Saturday, May 17, 2008

New and Improved: Now Twice as Bright

Interesting article on Space Daily yesterday: Astronomers have decided that the universe is actually twice as bright as they had previously thought.

It turns out that interstellar and intergalactic dust absorbs roughly half of the light energy emitted from distant luminous objects, re-emitting it later. So when we look at a distant object, we are seeing only half of the light that would reach us if space was completely transparent. So the universe is actually twice as bright as we perceive it to be. One implication of this is that energy generation in the universe is also twice what we thought.

A sentence that struck me was, "The Universe is currently generating energy, via nuclear fusion in the cores of stars, at a whopping rate of 5 quadrillion Watts per cubic light year - about 300 times the average energy consumption of the Earth's population." It seems as if the author is trying to emphasize the incredible amount of energy produced. But a cubic light year, while small on an astronomical scale, is incomprehensibly enormous by any human yardstick. The volume of the earth is roughly 270 billion cubic miles (270*10^9 mi^3). A cubic light year is about 218*10^36 mi^3. In other words, a cubic light year would hold nearly 10^27 earth volumes. And the population of earth is using a 300th of the energy generated in that volume. That is pretty mind-boggling! (Of course, we're living right on top of a star, astronomically speaking, so our energy density is way, way above any "average" cubic light year).

Our understanding of the distribution of matter in our galaxy and in the greater universe depends in large part on brightness measurements. Basically there are certain kinds of objects that emit light energy at a known rate or brightness. When we identify such an object (known as a standard candle) at a great distance, we can compare the light that is reaching us to the amount of light we believe it is emitting and arrive at a pretty good estimate of how far away it is. Most of our distance and some of our mass estimates for far away galaxies and stars come from this kind of inference. What I've been wondering, and the article doesn't address, is the degree to which this brightness research will force astronomers to reassess the distribution of matter and energy in our universe. You know, basic questions like where is everything, and how much of it is there?

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