Section III: Space Expansion (Hubble and Dark Energy), from "Is Space the Only Substance in the Universe?"

III.                   MORE SPACE TO EXPAND OUR THINKING

The Hubble Expansion

In 1929, Edwin Hubble made the dramatic discovery that the redshifts indicating increases of light wave lengths from 24 galaxies were proportional to distance. This implied that the universe was expanding, and that the galaxies were receding from us, with apparent velocities that increased the farther they were from us. These results were confirmed by further studies (Bahcall 2015). There was, and still is, no generally accepted law of physics to explain this phenomenon, and it is commonly attributed to a mysterious “dark energy.”

             The Hubble equation, using v for velocity of recession H₀ for the Hubble constant, and s for the existing distance to the receding galaxy (a physics notation tradition after Latin “spatium,” the source of our word space) is:

                                                                        v = H_0*s                                                           (1)

(Throughout this article, an asterisk * between letters or symbols will indicate multiplication, and will sometimes be inserted to clarify that these represent separate entities.)  

             The current value of the Hubble constant, measured by various methods, is estimated to be in the range of 72-74 km/second/megaparsec (Jackson, N.  2015), or about 22.1-22.7 km/second/million light years (one megaparsec being about 3.26 million light years). This is a very small amount of distance per second in proportion to the total current size of the “observable universe,” but there are so many millions of light years in the universe that over time it becomes significant.

Addition of New Space, and the Better of Two Possible Mechanisms for the Expansion

The apparent recession of the galaxies cannot be due to a familiar type of active motion. There is no known force, let alone a repulsive one, that increases with distance. Light being emitted today from the most distant galaxies in the “potentially observable universe” will never reach us because it can only travel at the speed of light, whereas those galaxies are now receding at a greater speed (Siegel 2018). The recession of galaxies is apparently some type of passive motion, which will be discussed below (see in Section IX, under the topic “Active and Passive Motion”).

             Two potential mechanisms could be responsible. The expansion of space is sometimes described as a “stretching” of existing space like a rubber band or balloon (Siegel 2021), and less commonly as adding more space to existing space. With either of those alternatives, the Hubble expansion should appear everywhere much as it does from earth, with the apparent speed of recession being proportional to distance.

             The observed red shift (wave lengths of light becoming longer) occurs as light travels through space. This would be consistent with either of the two potential explanations, the “stretching” of light waves, or the insertion of more space within the light waves, between the crests, so that the wave length increased. The “Nothing but Space” model explanation for redshift would be the latter.  New space units would be added, lengthening the waves in proportion to existing length.

             Of the two explanations, the addition of constant, discrete units of space would have the advantage of better explaining why the expansion is uniform throughout the universe. “Stretching” of space would seem to require a stretching force, acting everywhere the same, with the “stretching” having no limit and continually increasing in proportion to the existing amount “stretched.” No such force is known, and other stretchable objects such as springs become more resistant to further stretching, the more they are stretched. Thus the most practical explanation for galaxies receding from us at rates proportional to their distance, is an expansion of existing space. This means that each length in space is joined by an addition and becomes longer every second, so that at greater distances, the increases in distance from us per second are additive. 

Figure 1:  The addition of new space into each expanse of existing space would produce an effect similar to the Hubble expansion.

            In Figure 1, there are 5 expanses of existing space, each a set of 5 units represented by the lighter cubes, for 25 units of existing linear space. Each set represents a measure of distance, and the units are each three-dimensional, but we are only considering length in one dimension to measure distance. The black cubes represent one additional unit of space added to each of these sets of 5 per second. Let us therefore set an imaginary Hubble-like constant H_i  at 1/5 or 0.2 lengths/second, or one added length for every five per second (much greater of course than the actual H₀). The velocity of recession at the 25^th unit would be v= H_i*s= H_i*25=0.2*25 or 5 units per second. If the existing (pre-expansion) distance s were 4 times as long or 100 units and one new unit were still added per distance of 5, there would be 20 new units added to 100 existing units, for a recession velocity of 20 units/second at the 100^th unit. In other words, n times the starting distance would result in n times the recession rate, without changing H_i.  The velocity of recession would always proportional to distance. If new space were added in discrete, constant units like “volons,” it could pop up in between existing similar units, analogously to carbon dioxide bubbles in a carbonated soda or champagne.

             Siegel has noted that expansion does not seem to thin the energy density of space, and whatever potential energy causes the expansion does not seem to “stretch” or weaken, tending to cast doubt on the idea of “stretching” of existing space. However, he considers space as a “stage” rather than a substance, and says “It’s as though new space is getting created due to the Universe’s expansion,” rather than the other way around (Siegel 2021). One rather unusual suggested mechanism for the creation of new space is the deformation of bosons (Dil 2016).

            The occasional implications in physics literature of new space being added are usually not followed up by hypothesizing where this new space comes from, and what makes it appear. Other issues generally not addressed include its physical reality, what it does, and the potential impact of new space on thermodynamics and time.

            The Hubble equation (1) deals with linear distance from an observer anywhere in the “observable universe.” However, assuming as above that the “volon” units of space are three-dimensional, then not just distance but also the volume of the universe is increasing.