In the beginning…

First Light in the Universe: the discovery and subsequent and observations of the Cosmological Microwave Background radiation and what it tells us about cosmology.

I grew up a cradle Catholic and recall church doctrine about the creation of the universe. What struck me the most later in life, is our many scared texts around the world spoke about a great light at the beginning of time. After studying physics and astronomy, I came to see how close science comes to these religious ideas in describing the beginning of time. The last forty years has seen a revolution in our understanding of the structure and formation of the universe, driven by studies of Cosmic Microwave Background (CMB).  These new observations have given support to the hot Big Bang theory and helped astronomers in understanding the age, geometry, and make-up of the universe.  

The theoretical prediction of the CMB

We can confidently trace the origins of the universe using Einstein’s general relativity. The Belgian priest and astronomer Georges Lemaître and the Russian mathematician Alexander Friedmann independently discovered an important class of solutions to the general relativity field equations that modeled space-time (Tropp 1993). Lemaître applied these solutions to describe cosmic expansion and conceived the hypothesis of the “primeval atom”, which is the term he used to describe universe’s creation (Lemaître 1927). Lemaître was also the first to propose that a fossil radiation could now be detected in the form of cosmic rays left over from the super-dense cosmic quantum at the origin of the universe (Lemaître 1931).

The ideas of Lemaître, Einstein, and other early cosmologists play an essential role in the interpretation of the faint after glow of the Big Bang – the CMB presently seen by sensitive microwave antennae that is the “fossil” relic from the hot past at a time of only 3×10⁵ years after the beginning (Guth 1997).  Lemaître’s theory of relic cosmic rays did not attract any support.  As early as 1938, the notion that cosmic rays could be remnants of a primordial nucleus was undermined by research of Arthur Compton and Carl Anderson (Farrel 2005).   George Gamow (1948) later suggested the universe originated in a hot state and should have left an after glow of microwave radiation.  The temperature of this radiation would decline inversely to the factor by which the expansion had increased (T ~ 1/T) (Alpher, Bethe, and Gamow 1948). Alpher and Herman (1949) predicted that the radiation would now look like a thermal source at a temperature of  <5 K. Doroshkevich and I. D. Novikov (1964) wrote the first published recognition of the relic radiation as a detectable microwave phenomenon predicted a peak temperature of 3 K.

Discovery and subsequent detailed observations of the CMB.

In the 1960s, Arno Penzias and Robert Wilson (1965) were making all-sky measurements of radio noise using the Bell Labs 20-ft horn antenna (Figure 1).  While unaware of previous predictions of this relic microwave radiation, they detected a persistent microwave signal which they originally attributed to interference from the antenna or its radio amplifiers. They suppressed interference from the heat in the receiver itself by cooling it with liquid helium to −269 °C, only 4 °C above absolute zero. After reducing data they found a low, steady, mysterious noise that persisted in their receiver. This residual noise at a wavelength of 7.35 cm was evenly spread over the sky, and was present day and night (Penzias and Wilson 1965).

Prior to the Penzias and Wilson discovery, others had observed the CMB yet they did not attribute it to its ultimate source. In the 1950s, Emile La Roix and Tigran Shmaonov independently detected the CMB radiation during each of their radio all-sky surveys, but both dismissed their findings as noise introduced by minor defects in their instruments (Singh 2004). In the end, it was Penzias’ and Wilson’s persistence and determination that led to the discovery of the CMB radiation.  The uniformity they observed confirmed the approach of early cosmologists, which defines a homogenous and isotropic nature of the universe (Glendenning 2004). The high degree of uniformity of the CMB coming from all directions of the sky is persuasive evidence that it exists through the universe and is a relic from a time long before there were galaxies and stars.

In the same year as Penzias and Wilson’s discovery, Robert Dicke and Jim Peebles were preparing a specialized microwave antenna for the specific purpose of searching for the CMB radiation. Penzias and Wilson were unaware as to the significance of their discovery until they contacted Dicke. The two teams agreed to publish the findings and the interpretation as companion papers 1965 (Penzias and Wilson 1995; Dicke, el. al. 1965).  Penzias and Wilson were awarded the Noble Prize for the discovery in 1978. 

Other studies of the CMB soon commenced.  By the 1970s, the latest detectors were sensitive enough to detect potential heterogeneities in the CMB down to 1 part in 100. But detecting these fluctuations is difficult from ground-based receivers because the atmosphere absorbs microwave radiation at frequencies very close to the theoretical maximum of the CMB.  Measurements from high altitude balloons and satellites were needed to add more data points to the CMB blackbody curve. Astronomers attempted to study the CMB with large, helium-filled, high-altitude balloons. Dirk Muehler and Rainer Weiss (1973) made measurements of the cosmic background spectrum just above the peak using a liquid-helium-cooled detector carried 15 km aloft by a military reconnaissance balloon. In 1974, a balloon-borne broadband measurement extended well beyond the peak, but the observations strongly suggested that there was excess data.  Despite estimated uncertainties in the data collected from these early measurements of the CMB radiation, they pioneered new experimental techniques that would prove valuable in later years.  While they were inclusive, they at least showed that the spectrum was not too far from a blackbody (Figure 2).  

Early rocket and balloon-borne detectors proved problematic. The harsh environment of near-space altitudes caused detectors to fall apart and the balloons stayed aloft for only a few hours (Singh 2004).  Like others, George Smoot began to look for an alternative to balloon-borne detectors.  In 1976, Smoot and his team flew a twin-antenna Dicke radiometer to an altitude of 20 km aboard a U-2 aircraft. They found the radiation coming from one end of the sky had a wavelength that was 1 part in 1,000 different from the opposite half of the sky.  While this was not the anisotropy they were seeking, it led to the first measurement of the Solar System relative to the universe. This large scale anisotropy is attributed to motion of the earth relative to the radiation with a velocity of 390 plus or minus 60 km/sec. (Smoot, et. al. 1977).

