The Story of Astronomy (31 page)

Other techniques have been developed to measure galactic distances. In the 1970s Brent Tully and Richard Fisher discovered a relationship between the properties of the 21 cm hydrogen line observed in the radio spectrum of a spiral galaxy and its intrinsic luminosity of the galaxy. The hydrogen line is emitted from cold clouds of gas that lie between the stars in the disc of a spiral galaxy
and which, like the stars, rotate around the center. This rotational motion is detectable as a broadening of the 21 cm absorption line due to the Doppler effect, and the amount of rotation (and hence the observed width of the line) is driven by the internal gravity of the galaxy. The broader the line, the higher the galactic mass driving the motions, and thus the larger the intrinsic luminosity of the galaxy. It was possible from this broadening effect to calculate the absolute magnitude of a galaxy at the standard 10 kpc, and by comparing the absolute magnitude to the observed magnitude the distance of the galaxy could again be estimated. The Tully–Fisher technique can be used to measure distances up to 150 Mpc.

There is another very useful technique that can be used for measuring distant galaxies. It is based on the observation of the rare event called a supernova Type 1a. The supernovae are so bright that they can be seen in galaxies at distances of well over 1000 Mpc. These supernovae are due to a white dwarf in a binary system accreting matter from its secondary until it is at the Chandrasekhar limit of around 1.4 solar masses for a catastrophic core collapse in a supernova explosion. As the same mass is involved in each explosion, the event follows a similar pattern in terms of the intrinsic luminosity of the outburst and how this is related to the rate at which this brightness subsequently fades away. Thus
the observations of the apparent magnitude of a supernova explosion and the rate at which it fades enable a calculation to be made of its distance.

All of these distance-measuring techniques neatly overlap with each other so that a three-dimensional image of the universe is gradually being built up by astronomers. However, there is still one important technique that needs to be discussed; this is the redshift of the distant galaxies, and it is of great significance for it provides the key to the age of the universe.

During the 1920s Edwin Hubble (1889–1953) and Milton Humason (1891–1972) recorded the spectra of many galaxies using the 2.5-meter (100 in) telescope at Mount Wilson. As early as 1917 Vesto Slipher (1875–1969), working at the Lowell Observatory in Arizona, discovered that the spectra from the galaxies were noticeably shifted toward the red end of the spectrum. He rightly concluded that the galaxies were moving away from us. Hubble, however, used the Cepheid variables technique to estimate these distances and derive the distance–redshift relationship known as Hubble's law. This relationship can also be used to derive an estimated distance of a galaxy from its much more easily observed spectral redshift.

During the 20th century, a great deal of effort and research were devoted to an accurate determination of Hubble's constant, culminating in a key project using the Hubble Space Telescope. The best estimate for Hubble's constant is now generally taken as their result of 70 +/- 7 km/s/Mpc, published in 2001. The significance of this result lies in the fact that the inverse of Hubble's constant gives an estimate for the age of the entire universe.

Consider a distant galaxy rushing away from us. If we know its distance from us and its speed of recession, then the time it has taken to separate from us and the rest of the universe after the Big Bang is given by:

Time = (the distance from us) / (its speed away from us)

As Hubble's constant is simply the averaged ratio of speed/distance determined from a large number of galaxies, its inverse yields the required time. If 1/(
H
₀
) = (1/70) seconds Mpc/km, and using the unit conversion factors 1 Mpc = 3.09 × 10
¹⁹
km and 1 year = 3.156 × 10
⁷
s, we can estimate an age of the universe:

1 / (
H
₀
) = (1/70) × (3.09 × 10
¹⁹
) / (3.156 × 10
⁷
) years

= 1.4 × 10
¹⁰
years (or 14 billion years).

Hubble's constant thus provides a good first approximation for the age of the universe. However, as we shall see later in this chapter, the study of the expansion of our universe was going to throw up some surprises for astronomers.

We have seen how the spiral structure of our own galaxy, the Milky Way, has been discovered. The Sun lies well out in one spiral arm of the galaxy. At the center is a great bulge with the spiral arms lying in a disc around it. The bulge is surrounded by a halo of globular clusters consisting of old red stars, but the spiral arms contain younger blue-colored stars. Between the young stars in the disc of the galaxy are clouds of cold, molecular gas and dust which are the reservoir from which stars form. The existence of the dust had long been known from the dark patches apparent within the Milky Way, showing how the dust clouds can be so dense that they completely obscure the light from the stars behind them. The presence of vast quantities of gas was revealed from observations of the luminous nebulae surrounding clusters of newly formed young blue stars. The energetic ultraviolet light from the stars heated and ionized the gas atoms, which produced their own light when they later recombined, causing the nebulae to shine. Analysis of the emission from the gas enabled astronomers to measure the temperatures and densities of the gas nebulae; much of the radiation comes from “forbidden” transitions of ions of common elements that can never be observed in the laboratory since they require an extraordinary low density to occur.

Away from regions of active star formation, the gas clouds remain cold, and thus hidden to optical observations as they are either neutral or even molecular. The discovery of the extent of the neutral hydrogen in our galaxy and others had to await the development of radio astronomy after the Second World War. During the 1940s the theory of the 21 cm hydrogen line was developed. This is a spectral feature in the radio waveband that is generated by what is called a “spin flip” transition of electrons in hydrogen atoms. This is a spontaneous event that occurs very rarely—an individual atom may undergo such a change only once every ten million years—but it is still detectable because of the vast quantity of hydrogen in the galaxy. As radio astronomy developed radio emission lines from more complex molecules were discovered.

Observations of the 21 cm line were used to map out the density and distribution of gas in the plane of the galaxy at much further distances than could be obtained by studying the distribution of the stars. It is much easier to measure the Doppler shifts from the 21 cm line of different clouds of hydrogen than by amassing the shifts from thousands of individual stars, so the radio observations were also of major importance for determining the rate at which our galaxy rotates, and how this changes with distance from the center.

As we saw with the Tully–Fisher distance indicator, the rate at which a spiral galaxy rotates is directly due to its gravitational mass. Any individual object in orbit around the center of a galaxy—such as a neutral gas cloud, nebula or star—is responding to the gravity of all the mass at smaller radii. Thus by plotting the way the rotational velocity of objects in a galaxy changes with radius (known as a “rotation curve”), astronomers knew they could estimate the entire mass of a galaxy, and how it was distributed. Early attempts to determine the rotational velocity of galaxies used the Doppler shift observed from the absorption lines in stellar spectra, or the narrow emission lines of nebulae. By the late 1970s Vera Rubin (b. 1928) and her colleagues established a problem with the observed rotation curves of galaxies, which was later confirmed by more comprehensive observations using the radio 21 cm line as a tracer of the internal galactic dynamics.

All the results showed that the outer parts of the disc of all spiral galaxies (including our own) were rotating much faster than expected. They had sufficient speed to escape completely from the galaxy's gravity, but they remained attached. The only explanation was that there was far more gravitational mass in the galaxy than was indicated simply by the amount of stars and gas directly observed. A large quantity of invisible matter was required
that did not give off radiation at any wavelength; and in all spiral galaxies the amount of such “dark matter” was estimated to be greater than the visible mass by a factor of ten. But what was all this invisible matter?

The idea of “missing matter” was not a new concept, as it had been discovered as early as the 1930s when Fritz Zwicky (1898–1974) was studying clusters of galaxies. The motions of individual galaxies within a cluster are due to the gravitational attraction of the galaxy to the mass of the rest of the cluster. Zwicky was able to show that again, the galaxies were moving too fast to be responding simply to the gravity of the visible galaxies in a cluster. There had to be much more mass present, but this time it outweighed the observable components by a factor of over 100. Zwicky's original results have long since been confirmed for many clusters of galaxies, from observations of the internal dynamics of a cluster and also from studies of gravitational lensing.

Although it is well established that most of the gravitational mass in the universe is invisible at all wavelengths, the nature of the dark matter remains an open question. Possibilities range from “ordinary” matter which is comparatively well understood, such as brown dwarfs (failed stars), large planets, neutron stars or black holes; to far
more exotic (and hitherto undiscovered) subatomic particles such as axions or new types of neutrinos. The latter explanation proposes that a better understanding of particle physics is fundamental to explaining the motions of entire galaxies and clusters of galaxies.

Einstein's theories reinvented our interpretation of gravity not so much as Newton's “force at a distance,” but as the way in which space and time warp around the location of a massive object. An important consequence is that light traveling through space will sometimes find itself following a warped path, and indeed the observations of the deflections in the positions of stars whose light passed near the Sun during the 1919 total solar eclipse was the first observational confirmation of Einstein's theory. Today astronomers observe much more complicated distortions of the light as it passes by a large mass on its journey to Earth. Galaxies that lie behind (and at a far greater distance than) a cluster of galaxies sometimes have their image both magnified and greatly distorted as their light passes through the cluster—this is known as gravitational lensing. Often this distortion takes the form of multiple arcs and arclets. However, the amount of distortion traces the total gravitational rather than the visible mass, so a detailed mapping of the positions of such gravitational mirages
can reveal the presence and distribution of dark matter within a cluster.

Since the first studies of galaxies in the early part of the 20th century, it was obvious that the galaxies were not distributed uniformly on the sky—even when located well away from the obscuring effects of our Milky Way. This “clustering” of galaxies was first properly quantified in the 1950s by George Abell (1927–83), who created an extensive catalog of clusters from a detailed visual examination of photographic plates of the sky. His work also demonstrated that there was a range in cluster properties—not just in the number of galaxies, but also the shape and physical size of a cluster.

Abell's work—and that of other astronomers in the mid-20th century—was limited to the study of a projection of the sky onto two dimensions. Even so, it was clear that the distribution of clusters was also non-uniform in the sky, with regions where clusters themselves seemed to form immense structures known as “superclusters.” A full mapping of the true three-dimensional structure of the universe, however, involves the knowledge of the distance to all the galaxies. This is done most economically by
estimating the distance from a measured redshift via Hubble's law; even so, the determination of the redshift of a sufficient number of galaxies is a huge observational undertaking. For this reason, early surveys were necessarily limited to only small regions of the sky. One of the first attempts at a comprehensive redshift survey was begun by John Huchra (b.1948) and Margaret Geller (b.1947) in the 1980s, and it eventually grew to include over 14,000 galaxies. The resulting map showed some amazing structures, including the discovery of what became known as the Great Wall—a broad filament of clusters and galaxies about 200 million light years distant, which extends over 500 million light years long.

This was just the first (and not the largest) of many such walls and filaments now known to permeate the entire large-scale structure of the universe, and which surround regions of empty space of a similar size, known as “voids.” Today such surveys map out this cellular pattern of structure right across the universe.

The presence of dark matter was discovered by observing the way motions of astronomical objects are dictated by the gravity of an invisible mass. In the same way, the orbital motions of stars and gas at the very core of many galaxies (both spiral and elliptical) have revealed the presence
of immense masses at the center. Our own galaxy is thought to have a dark object at its center with a mass of around 2.5 million solar masses but contained in a region less than 20 light days across. Such supermassive black holes are found with masses up to several billion solar masses, and they are thought to lie dormant at the center of nearly all massive galaxies. A galaxy where the central supermassive black hole is still accreting matter is radically changed in appearance, having a very bright core, and is known as an “active” galaxy.