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From Einstein to Penzias and Wilson: this is how we learned that the universe is expanding

The idea of a universe swelling like a balloon does not usually attract our attention anymore. And not because we understand it, but because we are used to hearing it. When Einstein discovered this consequence of the general theory of relativity he could not believe it . To avoid this, he modified the equations by introducing a term outside the theory that stopped the expansion: the cosmological constant , which made the universe more acceptable to his mind. When astronomer Edwin Hubble later discovered the expansion of the universe, Einstein is reported to have said that the introduction of the cosmological constant had been the biggest mistake of his life.

An expanding universe implies a series of questions that a static one does not. Projecting the film backwards we will see the universe shrink to… what? Until seeing it converted into a point of infinite density and temperature. These reflections motivated the appearance of a new cosmology: the hypothesis of the Big Bang or Great Explosion. How can we imagine such an extraordinary event? To begin with, the expansion of the universe is usually represented in the mind with the image of a balloon inflating with galaxies painted on its surface. It is a misleading view of the universe because the balloon expands into something while the universe expands into nothing . That is the problem with analogies. They serve to visualize difficult concepts, but they do not represent reality.

The same goes for the Big Bang . When they talk to us about explosions we have in our heads bombs, blasting… The beginning of the universe cannot be seen like that either. It was an “explosion” completely different from what we could imagine, because everything was created in it: matter, energy, space and time. It is so strange to our minds that we accept with difficulty that it makes no sense to wonder what was before because the before did not exist . There was not time. I couldn’t even explode inside anything because there was no space. In fact, there is nothing beyond, because the universe is not inside any divine habitation. 

Now, if the universe originated with a formidable explosion, if everything began as a monumental firecracker, is it not possible that we can still hear the impressive initial chupinazo?

So thought in 1965 a cosmologist named Jim Peebles. Peebles set about calculating what would happen if the universe really had been born this way, and discovered that today we should be able not to hear but to see a microwave background radiation covering all of space. This radiation background would be like the echo of the initial tremendous explosion . Peebles wrote up his ideas in an article he submitted to Physical Review in March 1965, but the article was rejected.

The previous month, however, Peebles had been invited by John Hopkins University in Baltimore to speak about his work. On February 19, he presented his ideas and what happened next is one of those chains of coincidences that life gives us . Peebles’ talk was attended by a radio astronomer from the Carnegie Institution of Washington, Kenneth Turner. Turner was also an old friend of Peebles’s from their undergraduate days at Princeton.

Fascinated by the idea, Turner mentioned it to another radio astronomer friend of his, Bernard Burke. Burke, for his part, during an informal conversation with another colleague named Arno Penzias , asked him how the measurements were going on the new antenna that Bell Laboratories were building. Penzias mentioned to him that they had some problems because they had detected completely inexplicable signals. Burke remembered what Turner had told him and told Penzias that there was a group of theoretical physicists at Princeton who might be able to shed some light on the problem. Penzias called Princeton, and Peebles’ group of cosmologists set off for Crawford Hill , the site where Bell Laboratories was commissioning its new radio antenna. 

The rest is history.

This is how, in this very casual way, experimental proof was found that a long, long time ago, a great explosion marked the origin of the universe in which we live.


Silk, J. (2000) The Big Bang , Times Books

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