If you go up to the roof of a tall building and look into the distance, there will come a time when something will limit your vision . Perhaps it is the presence of other buildings, mountains or some kind of structure that covers what is behind . Perhaps it is the horizon itself, which limits the maximum distance of what you can see, or perhaps it is the very air of the atmosphere , which scatters light from distant objects until they are completely invisible.
All this can be translated to the case of astronomical observation , where in addition to observing distant objects is equivalent to observing ancient objects. Although the intergalactic medium is mostly empty, there are impediments to our astronomical observation of the most distant and oldest known objects. Still, there is a certain amount of gas and dust that is found occupying the regions that separate stars within a galaxy, galaxies within a cluster, and clusters within… the universe. The effect of this material, although it is present in very small quantities , can accumulate over billions of light-years of travel , scattering or absorbing light, hiding the most remote objects from us.
In addition and of course, it can happen that a relatively close galaxy hides another much further away. Although gravitational lenses can show us objects that are behind other closer ones, these structures distort the light and do not allow us to observe the entire region in detail.
But not only that, because in space other considerations arise that on Earth do not. On Earth, what prevents us from seeing in the distance is the presence of something corporeal located between what is observed and who is observing, whether it is an obstacle, the air of the atmosphere or the Earth’s surface itself. In space another impediment arises from the very fact that the universe is expanding . This expansion, which began with the Big Bang and now appears to be speeding up (rather than being slowed down, as we expected) by the presence of dark energy , in the process expanding the wavelength of any light traveling through the system. universe.
The wavelength is nothing more than the distance between two maxima or minima of a wave. In the case of sea waves or the waves that form in a lake when a stone falls, it would be the distance between the crests of the waves , or between the valleys that form between them. In the case of light, it is the distance between two maxima or minima of the electric or magnetic field that form it. The wavelength and the frequency, which is closely related to the latter, normally define the energy of a wave . In the case of a wave such as sound, the shorter the wavelength, the higher pitched a sound. In the case of light, the shorter the wavelength, the more “bluish” a light will be, and the longer the wavelength, the more “reddish” it will be. This is why, as the universe expands and the light traveling through it expands, this light has been seen reddened . That is, its wavelength has grown. This is what in astrophysics we know as ” redshift ” or “redshift” in English.
This red shift decreases the energy of the light emitted by stars, nebulae, supernovae and others, achieving, for the most distant objects, that it ceases to be detectable in the visible part of the electromagnetic spectrum . So in addition to the decrease in brightness due to distance, we also observe a decrease in brightness due to this redshift. However, with more sensitive telescopes and observing at different frequencies, most of these problems could be solved. What we could not get around is the obstacle of the cosmic microwave background .
This background constitutes the oldest light that we can observe. It comes from the moment when, at the origins of the universe, some 370,000 years after the Big Bang , the universe cooled enough to allow protons and electrons to form neutral atoms . When this happened, the countless photons that populated that young universe, which until now had been constantly bouncing off these charged particles, were finally able to travel freely , without interacting with anyone, bathing the universe with their light. This limit in our ability to observe light is not technological , it does not depend on having better telescopes or better detectors, but it is a limit of the universe itself . No light before that could travel freely across the universe and, billions of years later, reach our detectors.
But even if we can’t see older light, we can detect something older . On the one hand, the neutrinos that after approximately one second of the life of the universe underwent a similar release process to the one mentioned for the photons of light. On the other hand, the gravitational waves produced as a result of the Big Bang itself. We believe that both radiations will be able to be detected in the future, although in this case our technology is not yet sensitive enough to detect them today.
