Maybe the “bubble” idea was not a good way of explaining it…let’s talk some numbers.

The sun is radiating at some 4 x 10^26 watts (www.astronomynotes.com/starsun/s3.htm). There are about 100 billion suns in our galaxy, but most are not as big and bright as the sun. Let me make a very conservative estimate, and assume that there are “only” 1 billion sun-equivalent stars in the galaxy (if someone has a better number, please supply it!). Then the galaxy’s total power output is 4 x 10^35 watts. That is a lot of energy. Where is all that energy going? It is going “into space.” That is, the energy output of visible matter is powering the expansion.

Since all the numbers become “astronomical,” it is better to do the calculation on a per-kilogram basis. Our sun has a mass of roughly 2 x 10^30 kg (http://www.enchantedlearning.com/sub.../sunsize.shtml)
. So its energy output per kilogram is [4 x 10^26 watts]/[2 x 10^30 kg] = 2 x 10^-4 watts/kg, or 200 microwatts/kg. While the sun may be “typical,” it is not representative. For every kilogram of matter in a sun-like star radiating away energy, there may be 1,000 kg just sitting there, doing nothing. If we add in “dark matter” (questionable, but just to play it conservative), there is an additional factor of about 10 times as much non-radiant mass. So to get the power output per kilogram of the galaxy, I’ll divide the sun’s output by 10,000. Thus, the power output of the galaxy is estimated at 2 x 10^-8 watts/kg or 20 nanowatts/kg. Again, this is a very crude “guestimate,” and I welcome better numbers, but this will suffice for here and now. Finally, since I converted the expansion to an annual basis (#20), I’ll do the same for the energy output, so the estimate for the galaxy’s annual output is: [2 x 10^-8 watts/kg]*[3.15 x 10^7 sec/yr] = 0.63 joules/kg/yr.

Since j/kg/yr is not very intuitively helpful, we’ll change it to more familiar units. Star by converting to foot-pounds of energy per kg/yr: [0.63 j/kg/yr] * [0.73 ft-lb/joule] = 0.46 ft-lb/kg/yr. Now we get rid of the pounds & kilograms: [0.46 ft-lb/kg/yr] * [0.45 kg/lb] = 0.21 ft/yr, or about 2.5"/yr. In other words, the galaxy is radiating energy “as if” every bit of matter within it is “falling” 2.5 inches per year in earth’s gravitational field.

So there is one side of the equation: a rough estimate of the energy output of visible matter is 0.63 j/kg/yr, or -2.5 inches per year at the surface gravity of the earth.