Of Particular Significance

Hot Big Bang

© 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.

Before the Hot Big Bang: Inflation (maybe /probably/ maybe)

19 Responses

  1. What is the location (relative to us now) of this hot big bang? (How far from us here)

  2. Hi Prof Strassler, thank you for the succinct explainations here. Please the explain the source(s) of light in this statement ‘ The light which then became free to travel across the universe provides us with the “cosmic microwave background” ‘. What exactly does background mean, background of what? is it that if we travel 13billion light years we’ll be able to see it?

    Thank You.

  3. Dear Prof,

    Agreed with GEN. Great blog and superb website. After a rapid period of personal “inflation” I am suddenly a big fan, at least as much as an interested lay-person might be. Now some questions from a 20th century man trying to drag himself into the 21st century cosmology…

    Having gained my entire formal education in physics during the 70’s and 80’s, I was more or less led to believe that due to the explosive “big bang” (as it was then hypothesized) all matter created during or after that event was put in motion both cosmically and sub-atomically. Can motion in the cosmic sense still be thought of as a necessary property of matter after discounting the apparent relative motion imparted by inflation? I’d like to emphasize “necessary” . In other words, must all matter have an absolute large-scale motion as well as a relative one in an expanding universe?

    Thanks. JG

  4. Prof. Strassler, first of all, thank you for the wonderful blog. It’s one of the few that I read regularly. Being a non-physicist, I have a few naive questions. First, how long did the transition from inflationary to hot big bang take? Was this instantaneous everywhere, or more like the formation of crystals in solution? Second, how does the presence of dark energy transform to heat? And finally, what determined how much dark energy was left over after its transformation to heat (i.e., what determined how much dark energy is still present in the universe, or at least our observable patch?)

    Again, many thanks for the wonderful blog!


  5. That’s right (thanks a lot for the correction!):

    Cations are ions that flow to the cathode (the rod in a battery that has negative electric charge), so, they are positively charged ions.

    Ions produced by pure energy ionization are basically cations, as an influx of enough energy will free electrons from the outer layers of orbitals, the atoms will no longer be electrically neutral, as they lose some negative electric charge, they will become positively charged.

    Ions can be formed by other processes different from pure energy ionization, and these other processes are relevant to chemistry but not to the focus of our discussion.

    I use the term “pure energy ionization” to differentiate it from other kinds of ion formation.

    Kind regards, GEN

  6. We know that, up to a certain level, what really determines an atom is its nucleus, as we know that electrons can be “freed” to wander away (completely) from the nucleus, depending on the surrounding temperature or energy.

    An intermediate stage to this would be anions (atoms that lose some of the outermost electrons).

    1. I think you may mean ‘cations’; positively charged ions. (Anions are negatively charged ions.)

      While there would have indeed been ‘stages’ as the universe cooled they were not particularly important. The only element aside from hydrogen present in abundance was helium so only its two ionizations would have been important and they serve only to decrease (but not eliminate) the opacity of the universal plasma. It is with some justification then that we simplify this to the formation of hydrogen atoms, a transition that involved about 3/4 of the baryonic matter in the universe. There are some tweaks that are required to take into account the fact that the transition was not an instantaneous and ‘sharp’ one, but they aren’t usually worth considering.

  7. Indeed, I know that it is a complex chain of arguments, starting with the Alpher-Bethe-Gamow paper that correctly predicted in the late 1940s the formation (with the right relative amounts) of Hydrogen, Hellium and Lithium.

    In fact, it was that argument was the basis of Alpher’s doctoral thesis, if I’m not mistaken, and his defense of the thesis was quite an event, as it gathered a large crowd to witness his arguments.

    Kind regards, GEN

    1. I think you’re confused. Alpher-Bethe-Gamov is about nuclear physics; its about the formation of nuclei of those atoms during the first 3 – 20 minutes of the Hot Big Bang. That’s not about the formation of the atoms themselves, which occurs only 380,000 years later. In the interim, the electrons and nuclei move around independently, as a plasma.

      The formation of atoms was 1920s research, not 1940s research.


  8. I’m talking about the nuclei of Hydrogen, Hellium and Lithium. Complete atoms did not form until “recombination” happened, that is some 380,000 years after the “first moment that we have some info about our universe”, and that happened when the universe expanded to such a size that itself cooled below 3,000 Kelvin, which happens to be the temperature where atoms are cool enough for electrons to settle into orbitals within atoms.

  9. Matt, you mentioned the following: “100,000 years later, things had cooled enough for atoms to form…” We know that this happened some 400,000 years after our universe came to be. If I understand clearly what you have stated, there is evidence that points out that Hydrogen, Hellium and Lithium started to form around some 300,000 after our universe came to be … What is that evidence, and how that evidence tells us that it happened around that moment in time?

    1. Thanks — 100,000 should have been 380,000, I was careless.

      “after our universe came to be” — NOT SO! that should read “after the Hot Big Bang began”. We don’t know anything for sure about what’s before inflation.

      Your question is ambiguous: are you talking about the nuclei or about the atoms?

      In any case, the evidence is not trivial to put together; it’s a chain of arguments. We know how atoms form, so we know how much the universe has to cool before electrons will stick to nuclei. We know the temperature as a function of time according to Einstein’s equations and the measured current values of the density of the universe as a whole, the density of matter, and the current microwave background temperature. Put those two things together and it is straightforward to determine the time when atoms formed.

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