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Frontiers of Astrophysics
.
LARGE-SCALE STRUCTURE IN THE UNIVERSE
Probes of Large-Scale Structure
As seen on photographs taken with the largest telescopes, galaxies
appear to drift in the depths of space like motes in a sunbeam.
Everywhere they are clumped in groups containing a few galaxies,
and occasionally in clusters of a thousand or more. Some clusters
clump in superclusters 50 megaparsecs or more across. On even
larger size scales, however, groups and clusters of galaxies seem to
be distributed nearly at random, the number in a given volume of
space being about the same throughout the Universe.
This uniformity of the distribution of matter on very large scales
invites comparison between the observations and a simple model of
the Universe, or cosmology, derived for a uniform distribution of
matter from Einstein's General Theory of Relativity. According to
this model, the geometry of space-time is curved by matter, and the
curvature forces the matter to move: at any epoch the Universe must
be either expanding or contracting. Hubble's discovery in 1929 that
the Universe is actually expanding forces us to confront a bizarre
implication of the theory: that an expanding Universe must have
originated in a powerful explosion-referred to as the big bang
before which neither time nor space had any meaning.
In the half-century since Hubble's momentous discovery, astron
37
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38
ASTRONOMY AND ASTROPHYSICS FOR TEIE 1980's
omers have been probing the Universe in space and in time. Using
radio and optical telescopes, they have found objects so distant that
they are receding from us at 90 percent of the speed of light. With
microwave antennas, they have discovered a faint radio noise that
they interpret as the remnant of the big bang itself. From the theory
of the nuclear reactions that must have taken place during the first
3 minutes, they have calculated the abundances of key elements and
isotopes such as hydrogen, deuterium, and helium, which were
produced in the big bang; with ground-based telescopes and ultra-
violet spectrographs in Earth orbit they have verified that the actual
relative numbers of these atoms in space agree surprisingly well with
theoretical predictions.
The big bang has become the standard model with which to com-
pare observations. This is not to say that it is completely correct: the
data are imprecise; their interpretation may be in error; and the
theory could be wrong. A central problem for the future is the further
development of the big-bang model and its testing against all avail-
able observations.
The big-bang model requires that matter is distributed uniformly
on large scales. By using a variety of approaches, it is now possible
to test whether this is true. Observers have used apparent magni-
tudes as a rough measure of the distances of galaxies; plotting the
directions of galaxies in various distance ranges, they have found
that on scales exceeding 100 megaparsecs, galaxies are distributed
rather uniformly. One can obtain the precise location of each galaxy
in three dimensions by determining its red shift spectroscopically.
Recording the spectrum photographically is time consuming, but the
recent development of electronic array detectors has speeded up the
recording of spectra so greatly that red-shift surveys of thousands
of galaxies are now possible. The resulting three-dimensional distri-
bution appears to be uniform on the largest scales. It is anticipated
that red-shift surveys of much more distant galaxies will be com-
pleted during the 1980's.
X-ray and gamma-ray astronomy also tell us about the large-scale
distribution of matter. A diffuse background emission not attributable
to known sources appears in both spectral regions; its near isotropy
proves that it cannot originate within the Galaxy but must instead
originate at distances comparable with the size of the Universe itself.
The High-Energy Astronomical Observatory-1 (HEAo-1) x-ray ob-
servatory established that the x-ray background is highly isotropic
and that its spectrum between a few and about 60 kiloelectron volts
(keV) agrees closely with the radiation expected from a gas having
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Frontiers of Astrophysics
39
a temperature of about 500 million degrees, leading to the suggestion
that such gas is distributed uniformly between the galaxies. The
Einstein (HEAo-2) x-ray observatory, on the other hand, discovered
that individual quasars at large distances are powerful x-ray sources
in the few-keV range powerful enough, in fact, that quasars at even
larger distances than can be detected individually by the Einstein x-
ray observatory must account for a substantial fraction of the ob-
served x-ray background in the few-keV range. As some quasars
have also been found to be powerful gamma-ray sources, the gamma-
ray background may also be due to quasars. It is still not clear,
however, how quasar spectra would sum up so as to mimic the
spectrum of hot gas. The Advanced X-Ray Astrophysics Facility (AX~)
recommended in this report can observe sources 100 times fainter
than could the Einstein x-ray observatory and can thus determine
whether faint quasars account for the observed background at ener-
gies of a few keV. Measurements of faint quasars by the Gamma
Ray Observatory (GRO) will give similar information for the back-
ground at gamma-ray energies. If it proves that the x-ray and/or
gamma-ray backgrounds are actually due to quasars, the fact that
the background is highly isotropic requires that matter at great dis-
tances is distributed very uniformly. If, on the other hand, inter-
galactic gas is responsible for at least part of the x-ray background,
one can infer that it is distributed uniformly; moreover, the amount
of gas required is an important datum for the theory of evolution of
galaxies.
