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OCR for page 101
- ~
Opportunities
OBSERVATIONS FROM SPACE
Cosmologists are eagerly looking forward to the observations from
Earth orbit planned for the coming decade. A broad range of the
electromagnetic spectrum will be covered by the proposed missions-
the Gamma Ray Observatory (GRO), the Advanced X-Ray Astrophys-
ics Facility (AXAF), the Hubble Space Telescope (FIST), the Space
Infrared Telescope Facility (SIRTF), the Cosmic Background Explorer
(COBE), the Large Deployable Reflector (LDR), and an antenna in
space to extend the Very-Long-Baseline Array. Previous astronomical
satellites have brilliantly demonstrated that deeper exploration of
space, in many spectral regions, holds great potential for making new
discoveries, solving old problems, and raising important new ques-
tions. The recent results from the Infrared Astronomy Satellite (IRAS)
are an example of the scientific power of well-planned observations
from space. A list of planned studies and possible discoveries is long
and exciting; we can mention here only a few examples directly
relevant to current cosmological problems.
The many discoveries of IRAS highlight the untapped richness of the
infrared sky, so long obscured by atmospheric absorption and emis-
sion. For cosmology the infrared region holds special promise because,
as illustrated in Figure 13.1, this is where one may at last see the birth
of galaxies. The burst of starlight expected to accompany galaxy
101
OCR for page 102
102 COSMOLOG Y
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FIGURE 13.1 This figure shows the extent to which we can explore the universe
throughout the spectrum of electromagnetic radiation (in terms of either the redshift of
sources or, equivalently, how far back in time we see them). The darkly shaded areas
show the extent of our present knowledge. The lightly shaded area shows the region that
we can never view directly because the photons are either scattered by electrons or
collide with other photons, producing electron-positron pairs. The dashed boundary
surrounds the region where we may see galaxies in their early phase of development.
formation may have been redshifted by cosmological expansion into
the infrared region. The detection and study of primeval galaxies will
give us a major foothold in the little understood epoch between z = 103
and z = 1, and perhaps will also be the evolution of large-scale struc-
ture will be clarified. The National Aeronautics and Space Administra-
tion (NASA) is planning to capitalize on the success of lRAS by orbiting
a cryogenic telescope with an aperture of about 1 meter SIRTF. With
pointing and imaging capability, SIRTF will be more sensitive than
IRAS by factors of 109 to 103. Additionally, it will have more wavelength
coverage and spectroscopic capability. A major part of SIRTF's
scientific program will be a deep search for primeval galaxies.
OCR for page 103
OPPORTUNITIES 103
Also of great cosmological significance is the planned role of the
HST in the measurement of the extragalactic distance scale, the
expansion rate of the present universe (Ho), and the deceleration
parameter (q01. High spatial resolution (better than 0.1 arcsec) and
broad spectral coverage will allow more detailed observations of
nearby and distant galaxies, leading to better understanding of the
physical properties of galaxies including evolution. Thus, the HST is
expected to play an important part in improving the classical cosmo-
logical observations over the next decade.
The MST's unique angular resolution and ability to measure redshifts
at z > 1 suggest other observations of cosmological interest. A simple,
but important, observation will be to see whether the shape of galaxies
is evolving. Do the thin disks so prominent in most nearby giant
galaxies persist back to z—1? A1SO7 the HST will be a great help in
charting the way for deep redshift surveys. Because of the large
observing time required, the bulk of this work will be done from the
ground (see the following section), but calibrations and minisurveys
from space will be important benchmarks for these surveys. The HST
will be our best means for studying supernova events in deep space.
Currently, supernovae are being studied as possible cosmic distance
indicators and as a possible alternative to galaxies as probes for
measuring q0. The advantage of using supernovae is that there is a good
chance for theoretical understanding of the spectral and time depen-
dence of their flux without needing to assume that they are standards
of luminosity. One more example: MST's spectrometers operating at
ultraviolet wavelengths (inaccessible from the ground) will be able to
probe the thermal history of the intergalactic medium, which has been
strongly influenced by the formation of structure in the universe. Thus,
constraints can be set on the epoch of galaxy formation and on the
nature of dark matter.
