© Matt Strassler [March 15, 2014]
The Hot Big Bang is the period at whose end-stages we are living, during which the observable patch of the universe was initially dense and hot, and during which it has been expanding and cooling. The expansion has been slowing until recently. Do not allow yourself to be confused: The Hot Big Bang almost certainly did not begin at the earliest moments of the universe.
Some people refer to the Hot Big Bang as “The Big Bang”. Others refer to the Big Bang as including earlier times as well. This issue of terminology is discussed at the end of this article on Inflation.
- How hot was the Hot Big Bang at its hottest, before it began to cool?
- And how did it start?
We don’t know for sure yet. The Hot Big Bang may have begun when the universe became hot following a period of inflation, as explained in my article on the Era of Inflation. If so, the heat of the Hot Big Bang came from the dark energy that powered inflation, and the maximum temperature of the Hot Big Bang is related to the amount of dark energy that was available.
We don’t know yet If BICEP2’s recent result is correct and is being correctly interpreted [It is not, see here], we would now know roughly how much dark energy there was during inflation, so the maximum possible post-inflation temperature may now be known. It might have been
- as big as a large fraction of a percent of the Planck temperature (where the universe would have been hot enough to make black holes just from its own heat) or
- as small as the temperature corresponding to about the energy of the Large Hadron Collider (where it would barely have been hot enough to make Higgs particles)
and probably not lower.
Sometimes this “maximum temperature of the Hot Big Bang” is called the “reheating” temperature, but the “re-” in “reheating” is misleading. People used to assume the universe was hot before as well as after inflation, hence “reheating” — and you’ll find plenty of websites, books, videos and images for the public that still make this assumption — but the assumption is completely unjustified.
What happened next?
We are extremely confident that we know the main outlines and many details of what happened over the ensuing 13.7 billion years. The universe has been gradually expanding (the space itself has been becoming larger), and it has correspondingly been cooling and becoming more empty. Relative to the amazing event of inflation, the period thereafter has really been pretty uneventful, though there have been important milestones along the way [which may get articles of their own someday].
Within the first few minutes of the start of the Hot Big Bang:
- The Higgs field turned on (i.e., its average value became non-zero), assuring that many previously massless types of particles, including the quarks and electrons that are found in ordinary matter, developed a mass. Since that early time the Higgs field has been steady at that value, at least across the observable patch.
- The quarks, antiquarks and gluons which had been roaming free joined together to form hadrons, including protons and neutrons.
- The first atomic nuclei other than hydrogen formed, leaving the universe with a substantial amount of helium and little bits of deuterium (heavy hydrogen) and lithium. Later these were presumably the ingredients for the first stars.
380,000 years later, things had cooled enough for atoms to form, at which point the universe became the largely transparent place we know today. The light which then became free to travel across the universe provides us with the “cosmic microwave background”.
It was perhaps roughly a hundred million years later when the first galaxies began to form and the first stars shone. The timing and details aren’t yet established by any measurements yet, but efforts are underway.
Today, we live roughly 13.7 billion (13,700,000,000) years after the start of the Hot Big Bang. Notice I don’t say that “the universe is 13.7 billion years old” or that the beginning of the universe was 13.7 billion years ago… we don’t know that. What we do know is just that the Hot Big Bang began 13.7 billion years ago — but we don’t know if that moment was close to the beginning of the universe as a whole.