Theory
 
Overview
The theoretical research at KICP covers a wide range of topics from clusters of galaxies to modifications of Einstein's theory of general relativity. Our research emphasizes connection of theoretical models to observation and experiment, as each of our major theoretical thrusts is intimately connected with our experimental programs. Institute theorists are involved in developing new theories which may be constrained by data. Theorists are also working to optimize and compare competing observational strategies.

A crucial goal of modern cosmology is to understand why the conditions of our universe take on the particular states that we observe, rather than those of some very different kind of universe. This can be separated into two sets of issues:
  • What is the nature and origin of the matter and energy that comprise our current universe?
  • What determined the initial conditions from which the Universe evolved?

Answering these questions brings together ideas at the intersection of cosmology, particle physics, and gravitation. The theoretical research activities at KICP are broadly grouped into 3 areas:
 
The Expansion of the Universe -- Today and in its Infancy
KICP theorists are actively investigating the following issues:
  • inflationary cosmology (did the very young universe go through a phase of rapid expansion?)
  • the origin of the matter/antimatter asymmetry (why is the observable universe dominated by "normal matter" and not anti-matter?)
  • primordial nucleosynthesis (how did the first elements in the Universe form?)
  • dark energy (why is the Universe accelerating and what is the nature of the "dark energy" that is believed to be causing this acceleration?)
  • dark matter (what are the dark matter particles and how do they interact?)
  • are there alternatives to general relativity that might explain the observed acceleration and account for the dark energy?

Of the topics listed above, one of the most pressing is the attempt to understand the observed acceleration of the universe. It has by now been well established that we do not live in a conventional Universe dominated by matter. Even when dark matter is taken into consideration the Universe is accelerating and spatially flat - two phenomena which cannot be accounted for by the current matter density. The most straightforward explanation for these measurements is the existence of a small but nonzero vacuum energy, or cosmological constant. However, this possibility raises significant problems: the vacuum energy is much smaller than it has any right to be, and seems suspiciously close to the matter density. It is therefore worth considering more dramatic possibilities.

One such possibility is that of dynamical dark energy, which would be slowly varying but not perfectly constant. Even more dramatic is the possibility that Einstein was wrong, and that ordinary general relativity might be breaking down on cosmological scales. Both possibilities are currently under investigation, both in terms of underlying models and observational signatures.

Along with dark energy, inflation is one of the foundational ideas of contemporary theoretical cosmology. One of the principal research goals of the Institute is to advance our understanding the physics of inflation and its relationship to particle physics at high energies. We would also like to understand how we can best constrain the parameters of inflation with the Cosmic Microwave Background (CMB) observations and large-scale distribution of galaxies.

We are currently investigating both fundamental issues of relevance to inflation (quantum fluctuations, entropy, effects of string theory and extra spacetime dimensions) and the construction of detailed inflationary models (scalar fields and their interactions, production of density and gravity-wave perturbations, the process of reheating into matter and radiation). An important feature of inflation is that it makes specific and testable predictions for observables such as CMB temperature and polarization; as these data become increasingly refined, we hope to be ready to interpret them in terms of models of fundamental physics.
 
The Large-Scale Structure of the Universe
The large-scale structure of the Universe provides a laboratory in which KICP theorists can test models for inflation and the dark energy. Structure in the mass distribution of the universe is thought to originate as quantum fluctuations during the inflationary epoch that grow under gravity for most of subsequent expansion history and freeze after the expansion of the Universe started to accelerate. Although the mass distribution is not directly observable, structures manifest themselves in the anisotropy of the CMB, the abundance of galaxy clusters, the clustering of galaxies, and the distortion of astronomical images by gravitational lensing.

The CMB offers the earliest direct view of the inflationary density perturbations and hence some of the sharpest tests for inflationary models. In the future, CMB polarization may allow us to measure spectrum of matter fluctuations during inflation. It may also enable tests of the whole paradigm through limits on or detection of inflationary gravitational waves. Center theorists collaborate closely with experimentalists in the planning of future experiments to best exploit these capabilities.
The CMB also sets the initial conditions against which to compare more local large scale structure for tests of the dark energy. The abundance of massive dark matter halos offers a sensitive probe of the amplitude of mass fluctuations. KICP theorists work to understand the relationship between dark matter halos and observable objects such as luminous galaxies and clusters of galaxies. Observations of the clustering of these objects in the Sloan Digital Sky Survey coupled with numerical simulations of large scale structure are providing empirical constraints on these relations.

Finally, measurements of the gravitational lensing of galaxy images and CMB anisotropy offer opportunities to study the mass fluctuations directly. KICP theorists are involved in developing and applying methods for extracting information on the dark energy and inflationary spectrum as well as the relationships between luminous objects and dark matter halos.
 
The Formation and Dynamics of Galaxies and Clusters
KICP faculty, fellows and graduate students study formation and dynamics of galaxies and clusters. We exploit the potential of the nearby structures to probe the distribution and physics of dark matter and its interaction with baryonic matter in the Universe.
KICP theorists are working to construct and make detailed comparisons of sophisticated numerical simulations and new observations of galaxy clusters by the X-ray space telescopes and the radio instruments, such as the Sunyaev-Zel'dovich Array (SZA).
They are testing generic predictions of the standard galaxy formation paradigm against observations and building models to explain the observations in ways that are consistent with the theoretical predictions. Recent examples are the work of Kravtsov and collaborators who proposed a novel solution to the scarcity of dwarf satellites around galactic halos compared to the numbers predicted by theory --- the discrepancy often called "the missing satellites problem". The result puts the cold dark matter scenario on more solid ground.

The Institutes particle astrophysicists have been studying different strategies for detecting the products of dark matter annihilation in the Galactic halo substructure and in other nearby halos. These theoretical studies make direct predictions for what could be observed by atmospheric Cherenkov telescopes such as the KICP project, VERITAS. They are also exploring particle properties beyond the Standard Model through relics produced in the early Universe as well as more recent astrophysical events.
KICP theorists are studying how the observations of dwarf galaxy satellites in the Milky Way and the local group can help to put better constraints on the formation history of our galaxy as well as on its present structure. Others and are also working to put better constraints on the masses of black holes at the centers of galaxies and developing a better understanding of how galaxies and their components form.