Was the Early Universe Lumpy?

When the universe was only a few minutes old, was it smooth, like Cream of Wheat (yum!), or was it lumpy, like oatmeal? (Yuk!)  British cosmologist John Barrow and I explored this question in this paper, posted yesterday. Most cosmologists think that the matter in the early universe was smooth, not lumpy, and there's no compelling reason to believe otherwise, but it's always important to look at alternatives.

How can we even say anything intelligent about the universe when it was only a few minutes old? Our best probe is the production of elements in the early universe, which goes under the tongue-twisting name of "primordial nucleosynthesis." Most of the atomic nuclei on Earth were made in stars, but a small number, including helium, deuterium, and lithium, were manufactured in the first few minutes of the universe. And the amount of each element produced is exquisitely sensitive to the density of protons and neutrons when the universe was just a few minutes old. If the universe were lumpy rather than smooth, then the element abundances would fluctuate up and down in a predictable way, and we can average these out to get a prediction for what we would see today.

This is a very old idea, going way back to the 1970s. But there are a couple of reasons it's worth another look today. First, we have much better observations of the elements that were produced by primordial nucleosynthesis, especially deuterium, which is seen in gas clouds that appear to be pristine relics of the early universe. Second, the cosmic microwave background has given us a very precise estimate of the total neutron and proton density in the universe, a key parameter that is needed to accurately predict how much of each element gets produced in the early universe.

And our verdict? We were able to limit any "lumpiness" in the early universe to no more than a 10-20% fluctuation in the density. So our universe is Cream of Wheat, not oatmeal.