Roughly 13.7 billion years ago, the universe started expanding from a dense, hot volume. In the early universe, some 400,000 years after the Big Bang, conditions cooled enough to allow the formation of hydrogen atoms from free protons and electrons. After this early phase, known as "recombination," the universe began to take shape as objects - galaxies, stars, planets – coalesced from the elemental raw material left behind after the Big Bang.
Dark Matter's Crucial Role
Figuring out how stars, galaxies and other large-scale astronomical arrangements throughout our universe have taken shape is the focus of the science of structure formation. Observations reveal that galaxies are not randomly distributed but comprise a gigantic cosmic web. By studying the structure of this cosmic web and its changes over time, scientists can infer how much matter is in the universe and how rapidly it has been expanding. In addition, we also learn how galaxies – including our own Milky Way – have arisen and evolved throughout the long sweep of cosmic time.
Dark Matter Is Everywhere
A few key questions that confront scientists studying the universe's structure include: How did all of the unique objects in the universe originate from the nearly uniform conditions of the early universe? How did the universe evolve over time? And how will it continue to change in the future?
The Universe has structure on many scales. In the largest of scales, we see that the Universe is not random: galaxies are not haphazardly strewn around willy nilly. Rather, they form a tight, interconnected "cosmic web". To study the formation of this web, KIPAC scientists use experiments like Planck, BICEP2, POLAR, and Quiet II to study the Early Universe in an effort to understand the "seeds" that led to creation of the cosmic web we see today.
Our understanding of these initial conditions --- the state of the early universe --- is then used to perform state of the art numerical simulations, which KIPAC scientist use to determine how the tiny lumps in the early universe growth through gravity, creating the web-like structures that we see today. By combining these numerical simulations with observations of cosmic structure in the largest scales --- for instance using galaxy clusters, or experiments like the DES, LSST, and WFIRST --- KIPAC scientists can study both dark matter and dark energy; too little dark matter, or too much dark energy, and gravity would not be able to form the structures we see today. Conversely, too much dark matter, or too little dark energy, and the Universe would look much more structured (clumpy) than what we see today.
KIPAC scientists do not stop there, however. While we continue to explore how the cosmic web formed, there are equally pressing questions that we need to understand on smaller scales to understand how the Universe looks today. When and how did the first stars form? What was their impact on the cosmos? How do galaxies form, and how do they populate the gravity-generated cosmic web? Are most galaxies today forming stars, or is the most vigorous epoch of star formation behind us? From developing state of the art numerical simulations, to using observational tools like gravitational lensing to better understand the connection between galaxies and dark matter, KIPAC scientists aim to understand the Universe as a whole, from the largest structures in the cosmic web, to the structure of individual galaxies, and the interplay between them.