The Cosmic Microwave Background (CMB) is a relic radiation coming from the early universe. The story of this radiation is as follows. The universe in its primeval stages was made of a very hot1 plasma of matter (photons, electrons and baryons2), where photons could not travel much without bumping with electrons. As the universe expanded, the temperature dropped and the free path of photons increased. At some point, photons completely escaped from electrons and propagated freely. This is called the last scattering surface and these photons are what we observe! Actually, we see a snapshot of the physics of that early period of time. Thus, the CMB tells us about what was going on prior to the last scattering surface. Quite remarkably, the CMB radiation arrives at us at (almost) the same $2.73$K of temperature (homogeneous) in all directions (isotropic). This confirms the cosmological assumption that the universe is homogeneous and isotropic on very large distances. Check Wayne Hu’s pedagogical introduction.
Now the question is: what generated initially such almost homogeneous and isotropic universe? The most accepted answer is inflation: a period of a very rapid expansion of the universe, which brought a tiny patch to an astrophysical size. This rapid expansion rendered the content of that patch very close to be homogeneous and isotropic. The most powerful prediction of inflation is that the CMB contains tiny fluctuations (of the order of $1:10^5$). This prediction comes naturally when one considers quantum field theory3 in a rapidly expanding universe. These tiny fluctuations are responsible for all the structure we see in the universe–including ourselves. In the end we are all children of quantum fluctuations! See Baumann’s lecture notes for more details.
I study models of inflation and their signatures in the CMB. For example, I seek to find clarify unexplained features in the CMB (so-called anomalies) which may lead to new discoveries. I also look for characteristic signals of attractive4 models for inflation and theories of gravity.
- We know that because Einstein’s theory of gravity (general relativity) predicts that the universe expanded until its current size. This means that looking backwards in the past the universe shrinks and, as it becomes smaller and smaller, the temperature rises. ^
- Family of the proton. Particle made of three quarks. ^
- The result of joining classical field theory, quantum mechanics and special relativity. ^
- Attractive in the sense they are motivated from fundamental physics and that they are potentially falsifiable. ^
