More on inflation

Reading back over this morning’s post, I see I should have emphasised that cosmic inflation is not an alternative to the Big bang theory; it is simply a modification of the conventional big bang model.

What is the physical cause of the initial exponential expansion of the universe? Guth proposed that the expansion was propelled by a repulsive gravitational force generated by an exotic form of matter. Although this proposal was slightly flawed, the flaw was soon overcome by the invention of “new inflation” by Andrei Linde in the Soviet Union and independently by Andreas Albrecht and Paul Steinhardt in the US. (New inflation posits that the early universe went through the stage of exponentially rapid expansion in a kind of unstable vacuum state – a state with large energy density, but without elementary particles).

Nowadays, the evidence for the inflationary universe model is very strong. One of the coolest aspects of the theory inflation is an explanation for the large scale structure of the universe (galaxies etc.) Basically, the idea is that quantum fluctuations in the early universe could have been stretched by inflation to astronomical proportions, providing the seeds for galaxy formation. The predicted spectrum of these fluctuations was calculated by Guth and others in 1982.

These fluctuations can be seen today as ripples in the cosmic background radiation, although the amplitude of these faint ripples is only about one part in 100,000. The ripples were detected by the COBE satellite in 1992, and they have now been measured to much higher precision by the WMAP satellite and other experiments. The properties of the radiation are found to be in excellent agreement with the predictions of the simplest models of inflation [image].


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2 responses to “More on inflation

  1. ‘Basically, the idea is that quantum fluctuations in the early universe could have been stretched by inflation to astronomical proportions, providing the seeds for galaxy formation. The predicted spectrum of these fluctuations was calculated by Guth and others in 1982.’

    You write as if the stretching of the quantum fluctuations made them big enough to seed galaxy formation, which is totally misleading I fear.

    I studied cosmology a decade ago, and my understanding is the opposite of what seems to be implied by those sentences in your otherwise very nice post.

    General relativity (Friedmann-Robertson-Walker metric) predicts far too much curvature in the early universe, so the density fluctuations predicted by general relativity without inflation would lead to galaxy formation much too soon. With the Hubble telescope and others, the era of early galaxy formation can be determined and it is a lot later than general relativity predicts.

    In addition, the cosmic background radiation tells us what the fluctuations in radiation (and density of matter) were at 300,000 years after the big bang, when the temperature of the big bang fell below 3000 K allowing electrons and protons to combine into hydrogen, which made the universe transparent to most radiation. (At higher temperatures i.e. earlier times, the universe was basically ionised hydrogen gas, which was a strong absorber of all electromagnetic waves. Hence at earlier times than 300,000 years after the big bang, the radiation and matter temperatures were identical because they were in an equilibrium, but at all later times the radiation field temperature decoupled from that of the matter and decreased due to falling energy of photons received from 300,000 years emission time by distant matter as the universe expanded, i.e. the redshift effect.)

    Inflation was supposed to have occurred at very early times after the big bang (10^{-26} of a second or so), due to a phase change in the vacuum’s state, as you write in the post, which briefly allowed faster-than-light expansion.

    This extremely rapid ‘inflationary’ expansion epoch, at around 10^{-26} of a second into the big bang, is supposed to be wonderful because it would reduce the curvature of the universe thereafter, and would reduce galaxy formation rates subsequently. Galaxy formation requires curvature to make the quantum fluctuations grow. The role of inflation is to reduce the curvature of the universe on large scales by spreading the same amount of mass-energy over a bigger volume than suggested by the Friedmann-Robertson-Walker metric. Curvature (gravitational acceleration) is reduced if you spread the same amount of mass-energy over a bigger volume, just as gravitation would appear weaker if the Earth was made bigger in size but only contained the same mass.

    Inflation spreads out the matter over a bigger volume, hence it reduces curvature, which reduces the rate at which quantum fluctuations grow in size, which in turn reduces the rate at which star and galaxy formation is seeded.

    So the fact that as you write, ‘quantum fluctuations in the early universe could have been stretched by inflation to astronomical proportions’ is actually a bit misleading.

    The quantum fluctuations are actually reduced in size by inflation, because inflation reduces curvature, which in turn reduces the growth rate of quantum fluctuations.

    Inflation is not impressive because it makes no falsifiable predictions. It’s a false, epicycle-style piece of metaphysics.

    Instead of inflation reducing curvature by expanding the universe faster that light velocity during a phase transition in the vacuum state, what happens is that the universal gravitational constant is directly proportional to time since the big bang. This is a checkable prediction from a quantum gravity mechanism which reproduces all checked general relativity effects and which also predicts the gravitational constant G within the experimental error bars of the data.

    Hence at 300,000 years after the big bang, the universe was 46,000 times younger than it is now, so G was smaller by a similar factor. This is the correct reason as to why there the early universe was much flatter (less curved, i.e. less gravitational field strength) at early times such as when the cosmic background radiation originated.

    This explanation is the correct one for the observed slow rates of galaxy formation in the early universe, not inflation. There is solid evidence behind this model, because it is simple, makes checked predictions, and reproduces empirically confirmed aspects of quantum field theory and general relativity.

    Note that the variation in G with time prediction of this alternative theory was falsely attacked by Edward Teller in 1948 after a different theory of varying G was proposed by Dirac. Teller ignored the fact that electromagnetism is related to gravitation so should have a coupling constant that varies in the same way with time that G varies as. Teller claimed that varying G would vary the compression rate in the fusion rates in the stars (and obviously the fusion rate in first minutes of the big bang) in a way incompatible with observations, but this objection is totally false and based on Teller’s ignorance that the electromagnetic coupling will vary with time in the same way as G. The variation in electromagnetic coupling constant means that where gravitation is weaker (causing less compression of matter in the big bang fusion and in star fusion), Coulomb electromagnetic repulsion would similarly be weaker. Since it is precisely the Coulomb barrier that stops protons from easily being fused together by the short-ranged pion-mediated strong nuclear force, the variation in Coulomb force with time cancels out the effect of gravity constant G varying with time (so far as nuclear fusion is concerned). Everything works out!

  2. There are design problems.