A Quantum Fluctuation


Hello to those of you still reading despite the rather large hiatus RTU has been experiencing. To reiterate I am currently studying for my MSc degree in Theoretical Physics and with seven exams looming on the horizon I have had to neglect all forms of activity aside from burying my head in lecture notes.

Today I felt like breaking that silence to come to write about a process, that I like to think embodies the phrase ‘making a mountain out of a molehill’ in the truly cosmic sense. How, quantum fluctuations in the dawn of the universe were the seeds that developed into the stars, nebulae and galaxies we see today. The origin of such rich complex structure can be traced back to tiny quantum fluctuations in those first moments of our universe.


The paradigm most accepted by cosmologists today for describing the early universe is inflation. This is the idea that the early universe underwent a period of rapid expansion, small patches of spacetime were vastly stretched out and the universe grew at an accelerated pace. The theory of inflation is proposed for many reasons, including it’s ability to explain why the universe is observed to be flat and why it is observed to be homogenous. By a homogenous universe we mean it is, on large scales, the same in every direction we look. For example when we measure the Cosmic Microwave Background radiation (for our purposes here think of this as the background temperature of the universe) it is the same on every patch of the sky. Inflation allows these patches that are so far flung apart to have been able to be close enough together and ‘talk’ to each other in the early universe that they could have equilibrated their temperatures. I hope to do a more detailed post on the workings of inflation later. The only idea we need today is the rapid expansion of spacetime and the idea that the driver of inflation is a field. For a recap on the idea of a field see What is a Field?

Now as we know that the universe was undergoing such a rapid expansion, logically it must have been much smaller in the past. At very early times the universe was so small that the theory that ruled the realm was the theory of the small, Quantum Mechanics. If you want a refresher on Quantum Mechanics see the Laws of Quantum. In quantum physics, a quantum fluctuation is the temporary change in the amount of energy in a point in space, as explained in Werner Heisenberg’s uncertainty principle. Now this point is a little tricky but essentially what happens is that the field driving inflation experiences these quantum fluctuations, and everything else at this time is rapidly expanding, these quantum fluctuations are blown up too.


Now every point in spacetime has a region around it that contains events that can effect it. What I like to do is think of this region as a sphere, with the point of interest at the center. This region is called the Hubble radius (see point two in The Horizon and Beyond). These quantum fluctuations get blown up so large that they exit their initial Hubble radius and for the moment we put them to one side, worrying about what is inside only.

Now time progresses in the universe’s expansion, inflation ends and we enter the region thought of as the standard big bang cosmology, temperatures heat up, and particles emerge from the primordial soup that existed. For a punchy recount of these  stages of the universe see A brief history of the universe. In short, electrons fly around and eventually combine with nuclei to form atoms and small structure begins to come in existence. All the while the universe is still expanding, but at a much slower rate as when compared with during inflation. This means the Hubble radius or sphere that we were imagining earlier is also increasing and eventually it gets big enough that those quantum fluctuations outside that we now like to call cosmological perturbations (because they certainly aren’t quantum anymore!) get re-engulfed by the sphere.


[Replace the ‘co-moving horizon’ term with the ‘Hubble radius’ we were talking about, there is a subtle difference but for our purpose the concepts are interchangeable]

The perturbations affect (or if you’re well versed in the language of physics couple to) the density of the existing matter and then as a result experience the force of gravity, subsequently undergo gravitational collapse and this process over time is what forms the large scale structure in the universe today.  It was the quantum mechanical fluctuations in the initial universe that broke the initial smoothness if you like of the universe and provided the initial ‘clumpiness’ that when when coupled to the density of matter, over time caused a coalescing of matter into beautiful structures such as stars, planets and galaxies. The complexity that we see all around can be traced all back to simple fluctuations, that acted as the seeds of structure for the universe and in turn, essentially of life.


Read it at the source


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