A Hot Big Bang

You have probably heard the term "Big Bang" before when people start to talk about the origins of the universe, but when people tell you that the universe started really small (tinier than a grain of sand) and incredibly dense, and then suddenly expanded to what it is today, things become a little more difficult to grasp. The theory of a Hot Big Bang is the most widely accepted hypothesis for the origin of the universe, but it still leaves many questions unanswered. How and why did the universe expand? What caused the galaxies to form? But, perhaps the most daunting question is: "What existed before the Big Bang?" These are questions that are very difficult to answer. Astronomers are like archeologists who study the fossils of the universe. They form their theories based on what they observe, and luckily Big Bang theory seems to fit rather nicely to these observations. So what happened during the Big Bang?

Timeline of the Big Bang

Looking at the Horizon

First things first, let's explain why this timeline has such an odd shape. We mentioned before that based on its expansion, scientists approximate the universe to be about 15 billion years old. However, we can't see the whole universe, and we don't know how big it is. We can only see as far as light can travel, and right now we cannot "see" galaxies that are more than 15 billion light-years away because they have not yet entered our horizon. Have you ever watched the sunset at the beach, and noticed how the sun appears to just fall below the surface of the water? You know, however, that the reason the sun appears to drop is because as the Earth rotates, the sun drops lower in the sky as it becomes further and further from our line of sight. Finally, the sun sets on the "horizon" which is the farthest distance we can see due to the curvature of the Earth. We use this same term to refer to the universe because as far as 15 billion light years is all we can see--our horizon.

From the beginning...
So how did it all start? A very good question, and one that is highly debated. Most people agree that the universe started very small and very dense and underwent an initial inflation that lasted a infinitismle fraction of a second (thus the universe expanded much faster than the speed of light (and you thought nothing could move faster than the speed of light...). But the universe was still really hot, so hot that ordinary atoms couldn't even exist. Electrons caused very small packets of light called photons to scatter continuously, and if you can believe it, light was actually linked, or coupled, to the particles, causing the whole universe to glow. This is the stage that scientists like to call the primordial soup because the universe looked like a plasma "soup" of protons, electrons, neutrons, neutrinos, photons, etc.
(Just look at the picture on the right of Compton Scattering of photons off ionized particles.)

Anyhow, as you all probably know, things start to cool off as they expand, so as the universe continued to expand, it cooled off. Eventually, after about 300,000 years, it cooled off enough so that atoms could form. Photons are far more likely to scatter off ionized particles like protons and electrons (particles that have an electric charge) than neutrally-charged atoms, so finally the photons travelling through space could move in straight paths instead of constantly being scattered by electrons. As a result, the formation of atoms lead to the creation of the elements hydrogen and helium, and the now free-streaming photons made up a nearly uniform light radiation that filled the whole universe. The study of this radiation, which we call the Cosmic Microwave Background, may hold the key to some of the most pressing questions about the universe and its ultimate fate.
(The picture on the right shows photons travelling in straight paths when ordinary matter forms.)

The Formation of Galaxies and Galaxy Clusters
Some regions in this universe were very dense, and they had enough gravity to overcome the expansion and form galaxies and galaxy clusters. That is why we are careful to say that distant galaxies and galaxy clusters are expanding away from each other. For example, our own galaxy (known as the Milky Way) is really part of a cluster of galaxies. And unlike distant galaxies, the galaxies in what we call our local group are actually moving toward us due to gravity!

Below is a picture taken by the Hubble Space Telescope of the spiral galaxy NGC 4414.

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