Subatomic Particles Described on the Web
Cosmology Web Sites
Web-Based Interface Allows Students to Create
Models of the Universe
Frequently Asked Questions in Cosmology
by Ned Wright, UCLA
Astronomy images from various sources,
including HST, VLA; Astronomical Society of the Pacific Slide sets
about COBE, Compton Gamma Ray Observatory, Galaxies, Infrared, Life on
Mars, M31, Multiwave Sky, Past Astronomers, Quasars, Rosat, Seti, and
Search for Extrasolar Planets; plus artwork by William Hartmann
Microwave Anisotropy Probe (MAP) to be launched in 2000
http://physics.stanford.edu/linde.
A direct link to his 1994 Scientific American article on inflationary cosmology is at
http://www.sciam.com/specialissues/0398cosmos/0398linde.html
See http://physics.stanford.edu/linde for movies showing the process of self-reproduction of inflationary universes, as calculated by Prof. Andrei Linde of Stanford University.
The results of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale were announced in May 1999. Wendy Freedman, Barry Madore, and Robert Kennicutt were the Principal Investigators. In the Key Project, which took up 420 hours of Hubble time, they observed dozens of Cepheid variables in each of 18 galaxies, and used those Cepheid distances to determine the slope of Hubble's law. The value they derived for Hubble's constant is 70 km/s/Mpc, plus or minus 10%. The plus or minus 10% is the key advance. As Prof. Robert Kirshner of Harvard put it, we used to have a disparity of a factor of 2, which is like being uncertain if you have one foot or two, and now we are down to a toe.
At the American Astronomical Society Meeting in Chicago in June, values presented that corrected for the metallicity (the content of elements heavier than hydrogen and helium) brought the Hubble Constant to 68, still plus or minus 10%.
But Allan Sandage and his coworkers still don't accept this value. They have not given up their scientific defense of values for Hubble's constant less than 60.
Also, a group of radio astronomers using the VLBA have made direct measurements of the distance to the galaxy NGC 4258 by a new geometric method. They measured the motion of gas orbiting near the center of NGC 4258 (presumably around a giant black hole there). They are studying masers in the disk, which is about 2 light years across. With the VLBA, they were able to measure actual shifts across the sky (proper motions) of the masers at intervals of 4- to 8- months over more than 3 years. Comparing these proper motions with Doppler shifts earlier measured at the VLBA gives the distance to the galaxy, by trigonometry.
They get a distance to the galaxy of 23.5 million light years, compared with values of 27 to 29 million light years being determined for the same galaxy from Cepheid variables measured with the Hubble Space Telescope. This discrepancy is worrysome, and could carry over to Hubble's-constant determinations.
The observations were part of the Harvard Ph.D. thesis of Jim Herrnstein of the National Radio Astronomy Observatory. A couple of dozen other galaxies could also be studied with this technique.
NASA/STScI Press release, 5/25/99
The Hubble Space Telescope Key Project Team announced on May 25, 1999, 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.
"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.
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 Hubble's constant at 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.
"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.
A Nature of the Universe Debate, in memory of David Schramm, was held
in Washington, DC, on 4 October 1998. The title was:
"The Nature of the Universe: Cosmology Solved?"
Speakers were:
P. James E. Peebles, an astrophysical cosmologist's viewpoint
Michael S. Turner, a particle cosmologist's viewpoint
Owen Gingerich, introductory talk
Joseph I. Silk, introductory talk
Margaret J. Geller, moderator
Details are available at:
http://antwrp.gsfc.nasa.gov/debate/debate98.html
John Hawley of the University of Virginia and Katherine Holcomb have an algebra-only book on cosmology, "Foundations of Modern Cosmology" (Oxford University Press, copyright 1998). My colleague Karen Kwitter, who reviewed the book, says that it is a fabulous book and is very suitable for a non-majors but upper-level course on cosmology. The authors' website contains a lot of relevant information and photographs.
A slide set showing cosmic-background-radiation images is available from the Astronomical Society of the Pacific.
Recent data from the ground are available; the experiment contact I have is Max Tegmark, then of the Institute for Advanced Study, Princeton.