The cosmic microwave background radiation also gives information
about the large-scale structure of the Universe. Precise measurements
have revealed a smooth variation in its intensity over the sky that
is attributable to the Earth's motion through the cosmos. The ob-
served variation is unexpectedly large, corresponding to a velocity
of 500 km/see for the Local Group of galaxies with respect to distant
matter. The same measurements reveal no other certain variations
larger than 0.03 percent, indicating that the Universe was highly
uniform at the time the background radiation last interacted with
matter. Ground-based experiments indicate that the spectrum of the
microwave background radiation does not deviate significantly from
thermal, as predicted by the big-bang model, but a balloonborne
submillimeter experiment points to discrepancies that are difficult to
explain. Both variations in intensity with direction and deviations
from a thermal spectrum will be measured over the entire spectral
range with improved precision (about 0.01 percent) by the Cosmic
Background Explorer (COBE) mission planned by NASA.
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40
Expansion Time Scale
ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
The big-bang model predicts that galaxies should move away from
each other with velocities that are proportional to their separations.
Slipher and Hubble found evidence for such a relationship in exten-
sive measurements of the brightnesses and red shifts of galaxies
during the 1920's; the constant of proportionality between a galaxy's
velocity and its distance is called the Hubble constant. According to
relativistic models, the reciprocal of the Hubble constant (the "Hub-
ble time") is roughly equal to the present age of the Universe that
is, the time that has elapsed since the big bang.
Determining the value of the Hubble time requires the measure-
ment of the distances of remote galaxies, using a "ladder" of inter-
connected distance scales determined by different methods; each step
of the ladder reaches further into space. Hubble's own estimate for
it was 2 billion years. It has since been revised several times to 5,
then to 10, and then to 20 billion years; the latest estimates are
between 10 billion and 20 billion years. Each revision has been the
result of a major advance in understanding the properties of stars
or galaxies that are used to construct the ladder of distance scales.
The value of the Hubble time enters all cosmological calculations
in a fundamental way. To find its true value, each step of the ladder
of distance scales must be secure, and any contributions to the ve-
locities of galaxies that are not due to the expansion of the Universe
must be taken into account. An example of the latter effect is the
motion of the Local Group of galaxies revealed by study of the cosmic
background radiation; when this is taken into account, a more con-
sistent set of data for the Hubble time emerges.
Refinement of the distance ladder will take much more work.
Development of more precise astrometric methods, as recommended
in this report, will make possible a more accurate measurement of
the distance to the Hyades star cluster, the first step in the ladder
of cosmic distance scales. Because of its extremely faint limiting mag-
nitude, Space Telescope (ST) will for the first time resolve Cepheid
variable stars in the Virgo cluster, thereby eliminating an uncertain
intermediate step of the distance ladder. The continued deployment
of advanced optical detectors at ground-based telescopes will make
possible the rapid measurement of red shifts of galaxies at moderately
large distances, where the velocity field should be one of nearly pure
expansion; ST can determine the distances of the same galaxies by
comparing the brightness of their globular clusters with the bright
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41
ness of those in the galaxies of the Virgo cluster, whose distances
are known accurately.
The Early Universe
The cosmic microwave background radiation carries information about
the Universe before it was about 1/100,000 of its present age, so the
COBE experiment is fundamental to studies of the early Universe.
Other clues depend on the nucleosynthesis of various elements and
isotopes in the first 3 minutes. Theoretical predictions of their abun-
dances depend critically on the amount of ordinary matter present
during that period. If the amount is low, the resulting deuterium
abundance would be high and the helium abundance low; if the
amount is high, the opposite would be the case. Present information
on the abundance of deuterium and helium in interstellar space in
our Galaxy, taken at face value, indicates that the amount of matter
is too low by a factor of 10 for its gravitation to be able to halt the
expansion of the Universe.
However, helium has been produced and deuterium has been
destroyed in stars, so present abundances in the Galaxy may not be
the same as in the primordial gas that emerged from the big bang.
Abundances in intergalactic gas, if it exists, should be primordial.
Astronomers have discovered absorption-line systems in distant
quasars that probably originate either in clouds formed by the out-
ward ejection of thick shells of gas from the quasar itself or in in-
tergalactic clouds lying along the line of sight. In the first case, the
phenomenon would resemble the late stages occurring in the stellar
outbursts known as novae. In the second case, the clouds should
contain very little carbon or other medium-weight elements, which
are telltale signs of stellar nucleosynthesis, because such gas would
never have been inside a galaxy. The gas in such clouds would be
a good candidate for the study of primordial helium and deuterium.
Observations of helium and deuterium in such gas, however, must
be made at much shorter wavelengths than are accessible to ground-
based observatories; they require ST. With ST we can study helium
lines in clouds of red shift greater than unity and deuterium lines
in clouds of all but very low red shifts.
Groups, Clusters, and Superclusters
The grouping of galaxies on various size scales can be studied by
calculating the statistical correlations between the observed positions
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ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
of galaxies. In earlier studies, the apparent magnitudes of galaxies
were taken as measures of their distances, and their positions then
follow from their observed directions in the sky. The calculated
correlations between the positions derived in this way decrease
as an inverse power of the distances between pairs of galaxies.
