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baryon acoustic oscillations

The imprint of sound waves from the era of the cosmic microwave background, about 400,000 years after the Big Bang. These waves would imprint a standard distance between galaxies, with the distance increasing as the universe ages and expands. The details of this "scale length" would carry information about how the universe has expanded at different epochs since the Big Bang, which in turn would reveal details about the nature of dark energy.

Big Bang

The theoretical moment of creation for our universe. In this model, all matter, energy, space, and time sprang from a geometric point known as a singularity. Observations show that the Big Bang occurred about 13.7 billion years ago. The universe has been expanding since the moment of the Big Bang.

Big Crunch

A possible ending for the universe in which everything collapses back to a single point, like the one that began expanding at the Big Bang. For a Big Crunch to happen, the universe must contain enough matter for its gravity to overcome the expansion of the universe, reverse it, and pull everything back in.

Big Rip

A possible ending for the universe in which dark energy is so strong that it not only drives the galaxies away from each other, but it eventually forces all particles of matter away from each other. In this scenario, galaxies, stars, and planets all would disintegrate.

cosmic microwave background

WMAP image of cosmic microwave background
The "afterglow" of the Big Bang. It fills the universe as microwaves glowing at a temperature of about 2.73 degrees Kelvin (-454.4F, -270.3C). Tiny "ripples" in this afterglow, corresponding to temperature differences of a few millionths of a degree, indicate that the early universe was not completely smooth, but instead was "lumpy." The lumps were denser agglomerations of matter, and formed the "seeds" from which the first galaxies formed.

Cosmological Constant

A possible explanation for dark energy. First proposed by Albert Einstein to bring his equations into balance with the then-observed universe, it proposes that space itself produces a form of energy, known as the vacuum energy, that causes the universe to accelerate faster as it ages. Current models show that the observed dark energy is far too weak to be accounted for by theories of the cosmological constant.


The study of the past, present, and future of the universe, including the structure and evolution of energy, matter, space, and time.

dark energy

A description of our lack of understanding of how the universe works on the largest scales. It may be a "repulsive" force that is causing the universe to expand faster as it ages, a discrepancy in the laws of gravity, or some other phenomenon.

dark matter

A form of matter that emits no detectable energy, but that reveals its presence by exerting a gravitational pull on the visible matter around it, such as stars and galaxies. Dark matter appears to make up more than 80 percent of all the matter in the universe. The leading hypothesis says that it consists of a new type of elementary particles, usually described as "wimps" -- weakly interacting massive particles.

Doppler shift

A change in wavelength caused by an object's motion toward or away from the observer. An everyday example is the change in pitch of a siren or train whistle. When it moves toward you, the sound waves are compressed, so the pitch rises; when it moves away from you, the sound waves are stretched, so the pitch falls. The same thing happens with visible light and other forms of electromagnetic radiation. If a star or galaxy is moving away from us, its light is shifted to redder wavelengths; if it is moving toward us, its light is shifted to bluer wavelengths. Astronomers can use this shift to calculate the object's speed, and, with additional observations, to deduce its distance.

expanding universe

The universe is expanding as a result of the Big Bang. The rate of expansion has varied. Evidence shows that until about five billion years ago, the expansion was accelerating but at a slowly decreasing rate. Since then, the acceleration has increased, perhaps as a result of dark energy.


One of the four fundamental forces in the universe, it is a property of matter that "warps" the space around it, causing an attraction between two objects. The magnitude of gravitational attraction depends directly on mass and inversely on distance squared. In other words, if you move twice as far from an object, its gravitational pull on you will be only one-fourth as strong. Gravity was a more dominant force in the early universe, since the universe was smaller and there was less distance between objects. As the universe has expanded, dark energy appears to have become more dominant, pushing the galaxies apart at a faster rate.


The Hobby-Eberly Telescope Dark Energy Experiment. It will use an upgraded Hobby-Eberly Telescope, along with a suite of spectrographs known as VIRUS, to map a section of the universe as it existed 9 billion to 11 billion years ago. It will map one million galaxies or more and measure their motion through space, providing a probe of the nature of dark energy.

Hobby-Eberly Telescope

HET at night
The largest telescope at McDonald Observatory. Its primary mirror consists of 36 individual one-meter segments that are joined together like the tiles of a floor. They create a mirror with an overall diameter of 11 meters. Only 9.2 meters can be used for any individual observation, however, because of the telescope's unique design. Upgrades will increased the telescope's field of view by 2010.


