The History of the Universe

FAS Astronomers Blog, Volume 30, Number 6.

This is a story of the Universe. The Universe is big, really big. It also has an interesting history, although one where lots of things happened in the first three minutes or so before everything settled down to a 13.8-billion-year timeline stretching up to the present.

I guess we can call this the modern history of the universe. Most astronomers believe this history to be correct because of three general areas of research. I’ll explore more about these in future articles.

1. Einstein’s General Theory of Relativity.
2. Edwin Hubble’s discovery of the expanding universe.
3. The discovery of the Cosmic Microwave Background (CMB).

For a long time, or maybe before there was time, there was nothing (we think … maybe). Then, around 13.8 billion years ago, something happened, and everything began in a “Big Bang.” Although, this is somewhat of a misleading term. It wasn’t big – the universe was squeezed into an incredibly small volume at the beginning. It wasn’t a bang – there wasn’t an explosion in the traditional sense. The term was coined by Fred Hoyle during a 1949 BBC radio program. He didn’t believe that the universe began with a singular event sometime in the distant past. He was being a bit derisive, but he used the term, and it has stuck with us.

The universe’s history is usually stated in terms of eras (or epochs). The terms are often used interchangeably. For this article, I will use eras and then epochs to subdivide eras.

The Beginning

The history of the universe begins during one Planck time after the Big Bang (0 to 10^-43 seconds). We just don’t know what happened before then. At this point, the universe was unimaginably small, dense, and hot with a width equal to one Planck length, and a temperature equal to the Planck temperature. It is possible there was one single unified force governing everything. We call this the Planck era.

The Planck era lasted 10^-43 seconds. Then gravity separated from the other three forces (electromagnetic, strong and weak nuclear) beginning the Grand Unification Theory (GUT) era.

Four Forces and Cosmic Inflation

Shortly after, at around 10^-36 seconds, the strong force may have broken away from the electromagnetic and weak forces. The breaking of the strong force may have caused the universe to go through a rapid expansion (by 10^≈26 times) in a fraction of a second (10^≈-32 seconds). This is referred to as Cosmic Inflation where the universe grew from the size of a subatomic particle to the size of a grapefruit (or possibly the size of a basketball for those of us living in NC).

Alan Guth introduced the concept of cosmic inflation back in 1980, although Guth’s form of inflation didn’t quite work. Two years later, Paul Steinhardt, Andreas Albrecht, and Andri Linde modified Guth’s model and introduced “new inflation”, which fixed the problems with Guth’s original theory. Inflation has yet to be verified; however, it solves a few problems with the standard Big Bang, so today it is included in most models of the universe.

Once inflation ceased at 10^-32 seconds, the electromagnetic and weak forces remained coupled during the Electroweak era until 10^-12 seconds at which time the two forces split and took on their current form. Note that some articles overlap inflation with the electroweak epoch, others begin the electroweak epoch after inflation.

Age of Particles

By then, the universe cooled enough for the Particle era to begin creating various subatomic particles. Quarks and leptons are the most fundamental of these particles. Particles built from quarks are referred to as hadrons, which are subject to the strong nuclear force. Hadrons can be either baryons (fermionic hadrons) composed of three quarks, such as protons and neutrons, or mesons (bosonic hadrons) composed of a quark and anti-quark. This contrasts with leptons (i.e., electrons), that are not subject to the strong force. For more on this, see the standard model of particle physics.

The fundamental particles known as quarks began to form during the Quark epoch and the universe became a sea of quarks and gluons. This continued until hadronization took place at around 10^-6 seconds as quarks bound together to form hadrons (protons and neutrons) in the Hadron epoch. At around one second, the universe cooled some more, and leptons (i.e., electrons) formed during the Lepton epoch. Neutrinos began to spread across the universe in what is called Neutrino Decupling. There might be a faint afterglow of these neutrinos visible today called the Cosmic Neutrino Background, but it has cooled too much to be detected.

At some point, Baryogenesis and Leptogenesis took place. In both cases the universe was composed of almost an equal part particles (baryons and leptons) and anti-particles (anti-baryons and anti-leptons), most of which annihilated each other. However, there was a slight asymmetry between the two resulting in the dominance of particles over anti-particles and eventually matter over anti-matter.

