Another solid proof for the Big Bang was the discovery that Cosmic Background Radiation (CBR; also referred to as Cosmic Microwave Background [CMB] radiation in the diagram just above) peaks near the wavelength of 1 mm (1000 µm [micrometers]) which lies at the far IR/microwave boundary region of the EM spectrum. This is the peak wavelength expected from a radiant blackbody source whose temperature is now 2.73° K. George Gamow and his colleagues Ralph Alpher and R. Hermann had first predicted such radiation (their estimate of its peak was at 5° K) in 1948. The term "afterglow" can be applied to this radiation. The radiation consists of photons (whose electromagnetic wavelengths vary with the blackbody temperature distribution at any given time after the BB) created during the first BB minute as matter and anti-matter annihilated almost completely. We first discussed CBR on page 20-1 and suggested the reader consult this Wikipedia website for more information.
The CBR now evident as pervasive throughout space (both intragalactic and intergalactic) can be traced to an equilibrium state between nucleons, electrons and photons that was arrived at when the Universe had cooled to about 10 million °K approximately 6 months after the Big Bang. Evidence of what it was doing during the Radiation Era, up to Decoupling, had been lacking because of the opacity brought about by scattering and internal entrapment of photons (see page 20-1) within the early Universe during the next 350,000 years. At that time, as the temperature dropped to about 3000 °K, almost all electrons (the principal scatterers) and protons were able to combine as Hydrogen atoms that no longer scattered the photons so that light and other radiation emerged from the radiation "fog" which was fully lifted by 1,000,000 years after the BB. With the resultant transparent Universe, CBR first became detectable, displaying the higher temperatures it then possessed in the still early Universe. (If anyone had existed at that time, a near-infrared radiation detector would have been needed since that was the spectral range for a temperature of 3000° K; the CBR had cooled through the temperature range that would be in the Visible while the opaque Dark Age still dominated.) From Decoupling to the present time, the CBR has experienced a redshift of ~1200. This relativistic redshift resulting from expansion is involved in causing the temperature of 2.73° K that is recorded today.
The photon radiation throughout the Universe now being measured is a manifestation of the present-day Cosmic Microwave Background (CMB), inherited from the original radiation (much hotter and therefore then of much shorter wavelengths in the Infrared) released at the Big Bang. Astronomers commonly refer to the CMB as the general residue of photons that were produced and released during particle interactions in the first minute of the Universe - colloquially, the CMB is the remnant of the "burst" of radiation that marked the "explosion" of the Universe (but which really didn't explode in the sense of detonation of a nuclear device in which there is an initial "flash" of light). It is also referred to as the "afterglow" of the BB. This radiation seems to be very uniform and isotropic throughout the Universe. The vast majority of all photons found in the present Universe are tied up in the background radiation. However, despite their huge numbers, it is estimated that they comprise only about 1/50000th of the mass contained in all the galaxies. The present ~3° K value is consistent with a predictive model that requires very energetic high temperature radiation (mainly gamma rays, with much shorter wavelengths) that constituted the early CMB released soon after the Big Bang to cool drastically by adiabatic (no energy added or removed) thermodynamic expansion (a good Earth analog: expansion of an air mass is accompanied by release of heat with resultant cooling) within a Universe having at the least the presently observed spatial limits. Mechanistically, as space is stretched the original shorter wavelength photons experience a corresponding lengthening of their wavelengths into the microwave region and so lose energy (E = hc/λ) which in turn is expressed as a much lower temperature.
The extraction of a weak radio telescope signal (after receiver noise was subtracted) in the microwave region at 7.3 cm (4.1 GHz) was made in 1965 by R. Wilson and A. Penzias (for which they received the Nobel Prize in Physics; their discovery was somewhat accidental at first since they were trying to track down what they perceived as "noise" in their radio telescope). (Actually, a similar signal was first detected in 1961 by E. Ohm, then verified by B.Burke, but not connected to the CBR prediction.), with its correlation to cosmic background radiation. Correlation of Wilson and Penzias' discovery and its implication for the predicted background radiation was then confirmed by R. Dicke and his group at Princeton.
This accomplishment, along with the work by Hubble, the theory of General Relativity by Einstein, the pioneering concepts of a primordial singularity by Lemaitre, the Inflationary Model by Guth, and supporting contributions by numerous cosmologists, astronomers, physicists, and mathematicians, taken together, make up the foundation concepts that support and explain the Big Bang in its present form. Further discoveries will likely lead to refinements but the fundamental premises and the proper numbers predicted from the general model now seem to be solidly substantiated.
The value of satellites in understanding CBR is well illustrated by COBE (Cosmic Background Explorer), launched in 1987 (check out its current COBE website. Earlier attempts by Smoot and others to map the apparent non-variant (uniform) background radiation over the entire sky using balloons and aircraft, to make measurements above the atmosphere which blocks out (absorbs) radiation in the .001 to 0.1 m wavelength region of the spectrum, gave strong hints of radiation uniformity but were subject to imprecision. With COBE, the mapping process was greatly improved so that a detailed chart covering the full sky was assembled in just a year. COBE verified the high degree of uniformity of the present background in all directions and also confirmed that the general expansion is extremely uniform in all directions.
COBE took extremely accurate readings over much of the wavelengths involved in constructing a blackbody radiation curve. These measurements were then combined with those covering other wavelengths and obtained by different means to produce this classic blackbody radiation curve (see page 9-2 for a review of blackbody radiation) in which the COBE values were so accurate that error bars could be omitted. (When the COBE curve was first displayed to participants at an Astronomy conference, the audience was moved to give a standing ovation; such an extraordinary curve with all points precisely on the best fit version is the dream of all experimental scientists.) When compared with curves determined experimentally for blackbodies of different temperatures, the best fit was to a 2.726° K body; demonstrating that the CBR radiation fits that curve at better than 99% accuracy (an astounding achievement seldom attained in most scientific measurements).
