Let us briefly recap where we are at before returning to our story of the history of the early universe. Scientists have one theory, the Standard Model, that is astonishingly good at describing how three of the fundamental forces interact. They have a separate theory that explains how the fourth force, gravity, works. But they have a problem in that merging the two theories into a theory of everything is mathematically extraordinarily challenging. The Standard Model breaks down when gravity distorts space and time, and it cannot be extended to incorporate gravity using the mathematical tricks that were fundamental to its development.
Despite this, there are theories, including string theory I mentioned earlier, M-theory and quantum loop gravity, that are proposed as ways to unify gravity with the other three fundamental forces. Currently we have no way of testing the predictions each theory makes, and this means a widely accepted grand theory of everything does not exist. A general theory of everything would help us further probe the history of the early universe, but even without such a theory, there is quite a bit that scientists have been able to discover about the youthful universe. They have also been able to run computer simulations where they change the strength of the four forces and see whether atoms, and stars, would form. Not all values of the four forces would result in a universe where life could evolve, so we should be grateful they take the strengths they do. We do not know why the forces have these strengths, but I’m glad they do. If they didn’t, you wouldn’t be reading this book.
Insights from the Standard Model, the theory of general relativity, collisions between tiny particles in the world’s most complicated machine, computer simulations, and observations of galaxies across the heavens have allowed scientists to piece together the history of the early universe. With these insights we can step from energy to quarks and electrons to atoms of a whole host of elements, the first necessary parts of the universal history of you and me. What happened is what I summarize next. The history of the universe as it developed from a singularity to the stars we see when we gaze at the night sky.
Before our universe’s birth, scientists speculate there was a nothingness. When I say nothingness, you might imagine the emptiness of space, perhaps like the space between the Earth and the moon, or between the edge of our solar system and Alpha Centauri, our closest star system. But this space is not empty. It contains energy and force fields and on the Planck scale might look like a bubbling foam. Scientists assume that before the universe formed, there was nothing. Nothingness means no energy and no force fields. Somehow from this nothingness our universe materialized, and scientists do not know how. Some researchers have put forward theories, but currently there is no way of collecting data to test them. For instance, some mathematicians have argued that the nothingness from which our universe was born was unstable, and it collapsed to form the singularity that was our very early universe, but in truth we have no way of knowing whether this hypothesis is true.
The singularity that appeared 13.77 billion years ago and from which our universe grew may have been over a billion billion degrees Celsius and was very, very dense, with all the mass and energy of today’s universe contained in a single point that was smaller than the smallest particle seen in the universe today. The four fundamental forces of physics that now permeate our universe did not exist at its birth, and the force (or forces) at play in the first instant of our universe may have been simpler than those that govern our lives. In a fraction of a fraction of a second after the singularity appeared, there was a brief period of what scientists refer to as inflation that lasted less than a billionth of a second, during which time the universe expanded astonishingly quickly. During this period the universe grew by 1 followed by 78 zeros times. Even after this period of rapid expansion the universe would have only been somewhere between the size of a grain of sand and a basketball. In the remainder of the first fraction of a second of the universe’s life, the four fundamental forces emerged.
As the young universe expanded it began to cool, and as it did so the fundamental forces we see today began to reveal themselves. First to appear was gravity, followed by the strong nuclear force. Cosmologists do not agree whether the strong nuclear force separated from the electroweak force – the combined weak and electromagnetic forces – immediately before or after inflation, but its emergence filled the universe with a hot soup of quarks and gluons. Very shortly after the emergence of the strong force, the electroweak epoch ended, when the two remaining forces – electromagnetism and the weak nuclear force – appeared and electrons were formed. All this happened extremely quickly, and by the time the universe was 0.000001 seconds old, it had cooled sufficiently that neutrons and protons were able to form.
The hot soup of protons and electrons was opaque, and energy, in the form of photons, could not travel through it as easily as they traverse the universe today. The early universe by was consequently dark. Cosmologists study the universe by examining different types of electromagnetic radiation: gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves and radio waves. These different types of electromagnetic radiation travel across the universe as photons that are not only particles but are also waves (I explain this in the next chapter). Electromagnetic radiation lies on a scale defined by the distance between the peak of consecutive waves. Radio waves have the longest wavelength, and gamma rays the shortest. None of them could travel through the early universe, as the photons that carry energy would continually interact with the quark–electron soup.
By the time the universe was 380,000 years old, it had cooled sufficiently that the protons, neutrons and electrons could combine and form atoms of hydrogen and helium. At this point, the universe became transparent, and electromagnetic radiation could travel through it. The oldest radiation we can detect on Earth today has been travelling through the universe since it was 380,000 years old, a journey taking over 13.7 billion years. It is called the cosmic microwave radiation.
The cosmic microwave radiation is everywhere we look, and it has very similar properties whether we look towards the centre of our galaxy, or away from it. The model that ran time backwards from now towards the singularity at the birth of our universe predicted that such radiation should exist. Discovering it helped prove that the universe started with a singularity and a big bang.
Since the 1960s, when the cosmic microwave radiation was discovered, many research projects have mapped it, with a very high-resolution all-sky map being published in 2013. The map revealed the radiation was not completely identical in every direction but instead exhibited very small variations. There are points where it is slightly warmer, and others where it is a bit cooler. Because this radiation permeated the universe from its birth, this patchy temperature variation reflects slight differences in the distribution of energy and matter in the very early universe. Scientists believe that this variation early in the universe’s life is what resulted in the formation of galaxies and the large-scale structure of the universe we see today.
The detailed map of the cosmic microwave background also provided compelling support for the period of inflation, with the small fluctuations being caused by tiny differences in the distribution of matter during the universe’s very early history. The map also allowed cosmologists to calculate the age of the universe to a precision of 13.772 billion years old, plus or minus 59 million years.
Because the universe is so staggeringly vast, light that arrives on Earth may have been travelling through space for millions, or even billions, of years. When we look at an object in the night sky, we do not see what is happening to it now, but rather we see what it was doing when the radiation that we see left the object. Light departing the moon takes 1.3 seconds to reach Earth. Light from Alpha Centauri takes a little over four years and three months to get to Earth. If something catastrophic were to happen to the Alpha Centauri system, we would not know about it for over fifty-two months. When we gaze into space, we are consequently looking back in time, a bit like a geologist digging down through layers of time in rocks. The geologist is looking at a still picture of the past, while the astronomer sees a movie. If alien astronomers orbiting a star 70 million light years away are at this moment pointing their immensely powerful telescopes at Earth: they are eyewitnesses to tyrannosaurs charging across the Cretaceous landscape.