A simple model to explain this is based on gravitational clustering
of point masses, which are initially distributed at random but
which then move under their mutual gravitation as the Universe
expands. This model reproduces many of the features of the ob-
served clustering of galaxies, so that galaxies may have formed
early in the expansion of the Universe and clumped together later
by gravitation.
Recent observational work, however, has brought out an un-
expected new feature in the distribution of galaxies. Aided by red-
shift measurements, which furnish the distances of galaxies much
more accurately than estimates based on their apparent magni-
tudes, astronomers have found that groups of galaxies outside of
clusters are not sprinkled at random through space but instead
lie in great sheets between the clusters, leaving vast empty re-
gions between. To explain this may require a new theoretical
model. in which Galaxies formed rather late. At first, giant tur-
bulent cells of gas collided, compressing the gas into sheets; only
after the sheets formed did the galaxies condense from them and
then begin to clump together as in the earlier model.
Two kinds of data are required if we are to understand the
formation and clumping of galaxies. First, red-shift surveys em-
bracing a large number of galaxies are needed. For the nearer
galaxies, it is feasible to obtain red sniffs w^tn currently ava'^A-
able telescopes of moderate size, equipped with array detectors
and fast spectrographs. To penetrate more deeply into space,
however, large telescopes will be needed. Telescopes of the 5-m
class will make important contributions, but only a new tele-
scope of the 15-m class, such as the New Technology Telescope
(NTT), can measure the red shifts of galaxies at large distances
rapidly enough to accumulate the required number of galaxies.
The raw speed of NIT, made possible by its order-of-magnitude
increase in collecting area over the previous largest telescopes, is
critical for this project.
A A ~ ~ ~ ^ ^ ~ ~ ~ ~ ~ A _ A ~ ~-~
O ~
~ -id. ·.1 .1 ·1
Hidden Mass and the Fate of the universe
For the past 20 years, astronomers have been increasingly puzzled
by the ,ihidden mass,, problem: the matter that constitutes most of
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43
the mass of the Universe is invisible. The spectra of galaxies indicate
that, like our own Milky Way Galaxy, they contain normal stars;
however, the internal motions in galaxies are so large that they would
fly apart if the only gravitational attraction holding them together
were that of the stars we see. There must be additional mass present
in some form that is hidden from our immediate view- enough to
supply the gravitational attaction required for stability. The rotational
velocities observed in spiral galaxies demonstrate that the amount
of hidden mass inside a given radius increases approximately linearly
with radius out to distances of nearly 100 kiloparsecs. Similar results
emerge from studies of groups of two or more galaxies: their masses
must be at least 10 times greater than the masses of all the visible
stars in them.
Solution of the hidden-mass puzzle is a major goal of astronomy
in the decade ahead. The first task is to find how it is distributed.
The velocities of globular clusters in the outer reaches of galaxies
reflect the strength of the local gravitational field and hence the
distribution of mass in the parent galaxy. Since globular clusters in
even relatively nearby galaxies are extremely faint, spectroscopic
measurements of their velocities can be made only with a telescope
as large as NIT. Galaxies themselves can serve as probes of the
distribution of mass in clusters and superclusters of galaxies. Since
galaxies are much brighter than globular clusters, work on clusters
of galaxies is already proceeding with intermediate-sized telescopes.
However, measurements of velocities of galaxies in distant clusters
are essential to determine how the distribution of mass has changed
with time; this will require observations with NIT.
Various possibilities have been suggested to account for hidden
mass: diffuse gas, massive neutrinos, collapsed stars (white dwarfs,
neutron stars, black holes), and faint red dwarfs.
Diffuse gas can be ruled out as a dominant component of either
galaxies or clusters of galaxies through radio, optical, and x-ray ob-
servations; although 100-million-degree gas exists in clusters of gal-
axies, the amounts are not sufficient to hold the clusters together.
Massive neutrinos, if they exist, might fall into clusters of galaxies,
and possibly even into galaxies themselves, thus contributing to the
hidden mass.
Collapsed stars of various types could in principle constitute much
of the hidden mass; however, such stars are the descendants of
massive main-sequence stars and so would dominate the total mass
only if, at early epochs of star formation, massive stars dominated
the total mass of main-sequence stars. Just the contrary is observed
to be the case for star formation in our Galaxy near the Sun: faint
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ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
red dwarfs, which are of low mass, are so numerous that they ac-
count for most of the mass bound up in stars. One could speculate
that there were many more massive stars in the outer parts of galaxies
during the early stages of galaxy evolution, so that large numbers
of collapsed stars would exist there today. However, if that were so,
one would expect a higher concentration of heavy elements in the
outer parts of the galaxies, since massive stars synthesize heavy
elements and eject them into the interstellar medium; this is contrary
to observation.
