Chapter 27. Inflation

27.1 Introduction

Alan Guth, one of the theorists who came up with inflation.

The goals of this module: After completing this exercise, you should be able to:

1. List the observations that were inconsistent with the non-inflationary Big Bang theory.
2. Explain how an early, brief period of expansion resolves these inconsistencies.
3. Recognize the process by which scientific theories are updated.

In this module you will explore:

This activity looks at two features of the cosmic microwave background that were not predicted by the early versions of the Big Bang theory, and presents a possible theoretical explanation.

Why you are doing it: Some people think, if a prediction of a scientific theory is incorrect, that the entire theory is thrown out. The story of how inflation was added to the Big Bang theory is an excellent example of how this is frequently not how modern science works.

Astronomers have accepted the Big Bang theory in some form for almost half a century. The fact that technical advances like infrared cameras and orbiting observatories have upheld most of its predictions makes it likely that it will prevail even longer.

27.2 Background

The Cosmic Background Explorer's (COBE) Spectrum of the DMB

Western scientists assumed for hundreds of years that the universe was infinite and unchanging. In the early 1900's, though, theory and observations independently began to imply that the fabric of space and time is expanding after a creation event called the Big Bang.

One of the strongest pieces of observational evidence supporting the Big Bang theory is the cosmic microwave background (CMB). The CMB is the fossil light signal of the first blast of free light released as the universe expanded, predicted way back in the 1940's. The agreed upon value today is a chilly 2.725 K, close to the predicted value.

However, as is often the case, the same data that answer one question raise more new questions. The temperature of the CMB was too consistent everywhere; it didn't show the seeds of the future large-scale structure of the universe - yet we are very sure that galaxies and galaxy clusters do exist!

A series of missions to search for these seeds was successful with the Cosmic Background Explorer (COBE) in 1994, for which team leaders John Mather and George Smoot were awarded the 2006 Nobel Prize in Physics.

The second problem was that, as the resolution of the CMB improved and its fluctuations could be measured precisely by missions such as the Wilkinson Microwave Anisotropy Probe in 2003, the CMB showed the geometry of the universe to be flat, when the expectation was that it should be curved in some way.

Play the animation below. It starts with the best image available in 1994, and then morphs into the image in 2003. Notice how many more details are visible in the newer one!

Improvement of Resolution from COBE (1994) to the Wilkinson Microwave Anisotropy Probe (2003). http://www.nasa.gov/

27.3 The Temperature of the Cosmic Microwave Background (CMB) is Uniform

The Isotropy Problem.

The CMB's temperature was not only close to the predicted value, but the same 2.725 K everywhere the cameras looked, with variations of only 0.0002 K (after the signals from the motion of the Earth, the Sun, and our galaxy were removed.) Here's why the uniform temperature is a problem: the telescopes look back to the beginning of the observable universe in two opposite directions. These two regions were virtually the same temperature, yet the universe wasn't old enough for them to have ever been in contact with each other. This is called the isotropy or horizon problem.

Regions A and B in the figure, on our cosmic light horizons, are so far apart that light doesn’t have enough time to travel from one to the other in the age of the universe! (Light is the fastest way we know of for two separated locations to communicate with each other.) This is why the isotropy problem is also called the horizon problem.

27.4 The Universe is Flat

Possible Geometries of the Universe.

The mass in the universe curves the fabric of space-time, according to Einstein's theory of general relativity. If it curves positively, then the universe has enough mass to collapse back on itself. If it has exactly the right amount of mass, then it won't curve at all - it will be flat. If it doesn't have enough mass, than it curves negatively. If you can measure how much mass there is, then you can predict not only how the universe should curve, but also the future of the universe!

Another easier way is to estimate not the total mass of the universe, but the average density (mass per volume) throughout the universe. The mass density needed to exactly end up with a flat universe is called the critical density, also called Ω₀ (pronounced omega-sub-zero). If the true density is exactly equal to the critical density, then Ω₀ equals exactly 1 and the universe is flat. If Ω₀ is greater than 1, the universe is positively curved; Ω₀ less than 1 means that the universe is negatively curved. The measurement of fluctuations from the CMB showed that Ω₀ was almost exactly equal to 1.

There is no theoretical reason for Ω₀ to be equal to 1 and therefore for the universe to be flat, so this result is called the flatness problem.

27.5 Solving the Horizon and the Flatness Problems

Fortunately, there is a theoretical solution that elegantly solves both these problems. If the universe had a brief but intense expansion during the first second of the universe, two pieces of space that used to be next to each other would almost instantly become widely separated. This means that it would expand faster than the speed of light. Space expanding faster than the speed of light doesn't violate Einstein's rule that nothing can travel faster than light. Space expanding isn't the same as particles or photons moving - space isn't a 'thing.'

Since all regions of space started out at the same temperature, and if the expansion were the same everywhere, then they would also cool at the same rate, and end up at the same temperature when we measure it now. The tiny temperature fluctuations seen by WMAP are also consistent with inflation.

This extreme expansion would also have stretched the fabric of space-time so much that any overall curvature would get flattened out. This addition to the original, steady expansion idea of the Big Bang theory is called inflation. (You may have thought that this was a term only used in economics, but the volume of space can expand just like prices can rise!)

This animation is an analogy, which means that it's intended to help you understand inflation by using a simple model of the universe. It represents inflating space as three-dimensional, but our real universe has at least four dimensions – three directions in space and one in time (time runs forward). It also shows us looking at the universe from the outside, which isn't possible since nothing exists outside of the universe. This imagery is not helpful if you are thinking about the model that the universe has no center or 'outside', but it will help you visualize inflation! Click inflate to start the animation, and notice what happens to the curvature of the grid lines as this 'universe' inflates.

27.6 Connection to the Overall Big Bang Theory

As was said earlier, one answer - inflation - often brings up other questions. The cosmic microwave background observations suggest that the universe is flat, so this means that the mass density must be equal to the critical density. Here's the new problem: we can't find it all! Particles like neutrons, protons and electrons that make up you and me are only about 4% of the required mass. About a third of this 'missing matter' appears to be associated with galaxies, and the rest might be the 'dark energy' that started accelerating the universe 5-6 billion years ago.

Timeline of the Universe

The Big Bang theory isn't perfect, and no doubt will need more tweaking as more observations are made, but this is the latest and best story of the 'timeline' of the universe inferred by the Wilkinson Microwave Anisotropy Probe data you saw in the Background to this activity. Notice that inflation happens in the first trillionth of a second.

It's just so amazing that, for the first time, we can go back into the first second of time!

27.7 Quick Check Quiz

Indepth Activity: Inflation