In March 2016, the discovery of the farthest known galaxy, GN-z11 , was announced, just at the detectability limit of the Hubble Space Telescope. Located in the constellation Ursa Major, today we see it as it was 13.4 billion years ago, just 400 million after the big bang that gave rise to the Universe. It is located 32 billion light-years from us, which may seem incongruous given that our universe is less than 15 billion years old. However, we must remember something that we do not usually keep in mind when we talk about distances in the universe, especially if it refers to very distant objects: not only does light travel through space, but space itself expands as it travels. That is why the distance does not coincide with the time that light takes to travel through the universe.
Compared to the Milky Way, GN-z11 is 25 times smaller and has 1% of its mass , but new stars are forming (or were forming 13.4 billion years ago) at a rate 20 times faster. That shapeless mass discovered has a size of 500 light-years, little compared to the 100,000 that our own galaxy has. Is it a fragment of a protogalaxy or simply a zone of star formation in a still invisible galaxy? Or could it be a fragment thrown up by a colliding galaxy?
Numerous objects like this have been discovered in recent years, some of which have been identified as galaxies. Why is it important to observe them? For that we must look at the oldest moment that we can observe in the evolution of the universe, where the famous cosmic background radiation comes from 300,000 years ago. Until then the history of the cosmos had been characterized by a deadly boredom broken only by photons crashing into the space-filling plasma, free electrons, and nuclei of hydrogen and helium moving at frenetic speeds . Light was unable to “escape” from these continual collisions with matter, so the universe was opaque to radiation. But 300,000 years ago something happened that was going to change everything: the temperature of the universe dropped enough for the hydrogen and helium nuclei to trap the electrons that were flying around them, becoming neutral atoms , so that the photons stopped interacting with them. The matter. This moment of ‘decoupling’ of matter and radiation caused the universe to become transparent; that light escaped and today we can see it in the form of that radiation in the microwave range that floods the entire cosmos. It is a luminous border that does not allow us to observe anything that happened before, like a curtain that prevents any information from reaching us in the form of electromagnetic radiation about what happened in those previous 300,000 years. It is the oldest light that we can observe and marks the beginning of what is called the Dark Ages of the universe.
For the next 500 million years the history of the universe was boring again. The hydrogen and helium nuclei were trapping the electrons that until then had moved freely through space, giving rise to the first atoms. The universe, which was cooling as it continued to expand, thus became a gaseous sea where about a quarter was helium and the rest hydrogen – with some traces of beryllium and lithium, the element with which we build batteries. of our electronic devices. It was as if a dark and opaque mist covered every corner. However, something was brewing under that calm appearance: the ubiquitous force of gravity was silently doing its work. The first stars and galaxies were on their way. But there were still several million years to go.
But the time came when the Dark Ages ended and the Cosmic Renaissance began: stars, galaxies and quasars (incredibly bright objects whose engine is a huge black hole with a mass billions of times that of the Sun) appeared . Now, when did the first creative discharge take place in the universe? When did the first stars and galaxies appear? The discovery of these very young galaxies, formed a couple of hundred million years after the undocking, tells us that the Dark Ages were not so dark . What is certain is that 800 million years after the Big Bang, the date that marks the end of the Dark Ages , an accelerated process of creation began to take place that reached its peak, with most of the current stars and galaxies already formed, between 5,000 or 6,000 million years after the Big Bang.
Just as the morning mist dissipates with the first rays of the Sun, the light of the first stars changed the appearance of the universe. Hydrogen atoms -the essential component of this cosmic mist- lost their electrons, became ionized, so that the gas clouds ceased to be opaque and became transparent . From the Mauna Kea observatory, astronomers have been able to find both light areas and dark regions produced by opaque gas from the Dark Ages. For the second time in their history, the hydrogen nuclei were once again left alone, without their electron spinning around. This stage of reionization ended when the universe was a billion years old.
This is the prehistory of our universe as it has been reconstructed from the few observational pieces we have.