Faint red dwarfs could also account for the hidden mass, as large
numbers of them in the outer parts of galaxies would be consistent
with both the lower concentrations of heavy elements and the lower
light levels observed there. It may just prove possible to test this
hypothesis by using the recent discovery that red dwarfs are rela-
tively luminous sources of coronal x rays. AXAF will be able to detect
such red dwarfs by observing their integrated coronal x-ray emission
if they are numerous enough.
The hidden-mass problem is intimately connected with the ques-
tion of the ultimate fate of the Universe. According to the big-bang
model, the Universe will continue to expand forever if the amount
of matter in it is less than a critical value calculated to be between
0.5 x 10-29 and 2 x 10-29 g in each cubic centimeter. If the amount
of matter exceeds the critical value, the present expansion will reverse
at some time in the distant future, and the Universe will collapse
back into a singular state similar to the big bang. The observations
of deuterium and helium discussed earlier suggest that the amount
of ordinary matter is only 10 percent of the critical value, so that
only massive neutrinos could raise it above the critical value. A lower
limit on the total amount of mass in all forms is obtained from the
masses of clumps in the distribution of galaxies; current estimates
suggest that the aggregate amount of matter in such clumps may be
as much as 40 percent of the critical value. Since this is larger than
the upper limit on the amount of ordinary matter obtained from
observations of helium and deuterium, massive neutrinos may con-
ceivably account for most of the matter in the Universe. Massive
neutrinos are discussed further in the last section of this chapter.
EVOLUTION OF GALAXIES
The Study of Galaxies
Like the Galaxy in which we live, the 100 billion or more galaxies in
the visible Universe are fascinating systems in their own right. As
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45
the nuclear and gravitational energy stored in them is released, it is
likely that galaxies evolve toward objects evermore structured and
compact.
Among the variety of forms that galaxies take, Hubble discerned
several recurrent patterns spirals, ellipticals, lenticulars, and irreg-
ulars; these patterns have still not been completely explained theo-
retically. Ellipticals and lenticulars are nearly devoid of interstellar
gas and dust, while spirals and irregulars contain gas and dust, as
well as young stars formed recently from them. Until recently, the
gas and dust in spiral galaxies other than our own could be studied
with high angular resolution only at optical wavelengths, by imaging
the dark interstellar dust clouds and the luminous gas clouds heated
by bright young stars. Now the Very Large Array (VLA) radio tele-
scope can image galaxies both in the 21-cm line produced by inter-
stellar atomic hydrogen and in the synchrotron radiation produced
by relativistic electrons gyrating in interstellar magnetic fields; it can
thus trace the distribution and state of the interstellar medium with
angular resolution comparable with that of optical telescopes.
As in all fields of astronomy, spectroscopy is the key to deeper
understanding. Ground-based optical spectroscopy of galaxies dem-
onstrates that a major component of most galaxies is stars of various
masses and ages, like those in our Galaxy. However, present ground-
based telescopes are hard pressed to obtain the spectra of extremely
faint subsystems of galaxies, such as individual giant stars, regions
of ionized gas, and globular clusters; they are too small to permit
collection of photons at a sufficiently high rate. NTT, with its order-
of-magnitude increase in collecting area, can obtain the spectra of
such objects, thus making possible a whole new range of studies
related to chemical composition, distribution of stellar masses, and
rotational and random velocities within galaxies. For a galaxy of a
given red shift, NTT will make possible studies with much higher
spectral resolution; for the same spectral resolution, it can carry out
studies on galaxies of much higher red shift. The latter capability is
crucial for analysis of objects of large red shift that will be discovered
by ST.
One of the most striking capabilities of the new instruments rec-
ommended for the 1980's is the systematic exploration of the de-
pendence of various galactic properties on red shift at greater and
greater cosmological distances. Big-bang models of the Universe pre-
dict such a dependence because the evolution of galaxies with time
translates into changes with lookback time, and hence with red shift.
ST and NTT will be able for the first time to observe galaxies with red
shifts substantially exceeding unity, corresponding to lookback times
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ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
that are more than half of the Hubble time. ST can image such distant
objects because the sharpness of its images makes them stand out
against the background, and NIT can obtain their spectra because it
has a much larger collecting area than present large telescopes. If
the matter comprising the inner parts of galaxies has already settled
into an equilibrium state within considerably less than a billion years
after the big bang, the forms of galaxies would not depend sensitively
on red shift out to red shifts of 10 or so. However, the evolution of
stars and the conversion of interstellar gas into stars proceeds much
more slowly and should be observable at much lower red shifts. The
spectra of isolated elliptical galaxies should manifest subtle changes
that reflect the evolution of the stars that they contain, while isolated
spiral galaxies should in addition manifest the progressive depletion
of interstellar matter, as well as its enrichment in heavy elements
produced by supernova explosions. A major indirect effect will be
the reduction in the number of short-lived massive stars as the gas
required to form them is depleted. Failure to observe such basic
predictions of big-bang theory would force major revisions in current
thinking.
