4.1 Cosmology Basics
4.1.1 What is cosmology?
Cosmology is the study of our universe as a whole, and thus focuses on the largest scales accessible to science. It strives to answer a number of “big questions”: What is our universe made of? How did it begin? How did the matter assemble into the structures we see today? What is its ultimate fate? In order to address these questions, cosmologists use a wide variety of astronomical observations and draw on theory from across essentially all fields of physics, ranging from general relativity to quantum field theory.
One remarkable feature of the study of cosmology is that it provides insights into particle physics – i.e., the study of the smallest scales in physics – that could never be observed in a terrestrial laboratory. The tremendous energy in the hot, early universe shortly after the Big Bang far exceeds the energies that could ever be produced by the most sophisticated particle accelerators on Earth. Now that cosmology is becoming a precision science, particle physicists are using cosmology as a tool, using the study of the largest scales in physics to learn more about the smallest.
4.1.2 The standard cosmological model
Over the past few decades, cosmology has evolved from a highly speculative field dubbed “A search for two numbers” by Sandage (1970) into a full-fledged observational science, with vast quantities of data supporting a detailed theoretical model that is now well-established enough to be known as the “standard cosmological model”. We are commonly said to be living in the age of precision cosmology. However, many aspects of the standard cosmological model are quite surprising, and have led to more questions than answers. The basic premises of the standard cosmological model are as follows:
• Our universe is expanding – the gravitationally bound structures in our universe (e.g. clusters of galaxies) are all moving away from each other.
• Our universe used to be much hotter, denser, and smoother than it is today - the early universe was a hot soup of quarks and elementary particles.
• The large-scale structures in our universe grew through gravitational instabilities seeded by quantum fluctuations in the early universe.
• These quantum fluctuations grew to macroscopic size during a phase of rapidly accelerating expansion of the early universe called “inflation”.
• Ordinary matter (protons and neutrons, a.k.a. baryons) only make up 4% of the mass density in our universe.
• About 21% is made of “dark matter”, a mysterious substance that is gravitationally attractive but does not interact with light.
• About 75% is made of “dark energy”, an even more mysterious substance that effectively gives rise to a repulsive gravitational force and is causing the current expansion of our universe to accelerate.
This basic picture is consistent with a wide variety of different types of measurements, ranging from the tiny fluctuations in the microwave radiation produced by the early universe to the rates at which distant supernovae are moving away from us to the clustering patterns observed in the distribution of galaxies today. The agreement between all of these different measurements is quite remarkable, and has forced scientists to take seriously these rather preposterous concepts of dark matter, dark energy, and inflation.
Today the study of cosmology is focused on gaining a deeper understanding of the physics behind these concepts. What triggered inflation and how did it stop? What is the nature of dark matter? How much of the dark matter might be made of neutrinos? What sort of substance could have such bizarre properties as dark energy? Is dark energy a substance at all, or is the observed acceleration really due to a breakdown of general relativity at cosmological distance scales? These are the types of questions currently being investigated.
Go team science, and good luck with the thesis Molly, even though I know you have it in the bag.