Hydrogen is the simplest atom that exists, it contains only one electron and one proton (let's compare it with any other, for example californium – there is an element that is called that – which has 98 protons and 98 electrons or with uranium with 92 decade). Hydrogen is also the most abundant, it represents three-quarters of the total mass of what the things we see are made of, baryonic matter. The keys to the complexity of the universe are written in the simplicity of hydrogen and much of the cosmic history that we can reconstruct is basically a soap opera starring the electron and the proton, their tumultuous couple relationship and the 21.1 cm of The energy that separates them when they are very close together.
The oldest signal that we have been able to measure in the cosmos is related to the process by which electrons bonded to protons to form the first hydrogen atoms. This union that occurred just 380,000 years after the Big Bang left a mark on the entire universe, it is the most remote thing we have looked back in time and we know as the Cosmic Microwave Background (see, for example, the PLANCK mission). From that moment, until the stars are formed and we begin to see their light, we enter the age of dark ignorance.
The 21.1 cm is key because that is the signal that the cosmos would be sending us redshifted (like a spring that is very stretched by the expansion of the universe)
To be able to see what happened in that long dark period of which we know practically nothing, we must detect the only thing that existed: extremely remote, trillions of clouds of neutral hydrogen gas. During this first relationship in the form of neutral hydrogen between the proton and the electron that lasted about 660 million years and until our protagonists divorce again, separating into the electrons and protons that we call ionized gas, we have not been able to detect a signal yet. That is where those 21.1 cm are key because that is the signal that the cosmos would be sending us red-shifted (like a spring highly stretched by the expansion of the universe)
How is this signal produced? Let's think of simple hydrogen with a positive charge and a negative charge that in its most relaxed, fundamental configuration, are located very close to each other. Imagine, for example, that you are lying on the sofa watching a documentary about lions. In that sofa they can be in two states, both with the heads on the same side or both with the heads on opposite sides of the sofa. Obviously, one state is not the same as the other, we call the one with the parallel heads its excited state because it tires and although the electron can spend about 10 million years like this, it ends up grabbing the cushion and going to the other side of the sofa, its state fundamental, where they have antiparallel heads. These two energies are very similar, but the passage from one to the other is manifested in the emission of radiation that has a well-known wavelength , 21.1 cm, which when transformed into frequency gives us 1420.4 megahertz (MHz), that is, we are talking about waves. radio. It is a large wave and as such it is associated with a very small energy, it is 26 orders of magnitude less than the 89 calories of energy that a banana is capable of providing. This would be like comparing the size of the universe with the length of a meter, and stretched by the expansion of the universe at the time we want to measure it would be displaced at lower frequencies between 10 and 210 MHz.
If we can detect the one that comes from the first hydrogen clouds formed in the universe before there were even stars, we would have an accurate sign of the beginning of the universe
This wavelength is one of the most precisely known quantities in astrophysics, its existence was predicted in 1944 by the astronomer Hendrick van del Huslt as a way to detect cold atomic hydrogen and it allowed us to see the structure for the first time spiral of our galaxy. The plates of the Pioneer 10 and Pioneer 11 space probes bear the figure of two humans measured on the scale of this hyperfine transition of the hydrogen atom that is also used to map gas clouds in our relatively close environment. If we can detect the one that comes from the first hydrogen clouds formed in the universe before there were even stars, we would have an accurate sign of the beginning of the universe.
And now let's talk about the problem. Let's imagine that we can build a radio device capable of tuning in that signal. Suppose further that just by moving the dial, which was done in old radios, we tune not different stations but the signals broadcast at different times. What are we waiting for to build it? Well, we are already doing it, that device is called the Square Kilometer Array (SKA) , Spain will have an important contribution and it is expected that it will revolutionize the field allowing for the first time to detect giant hydrogen clouds very far away in time and space. There is another part of emission from these clouds that we cannot tune with instruments on Earth for two main reasons, one that down here we make a lot of noise at those frequencies and another that is blocked by the presence of the ionosphere (a layer of the atmosphere between the 80 and 400 km of altitude that reflects the radio waves, where the northern lights are also formed and the meteoroids disintegrate). Our hope, perhaps the only one, to tune that signal above 30 MHz would be to do it with radio telescopes placed on the far side of the Moon. None of the future space missions to our satellite still contemplates that possibility to do pure, basic, fundamental science, not space mining or tourist trips, from the Moon. Let's hope we don't miss that opportunity to illuminate our knowledge about that dark period in the life of the universe, test our cosmological models, understand how the universe works in its most fundamental way, and learn physics that we haven't even had a chance to do yet. To imagine. Meanwhile we remain very attentive to the surprising discoveries that we hope will be produced with SKA .
Eva Villaver is a researcher at the Astrobiology Center, dependent on the Higher Council for Scientific Research and the National Institute of Aerospace Technology (CAB / CSIC-INTA). Cosmic Void
is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name "cosmic vacuum" refers to the fact that the universe is and is, for the most part, empty, with less than 1 atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillion atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up of Pablo G. Pérez González , researcher at the Center for Astrobiology; Patricia Sánchez Blázquez , tenured professor at the Complutense University of Madrid (UCM); and Eva Villaver , researcher at the Center for Astrobiology.
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