Formation of Galaxies
The first relativistic models of the big-bang Universe were derived
by Friedmann in 1922. For simplicity, he assumed that matter is
distributed absolutely uniformly. Although this assumption conflicts
with the existence of stars and galaxies, the model is useful because
matter is in fact distributed quite uniformly when averaged over
large distances. Still, the origin of galaxies in a big-bang model is an
unresolved problem.
Many properties of galaxies can be explained at least qualitatively
if it is assumed that they originated in small fluctuations in the
amount of local matter in the early Universe. At that time, the be-
havior of matter was governed by the pressure exerted by the cosmic
background radiation. Two types of density fluctuations could have
existed. One type, so-called isothermal fluctuations, would have led
to gravitationally unstable clumps of matter if they had involved
more than 105 to 106 solar masses; another type, adiabatic fluctua-
tions, would have led to gravitationally unstable clumps if they had
involved more than 10~3 to 10~4 solar masses. In both cases, instability
would have set in about 100,000 years after the big bang, and as a
result, the matter in the fluctuations would soon cease to participate
in the cosmic expansion, would then become more dense as self
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47
gravitation drew the gas together, and would ultimately form discrete
gas clouds of various masses.
The Cosmic Background Explorer (COBE) satellite will yield impor-
tant information on the proposed instability process by observing
the disturbances in the background radiation that would accompany
any density fluctuations in the early Universe. Adiabatic fluctuations,
which involve variations in temperature and hence in the intensity
of the cosmic background radiation, would result in intensity vari-
ations on angular scales of a few degrees if the masses involved in
the fluctuations are about those of clusters of galaxies. Complemen-
tary information about fluctuations on the smaller angular scales
corresponding to individual galaxies (less than a degree) will be
obtained by the Large Deployable Reflector (LDR) in space. The theory
of adiabatic fluctuations has been worked out in detail for the case
in which there is a random collection of initial fluctuations of various
sizes and masses. Fluctuations involving 10~3 to 10~4 solar masses,
usually identified with groups and clusters of galaxies, should form
clouds first; the formation of galaxies would have taken place later
within these clusters and groups. Clusters and superclusters con-
taining more than 10~3 to 10~4 solar masses must have formed through
later gravitational clustering of the original mass aggregations of this
size.
Alternatively, isothermal fluctuations may have dominated the in-
itial stages of galaxy formation. In this case, the first objects to form
must have had masses from 105 to 106 solar masses, and galaxies
must have been built up later by gravitational clustering of these
smaller objects. The fact that globular clusters containing 105 to 106
solar masses are so common would be a natural result of isothermal
fluctuations. If galaxies formed out of objects having 105 to 106 solar
masses, then groups and clusters of galaxies must have formed sub-
sequently through gravitational clustering of the galaxies themselves.
This process can be modeled with computers by treating each galaxy
as a point mass and calculating its gravitational interactions with its
neighbors. Extensive simulations of gravitational clustering have been
carried out in this way during the past decade; the results agree with
observations in some respects, but they do not predict the large holes
devoid of galaxies that have been observed between clusters. It is
still uncertain whether galaxies or clusters of galaxies originated first.
None of the existing computer simulations of either galaxy collapse
or clustering addresses the origin of the fluctuations themselves.
Current attempts to answer this important question, based on Grand
Unified Theories of elementary particles, are encouraging.
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ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
for obtaining radial-velocity measurements of the required precision;
sustained programs of observations of many candidate stars are now
required. Astronomers have expended great effort to make milli-
arcsecond-position measurements on nearby stars, but until recently
the required precision has not been available. With the development
of the optical astrometric techniques recommended in this report,
however, it should be possible to observe many stars with a precision
exceeding 1 milliarcsecond and by this means to detect Jupiter-sized
planets around nearby stars, if they exist. Space astrometry should
ultimately yield much higher positional accuracy, leading to the still
more interesting prospect of detecting Earth-sized planets. Far-in-
frared interferometric observations from space could also reveal planets
around nearby stars.
Search for Extraterrestrial Intelligence
Even if other planets are detected, it will still be difficult to infer
whether life is present; to do so directly would require imaging the
planet itself with as yet undreamed-of resolution. Only if the planet
harbors intelligent life capable of producing electromagnetic signals
detectable at Earth is there at present any hope of finding life outside
our solar system. It is a remarkable fact that radio and television
signals generated copiously on Earth could be detected at distances
of many parsecs by civilizations that, like ours, would otherwise
have no way of knowing of the existence of details of the planet that
is our home.
Should the human race search seriously for signals from other
possible civilizations? Much has been written about this question,
both on a technical and a philosophical level. Reception of intelligent
signals from space could have a dramatic effect on human affairs,
as did contact between the native peoples of the New World and
the technologically more advanced peoples of Europe. The effects
would be beneficial, if the information could be deciphered and
should prove generally useful; on the other hand, they could be
harmful if humanity is not ready to use the information wisely.
