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MSSL Astrophysics Group Beginner's Guides

A Beginner's Guide to the Cosmic Microwave Background

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In the Beginning

The current theory of the origin of the universe is the Big Bang Theory. This theory has been supported by the discovery of the Cosmic Microwave Background which adds further evidence and explanation.

When the universe was very young it was also very hot and a great deal smaller than it is today. It was so hot that all the matter within it was a plasma. This plasma coupled with photons and this was called the photon-baryon fluid.

This fluid swirled around and mixed together which at times meant that some areas of the universe had more of these coupled molecules than others. It is statistically more likely that the density of the photon-baryon fluid is going to be different in different places than it is that the fluid would be a completely even density across the galaxy. This is thought to be essential to the creation of large formations like galaxies and galaxy clusters.

Inflation

After the initial Big Bang which is still fairly unknown, there was a period of time which was known as inflation, during which time the universe expanded rapidly and most importantly its expansion accelerated. It is thought that this was the only time in the history of the universe that the rate of expansion was accelerating.

The Photon-Baryon Fluid

At this time the universe was very, very hot and a great deal smaller than it is today. The extreme heat meant that all the matter was in a plasma state and was also mixed with the photons. This formed a "fluid" where matter and photons coupled with each other. This was called the photon-baryon fluid.

This fluid swirled around randomly which meant that sometimes as the molecules moved there were more in one place than there were in others. This is just like if you were to rip up a piece of paper into lots of equally sized sections, put them into a cup and then tipped them on the floor. You are very unlikely to see a well organiused pattern with each piece of paper the same distance from each other. They are more likely to be jumbled up; more on one side, some on top of each other etc. etc.. This is what the photon-baryon fluid was doing except that all the molecules were moving all the time.

Gravitational Fluctuations

Each molecule has its own gravitational attraction. Everything that has mass is pulled towards everything else that has mass. This can be a little speck of dust, or the Earth. Obviously we do not feel ourselves being dragged towards the speck of dust, but we can't jump off the Earth, so the more mass something has, the greater the attraction.

This was the same for the photon-baryon fluid. The areas of more mass (more photon-baryon molecules clumped in one area, were the areas of greater gravitational attraction. As the forces were so small at this time and the temperature and speeds of all these molecules so great, these areas of greater density changed all the time. This principle is thought however to be the beginnings of large structures in the universe, like galaxy clusters and super clusters.




The Last Scattering


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The Last Scattering

Over time the universe continued to expand outwards and consequently get cooler. The reason for this is that no extra energy was being added to the universe and yet the same energy was therefore being spread over a greater volume. This means that there was less energy per unit volume and therefore a lower temperature. The only reason that the matter on the universe was a plasma was because of such a great temperature. Eventually when the universe cooled to a certain temperature there was no longer enough energy to keep the electrons and protons apart and so they combined to form hydrogen and helium. This point in time was (inappropriately) called 'Recombination' or 'The Last Scattering'. The process of the electrons and protons combining meant that the photon-baryon fluid was destroyed and the photons were released.

Brief History of Universe from Wayne Hu's Homepage

The polarisations, intensity and wavelength of the photons tell us a great deal about the state of the matter at this time. This is seen in the anisotropies (described later).

From this point onwards the more dense areas of matter tended to stay more dense as there was not as many high energy particles around to break the attractions between the atoms in these areas. This means that the more dense areas at this time attracted other atoms around them, which created a greater gravitational attraction and attracted yet more atoms and so on.

These tiny differences in densities, or gravitational differences therefore went to make up the huge super clusters and clusters of galaxies that we know exist today! That is why the last scattering is such an important period of time in the structure of the universe.

An Expanding Universe


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An Expanding Universe

The present theory of the creation of the universe, the Big Bang Theory states that the universe is expanding. It is expanding in such a way that everything is moving away from everything else.

If you imagine a deflated balloon and you draw lots of dots on this balloon. When you blow the balloon up, the dots get further away from each other. If you imagine that each of these dots is a galaxy, this is what is happening to the universe.

There is another important effect which is matched with this balloon analogy. If you measure the size of the dots when the balloon is blown up the dots are much bigger suggesting that the galaxies are larger, which is true.

This process can be taken to even smaller scales. Imagine looking at a photon of light, which is a wave packet. If you drew this on to an inflated balloon and then blew it up, what happens? The length of the wave increases which means that the wavelength increases, which means that it shifts further to the infra-red side of the electromagnetic spectrum.

This results in all the photons that were released at the time of the last scattering being stretched to the infra-red side of the spectrum.


The Horizon


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The Horizon

On Earth there are many reasons why we can't see the South Pole from the North Pole. Hills, mountains and even air get in the way, the light from the south pole only has so much energy and this can be absorbed when it collides with air molecules meaning that it does not reach us at the North Pole. However, on top of all these reasons another one is because of the curvature of the Earth. The limit we can see across the Earth is called the horizon.

