Thursday, October 26, 2017

Some clarifications on the cosmic background radiation

In 1948, Ralph Alpher and Robert Herman (both in George Gamow’s team) came to the conclusion that if the universe had come out of a Big Bang and had expanded since that point in time, there should exist a cosmic background radiation in the frequency of microwaves (or what means the same, at a temperature of about 5K, 5 degrees above absolute zero). Alpher and Gamow had published that same year another prediction about the average composition of the cosmos, starting from the Big Bang theory.
In 1964, Arno Penzias and Robert Wilson were working with a newly built very powerful radio telescope and detected a background noise that could not be eliminated. First they thought that it would be of terrestrial origin, but once all the possible sources of noise had been taken into account, the effect persisted. Then they came to the conclusion that such noise could not come from the solar system or from our galaxy (for in that case it would be more intense in one direction than in another), and that its origin had to be cosmic. The temperature of that radiation (that is, its frequency, considering the Wien equation) turned out to be about 3K. Robert Burke of MIT suggested to Penzias that such noise could be the cosmic background radiation predicted by Alpher and Herman. This was in fact confirmed. For their discovery, Penzias and Wilson received the Nobel Prize in 1978.
Along with the argument based on the average composition of the universe, the cosmic background radiation gave the accolade to the Big Bang theory, which became the standard cosmological theory (although see an earlier article on this blog).

Arno Penzias
In the half-century since its discovery, the cosmic background radiation has become one of the best-studied objects in the universe. Thus, we now know that its temperature is equal to 2.72548 K, which corresponds to a frequency peak of 160.23 gigahertz (in the microwave region). Its intensity is almost the same in all directions, but tiny variations are detected, of the order of one part in 100,000. These differences are represented in the maps that usually appear on the front page of newspapers, with red (hotter) and blue (coldest) areas, but the differences are very exaggerated, because they affect the fifth decimal place of the temperature (the 8 in the value indicated above).
Although the temperature of the cosmic background radiation is very well known, a few more things are usually said about it that should be taken with a grain of salt. For example, it is often stated that the average temperature of the cosmos at the time of its appearance was 3000K. This figure is obviously an approximation (the three zeros make that clear to an experienced observer). It is also often said that this radiation arose 380,000 years after the Big Bang. This value is still less reliable than the previous one, since on the one hand it depends on it, and on the other hand it is associated with the standard cosmological model, which in turn depends on six independent variables, whose values are obtained by simulations and parameter adjustment using mathematical methods, i.e. they aren’t measured directly.
Robert Wilson
The microwave background radiation (this is its official name) is the farthest object we can see in the universe. The observable universe reaches in theory a little further (up to about 46 billion light years from us), but we cannot see beyond the cosmic background radiation, which covers that farther part. If we could see what was behind, we would be looking at what the universe was like before this radiation appeared, but since the universe was by then opaque, that part is beyond our reach, because light could not pass through.
The isotropy of the cosmic background radiation (the fact that its temperature is practically the same in all directions) was one of the main reasons why Alan Guth proposed in the 1980s the inflation theory of the Big Bang, which states that the universe expanded inordinately for an infinitesimal fraction of a second, shortly after its origin in the Big Bang. This theory, although widespread among cosmologists, has so far been unable to obtain experimental confirmation.

The same post in Spanish
Manuel Alfonseca

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