A Big Problem With the Big Bang
The universe has a lithium problem.
Within a few minutes after the Big Bang, the hot soup of the nascent universe expanded and cooled enough to form hydrogen and helium, the simplest elements on the periodic table. Our best models predict that the early universe was roughly 75% hydrogen and 25% helium, with trace amounts of heavier elements. This matches current observations, except for the fact that we should be able to detect three times more lithium than we do.
Known as the cosmological lithium problem, this inconsistency raises some concerning questions about the early universe.
How Hubble Changed the Game
In 1923, Edwin Hubble sat in the Mount Wilson Observatory and pointed the 100 inch Hooker Telescope at a fuzzy patch in the sky called the Andromeda Nebula. Astronomers believed this was merely a collection of hot gas and dust. Hubble, though, used the telescope to resolve distinct stars, providing the first strong evidence that this cloud was, in fact, a separate galaxy.
And Hubble kept going. He found a Cepheid variable star within Andromeda. These stars, known as standard candles, have cyclical brightness, making them useful for measuring distances in space. The period of their brightness cycle is directly related to their intrinsic brightness. The brighter the star, the longer the period. By knowing the period, astronomers know the actual brightness. Then, by calculating their observed brightness, they can estimate how much is lost due to distance. When Hubble performed these calculations, he found that Andromeda’s Cepheid variable stars were much further away than stars in the Milky Way. The conclusion was inescapable: the Milky Way was not alone.
If he had stopped here, this would have been enough to etch his name into the pantheon of scientists who changed our understanding of the universe and our place in it. But he wasn’t finished. Hubble pointed the Hooker Telescope at other nebulae, and performed the same technique with Cepheid variable stars. He pored through the data for years and found that the universe was full of galaxies.
Then came his greatest discovery. His data showed that the light was red-shifted, meaning stretched into longer wavelengths. This occurs when the source of the light is moving away from the observer, much like how the sound of an ambulance is shifted to a lower pitch after passing. He reasoned that the galaxies, then, were moving away from us. The data also showed that the further away a galaxy was, the more red-shifted its light, an idea known as Hubble’s Law. So not only was the universe expanding, it was accelerating.
Big Bang Hot Model
A Belgian priest and MIT physics Ph.D., Georges Lemaître published a paper in 1927 that provided solutions to Einstein’s General Relativity in an expanding universe. Though he didn’t know it at the time, his solutions matched Hubble’s observations. A few years later, famous astronomer Sir Arthur Eddington realized the genius of this paper and had it translated into English. The idea of an expanding universe was still hard for many to swallow, but Lemeitre’s math and Hubble’s observations were too much to ignore.
Lemaître then analyzed the logical consequences of an ever expanding universe. It stands to reason that, if you turn back the clock, the universe would be denser and hotter. Go back far enough and it reaches a single point, which he called the primeval atom. From this point, the universe exploded, creating both space and time. In 1949, English astronomer Fred Hoyle mocked this idea by calling it the Big Bang. Not only did the name stick but so did the science.
Today, Big Bang cosmology is a robust field. Though many questions remain unanswered, the Big Bang is widely accepted as having happened around 13.8 billion years ago.
Evidence for the Big Bang
Perhaps our greatest piece of evidence is the cosmic microwave background (CMB). Accidentally discovered in 1965, the CMB is the leftover radiation from the immediate aftermath of the Big Bang. The extremely hot temperatures during this period produced highly energetic radiation, which became red-shifted over 13.8 billion years, as the universe expanded and cooled, creating lower frequency microwave radiation that permeates the entire known universe. Everywhere we look, the CMB is more or less uniform as predicted by our Big Bang models.
Another good piece of evidence is the observed expansion. Hubble’s observations and Lemaître’s math were the first proof of the universe’s expansion and acceleration. Since their seminal work, the Hubble Constant, the rate at which it expands and accelerates, has been measured at about 70 kilometers per second per megaparsec. The Hubble Constant is still being refined, with different schools of thought producing slightly different answers. This feud is known as the crisis in cosmology.
Lastly, our observations show an abundance of light elements, again predicted by our models. Within the first 3 minutes after the primeval atom or singularity exploded in the Big Bang, nucleosynthesis formed the first elements. Naturally, there was an abundance of the simplest elements: hydrogen and helium. Calculations suggest there should be about 75% hydrogen, the lightest element, 25% helium, the second lightest element, and small amounts of heavier elements like lithium and beryllium. For the most part, this is what our observations reveal, except for the missing lithium.
The Lithium Deficiency
A second after the Big Bang, the universe was 10 billion degrees C. After 99 seconds, the temperature dropped to 1 billion degrees, cool enough for the strong nuclear force to work. This force binds quarks together to form protons and neutrons, as well as binding protons and neutrons to form the nucleus of atoms. The bulk of the matter and energy went into hydrogen and helium, but .00000007% should have formed lithium, the next heaviest element with 3 protons.
When we analyze light from the stars, including light passing through the interstellar medium such as dust and gas clouds, we can see the predicted amounts of hydrogen and helium. But we should see 3 times more lithium. Analysis is done by spectroscopy, in which the light is spread out in a spectrum, allowing scientists to analyze the presence or absence of particular wavelengths associated with different elements.
So what’s the answer to the riddle? Nobody knows for sure, but an array of solutions have been proposed.
One of these is that lithium sinks over time. Some observations show that older stars do not have enough lithium, while younger stars have too much. Over long periods of time, turbulence in the star might draw lithium down, hiding it from view.
It’s also possible that the fundamental constants have changed. What we consider constants today, such as the speed of light, the electric charge, the fine structure constant, the gravitational constant, the strong coupling constant, might have been different in the early universe. Maybe they evolve over time. Maybe the intense heat and density of the early universe gave them different values. If this is true, couplings explained by the standard model might’ve behaved differently.
Maybe the cosmological principle is wrong. It states that the universe is both homogeneous and isotropic, meaning it has the same composition and properties. If this is wrong, then the early universe was not uniform, resulting in different amounts of lithium fusing than expected.
Or maybe our understanding of the Big Bang is flawed. So much evidence supports the Big Bang that this is unlikely, but we certainly need more evidence to solve the universe’s lithium deficiency.
