by Sarah Scoles
BBN -- Boy Band Network? Baby Back News? Big Bad Notepad? Barometer-Based Neuroticism?
Big Bang Nucleosynthesis. Big Bang nucleosynthesis is what it sounds like: it refers to the first formation of nuclei, which started about 3 minutes after the Big Bang. Hydrogen, deuterium, helium-3, helium-4, lithium-6, lithium-7, and beryllium all formed between 3 minutes and 20 minutes after the universe's beginning, after the temperature was low enough to allow quarks to join up and become protons and neutrons, and protons and neutrons to join up into nuceli.
Now, 13.7ish billion years later, we have developed science and can figure out both what we think the relative abundances of those first elements were, and what they actually appear to be. Any discrepancies between the two will help us learn more about the rights and wrongs of our models of the early universe.
One "wrong" that we know of is that, so far, our measurements of the amount of primordial lithium in the universe are about 4 times lower than they "should" be. A new paper by Midwestern astronomers Howk, Lehner, Fields, and Mathews, however, says, "Maybe not. Maybe we've just been measuring the wrong thing."
So, first, let's ask this: How do astronomers measure the amount of lithium formed when the universe was young if the universe has been progressing and producing, you know, lithium ever since?
Second, let's ask, "On what do astronomers base their predictions of light element abundances?"
They only need one piece of information: the number of photons versus the number of baryons (regular matter, like protons and neutrons--atom stuff).
In other words, how much radiation was there compared to how much stuff there was?
With modern equipment, we don't have to guess about that any more: the WMAP satellite, which makes measurements of the cosmic microwave background, can actually measure the photon-baryon ratio.
Its measurements largely satisfy us in their agreement with our ideas. The experimental photon-baryon ratio, when used to predict the abundances of light elements, made predicitions that mostly matched the experimental abundances. Helium-4? Awesome. We were right. Helium-3? Even better. Go us. But lithium? Nope.
Let's step back and ask where these experimental abundances came from.
Astronomers want to find objects whose compositions will reflect the composition of the early universe, and since they are unlikely to find any objects whose compositions have been completely unaffected by processes that have occurred since the Big Bang, they need to be able to figure out how, and how much, those processes would have changed the abundances.
Historically, they have chosen old dwarf stars in the Milky Way's halo, as these stars presumably preserved the primordial chemical state of the universe fairly well. But this abundance was lower than predicted, and that led to the informative term "The Lithium Problem." Either BBN was kind of wrong, or we misunderstood the processes that destory lithium inside stars, and more gets destroyed each year than we think.
Howk and colleagues, however, looked somewhere else: the Small Magellanic Cloud, a small galaxy orbiting the Milky Way. And not at dwarf stars, but at the gas between stars.
After taking into account things like how much lithium is ionized and how much is trapped inside dust grains, they determined how much lithium there was in the Small Magellanic Cloud's interstellar gas, compared to how much other stuff there was.
This abundance matched BBN's predictions, meaning that our ideas about the Big Bang, the moments after the Big Bang, the formation of the first subatomic particles, and the formation of the first atomic particles is (or could be) true.
Well, if the lithium abundance is "right" in the gas of some other galaxy, why is it so "wrong" in the old stars in our own galaxy? And how do we know that just because the other galaxy's contents matched what we expected that that answer is "right" and our previous answer was "wrong"?
Well, the authors admit that they can't know that. They say in the paper that maybe the lithium abundance in the SMC gas is no more primordial than in the dwarf stars, and that it is still possible that we don't have BBN, or our conception of the early universe, quite right. It is also possible that we don't exactly get what processes affect lithium in stars. It is also possible that we don't understand, in general, how and why lithium may have been produced or depleted in general.
So not exactly "yay."
But still. What's interesting is that the photon-baryon ratio was directly measured and is pretty well nailed down, and we think we understand BBN pretty well because it didn't happen right after the Big Bang (when we have a lot of speculation and also equations but also speculation) about the conditions and the way that the laws of the universe manifested themselves.
The early universe is an interesting place, because it is so foreign and hot, like a tiny island nation. While we can never go back and witness what happened, the conditions of way-back-then are the ones that still determine the way the universe is today. If there were more photons, if there were fewer baryons, if the universe had expanded faster or slower, etc. Had nucleosynthesis produced different amounts of light elements, the universe may have evolved quite differently, and may be quite different now. We know how it is now. We want to know how it was back then, so we can see how we got here. It's like asking your parents to tell you stories about the time before you could form permanent memories.
Howk JC, Lehner N, Fields BD, & Mathews GJ (2012). Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud. Nature, 489 (7414), 121-3 PMID: 22955622