Negative Pressure, Exotic Particles, and the Emergence of Something out of Nothing
So, lets quickly recap. We have evidence that the Universe is expanding, and that the distribution of galaxies is both homogeneous (smooth) and isotropic (the same in all directions). What’s more, our theories of universal evolution tell us that the universe appears to be made up of 50,000 regions that are causally disconnected; they are independent. But since the universe is also homogeneous, this means that for some mystic reason these 50,000 regions managed to evolve in practically the same way as each other.
Physicists do not like mysticism and so they treated these phenomena as paradoxes, mysteries that need to be solved, and a neat solution was proposed in the late 70s/early 80s dubbed inflation. This is the very rapid expansion of the universe at early times, which allowed the universe to be much smaller than we thought it could be, and expanding to the size necessary to account for the subsequent evolution, without upsetting the status quo of the standard Big Bang theory. All this was discussed in part III of this series, and we left off at the point where we asked “How does inflation solve the origin of structure puzzle”, and we shall pick up there.
To be able to answer this question, we need to understand the process and nature of inflation a bit better. The equations that `govern’ or `describe’ the expansion of the universe are known as the acceleration (or Raychaudhari) equation, and the Friedman Equation. These equations relate the rate at which the universe is expanding to the content of the universe. So, if we know that the universe is composed mainly of radiation, the Friedman equation would then tell us the rate at which the universe is expanding, and the Raychaudhari equation would tell how that rate was changing.
Combined these equations tell us that if we want the expansion to accelerate, as is necessary for inflation, then we need something which has negative pressure or negative energy. Since negative energy means negative mass that makes it too weird to make sense (even for cosmologists) so focus was placed on finding something with negative pressure.
So what is negative pressure? To get an intuitive understanding of this, imagine that you are blowing air into a glass bottle which cannot expand. As you blow into the bottle you are forcing the air molecules closer together, this is known as positive pressure. Negative pressure (you guessed it) is like sucking air out of the bottle, you are forcing the air molecules away from each other.
So negative pressure forces things away from each other, it acts like anti-gravity, which means that negative pressure could have caused the original expansion of the universe! So the question is what has negative pressure?
We know that everything we have encountered in the universe (be it in the form of matter or radiation) has normal positive pressure, so we really are looking for something as yet unencountered… something exotic.
I struggled with this bit. I wonder if people are aware of what is known as the particle-wave duality? Sometime in the 1800s Thomas Young showed that light behaved as a wave, it spreads out like ripples on a pond. In the late 1800s Thomson showed that light also behaves like a particle a.k.a a photon. Both theories are correct, light behaves as both a wave and a particle.
To get the universe to expand very very rapidly the average pressure must be negative, which means that you would need to fill it with a particle that pushes everything away from it. Such a particle has never been observed in nature (hence is exotic), but is a necessary component of fundemental theory, it is known as a scalar particle (wave) and in the context of inflation we call it the inflaton.
Initially introduced to solve the paradoxes of Big Bang cosmology, it was found that the inflaton has another trick up its sleeve … the ability to create the ‘wells’ into which matter will eventually fall and form structure (like galaxies). To achieve this we need to speak a bit of quantum field theory (dont stop reading I shall try to simplify this as best I can!).
The basic idea is that on very small length scales things don’t act normally … on a tiny level everything around us is fluctuating, an electric field may have a specific amplitude (power/strength/value) to the naken eye, but look close enough (hypothetically) this amplitude is changing, it is sometimes less and sometimes greater than the average value that we measure. This happens so rapidly however that you would never notice. Does this make sense? Check out the next figure …
So if the fluctuations are wiggling rapidly, and dont have an effect, how can it lead to ‘fixed’ tangible wells into which matter will eventually fall into… read on.
The Quantum to Classical Transition (the Emergence of Something out of Nothing):
Remember that the universe is expanding extremely rapidly? Ok, well since the inflaton is also a wave, it stretches out across the universe and gets stretched out or magnified by the rapid expansion. So pretty much as soon as a little wiggle appears on the background (the average value of the wave or the classical picture above) it gets blown up to ‘proper’ proportions. These ‘proper’ proportions are what we call classical, their value becomes fixed and the wiggle stops wiggling.
Since the wiggles can be any length when they get transformed to classical size we end up with a range of different sized peaks and troughs across the universe, and because they are classical they survive through the end of inflation, and effectively determine the pattern of galaxies we see across the sky.
Next up: how do we test this idea we call ‘inflation’?