The technology is now available to make significant searches of
this kind. The 300-m radio telescope of the National Astronomy and
Ionosphere Center at Arecibo, Puerto Rico, is capable of receiving a
message beamed at us from any of the hundreds of billions of stars
in our Galaxy, provided the civilization sending the message were
transmitting with a facility similar to that at Arecibo. Several searches
for such extraterrestrial signals have already been undertaken, so far
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91
with negative results, but the rate of improvement of communica-
tions technology is so rapid that each search has been far more
sensitive than its predecessors.
We are entering an era when it is technically possible both to
detect planets around nearby stars and to detect signals from intel-
ligent life on planets immensely farther away, even if we cannot
detect the more distant planets themselves. Both investigations would
bear directly on important scientific questions. Our interest in the
tiny fraction of the matter in the solar system that condensed into
planets is heightened by the fact that life has developed on at least
one of them. Have condensations to planets and the origin of life
occurred elsewhere as well? And has that life evolved into com-
municative intelligence, with which we human beings might be able
to enter a conversation about life in the Universe?
These questions reach far beyond astronomy, and even beyond
science as we currently think of it. Yet astronomers, who are in a
sense commissioned by the public to keep an eye on the Universe,
feel bound to ask them and to point out how we might begin to try
to answer them. It is for these reasons that the Committee recom-
mends that in the 1980's an astronomical Search for Extraterrestrial
Intelligence be initiated as a long-term effort.
ASTRONOMY AND THE FORCES OF NATURE
Energy Sources in the Universe
In the 1970's, physicists have made substantial progress toward re-
alizing an age-old dream-the understanding of all the forces in
nature as different aspects of a single fundamental force. A theory
that unifies electromagnetic and weak nuclear forces has been suc-
cessfully developed along with a comprehensive theory of the strong
nuclear force; new theories aimed at unifying both of these theories
are now being proposed. Astronomical data have played a role in
these developments and may play an even greater role in the future.
Newton's law of gravitation, formulated in precise mathematical
terms, set the stage for the investigation of the forces of nature that
continues today. We now realize that chemical energy, such as that
released in the burning of fossil fuels, is due to the action of electrical
forces within atoms. Holding electrons in orbits around nuclei just
as gravitation holds planets in their orbits around the Sun, these
forces release energy whenever an electron drops into a lower orbit.
Magnetic forces result from the motion of electrically charged par
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ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
ticles. In the 1860's, Maxwell unified electrical and magnetic forces
in a single theory, called electromagnetic theory, which also explains
electromagnetic radiation as a wave that sustains itself through a
constant interplay between electrical and magnetic energy. By the
enr1 of the nineteenth century, both gravitational and electromagnetic
forces were well understood at a certain level.
O
Early in the twentieth century a series of important experiments
revealed that the orbits of electrons are qualitatively different from
those of planets. The position of a planet can be predicted precisely
from a knowledge of the gravitational force acting on it, but the best
one can do with an electron is to predict its probability of being at
various possible positions. The impossibility of doing any better,
embodied in Heisenberg's Uncertainty Principle, is an essential fea-
ture of what is now known as quantum theory. Today, the melding
of electromagnetic theory and quantum theory, called quantum elec-
trodynamics or QED, is unchallenged in its ability to describe elec-
tromagnetic phenomena. A shining goal of contemporary physics is
to bring the understanding of all the forces of nature up to the
standard of QED.
Sunlight is electromagnetic radiation, and the form in which the
energy of sunlight is stored by plants is chemical energy; both forms
of energy are embraced by QED. What about the energy stored in
the Sun, which it emits as sunlight? Early suggestions included elec-
tromagnetic radiation trapped within the Sun, chemical energy stored
in its atoms and molecules, and the energy due to the gravitational
attraction between all of its atoms. However, none of these forms
of energy is adequate to keep the Sun shining for its known age of
4.5 billion years. The solution to this problem was reached in the
early 1920's, when it was recognized that a new form of energy
discovered in the laboratory, nuclear energy-which is released, for
example, when the nuclear force between hydrogen nuclei (protons)
draws them together to form helium nuclei-could keep the Sun
shining for many billions of years.
Nuclear interactions come into play only at very high temperatures;
only then do nuclei have sufficient speeds to overcome their mutual
electrical repulsion. Thus, nuclear forces play a role in astronomy
only where matter is extremely hot, as in the interiors of stars or in
the searing heat of the big-bang explosion. Laboratory studies of
nuclear reactions show that there are actually two types of nuclear
force, strong and weak; the latter is associated with an unusual
particle called the neutrino.
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Two Puzzles: Solar Neutrinos and Hidden Mass
93
Neutrinos can penetrate the entire Sun, so weak is the force with
which they interact with matter. Detectors placed beside nuclear
reactors, which are copious sources of neutrinos, can record only a
minute fraction of those emitted. Despite the great difficulty of de-
tecting them, the role of neutrinos in astronomical research has be-
come increasingly important.
