Each bulleted item links to a representative page (rather shaded towards my favorite universities). Similar pages exist at other universities in each topic area. The pages chosen here present one perspective and should not be taken as the definitive description of that research area. For other perspectives you can visit a comprehensive list of graduate programs in the US. At each university you can search for a page on research interests for that department.
In practice most of these branches of physics employ both theorists and experimentalists. Occasionally an individual wears both hats.
Condensed Matter Physics
Condensed matter physics is the field of physics that deals with the macroscopic physical properties of matter. In particular, it is concerned with the condensed phases that appear whenever the number of constituents in a system is extremely large and the interactions between the constituents are strong. The most familiar examples of condensed phases are solids and liquids, which arise from the bonding and electromagnetic force between atoms. More exotic condensed phases include the superfluid and the Bose-Einstein condensate found in certain atomic systems at very low temperatures, the superconducting phase exhibited by conduction electrons in certain materials, and the ferromagnetic and antiferromagnetic phases of spins on atomic lattices.
Condensed matter physics is by far the largest field of contemporary physics.A lot of progress has also been made in theoretical condensed matter physics. By one estimate, one third of all American physicists identify themselves as condensed matter physicists. Condensed matter physics has a large overlap with chemistry, materials science, nanotechnology and engineering.
One of the reasons for calling the field condensed matter physics is that many of the concepts and techniques developed for studying solids actually apply to fluid systems. For instance, the conduction electrons in an electrical conductor form a type of quantum fluid with essentially the same properties as fluids made up of atoms. In fact, the phenomenon of superconductivity, in which the electrons condense into a new fluid phase in which they can flow without dissipation, is very closely analogous to the superfluid phase found in helium 3 at low temperatures.
Atomic, Molecular and Optical Physics
Atomic, molecular and optical physics is the study of matter-matter and light-matter interactions on the scale of single atoms or structures containing a few atoms. The three areas are grouped together because of their interrelationships, the similarity of methods used, and the commonality of the energy scales that are relevant. Physicists sometimes abbreviate the field as AMO physics. All three areas include both classical and quantum treatments.
Individual atoms or small groups of atoms are the focus in this field. Some researches use single trapped atoms to make extremely precise measurements to confirm theories about Quantum Mechanics or Relativity. Others are exploring the interaction between atoms (or molecules) or the interaction between light and atoms. Lasers and laser physics are an important part of this research area.
Nuclear physics is the branch of physics concerned with the nucleus of the atom. It has three main aspects: probing the fundamental particles (protons and neutrons) and their interactions, classifying and interpreting the properties of nuclei, and providing technological advances. Nuclear physics includes the fission and fusion reactions that provide nuclear power, as well as the nuclear synthesis process that fuels the sun and other stars.
High Energy Particle Physics
Particle physics is a branch of physics that studies the elementary constituents of matter and radiation, and the interactions between them. It is also called high energy physics, because many elementary particles do not occur under normal circumstances in nature, but can be created and detected during energetic collisions of other particles, as is done in particle accelerators.
The current state of the classification of elementary particles is the Standard Model. It describes the strong, weak, and electromagnetic fundamental forces, using mediating gauge bosons. The species of gauge bosons are the gluons, W- and W+ and Z bosons, and the photons, respectively. The model also contains 24 fundamental particles, which are the constituents of matter. Finally, it predicts the existence of a type of boson known as the Higgs boson, which has yet to be discovered.
Another major effort in theoretical particle physics is string theory. String theorists attempt to construct a unified description of quantum mechanics and general relativity by building a theory based on small strings, and branes rather than particles. If the theory is successful in this, it may be considered a Theory of Everything.
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature and chemical composition) of astronomical objects such as stars, galaxies, and the interstellar medium, as well as their interactions. The study of cosmology is theoretical astrophysics at the largest scales.
Because it is a very broad subject, astrophysicists typically apply many disciplines of physics including, but not limited to, mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics. In practice, modern astronomical research involves a substantial amount of physics.
Physics Education Research
This is a relatively new field in physics departments. The practitioners are physicists who observe students learning physics and examine the difficulties encountered. Their work is changing the face of the college physics courses nationwide.
A growing number of departments support degree candidates who want to straddle two disciplines in their research. Topics like biophysics are indicative of the programs available.