What is the Big Bang?
While in the popular imagination the "Big Bang" sounds like an explosion at the beginning of the Universe, the reality is a little less dramatic. The Big Bang model encapsulates the idea that the Universe began in a hot, dense state, from which it expanded rapidly. As the Universe expanded, it cooled - just like any expanding gas. (You can see this yourself by letting air out of a bicycle tire: as the air spreads out of the opening, you will feel it cold against your hand.) In fact, the Big Bang model predicts that sufficiently early in the Universe's history, the laws of physics (at least as we currently understand them!) break down, so that we cannot even describe the Universe in its earliest moments. Fortunately, modern physics can describe the expansion at later times, although many important uncertainties do remain.
Image: Snapshot of an artist's animation of the big bang.
Credit: NASA's Goddard Space Flight Center /CI Lab
What was the early Universe like?
As the Universe expanded and cooled, its composition evolved. When it was extremely dense and hot, the Universe had so much energy that particles of all sorts would pop into existence but shortly thereafter be annihilated by other particles in the cosmic soup. However, over time more and more particles became stable, including protons and neutrons, which combined into the nuclei of atoms a few minutes after the Big Bang. The simplest of these nuclei - a single proton - could form a hydrogen atom by combining with an electron. But, early in its history, when the Universe was dense and hot, the photons - particles of light - had so much energy they would immediately destroy any hydrogen atoms that formed. Because the Universe has about two billion photons for every proton, almost all the protons and electrons were kept apart. Fortunately for the matter, as the Universe expanded, these photons gradually lost more and more of their energy. This is the redshift: photons are stretched out by the expansion, which makes them lose energy and (at least for photons that produce visible light) become more red.
Image: An artist's animation depicting the life of a photon, or particle of light, as it travels across space and time, from the very early universe ESA Planck satellite.
Credit: NASA/JPL-Caltech
What is the Cosmic Microwave Background?
About 400,000 years after the Big Bang, photons had lost enough energy that they could no longer break hydrogen atoms apart. It was at this time that the Universe transitioned from a sea of separate protons and electrons to a sea of neutral hydrogen. But what of the photons? Once the hydrogen atoms formed, they no longer interacted with the vast majority of photons. So these photons - two billion per atom! - simply traveled in straight lines after that point, even until the present day. Now, the Universe's expansion has robbed them of so much energy that they appear as microwaves to us, making the cosmic microwave background. Because the photons have traveled undisturbed for almost 14 billion years, when astronomers take an image of them, they see the Universe as it was just 400,000 years after the Big Bang! This is the most distant light we can see, but it is extremely important for cosmology because it provides a baby picture of the Universe.
Image: ESA Planck has imaged the most distant light we can observe, called the cosmic microwave background, with unprecedented precision.
Credit: JPL, ESA, the Planck Collaboration
What triggered the formation of structures like galaxies in the Universe?
This first thing one notices about this baby picture is its uniformity: at the time of the cosmic microwave background, the Universe was the same everywhere to a few parts in 100,000! That is surprising in several ways, but the most important is that these tiny fluctuations nevertheless formed the seeds from which galaxies would later form. We will tell the story of how those seeds grew into galaxies shortly, but for now we must ask what generated even these tiny fluctuations. The leading explanation today is cosmic inflation, a period of extraordinarily fast expansion very early in the Universe's history. The mechanism is important for us because the end of inflation isn't quite uniform across the Universe: each part of the Universe ends the process at a slightly different time, which generates ever-so-slight differences in the matter density of each region. Fortunately, those tiny variations will grow over time, as we will explore on the next page.
Image: ESA Planck mission has imaged the oldest light in our universe and the bottom map shows the largest-scale features of the map.
Credit: JPL, ESA, the Planck Collaboration
