The Evidence for Big Bang

In 1929, Edwin Hubble presented the evidence for the expansion of the universe. He observed that the distances to far away galaxies were strongly correlated with their redshifts—an idea originally suggested by Lemaître in 1927 6. Hubble observed that nearly all galaxies and clusters are moving away from us at a velocity ν proportional to their distances d from the Earth, so $\nu=H_0 d$ where $H_0$ is the Hubble’s constant that determines the rate of expansion 7.

Assuming that we are not at the center of the Big Bang explosion, the only possible interpretation is that all observable regions of the universe are receding from each other. The further they are from us, the faster they are moving away. Known as Hubble’s law, the relationship between the distance to a galaxy and its recessional velocity is shown in the Figure 2.

 The term redshift is used by Astronomers to describe how far away a stellar object really is. Astronomical distances can be measured by examining the absorption or emission lines in the spectrum produced by the star or galaxy. The spectrum is unique for each atomic element and the absorption or emission lines always have the same spacing. When an object in space moves toward or away from us, the absorption or emission lines will be found at different wavelengths than where they would be if the object was not moving (relative to us). Light is blueshifted when an object in space moves toward us as its light waves are compressed into higher frequencies or shorter wavelengths, and redshifted when it moves away from us as its light waves are stretched into lower frequencies or longer wavelengths. The light from most objects in the universe is redshifted as seen from the earth. Only a few local objects like planets and some nearby stars, are blueshifted. This shows our universe is expanding.

If the galaxies are moving apart, they must have been closer together in the past and from the present rate of expansion one could estimate they were almost on top of each other 10 to 15 billion years ago.

 Figure 2. Recent observations of the red-shift of galaxies. Picture credit: The universe in a nutshell by Stephen Hawking.

Hubble’s discovery was the first observational support of the Big Bang theory. The observed velocities of distant galaxies appeared to show that the universe was expanding in a manner consistent with the Friedmann-Lemaître model. Reversing the expansion of the universe backwards in time using general relativity implied a universe starting as an infinitesimally small region with infinite density and temperature at a finite time in the past. This event is called the Big Bang, but the term can also refer to the early hot, dense phase during the birth of our universe.

In 1948, George Gamow and his colleagues theoretically predicted that the radiation from this very hot stage of the Big Bang should still be around today. They calculated the temperature necessary for the early universe to yield the observed abundance of He. Ralph Alpher and Robert Herman expanded on the earlier work and predicted that if Big Bang occurred, there would be residual background radiation from the hot, dense days of the early universe. They calculated the temperature would currently be about 5°K some 14 billion years after its creation.

 Penzias and Wilson initially had no idea what they had detected. But they realized the significance of their accidental discovery when a friend (Professor Bernard F. Burke of MIT) directed Penzias to the theoretical work of James Peebles in Robert Dicke’s group in Princeton on the possibility of finding radiation left over from Big Bang. Penzias contacted Robert Dicke who sent a copy of Peebles’ preprint. It was Dicke who correctly interpreted that Penzias and Wilson’s signal could be the CMB as predicted by some cosmological theories. In Wilson’s own words: “Shortly after sending the preprint, Dicke and his coworkers visited us in order to discuss our measurements and see our equipment. They were quickly convinced of the accuracy of our measurements… We agreed to a side-by-side publication of two letters in the Astrophysical Journal – a letter on the theory from Princeton and one on our measurement of excess antenna temperature from Bell Laboratories. Arno and I were careful to exclude any discussion of the cosmological theory of the origin of background radiation from our letter because we had not been involved in any of that work. We thought, furthermore, that our measurement was independent of the theory and might outlive it. We were pleased that the mysterious noise appearing in our antenna had an explanation of any kind, especially one with such significant cosmological implications”. — Nobel Lecture, 8 December, 1978 Many people believe it was a gross oversight of the Nobel committee that Dicke and Peebles’s contribution was not recognized along with Penzias and Wilson’s.

In 1964, Penzias and Wilson measured the remnant temperature of the universe at its current state to be about 3°K using radio telescopes. A very striking aspect of their observation was the fact that their signal appeared to be uniform in all directions. The high degree of isotropy indicated that the signal originated far beyond our Milky Way galaxy, or equivalently, very early in time. The signal source had to be enormously powerful to be detectable now. With help from Robert Dicke and James Peebles of Princeton University, Penzias and Wilson inferred that this radiation is the afterglow of the Big Bang that Gamow and his group predicted.

Nevertheless, one still needs an enormous leap of faith to believe that Penzias and Wilson indeed stumbled on to the primeval afterglow since they managed to observe only one small part of the spectrum of radiation after all. Interestingly, the evidence lay in the phenomenon called blackbody radiation 8 whose spectrum and characteristics were very well known following Max Planck’s famous derivation of its formula in 1900. According to the Big Bang model, the early universe was very hot and packed with particles and light (photons). The particles constantly collided with the photons, absorbing and re-emitting them. It was realized that CMB should resemble a blackbody spectrum, and the spectrum’s characteristic shape should be preserved while light traveled through the expanding space.

 Figure 3. Data from COBE showed a perfect fit between the black body curve predicted by Big Bang theory and that observed in CMB.

In 1991 the COBE (Cosmic Background Explorer) satellite made a precise measurement of the background radiation from Earth’s orbit. The results showed that the CMB spectrum (Figure 3) agrees with a nearly perfect blackbody with a temperature of $2.725 \pm 0.002$°K. The plot shows thirty-four data points along with the best-fit blackbody spectrum. The data fit the blackbody spectrum so perfectly that the theoretical blackbody curve hides the error-bars of the data points.

COBE’s blackbody data is considered the finest fit between theoretical and observed results in the history of astronomy.

6 Measurements of redshift of galaxies and its significance were understood before 1917 by James Edward Keeler (Lick and Allegheny), Vesto Melvin Slipher (Lowell), and William Wallace Campbell (Lick) at other observatories. In a classic paper presented to the American Astronomical Society (August 1914 meeting), Slipher showed out of 15 galaxies, 11 were clearly redshifted. He received a standing ovation.

7 The German astronomer, Carl Wilhelm Writz, was technically the first person to provide evidence of the expansion of the universe. He published his results in 1922, several years before Edwin Hubble’s landmark publication.

8 Radiation has blackbody spectrum when an object or system absorbs all radiation incident upon it and re-radiates energy but does not reflect it (hence blackbody) which is characteristic of this radiating system only and not dependent on the type of radiation which is incident upon it.