It is probably not an overstatement to assert that anyone who does not adequately understand the phenomena of redshifted light in space, emanating from luminous celestial objects and received by observers on Earth, cannot really understand cosmology. Why? Because, depending upon how such redshifts observed from galaxies are interpreted, the universe is either 1) an expanding finite sphere which possibly originated from a microscopic Big Bang; or 2) it is infinite in space and time and has no geometrical shape or systematic motion on a cosmic scale.
To paraphrase Professor Herbert Dingle (President of the Royal Astronomical Society from 1951-53), without the galactic redshift phenomena “Cosmology would scarcely exist as a scientific subject.” In other words, the entire subject of current cosmology is primarily premised on the perception and interpretation of galactic redshifts…and the deductions and extrapolations based thereon.
The remaining discussion is largely devoted to what causes a galactic redshift phenomenon, the fascinating and fantastic deductions that have followed from various interpretations of galactic redshifts, and the validity or invalidity of these speculations. The little-known issues, discussions, and commentary along the way should prove to be thought provoking and insightful, as well as controversial.
What is not controversial, however, is the objective description of wavelengths of light (Figure 1) and the redshifted light itself which we observe on Earth. Light received on Earth from any luminous object in space (i.e., a star) through a telescope “is a composite of many individual colors or wavelengths.” When this composite beam passes through a glass prism, the individual light rays are bent (refracted) and their colors are separated out (according to their individual wavelengths) in an ordered and visible rainbow-like sequence. From the longer wavelengths associated with the red end of the rainbow-like spectrum, the wavelengths of light progressively diminish in length to the shorter wavelengths associated with the blue and violet end of the rainbow-like spectrum (Figure 2). Hence, the color of a “position in the [rainbow-like] spectrum indicates the wavelength of [its] light.” This spectrum of light can then be analyzed on Earth by a sophisticated optical instrument known as a spectroscope.
In the Earth laboratory, when detected by a spectroscope, each atomic element which is heated to incandescence has its own unique signature of colored lines that are part of a normal rainbow-like spectrum. For example, in the Earth laboratory the element calcium is identified by two violet colored spectral lines (called “H” & “K” lines) in the violet section of the normal rainbow-like spectrum (Figure 2A). This spectrum is sometimes called an “emission spectrum.”
On the other hand, in local space when calcium is vaporized by the intense heat of the Sun or another star, the two violet (H&K) lines of calcium are absorbed by the hot incandescent atmosphere of the star. Therefore, they are not received through the telescope on Earth as thin violet lines in the rainbow-like spectrum received from such star. In their place are seen two dark lines called “absorption lines,” which indicate the presence of vaporized calcium in the star’s incandescent atmosphere. In a like manner, the star’s spectrum will also show many other patterns of dark absorption lines which indicate that other atomic elements are also vaporized and absorbed in the atmosphere of the star. When all of these dark absorption lines are superimposed upon the star’s rainbow-like spectrum of light the resulting total pattern of dark absorption lines is called an “absorption spectrum.”
Thus, every luminous body in space has two types of spectra: a continuous rainbow-like spectrum and an absorption spectrum of dark absorption lines superimposed upon it. For instance, “…[T]he [Sun’s] solar spectrum is a continuous [rainbow-like] spectrum on which is superimposed a pattern of dark…lines.”
In the local space of the Milky Way galaxy, the absorption spectrum of each luminous body (i.e. a star) usually remains superimposed at or close to its normal location on its associated rainbow-like spectrum (Figure 3A). But sometimes the entire absorption spectrum of a star in local space can be observed to be displacing or shifting somewhat toward the red end or blue end of its associated rainbow-like spectrum. If the entire absorption spectrum abnormally shifts toward the red end, this “light shift” phenomenon is called a “redshift” (Figure 3B). If it abnormally shifts toward the blue end, this is called a “blueshift.” (Figure 3C).
