Few questions hold greater importance in science than how it all began. What did the beginning of
the universe look like? Why did it happen in the first place? And although we are a long way off
from having a complete answer, scientists have learned enough that a theory has been
conceived – the Big Bang. By examining stars and the spectrographic data given off by matter when
light passes through it, we have assembled the jigsaw pieces of the Big Bang, and have begun to
form a picture of those first explosive moments. But here’s the thing about jigsaws. Sometimes
you find a piece that just doesn’t fit. At that moment you realise with a sinking heart;
you made a mistake somewhere along the way. And you have no idea where.
Is this what’s happening to the Big Bang Theory? Or are we only looking at our puzzle piece the
wrong way round? It all comes down to an element we use every day, in medicines,
alloys and most commonly, the batteries in our phones. One that is strangely missing,
from the universe; at least in the amounts we would expect.
So, here’s the big question: Where is all the Lithium?
I’m Alex McColgan, and you’re watching Astrum. Join with me today as we examine the Lithium
Discrepancy, a mystery that continues to confound scientists to this day, and might
force us to reevaluate our models if we ever hope to answer the question of where we came from.
To begin with, let’s cover some of the basics about Lithium. Lithium is a relatively simple
element. It’s one of the alkali metals, and for a metal is surprisingly soft; capable of being
cut with just an ordinary steel knife. You might have seen it used in science experiments in school
as it reacts quite satisfyingly when mixed with water, producing lithium hydroxide and hydrogen
gas. More commonly, it’s used in phone batteries, laptop batteries, and in other electronic devices
due to its high electrochemical potential. But the feature of Lithium that makes it so important
for our discussion is its internal structure – it’s made with just 3 protons in its nucleus,
meaning it’s one of only a handful of very light weight elements that could be created
in the early moments of the universe. But it’s the expected presence of Lithium
in the universe’s beginning that causes our problem with the Big Bang model. So,
what do we think was going on back then, and how exactly does Lithium prove it’s wrong?
The universe’s origin is murky to us. After the discovery in the 1920’s that our universe was
expanding, a Belgian cosmologist Georges Lemaître theorised that if the universe was getting larger
and cooler, it used to be smaller and hotter, with all that energy being squeezed into a
smaller and smaller area. The logical conclusion to this was that if you rewound the universe back
to its very beginning, all of the universe’s energy would exist in a single point in space.
This singularity would be infinitely hot and dense – which makes sense. Based on that heat,
this idea is known as the hot big bang model. But it’s this infinity that causes the murkiness to
creep in. Einstein’s theory of relativity can be used to predict the motions of objects under the
effects of gravity, but no equation tends to function when you start adding infinity into
the mix. Hence, at that very first moment of the universe’s origin, we don’t have the equations
to describe very well what was going on, or what might have caused the universe to start expanding.
And with an infinitely hot and dense singularity, there’s no way of knowing what might have gone on
before it – any evidence or signals from before that time would have been met with the hottest
incinerator the universe has ever created. It happened somehow. That’s all we can say for sure.
At this point, there would be no Lithium. The very start of
the universe would have been simply too hot. After that, though, even just one second after
the universe begins to expand, the picture becomes much clearer. 1 second after the
universe’s expansion began, the temperature of the universe dropped down to a mere 10,000,000,000,000
degrees C, which actually is only 1000 times the temperature at the centre of our Sun. This was
still hot enough that matter couldn’t form yet – the strong force, the fundamental force that
holds together the nuclei of atoms, would have been overwhelmed by this intense energy. So,
everything would have existed in the form of radiation instead, the remnants of which we can
still see today in the Cosmic Background Radiation – discovering this became one of the proofs that
scientists are onto something with this model. The universe didn’t remain as just radiation
for long. Just 99 seconds later, it would have cooled to 1,000,000,000,000 degrees C,
and the strong force would have kicked in. The universe’s first protons by then
had started forming, and began binding with neutrons, creating hydrogen and the 1 proton,
1 neutron isotope of hydrogen known as deuterium. Hydrogen and its isotopes (versions of hydrogen
that had the same number of protons, but more or less neutrons) were the most common element in
the universe, but hydrogen wasn’t the only one. Everything was still filled with so much energy
that the hydrogen protons frequently smashed into each other, underwent nuclear fusion,
and soon created the larger, more complex element helium (with 2 protons). Once you
run the equations, in the primeval universe matter was nearly 76% hydrogen, and nearly 24% helium.
But sometimes, very rarely, another proton would merge with the other two and would form isotopes
of Lithium, bringing the number of protons up to 3 in these atoms. By mass, this made
up only 0.00000007% of the matter in the early universe. But here is where the mystery comes in.
By examining the spectroscopic data from stars, or by evaluating that same light as it passes
through clouds of matter, we can evaluate the distribution of elements in the universe today.
