By Ryan Ridden 13/01/2016

In our universe the incredibly tiny and the absolutely enormous are intertwined.

In our everyday life there appears to be a clear distinction between the microscopic and macroscopic worlds, however, in many ways this is an illusion. We can begin to see how the two worlds connect by looking at the fundamental rules of the universe, as uncovered by physics and the large scale universe, observed through astronomy.

The clearest connection between the micro and macro worlds can be found 13.7 billion years ago at the beginning of the universe—at the Big Bang. In the Big Bang model, the universe itself started in a very small space, the entire cosmos compressed into an infinitely small point called a singularity. In this model connections between the micro and macro become apparent, but what exactly are they?

A Micro Beginning

The first is the most obvious and something that is yet to be grounded in experiments and observations. If the universe started out in a singularity (or something equally as small) it would have been governed by the laws of the very small (quantum mechanics), and because it would have been so dense it would have also been governed by the laws of gravity (general relativity).

As you may well know, no one can work out how general relativity and quantum mechanics fit together, but that’s not important for now. The important part to note is that quantum mechanics is involved, which means that random changes known as quantum fluctuations are allowed. It’s thought that these fluctuations could have disrupted the fragile state of the singularity, causing the Big Bang itself!

As I mentioned earlier this remains solely as a theory, but like many abstract theories it rests on the back of known physics to find its plausibility. So from this theory we can say that quantum mechanics was responsible for the universe, micro produced macro. But you may not be satisfied with this and would rather something grounded with observations—like a good skeptic should!

A Sea of Particles

The second connection again comes from the Big Bang, but this time we will be thinking about what happens after the bang. After the bang the universe was very hot and dense, filled to the brim with a sea of the fundamental subatomic particles. This sea should have looked almost exactly the same in every direction.

In reality quantum mechanics rears its curious head once more and those quantum fluctuations come back into play, creating absolutely tiny variations in the density of the subatomic sea. As the universe expanded and cooled fundamental particles began to clump together to form protons, neutrons and with them the first nuclei. Electrons were yet to bind to the newly formed nuclei and zoomed around the universe bumping off other electrons, nuclei and photons (light).

With the stage set we can now return to the tiny variations in the sea of particles. As time passed gravity pulled a mysterious particle that only feels gravity (dark matter) into the denser regions. This made the dense regions denser allowing gravity to pull in the other particles zooming around, the electrons, photons and nuclei. As the particles collapse under gravity they begin to push on each other, until the force of pressure pushing outwards was greater than the force of gravity pulling inwards. These particles would then rush out at half the speed of light away from the dense point in space, creating a spherical shell and the cycle would repeat again. A series of collapses and repulses, pushing around the sea of particles.

While the sea of particles was being pushed around, the universe continued to expand and cool and at 380,000 years after the Big Bang electrons were caught by the nuclei and the first atoms formed. Now that electrons weren’t running all over the place they no longer bumped into the nuclei and photons, greatly reducing pressure in the universe. For the first time light could travel in straight lines without being rudely interrupted by meandering electrons. With little pressure remaining, the collapsing and expanding processes around dense parts of space came to a halt, allowing the universe to enter a new era.

This is the theory of the later parts of the Big Bang, so all that is left to see if there is supporting evidence and think about how micro can influence macro.

Old Light and Galaxies

The supporting evidence comes in two parts, the cosmic microwave background (CMB) and galaxy clusters. The CMB simply put, is the first light that traveled freely in the universe once atoms formed. It gives us a snapshot of how the universe looked 380,000 years after the Big Bang, a baby picture of sorts. When the CMB was discovered by accident in 1964, it looked exactly the same across the entire sky, since then more detailed surveys found the CMB has tiny variations, around 10 parts per million!

CMB surveys
Three major surveys of the CMB. With each new survey more detail is found allowing us to learn more about the beginning of the universe.

These detailed maps of the CMB show the early universe had a multitude of tiny variations. Moreover these variations fit with the picture we developed earlier; small variations in the early universe pulled in matter and pressure pushed it away, creating lots of intermixing spherical shells.

A map of the 50 million galaxies found in the Sloan Digital Sky Survey.
A map of the 50 million galaxies found in the Sloan Digital Sky Survey. With all these galaxies the structure of the universe is clear to see.

Even more surprising is that the formation of galaxies was determined by the pushing and pulling process. When atoms formed and the pushing and pulling stopped, matter was arranged around the dense points in space like big spheres which had a radius of about 240,000 light years, a distance known as the “Sound Horizon”. In these enormous spheres of matter, gravity pulled away to make the first stars and then the first galaxies.

If you were to conduct a massive galaxy survey to measure the large scale structure of the universe, like in the Sloan Digital Sky Survey, you will find something very interesting. The structures, or galaxy clusters, are often separated by the sound horizon which has expanded along with the universe to be 489.2 million light years. The small determined the very large.

Connected Worlds

From all this it’s clear to see that the very structure of the universe, including all of its galaxies as well as you and me are a result of tiny fluctuations at the beginning of the universe. This is the story of us, the story of our universe—micro to macro.

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