When you look up at the sky, you might guess that patches of the sky far away from one another have been evolving independently from each other. However, previous CMB measurements have indicated that this isn’t the case. In fact, all regions of the sky seem to know something about one another. No matter how far apart, each part of the sky is almost exactly the same temperature. This would be a quandary for Einstein himself, as his theory of relativity tells us that these parts of the sky are too far apart to have had any communication since the beginning of the universe. In the 1980s, a model called inflation was suggested to explain this surprisingly uniform sky temperature, it says the universe we observe today comes from a single patch that was rapidly stretched at early times. Although our current understanding of the early universe seems to be consistent with this inflation model, we have not yet observed the “smoking gun” of inflation. This is a primary objective for the Simons Observatory. The smoking gun that we are searching for is B-mode polarization pattern of CMB photons, a particular pattern which looks “swirly” to the eye. If we see this pattern, we will be able to draw conclusions about the inflationary model of the early universe.
A variety of cosmological measurements (of which measurements of the CMB is included) have provided us with evidence that ~96% of the composition of our universe is not like the kind of matter we interact with everyday (baryonic matter: protons, neutrons, electrons, etc.), but is instead dark matter (~23%) or dark energy (~73%).
A number of cosmological observations (e.g. the velocities of galaxies in galaxy clusters, galactic rotation curves, measurements of gravitational lensing; the energy distribution of the CMB) show effects that would be produced if there was a lot more matter than what is actually seen. The proposed explanation for these observations is a new, yet undetected type of particle (or particles) deemed Dark Matter (DM). The nature of DM remains one of the largest mysteries in physics today. The unprecedented sensitivity of the new generation of CMB experiments will enable scientists to identify and study small, subtle features in the CMB pattern, thus providing a probe into the properties of DM that is complementary to those from ongoing particle physics experiments.
One way that DM can be probed is through gravitational lensing. When an object is observed through a magnifying lens its image increases in size and becomes distorted. The same phenomenon happens in the cosmos, where the lens is represented by massive structures and the distorted objects are galaxies and the background CMB pattern. According to the Einstein’s theory of General Relativity, the light that propagates in the universe feels the gravitational potential sourced by massive objects (stars, galaxies, galaxy clusters) and in response its path is altered. This gravitational lensing effect is one of the most promising tool that cosmologists can use for mapping the distribution of total matter in the universe, especially the invisible dark matter.
The fact that our universe is accelerating as it expands posed a surprising challenge to our understanding of the laws of physics when it was discovered nearly 20 years ago. This fact is still one of the greatest puzzles in the universe and we haven’t really got passed giving it a name, Dark Energy (DE). Understanding DE is one of the driving scientific goals for a whole class of next generation cosmological observatories.
Though we have no concrete understanding of DE, two classes of phenomenological explanations have been proposed to explain it. First are new models of the gravitational interaction which alter Einstein’s General Relativity. The second proposal is that there is another completely new class of energy which behaves fundamentally differently than anything we have ever observed and pushes all things apart like a sort of anti-gravity.
By observing small distortions in the energy and trajectory of CMB photons as they travel through the forming structures in the universe over billions of years, we can learn about the mechanism driving the acceleration of our universe, possibly discovering if either of the suggested explanations is likely to provide the correct picture of our universe, or if we have to start from scratch!
You might have heard of protons, neutrons, and electrons, which are the building blocks of the matter around you. However, you might not of heard of neutrinos, the electron’s shy cousin. Neutrinos rarely interact with particles like the protons, neutrons and electrons, and are therefore difficult to study. Even though physicists have known about neutrinos for over 80 years, we still don’t know how heavy they are (i.e. we don’t know their mass). We do know that they have a (tiny) mass from experiments that have observed neutrinos changing their flavour along their path from the emitting source to the observer (neutrino oscillations). This phenomenon is only possible if neutrinos have a non-zero mass. Neutrinos are the only standard model particles of unknown mass (and we really want to fill this gap!) and measuring the neutrino mass scale is a fundamental step towards unveiling other properties of these unique particles.
Dark Matter (which is partly made up of neutrinos) is the stuff that primarily dictates how the universe’s constituents clump together on very large scales. If neutrinos have a large mass, they will make up a large fraction of the DM and if they have a small mass they will make up a small fraction of the DM. If a large fraction of the DM is made up of neutrinos, then we expect that the large scale stuff in the universe to look washed out; however if a small fraction of the DM is made up of neutrinos, we expect the large scale stuff in the universe to look very clumpy. So, by looking at the large scale structure of the universe, or how matter clumps together on the universe’s largest distance scales, we can deduce what fraction of dark matter is composed of neutrinos and eventually their mass.
Some of the CMB photons we will collect have traveled through galaxy clusters (large groups of stars, galaxies and hot gas bound together by gravity). Inside the clusters, CMB photons scatter off electrons in the hot gas and increase their energy by a tiny, yet detectable amount. The change in energy of these photons has a very peculiar signature, and is called the Sunyaev-Zeldovich effect (or SZ effect).
Detailed analysis of the SZ effect for a particular galaxy cluster can teach us about the properties of that cluster. In particular, we can use this to learn about the gas composition and velocities of individual galaxy clusters, probing some of the largest distance scales in the universe. If we combine the information from our SZ studies with other observations of the same clusters, we can learn about about how these galaxy clusters evolve.
Additionally, we can use our SZ information from many clusters to obtain a statistical description of the distribution of clusters depending on their mass and distance from us. This will give us information about the evolution of our universe and the interactions at play between the different components of the cosmic inventory.