The further away an object is, the longer it has taken the electromagnetic radiation to reach us. By looking at objects that are different distances away, scientists can consequently piece together the history of the universe. The observable universe has grown from the small local neighbourhood we were in when the universe became transparent. The cosmic microwave background is the first light from the bit of the universe our current neighbourhood was in when the universe became transparent. All the galaxies we can see spread out over billions of light years are the matter that was packed into a small locality when the universe was 380,000 years old.
One way to visualize this is to imagine putting a dot of ink on an uninflated balloon. The surface of an uninflated balloon is a bit like the whole universe, but in two dimensions. When the universe became transparent and light could start to travel through it, our neighbourhood was like the ink dot on the uninflated balloon. Now imagine blowing the balloon up. As the surface of the balloon stretches, the ink dot becomes bigger. Inflating the balloon mimics the universe expanding. The stretched ink dot is like what we can see now, the observable universe. There is a lot of the balloon we can’t see, and that is equivalent to the unobservable universe. The cosmic microwave background can be thought of as the light that was in the ink dot on the uninflated balloon. The galaxies represent all the matter that existed in the ink-dot region of the uninflated balloon. Cosmologists know the universe is still expanding and the rate of expansion is accelerating. This is like the balloon being blown up at an ever-increasing rate. The ‘universe as a balloon’ analogy is useful, but not perfect. The surface of an expanding balloon is curved, while scientists have shown that the universe is not curved but flat. The balloon analogy has other limitations too. Over-inflated balloons pop. Let’s hope the universe doesn’t eventually do the same.
By the time the universe became transparent it had expanded quite a lot from the singularity it started out as and was about 1.4 million light years across. The reason we cannot see the edge of the universe is that the observable universe we see today was not on the edge of the universe when it became transparent. When the universe was 380,000 years old the bit that became the universe we observe today was surrounded by matter and energy in all directions. We have no idea whether we are closer to the edge or the centre of the universe, and perhaps we will never know.
Once the universe had become transparent and hydrogen and helium atoms were common, gravity then began to work its magic. The first atoms and molecules were attracted together until they formed the first stars. These stars first burned when the universe was about 100 million years old, and they were many times larger than the sun, but their lifespans would have been shorter.
Stars burn bright because the force of gravity pulls atoms of hydrogen and helium closer together, heating them up and making them energetic enough that they start to collide. These collisions can break the bonds between nuclei and electrons, eventually forcing the nuclei together, where they can form the nuclei of heavier elements such as nitrogen and oxygen.
Brighter stars use up all their hydrogen and helium fuel more quickly than those that are smaller, but all stars above a certain size turn their fuel into nuclei of heavier elements such as nitrogen, oxygen, carbon and iron. As these stars near their end, collapsing in on themselves, gravity becomes so strong it pushes numerous protons and neutrons together to form the nuclei of the even heavier elements such as uranium and plutonium. It is fascinating how atomic nuclei of heavier atoms can be formed from hydrogen via the weak nuclear force and gravity.
In the centre of a star, hydrogen nuclei are turned into helium nuclei and something called a helium core forms. The nuclear fusion in a star is triggered because gravity pushes atoms closer together, making them highly energetic. The fusion generates a lot of energy in the form of heat and light that exerts pressure, preventing the outer layers of the star collapsing in on themselves. For most of a star’s life, the effects of gravity pushing atoms towards one another, and the pressure created by nuclear fusion emitting energy, are approximately in balance. Nonetheless, towards the end of a star’s life things become unstable. Exactly what happens at this point depends upon the mass of the star. In the case of the sun, it will continue to get steadily hotter and larger over the next 5 to 6 billion years until a time when it will go through cycles of expansion and contraction. At its largest size it will extend to the orbit of Jupiter, before contracting again to be smaller than it is today. These pulsing contractions and expansions will lead to the helium core becoming heavily compressed and being raised to a temperature of 100 million degrees Celsius. The helium is converted into carbon and oxygen and other heavy elements, generating huge amounts of energy that cause the sun to expand to a vast size once again. Eventually these cycles of expansion and compression cease, with the heavier elements created in the sun’s death and any unburned hydrogen forming a cloud of dust. The Earth will be swallowed and burned up during one of the sun’s dying expansions and our planet will cease to be.
When stars that are greater than about eight times the mass of our sun run out of fuel, gravity causes them to collapse until they form supernovae – the most violent explosions in the universe. The star that goes supernova explodes and as it does so it produces elements heavier than iron. The elements, dust and debris from a supernova permeate space and can form nebulae from which the next generation of stars is made. Heavy elements are quite abundant on Earth and elsewhere in our solar system, providing evidence that our sun cannot be a first-generation star. In fact, scientists know that the universe was already over nine and a quarter billion years old when our solar system formed. The Earth may be formed from the debris of a second-, third- or fourth-generation star – we do not know. What we do know is that the heavier elements made from the death of earlier generations of stars were necessary to produce our home planet and the emergence of life.
At this point in our story we have gone from a singularity to our neighbourhood in the universe where some of the hydrogen and helium formed earlier in the universe’s life has been converted, via a process known as nucleogenesis that happens in the heart of stars, to heavier elements. Our next step on the journey is to investigate how these heavier elements combine to form molecules like water, carbon dioxide and methane – groups of atoms of different elements that have combined via the electromagnetic force.
In this chapter we have covered how the following things had to happen for you, and me, to exist. 13.77 billion years ago a very hot, very dense, singularity consisting of large amounts of energy had to form. The universe began to expand and started to cool, with its expansion and cooling continuing to this day. As the universe started to age, but still within the first blink of an eye, the four fundamental forces emerged, and energy was converted to matter in the form of the fundamental particles. The link between energy and matter is critical to our universe’s functioning, and to you and me. Quarks soon started to form protons and neutrons and then the nuclei of the lightest elements. As the universe continued to cool further, the nuclei combined with electrons to form the first atoms. These were eventually pulled together to form the first stars, creating local pockets of intense heat that created heavier elements, which eventually, in our neck of the cosmic woods, combined to create our sun and solar system. I wish I’d been taught this at school, as every child should know what remarkable knowledge physicists have of how and why atoms of the periodic table came to be. Chemistry is the next step in our history, and in particular why chemical reactions happen. You and I are made of complex chemistry, so understanding how and why atoms interact as they do is crucial to figuring out why we are here.