The Size of Planets, the Age of the Universe, the Ignorance of the Masses

[This post is a bit long and diffuse… I may hack it up into bite-sized pieces later…]

Just because my job has ‘astro’ in the title, doesn’t mean I know enough to comment on whether or not Pluto is a planet. And there’s plenty of other science-in-the-news…

The International Astronomical Union (IAU) has decided that a planet is anything with enough gravitational pull to make itself round. As a scientific organization, the IAU probably had to go for something like this — a more or less physical definition — along with the sociological desideratum of preserving Pluto’s planetary status. The other — perhaps more sensible — option would have been to declare “planet” to be a category something like “race” or “pornography”: not actually well-defined by some set of principles, but nonetheless “we know it when we see it”. We could then just declare the same old 9 planets, including tiny but venerable Pluto, and move on. Instead, with the current definition there are 12 planets, and astronomers will probably find a lot more over the coming years. I’m not sure if we should bother changing the textbooks quite yet.

More meaningful to me is the age of the Universe. Astronomers at the Carnegie Institution and elsewhere have observed eclipsing binary stars in a nearby galaxy, and thereby determined the stars’ masses. With careful modelling, they’ve then been able to predict how luminous those stars should be, and by comparing that to the stars’ observed brightness, determine the distance to the galaxy. Their result puts the galaxy about 15% further away than previous (less direct) measurements; if correct — and if we can distinguish the galaxy’s cosmological “motion” from its attraction to other nearby galaxies (such as our own) —
this wouldn’t impact merely the distance to this one object, but would revamp the entire cosmic distance scale, lowering the Hubble Constant which measures the expansion rate of the Universe, and finally making the Universe about 15% older than we thought.

On the one hand, 15% isn’t that big a change in a quantity, the Hubble Constant, that used to be uncertain to about 50% as recently as a decade ago. On the other hand, recent measurements from a variety of quite disparate sources have confirmed its higher value to better than 10% or so. But it’s an intriguing possibility that could push the details of the Hot Big Bang model in intriguing ways, but almost certainly without getting rid of the weirdest features of the models, such as the unexplained, exciting, and increasingly solidly measured Dark Energy. (As usual, Ned Wright’s Cosmology Tutorial is an excellent starting point if you’re perplexed by my jargon.)

In other cosmology news, the lucrative and prestigious Gruber Prize in Cosmology has been awarded to the COBE team, which first measured the fluctuations in the Cosmic Microwave Background that’s since enabled us to absolutely confirm the hot Big Bang theory, measure the curvature of the Universe and the mass of its contents.

Finally, the country of my birth is populated by ignorant savages. (Original reference here.)

One response to “The Size of Planets, the Age of the Universe, the Ignorance of the Masses”

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