The Cosmic Microwave Background (CMB) WMAP_POL Picture from here Large scale structure Early universe field theory
Lyman alpha systems Clusters Cluster Gravitational Lensing

What is Theoretical Cosmology?

Theoretical cosmologists study the evolution and composition of the universe as a system, constructing theories and confronting them with experiments and observations. Many of the quantities of interest are average properties; consequently, theoretical cosmology deals to a large extent with statistical predictions and measurements. In recent years, advances in measurements and computational techniques have brought us to the brink of quantifying precisely these fundamental properties.

What is known?

There are some fundamental things about our universe which we do know with high confidence:
  • Our universe is approximately homogeneous (the same at all points in space) and isotropic (the same in all directions in space) on large scales and at early times.
  • Our universe is expanding (discovered by Hubble in the 1920's).
  • Space is flat on the average.
  • The mass energy in the universe is approximately 1/4 matter and 3/4 "dark energy", less than 1/5 of the matter is baryonic, the rest is "dark matter" (see figure on right).
  • The universe is approximately homogeneous and isotropic on large scales or at early times.
  • The distribution of matter in the universe today is statistically consistent with growth from a close to scale invariant set of perturbations at an early time under the influence of gravity.
Universe_pie Picture from here

What is under study?

Specifics of this emerging paradigm are still under intense investigation. These include:
  • Exactly how fast is the universe expanding? In other words, what is the Hubble constant, H0?
  • Exactly how much matter is there in the universe? What is it? There is strong evidence for dark matter implied by, for example, the motions of astronomical objects. Is this matter some exotic particle from high energy physics or a quantum black hole or something more prosaic? What is this dark matter made of?
  • Is the dark energy a (cosmological constant), i.e. an ambient 'vacuum' energy density in the universe? Or is it not a cosmological constant but something varying? How would particle theory produce this? And if it is not a constant, how will it evolve (and thus what is the ultimate fate of the universe - will it expand forever or recollapse in a ``big crunch'')?
  • Why was the universe approximately homogeneous and isotropic at early times? Did inflation occur?
  • How did departures from this purely homogeneous and isotropic universe arise? What were the seeds for the formation of structure (stars, galaxies, clusters of galaxies)?

These various questions fall into the areas of cosmological parameter estimation (Hubble constant, matter density, dark energy density, etc.), dark matter searches, early universe model building (inflationary theory, theories of dark energy, string cosmology).

What are the new frontiers?

We now have an incomplete, but highly predictive, theory which connects the structure we see in the universe today to the exotic high-energy physics of the early universe. Within this paradigm calculations can be carried out with high precision (often numerically) and their predictions compared to the wealth of observational and experimental data we are rapidly accumulating. In particular, with many of the parameters/properties mentioned above approximately known, initial conditions are now in place to ask other questions:
  • Structure formation
    What is the (statistical and precise) distribution of structure today and how is it evolving? On large scales, on small scales? Are there simple ways to characterize and understand the emergence of regular structures and regularity from the complexity due to gravitational collapse?
  • Galaxy properties
    Can we extend our understanding of what matter is doing to an understanding of what luminous (more often observed) matter is doing? Dark matter halos can be reliably simulated using numerical simulations of dark matter and gravity only, and galaxies populate these halos often in a many to one manner. How do they do so and what galaxy properties determine the association to the host dark matter halos (collapse time, recent merger activity)?
  • Galaxy formation
    How do galaxies form? What is the role of black hole growth and quasar activity in the history of galaxies? How do objects at high redshift evolve into those we see today; what are the progenitors of today's galaxies of different sorts, of clusters? How did the history of galaxies affect their surroundings (injecting metals, for instance)?
  • Reionization
    How and when did the first luminous objects form and how did reionization occur? What is the respective role of stars and quasars in producing ionizing photons? Were there early supermassive stars (PopIII) and if so, when did they stop forming and what happened at the end of their lifetimes?

Measurements addressing these questions also can shed light on cosmological parameters, etc., and vice versa. Different theories (e.g. different types of matter) predict different results for each of the properties above, and different cosmological histories.

Observational Input

The cosmological history and other properties can be measured in several ways. For example,

  • The expansion of the universe can be studied by measuring redshifts of cepheids, just as Hubble did in the 1920's. The cepheids fluctuate in luminosity on a time scale related to their luminosity. Knowing their absolute luminosity allows one to calculate their distance. The Hubble Space Telescope has a Key Project to measure the Hubble Constant H0. An introduction can be found in this Scientific American article by W. Freedman. Also, some recent review articles are at this link, here and here (a recent workshop summary). (Hubble diagram taken from this paper.)

  • Type I supernovae are well characterized and can be used as another standard candle (object with 'known' absolute luminosity). Again, if their absolute luminosity is known, their distance from us can be deduced. High redshift supernovae searches are being pursued by the supernova cosmology project and the high-z supernova search (the latter is the source of the figure at left). They can be used to measure the energy density of the universe, and the cosmological constant. (This picture is from Goddard.)

  • Large scale structure refers to the distribution of matter on very large scales in the universe, around and above 10 Mpc. This can be studied by galaxy surveys, looking at the spectra of quasars as their light travels through hydrogen clouds to reach us (Lyman alpha systems) and comparing with simulations such as these, and weak lensing, quasar clustering and peculiar velocity measurements. The relation of the hydrogen clouds to other structures at different redshifts is schematically pictured by Z. Haiman.

  • Cluster Clusters of galaxies are used to study the total matter energy density of the universe by measuring their baryon content (cluster baryon fraction) and their abundance and evolution (see this review article or this or this for recent work). They can be used to measure the expansion rate H0. via the Sunyaev-Zeldovich effect (a 1996 review and a more recent review). Galaxy clusters can also be gravitational lenses.

  • Gravitational lensing: the statistics of number of lenses gives limits on the cosmological constant (a classic reference is here and a data base of lenses is here). Time delays from gravitational lenses can be used to measure H0, the Hubble constant (see e.g. this review article or this one). There is a wiki on measuring the Hubble constant via lensing. Weak lensing as above can be used to study large scale structure. The picture source is here.

  • COBE The CMB (Cosmic Microwave Background) is made up of photons coming to us from the time when they decoupled from matter. The photons have the imprint of what the charged matter was doing right before decoupling, as well as the imprint of the gravitational potential they have travelled through in the subsequent 10 billion plus years to reach us. The very small size of the fluctuations in the CMB means that calculational approximations are well controlled, and the resulting predictions are consequently very accurate. The LAMBDA site maintains a list of current and upcoming experiments and data. Many review articles on the CMB at various levels can be found here.
  • Baryon acoustic oscillations (BAO), from the same fluctuations sourcing the CMB, also leave traces in the power spectrum (clustering) of objects, e.g. galaxies, at later times, and can be used to probe both the Hubble parameter and the angular scale at a given redshift, giving clues to any evolution of dark energy (if any evolution occurs). In low redshift galaxy data (SDSS and 2DF) a small BAO signal is expected and seems present, many other galaxy surveys are underway or planned as are searches for the signal in other phenomena besides galaxies. A BAO resource list is here. (See also Daniel Eisenstein's page.)

  • ETC! These are just some examples, many other useful methods exist and more are being invented all the time.

Further Reading

list compiled by M. White and J. Cohn

Last modified March 17, 2006.

This page was developed in collaboration with Martin White.

Some of the material described in this web site was developed while J. Cohn was supported by the National Science Foundation under Grant No. NSF PHY98-96019.
"Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF)."