Cosmic Times

Inflation in the Universe

The primary message of this article is to show that Big Bang theory has gone through changes since it was first conceived. Observations of the cosmic microwave background posed problems for the theory as it stood, so theorists developed Inflation Theory to better account for those new observations.

The Problem

In the article titled "Universe's 1st Baby Picture", we found that the cosmic microwave background was very, very smooth. There were density differences, but as we discuss in the article and the article notes, those differences are very small. In fact, the CMB was so uniform that it posed a problem for Big Bang theory. In cosmology, this is called the Horizon Problem.*

It is not unusual for a scientific theory to undergo revisions when new observations conflict with its predictions. In fact, this is the very heart of the scientific process. Students may be surprised that, more often than not, theories generally undergo revision rather than being discarded wholesale when new observations do not support the theory. Theorists work to incorporate the new observations into the theory with as little change as possible to the original theory, because the theory presumably worked pretty well for observations prior to the new observations.

Big Bang theory is no different from any other scientific theory, and even before the COBE results, it was becoming clear that the background was too uniform. The reason this was a problem was that regions of the CMB that should not have had time to "talk" to each other were essentially in thermodynamic equilibrium. This was a problem.

To better understand this problem, consider a mug of hot coffee. If we drop an ice cube into it, we know that eventually the ice cube will melt and warm up while the hot coffee will cool down a little, and liquid in the mug will come to the same temperature throughout. Once the liquid is all at the same temperature again, scientists would say that the liquid is in "thermodynamic equilibrium".

For this illustration, let's say that it takes 15 minutes for the liquid to come back to thermodynamic equilibrium. Up until that 15-minute mark, different areas inside the coffee mug will have different temperatures. Early on, the differences are especially pronounced. During the first minute, the coffee will be relatively cool near the ice cube, but the coffee toward the bottom of the mug will still be close to its original temperature. The way astronomers might describe this is that different parts of liquid in the mug have not had time to "communicate" with each other.

Illustration of ice melting in a coffee mug

Thermodynamic Equilibrium in a Coffee Mug

Add an ice cube to a mug of hot coffee.

Just after the ice is placed in the mug, the coffee at the bottom of the mug will be close to its original temperature, while coffee immediately surrounding the ice cube will be cooler.

Right as the ice cube disappears, let's say at minute 5, there will be a temperature difference between coffee in different parts of the mug.

Eventually, all of the coffee in the mug will be at the same temperature. This final temperature will be slightly lower than the original temperature of the coffee. However, the final temperature will be significantly higher than the original temperature of the ice, because there is so much more coffee than ice.

(Image credit: NASA's Cosmic Times)


Taking the coffee mug analogy one step further, let's imagine that we measure the temperature at all parts of the mug 10 minutes after adding the ice cube. We know that the liquid in the mug should be at different temperatures depending on where we measure, so if our measurement showed a uniform temperature everywhere in the coffee mug, we would be very surprised. The Big Bang model, as it existed before Inflationary Theory, predicted that the Universe should not have been in thermodynamic equilibrium when the CMB was emitted. In other words, the Universe would be so large that parts of the Universe should not have been able to communicate before the CMB was created. So we had every reason to believe that the CMB measured in opposite directions should have shown different temperatures. The observations, however, found that the temperature was uniform in every direction.

The Universe without Inflation:
Prediction versus observation

What we expected

Below is a cut-away of the coffee mug with colors indicating the temperature gradient that we might expect to measure 10 minutes after we add the ice cube. The liquid in the mug has not had time to reach thermodynamic equilibrium, so there is a temperature gradient, with blue indicating cooler temperatures and red indicating hotter temperatures.

Toward the top of the mug, the coffee is cooler because of the melted ice cube. Toward the bottom of the mug, the coffee is hotter because it has not had time to mix with the cooler liquid at the top of the mug.

Illustration expected temperature gradient of coffee after ice has melted but before liquid has come to thermodynamic equilibrium

(Image credit: NASA's Cosmic Times)

What we measured

Below is a cut-away of the coffee mug with colors indicating the temperature gradient that we measured at 10 minutes after adding the ice cube. The consistent temperature was not expected because coffee should not have had time to come to thermodynamic equilibrium.

This is analogous to the measurements of the CMB prior to Inflation Theory – cosmologists needed to explain why the temperature was so consistent when the Universe should not have had time to come to thermodynamic equilibrium before the CMB was emitted.

Illustration of measured temperature in coffee mug with no gradient

(Image credit: NASA's Cosmic Times)


The Solution: Inflation

Inflation Theory answers the Horizon Problem by proposing that early in the Universe's history, a small parcel of space expanded rapidly. This expansion increased the size of the Universe by a factor of 1026 in a fraction of a second. This might sound like it violates relativity, because the expansion occurs faster than the speed of light. However, the expansion is of space only, with no matter or information carried between points at faster than the speed of light.

The Horizon Problem is solved with inflation because prior to the episode of inflation, those regions that are on different sides of the Universe today were actually much, much closer than they would have been without Inflation. That means that parts of the CMB that look very distant today actually would have had time to "communicate" early in the Universe's history because they were much closer then and in thermodynamic equilibrium. This explains why their temperatures are so similar now.

Returning to the coffee mug analogy, this time we will take a much smaller mug of coffee and add an ice cube. Because there is less liquid that needs to come to thermodynamic equilibrium, it will happen much faster. Imagine that thermodynamic equilibrium occurs 5 minutes after adding the ice cube. At 7 minutes after adding the ice cube, increase the size of the mug by a factor of 4, and measure the temperature everywhere in the mug a few minutes later. We wouldn't be surprised if the temperature was uniform throughout the coffee in the mug, because it was in thermodynamic equilibrium before the expansion. This is what happened in the universe, except the universe expanded by a factor of 1026!

Illustration of ice melting in a coffee mug

The Universe with Inflation

Add an ice cube to a small mug of hot coffee.

Just after the ice is placed in the mug, the coffee at the bottom of the mug will be close to its original temperature, while coffee very close to the ice cube will be at a lower temperature.

However, with a smaller mug, the coffee and ice cube will come to thermodynamic equilibrium much faster; let's say 5 minutes after dropping the ice cube into the mug. Now, all parts of the coffee are the same temperature.

In Inflationary Theory, after the early universe came to thermodynamic equilibrium, but before the cosmic microwave background was emitted, the universe underwent a rapid expansion. So, in our coffee-mug model, our small mug of coffee expands sometime after it comes to thermodynamic equilibrium.

Note: The illustration scale and timing are not to scale with the Universe's period of inflation.

(Image credit: NASA's Cosmic Times)


*Big Bang Theory had three different problems that are accounted for in Inflation Theory: The Flatness Problem, The Horizon Problem, and The Monopole Problem. We concentrate on the Horizon Problem here because it is the easiest to understand. For more information on the other problems and how they are solved with Inflation Theory, follow the links under "Other Resources" below.

Other resources

The following webpages have more detailed information:

A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC