Hubble measures the expanding Universe

Hubble Measures the Expanding Universe

Latest results from the Hubble Space Telescope pin down the age of the Universe

FROM A NASA HQ PRESS RELEASE

May 25, 1999: The Hubble Space Telescope Key Project Team today announced that it has completed efforts to measure precise distances to far- flung galaxies, an essential ingredient needed to determine the age, size and fate of the universe.

Right: A NASA Hubble Space Telescope (HST) view of the magnificent spiral galaxy NGC 4603, the most distant galaxy in which a special class of pulsating stars called Cepheid variables have been found. Researchers found 36-50 Cepheids and used their observed properties to securely determine the distance to NGC 4603. Observations of distant Cepheids such as those in NGC 4603 also help astronomers to precisely measure the expansion rate of the Universe (more information).

"Before Hubble, astronomers could not decide if the universe was 10 billion or 20 billion years old," said team leader Wendy Freedman of the Observatories of the Carnegie Institution of Washington. "The size scale of the universe had a range so vast that it didn't allow astronomers to confront with any certainty many of the most basic questions about the origin and eventual fate of the cosmos. After all these years, we are finally entering an era of precision cosmology. Now we can more reliably address the broader picture of the universe's origin, evolution and destiny."

The team's precise measurements are the key to learning about the universe's rate of expansion, called Hubble's constant. Measuring Hubble's constant was one of the three major goals for NASA's Hubble Space Telescope when it was launched in 1990.

Hubble Constant image
For the past 70 years astronomers have sought a precise measurement of Hubble's constant, ever since astronomer Edwin Hubble realized that galaxies were rushing away from each other at a rate proportional to their distance, i.e. the farther away, the faster the recession. For many years, right up until the launch of the Hubble telescope -- the range of measured values for the expansion rate was from 50 to 100 kilometers per second per megaparsec (a megaparsec, or mpc, is 3.26 million light years).

The team measured the Hubble Constant to be 70 km/sec/mpc, with an uncertainty of 10 percent. This means that a galaxy appears to be moving 160,000 miles per hour faster for every 3.3 million light-years away from Earth.

Left: Astronomer Wendy Freedman and her collaborators have used pulsating stars called Cepheids to measure the distance to galaxies like the Fornax cluster barred spiral galaxy NGC1365. The ground based photo (left) shows an inset locating the HST image (right) which Freedman and team have used to identify some 50 Cepheids. Their distance and velocity measurements of these and other Cepheids determine Hubble's constant to be about 70 kilometers per second per megaparsec.

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"The truth is out there, and we will find it," said Dr. Robert Kirshner of Harvard University. "We used to disagree by a factor of two; now we are just as passionate about ten percent. A factor of two is like being unsure if you have one foot or two. Ten percent is like arguing about one toe. It's a big step forward." Added Robert Kennicutt of the University of Arizona, a co-leader of the team: "Things are beginning to add up. The factor-of-two controversy is over."

The team used the Hubble telescope to observe 18 galaxies out to 65 million light-years. They discovered almost 800 Cepheid variable stars, a special class of pulsating star used for accurate distance measurement. Although Cepheids are rare, they provide a very reliable "standard candle" for estimating intergalactic distances. The team used the stars to calibrate many different methods for measuring distances.

"Our results are a legacy from the Hubble telescope that will be used in a variety of future research," said Jeremy Mould of the Australian National University, also a co-leader of the team. "It's exciting to see the different methods of measuring galaxy distances converge, calibrated by the Hubble Space Telescope."

Combining Hubble's constant measurement with estimates for the density of the universe, the team determined that the universe is approximately 12 billion years old -- similar to the oldest stars. This discovery clears up a nagging paradox that arose from previous age estimates. The researchers emphasize that the age estimate holds true if the universe is below the so-called 'critical density' where it is delicately balanced between expanding forever or collapsing. Alternatively, the universe is pervaded by a mysterious 'dark force' pushing the galaxies farther apart, in which case the Hubble measurements point to an even older universe.

The universe's age is calculated using the expansion rate from precise distance measurements, and the calculated age is refined based on whether the universe appears to be accelerating or decelerating, given the amount of matter observed in space. A rapid expansion rate indicates the universe did not require as much time to reach its present size, and so it is younger than if it were expanding more slowly.

The Hubble Space Telescope Key Project Team is an international group of 27 astronomers from 13 different U.S. and international institutions. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD.

Web LinksÂ

HUBBLE COMPLETES EIGHT-YEAR EFFORT TO MEASURE EXPANDING UNIVERSE, NASA HQ press release, May 25, 1999

The Expansion Rate and Size of the Universe., W.L. Freedman. From Scientific American, March 1998.

The Shapley-Curtis Debate in 1920.What is the scale of the Universe? What was the debate, why was it important, and how was it resolved? From Astronomy Picture of the Day.

75 years later: the 1996 debate on the size and age of the Universe. What is the scale of the Universe? What was the debate, why was it important, and how was it resolved? From Astronomy Picture of the Day.

Edwin Powell Hubble -- Biographical Memoir

The Hubble Constant -- from a NASA Space Science Short

Hubble's Constant and the Expanding Universe (I) -- Astronomy Picture of the Day, May 13, 1996

Hubble's Constant and the Expanding Universe (II) -- Astronomy Picture of the Day, May 14, 1996

Edwin Hubble Discovers the Universe -- Astronomy Picture of the Day, Feb 17, 1996

The Cepheids of M100 -- Astronomy Picture of the Day, Jan 10, 1996

PG 1115: A Ghost of Lensing Past -- Astronomy Picture of the Day, Nov 2, 1998

Cosmic Gamma-ray Bursts -- News and Research

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