THE VISIBLE UNIVERSE
According to modern theory, there is an observational limit to the universe, which represents the boundary of the visible universe. When scientists began to study distant galaxies during the first half of the twentieth century, they made a remarkable discovery: the light emitted by a galaxy is shifted progressively toward the red end of the electromagnetic spectrum as a function of its distance from the earth.
This is known as the cosmological red shift.At first, this phenomenon had no explanation. One difficulty it presents is that the cosmological red shift is more or less spherically symmetrical. The same red shift phenomenon is observed regardless of the direction in space in which a distant galaxy might lie from the earth. The most widely accepted explanation of this phenomenon was proposed initially by Edwin Hubble, whose name is memorialized in the famous Hubble telescope.
He suggested that the red shift phenomenon is due to a luminous Doppler shift that is comparable to an acoustic Doppler shift, which is part of our common experience and can be characterized by the sound of a train whistle when it passes a stationary observer. Anyone who has experienced this phenomenon has noticed that the sound of the whistle changes as the train passes: As the train approaches, the whistle is higher pitched than after it passes. The seeming change in sound is due to the motion of the train, which serves as the sound’s source.
Generally, we can say that any sonic object that moves toward an observer will emit sonic wavelengths that are shorter than those emitted when the object moves away from the observer. In this case, the pitch of the sound is perceived as higher when the object is approaching and lower when the object is receding. The degree to which the wavelengths are shortened or lengthened is related directly to the speed of the moving object as compared to the speed of the sound waves emitted by that object.
Hubble proposed that the cosmological red shift is due to a similar “luminous” Doppler shift, which, in this case, has to do with the light waves emitted by distant galaxies. According to this explanation, these galaxies must be moving away from Earth at a speed that increases with increasing distance, so that emitted wavelengths are lengthened increasingly, or shifted toward the red end of the spectrum, as a function of distance.
Once this proposal was made, theorists were able to predict the recession speeds for different galaxies. In doing so, they found that some of the distant galaxies have recession speeds that are an appreciable fraction of the speed of light. They then calculated the distance at which the recession speed would actually equal the speed of light. This distance scale, estimated to be roughly 1028 centimeters, became known as the Hubble radius, or the radius of the visible universe. It represents the radius of that spherical portion of the universe visible from earth. Any galaxy that might lie on the surface or outside the Hubble sphere will not be observable from the earth, even in principle, because its galactic recession speed will either equal or exceed the speed of light. This means that light emitted by such galaxies will never reach Earth.
Whether or not galaxies exist beyond the Hubble radius is still a matter of dispute. Some theorists have proposed that the universe is actually infinite, and that the Hubble radius merely determines the finite portion of the universe visible from Earth. Others have proposed that the Hubble radius marks the boundary of the created universe itself—nothing whatsoever exists outside the Hubble sphere, not even time or space. In actuality, Hubble’s explanation of the cosmological red shift is not the only possible explanation; it is simply the most popular. Proposed are a number of complementary explanations that do not require any form of galactic recession and which account for the red shift phenomenon using mechanisms other than a luminous Doppler shift.
No matter which theoretical model is used to explain it, however, the red shift phenomenon itself is not in dispute. It is an empirical fact confirmed by thousands of astrophysical observations. This summarizes our modern scientific understanding of what is meant by the visible universe. Next, we must examine the ancient wisdom concerning the visible universe: the universal wisdom.