This year I am on sabbatical at the Max Planck Institute for Physics in Munich, Germany, which by a happy coincidence was also the site of the 17th Texas Symposium on Relativistic Astrophysics held here two weeks ago (December 12-15, 1994). I was able to attend the Symposium, to learn quite a bit about the present state of astrophysics, and to contribute a paper co-authored by SF writers Forward, Benford, and Landis and wormhole theorists Visser and Morris [see my recent AV column on this in the mid-December-94 Analog].
The Symposium bears the "Texas" label because in 1962, after astrophysics had just been jolted to its roots by the discovery of quasars, a special meeting was held in Dallas, Texas to bring general relativity theorists together with astronomers and astrophysicists in an attempt to understand the startling new quasar phenomenon. Ever since, the symposia have been held every two years in a variety of locations in and out of Texas. Munich was host to the most recent Texas Symposium because it has become a center of work in x-ray astrophysics, both theoretical and observational, as well as the home base of the European Southern Observatory. The conference was held at a time when the moon was bright (about 2/3 full), because it is easier to get the attention of astronomers at such times.
The "relativistic" part of the conference title literally refers to
phenomena that involve very high energy processes or collapsed states of
matter: gamma, x-ray, and gravity wave astronomy, supernovae, neutron stars,
jets from active galactic nuclei, quasars, black holes, and exotica like
cosmic strings and wormholes. For the purposes of the Texas Symposium,
however, the term has been generously expanded to include areas that are
only marginally relativistic: radio astronomy, microwave background measurements,
gravitational lensing, neutrino astronomy, deep exposure surveys of the
galaxy populations, and dark matter searches. The major topics covered
by the 17th Texas Symposium therefore provide an excellent overview
of the state of some of the most interesting areas of astrophysics. In
this present column I will provide an overview of two of these areas, cosmology
and gravity wave detection. In my next column [Analog,
October-1995] I will give an overview of another area covered in the
symposium: gamma ray bursts.
§ Observational Cosmology: Closing in on the Constants of the Universe - In the 1930s Albert Einstein expanded his fundamental theory of gravity (general relativity) into a formalism for theoretical cosmology that is able to treat whole universes as physical objects described by a few equations. Within these equations, the state and ultimate fate of a given universe are described by the values of a few parameters: H0,W0, WB, t0, and L. None of these quantities are particularly well determined at present. Many of them are expected to change as the universe expands and changes, and in these cases the "0" subscript indicates their present-time value. Let me begin by explaining what they mean.
The Hubble parameter H0 has a present best value of about 66 ± 30 km/sec per megaparsec (1 megaparsec = 3,261,633 light years) and describes the present expansion rate of the universe. In other words, a galaxy 100 million parsecs away should be receding from us at a speed of about 6,600 km/second due to the expansion of the universe.
The density parameter W0 has a present best value between about 0.1 and 2 (theorists love W0=1) and describes the ratio of the actual present mass-density of the universe to the "critical" mass density rc that would exactly "close" the universe (about 10-29 gm/cm3). The value of W0 implies the ultimate fate of the universe, whether it will expand forever (W0<1) at about the present rate, or stall in a parabolic expansion that diminishes asymptotically to zero (W0=1), or reverse its expansion and recontract to a Big Gnab singularity (the ultimate form of recycling) sometime in the future (W0>1).
The baryon density parameter WB has a present best value between about 0.01 and 0.2 and describes the fraction of W0 that is provided by ordinary matter (galaxies, stars, planets, gas clouds, atoms, protons, electrons, etc). The well established fact that WB<<W0 implies that most of the mass of the universe, between 80% and 99%, is not in the form of ordinary matter. This is called the Dark Matter Problem.
The age of the universe t0 has a present best value of about 0.8 ± 0.5 × 1010 years. It is the elapsed time since the Big Bang. The present age t0 is expected to be smaller than the "extrapolated age" given by 1/H0 (about 1.2 × 1010 years) because the universe should have expanded rapidly at first and then decelerated under the pull of the gravitational mass it contains. Theorists would prefer for the product H0t0 to be between about 0.9 and 0.7. Values less than 2/3 or greater than 1 cause them severe problems.
The cosmological constant L, the mass-energy density of free space, was once described by Albert Einstein as "my biggest mistake". In formulating his original cosmology formalism, he discovered that the mathematically described universes emerging from his formalism were dynamically unstable and underwent rapid expansion or collapse. They were therefore inconsistent with the pre-Hubble "steady-state universe" orthodoxy of the time. In desperation, Einstein was driven to give free space itself an energy density rcL that could be positive or negative. This allowed him to produce steady-state solutions to his equations. After the expansion of the universe was discovered by Hubble, Einstein considered this introduction of the "unneeded" constant L an embarrassment.
Today the cosmological constant L continues to be an embarrassment to cosmologists, but for other reasons. Like Einstein, they would like to have L=0 so that it could be dropped as a parameter, but all attempts to predict its value from standard field theories or from emerging theories of quantum gravity have lead to L values that range from infinite to too large by a factor of 10120. No theory gives a value of L small enough to be consistent with observations. This is a major unsolved problem of modern cosmology.