During the next few minutes, the universe cooled enough so that Nucleosynthesis occurred. Protons and neutrons combined to form the hydrogen (75%) and helium (25%) nuclei we see today. This process took place from around 3 to 20 minutes after the Big Bang, although some sources say it happened earlier between 10^-3 seconds and 3 minutes.

Photons and the CMB

The universe was, however, still dominated by photons, much of which were formed by the annihilation of particles and anti-particles. During this Photon era the universe was a dense sea of photons along with electrons and nuclei. Photons of light bounced around off free particles and couldn’t travel very far, making the universe opaque.

Eventually, the universe expanded and cooled to the point where electrons and nuclei could attract each other. Neutral atoms of hydrogen and helium formed beginning the Recombination era at between 240,000 to 300,000 years. Then, some 380,000 years after the Big Bang, as free particles combined into atoms, light, for the first time escaped and traveled through the universe. We see this light today as the Cosmic Microwave Background (CMB).

The CMB is a very faint background “noise” that resembles a blackbody radiating at around 2.7^o K. The CMB was accidentally discovered by two astronomers, Arno Penzias and Robert Wilson, in 1965. At the time, it appeared to be uniform (to one part in 100,000). However, three satellites (COBE, WMAP, and Planck) have measured the minute temperature differences in the CMB from which we think the large-scale structure of the universe including galaxies, galactic clusters, and cosmic voids evolved.

Dark Ages, Stars, and Galaxies

For a long time, neutral hydrogen atoms floated in space. Although the universe was transparent, there were yet to be stars and galaxies, so there was very little visible light. This began what is known as the Cosmic Dark Ages. Eventually gravity pulled these atoms together and the first stars formed. The problem was that the neutral atoms in the first stars (and in interstellar space) absorbed (or scattered) photons of light making it difficult for us, many years later, to see these stars. So, the fog of darkness associated with the dark ages continued.

The first stars formed around 100 to 250 million years after the Big Bang. These were large Population III stars built from primordial hydrogen. They burned fast and died quickly as supernovae seeding the universe with heavier elements. Because of their size, they were hot and emitted energetic ultraviolet light rather than the visible light we see from our Sun. This light eventually broke down the interstellar neutral hydrogen atoms creating, again, ionized protons and electrons. These ionized particles no longer absorbed (or scattered) photons of light and the fog of the dark ages disappeared. This began what is known as the Reionization era, which started at around 400 million years and lasted until around one billion years after the Big Bang.

The universe was much less dense at this point than during the earlier period of recombination. Then there was such a thick soup of plasma (photons, electrons, protons) that photons of light scattered off the ionized particles. During the dark ages, neutral hydrogen atoms were able to absorb photons. In the reionization era, the ionized particles were more thinly distributed, allowing photons and light to escape rather than scatter.

Over time, gravity pulled many of these early stars together into galaxies and galactic clusters. Early stars exploded in supernovae leaving stellar black holes at their cores. It is possible that these black holes migrated toward the center of galaxies creating the super massive black holes we see today.

As young massive black holes formed, gravity pulled so much material toward them that huge accretion disks developed around the black holes. The temperature of these disks climbed to incredible levels radiating vast energetic streams, which we see today as distant Quasars (Quasi Stellar Objects). Quasars are one of the most powerful objects in the universe and outshine their host galaxies by up to 100 times.

Density of the Universe

Energy dominated the density of the early universe. The universe was hot and dense and although particles and matter formed, they were no match for the energy of the universe.

As the universe expanded, the energy (photons) density declined due to the increase in volume and the stretching of light waves (lower frequency). This allowed matter, whose density declined by only volume, to take over and become the dominant form some 50 to 70 thousand years after the big bang.

As the universe continued to expand, the density of matter (both baryonic and dark) decreased. Around 9 billion years after the Big Bang, dark energy began to dominate, and the expansion of the universe started to accelerate.

Modern Times

Sometime around 4 ½ billion years ago, a cloud of dust and gas was disturbed by a nearby supernova. The gas began to rotate, and an average sized yellow star appeared at its center. Soon planets formed around the young star and our solar system was born. For a time, some of the planets migrated in and out from the Sun disrupting material that had failed to condense into other planets. Rocks and dust came crashing down into the inner part of the solar system during what is known as the Late Heavy Bombardment creating the craters we see on the Moon and elsewhere. This material might have provided one of the planets (the Earth) with water eventually leading to the formation of life.

The story continues with the Geological ages and epochs of the Earth, but I’ll leave that for another time.