How is Cosmic Background Radiation scientifically measured?

Question

Seems cosmic background radiation is a significent factor in deterimining the origin of our universe. Does it allways follow a Big Bang?

Answer 1

In cosmology, the cosmic microwave background radiation (most often referred to by the acronym CMB but occasionally CMBR, CBR or MBR, also referred to as relic radiation) is a form of electromagnetic radiation discovered in 1965 that fills the entire universe. It has a thermal black body spectrum at a temperature of 2.725 kelvin. Thus the spectrum peaks in the microwave range at a frequency of 160.2 GHz, corresponding to a wavelength of 1.9 mm.

Measurements of cosmic background radiation are critical to cosmology, since any proposed model of the universe must explain this radiation as we observe it. Although the general feature of a black-body radiation spectrum could potentially be produced by many processes, the spectrum also contains small anisotropies, or irregularities, which vary with the size of the region examined. They have been measured in detail, and match to within experimental error that would be expected if small thermal fluctuations had expanded to the size of the universe we see today. As a result, most cosmologists consider this radiation to be the best evidence for the Big Bang model of the universe.

Measurements of the CMB have made the inflationary Big Bang theory the standard model of the earliest eras of the universe. The standard hot big bang model of the universe requires that the initial conditions for the universe are a Gaussian random field with a nearly scale invariant or Harrison-Zel'dovich spectrum. This is, for example, a prediction of the cosmic inflation model. This means that the initial state of the universe is random, but in a clearly specified way in which the amplitude of the primeval inhomogeneities is 10^-5. Therefore, meaningful statements about the inhomogeneities in the universe need to be statistical in nature. This leads to cosmic variance in which the uncertainties in the variance of the largest scale fluctuations observed in the universe are difficult to accurately compare to theory.

During the first few days of the universe, the universe was in full thermal equilibrium, with photons being continually emitted and absorbed, giving the radiation a blackbody spectrum. As the universe expanded, it cooled to a temperature at which photons could no longer be created or destroyed. The temperature was still high enough for electrons and nuclei to remain unbound, however, and photons were constantly "reflected" from these free electrons through a process called Thomson scattering. Because of this repeated scattering, the early universe was opaque to light.

When the temperature fell to a few thousand Kelvin, electrons and nuclei began to combine to form atoms, a process known as recombination. Since photons scatter infrequently from neutral atoms, radiation decoupled from matter when nearly all the electrons had recombined, at the epoch of last scattering, 379,000 years after the Big Bang. These photons make up the CMB that is observed today, and the observed pattern of fluctuations in the CMB is a direct picture of the universe at this early epoch. The energy of photons was subsequently redshifted by the expansion of the universe, which preserved the blackbody spectrum but caused its temperature to fall, meaning that the photons now fall into the microwave region of the electromagnetic spectrum. The radiation is thought to be observable at every point in the universe, and comes from all directions with (almost) the same intensity.

In 1964, Arno Penzias and Robert Wilson accidentally discovered the cosmic background radiation while conducting diagnostic observations using a new microwave receiver owned by Bell Laboratories. Their discovery provided substantial confirmation of the general CMB predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K—and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded a Nobel Prize for their discovery.

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the Big Bang's predictions regarding the CMB. COBE found a residual temperature of 2.726 K and in 1992 detected for the first time the fluctuations (anisotropies) in the CMB, at a level of about one part in 105. John C. Mather and George Smoot were awarded Nobels for their leadership in this work. During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably BOOMERanG, found the universe to be almost spatially flat by measuring the typical angular size (the size on the sky) of the anisotropies.

Answer 2

The Cosmic Microwave Background Radiation is a confirmation that the Big Bang model is correct.  If the Universe started hot and dense, then about 370,000 years later, when the Universe became cool enough for electrons to bind to protons (forming hydrogen), then at that moment, photons wouldn't bump into things and go nowhere, but in fact would be free to go cosmological distances.  The Universe became transparent.  The Big Bang theory predicts the temperature that this should happen, and also predicts the spectrum shape.  It should be an essentially perfect black body radiation curve.  Then, the Universe expanded, and we expected that the original radiation's wavelengths would be stretched - red shifted into the microwave part of the spectrum.

So you can say that the CMBR always follows a Big Bang, but that's a kind of strange way to talk about something that happened exactly once (that we know of).

You can detect the CMBR from the ground. You should be able to see it in every direction. In fact, if you tune your NTSC TV (soon to be obsolete) to an unused channel, some of the snow you see is from the CMBR. But from the ground, it's pretty noisy.  So you can't see that any part of the CMBR is different than any other part.  So, NASA launched first the COBE satellite to map the whole sky with enough to see some changes, then some scientists launched a balloon in Antarctica for a few weeks - called Boomerang, which measured the changes more accurately, but for a small part of the sky, and then NASA launched the WMAP satellite to achieve this higher accuracy but for the whole sky.  WMAP is still operating.  An even more sensitive satellite called Plank is scheduled for launch in a few years. These slight changes in the CMBR represent places where the Universe was slightly more dense and less dense.  From these density abnormalities - places where matter wasn't distributed in a uniformly smooth way, matter was able to clump into stars and galaxies.  So, you and i are the product of abnormalities.