The COBE was designed specifically as a cosmological satellite, to
make detailed measurements of the 3-K radiation and to look for an
infrared background flux. High spectral accuracy will permit a search
for distortions in the sensitive region over and around the blackbody
peak (A ~ 2 mm), and large-scale (>7°) anisotropy will be accurately
measured at A = 9, 6, and 3 mm. Because of limitations on the size of
its antennas, COBE will not look for anisotropy at small angular scales.
AXAF was highly recommended by the report of the Astronomy
Survey Committee as an instrument sure to make important contribu-
tions to broad areas of astronomy, including cosmology. Since the hot
plasmas at the cores of some clusters of galaxies are strong x-ray
emitters, AXAF will be able to make detailed measurements of these
OCR for page 104
104 COSMOLOGY
sources at redshifts of z = 1 to 2. Hundreds of sources per square
degree are expected, with the number depending on the cosmological
parameter q0 and possible evolution. AXAF's ability to carry out
detailed studies of these distant sources promises important new data
from a little-known cosmological epoch. In addition, AXAF will
provide much better data on nearby clusters of galaxies than was
possible with the Einstein satellite. It will measure accurate tempera-
ture gradients as well as density gradients in the hot plasma in rich
clusters. These will yield model-independent measurements of the
gravitational potentials of the clusters and thereby trace the possible
dark matter in the outer regions of the clusters.
The LDR is currently envisioned as a 30-m telescope, with diffrac-
tion limited at A ~ 30 ~m. Two important cosmological observations
are being anticipated: a sensitive measurement of small-scale
anisotropy in the 3-K radiation and a search for primeval galaxies at z
~ 3 using the reflector as a light bucket at A ~ 1-4 ~m. Currently, LDR
offers our best hope of pushing small-scale anisotropy measurements to
levels of ATIT ~ 10-6, in pursuit of the primordial density fluctuation
spectrum. Above the atmosphere LDR offers the low-noise, broadband
capability needed for sensitive measurements near the peak of the
spectrum at A ~ 2 mm.
CONTINUED GROUND-BASED OBSERVATIONS
Most of what we know about the universe has been learned from
interpreting observations made with ground-based instruments. Many
of the data come from large telescopes at major observatories, but
some important contributions have been made with small, special-
purpose instruments. Always, the role of the theorist with a good
understanding of the observations is an essential one, perhaps more so
than in most areas of physics. The prospects for exciting ground-based
work over the next decade are excellent; there is no shortage of
important problems.
Because of the crowded schedule of broad-based science for the
HST, only critical cosmological observations of relatively short dura-
tion can be made. Ground-based observatories will continue to be our
main sources of data about the universe. The rapid pace of develop-
ments in extragalactic astronomy indicates that we are only just
entering the age of discovery. The Astronomy Survey Committee
discusses a broad range of opportunities for ground-based telescopes;
here we emphasize only a few of particular current interest to cosmic
physics.
OCR for page 105
OPPORTUN/T/ES 1 05
Two major themes of current work are to measure q`, and to
understand the origin and evolution of large-scale structure in the
universe. Recent redshift surveys of large numbers of nearby galaxies
have greatly increased our understanding of kinematics and galaxy
clustering in the local universe. It is important to extend this under-
standing to redshifts of z—I if possible. The joint distribution of galaxy
redshifts and magnitudes will measure large-scale clustering of galaxies
and afford a much clearer understanding of the evolution of structure in
the universe, which depends on q`' and on the nature of dark matter.
Such a survey is technically feasible with current and planned tele-
scopes. Curiously, the interpretation of data from a deep survey
program would be limited in part by our lack of systematic, baseline
knowledge of nearby galaxies. Such fundamental studies are well
within the reach of present technology, but they have not been done.
There is a perception among observers that such long-term programs,
however important, cannot be undertaken because of uncertainties in
funding and the allocation of telescope time.
Currently, our deepest look into the big bang is provided by
measurements of the abundance of light nuclei. Astronomical obser-
vations of these abundances need to be extended to more sources and
to even better accuracy. More theoretical work must be done to find
and understand all possible astrophysical production and destruction
mechanisms. As a cornerstone of our current hot big-bang cosmolog-
ical model the nucleosynthesis argument must be as sound as possible.
Similarly, there is still much to be learned from further studies of the
3-K radiation. The spectrum near the blackbody peak needs to be
measured still more accurately, large-scale anisotropy measurements
can be improved (especially at millimeter wavelengths), and better
polarization searches can be made. Fine-scale anisotropy measure-
ments, of great importance to the understanding of primordial fluctu-
ations, should be pursued from aircraft or balloons if necessary. Little
is known about anisotropy on intermediate scales (~1°~; all angular
scales are potentially interesting and should be probed to the highest
possible precision.