The universe, like our Earth has a horizon, but for a slightly different reason. Because of the vast distances involved and the fact that light travels at a finite speed, some galaxies that are very far away are not visible because their light simply has not reached us yet.

Another important property is that because light from the horizon has taken such a long time to get here, it effectively means that we are looking into the past. Looking at a wide angle view of the horizon means that we are receiveing light from the time of the last scattering. We cannot see before then as the photons were not free to travel as they were bound in the photon-baryon fluid.

This also means that over time the horizon of the universe will extend so we can see further.

COBE


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The COsmic Background Explorer - COBE



COBE is a satellite that was launched on November 18, 1989 by NASA's Goddard Space Flight Center. It was equipped with three instruments called DIRBE, DMR and FIRAS.

Its objective was to look for and map as accurately as possible the cosmic microwave background. The DMRs (Diffuse Microwave Radiometers) were designed to search for the primeval fluctuations in the densities of matter at the last scattering, which were discussed earlier.

It must be noted however, the accuracy of COBE's vision. When looking at the sky COBE sees the CMB in a very blurred way. So much so that if it was to look at the Earth it could only detect the large continent forms. Saying this, the maps that COBE produce are still very useful. Another satellite launched by NASA called MAP and a satellite by ESA called Planck are planned to be launched, both which will look at small angles along the sky at the CMB.




The Anisotropies from the Beginning


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The Anisotropies From The Beginning

What COBE saw was a revelation. As well as the constant temperature background brought to light by Penzias and Wilson, there was a slight fluctuation in this temperature, in the order of tens of microKelvin. 1 microKelvin is 0.000001 or 1E-6. These fluctuations are thought to be the gravitational instabilities that existed when the universe was only about 300,000 years old, at the time of the last scattering.

It is also believed that wherever in the universe you observe this you would see the same. This means that scientists believe the universe to be isotropic. It is also thought that (on large scales) the universe is the same density everywhere and so it is homogeneous.

Whether the universe is homogeneous and isotropic could greatly influence which theory of the origin of the universe is supported.


How are the Anisotropies Created


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How are the anisotropies created?

Back in time to just before the last scattering the gravitational instabilities were fluctuating as they had been for thousands of years before that. Each gravitational instability creates a force around it which means that to get out of the 'potential well' you have to expend energy.

Picture shows areas of these fluctuations on the sky, blue is hot, more dense areas of matter, red is cold less dense areas of matter.

The particles that are in this potential well oscillate as if they were stuck to either end of a spring. The greater the potential well (the more photon-baryon particles there are around them creating a greater gravitational force) the more tightly forced together the 'spring' between them is. This is effectively a sound wave. However, to oscillate in and out the particles need energy and expel energy in the form of photons. This was happening since after inflation except it is not possible for us to see it as the photons were absorbed elsewhere in the fluid.

When the universe cooled enough, at the time of the last scattering these photons were able to travel freely. Because the universe was so hot they were high energy photons which have shorter (more blue) wavelengths. These photons were all very, very nearly the same wavelength, but because different energies were involved in oscillating different particles in different areas the wavelengths varied a tiny amount. This is why Penzias and Wilson picked up a constant background temperature, because their equipment simply was not sensitive enough to detect the differences. This means that as the universe expanded, the wavelengths of light stretched to the infra-red/submillimetre, but still kept the tiny differences that they had initially. This created the anisotropies with slightly hotter regions being more dense areas of matter at the last scattering and slightly cooler regions being less dense. This is also what COBE detects.

The Fate of the Universe


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The Fate of the Universe

What is going to happen to the universe? Hopefully the CMB can help us determine this.

Einstein proposed that gravity was merely a curvature of space-time. This curvature of space-time is caused by mass, so the more mass that there is in the universe, then the more space-time is curved.

There are three options:

An open universe
That there is not enough mass in the universe to halt expansion so the universe will continue to expand forever.


A Flat Universe
That there is just enough mass in the universe to reach a steady state (unlikely result).


A Closed Universe
That there is enough mass in the universe to curve space-time enough that it can halt the expansion of the universe.



We still do not know if the mass will cause a curvature which is similar to our Earth, so that we can go off one side and come on the other, or curved in another sort of way

Hopefully the CMB photons can help us with this as the distances that we need to travel to determine this are too great for us. If the universe is closed then it should focus the CMB background like a lens more that if it wasn't.

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Contents
Penzias and Wilson and the antennaIntroduction
Photon-Baryon FluidIn The Beginning
The Last Scattering
An Expanding Universe
The Horizon
COBE
The Anisotropies from the Beginning
How are the anisotropies created
The Fate of the Universe


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Frames test Cosmic Microwave Background Polarisation

Cosmic Microwave Background - A Beginners Guide