > Maybe we do see it, maybe the acceleration of the universe we see today is the inflation field at work having undergone a phase change when the universe was 10^-35 sec old and switched into a much lower gear. Or maybe not. Andrei Linde thinks the inflation field decayes away like radioactive half life, and after the decay the universe expanded at a much much more leisurely pace. But for that idea to work Guth's the inflation field had to expand faster than it decayed, Linde called it "Eternal Inflation". Linde showed that for every volume in which the inflation field decays away 2 other volumes don't decay. So one universe becomes 3, the field decays in one universe but not in the other 2, then both of those two universes splits in 3 again and the inflation field decays away in two of them but doesn't decay in the other 4. And it goes on like this forever creating a multiverse. If any of this is true we may be able to prove it because Eternal Inflation would create gravitational waves with super long wavelengths that would produce very slight changes in the polarization of the cosmic microwave background radiation that we should be able to detect before long, assuming they exist. 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