If you wanted to build a universe, how would you go about it?
Well, there are several approaches possible. For example, you could have unique components for everything in it. Human particles would make up humans. Star particles would make up stars. Plant particles would make up each different type of plant.
Or you could just start with one material - a "universal clay" - and mold all of the components from this compound. The differences would not be in the material used but in how it was sculpted and treated.
Or you could select a few elements - such as earth, fire, water and air - and blend them together in different proportions to create everything around us. The difference between a human and dandelion would be in how much of each of these different components is present and how they are arranged. Transformation between, say, gold and lead could be accomplished by redressing the balance between these components.
But the universe we occupy is both simpler and more complex than any of these models. It starts with energy, which is equivalent to matter and vice versa. This is the real meaning behind Einstein's equation - energy and matter are just two forms of currency with the speed of light squared being the conversion rate.
With the massive amount of energy released in the big bang, it was possible for some of it to coalesce into sub-atomic particles which eventually combined to give us protons and neutrons among other species. Electrons belong to a different class, but these three components are the basis for all of the matter around us. Atoms are composed of a nucleus containing protons and neutrons surrounded by a cloud of electrons.
Initially, in our universe, all of the atoms were one of two isotopes of hydrogen - hydrogen or deuterium. Hydrogen is a bit quirky as it is the only element with names for its individual isotopes. There is a third one called tritium.
The difference between the three isotopes of hydrogen is the presence of a neutron. Ordinary hydrogen atoms, which make up about 89 per cent of the atoms in the universe, are composed of a single proton and a single electron. The proton is positively charged and attracts the negatively charged electron. These two particles orbit each other although as the proton is much heavier (1,836 times), it is the electron which we see moving wildly about.
A small percentage of the hydrogen atoms (about one per cent of the atoms in the universe) are deuterium having a nucleus composed of a proton and a neutron. As the neutron carries no charge, it has little impact on the number or velocity of the electrons. It just makes for a heavier nucleus.
In tritium, the nucleus contains two neutrons but still only one proton. Indeed, the defining characteristic of all hydrogen atoms is the presence of only a single proton in the nucleus.
Put two protons in the nucleus and we have a helium atom. Helium makes up about nine per cent of the atoms in the universe. It is generated by the fusion of hydrogen atoms in the core of stars. Simplistically, two deuterium nuclei join together to give helium - although it is actually much more complicated. Because helium nuclei are emitted during certain types of radiation, they have a name and are called alpha-particles.
It is the fusion together of helium nuclei which allows us to build more complex atoms. A carbon atom is generated by fusing three helium nuclei. An oxygen atom requires four and so on.
One of the subtle points about the elements making up everything around us is that, with a few exceptions, even-numbered elements are more abundant than their neighbouring odd-numbered elements.
That is, oxygen is more abundant than either nitrogen or fluorine when considering the universe as a whole.
This building up of elements from hydrogen and helium nuclei occurs in the heart of a star. The rate is limited by the difficulty in generating beryllium, but the overall effect is to generate all of the atoms in the periodic table.
There are 90 or so naturally occurring elements and we have added another 28 to the table through synthetic means.
So, how do you go about building a universe?
You start with a huge amount of energy and from that you create elementary particles such as the proton, neutron and electron. By combining these sub-atomic constituents together, you get simple atoms such as hydrogen and helium. But put them in the oven to bake - in the heart of a star at hundreds of thousands of degrees under extreme pressure - and you can generate all of the elements across the periodic table.
Roughly 4.5 billion years after that, chemists will have discovered all of the elements and organized them into the periodic table.
