Saturday, November 21, 2009

Evolving Chemicals

ResearchBlogging.orgNo matter the context, to Creationists, "Evolution" is a dirty word that they have to deny exists or minimize.

I already have an entire series of posts about stellar evolution. But another "evolution" Creationists try to deny is the so-called "chemical evolution". What this might mean differs depending on which Creationist you're talking to (or more commonly, by how far you've forced them to move their goalposts), but one context they frequently mean is the process by which the universe became enriched with heavy elements via the processes associated with stars.

Now, take a look at this picture from a recent survey paper:

It doesn't take a χ2 fit to see what's going on here. The X axis has two different sets of units that display essentially the same thing. Along the bottom it has the redshift which essentially gives a measure of distance. Applying our understanding of cosmology, we can alternatively display that as a time since the Big Bang. So things all the way to the left are things that happened in our local universe and are going on (in a cosmological time scale) now. Things way off to the right are things that are far, far away, and as such, their light has taken a long, long time to reach us.

These far away things tell us what the universe used to be like. The Y axis is a measure of the "metallicity" (the percent of elements heavier than helium) of the objects being studied, which, in this review paper by Sandra Savaglio at the Max Plank Institute, is Gamma Ray Bursts (GRB) and Quasars (QSO). This is normalized so that the metallicity of the Sun is 1 and the whole thing is on a logarithmic scale.

Putting all of this together it becomes painfully obvious: The amount of heavy elements in the universe has been increasing. It has a lot of scatter to it (which is to be expected since some galaxies in which these objects exist would be more active than others), but the overall trend definitively shows that the metal content increases as you move to the present (to the left)!

Of course, I'm already expecting a Creationist to stop by and try to point out just how far off some of those GRB data points are from the dashed-trend line (the solid one is the fit of a model). However, there's a reason to expect this too: The author suggests it's "due to the different regions probed by the two populations. GRBs tend to occur in regions with high star-formation, therefore in regions closer to the galaxy center, where metallicity is on average larger than in a random galaxy sightline."

For this reason, GRBs may not be the best detector of metallicity (since they're prone to being in regions polluted by high rates of star formation and death and getting extra enriched compared to the rest of the universe), but the quasars definitively reveal the trend and Creationists, yet again, will have to move their goalposts.
Sandra Savaglio (2009). The Cosmic Chemical Evolution as seen by the Brightest Events in the
Universe arXiv arXiv: 0911.2328v1

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