The current theory of stellar energy generation predicts that large
numbers of neutrinos are produced in the fusion of hydrogen to
helium in the deep interior of the Sun. Because this theory is critical
to our understanding of stellar structure and evolution generally, it
is important to test this prediction by measuring the flux of solar
neutrinos at the Earth. The observed flux of neutrinos is less than
one third of that predicted from the most carefully constructed models
of the solar interior. Among various proposed explanations of this
discrepancy is the possibility that neutrinos behave differently from
what has been assumed until recently.
In a completely different area of research, it has been proposed
that the problem of hidden mass in galaxies might be resolved if the
rest mass of neutrinos were not zero, as usually assumed. From
calculations of the number of neutrinos produced in the big bang,
one finds that neutrinos could supply the hidden mass in galaxy
clusters if they possess a rest mass about 1/10,000 that of the electron.
There are thus two astronomical problems that might be resolved
if neutrinos prove to have properties not previously known. Theo-
retical physicists have recently suggested a resolution of both of these
problems. The recently developed unified theory of weak and elec-
tromagnetic forces is based on a principle called gauge invariance
and is therefore referred to as "the gauge theory of weak and elec-
tromagnetic interactions." So far it has succeeded in explaining all
the various phenomena involved with both electromagnetic and weak
nuclear forces.
The gauge theory of weak and electromagnetic interactions in its
original form says nothing about the problems of solar neutrinos or
hidden mass. However, pursuing the principle of gauge invariance
behind it, physicists have constructed a theory of the strong nuclear
force, called quantum chromodynamics, or QCD. This theory pos-
tulates the existence of elementary particles that combine to form
protons and neutrons, called quarks. The success of QCD in explain-
ing the results of experiments in elementary-particle physics gives
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ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
increasing confidence that it is the correct theory of the strong force
that binds neutrons and protons into atomic nuclei.
Spurred by the success of the gauge theory of weak and electro-
magnetic interactions and of QCD, physicists are now trying to find
an even more general gauge theory, called "Grand Unified Theory,"
that incorporates both. Some theories of this type predict that there
should be the three types of electrons that are actually observed, as
well as three corresponding types of neutrinos, called e, mu, and
taut In some versions of the theory, e, mu, and tan neutrinos are
regarded as three aspects of the same basic neutrino, which has a
finite rest mass and which oscillates back and forth among its three
aspects. Although the nuclear reactions in the Sun emit only e neu-
trinos, according to some Grand Unified Theories neutrino oscilla-
tions would be expected to occur long before the neutrinos reached
the Earth, so that at the Earth one would observe a random mixture
of e, mu, and tan neutrinos. Since the Homestake Mine apparatus
is sensitive only to e neutrinos, a factor-of-3 discrepancy would
thereby be explained.
Oscillations can occur only if neutrinos have a finite rest mass. If
the value of the rest mass were in the right range, it would have a
dramatic bearing on our understanding of the hidden-mass problem
and of the ultimate fate of the Universe.
Theories involving several different types of neutrinos are con-
strained by calculations of the properties of the early Universe. If
there were more than about four types of neutrinos, their contri-
bution to the gravitational acceleration in the early Universe would
have been so great that there would not have been sufficient time
for primordial neutrons to decay; there would then be more helium
in the Universe than is actually observed. Thus, current astronomical
observations eliminate some versions of Grand Unified Theories.
A critical experiment endorsed earlier in this report will help to
shed light on the true nature of neutrinos. The gallium solar neutrino
experiment will be sensitive to neutrinos of much lower energy than
those measured by the 37C1 detector in the Hamestake Mine. The
flux of such lower-energy neutrinos can confidently be calculated
from the observed luminosity of the Sun, independently of the details
of solar models. If there is a discrepancy between the predicted and
observed values of the solar neutrino flux in the gallium experiment,
it could be an indication that neutrinos oscillate and have a finite
neutrino rest mass.
There may also be powerful sources of high-energy neutrinos among
the many sites of violent activity observed to occur on both stellar
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Frontiers of Astrophysics
95
Chlorine solar-neutrino detector deep ire the Homestake Mine, Lead, South Dakota. (Photo
courtesy of R. Davis, Jr., Brookhave~z National Laboratory)
and galactic scales. Despite the difficulty of detecting such neutrinos
and the weak fluxes to be expected because of the distances to the
sources, the study of energetic-neutrino detectors with possible as-
tronomical applications is appropriate for the coming decade. An
interesting possibility for such study is the proposed observation of
neutrino-induced reactions in seawater employing arrays of photo-
multipliers to detect the associated Cerenkov radiation.
Before the First Three Minutes
Although astronomical data now available appear to be in agreement
with the predictions of big-bang cosmology, the big-bang model
cannot yet be considered conclusively proven, so that it is of the
greatest importance to test its predictions however possible. In par-
ticular, the model predicts that the cosmic microwave background
originated as high-temperature radiation in the first few minutes of
time. As the Universe expanded, according to this view, the radiation
cooled to its present observed temperature, about 3 degrees above
absolute zero. When the Universe was about 1/10,000 of a second
old, its temperature was a trillion degrees, so hot that the radiation
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ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
present created about 100 million proton-antiproton pairs for every
proton now observed in the Universe. As time passed, these pairs
annihilated, leaving behind only the very small fraction by which
the number of protons exceeds the number of antiprotons. Had this
excess not existed, the number of protons in the present Universe
would have been 10 billion times smaller, and there would not have
been sufficient matter in the Universe for the formation of galaxies,
stars, and planets.
