Cosmology Primer: The Measured Universe
Astronomy, arguably the oldest science, has traditionally depended on observations of phenomena in the sky. The observational nature of the discipline distinguishes it from experimental sciences like physics or chemistry, in which we can control the conditions under which we measure the systems of interest, gradually altering configurations and repeating the experiments as necessary. When it comes to phenomena outside our Solar System, we have to take what the universe gives us. The difference between experimental and observational science is much like the difference between asking someone questions versus eavesdropping on their conversations -- asking our own questions allows us to be precise and ask for clarification, but eavesdropping can allow us to learn secrets that people would never have revealed under direct interrogation.
In looking out at the universe, our most straightforward tool is ordinary light, the same messenger that allowed our ancestors to chart the stars and planets. We now know, of course, that visible light is just one form of electromagnetic radiation. Other forms stretch from very long wavelengths (infrared light and radio waves), through visible light, down to very short wavelengths (ultraviolet light, X-rays, and gamma-rays). All of these wavelengths of light are emitted by objects in the universe, and astrophysicists take advantage of all of them to get as complete a picture of the universe as possible, as discussed in the page on the luminous universe.
Galileo, the first person to observe the sky through a telescope, demonstrated the existence of objects (such as the moons of Jupiter) that would never have been found by the naked eye, and we continue to follow in his footsteps. The basic principle of a telescope (working in ordinary visible or infrared light) is simply to collect a large number of photons (light particles) and focus them to a detector. Each individual photon is distinguished by three characteristics: its frequency (or equivalently wavelength, or energy), the direction from which it arrives, and the time at which it is observed. Different detectors may keep track of some or all of this information; studying the distribution of photon frequencies is spectroscopy, the precise directions from which the different photons arrive is of course imaging, while studying the amount of light arriving as a function of time is photometry. Modern telescopes strive for better views of the universe both by increasing the size of their light-collecting area -- typically with large mirrors -- and by being located in places where interference from the Earth's atmosphere is minimized -- either in dry, high-altitude climates, or somewhere in orbit outside the atmosphere entirely.
Telescopes using visible light continue to produce startling discoveries. The Sloan Digital Sky Survey (SDSS), for example, makes use of a dedicated telescope at Apache Point in New Mexico to survey a large area of the deep sky for galaxies and quasars as well as ordinary stars. The SDSS first images the sky, and then does spectroscopy on the most interesting objects found so as to determine their cosmological redshift (as explained in the page on the expanding universe). Since redshift is proportional to distance according to Hubble's Law, the result is a three-dimensional map of the large-scale structure of the universe. Both in the SDSS and in future surveys, an important goal will be to study large-scale images of galaxies to search for the slight distortions of images due to the gravitational lensing of background objects by galaxies in the foreground.
In a complementary vein, a satellite such as the Hubble Space Telescope (HST) can peer extremely deeply into one region of the sky to reveal galaxies in the early stages of their evolution. The great advantage of being in orbit is not that you observe more photons, but rather that the absence of atmospheric distortion allows for the collection of more precise and detailed images. Such improved resolution can be crucial, for example, in the study of distant supernovae, where it is important to distinguish the supernova event from the light of the surrounding galaxy. A successor to the HST, the James Webb Space Telescope, is currently under development by NASA; it will be able to look even deeper into the universe, to help us understand how early galaxies were assembled from relatively smooth distributions of gas and dark matter.
At longer wavelengths, radio telescopes have become an indispensable part of observational astronomy. Several Nobel prizes have gone to discoveries made with radio telescopes, from the discovery of pulsars (rapidly rotating neutron stars) to the first observations of the microwave background itself. Along with visible light, the radio band is one in which radiation can readily penetrate the Earth's atmosphere, so ground-based telescopes are extremely valuable. Nevertheless, just as with visible light, there is much to be gained by minimizing interference with the atmosphere. It is therefore common to place radio telescopes in locations where the atmosphere is both thin and stable; popular choices include the high plains of Chile, and the extreme cold of the South Pole. More dramatic methods of minimizing atmospheric interference include placing radio telescopes on long-duration balloon flights, or simply putting them on satellites completely above the atmosphere. These techniques are especially useful when observing the microwave background; the CMB is at such a low temperature that tiny amounts of contamination from the Earth itself can be troublesome to experiments. The Planck satellite, a planned collaboration between NASA and the European Space Agency, will hopefully provide maps of the CMB of even higher precision than those produced to date. Another goal is to obtain high-precision measurements of the polarization of the microwave background; as explained in the page on the really early universe, such observations may provide crucial clues to the nature of inflation.
