Chapter 18Our Expanding Universe Chapter 18 makes the “grand leap” to the universe as a whole, and introduces ideas about other galaxies that are less than a century old. The existence of other galaxies of stars was debated without resolution in 1920, but is now common fact. 1. The nature of the universe is revealed by galaxy redshifts and the 3K microwave background. 2. At present the term “redshift” is synonymous with “distance,” although there is still some debate in lesser quarters.
Until after the Great Debate of 1920 between Harlow Shapley and Heber Curtis into the nature of the spiral nebulae, it was not obvious that there were other galaxies beyond the Milky Way. Shapley, the “winner” of the debate, actually argued that spiral nebulae were part of our Galaxy. Curtis, the loser ultimately proven correct, argued that the spiral nebulae were other galaxies lying beyond the borders of the Milky Way. In 1920 the best views of “spiral nebulae” were ambiguous about whether or not they contained stars.
This is one of the best “pictures” of our Galaxy from the Sun’s location as sketched by Sergei Gaposhkin from Australia (1957),. The lower view is Sergei’s attempt to step outwards by 1000 parsecs from the Sun. Bulge Disk
Since the Sun is located just above the central line of the main disk of the Galaxy, only lines of sight upwards from the main disk give the best views of the surrounding universe of galaxies.
The rectangular area below lies upwards from the Galactic plane, thereby sampling the universe.
A close-up of the region of sky where the Hubble Space Telescope Ultra Deep Field image lies.
Every “fuzzy” object in this image is a distant galaxy. Foreground stars in our own Galaxy have associated diffraction spikes because they are point-like images. Star
Schematics of the meanings for “homogeneous” and “isotropic.”
Bubbles, Voids, and Strands. Surveys of the spatial distribution of galaxies indicate that they are not as “homogeneous” as sometimes thought. The location and brightness of galaxies in this view indicate the patchy nature of their distribution.
Distance surveys in selected bands of sky further accentuate the patchy nature of the distribution of galaxies, which is marked by bubbles containing very few galaxies surrounded by denser strands rich in clusters and superclusters of galaxies.
The 2dF Galaxy Redshift Survey. The largest voids measure 100 Mpc across.
How are distances to galaxies determined? Distances to nearby solar system objects are measured by radar, nearby stars by parallax, then cluster main sequence fitting, the Cepheid PL relation, and finally Type Ia supernovae.
The standard candles used to measure distances to galaxies are: For “nearby” galaxies, Cepheid variables, since their periods of pulsation correlate with their intrinsic luminosities (period-luminosity relation). (similar to using the “watts” label on a light bulb to determine how bright it is) For distant galaxies, Type Ia supernovae, since their peak brightness is always the same. They are also the brightest class of supernovae. (similar to, say, all supernovae appearing as bright as a “150 watt” light bulb)
Determining distances to galaxies using Cepheid variables. Step 1. Identify a Cepheid from its changes in brightness with time. Step 2. Measure its mean brightness and period of variability. Step 3. Use the Cepheid period-luminosity relation to establish its intrinsic brightness and calculate its distance.
Lastly, there is a relationship between a galaxy’s redshift and its distance. Cluster elliptical galaxies of different redshift, and how they correlate with distance for an outdated Hubble constant of 50 km/s/Mpc.
Note how size and redward shift of the spectral lines correlate exactly for bright ellipticals.
Conclusion: We live in an expanding universe.
A baking raisin bread analogy is often used to picture an expanding universe. It does not matter which raisin represents the Sun. From every raisin the nearest raisins all appear to increase their distances with time, at a rate proportional to the distance from the reference raisin.
The meaning of “redshift.” Spectral features are shifted to longer wavelengths according to a simple formula: (λobserved – λrest)/λrest = velocity/c. The spectrum is “stretched” to the red, not simply shifted. There is no shift for λ = 0.
The Hubble Law is not a peculiarity of our local galactic neighbourhood, but reflects an actual expansion of space, much like the expanding raisin bread, referred to as the “Hubble flow.” Velocities and distances are often found with the standard Doppler equation: and For large velocities (approaching c), the relativistic velocity and distance equations become: and For reference purposes only.
The simple formula z = v/c breaks down as speeds approach the velocity of light, and the relativistic version must be used.
How observed redshift z correlates with recession velocity v when the relativistic formula is used. Note that z = 5 does not mean a galaxy is moving at 5 times the speed of light. It is actually moving at 95% of the speed of light.