There are also experimental hints that L
may not be zero. Using the Casimir effect [see recent AV columns in Analog
Mid-December, 1990 and February, 1995], laboratory experiments have successfully
removed energy from the volume of space between a pair of parallel conducting
plates, giving that volume an energy density less than that of empty space.
It is unclear whether the alternative of a negative energy density or of
a non-zero baseline free space energy density (L>0)
is more palatable in accommodating this result.
Much of the theoretical and observational work presented at the 17th Texas Symposium on Relativistic Astrophysics was related to the values of these parameters. Uncertainties in the constants of the universe were being trimmed down from many directions. Wendy Freedman described a program that is using the Hubble Space Telescope to gather data on variable Cephid stars in distant galaxies to determine H0 to ±10% accuracy. The present preliminary results of her collaboration give a value of the Hubble parameter of H0 = 80 ± 17 km/sec/megaparsec. Theorist Mike Turner, in a later talk, pointed out that H0 > 85 ± 7 km/sec/megaparsec forces a non-zero cosmological constant (L>0), and that Freedman's new value is perilously close to this range.
§ COBE: Perhaps the worst nightmare of students is to oversleep on the morning of a big exam. The corresponding nightmare of scientists is to oversleep due to jet lag on the morning of an address at an important conference. At the 17th Texas Symposium this happened to David Wilkinson, who was present on the latest results from the Cosmic Background Explorer (COBE). His talk was delayed, but fascinating.
The cosmic microwave background was produced about 100,000 years after the initial Big Bang, when the universe emerged from its plasma phase and protons combined with electrons to form a hot but electrically neutral medium. The photons from this event that reach us today have been Doppler shifted downward in frequency by a factor of 1000 and transformed from infrared light to microwaves. They constitute the majority of photons in the universe and form a "microwave radiation bath" through which, according to COBE data, the Solar system moves with a velocity of 369.5 km/s or 0.123% of the velocity of light.
The microwave background in a 7º angular region of sky has been found by COBE to fluctuate in the intensity by few parts in 106 (the so-called "Face of God" in the initial COBE press reports). The size of these fluctuations has placed tight constraints on cosmological models (and eliminated some of them). So far, the COBE results are consistent with the standard Big Bang model and the inflation scenario. For example, COBE shows a fluctuation power exponent of 1.11±0.6 while inflation predicts 1.00.
Perhaps the most exciting part of Wilkinson's talk, however, were the fluctuations of the microwave background that COBE did not observe because of angular resolution. New calculations predict a "ringing" in the decoupling of the microwave background, a definitive wiggle pattern in the measurements that should be observable at angular scales of <2o. The information content of the predicted wiggles is very rich, and would permit unambiguous extraction of H0, W0, and WB to an accuracy that would depend primarily on the quality of the measurements. Many groups are now attempting these measurements from mountain tops and balloons, but what is clearly needed is a new satellite measurement to perform COBE type measurements with better angular resolution. The European Space Agency satellite COBRA/SAMBAS will be launched soon, and is designed to do this.
At the 17th Texas Symposium the fundamental parameters of our universe are still uncertain to about a factor of 2. I suspect that by the time of the 18th Symposium, two years from now in Chicago, they will have been measured by overlapping methods to 10% or better. And I am betting that the cosmological constant is different from zero.
§ Gravity Wave Detectors - Gravity waves are propagating disturbances in space itself that are produced by rapid movements of massive objects, e.g., black holes or neutron stars in decaying binary orbits. So far, gravity waves have been detected only indirectly by observing the spin-down rate of binary pulsars as the system loses energy due to gravity wave emission. Presently a new major facility funded by the U. S. National Science Foundation for the direct detection of gravity waves, LIGO, the long baseline gravity wave interferometer, is under construction in Washington State and Louisiana. At each LIGO site there will be a right-angle pair of 2 km long multi-pass laser interferometer arms. The detector will record shifts in the laser interference pattern as incoming gravity waves alter the lengths of the detector arms by a distance of about 4 × 10-16 cm. It is expected that LIGO will produce the first direct detection of gravity waves and will open a new window on the universe, creating the new field of gravity-wave astronomy.
At the 17th Texas Symposium on Relativistic Astrophysics, Kip Thorne gave an overview of the new detectors that will be coming on line after LIGO. There are two more ground-based gravity wave detectors: VIRGO, a 3 km interferometer to be located at Pisa, Italy, and GEO, a 0.6 km interferometer to be located at Hannover, Germany.
On the longer time scale, space-based gravity detectors are now being planned. LISA is an ensemble of 6 laser interferometer modules to be funded by the European Space Agency and boosted into a solar orbit 20º behind that of the Earth by an Ariane 5 launcher, perhaps in 2015, with technical studies beginning this year. A somewhat more modest NASA/JPL project, SAGITTARIUS, would place a similar set of interferometer modules in a near-Earth orbit. Both of these space-based detectors, because of their much greater interferometer lengths and their isolation from Earth-based vibration, would be far more sensitive than their Earth-bound counterparts and will have a much better overlap with theoretical predictions of gravity wave producing astrophysical events.
A new window is opening on the universe, and we can expect to see some
strange, puzzling, and interesting sights.
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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