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Inflation as a Ski Slope If inflation took place, it must have been triggered by a hypothetical “inflationary energy,” caused by a field called “the inflaton” that would have permeated space. Different versions of inflation theory propose different relations between the strength of the inflaton field and the density of inflationary energy. Two of those relations are plotted here. One ( blue at left ) is akin to the traditional textbook models of inflation; the other ( pink at right ) requires very special starting conditions and thus seems implausible. This analogy with two ski hills offers an idea of why the second class of models—the kind of inflation that has not been ruled out by recent data—is hard to swallow.  PARADIGM SHIFT Given all These problems, the prospect that inflation did not oc - cur deserves serious consideration. If we step back, there seem to be two logical possibilities. Either the universe had a begin- ning, which we commonly dub the “big bang,” or there was no beginning and what has been called the big bang was actually a “big bounce,” a transition from some preceding cosmological phase to the present expanding phase. Although most cosmolo- gists assume a bang, there is currently no evidence—zero—to say whether the event that occurred 13.7 billion years ago was a bang or a bounce. Yet a bounce, as opposed to a bang, does not require a subsequent period of inflation to create a universe like the one we ind, so bounce theories represent a dramatic shift away from the inflation paradigm. A bounce can achieve the same end as a bang plus inflation because before the bounce, a span of slow contraction extending for billions of years can smooth and flatten the universe. It may seem counterintuitive that slow contraction has the same efect as rapid expansion, but there is a simple argument that shows it must be so. Recall that without inflation, a slowly expanding uni- verse would become increasingly curved, warped and nonuni- form with time from the effects of gravity on space and matter. Imagine watching a film of this process run backward: a large, highly curved, warped and nonuniform universe gradually con- tracts and becomes lat and uniform. That is, gravity works in reverse as a smoothing agent in a slowly contracting universe. As in the case of inflation, quantum physics amends the sim- ple smoothing story in bounce theories as well. Quantum fluctua- tions change the rate of contraction from place to place so that some regions bounce and begin to expand and cool before others. Scientists can construct models in which the rate of contraction gives rise to temperature variations after the bounce that are con- sistent with the pattern of hot and cold spots observed by the Planck satellite. In other words, contraction before a bounce can do what inflation was supposed to do when it was first invented. At the same time, bouncing theories have an important advantage compared with inflation: they do not produce a multimess. When the contracting phase begins, the universe is already large and classical (that is, described by Einstein’s gen- eral theory of relativity), and it bounces before it shrinks to a size where quantum effects become important. As a result, there is never a stage, like the big bang, when the entire universe is dominated by quantum physics, and there is no need to invent a quantum-to-classical transition. And because there is no infla- tion during the smoothing to cause regions that undergo rare, large quantum fluctuations to blow up in volume, smoothing via contraction does not produce multiple universes. Recent work has produced the first detailed proposals for describing how the universe could have transitioned from contraction to expansion, enabling the construction of complete bouncing cosmologies. NONEMPIRICAL SCIENCE? Given The issues with inflation and the possibilities of bouncing cosmologies, one would expect a lively debate among scientists today focused on how to distinguish between these theories through observations. Still, there is a hitch: inflationary cosmol- ogy, as we currently understand it, cannot be evaluated using the scientific method. As we have discussed, the expected out- come of inflation can easily change if we vary the initial condi- tions, change the shape of the inflationary energy density curve, or simply note that it leads to eternal inflation and a multimess. Individually and collectively, these features make inflation so flexible that no experiment can ever disprove it. Some scientists accept that inflation is untestable but refuse to abandon it. They have proposed that, instead, science must change by discarding one of its defining properties: empirical testability. This notion has triggered a roller coaster of discus- sions about the nature of science and its possible redefinition, promoting the idea of some kind of nonempirical science. A common misconception is that experiments can be used to falsify a theory. In practice, a failing theory gets increasingly immunized against experiment by attempts to patch it. The theory becomes more highly tuned and arcane to it new obser- vations until it reaches a state where its explanatory power diminishes to the point that it is no longer pursued. The explan- atory power of a theory is measured by the set of possibilities it excludes. More immunization means less exclusion and less power. A theory like the multimess does not exclude anything and, hence, has zero power. Declaring an empty theory as the un questioned standard view requires some sort of assurance outside of science. Short of a professed oracle, the only alterna- tive is to invoke authorities. History teaches us that this is the wrong road to take. Today we are fortunate to have sharp, fundamental ques- tions imposed on us by observations. The fact that our leading ideas have not worked out is a historic opportunity for a theo- retical breakthrough. Instead of closing the book on the early universe, we should recognize that cosmology is wide open. |
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