Cosmology Primer: The Luminous Universe

Our Sun is a star, much like the other stars in the night sky except much closer to us. Astronomical distances are so immense that scientists measure them in terms of the time it takes light to travel between objects. Light from the Sun takes about eight minutes to reach the Earth, so we say the Sun is eight light-minutes away; this is equal to ninety-three million miles. The nearest star, in contrast, is over four light-years away -- almost twenty-four trillion (24×10¹²) miles.

Stars are not distributed evenly throughout the visible universe. They are grouped into galaxies, collections of millions of stars orbiting each other under their mutual gravitational attraction. Our galaxy, the Milky Way, has approximately one hundred billion (10¹¹) stars. Many of these stars are likely to have their own planets orbiting around them; astronomers have discovered numerous planets outside the Solar System, although not a sufficient number to draw reliable conclusions about their abundance. In addition to the stars and planets, galaxies contain large clouds of gas and dust, often in the process of collapsing to form new stars.

Galaxies come in many different forms, depending on their age and history as well as the distribution of stars, gas, and dust. The image on the right is part of the Deep Field image from the Hubble Space Telescope, and shows a wide variety of galaxies. Among the different kinds of galaxies, three varieties are especially fundamental: spirals, ellipticals, and irregulars. Spiral galaxies are large disks containing both stars and dust clouds, typically orbiting around a central bulge of stars. Seen face-on, these galaxies exhibit dramatic spiral arms, indicating regions where stars are being formed. Elliptical galaxies, in contrast, are dense with stars but have relatively little dust, and are similar to the central bulges of the spirals. Irregular galaxies do not feature the well-defined shapes of spirals and ellipticals, and are often smaller galaxies.

While stars orbit each other in galaxies, galaxies themselves often orbit within collections of other galaxies. Our own Milky Way has a number of satellite galaxies, including the Magellanic Clouds visible in the Southern Hemisphere sky. There are also larger collections of galaxies: groups, clusters, and superclusters. The Milky Way is a member of the Local Group, in which the other large member is the Andromeda Galaxy, M31. Superclusters are the largest gravitationally bound systems, but the distribution of galaxies on scales larger than superclusters is nevertheless not completely uniform; the deviations from perfect regularity form the large-scale structure of the universe. The galaxy map from the Sloan Digital Sky Survey shows evidence of large-scale structure.

Stars and galaxies are the most obvious features of the universe, at least if we are observing it using ordinary visible light. In an effort to learn as much as we can, astronomers will often turn to other forms of light. Light can be thought of as either an electromagnetic wave, or as individual particles called "photons." Radiation with a longer wavelength than visible light (infrared and radio waves) correspond to lower-energy photons, while shorter wavelengths (ultraviolet light, X-rays and gamma rays) are high-energy photons.

Observations in different wavelengths give evidence of a violent universe in a constant state of flux. Infrared and radio observations show regions of gas and dust collapsing to form new stars. X-rays and gamma rays, at shorter wavelengths and thus higher frequencies, are formed by very high-energy processes in extreme environments. It is from such observations that we infer the existence of black holes -- regions of spacetime where the gravitational field is so strong that nothing entering can ever escape. In the observed universe, black holes seem to come in two major types: supermassive black holes (over a million times the mass of the Sun) at the centers of large galaxies, and smaller black holes (a few times the mass of the Sun) scattered throughout our galaxy. We suspect that these smaller black holes result from supernovae -- the explosions of stars at the end of their life cycles. Supernovae are rare (perhaps one per galaxy per century), but by observing thousands of galaxies at once we are guaranteed to discover a respectable number on demand; surveys of just this sort have been crucial in making the case for dark energy, as explained in the page on the dark universe.

Most of the photons in the universe are actually not emitted by stars, gas, or any other object in the contemporary universe. Rather, they are the low-energy photons left over from the Big Bang, as described in the page on the early universe. This relic radiation is primarily in the microwave region of the spectrum, and is known as the Cosmic Microwave Background (CMB). Photons from the CMB pervade space, providing a background buzz that serves as a constant reminder of the hot, dense state in which our universe began.

Since ancient times, almost everything we have learned about the universe has come through observing light of various forms. Modern astronomers, however, are increasingly taking advantage of other ways to probe the universe. In addition to ordinary photons, numerous other kinds of particles are constantly bombarding the Earth: protons (and atomic nuclei) that make up cosmic rays, ultra-light neutral particles known as neutrinos, and the gravitational equivalent of electromagnetic waves, known simply as gravitational waves. Cosmic rays have proven to be an invaluable window onto energetic processes in the universe, leading to important insights into particle physics (such as the discovery of the muon, a heavier cousin of the electron); today, the origin of the highest-energy cosmic rays remains a deep mystery. Neutrinos from astrophysical sources have likewise led to significant discoveries; it was the shortage of neutrinos emitted from the Sun which gave the first clue that these particles might have small masses, rather than being strictly massless. Gravitational waves have never been detected directly, although new observatories are coming online with the goal of doing exactly that; however, the effect of gravitational waves has been observed in the loss of energy from orbiting neutron stars, for which Hulse and Taylor were awarded the Nobel Prize.

Next: The Dark Universe