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
Thematic thread on Standard Cosmology: Preceding Next
Manuel Alfonseca
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