The basic observational evidence of the expanding universe is that light from distant
galaxies is shifted in wavelength toward the red end of the spectrum. What is
the evidence of a hot Big Bang? It is the microwave background radiation, a small
remnant of radiation left over from the hot Big Bang. As we shall see, this microwave
background radiation is crucial to making detailed predictions in Big Bang
cosmology.
In the late 1940s George Gamow and colleagues pointed out that if the universe
began with a hot Big Bang, as they thought likely, the blackbody radiation emitted
at that time should still be present. The universe has expanded so much since the Big
Bang that all short-wavelength photons today have wavelengths that are so stretched
that they have become long-wavelength, low-energy photons. This cosmic radiation
field would look like the radiation emitted by a blackbody at a very low temperature.
Gamow had predicted a current temperature of about 5 K. At the time Gamow
made this prediction, equipment capable of detecting such radiation was not available,
so nothing came of the suggestion that the radiation might still be bouncing
around space. In the early 1960s Robert Dicke at Princeton University had arrived
independently at a prediction of such radiation at 10K by a different route. Dicke
and his colleagues began designing an antenna to detect this microwave radiation.
Meanwhile, just a few miles from Princeton University, Arno Penzias and Robert
Wilson of Bell Laboratories (Fig. 7.8) had modified a ground-based radiometer that
had been used to detecting signals from Echo satellites, into a low-noise radio antenna
of 7.35 cm long. They were bothered by an excess of noise that they could
pinpoint – it did not vary with the time of day or the season. Assuming it was
due to radiation in thermal equilibrium, they calculated the antenna temperature of
this blackbody radiation. They first applied Wein’s displacement formula, λpeak = 0.51 cm/T (K), with λpeak = 7.35 cm, and found that T = 0.51/7.35 = 0.07 K.
This was unreasonably low, so they assumed they were below the peak and on the
low frequency or long-wavelength tail of the blackbody spectrum. With this last
assumption and the measured value of the energy density per unit frequency, they
found the predicted antenna temperature to be ∼3 K, and became convinced that
the noise was not caused by their instrument. They conjectured that it might be extraterrestrial.
After learning about the work of Dicke and his colleagues at Princeton
University, Penzias and Wilson came to realize that they had discovered the background
radiation left over from the hot Big Bang.
Since those pioneering days, scientists have made many measurements of the intensity
of the cosmic background radiation at a variety of wavelengths. The most accurate
measurements come from the Cosmic Background Explorer satellite, which
was placed in orbit around Earth in 1989 (Fig. 7.9). Data from COBE’s spectrometer
(Fig. 7.10) demonstrate that this ancient radiation has the spectrum of a blackbody
with a temperature of 2.73 K. This radiation field, which fills all of space, is commonly
called the cosmic microwave background.
We will learn later that this cosmic background radiation was released about
half a million years after the expansion began, at a time when hydrogen atoms had
cooled to 3000 K, the temperature at which atoms are stable. The universe was then
a neutral gas of atoms, and the electromagnetic radiation present at that time could
travel without being absorbed. Following that period, very little additional electromagnetic
radiation has been formed, since neutral atoms do not radiate nearly
as readily as charged particles. The spectrum of the microwave radiation now ob7.5
The Microwave Background Radiation 127
served, therefore, reflects the temperature t = 105 years. However, it is extremely
redshifted, and so we now measure 2.73K as its temperature, rather than the few
thousand Kelvins characteristic of dissociation of atoms.
Some of you may wonder how the cosmic background radiation has the spectrum
of a blackbody while the space is expanding. So before we continue further, let us
digress a moment to take a look at the effect of expansion on the spectrum of the
cosmic radiation.
We may visualize the effect of the adiabatic expansion of the universe in the
following way. Imagine a box made of perfectly reflecting mirrors and blackbody
radiation from a hot source is directed to the box. Next, the box is closed so that
there can be no leakage of the radiation. The radiation will be trapped and bounce
back and forth between the walls indefinitely. Now let the walls of the box slide
outward so that the volume of the box increases. As radiation strikes the moving
walls, it undergoes Doppler shifts to the red, so the wavelengths of the entire radiation
increase. But the wavelengths increase in such a way that the distribution
among them still corresponds to the radiation curve for a blackbody. The effect of
the moving walls, as the box becomes larger, is to change the radiation from that
corresponding one temperature to that for a blackbody at a lower temperature.
