The most important image in history

\section*{Beyond the last galaxy}

Due to the limited speed of light, looking far out into space means looking back in time. The farther you look, the longer back in time you will see. In 1989 the COBE satellite was launched in orbit around the earth, bringing a telescope designed to look as far away as possible. Beyond the last galaxy, before the first stars were born, they saw the glowing dust of the big bang. For this John C. Mather and George F. Smoot were awarded the 2006 Nobel prise in physics.

It started in the beginning of the 20:th century when Hubble found that distant galaxies all moved away from us. Backtracing the motion of the galaxies he saw that about 14 billion years ago, all matter was concentrated in a very small volume and rapidly moving away from each other. This observation gave rise to the big bang theory, that the universe exploded into existence from a single point.

\subsection*{Decoupling: When the universe turned transparent}
Looking back in time with the COBE telescope we will eventually arrive at the forming of the first stars and galaxies. Before that the universe contained only hydrogen gas, the main ingredient of stars. But hydrogen is a transparent gas, COBE will see through it, so we can look even further back in time. As we continue back in time the universe grows hotter and hotter, making the hydrogen atoms move faster and colide more violently. A hydrogen atom consists of a proton in the nucleus and an electron orbiting around it, but when the gas gets hot enough, around 3000 degrees Celsius, the electron will be knocked away. At higher temperatures the electron and proton will not orbit each other in pairs, but rather be a mess of protons and electrons bouncing around each on it's own. This is called a plasma, and it is not transparent as the free electrons will absorb any light passing through. So continuing back in time the hydrogen gas will eventually be hot enough to form a plasma. This transition is called the decoupling. That is how far back in time we can see. Beyond the last star, and behind the transparent hydrogen gas, we will see a 3000 degrees hot plasma of protons and electrons.

\subsection*{Blackbody radiation}
How does a 3000 degrees hot proton-electron plasma look? The answer is simple: it glows. In fact, everything glows when it gets hot. A spot on a stove glows deep red if you leave it on, iron glows yellow when heated. Even the white light of the sun comes from the fact that the surface is 6000 degrees hot. This is called blackbody radiation. So this plasma glowed yellow, the color determined completely by its temperature. From this, one would expect the night sky to be glowing yellow, not black, but the expansion of the universe solves this inconsistency. Even though the 14 billion years old light emitted from the plasma has travelled right through the universe, it still has been subject to changes in the space-time fabric itself. Since the decoupling of the universe, the space itself has expanded to many times its own size and the traveling light from the plasma has expanded with it. Now, light that expands gets less intense, actually changing color towards red. So the yellow light from the plasma will have changed since emitted: first it turned red, and then, as the universe continued to expand the light went into infrared and then continued away from visible light. Today, after 14 billion years, we can expect the glow to be far into the microwave spectrum, corresponding to a much colder blackbody. Luckily we can still recognize it as blackbody radiation from the distribution of colors, only that it will seem to be colder than it was then.

What COBE measured was just this. The background of the universe seems to be a black body of the temperature 2.7 degrees above absolute zero, that is around -270 degrees Celsius. This light is in the microwave region of the spectrum, far from visible light, which is why this is called the cosmological microwave background. The distribution of the colors was exactly the one predicted, confirming the big bang model. This, on its own, was a great success, but there was more.

\begin{figure}[h]
\centering
\includegraphics{cobe} % name of the file - without extension
\caption{The prediction of the theory (line) were confirmed by the COBE experiment (dots) with a very convincing accuracy.}
\end{figure}

\subsection*{A problem with communication}
According to the very simplest big bang model where the universe expands at constant speed in the early universe, the different parts of the universe would have moved away from each other so fast that they did not have time to communicate. Not even using light, the fastest possible way of communicating, could they exchange information. So for us today, looking in opposite directions, we will see two parts of the universe that have never had the chance to communicate. But then, how come that they look almost the same? Actually, in this simple model, there is no good reason why they should be even similar! And this is a sign that the model should be corrected.

The proposed explanation was the following. In the very early universe, long before the decoupling era when the universe became transparent, the universe expanded at a more moderate speed. This would allow different parts of the universe to communicate, and preassure and temperature would have time to even out. After that, the universe entered a brief but extremely fast expansion phase, blowing up to $10^{30}$ times its own size in about $10^{-31}$ seconds! This phase is called the inflation. Even though the universe has smoothed out its temperature, it still will never be perfectly even, there will always be small fluctuations. These small fluctuations however will expand with the inflation and will give rise to large different regions of the universe with slightly different temperature. As the universe then continues to expand at a normal rate those regions will not be given time to comunicate and even out the differences. So when the universe eventually arrives at the decoupling it will have an approximately even temperature distribution, but the small fluctuations from just before the inflation will still be there.

In that model we today expect to see almost the same temperature in every direction, but with small fluctuations from different regions. Indeed COBE registered also these fluctuations, giving a snapshot of how the universe looked just before the inflation, only a fraction of a second after the big bang itself.

The official motivation for the Nobel price is ''for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation''.

\begin{figure}[h]
\centering
\includegraphics{cobe-max} % name of the file - without extension
\caption{The famous snapshot of the universe a fraction of a second after its creation.}
\end{figure}