What caused the excess of matter over antimatter implied by this
big-bang scenario? Until recently, physicists had regarded the excess
as a fact as inexplicable as the existence of the Universe itself. Re-
cently, it has been suggested that Grand Unified Theories provide
an explanation: very heavy particles present in the first 10-38 sec of
the history of the Universe decayed, creating in the process slightly
greater numbers of protons than antiprotons. This prediction can be
tested in a straightforward way, for if protons can be created they
must also decay. As the lifetime of the proton estimated from Grand
Unified Theory is 100 billion billion times the age of the Universe,
physicists are not concerned that the Universe will soon evaporate.
On the other hand, the predicted proton lifetime is sufficiently short
that one such decay will occur in a ton of material each year. Ex-
periments are now in progress to detect such events.
The Limits Of Gravitation
Gravity keeps us on the Earth, binds the Earth to the Sun, and slows
the expansion of the Universe. Newton described it as a force, while
Einstein, in his General Theory of Relativity, interpreted gravitational
forces in terms of the curvature of space-time. Einstein's theory,
unlike Newton's, is believed to be valid for very strong gravitational
fields and for bodies moving close to the speed of light; it is therefore
crucial for an understanding of systems such as neutron stars, black
holes, and the expanding Universe.
The General Theory of Relativity predicts that when any non-
spherical body collapses to form a compact object or a black hole, it
emits a new form of energy called gravitational radiation. Although
this radiation is predicted to be extremely difficult to detect, several
research groups are now building detectors thousands of times more
sensitive than those available during the 1970's. Parallel efforts to
calculate the amount of gravitational radiation emitted by collapse
indicate that, if the planned development of new instrument concepts
succeeds, we might hope to detect an event within two decades-
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Frontiers of Astrophysics
97
even earlier if there should be a new supernova within the Galaxy.
The recently confirmed, slow decrease in the orbital period of the
binary pulsar has already been interpreted as the result of gravita-
tional radiation from a close pair of neutron stars.
While efforts to develop a quantum theory of gravitation have not
yet succeeded, there is reason to believe that quantum effects should
occur near black holes, where space-time curvature is high. The
quantum theory of elementary particles predicts that even in vacuum,
particle-antiparticle pairs are constantly being produced and anni-
hilated in an interval of time too short to observe. If this effect should
occur near a black hole, one member of the pair may fall into the
black hole before the pair annihilates. Zero-mass particles, including
photons, are created similarly; the black hole thus appears to the
outside world as a source of radiation, ultimately evaporating as a
result of the energy lost. Black holes of all sizes could have been
created in the big bang; in particular, those having masses about of
10~5 g (the mass of a small mountain on Earth) would just be evap-
orating now, giving rise at the ends of their lives to bursts of gamma
radiation. Such radiation from evaporating black holes has been
searched for, and, although the Gamma Ray Observatory will con-
tinue the quest, so far none has been found. It thus appears that
primordial black holes with masses less than that of a mountain
cannot make up a significant fraction of the mass of the Universe.
The theory of black-hole evaporation depends on the quantum
nature of strong nuclear forces but not on the quantum nature of
gravitation. Although no convincing theory of gravitation that in-
corporates the quantum principle has yet been produced, it is con-
jectured that the quantum effects must become important whenever
the radius of curvature of space-time becomes less than the so-called
Planck length, 10-33 cm. Such conditions are thought to have occurred
in the Universe at times before 10~3 sec and at temperatures above
1032 deg. Because the energies and temperatures characteristic of
Grand Unified Theories are remarkably close to these values, some
physicists believe that a theory should be possible that incorporates
all four forces in nature into one "Super-Grand" force at energies
only slightly higher than those relevant to Grand Unification. A
prime hope for such a theory is that it will yield, almost as a by-
product, the correct theory of quantum gravitation. Attempts in this
direction have so far met with little if any success, but the devel-
opment of such a theory could be considered to be the ultimate
challenge to physics at present.
The notion of force, as a law governing matter once created, fails
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ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
to take account of the process of creation itself. Is it possible, as
astrophysics pushes the frontiers of time back to the moment of
cosmic creation, that the existence of the Universe will be recognized
as a consequence of the nature of the fundamental force? Is it possible
that the potential existence of the world somehow calls it into exis-
tence? Such questions, once believed outside the range of science,
are now arising in scientific thought.
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The primary mirror for the Space Telescope being inspected after
figuring. Photo courtesy of the National Aeronautics and Space
Administration)
Representative terms from entire chapter:
cosmic rays