A variant of this includes measurements made by other CMR measuring experiments (different systems).
The two plots differ because of different brightness and frequency units and log values are used in the second diagram.
COBE allowed the mapping of radiation in the early stages of the Universe, (specifically, at the close of the Radiation Era some 300,000 [perhaps to 500,000] years after the Big Bang, when the plasma in the expanding Universe had cooled sufficiently to become transparent to photons) to an accuracy such that it showed variations in temperature and density as slight as 1 part in 10000 during the first billion years after time zero. Said another way, COBE proved the residual radiation after the Big Bang was smooth to within a fluctuation of 0.01 percent. (Had it been notably rougher, such irregularities would have forced the Universe either to collapse on itself or develop mostly black holes instead of stars.) It also established a range of +/- 30 microKelvins as the range of differences around the average CMR temperature; these irregularities are of the order of 1 part in 100000. The maps below show the broad distribution of these minute temperature differences (ripples) across the early Universe as detected by COBE's DMR (Differential Microwave Radiometer) using data collected at 53 and 90 GHz. The blues represent slightly cooler and reds slightly warmer temperatures - thus also define regions of greater and lesser densities.
The top map is the "raw" data plot in which the dipole effect caused by the Doppler motion of the Milky Way galaxy has not been removed. The middle map results when the dipole effect is eliminated, but the radiation from the Milky Way (central band) has not been compensated for. The bottom map is the final product with both dipole and galaxy effects removed - this is the one usually cited as the model for CMB distribution. Another such plot, using different colors, recasts the distribution in terms of the northern and southern hemispheres of the celestial sphere:
These small differences were, however, vital in allowing matter to break from the initial extreme uniformity into regions of slightly cooler, denser conditions where the protogalaxies could begin to form. Eventually, in the early Universe these seed fluctuations promoted localized clotting of particles that became gravitational centers whose growing attraction of more matter led ultimately to development of the billions of galaxies that populate the Cosmos as we now know it.
COBE has allowed an estimate of the total energy in the Universe by sampling yet another part of the spectrum. This results from painstaking analysis of radiation in the far Infrared using the Diffuse Infrared Background Experiment instrument onboard. This measures heating of the dust distributed throughout the Universe, using windows at 140 and 240 µm. However, the overall background is "contaminated" by dust and other sources within and around the Milky Way, the Earth's atmosphere, and other sources, which require correction. The procedure is indicated in this figure:
The upper panel shows a sky map of the Infrared radiation for the whole Universe with a bright central band representing the Milky Way contribution. The central projection is the change after Zodiacal light is removed. The bottom panel is the residual Infrared radiation for the Universe after the Milky Way Galaxy's influence has been removed. The net effect is that there is much more starlight in the Universe as "fossil radiation" than heretofore suspected owing to the masking by dust (ranging from near-Earth to intergalactic) whose influence is now accounted for with this corrective DIRBE inventory.
In April, 2000 a group of scientists presented the results of project BOOMERANG (acronym for Balloon Observations of Multimetric Extragalactic Radiation and Geophysics) One output was a more detailed map of 3% of the sky which shows variations (with a 35x improvement in resolution) in CBR at the end of the Radiation Era - which also signals the beginning of the Decoupling Era marked by the recombination of protons and electrons to form Hydrogen atoms. This map was constructed by measurements obtained with a passive microwave telescope suspended on a balloon for 11 days at approximately 36400 meters (120,000 ft) above the Earth's atmosphere over the Antarctic. The variations depicted are in units of microKelvins.
Here are several more maps from this experiment using radiation detected at different wavelengths. The upper and lower left maps are at 90 and 150 MHz respectively; the two right maps are differences between 90 - 150 (top) and 150 - 240 (bottom) MHz.
The Boomerang scientists envisioned the early Universe to be full of 'sound waves' compressing and rarefying matter and light, producing 'acoustical peaks'. They used this model to calculate the distribution of Dark Matter (30%) and Dark Energy (65%), with the remainder about 4.5% Ordinary Matter, within the components of the physical Universe.
COBE and Boomerang results are confirmed, with more detail, by the CBI (Cosmic Background Interferometry) experiment run jointly by CalTech and the NSF. The CBI is located in dry air in Chile's Atacama desert, at an altitude 0f 5080 m (16.700 ft). It started data collection in 1999. Here is an onsite photo of this sensitive instrument:
Thirteen 1 meter diameter dish antennae are synchronized in an array with a broad frequency baseline from 26 to 36 GHz. Each dish receives a different wavelength signal, and interferometry is used to integrate the data from which a power spectrum is produced, as shown by the solid line curve, with values from other CMR instruments also shown:
This next figure is a map of the background radiation over an area equivalent to about 2.2 degrees in declination units (2 widths of a full Moon). The differences being measured are temperature values in microKelvins (µK) that vary around the mean sky temperature of 2.73.. °K.
What is being sensed are small temperature differences (range of ~100 µK) when the CBR was around 3000° K. The yellows indicate slightly hotter regions compared with cooler reds and blacks. Associated with these differences are variations in material density - the hotter regions have higher densities, indicating matter has already begun collecting and interacting to generate heat (possible indication that early stars had formed). This observation supports the idea that matter in the Universe at this early time was unevenly distributed, thus pointing to the first stages of (increasing) density/gravity variations required to initiate the process by which galaxic clusters form.
The results from COBE proved of such import to understanding the early Universe, especially the small but vital fluctuations it detected, that a more sophisticated satellite, WMAP (Wilkinson Microwave Anisotropy Probe), was launched in July of 2001. Background information on this important new astronomical observatory can be found at NASA Goddard's WMAP site.