And the earth was without form, and void; and darkness was upon the face of the deep. (Genesis 1:2)
One billion years have passed on the Cosmic Calendar since the Big Bang. Much of that time has been spent in the darkness known as the Dark Ages. The universe created no new light since the time that the photons that were released during Recombination, the time when the primordial plasma of electrons and atomic nuclei coalesced into a sea of hydrogen and helium atoms. The universe continued to expand and cool. From this beginning, we might expect the universe to end in frozen darkness, yet when we look to the heavens we see myriad stars, blazing stellar furnaces.
What happened to change the cold, dark fate of the universe?
The early universe was also very homogeneous. The matter and energy in the universe was spread out smoothly, almost perfectly so. Small fluctuations did exist,1 yet the universe was so smooth—How smooth was it?—it would be like hiking the 4,500 kilometers2 from Los Angeles to New York in an alternate universe where the terrain was flatter than Kansas, like glass. The only landmark to relieve the monotony of the months of hiking across glassy plains would be a single blip of a hill perhaps seven stories tall.3
Yet the universe isn’t so plain and boring now. Everywhere we look we see a riot of diversity and complexity. How did unrelenting plainness become practically infinite diversity?
The Dark Ages weren’t an uneventful chapter in our story. Things were taking shape in the darkness.
Some of the answers to our questions may lie in the colossal inflation of the universe that happened during the Electroweak Epoch. If inflationary theory is correct, those tiny fluctuations in the early universe became the seeds for the structure that we see today. When the universe began to inflate to about 1026 times its size, these quantum fluctuations smaller than atoms got caught up in inflation. They grew to galactic proportions and started in motion the formation of structure.
In its early history, the universe was dominated by the expansion of the Big Bang. As time went on, another force began to assert itself at large scales: gravitation, the force that attracts all particles of matter to each other.4 Inflated quantum fluctuations created some places of greater density where more matter was packed into a small space. Because these places of greater density had more matter, they had greater gravity. Because they had greater gravity, they could attract even more matter. An so on.
In the inky darkness, the first structures began to take shape.
The first structures to form were what we see today as galaxy clusters. Dense clouds of hydrogen and helium gas and dark matter collapsed in on themselves due to the force of their own gravity. Like a runaway train, nothing was yet able to stop this collapse.
Smaller parts of these huge clouds were denser than others and began the process of gravitational collapse at a smaller scale. As the clouds of atoms and dark matter collapsed, they began to spin, forming the first nascent galaxies, including our own Milky Way. As atoms fell into these swirling vortices, they collided with each other. These collisions created heat. The universe began to warm up again.
At the heart of most large galaxies is a supermassive black hole 105 to 1010 times as massive as the sun.5 A black hole is a region of space that is so dense that its tremendous gravity captures everything that comes too close. Even light is unable to escape its gravity once it gets too close.
These monsters had a voracious appetite, consuming tremendous amounts of matter in the early universe. As the black hole eats, it is thought to create very dense regions where the infalling matter has been packed together very tightly. This somehow releases tremendous energy and light. These bright phenomena are known as quasars, the brightest objects in the visible universe. The brightest quasar in our sky would be as bright as the sun if it were 33 light years away, almost 2 million times as far away as the sun. This particular quasar is therefore about 2 trillion times as bright as the sun or 100 times as bright as the average galaxy!
Of the 100,000 known quasars, the nearest to us is 780 million light years away. Most of them are much farther. Quasars were therefore more common in the early universe. Once they run out of matter to consume, quasars turn off. It seems that the age of quasars is over.
Parts of these galaxies were denser than others, so the process of collapse repeated itself yet again at smaller scales. Clouds of gas inside galaxies began to collapse in on themselves and spin just like their parent galaxies had. These spinning clouds of gas were the beginnings of the first stars. As they collapsed, they generated even more heat, enough to strip the electrons from the nuclei of atoms. These proto-stars eventually got dense and hot enough that the bare atomic nuclei began to fuse together to form heavier elements when they collided. When nuclei fuse together, some of their mass is lost and is converted directly into energy and light. Nuclear fusion is what lights the stars.
The stellar furnaces had been lit.
Nuclear fusion is also what finally stops the progress of gravitational collapse. Stellar radiation and heat exerts an outward pressure that balances against the inward pull of gravity. This dynamic equilibrium holds stars together and prevents them from collapsing altogether.
The radiation from newly formed stars began to heat up the gas between the stars. These gaseous atoms also lost their electrons and became ions again. The atoms were ions when they first formed, before they trapped electrons during Recombination. Today, almost all of the visible matter in the universe is ionized. For this reason, this period of reheating is known as Reionization, the time when the universe lit itself again and came out of the Dark Ages. On the Cosmic Calendar, this period began on January 5th and lasted until about today, the 27th (13,550–12,700 million years ago).
That is how the cosmos came to have the shape and light that we see.
As I began with a biblical passage, I must end with the Galaxy Song. How amazingly unlikely is our birth indeed!
We will continue our story at the end of August.