Redshift-distance relation (again). Slope = Rise/Run = H0
The inverse of the Hubble constant (which is the slope of the Hubble relation) has units of time and is called the Hubble Time. It is an estimate of the age of the universe (backwards extrapolation), provided that the expansion began at some point in the past and has been continuing at the same rate ever since. For H0 = 71 km/s/Mpc. The textbook uses H0 = 22 km/s/Mly (the same). The Milky Way’s globular clusters are all less than 14 billion years old.
Penzias and Wilson with the radio horn used to discover the 3K microwave background radiation.
The 3K background revealed by the COBE satellite, displays a “Doppler shift” attributed to mass asymmetry in the early universe when matter separated from radiation.
The 3K microwave background with the Doppler shift removed, as recorded by WMAP.
The 3K microwave background matches the radiation from a black body with T = 2.728 K.
The 2.728 K background is the constant faint glow from the universe when T = 3000 K, now redshifted by z≈ 1000.
Conclusion: We live in an expanding universe that has a background glow.
Next Step? Big Bang cosmology?
The temporal development of the universe according to the Big Bang model (logarithmic units).
Astronomical Terminology Shapley-Curtis Debate. A 1920 debate in Washington about the nature of the spiral nebulae. Hubble Law. The relationship correlating increasing distance with larger recession velocity for galaxies. Hubble Constant. The slope of the Hubble relation: H0 = 71 km/s/Mpc = 22 km/s/Mly. Hubble Time. The inverse slope of the Hubble relation, yielding an estimate of 13.8 billion years for the age of the universe. Relativistic Doppler Shift. How redshift z = Δλ/λ is converted to recession speed v for velocities near c. 3K Microwave Background. The faint glow from the entire sky (black body T = 2.728 K) attributed to the recombination era following the Big Bang. Big Bang. Fred Hoyle’s derogative term for the supposed origin of the universe in an explosive fireball 14 billion years ago.
Astronomical Terminology (continued) Homogeneity. A term describing the similar appearance of the universe in all directions for all observers. Isotropy. A term describing how the universe appears to be roughly the same no matter in which direction one looks. Distance Ladder. A term describing the various methods by which astronomers determine distances to objects in space, beginning with the closest and ending with the most distant. Scale Factor. The constant of proportionality describing the expansion of the universe, namely the separation of galaxies in the past in terms of their present separations. Light Element Production. A model of the early universe describing the production of various isotopes of hydrogen (1H and 2H, deuterium), helium (3He and 4He), and lithium (7Li) in the Big Bang.
Sample Questions 13. What are peculiar velocities, and how do they affect our ability to measure H0?
Answer. Peculiar velocities are speeds that galaxies exhibit in addition to those expected from the general expansion of the universe. They result from the gravitational attraction of galaxies to each other over large scales, and need to be taken into account when determining how a galaxy is moving relative to us in the absence of local gravitational effects.
Answer. It is the leftover energy from the early universe. At early times the universe is assumed to have been very hot, but a few hundred thousand years after the Big Bang the fireball had cooled considerably, although still at high temperatures. When the fireball had cooled to T≈ 3000 K it became transparent to energy. The thermal energy of the hot universe then escaped into space as black body radiation and is seen now as the highly-redshifted light from that early epoch.
Chapter 19Galaxies Now that the grand nature of the universe has been described, Chapter 19 steps backwards to look into the nature of individual galaxies and their environs. 1. The main galaxy types are described, pointing out how they differ in terms of the relative frequency of old and young stars, and interstellar gas, within them. 2. Imaging of galaxies and galaxy clusters reveals that it is a “violent” universe. Evidence for collisions between galaxies is everywhere, although keep in mind that by “collisions” we are talking about events that take place over billions of years, not mere seconds or minutes.
The Hubble Sequence: The familiar “tuning fork” diagram developed by Edwin Hubble is an attempt to link the main galaxy types of elliptical (E), spiral (S), barred spiral (SB), and irregular (Ir) classes. The lenticular galaxies (S0, SB0), added later, were a supposed link between spheroidal E galaxies and flattened S and SB galaxies, but unfortunately the diagram was also pictured as an evolutionary sequence. Thus, elliptical galaxies are often referred to as “early-type” galaxies, much like “early-type” OB stars.