10. Reader Replies to "Why is the Sky Dark at Night?"

Age versus expansion

[ There was recently ] a very interesting debate over the theories why space is dark. Steady state vs expanding universe, relevance of redshift and the accuracy of textbooks. It was posted on digg.com. Enjoyed your tutorial!

Google groups:"Why is space dark?"Thanks for passing this along. I noticed many of the contributors dismiss redshift as a plausible explanation, arguing that gamma "rays" and other very short wavelength radiation would be shifted into the visible part of the spectrum, thus keeping things balanced. This argument is flawed, because wavelength issues aside, redshift causes an overall, substantial reduction in energy per unit of volume (because the volume is steadily increasing), even when all energy sources are accounted for.

The present temperature of the universe represents a balance between the energy provided by the stars in converting matter to energy, and the expansion of the universe, which causes that energy to occupy a steadily increasing volume over time.

One argument against the age of the universe as a factor is that at very long ranges (and time intervals), redshift values are so high as to prevent any significant contribution to the present temperature, and those ranges are less than the age of the universe. This means that, in an infinitely old universe that is otherwise like this one (quite a stretch, I know), there would be an "extinction range" or energy horizon, beyond which no significant radiation could pass, and that range is less than the age of this finite-age universe.

Indeed, evidence for the central role played by redshift can be found in the remnant radiation from the "Big Bang", an extraordinary outpouring of energy unmatched by any more recent process. Because of redshift alone, that explosion is detectable as a glow of a mere 2.7 Kelvins, not a significant factor in present universal heating. It's hard to argue that the universe's age is the primary factor when you consider the ratio of temperatures between the Big Bang and the present (suggesting that a steady-state, expanding universe would show a similar horizon effect over, say, 15 billion years).

I could have included the universe's age as a factor in my article, but I decided to avoid too much detail in a piece meant for general readers. But for the reasons offered above, prevailing redshift values, plus the described horizon effect, would limit the temperature in a universe that was infinite in age but expanding (if one cares to imagine such a thing).

Some of the contributors mention interstellar dust as a factor, but this would only briefly delay the heating of all matter. It's important to realize that in a closed system with a powerful energy source like stars (and assuming a steady-state universe), everything gets to the same temperature relatively quickly.

As to the related issue of dark energy, please see my article on that topic here:

The Physics of Dark Energy