A redshift received from a luminous body located within the local space of the Milky Way galaxy, always indicates that the light waves emitted by such body are being received on Earth less frequently (per second) than normal. Thus, the wavelengths received form such body are said to have a lower frequency than normal light waves (Figure 3B). The term “magnitude of redshift” refers to the measurable distance that a superimposed absorption spectrum abnormally shifts toward the red end of a normal rainbow-like spectrum (compare Figure 3A and Figure 3B).
On the other hand, a blueshift received from a body located within the local space of the Milky Way galaxy indicates that the wave lengths emitted by such body are being received on Earth more frequently (per second) than normal. Thus, the wave lengths received from such body are said to have a higher frequency than normal light waves (Figure 3A). The term “magnitude of blueshift” refers to the measurable distance that a superimposed absorption spectrum abnormally shifts toward the blue end of the normal rainbow-like spectrum (compare Figure 3A and Figure 3C).
What causes these abnormal shifts of an absorption spectrum? In other words, what do they indicate? Light shifts received from a body located within the local space of the Milky Way galaxy, basically only indicates one thing. They only indicate the total line-of-sight (or linear) relative velocity between a co-moving light source (i.e., a star) and a co-moving observer (i.e., on Earth). Any magnitude of redshift indicates a relative velocity of mutual separation, and any magnitude of blueshift indicates a relative velocity of mutual approach. These optical effects are called the “Doppler velocity effects of light.”
What about light shifts received from other galaxies? Other galaxies are also stellar systems and (like the Milky Way galaxy) are dominated by relatively small stars like our Sun. Thus the “spectra [of other galaxies] resemble that of the Sun.” But, as Edwin Hubble also pointed out:
“Although the spectra of the sun and of the nebulae [galaxies] exhibit the same pattern of absorption lines, there is one remarkable difference. The lines in the nebular [galaxy’s] spectra, in general, are not in their normal positions; they are displaced towards the red end of the spectrum to positions representing wave lengths somewhat longer than normal. The entire pattern of absorption lines, all details in a [galactic] spectrum, appear to have been shifted towards the red.”
Hubble also pointed out that such galactic redshifts are generally of much higher magnitude than those observed from local stars, and (most importantly) that such observed galactic redshift magnitudes are generally proportional to the estimated distances of their source galaxies from the Earth.
In other words, the basic differences between light shifts received from local stars in our Milky Way galaxy and light shifts received from distant galaxies, is that the light shifts from distant galaxies: 1) are almost all redshifts (rather than an even mix of blueshifts and redshifts); 2) they are generally of much higher magnitude than light shifts observed from local stars; and most importantly 3) their magnitudes are generally proportional to the estimated distances of their source galaxies from Earth.
It is only when we consider light shifts observed from other distant galaxies (i.e. galactic redshifts) that interpretations of what causes them and what they indicate becomes controversial. Since almost all light shifts observed from other galaxies are redshifts, rather than blueshifts, we shall confine most of our remaining discussions to galactic red shifts.
The controversy concerning galactic redshifts boils down to just two possible interpretations. Both Arthur Eddington and Edwin Hubble agreed on this point. In Hubble’s and Eddington’s words:
“If red-shifts are produced in the nebulae [galaxies], where the light originates, they are probably the familiar [Doppler] velocity-shifts, and they measure an expansion of the universe. [On the other hand], if the nebulae are not rapidly receding, red-shifts are probably introduced between the nebulae and the observer [and just measure distance].”
“If the loss [of energy] occurs during the passage of the light from the nebula [galaxy] to the observer, we should expect [the magnitude of redshift] to be proportional to the distance; thus the red-shift, misinterpreted as a velocity, should be proportional to the distance.”
In other words, the “64 trillion-dollar question” is: 1) Do galactic redshifts indicate relative (Doppler) velocity, i.e. that the galaxies are rapidly receding from the Earth, and thus that the universe is expanding? Or 2) do they just indicate a loss of light energy during the vast distances that light must travel from distant galaxies to the Earth? If galactic redshifts just indicate the loss of light energy during the vast distances which light travels from a distant galaxy, then they do not indicate an expansion of the universe.