You may have heard of the Cosmic Microwave Background , a kind of light that seems to come to us from all directions in the cosmos and has sometimes been called “the echo of the Big Bang” or “the light of the Big Bang”. However, this cosmic background is neither of these things, although this of course does not make it any less important or interesting. But let’s talk about what this phenomenon is.
The Cosmic Microwave Background is the oldest light we can (and could) observe in the entire universe. No matter how much our telescopes improved and no matter how many new techniques we discovered, we would not be able to detect any older photons . Maybe other particles, but no photons. This is because the limitation is not technological, but physical. Before the light that makes up the MCF was emitted , the universe was opaque to all light and any photon that wandered freely through the universe was absorbed and re-emitted by some of the other particles that populated it (electrons and protons, for example), so that they could not travel freely for long. But maybe we should go back further in time.
From the soup of particles that filled the entire universe immediately after the Big Bang, only hydrogen, helium and lithium nuclei, electrons, photons and neutrinos remained, after a few minutes. However, even though the universe had cooled enough to stop the formation of heavier atomic nuclei after just 20 minutes of life , it was still too hot to allow electrons to join these atomic nuclei to form neutral atoms. . Therefore, during the next days, months and years, the atomic nuclei and electrons had too much energy and after each one of the countless collisions that made them interact they would separate again . As these charged particles were free, they could freely interact with the photons that also populated that universe, thus preventing them from traveling long distances without colliding with anything .
It took about 370,000 years for the temperature of the universe to drop enough to finally allow nuclei and electrons to combine. When this happened, the photons were finally free , since it was much harder for them to interact with the neutral atoms than with their charged constituents. It is this primordial light , which was trapped during that first third of a million years , that we now detect and recognize as the Cosmic Microwave Background .
These microwaves were first detected in 1964 by accident . Americans Arno Penzias and Robert Wilson were testing Bell Labs’ new radio telescope when they detected a constant, very low-energy signal that they could not eliminate or explain in any way . This signal, which was in the microwave range, was equivalent to the emission that would be made by a body that spanned the entire sky and had a temperature just above absolute 0 . This turned out to be the Cosmic Microwave Background. However, at the time that light was emitted in the early universe, it was not at such a low temperature or in the microwave range. At that time the average temperature of the universe was around 2700 ºC , so the light emitted by this primordial soup of nuclei and electrons would have emitted a light with a slight reddish glow , similar to that of the star Proxima Centauri. Red light has a much shorter wavelength than microwaves.
The decrease in temperature and the dilation of the wavelength of this cosmic background has been due to the expansion suffered by the universe since it was emitted . In the roughly 13.4 billion years since its emission, the universe has expanded to be hundreds of times larger than it was then . This radiation can be detected unintentionally by radio antennas, as happened to Penzias and Wilson, so it is believed that a small contribution of the white noise that we observed in an old television without tuning could come from this Cosmic Microwave Background. . However, that contribution, even being detectable, would be very small, with most of that noise being reflections of other radio waves that our television does not process correctly.
The image that serves as the cover for this article and that you have above shows the fluctuations in this cosmic background, that is, the small differences in temperature of the light coming from each point in space. Therefore, it can be considered as a map of the entire universe seen from Earth, although it must be understood carefully. These fluctuations are incredibly exaggerated in this image. If we were to take the original signal and translate it into a light that our eyes can see, green for example, which is roughly in the middle of the range of wavelengths visible to humans, we would see a completely smooth image . We would not see any fluctuations or imperfections, because the reality is that this cosmic background is tremendously uniform. We exaggerate the current fluctuations to be able to visualize them and be able to work with the important information they provide us.
This image, which may seem incapable of containing too much information , is one of the strongest evidence we have that the universe we live in began with a Big Bang and is also an essential tool for knowing the energy content of the universe, that is, what percentage of it is made up of ordinary matter or dark matter or dark energy.
