THE BIG BANG

Cosmology 101

One of the most tantalizing questions we can ask in astronomy is, “How did all this material that we detect in the cosmos get started?” To answer it, astronomers look back across 13.8 billion years to a time when the universe was in a seething, hot, dense state of existence. How long it was that way and what came before it are still unknown. Yet, from that first state of existence sprang the entire universe we know today. The origin of the universe and how it has evolved to the complex and large structures we see today are the subject of a science called cosmology.

The Beginning

The Big Bang—the birth of the universe—occurred some 13.8 billion years ago. It was an event that filled all of the space there was at that time with matter and energy. It was the beginning of space and time. The Big Bang wasn’t an explosion, as the name seems to suggest. Rather, the birth of the universe set off an expansion of space and time that continues to this day. The most primordial particles of matter were created in the event.

The first second after the Big Bang, the entire universe was a soup of subatomic particles, superheated to 10 billion degrees. In that first second, amazing things happened:

• The force of gravity separated out from the electronuclear force and was joined soon thereafter by the electromagnetic force.
• The universe changed from being a hot soup of quarks and gluons (elementary particles), and protons and neutrons began to form.
• At the ripe old age of one second, the newborn universe was cool enough that it began forming deuterium (a form of hydrogen) and helium-3. At this point, the newborn universe had doubled in size at least ninety times!

Over the next three minutes, the infant universe continued to cool down and expand, and the creation of the first elements continued. For the next 370,000 years, the universe continued its expansion. But it was a dark place, too hot for any light to shine. There existed only a dense plasma, an opaque hot soup that blocked and scattered light. The universe was essentially a fog.

The next big change in the universe came during the era of recombination, which occurred when matter cooled enough to form atoms. The result was a transparent gas through which the original flash of light from the Big Bang could finally travel. We see that flash today as a faint, all-encompassing, distant glow called the cosmic microwave background radiation (sometimes shortened to CMB or CMBR). The universe was leaving its cosmic dark ages behind. Gas clouds condensed under their own self-gravity (possibly helped along by the gravitational influence of dark matter) to form the first stars. These stars energized (or ionized) the remaining gas around them, lighting up the universe even more. This period is called the Epoch of Reionization.

From the Big Bang to You

1. Pre–Big Bang: quantum density fluctuations
2. Pre–Big Bang: cosmic inflation
3. 13.8 billion years ago: the Big Bang
4. 13.4 billion years ago: the first stars and galaxies
5. 11 billion years ago: the Milky Way Galaxy starts to form
6. 5 billion years ago: the Sun begins to form, along with the planets
7. 3.8 million years ago: the first life appears on Earth
8. 2.3 million years ago: the first humans appear
9. Modern time: you were born

The First Galaxies

By the time the universe was nearly 400 million years old, the first stars and galaxies were forming. As the expanding cosmos cooled, dark matter condensed into clumps. This spurred the accretion of gas into dense regions, eventually forming stars. The galaxies looked nothing like the spirals and ellipticals we see today. The primordial galaxies were more like shreds of light-emitting material. They arose from fluctuations in the density of the early universe.

The universe continued to expand, but that expansion began slowing down for the next few billion years down under the gravitational influence of matter in the form of stars and galaxies. Then, when the cosmos was about 5 or 6 billion years old, something interesting happened. Dark energy—which had been around for most of the history of the cosmos—began to accelerate the expansion of the universe. This mysterious force seems to be a property of otherwise “empty” space, and its effect has been measured through observations of distant supernovae by ground-based as well as space-based observatories such as the Hubble Space Telescope observations of distant early supernova explosions. Dark energy acts opposite gravity and continues to speed up universal expansion today.

Galaxy Evolution

From the time they formed until the present day, galaxies have been the major sites of star formation in the universe. Our own galaxy was born about 10 billion years ago, and it has grown—as many other galaxies do—through collisions and mergers. As this process has occurred, the Milky Way’s regions of star formation have been busy, creating new generations of stars from the materials left behind by the first populations. Our own solar system was formed about 5 billion years ago, some 9 billion years after the first moments of the Big Bang. It will exist for another 5 billion years, until the Sun dies. Our galaxy will eventually enter into a multi-billion-year-long dance with the Andromeda Galaxy, an event that will merge the two into one large elliptical galaxy looking nothing like the spirals that combined to build it.

The universe’s billions of galaxies exist in superclusters that form giant sheets and filaments of matter that stretch across the cosmos. While our universe is not going to collapse in on itself, this cosmic web of existence will continue to expand for tens of billions of years into the future.