Discovering a Runaway Universe

Astronomer Edwin Hubble first measured the expansion rate of the universe in 1929. Using the 100-inch Hooker Telescope on Mount Wilson in California, Hubble discovered that the farther a galaxy is from us, the faster it appears to be receding into space away from us. Hubble uncovered this linear relationship by plotting his observations of 24 nearby galaxies – their observed velocities vs. their distance. That plot revealed that the universe was expanding uniformly in all directions. The rate of that expansion, a value called the Hubble Constant, is an essential ingredient in determining the age, size, and fate of the cosmos.

Though exceptional for their time, Edwin Hubble’s original distance measurements were imprecise and gave an approximate age of the universe between 10 billion to 20 billion years.

Nearly a century after Edwin Hubble’s momentous discovery, the telescope named in his honor refined those distance measurements. Astronomers use the telescope to measure distances by comparing the brightness of a known object in our galaxy (like a star or a supernova) to that of similar objects in a distant galaxy. They then couple those distances with the best galaxy-velocity measurements obtained from other telescopes. After more than three decades of observations, teams using Hubble’s extraordinary capabilities measured the expansion rate to a precision of just over 1%, about eight times more precise than they originally anticipated! This value for the Hubble Constant puts the age of the universe at about 13.8 billion years old. However, there is a twist…

Ground-based image of the Andromeda Galaxy stretches from lower left to upper right. A white arrow points to the location of the Hubble observations. Above the galaxy are four boxes containing Hubble images of the variable star at different luminosities.

Astronomers use cyclical changes in the brightness of Cepheid stars to determine astronomical distances. The arrow points to a Cepheid star in the Andromeda galaxy observed by Hubble (inset boxes).

NASA, ESA, Hubble Heritage Team (STScI/AURA), R. Gendler

In 2011, Hubble observations, along with those of ground-based observatories, surprised astronomers by revealing that the universe is not just expanding, but accelerating – a discovery that won the 2011 Nobel Prize in Physics. Many scientists believe an invisible force, called “dark energy,” causes this acceleration. We can think of dark energy as an “antigravity” or repelling force that pushes galaxies apart by stretching space at an increasing pace. Although current technology does not allow us to directly measure dark energy, we can characterize it by observing its effect on normal matter in the visible universe. From these observations, scientists estimate that dark energy is about 68%, dark matter is about 27%, while normal matter and energy are only about 5% of the entire universe. By studying how dark energy behaves over time, astronomers hope to gain a better understanding of what it is and how it might affect the future of the cosmos.

When the Hubble Space Telescope launched in 1990, one of its main goals was to measure the rate at which our universe is expanding.
NASA; Producer & Director: James Leigh

A circular, gold Nobel Prize Medal sits against a black background.

Hubble science team member Adam Riess and fellow astronomers won the Nobel Prize in Physics from the Royal Swedish Academy of Sciences on October 4, 2011 for discovering that the expansion of the universe is accelerating. The astronomers used Hubble data, as well as data from several ground-based telescopes.

NASA

Edwin Hubble found a Cepheid star in the Andromeda galaxy that proved that galaxy was beyond our own Milky Way galaxy.
NASA, ESA, and Z. Levay and G. Bacon (STScI)

A portion of the Hubble Deep Field. Two observations of the same part of the sky. 1995 reels a bright star that is no longer there in the 2002 observation. This animated gif cycles between the two images making the supernova appear to blink on and off.

Certain supernovae have a characteristic maximum brightness that astronomers can use to calculate their distances from Earth. Refining celestial distances enables astronomers to better calculate the expansion rate of the universe. The arrow points to a distant supernova discovered in an area of the sky first imaged in 1995 called the Hubble Deep Field. Astronomers found the supernova when they targeted the same area of sky again in 2002 and saw a change.

NASA and J. Blakeslee (JHU)

Dark energy is just one of the mysteries Hubble is trying to uncover.
NASA; Producer & Director: James Leigh

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Image is filled with distant galaxies in colors of yellow-gold, red, blue, white. Lower left holds a face on spiral galaxy.

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