Modern Cosmology -
Part IV: The Early Universe and Primordial Nucleosynthesis
Jacobs University Bremen

Extragalactic distance ladder

Caption: When the universe formed with the Big Bang, it was very hot. In order to understand what occurred during the first few hundred-thousand years of the universe's history, you must first understand what heat really is at a microscopic level: Heat is a form of energy, just as light and momentum are forms of energy. On very small scales, this heat energy is represented in the momentum of particles. For example, take an atom of hydrogen: If it is very cold, then it does not move very quickly. If, however, you were to take an atom of hydrogen from the core of the sun (16 Million K), it would be moving very quickly.

With this definition of heat in mind, when the universe was approximately 10-43 seconds old, it was 1032 K. At this age, electrons did not even exist, much less were they free of atoms, which had also not formed. This is the age that is studied by "Quantum Cosmologists". At this age and temperature, matter and antimatter existed in almost equal amounts, but they were both dominated by the background energy of the universe. The laws of physics as they are presently understood that make up the so-called "Standard Model" break down as quantum cosmologists try to make sense of what the universe was like at this time, which is known as the "Planck Epoch", in which the laws of quantum gravity (gravity at sub-quark scales) should have dominated. Since there is not yet an intact theory of quantum gravity, very little is understood about this time period in the universe's distant past.

After this phase, the universe consisted of quarks, gluons, leptons and photons (called a "quark soup"). At the time the universe cooled down to temperatures of 150 MeV, quarks have been confined and hadrons appear.

Particle Physics and the Early Universe
1. Mid-Term Review Solutions
==> Midterm Review Solutions
Mid-Term Solutions
2. Standard Model of particle Physics and the Early Universe
Given our understanding of the current state of the universe, and our knowledge of the appropriate laws of physics, we can extrapolate backwards in time to say what the early universe must have been like. Fortunately, we can then use current observations to test whether such an extrapolation is valid; the answer is that it is remarkably accurate.

The cosmic microwave background (CMB) is the leftover radiation from the Big Bang. When the universe was much smaller it was much hotter and denser. The ordinary matter that today resides in the form of stars, gas, and dust was packed together at incredible densities and temperatures, so much so that electrons moved freely rather than being attached to individual atomic nuclei (much like conditions at the center of a star). This hot plasma gave off copious amounts of radiation, just like any other hot object, and we can detect that radiation today. The plasma was also opaque; any photon would only travel a short distance before bumping into a free electron. At 380,000 years after the Big Bang, the temperature had dipped below about 3,500 Kelvin, cool enough for electrons to recombine with nuclei to make atoms, and the universe suddenly became transparent. As the universe expanded and photons redshifted to longer wavelengths, the radiation subsequently cooled to about 2.725 Kelvin, which is what we see today. The CMB was first discovered by Arno Penzias and Robert Wilson in 1965.

Caption: Supersymmetry (often abbreviated SuSy) is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners. In a theory with unbroken supersymmetry, for every type of boson there exists a corresponding type of fermion with the same mass and internal quantum numbers, and vice-versa. So far, there is only indirect evidence for the existence of supersymmetry. Since the superpartners of the Standard Model particles have not been observed, supersymmetry, if it exists, must be a broken symmetry, allowing the superparticles to be heavier than the corresponding Standard Model particles (at left hand side). These particles, however, would be created in the very universe, when the temperature exceeded a few TeV.

In a universe dominated by matter and radiation (as opposed to dark energy), the mutual gravitational pull of all the particles tends to slow down the expansion rate as the universe expands. When the universe was smaller and more dense, it therefore follows that the expansion rate was much larger than it is today. Indeed, as we extrapolate the universe further back in time, we reach a point where the density, temperature, and expansion rate were all infinitely large. This point is a singularity, which we refer to as the Big Bang (although that term is also used for the entire cosmological model that includes the later universe as well). At the Big Bang, our knowledge of what happens gives out; the fact that physical quantities become infinite is a sign that we don't know what is going on. Presumably, in the real world there is no singularity; instead, something happens that cannot be described by physics as we currently understand it.
The Early Universe
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Exercises V
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Exercises IV - Solutions
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Lecture Notes: Part IV
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