The dominant Big Bang model of contemporary cosmology is based partly on Einstein’s general theory of relativity and partly on the Standard Model of particle physics, which in turn is based on quantum mechanics and quantum field theories. By applying the mathematical equations of these theories to observations and measurements of the universe today, cosmologists can predict—or rather, retrodict—what the universe was like at times further and further into the past.
The basic strategy is this. First, estimate the average density of matter and energy in the observable universe as it is today. Then use general relativity, together with the Hubble constant, to retrodict how the size and density of the observable universe has changed over time. By calculating the average density at any given time in the past, we can also estimate the average temperature at that time, since the compression of matter and energy results in higher temperatures; and conversely, allowing a system to expand results in lower temperatures. Finally, when the average temperature and density at any given time has been determined, the Standard Model of particle physics can be used to predict what sorts of particles existed at that time.
In this way, we can rewind history (so to speak) all the way back to a fraction of a second after the Big Bang. Below is a brief summary of important phases in the history of the universe, according to these calculations:
Retrodictions based on general relativity imply that the universe (or at least the presently observable part of itAs mentioned in a “fine print” section on the previous page, the universe might be infinite in size, in which case it would have been infinitely large even at the time of the Big Bang. Nevertheless, general relativity implies that the presently observable part of the universe was infinitesimally small at the beginning of time.) was initially smaller than one Planck length—a unit of length approximately equal to 1.6 × 10-35 meters, which is more than a billion, billion times smaller than the diameter of a proton. The density and temperature of the universe at that time would have been incomprehensibly high, and the Standard Model of particle physics almost certainly does not apply to such extreme conditions. For this reason, it is impossible to determine what the universe was like during this brief initial phase, but theorists speculate that even elementary particles could not have existed near the beginning of the Big Bang.
No one knows whether the universe was ever really that small, of course. General relativity is probably invalid at the microphysical level where quantum effects are significant; so it may yield incorrect predictions about the earliest moments after the Big Bang. If the universe actually did begin in the way general relativity suggests, though, it would have been far too hot to be described adequately by the Standard Model of particle physics.
During this phase, the universe rapidly expanded and cooled to 1015 K or so. (That’s a quadrillion degrees Celsius.) According to the Standard Model, this absurdly high temperature is nonetheless cool enough for elementary particles—quarks and leptons—to form.
Some theorists speculate that the universe underwent a brief period of extremely rapid expansion, called cosmic inflation or the inflationary epoch, sometime during this period. This inflation hypothesis is one possible explanation for the low-entropy condition of the early universe, as mentioned in chapter 3.
During this phase, which ended about 1 second after the universe came into existence, the universe had cooled to less than a trillion kelvins. That’s still ridiculously hot, but it’s cool enough for the strong force to bind quarks together, forming protons and neutrons.
This is the earliest phase for which the predictions of the Standard Model can be experimentally tested. Temperatures up to several trillion kelvins have been produced by accelerating atomic nuclei to nearly the speed of light, then slamming them together head-on inside the Large Hadron Collider.The CERN facility set a record in August 2012, achieving temperatures around 5.5 trillion kelvins by colliding lead ions in the LHC. See this press release for more information. In these experiments, physicists found evidence that the colliding protons and neutrons had briefly “melted” into a soup of quarks and gluons. Quarks and gluons cannot be directly observed, because they quickly recombine to form new composite particles. But some exotic particles detected in the LHC were of types predicted to form out of quark-gluon plasma, providing evidence that the protons had indeed “melted.” Although this doesn’t prove that the theory provides an accurate description of the early universe, it does indicate that the Standard Model yields reliable predictions even for temperatures up to a few trillion kelvins.
Approximately three minutes after the Big Bang, the universe had expanded and cooled enough for the strong force to hold protons and neutrons together, forming larger atomic nuclei through nuclear fusion. The process of nuclear fusion (or nucleosynthesis, as it is called in this context) lasted until around 20 minutes after the Bang. By that time, the universe had expanded and cooled too much: its temperature and pressure were too low for nuclear fusion to continue.
Because the period of nucleosynthesis lasted only a few minutes, there wasn’t enough time for large nuclei to form. The majority of atomic nuclei formed during this period would have been isotopes of hydrogen, followed by helium and perhaps small amounts of lithium. This is an especially significant claim of the Big Bang model, because it is one of very few retrodictions about the early universe that we can test by observing the universe today. Using spectroscopy to determine what types of atoms are most abundant in stars and galaxies throughout the universe now, astronomers have determined that hydrogen is by far the most common element, followed by helium, just as we would expect if the Big Bang model’s account of nucleosynthesis is correct. Cosmologists consider this an important piece of evidence supporting the Big Bang model.
Although the first atomic nuclei formed a few minutes after the Big Bang, the universe would have to expand a lot in order to cool enough for electrons to stick to those nuclei and form whole atoms. According to the Big Bang model, the universe was filled with glowing-hot plasma for hundreds of thousands of years after the period of nucleosynthesis ended. (Plasma is a state of matter that occurs at high temperatures. It’s like a gas, except that the electrons are moving too fast to stick to the atomic nuclei.) It took nearly 380,000 years for the universe to cool down to only 4,000 kelvins—still more than twice as hot as the flame of a butane torch. At that temperature, electrons could finally attach to protons, forming neutral hydrogen atoms.
The CMB comes from all directions, so it can be visualized as a sphere surrounding us. This interactive 3-D map of the CMB was created using data from the Planck space telescope. The colors represent subtle variations in the microwave radiation, which correspond to slight differences in the temperature and pressure at various places in the early universe.
This phase of the Big Bang model also yields a measurable prediction about the universe we observe today. Hydrogen gas is transparent to light, but hydrogen plasma is not: photons are scattered by the free electrons in plasma. When the universe finally cooled to the point where hydrogen plasma turned into hydrogen gas, light which had been trapped by the glowing plasma was suddenly released. In other words, photons that had been bouncing randomly inside the plasma were suddenly free to travel in a straight line (or geodesic) through spacetime. Although this happened billions of years ago, according to the Standard Model, some of the light released at that time is just reaching us now, coming from somewhere billions of light years away. It isn’t visible light, though. It originated as infrared light from the hot plasma; but as it traveled for billions of years through the ever-expanding universe, the stretching of spacetime also affected its wavelength, stretching infrared photons into microwave photons. This faint glow of microwave light from the most distant reaches of the observable universe is known as the cosmic microwave background (CMB).
Early proponents of Lemaître’s Big Bang theory predicted the existence of the CMB in the late 1940s; the radiation itself was first detected in 1964.American cosmologists Ralph Alpher and Robert Herman predicted the existence of microwave background radiation in 1948 (in this article), but the prediction was not taken seriously by most of the scientific community until astronomers Arno Penzias and Robert Wilson accidentally discovered the CMB in 1964. While using an ultra-sensitive antenna to scan the skies for radio signals, Penzias and Wilson encountered a persistent microwave “noise” that seemed to come from every direction. They later won the 1978 Nobel Prize in Physics for their accidental discovery. For more information about this felicitous accident, see this. This discovery was perhaps the single most important piece of evidence that persuaded the scientific community to accept the Big Bang model.
After the glowing hydrogen plasma cooled into hydrogen gas, the universe remained dark for hundreds of millions of years—a period affectionately termed the “Cosmic Dark Ages.” Then, sometime around 300 million years after the Big Bang, new light began to shine. Stars and galaxies were forming. We’ll examine that process on the next few pages.