THE SKY AFTER THE MYSTERIOUS BIG BANG
The image above shows how it looked. It is called Cosmic Microwave Background, in the literature often abbreviated CMB. In the same way as the sphere of the Earth can be placed in an ellipse, so can the sphere of the sky in order to capture all directions. The image is shown in a galactic coordinate system, with our galaxy the Milky Way placed as the equator.
It is the oldest image of the universe or with other words the map of the youngest light of the universe, that is, when the universe was 380.000 years old. Today, our universe is 13,8 billion years old. On such a large time scale we need to go almost the same distance back in time to see the first light, and this is for now as close to the Big Bang as we can see.
UNDERSTANDING THE IMAGE
An old friend of mine, physicist Filip Kozarski, whom I have met during my studies in Munich, just after he returned from Berkeley, has recently held a lecture on the famous image and we had a short chat afterward.
In order to understand the image, we need to start with some basic properties of light.
Light is electromagnetic radiation. It travels at a finite speed and needs about 1 second to the Moon. Electromagnetic radiation is a form of energy that is all around us and takes many forms of different wavelengths. If the wavelength is short, the energy of the radiation is high and vice versa, radiations with longer wavelengths have lower energy levels. Depending on the wavelength, radiation has different colors. With longer wavelengths, the color gets more reddish. Every object radiates electromagnetic radiation according to its temperature.
The correlation between light and color can easily be demonstrated by comparing the sun and the fire. They both radiate in the wide spectrum of the electromagnetic radiation. However, the sun illuminates its surroundings much more than it heats it in contrast with fire that heats more than it illuminates. This is because the temperature at the surface of the sun is much higher than that of the fire. The wavelength output of the sun has more energy, so the wavelength of fire radiation is longer, which is the reason why in our eyes the fire is redder than the sun.
WHERE DOES THE LIGHT COME FROM?
At the very beginning, the universe was very hot and countless nuclear reactions were taking place. Eventually, the things have cooled down and the temperature has dropped to 3.000K ( 2.800°C). This temperature finally allowed the electrons to get captured by nuclei and hydrogen atoms were formed. This time in the timeline of the Big Bang is called Recombination. Prior to that, the light (electromagnetic radiation) could not travel much, since it was constantly bumping into free-floating particles. The light was trapped. But now that the electrons got caught, the light is free and can travel forward, since about 380.000 years after the Big Bang.
This explains why we can not see any light younger than that. And the light that young can be seen thanks to the finite speed of light. The light needs 1 second to the Moon, which means that when you look at the Moon you see 1 second in the past. In the same way, it took 13,8 billion years for the light from recombination to come to us. But it is here now and we can see it. And if we look enough light years back, through all the stars and galaxies, we see that moment of recombination when the atoms began to compose and the light was able to start traveling freely. In other words, we see this last surface, and the radiation from it makes up the Cosmic Microwave Background.
The radiation at the time close to Big Bang had a temperature of 3.000K. Since then space has expanded almost 1.100 times, the distances between galaxies got bigger and the light now needs more time to travel from one galaxy to the next. With the distance and space expansion, the wavelength of radiation has stretched according to the expansion of the space and the radiation has cooled down from 3.000K to 2,7K (2.800°C to -270°C). With the temperature so close to absolute zero, the radiation mostly consists of the microwave spectrum.
The temperature is almost uniform and not being very precise, the picture would be one-colored. Only with very sensitive instruments, we can detect fluctuations, which is what gives more colors to the picture. The differences in temperature are represented by cold and warm colors and are very very small, of the order of 1 in 100.000. By studying these fluctuations, we can learn about the origin of galaxies and how the first stars formed.
HOW DID WE GET THE IMAGE?
While the microwave radiation is invisible to the naked eye or using optical telescopes, microwave telescopes are able to detect the faint signal.
It was in the year 1964 when two scientists Arno Penzias and Robert Woodrow Wilson wanted to observe our universe in the radio spectrum. The problem was that there was lots of noise standing in their way. They did not know what it was and they ended their observations concluding that there was noise everywhere. Today we know what that noise is. That noise is our image, the CMB. The leftover radiation of the Big Bang.