Why do astronomers insist on using ergs?
I’m a long-time member of my local amateur astronomy club, the Astronomical Society of Northern New England (ASNNE), and last night we hosted Dave Batuski from the University of Maine who gave a talk on the present state of precision cosmology. Of particular interest is the nature of the so-called ‘dark energy’ that is entirely different from dark matter, and that seems to make up about 73% of the apparent mass of the universe. Based on measurements of the accelerating expansion of the universe, the density of this energy, as given by Batuski in his talk, is about 10-8 erg/cm3. Quantum field theory apparently (I have long since forgotten so I’m trusting his numbers) predicts 10120 erg/cm3, a glaring difference. The quantum prediction is based on spontaneous pair creation.
In any case, my first question is: why do astronomers insist on still using ergs (not to mention cubic centimeters)? According to Batuski a single erg is about as much energy as is exerted by a flea when jumping. As a physicist and mathematician I understand that some things are just intuitively easier to interpret using some units over others, but, to some extent, this is a matter of conditioning. As such, it would make sense for astronomers and physicists to speak the same language, considering that, in many cases, they are in the same department. While it may seem I am biased against ergs simply because I am a physicist and mathematician, there is a practical side to all this. I teach my students to focus on SI units in my courses since it makes it much easier to check one’s calculations if no conversions are required. Since, officially, combined units such as Joules, Newtons, Watts, etc. are all interpreted in terms of the base kilograms, meters, seconds, etc., it seems most rational to me to do that everywhere.
Now, as for the science itself, Batuski, of course, took the line that the quantum prediction was flat-out wrong. While I am sympathetic to the idea that observational evidence trumps theory every time, the measurements being made are so phenomenally difficult that this acceleration wasn’t discovered until 1998. The corrections due to localized mass distributions, lensing effects, etc. are all so difficult (forget even the dark matter problem) that I would be hard-pressed to be absolutely assured that the astronomers’ figure of 10-8 erg/cm3 was dead-on correct. I’m not saying the quantum field theorists’ figure is any more reliable, simply that it seems absurd (though rather typical, in fact – see Dennis Overbye’s Lonely Hearts of the Cosmos) for the astronomers to be so self-assured about that number.
My own theory is this: a true theory of quantum gravity may suggest that the fabric of spacetime itself is quantized and I wouldn’t be surprised if, in some manner similar to spontaneous pair creation, though not symmetric, these spacetime quantons are self-replicating in some manner but not annihilating. I envision a cellular-like structure similar to a growing mass of bubbles. Of course, we’d need a full theory of quantum gravity to even theoretically play around with this idea, but it’s an intriguing prospect nonetheless since it might be able to explain these anomalies. I honestly think Ken Wharton’s on to something, but only time will tell.