Which of the following elements was not present in the very early universe before the formation of stars? A) Helium. B) Hydrogen. C) Uranium. D) Lithium.
Okay, so, in this question, we are discussing the very early universe. So, let’s start by recalling how the universe began. The universe began with an event known as the big bang. And everything as we know it in the universe was all concentrated in a very, very small, very dense, very hot region. In fact, that was the universe. It was extremely tiny right at the beginning.
And then, in a very short period of time, the universe expanded; it got larger. And as this extremely hot, tiny, dense region began to expand, initially lots of electromagnetic radiation started bouncing around inside the universe. Because at this point of the universe’s life, the energy density in the universe was very high. In other words, there was lots of energy flying around in the form of electromagnetic radiation.
But then, as the universe expanded even further, it obviously got larger, but it also started cooling down. And eventually, it cooled enough for matter to start to form, specifically protons, neutrons, and electrons. And of course, there was a large amount of radiation flying around during that time also. Now, specifically with the existence of the individual protons in the universe, we can see that one of our answer options cannot be the answer to our question.
Because a proton can be said to be the same thing as a hydrogen nucleus. Because we can recall that a hydrogen atom will only ever have one proton in the nucleus, which means that a proton flying around by itself is also a hydrogen nucleus. And this tells us that hydrogen was present in the very early universe, when we’re looking for elements that were not present. Which means that we can cross out option B, as this is not the answer to our question.
Anyway, moving further on in time, the conditions in the universe were hot enough for nuclear fusion to occur. Because these protons flying around would sometime combine with each other and sometimes with a neutron or a couple of neutrons in some instances. And so, we would have some particles consisting of two protons and potentially one or two neutrons also. And particles such as this are the same thing as the nucleus of a helium atom because we can recall that helium nuclei have two protons in them. Which means that in the very early universe, helium also existed. Hence, option A is not the answer to our question.
And also in some cases, we would have three protons joining together with often some number of neutrons. And the element containing three protons in its nucleus is known as lithium. Therefore, before the formation of stars in the very early universe, lithium was also present. Admittedly in very small amounts because the universe was still mainly hydrogen and little bits of helium and very small amounts of lithium. But lithium was still present.
On the other hand, it is extremely difficult to create a uranium atom with nuclear fusion because each uranium atom contains 92 protons. And so, the formation of uranium will have not happened until much later, when massive stars were formed in the universe. And they went through their entire life cycle billions of years until they reach the ends of their lives. At which point, these stars would undergo a supernova explosion. And this would create lots of heavy elements, uranium being one of them. Which means that at this point, we found the answer to our question. The element that was not present in the very early universe out of the list that we’ve been given is uranium.