And what we see roughly matches this prediction. Our own Sun is, by mass, 71% hydrogen, 27% Helium,
and 2% other heavy elements, which makes sense as the Sun would have had time to undergo more
nuclear fusion in its lifetime, slightly altering the ratios of these elements. These
ratios are mirrored throughout the universe as a whole – another strong proof that the
hot big bang model and our equations are right and that the universe did evolve in this way.
And yet our predictions about Lithium are wrong. Although the prediction for the amount of
Lithium in the early universe is really low, the prediction is actually still too high compared to
what we see: 3 times too high. Or, to put it another way, as we study the spectroscopy of
primordial stars (the further away the star, the earlier in time the light would have left it), we
don’t see anywhere near the amount of Lithium we ought to be seeing if our prediction was correct.
There are further mysteries surrounding Lithium. Lithium-7 is the isotope that we tend to see in
the early universe. That means a Lithium atom that has 3 protons and 4 neutrons in its nucleus.
But there is another isotope of Lithium, known as Lithium-6, with one fewer neutron – that is meant
to be much less stable and much less common than its counterpart, supposedly making up only 2 out
of every 100,000 lithium nuclei. While the total amount of Lithium in the universe is too low,
the ratio of Lithium-6 to Lithium-7 is much too high – Lithium-6 is a staggering 1000 times more
prevalent in the early universe than it should be. So what does this mean for the hot big bang model?
Is it really back to the drawing board for our most trust theory
about the universe's origins? Not necessarily.
Two possibilities exist. One that involves re-writing the big bang theory as we know it.
But, there is another explanation.
Given the importance of Lithium in technology, we could definitely do with more of it. But
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When faced with something like this, one of two things must be true.
Either our observations are wrong, or our models are wrong. Let's consider each possibility.
It’s difficult to throw out the hot big bang model completely.
Nothing else does as good a job at explaining the existence of the Cosmic Background Radiation,
or the amounts of Hydrogen and Helium that we see in our universe. The quantities of those
elements check out almost exactly with the hot big bang’s predictions, which seems too perfect
to be a coincidence. That said, scientists are becoming less certain about the model as it
stands. Stephen Hawking, who was once one of the leading advocates of the idea that our universe
began with a singularity, later went off the idea. In his book A Brief History of Time, he stated:
“…[Roger Penrose’s and my] work became generally accepted and nowadays nearly
everyone assumes that the universe started with a big bang singularity. It is perhaps ironic that,
having changed my mind, I am now trying to convince other physicists that there was
in fact no singularity at the beginning of the universe – as we shall see later,
it can disappear once quantum effects are taken into account.”
Those quantum effects are too complicated to go into for this video, and this doesn’t
necessarily solve the Lithium problem even if true; but it does highlight that the origin of
the universe is an evolving branch of physics and cosmology, where the full explanation for
it has not yet been found. Perhaps our models will one day adjust in such a way that the
Lithium problem falls away, even as the broad brush strokes of the theory remain in place,
letting the jigsaw piece of the Lithium numbers fit nicely with Hydrogen and Helium.
The other option is that there is something wrong with our observations, and there may well be some
grounds to this. Because of the small amounts of Lithium being discussed, it is quite difficult
to see the exact amounts of it that exist in the interstellar medium once you look far back
enough. The only way to probe the interstellar medium’s makeup is to shine a light through it
and then see which lines in your spectrograph get blocked out. This can tell scientists what
elements might make up a particular nebula or dust cloud. But this gets harder and harder to
do the further away the target is. Eventually, it becomes completely impossible to get an exact
estimate. Instead scientists are left looking at the spectrographs of primordial stars, which are
not necessarily representative of the universe as a whole at that time. A star is a special
case – something going on inside there might be altering the amount of Lithium present within,
converting the Lithium into something else, while also boosting the amount of Lithium-6 (although
it’s difficult to guess what that process might be). If our observations could improve and we
could better measure the amount of Lithium in the early universe as a whole, perhaps the numbers
would come back into line on their own. Maybe the Lithium Problem never existed in the first place.
Ultimately, no answer to how the universe came to be will ever be complete until all
the available data matches the theory’s predictions - whether that’s Lithium,
or other problematic elements in the universe, like the overabundance of Gold out there (which I
talk about in this video here). All these numbers have to be brought into line. There is still so
much to learn about the universe’s origins. I do find it incredible just how much we are
able to learn and deduce about events that happened billions of years ago, back before
matter itself had properly formed, using just our telescopes, and our minds. That picture - the
grand picture of how it all began - is waiting to be assembled. However the universe came to be,
it is built on a foundation of an underlying order, that allows us to unlock its principles,
tease out its mysteries, and reconstruct the configuration of its individual pieces.
One day, I am confident we will solve the Lithium Problem, too. How can we resist,
when the answer to our ultimate origins awaits? Some questions are just too important to leave
unanswered, and I, for one, can’t wait for the day this jigsaw is ultimately completed.
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