In the late 1960s, after the detection of space-filling microwave background radiation, it became clear to cosmologists that the origin of the universe had begun with a big bang . Over time, evidence has accumulated that nearly 14 billion years ago, everything we know arose from a single, explosive event.
There are three great experimental tests that make us think that it was so: first, the discovery of the expansion of the universe by the North American astronomer Edwin Hubble around 1925; second, the aforementioned microwave background radiation, which constitutes the current ‘echo’ of the Big Bang; and third, the composition of the universe as three quarters hydrogen, one quarter helium, and a few traces of lithium and beryllium (this is called primordial nucleosynthesis). This is so because three minutes after the big bang, the temperature of the universe would have dropped enough to stop any nuclear reaction. Calculations made around 1950 by, among others, Fred Hoyle, William Fowler and Geoff and Margaret Burbidge, showed that in this case the proportion of helium in the universe must be 25%. Interestingly, these four astrophysicists were staunch critics of the Big Bang. In fact, the name was given to him by Hoyle during his famous 1950s BBC radio show, The Nature of the Universe .
When we predict the amount of hydrogen and helium generated by the Big Bang, we do it really well. Even with the amount of deuterium, a type of hydrogen – an isotope in technical terms – that has one proton and one neutron instead of just one proton. Theory predicts that for every million hydrogen nuclei 25 deuterium nuclei were formed, and observations show that this prediction is correct to within 1.6%. But with lithium, the theory fails miserably: it predicts that for every ten billion hydrogen atoms, around five lithium atoms were formed (we are talking about an isotope of lithium, Li7, which has three protons and four neutrons and is the form most abundant of this element). However, the best observations we have show that there are fewer than two lithium atoms for every ten billion hydrogen atoms. This is known in cosmology as “the missing lithium problem” and is an unknown that raises serious questions about whether we know what happened in the cosmos from about 10 seconds to 20 minutes after the Big Bang.
At that time the universe was very hot and expanding rapidly. At that time, electrons and photons moving freely through space formed a plasma with the first atomic nuclei of hydrogen, helium, lithium, and beryllium. From then until the first stars began to shine, the chemical composition of the universe was that simple.
There are various explanations for such a large discrepancy. One of the latest is that of some Chinese researchers where they defend that the nuclei in the first moments of their existence behaved differently from what was believed. To find out what was happening, they say, it is necessary to apply a new formulation of physics that studies systems with a large number of components -such as gases, made up of millions and millions of atoms- that was introduced in 1988 by the physicist Brazilian of Greek origin Constantino Tsallis, and which is used to describe complex systems that are out of balance, which are those that can undergo changes spontaneously; Just the kind of situation the universe was in at the time. And apparently their accounts match the observations .
But this is not the only explanation at play. European astronomers say that the origin of the discrepancy lies elsewhere, in the way we have to account for the lithium present in ancient stars. Astronomers calculate it using a very simple method: by looking at the intensity of spectral lines , the fingerprints left by chemical elements in starlight. From them, the abundance of a chemical element can be deduced: the more intense it is, the greater the quantity there is. Well, according to this team of astronomers, led by Andreas Korn from Uppsala University (Sweden), these measurements can be misleading because their observations have shown that the proportion of lithium present in the atmosphere of stars decreases as age, since it sinks inside due to the action of gravity. It’s like the double chin in humans: gravity pulls the face down to form the classic “crow’s neck”.
But it does not stop here. A group of researchers from the Astrophysical Institute of Andalusia states that the lithium that was not produced in primordial nucleosynthesis was formed in novae . Apparently, in these bursts, enormous amounts of beryllium-7 are generated, which decays and transforms into lithium after just over 50 days. In light of this discovery, Spanish astronomers point out that part of the lithium in the Universe could have been generated in novae, which appear when one of them is a white dwarf in a binary star system and tears off part of the outer layers of its companion: then the white dwarf explodes in a thermonuclear flare with which it increases its brightness up to 100,000 times.
Unfortunately, and despite all efforts to solve it, the Universe still has a lithium problem . Or rather our theories have it.