The cosmological inflation theory always sounded weird to me. I’ve been reading a bit about it, trying to get my head around what they’re claiming. And I’m unconvinced that it’s a valid theory, even though it’s currently winning at prediction.
The classical big bang theory certainly has problems that need addressing. Now, there’s no doubt that there was a bang which was big. Everything we see in the universe is flying apart from everything else, and behind everything we can see is a thermal glow which, if you account for redshift, appears to come from hot gas just at the point where it cools enough to be transparent. (This is around 3000 Kelvin, about as hot as a halogen lamp filament). So there’s no way around the conclusion that, thirteen point something billion years ago, the entire observable universe was packed into a much smaller volume which was dense and hot, and that it exploded out at terrific speed. That much is clear.
The problems arise when you try to extrapolate what came before the ball of dense hot gas, which clearly was already expanding. The math says that it must have all expanded from a volume that was much smaller and hotter still. In fact, the classical mathematical solution to the big bang insists that the initial explosion must have taken place in a region that wasn’t just tiny, or even infinitesimal, but in no volume at all: an absolute single point where density and temperature equalled infinity.
This answer is nonsensical. Scientists are now rightly rejecting it, as many of them also reject the notion that the center of a black hole must be a singularity of zero size. Clearly both are an oversimplification.
The trouble is, if you postulate anything else, the consequences run up against observed data. The key point of observation is that the universe, as far back as it’s possible to see, has an essentially uniform density and temperature in all directions. It appears very much as if the volume of hot gas which existed at the earliest moment we can see was in a state of thermal equilibrium with itself, as if all energy differences had been allowed to settle down and blend themselves together at a smooth common level of heat.
But such a blending could not have happened. There was no time for it to happen in. And worse, the uniformity includes regions which could not have interacted with each other, because light is only now managing to cross from one to the other, or hasn’t even done so yet. If we look at opposite sides of our sky, we see as far as light has travelled either way in all of history, which means the total distance from one side to the other has only been crossed halfway. Since no influence can move faster than light, the two opposite sides cannot possibly have exchanged any energy with each other. It’s true that they may once have only been a foot apart, but they moved away from each other so close to lightspeed that the light from one side still hasn’t reached the debris of the other. This means they cannot have exchanged any form of energy.
The smallest random quantum fluctuations back then should loom large today as differences in the cosmic backround temperature, and in the density of galaxies. The differences are too small to account for without some means of smoothing them away. It can’t have been ordinary thermal equilibrium. What could it have been?
Similar questions apply on more esoteric levels too, like why the curvature of space appears to be so near to the ideal value that makes it absolutely flat. It’s not that it’s close today that’s the problem, since our measurement of it is not all that precise... it’s that any small departure would increase over time, so if it’s close now, then in the distant past it must have been ultra-close with improbable precision.
The inflation theory is an attempt to answer these questions. It postulates that some unknown repulsive force caused space itself, and the matter in it, to undergo some kind of self-generating expansion which kept the universe hot and dense while it grew at an immense rate — an increasing rate that compounded exponentially. Then, at some point, it ran out of gas and the universe started coasting outward in a conventional way, as we now observe.
The math they postulate for this process would have the effect of ironing out the irregularities that preceded it, making everything steadily smoother and flatter as long as it continued. And it allows for a time before the inflation started, in which parts of the universe that are now inseparably distant could have achieved energy equilibrium. It would only have required a brief instant of delay between the initial creation, when all the matter may have been trapped by gravity in a very small initial volume, and the commencement (by entirely hypothetical means) of inflation.
It might even mean that the initial appearance of the pre-inflated universe could be possible as a quantum fluctuation in vacuum, because the amount of energy that needs to appear from nothing might be comparatively tiny. In these versions, the universe’s gravitational field constitutes a store of negative energy, and the total net mass of the cosmos is zero.
But if you run it backwards to see what initial conditions could have worked out that way, many say that it falls victim to the same problems as a conventional big bang. Whatever state preceded it has to have been already uniform to an utterly unnatural degree. Some say that it actually makes the problem worse.
But aside from that, the whole theory just seems like the ultimate in ad-hockery, a contrivance of arbitrary rules and conditions based on imaginary physics, tuned to “predict” observed results. And it makes some claims that are hard to credit, like that the inflationary expansion was in some sense faster than light. Apparently this is a mathematically allowed solution to the equations of general relativity.
We already knew that we can’t account for the ultimate question of why there is something and not nothing. Even a religious hypothesis doesn’t have an answer for that. But even leaving that aside, it’s looking like we’ve really got no answers as to what physical conditions must have existed in the early part of the big bang — at a point where matter and energy had fully come into being and started obeying the physical laws of our cosmos, but before what can be observed.
I’d say both the conventional big bang theory and the inflation theory must be missing something essential. There’s something big going on there which we’re totally failing to see yet. Probably something that will look blatant and obvious in hindsight someday. Maybe brane theory, or something equally far out, can provide a missing piece if it develops enough.
The inflation theory may be half true. I’m sure some parts are valid. Maybe even most parts. The part where energy and gravity cancel each other out and can therefore be created together in arbitrarily large quantities, for instance, sounds pretty attractive. But I think we’re probably still missing some key piece of context in which these parts can make a good overall theory.
It’s clear that the inflation hypothesis in its current form is incomplete... I wouldn’t be surprised if whatever comes along to complete the picture ends up discarding a large part of it in the process, and the hypothetical inflation stage seems like a prime candidate for something that might turn out to not be needed anymore if we only knew what was missing.
UPDATE, 2020: David Fajman of the U of Vienna is now building a mathematical theory which says the smoothing of the universe may arise naturally from general relativity, with no need to hypothesize inflation. I don’t quite understand how — something to do with gravity waves dissipating and smoothing away early irregularities in spacetime curvature. If this pans out, the whole inflation theory might collapse. And while we’re at it, Dark Energy is another cosmological theory that is under threat of collapsing in a similar way... it also might end up with a mundane explanation.