Primordial magnetic fields and relic gravitational waves: messengers of the first microseconds
Axel Brandenburg (Nordita)

Our detailed understanding of cosmology rests on only a few firmly
established observational probes. One of them derives from the expansion
history pf the universe and the clustering of galaxies observed in
the large-scale structure of galaxies observed today. The other is
the measurement of temperature anisotropies and polarization in the
cosmic microwave background that formed when the universe was 400,000
years old. Finally, there is the measurement of the abundances of light
elements in the universe that constrain the physics within the first three
minutes. However, we lack measurements of the first microseconds when weak
and electromagnetic forces decoupled and particles attained their masses
and quarks got confined inside nucleons. During these times, physics
beyond the standard model must have determined the matter-antimatter
asymmetry, produced dark matter, and led to the emergence of neutrinos. We
can probe these very first moments through measurements of primordial
magnetic fields and relic gravitational waves. The evolution of magnetic
fields can be traced in an evolutionary diagram of magnetic field versus
length scale and it follows a characteristic path. Relating the start- and
endpoints to each other is an important theoretical accomplishment. At the
same time, observational constraints in radio and gamma ray frequencies
begin to narrow down the allowed parameter space in the evolutionary
diagram. In this colloquium, I will discuss several of the theoretical
and observational discoveries that make primordial magnetic fields
and gravitational waves powerful probes of the first microseconds of
the universe.