Cosmological Inflation

The theory of inflation came to be after cosmologists found a few faults in our understanding of expansion. It has always been one of the craziest (in a good way) topics to me, because so little is understood about it. Yet, it seems to fit exceptionally well with our model of the universe.

The three problems from last week

Here’s the three problems summarised

  1. Our observations show a universe over 27 billion light years across, and the universe isn’t old enough to have expanded that much.
  2. Our universe seems to be just about flat, meaning it’s at the critical density. However there’s such a slim chance of that happening.
  3. There are fluctuations in density that expansion alone can’t explain. How did they get there?

Enter Inflation (and a Mexican hat diagram)

Inflation is essentially exponential expansion. Loads of posters of expansion of the universe usually have inflation on them, and look like the picture below. The inflation period is at the very start,between 10^-36 seconds and around 10^-33 seconds:

Credit: NASA

The theory was proposed by Alan Guth, a cosmologist who developed the idea whilst at Cornell. He proposed that if a field in the universe had experienced a false-vacuum state, then it would create exponential expansion. This idea is backed up by Einstein’s theory of general relativity, which has long been proven.

Cool, but what’s a false vacuum?

This is where I introduce my Mexican hat diagram, created using my amazing tech skills:

Scalar Field.png

At this time in the universe, everywhere was so hot that all fundamental forces were unified into one. We can model this grand-unification of forces using a field, rather than a particle, and this is what we believe the field pattern would look like.

I’ve plotted energy density against strength of field. At the very beginning, the universe starts off at high strength and high energy density.

As time increases and the universe expands (normally), the energy density and field strength lower. The field moves down the red line, jumping from one side to the other, until we hit a bump in the very middle of the graph. Whilst staying on this bump, we’ve entered a false vacuum and will expand exponentially until we slide to a true vacuum.

The importance of the false vacuum

Whilst we’re in a false vacuum, what we get is a discrepancy between the energy density we should have, and the energy density we currently have. This creates a negative pressure and thus, exponential expansion.

Interestingly, the difference in energy between the false and true vacuum is so large that it’s equivalent to energy released if all the universe’s galaxies combined with their own antimatter twin!

Solving the three mysteries

Here’s how inflation answers the three mysteries.

Firstly, with inflation, our universe has had a period of exponential expansion, so looks bigger than it should be. With inflation, we can find a time when the universe was all in contact with each other that makes our observations fit with our calculations.


Secondly, inflation makes our already gigantic universe even larger. Despite the universe looking flat, it may just be the case that we’re just looking deep enough!

This argument may seem difficult to grasp if you’re a Flat Earther.

And lastly, inflation results in the space between particles enlarging and separating. This actually smooths out fluctuations and creates a very homogeneous universe. In this inflation era, only small quantum fluctuations stay prominent (these are basically matter/antimatter pairs, they appear and disappear all the time).

But after inflation, we’re left with such a huge amount of energy from the false vacuum (called potential energy of inflation). This all gets converted into real matter (by E=mc²), and is homogeneously distributed across the universe due to the uniform nature of the false vacuum energy.

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