An important feature of the cosmic microwave background radiation is its intensity.
At any given wavelength, the cosmic background radiation is extremely
isotropic on the small scale. In directions that differ by only a few minutes of arc,
any fluctuations in its intensity is less than 1 part in 10,000. On the other hand,
a large-scale anisotropy in the cosmic background radiation has now been established,
in the sense that it is slightly hotter in one direction than in the opposite
direction in the sky. This is due to our own motion through space. If we approach
a blackbody, its radiation is Doppler-shifted to shorter wavelengths and resembles
that from a slightly hotter blackbody. When we move away from it, the radiation
appears like that from a slightly cooler blackbody. This effect has been observed in
the microwave background.
The measurements of this relative speed are very difficult because the difference
in intensity is very tiny compared with the radiation from Earth’s own atmosphere.
Hence the measurements must be made from high-flying balloons, aircraft, or spacecraft.
The data indicate that our Galaxy and the Local Group is moving at a speed
of about 600 km/s with respect to the microwave background (or with respect to the
uniform expansion of the universe as a whole), toward the general direction of the
Virgo and Hydra cluster. This can be thought of as a peculiar motion of the Local
Group, superimposed on the general expansion.
The uniformity of the radiation tells us that at an age of less than a million years
the universe was extremely uniform in density. But at least some density variations
had to be present to allow matter to gravitationally clump up to form galaxies
and superclusters of galaxies. The isotropy of the microwave background radiation,
therefore, puts interesting constraints on theories of supercluster, cluster, and galaxy
formation.
COBE’s Differential Microwave Radiometer, a set of very sensitive and stable
radio receivers, was designed to analyze this problem by mapping the cos128
7 Introduction to Cosmology
mic background far more precisely than is possible from Earth’s surface. Even
above the atmosphere, the cosmic background fluctuations are swamped by radiation
fluctuations due to foreground stars and dust clouds in the Milky Way and
other galaxies. Such interference must be identified and subtracted from the measured
signal. Hundreds of millions of observations were processed in this fashion
to produce a single map. This map was then analyzed statistically and revealed the
presence of fluctuations of (30±5)×10−3 K in the temperature of the background
radiation. Indeed, COBE had detected the non-uniformity in the microwave background,
amounting to about 30 millionths of a Kelvin. This may be sufficient to seed
the formation of large-scale structures in the universe, especially if there is a great
deal of nonluminous or “dark matter” present in the universe. This invisible matter,
whose nature is not yet confirmed, supposedly provided an added gravitational
force needed to pull the gas together into galaxies within a reasonable time.We will
explore the dark matter problem and ideas of structure formation in a later chapter.
Proponents of inflationary scenarios assert that the size of the observed fluctuations
is consistent with their being produced by microscopic quantum effects that
were magnified by the rapid pace of inflation or by gravitational waves generated
during inflation itself. According to inflationary scenarios, the universe expanded
rapidly for a brief instant just after the Big Bang. More detail will be introduced
later.
7.6 Additional Evidence for the Big Bang
The cosmic microwave background radiation and its spectrum provide the strongest
evidence in favor of the Big Bang theory. There are additional bits of evidence com-
ing from measurements on the abundance of helium and deuterium in the universe.
Helium is formed from hydrogen in the interiors of stars during their lifetimes. On
the other hand, the Big Bang cosmology predicts that helium was also formed from
hydrogen in the early stages of the Big Bang. These circumstances led to a criterion
for judging whether the Big Bang ever occurred. If nearly all the helium in the
Universe were primordial, that would be good evidence in favor of the Big Bang.
If it turned out that all the helium that exists today had been manufactured in the
interiors of stars, none would be primordial, and confidence in the Big Bang theory
would be weakened.
The answer to this problem was to measure the helium content of old and young
stars. The young stars were formed from an interstellar medium containing primordial
helium, if any, plus all the helium that was added to the universe subsequently
in many generations of stellar evolution. The old stars were formed when the galaxy
was young, before the interstellar medium had been enriched by helium formed in
stellar interiors. Their helium content is primordial only. Therefore the comparison
of the helium content in the two groups of stars tells how much helium is primordial,
and how much has been added as the product of reactions in stellar interiors.
