March 17, 1999
Measuring the Expansion Rate of the Universe
Quasar PG 1115+080, split into four distinct images (numbered 1-4) by the gravitational field of the massive foreground galaxy seen in the center of the image. "Gravitational lensing," as illustrated here, allows astronomers to measure the distances to objects far beyond our local cosmic neighborhood. For instance, quasar PG 1115+080 turns out to be approximately 8 billion light-years from Earth. The foreground galaxy is at 3 billion light-years. The image was taken in infrared light by NASA's Hubble Space Telescope (HST). The linear streaks in the image are artifacts of the HST camera.
Image Credit: Christopher D. Impey (University of Arizona) and NASA/Space Telescope Science Institute.
Gravitationally lensed quasars like the one shown above offer astronomers a new method to measure the Hubble constant (H). This constant, named after the American astronomer Edwin P. Hubble (1889-1953), indicates how fast the universe is expanding. The universe's expansion rate, in turn, allows astronomers to estimate the age of the universe, as well as to narrow down theories about its eventual fate--namely, whether it will expand forever or if gravity will bring the expansion to a halt and cause it to re-collapse.
The Hubble constant, as determined from observations of gravitationally lensed quasars, is 48,000 miles per hour for every million light-years in distance, according to a team of astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) and the University of Arizona (UA). This means that galaxies that are 1 billion light-years distant recede from us at a speed of about 48,000,000 miles per hour, those twice as far away recede twice as fast, etc.
In the language of astronomers, the Hubble constant measured by the CfA/UA team is 70 km sec-1 Mpc-1 (meaning 70 kilometers per second per million parsecs; one parsec corresponds to 3.26 light-years; one light-year equals 5.9 trillion miles).
Determining H
Determining H requires two separate measurements: one that is very difficult, the other that is more routine. The difficult measurement is getting the distances to faraway galaxies and quasars. The routine one is obtaining the speeds at which they recede.
The CfA/UA team was able to measure the distance (D) to quasar PG 1115+080 because the gravity of the massive foreground galaxy bends the light rays from the quasar in such a way that here on Earth we see four distinct images. The light of each of the images takes a slightly different path around the foreground galaxy. By estimating the differences in the path lengths of the four light beams (this requires a model of the mass distribution of the foreground galaxy) and knowing the speed of light, the distance to the quasar can be calculated.
The second measurement, that of the quasar's recession speed (v), was obtained by splitting its light into wavelength components and comparing the resulting spectral pattern with standard laboratory patterns. The CfA/UA team found that the quasar is receding at a speed of roughly 380 million miles per hour. By repeating these measurements for about two dozen distant, lensed quasars and averaging the results, the team obtained the value for the Hubble constant of 70 km sec-1 Mpc-1 stated above (note H = v/D).
Professor Christopher Impey of the UA Steward Observatory points out that this value of H is actually an upper limit. Choosing different, but still realistic mass distributions for the foreground lensing galaxies lowers the value of H somewhat. Nevertheless, the CfA/UA result falls within the range for H obtained by other teams using different methods for measuring the distances to faraway objects. The range is 60 to 75 km sec-1 Mpc-1.
The Meaning of H
A value of H in the range from 60 to 75 km sec-1 Mpc-1 suggests that the universe is between 13 billion and 16 billion years old--if, and this is a big if, the matter in the universe is sufficient to eventually bring its expansion to a halt without causing a re-collapse. This amount of matter is referred to as the "critical" density. (If the actual density of the universe is greater than the critical density, gravity will eventually bring about a re-collapse. If it is less, gravity is too weak and expansion will continue forever.)
All measurements to date of the matter in the universe (i.e., matter contained in galaxies, hot gas enveloping galaxy clusters, and diffuse gas existing between galaxy clusters) indicate that the matter density of the universe is at most 10% of the critical value. If correct, the age of the universe is then only about 9 to 11 billion years (based on H = 60 to 75 km sec-1 Mpc-1).
This spells trouble for our current theories of the universe, for the oldest stars have ages of approximately 16 billion years. Obviously, the universe cannot contain stars older than itself.
There are two possible ways out of this dilemma. Either there exists more matter in the universe than astronomers have found, which would raise the universe's density and bring its age closer to the 16 billion years required by the oldest stars; or there exists a mysterious force that counteracts gravity over cosmic distances, which would also raise its age. Einstein postulated such a force, though later retracted it and called it the "biggest mistake" of his life. Some theorists speculate that such a repulsive force could result from quantum effects in the vacuum, but there is no evidence for it.
Thus, two of the great mysteries of the universe--its age and its eventual fate--remain unsolved. Astronomers hope that the next generation of more powerful space telescopes, to be launched during the first decade of the second millennium, will bring them closer to answers.
More Cool Stuff
- For more information about the work reported here, known as the CfA-Arizona Space Telescope Lens Survey (CASTLES), go to press release 98-37 of the Space Telescope Science Institute, dated October 26, 1998:
- http://oposite.stsci.edu/pubinfo/pr/1998/37/pr9837.html
- To learn about two other projects also designed to determine the Hubble constant--the "HST Key Project on the Extragalactic Distance Scale" led by Wendy Freeman of the Carnegie Observatories in Pasadena, CA, and a project led by Allan Sandage, also of the Carnegie Observatories--see NASA press release 96-94, dated May 9, 1996:
- ftp://ftp.hq.nasa.gov/pub/pao/pressrel/1996/96-94.txt
- For yet another project on the Hubble constant, that of the High-Z Supernova Search Team led by Peter Garnavich of the Harvard-Smithsonian CfA, whose results support the conclusion of many astronomers that the universe is likely to expand forever, go to our Observation of the Week of June 3, 1998:
- http://observe.ivv.nasa.gov/nasa/ootw/1998/ootw_980603/ob980603.html
As part of its Learning Technologies Project (LTP), NASA supports a number of educational Web sites that have excellent material on the space sciences:- Jet Propulsion Laboratory (JPL) sponsors Telescopes In Education (TIE), a project that focuses on providing the tools students need to make hands-on discoveries in astronomy and astrophysics.
- http://tie.jpl.nasa.gov/tie/
- The Smithsonian Astrophysical Observatory has developed an integrated science curriculum for grades K-6 that brings the Spirit of Inquiry into the everyday life of the classroom with Everyday Classroom Tools.
- http://hea-www.harvard.edu/ECT/
- Other LTP sites that are dedicated to space science can be found at:
- http://learn.ivv.nasa.gov/education/topics/space_sci.html