A sudden and dramatic expansion of the universe after the Big Bang. The "force" that propelled inflation may be related to dark energy.


A unit of length used by astronomers to measure great cosmic distances. One light-year is equal to the distance light travels in one year, which is about 5.88 trillion miles or almost 800 times the diameter of our solar system. The nearest star is a mere four light-years away, while the nearest large galaxy lies about 2.5 million light-years from Earth. Astronomers also use another unit, the parsec, which is equivalent to 3.26 light-years.


The apparent shift in position of an object relative to background objects when observed from two different locations. An everyday demonstration of parallax is easy to try: hold a finger in front of your face, and without moving it, wink one eye and then the other. When you see the position of your finger change with respect to the background, you are seeing parallax. Astronomers measure the parallax of a stars close to the Earth using Earth's orbital diameter as a baseline. The astronomer observes a star at six-month intervals, when Earth on opposite sides of the Sun. Astronomers look for a shift in the position of the star, and if they find one, they can use simple geometry to measure the star's distance from Earth.


A unit of distance equal to 3.26 light-years. The name means "PARallax-SECond," and it refers to a way to measure the distances to other stars. The most accurate way to measure the distances to close stars is to use basic geometry. Astronomers measure the position of a star in the sky at six-month intervals, when Earth is on opposite sides of the Sun. If the star is close, then it will appear to shift a bit compared to the background stars. It's the same effect you see if you hold your finger in front of your face and look at it with first one eye, then the other: the finger appears to move against the background of objects. This effect is called parallax. If a star has a parallax of one second -- in other words, it appears to shift back and forth across the sky by exactly one second of arc (1/3600 of a degree), then its distance is one parsec.


A proposed new field that would account for dark energy. Magnetic, electric, and gravitational fields already permeate the universe, and some scientists suggest that dark energy could be in the form of a yet-undetected field.


A shift in the spectrum of an object's light caused by its motion away from the observer. HETDEX will measure the redshifts of one million or more galaxies to determine their speed and distance, which will serve as a probe of the properties of dark energy.

relativity, general

"Space tells mass how to move" while "mass tells space how to curve" -- J.A. Wheeler. Einstein created this model, which describes gravity as a curvature in space-time, which is the four-dimensional fabric of our universe. His theory is the best model for gravity so far, and has been confirmed in experiments and observations. According to the theory, regardless of one's point of view (as measured by speed and direction), physical law and the speed of light are unchanged. This implies that measurements made in time and space are not absolute, but relative to your particular point of view or reference frame. General relativity led to concepts and theories such as black holes, parallel universes, worm holes, and space-time.


A technique used by astronomers to determine the properties of celestial objects by breaking their light into its individual wavelengths or color and analyzing the intensity of each wavelength. Each element absorbs and emits light in a unique set of wavelengths, so the astronomer can sift through the spectrum of a star and determine what elements are present in the star. Spectroscopy also allows astronomers to measure an object's motion toward or away from Earth, its temperature, and other properties. HETDEX will use spectroscopy to measure the properties of at least one million galaxies, including their motion and distance.


spectra redshift
A wavelength or frequency band of electromagnetic radiation that contains information about the source of energy. An optical instrument called a spectrograph refracts the light into a spectrum of color with a grating or prism.


The explosive death of a star. There are two basic varieties of supernovae. One forms when the core of a massive star collapses and its outer layers blast into space. The other forms when a white dwarf star (the exposed, dense core of a once-normal star like the Sun) steals gas from a companion star. When enough gas builds up, it triggers a runaway reaction that blasts the star to bits. The brightness of a white-dwarf supernova, known as a Type Ia, varies in a predictable way, making them good "standard candles" for measuring distances to remote galaxies. Type Ia supernovae are used as a probe for evaluating different models of dark energy.

vacuum energy

A possible explanation for dark energy. First proposed by Albert Einstein to bring his equations into balance with the then-observed universe, it proposes that space itself produces a form of energy, known as the cosmological constant, that causes the universe to accelerate faster as it ages. Current models show that the observed dark energy is far too weak to be accounted for by theories of the cosmological constant.


VIRUS close-up
Visible Integral-Field Replicable Unit Spectrographs. A set of spectrographs that will be used with the Hobby-Eberly Telescope for the HETDEX project. Each of the 150 or so units will contain bundles of optical fibers that will direct the light from distant galaxies to a spectrograph, which will split the light into its component wavelengths or colors.