The helium content of young stars can be determined directly from the intensities
of the helium absorption lines in their spectra. This absorption takes place only in
the atmosphere of the star, so the intensity only gives the amount of helium in the
star’s outermost layer. However, in a young, unevolved star, the helium is dispersed
uniformly; the amount in the atmosphere is an accurate indicator of the amount in
the entire star. Only the hot stars—O and B type—can be used for this purpose,
because helium lines appear only with significant intensity in the spectra of these
stars. The spectroscopic studies indicate that about 30 percent of the mass of young
stars consists of helium.
When we come to old stars, the helium absorption lines cannot be used in the
same way to determine helium content, because a population of old stars does not
include O and B types, which are massive and live only a short time–not more
than 100 million years. But another method is available. The ages of old stars in
globular clusters can be determined by fitting Hertzsprung-Russell (H-R) diagram
computed for globular clusters of various ages to the observed H-R diagram. (H-R
diagram is a diagram on which the absolute magnitude or luminosity of stars is
plotted against spectral or surface temperature.) Computations on stellar structure
show that the position of a star on the H-R diagram depends to some degree on
its chemical composition. In particular, the luminosity (and so the star’s position
on the H-R diagram) is strongly dependent on the helium content of the star. By
carefully matching the observed and theoretical H-R diagrams for a globular cluster,
it is possible to determine not only the age of the stars in the cluster but also their
helium content. Helium contents between 22% and 26% are found in this way. In
other words, the helium content of old stars is a little less than the helium content
of young stars, but close to it. This agrees with the prediction of the Big Bang
theory that most of the helium in the universe was made shortly after the Big Bang;
only a small amount was contributed subsequently by nuclear reactions in stars. The
quantitative agreement between the predicted and observed amounts of primordial
helium is impressive. These findings significantly strengthen the case for the Big
Bang theory. The Big Bang nucleosynthesis of the light elements is important and
will be explored in Chapter 11.
Astronomers have found direct evidence of primordial helium in the spectrum of
ultraviolet light emitted by a distant quasar. As the emitted light traverses the vast
expanse of space between the quasar and Earth, it encounters intergalactic helium
and hydrogen. Gas completely ionized by the quasar light cannot absorb any more
radiation, so the light passes unimpeded, as if it were traveling through a transparent
medium. This appears to be the case for diffuse hydrogen that is easily stripped of
its one electron.
It takes more energy to ionize a helium atom, which has two electrons. Although
the quasar beacon fully ionizes most of the helium it encounters, some of the atoms
manage to retain one of their electrons. When the radiation passes through singly
ionized helium, the ions absorb light of a particular wavelength, leaving behind a
fingerprint—a dark line, or gap, in the quasar’s spectrum. But because of the redshift
of light caused by the expansion of the universe, gaps due to helium ions at different
distances along the line of sight to the quasar will appear at different wavelengths
to an observer on Earth. Thus, the helium ions collectively create a series of dark
absorption lines in the quasar spectrum.
The HUT (the Hopkins Ultraviolet Telescope), part of the Astro 2 Observatory
that flew aboard the space shuttle in March 1994, recorded a series of such dark
lines in the spectrum of the quasar HS 1700 + 64, which lies about 10 billion lightyears
from Earth. The singly ionized helium detected by HUT represents only a tiny
fraction of the total amount of helium that resided in the early universe, because
most of the gas is completely ionized.
The existence of deuterium provides even stronger support of the Big Bang. An
ordinary hydrogen nucleus consists of a single proton. In deuterium, a proton and
a neutron are bound together. Deuterium is a form of hydrogen and not some other
element because the addition of a neutron to the nucleus does not alter its chemical
properties. The nucleus still has a charge of +1, and it will still form an atom in
which there is a single electron.
Deuterium is not very abundant in our universe. There is roughly about one deuterium
atom for every 30,000 atoms of ordinary hydrogen. Yet the existence of even
tiny quantities of deuterium provides scientists with significant evidence about the
Big Bang. The deuterium nucleus is relatively fragile, and it cannot be created in
stars. The high temperatures in stellar interiors would cause deuterium nuclei to
break apart as soon as they were formed; thus, the only place that deuterium could
have been created is in the Big Bang.
Tai L. Chow
Author
Gravity, Black Holes,
and the Very Early Universe
An Introduction to General Relativity
and Cosmology
The Big Bang was an actual, observable event in the history of our universe.
Grumpy