The critical question of the nature of the dark matter that appears to
dominate the present universe must be addressed by predicting and
searching for signatures of the various candidates. Some possible
signatures that have been suggested include x rays from accreting black
holes, infrared radiation from very-low-mass stars' ultraviolet photons
from the decay of massive neutrinos, direct detection of magnetic
monopoles, and the photons from axion decay induced by magnetic
fields. Theoretical studies will continue to impose constraints' such as
OCR for page 106
106 COSMOLOG Y
the limits on cosmological monopole flux imposed by the existence of
a galactic magnetic field (the Parker limit). Particle accelerators are not
generally regarded as astrophysical observatories, but the discovery of
a stable weakly interacting, massive particle could have a profound
effect on cosmology.
PARTICLE PHYSICS AND COSMOLOGY
Conventional cosmology, if correct, places some important con-
straints on particle physics; examples are the allowed number of
neutrino types and the allowed ranges of masses and half-lives of
neutrinos. The new particle physics has generated some exceedingly
stimulating ideas in cosmology and has great potential for influencing
future thinking and directions; for example, the discovery of Higgs
particles would be of major importance in lending credence to the
inflation scenario. Within the decade the width of the neutral interme-
diate-vector boson Z° and the partial width due to neutrino pairs may
be measured. Since the number of neutrino types affects nucleosyn-
thesis, the measurement directly tests the big-bang model.
Many particle-physics experiments of interest to cosmologists do not
use accelerators. One class of such experiments tests the predictions of
theories, such as Grand Unification, which have implications in cos-
mology. Examples are the searches for proton decay and for an electric
dipole moment of the neutron. Other experiments, such as those
attempting direct detection of dark-matter candidates, offer the hope of
a decisive resolution of important cosmological problems.
THEORY
Given the limited and indirect observational basis of cosmology, it is
essential that theorists range broadly in their search for interpretations
and for crucial observational and experimental tests. Fortunately, the
field is sufficiently exciting to attract excellent theorists in graduate
school and from other areas of physics and astronomy. It is impossible
to anticipate where theory might go in the near future, but we briefly
mention a few of the current promising ideas.
On the particle-physics side, the successful quantization of gravity
seems essential for penetrating the mysterious Planck era. Perhaps
only then will physics be able to address the question of initial
conditions. Currently quantum gravity enjoys great popularity among
gravitation, particle, and cosmological theorists. There has recently
been much study of universes with more than four dimensions,
OCR for page 107
OPPOR TUNI TIES 107
motivated in part by supergravity theories, which attempt to unify
gravity with the other three forces. (See the discussion under Quantum
Gravity in Chapter 8.) An intriguing possibility is that these theories
might lead to an understanding of the origin of space-time itself.
Another difficult task is to develop the theory and consequences of
symmetry-breaking transitions in the early universe. For example, the
time-dependent transition that may cause inflation needs to be better
understood.
On the astrophysical side, one attempts to understand the structure
of the universe as it is now and to infer from that what it must have
been like in the past. Essential to such a program are detailed studies
of the complicated processes occurring during the nonlinear develop-
ment of a multicomponent system of radiation and one or more
dark-matter candidates. The processes must be understood from the
present back to a time before the radiation decoupled from the matter.
Such studies may invoke a wide variety of possible scenarios, but they
must mesh with a rich texture of observations. In many cases extensive
numerical computation is essential, and here a barrier to progress is the
somewhat irregular and informal coupling of theorists to the frontiers
of progress in computing technology. We also expect analytic methods
to continue to provide new ideas and important guidance for observers.
Finally, we must bear in mind that the search for viable alternative
cosmological models should continue. As an example, cold big-bang
models in which the microwave background was produced by stars and
thermalized by dust at an early epoch cannot be dismissed; they can
give a present ratio of photons to baryons in agreement with the
observed value. A major difficulty with such models is that no natural
way to produce the observed deuterium has yet been found. Another
class of nonstandard models are those that were initially chaotic rather
than smooth. Is it possible that some process like particle production
smoothed them out? What fraction of such models could evolve to
resemble the present universe? What is the effect of an inflationary
epoch on such models?
Representative terms from entire chapter:
cosmological observations