The anisotropic variations Smoot’s team was searching for were thought to have led to the formation of galaxies in the early universe.  These anisotropies were predicted to have been very irregular and spread throughout the entire sky. Earlier observations found that the CMB had an extremely uniform temperature, excluding the distortion caused by the Doppler Effect resulting from the Earth’s own movement within the universe.  This result contradicted observations of large scale structures, such as clusters of galaxies, which indicate the universe was relatively heterogeneous on the small scale when the CMB was released. If the universe was heterogeneous at the time of the emission of the CMB, then they would be observable today through weak variations in the temperature of the CMB.

Even while Smoot had been working on the U-2 missions, he suspected that a satellite-based detector might be the only way to detect these anisotropic variations. NASA unified separate proposals from Smoot’s team and the Jet Propulsion Laboratory (JPL) and funded the development of the Cosmic Background Explorer (COBE). Launched in 1989, COBE carried the Far InfraRed Absolute Spectrophotometer (FIRAS), designed to measure extremely small deviations (0.1%) of the CMB radiation from a blackbody spectrum (Mather, et. al. 1990). The COBE team released the first spectrum of the CMB radiation measured from a satellite (Figure 3) in 1990. The size of the error boxes indicated estimated uncertainty of less than 1% (Mather, et. al. 1990).  COBE’s measurements found the CMB to be characterized by a thermal source of 2.726+0.005 K, which is very close to the predicted value (Smoot, et. al. 1992).  COBE’s satellite detection of large-scale CMB in 1992 brought the most compelling evidence for the hot Big Bang theory and put an end to the many competing ideas on the creation of the universe which had been circulating at the time.

In 1992, Smoot’s team discovered the long-sought variations in the early universe. They used COBE to map the intensity of the CMB radiation with an angular resolution of 7^o and found small density fluctuations that match those predicted for the growth of galaxies and clusters of galaxies that are observed today.  The variations are extremely small, only 1 part in 10⁵, but they provide an insight into how galaxies grew out of variations in the density of matter of the early universe (Smoot, et. al. 1992). These density fluctuations confirmed predictions by the hot Big Band theory.

COBE’s confirmation of small areas of anisotropies in the CMB radiation led to a new area of study in cosmology, requiring the observation of this diffuse radiation by using more sensitive detectors. The Wilkerson Microwave Anisotropy Probe (WMAP 2008), launched in 2000, measures the CMB with much greater precision than COBE. The WMAP mission measured the relative CMB temperature over the full sky with an angular resolution of at least 0.3° (Figure 3) and provided the first high-resolution observations of the CMB anisotropies (WMAP website 2008).

WMAP provides strong observational evidence for the inflationary cosmology model. CMB polarization results provide experimental confirmation of cosmic inflation favoring the simplest versions of the theory (WMAP website). WMAP data also indicates the large scale geometry of the Universe is flat; that is, it obeys Euclidean geometry with light traveling along straight lines and the angular sizes of objects are inverse proportional to their distances.  Previous the WMAP, the BOOMERANG detector flown in 2000 aboard a high-altitude balloon above Antarctica mapped the CMB with 40 times more resolution than COBE.  It detected small CMB temperature anisotropies of 0.75” across, a value consistent for a flat universe (COCG website 2005).   The WMAP Team also observed angular diameters of typical anisotropies that are predicted by flat space (Hinshaw, et. al. 2008). The overall flatness of the universe means that the average total density (matter and energy) is equal to the critical density. In other words, the ratio of the average density to the critical density, Ω(total) is 1.0 in a flat universe.  Previous measurements had suggested the matter density of the universe, Ωm, is only about 0.3 (Hinshaw, et. al. 2008). The remaining density (Ω_Λ=0.7)_, can not be uniformly distributed gravitating matter (baryonic matter) because it would have adversely affected the growth of large-scale structures, such as galaxy clusters from the initial variations in the density of the universe (Singh 2004). This remaining density parameter, approximately 70% of the composition of the universe, is likely to be the repulsive dark energy previously suggested by Type Ia Supernovae observations (Goldhaber and Perlmutter 1998).

The WMAP also refined our estimate for the age of the universe. WMAP gives a Hubble Constant H₀ = 70.1 ± 1.3km/s /Mpc, very similar to that estimate by the Key Project team (Freedman 2001).  This puts the age of the Universe at 13.73 ± 0.12 billion years old (Hinshaw et. al. 2008).

Conclusion:

The cosmicmicrowave background radiation (CMB), fills the entire universe with a thermal black body spectrum at a temperature of 2.725 K. Most cosmologists consider this radiation to be the best evidence for the hot Big Bang model of the universe.  It was predicted in 1948 by Gamow, Alpher, and Herman and first detected by Penzias and Wilson in 1965.  Since first detecting the CMB, we have conducted hundreds CMB studies to measure and characterize the signatures of the radiation. Recent observations by orbiting detectors, such as COBE and WMAP, have given support to the Big Bang theory and helped astronomers in developing a standard model of cosmology in which the universe consists of baryonic matter, dark matter, and dark energy.  CMB observations allow us to accurately estimate the age and geometry of the universe, the motion of the earth relative to the universe and define key cosmological constraints.  

References

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