In contrast to radio waves and visible light, higher-frequency waves of ultraviolet light and X-rays are unable to penetrate through the atmosphere, and it is necessary to go into orbit to observe them directly. NASA's Chandra satellite has provided an unprecedented view of the X-ray sky; future missions are planned that will inventory black holes throughout the universe, map out hot gas in clusters of galaxies, and image the innermost regions close to black holes themselves. Once we consider extremely high-energy gamma rays, however, the number of photons emitted by interesting sources becomes very small. The planned GLAST satellite will feature a large area telescope to collect as many gamma-ray photons as possible. Alternatively, we can take advantage of the fact that high-energy gamma rays give off a kind of secondary (Cerenkov) radiation when they collide with the atmosphere. This radiation can be detected, and from that we can reconstruct information about the original gamma rays; this technique has been successfully used at the Whipple observatory in Arizona, and is the basis for its successor, the VERITAS observatory.
As mentioned in the page on the luminous universe, photons are not the only means by which we can observe the universe. Other kinds of particles which we are able to observe include cosmic rays and neutrinos, with very different techniques applicable in either case. Cosmic rays, which are thought to be individual protons or atomic nuclei that have been accelerated to tremendous energies, are similar to very high-energy gamma rays, in that the number of particles is extremely low. But because the energy of each particle is so large, it is again possible to observe secondary effects when the cosmic rays interact with the atmosphere. The Pierre Auger observatory, which aims at understanding cosmic rays at the very highest energies, uses two complementary techniques: direct detection of Cerenkov radiation in an array of detectors on the ground, and observations of flashes in the atmosphere caused when secondary particles cause nitrogen to fluoresce.
Neutrinos are also observed using several different techniques. One type of detector uses the fact that a neutrino can interact with an atomic nucleus to turn it into a different element entirely, such as chlorine being converted to argon. It was a detector of this type at the Homestake mine in South Dakota that first provided evidence for an anomalously low flux of neutrinos from the Sun. Alternatively, a neutrino can knock an electron out of an atom, and the electron in turn can travel through a medium such as water and give off detectable Cerenkov radiation. This technique is only sensitive to higher-energy neutrinos, but has the advantage of providing information about the direction and timing of the event; the Super-Kamiokande observatory in Japan used this technique to detect neutrinos from Supernova 1987a in the Large Magellanic Cloud. An advanced version of this technique, used by the Sudbury Neutrino Observatory, uses heavy water (in which ordinary hydrogen is replaced by deuterium) and is sensitive to the dissociation of the deuterium nucleus by a high-energy neutrino. Similar techniques on a very large scale look for neutrinos passing through large bodies of water or through the Antarctic ice sheets, such as the planned Ice Cube facility.
Meanwhile, physicists continue to work on ways to detect particles and waves that have never been directly observed in the laboratory. An obvious example is provided by gravitational radiation, which is emitted by rapidly-accelerated masses such as very close compact binary stars or objects falling into black holes. The LIGO observatory, currently in the early stages of collecting data, consists of facilities in Louisiana and Washington, each with two evacuated arms four kilometers in length, arranged at right angles. Laser light is bounced off of mirrors suspended at the ends of each arm, and sensitive detectors search for tiny displacements due to the stretching of spacetime characteristic of gravitational waves. A future project is the LISA observatory, similar in spirit to the ground-based observatories but consisting of three satellites flying in formation at distances of five million kilometers from each other. Like the Planck mission to observe the CMB, LISA is a joint effort between NASA and the European Space Agency.
It goes without saying that we would like to directly observe the mysterious dark constituents of the universe: dark matter and dark energy. Unfortunately there are many different models for what these might be, and correspondingly many ways we might try to detect them (and the very real possibility that they might never be directly detectable). As discussed in the page on the dark universe, the leading candidates for dark matter are either supersymmetric neutralinos or axions; in both cases there are active programs underway to detect such particles directly. For neutralinos, aside from finding evidence for supersymmetry at high-energy particle accelerators, the best bet is to build extremely sensitive devices that would register each time a passing neutralino scattered off a nucleus in the detector -- a very rare event indeed. To shield as best as one can from sources of external noise, such detectors are typically built deep underground, so that the dirt above serves as a barrier to unwanted particles such as cosmic rays. Axions, meanwhile, can be converted to photons in a precisely-tuned magnetic field, and experiments are underway to search for such an effect. Unfortunately, the magnetic field must be tuned in a way that depends on the mass of the axion, which is one of the thing we don't know with great precision.
For dark energy, meanwhile, there are three main possibilities: it is a strictly constant vacuum energy, a slowly-varying dynamical component, or a breakdown of general relativity on cosmological scales. If the dark energy is simply vacuum energy, there is no way to detect it directly; the best we can hope for is to attain a better understanding of why it has the value it does, perhaps through hints provided by particle physics. If it is dynamical, however, it may be possible to detect long-range forces due to gradual variations in the dark energy. Finally, if gravity is to blame, we may be fortunate enough to detect a breakdown of general relativity in the Solar System due to the same effects that are making it break down in cosmology; unfortunately, the specific effects we might see will depend on the details, which are far from clear at this point.