Chapter 20.

Structure of the Universe

20.1. Components and Organization of Universe

• Galaxies are great islands composed of stars scattered throughout the Universe; they are the building blocks or basic units of which the Universe is composed
• Our Galaxy is only one of billions of galaxies
• Galaxies, like stars, exert sufficient gravitational attraction to be bound to one another permanently
  • Galaxies occur in pairs
  • Small groups of a few tens
  • Clusters of hundreds to thousands
  • Superclusters composed of a number of clusters, small groups, and individual galaxies
• Galaxies constitute the bulk of the matter that is emitting visible light
  • Through most of this century, visible galaxies were thought to contain the bulk of the Universe' matter
  • Today, growing evidence suggests that visible galaxies may be but a small percentage of all the matter in the Universe

20.2. Observed Structure of the Universe

• Hubble sequence of galaxies
  • Edwin Hubble (1889-1953), American astronomer, devised a classification scheme for galaxies
  • Arranged in orderly progression shaped like a tuning fork
  • Sequence runs from essentially spherical configurations through lens-shaped systems to very flat spiral systems to irregular ones
  • Four major classes are ellipticals (E), lenticulars (S0), spirals (S normal and SB barred), and irregulars (Irr); classification criteria
  • Degree of flattening; ellipticals graded from 0 to 7, spherical to most flattened
  • Relative size of the nucleus compared to the disk; spirals graded a to c, large to small
  • How tightly the spiral arms are wound; spirals graded a to c, tight to open
  • Population and types of stars
  • Relative amount of interstellar matter (gas and dust)
  • Properties of classes of galaxies
    • Galactic Property, Elliptical (E), Lenticular (S0 & SB0), Spiral (S & SB), Irregular (Irr)
    • Mass (MGal)a, 10-6 to 5, 0.1 to 2, 0.1 to 2, 10-4 to 0.2
    • Radius (RGal)a, 10-2 to 5, 0.2 to 2, 0.2 to 2, 0.05 to 0.3
    • Luminosity (LGal)a, 10-5 to 7, 10-3 to 2, 0.1 to 2, 10-5 to 0.1
    • Mass/Luminosity (solar units)b, 5 to 80 giantsc ,1 to 5 dwarfs, 5 to 50, 2 to 20d, 1 to 3
    • Typical rotation velocities (km/s), <100, 100 to 200, 100 to 300, 50 to 150
    • Color, Red, Red, Red (nucleus) blue (spiral arms), Blue
    • Spectral class of central regions, K, G, F to K, A to F Irr I, G to K Irr II
    • Stellar ages, Old, Old, young, Old (halo and nucleus) young (disk), Young some old
    • H I in total mass (%), <0.1, <1, 1 to 10e, 15 to 50
    • Dust content, Very little or none, Little to none, Some, Some
      • a Units of our Galaxy: MGal = 200 x 109 Mo, RGal = 5 x 104 ly, LGal = 20 x 109 Lo
      • b Solar units: (M/L)o = 1 c Highest in giant ellipticals d Highest in center e Decreases from Sc to Sa.
• Expansion of the Universe
  • Cosmological redshift
    • 1912, found that absorption lines in composite spectrum of all the stars in Andromeda galaxy were blueshifted, indicating a velocity of approach
    • 1928, large redshifts in the absorption lines of all but 5 of 41 nearby galaxies had been found (the 5 having blueshifted spectra)
    • Even larger redshifts have since been found for fainter galaxies
    • Hubble estimated distances for a number of galaxies whose redshifts had been measured and found that a straight-line relationship existed between their redshifts interrupted as recessional velocities and their distances, in the sense that the farther away a galaxy is, the faster it is moving away from us; only exceptions were several nearby galaxies that are blueshifted
    • Hubble showed that for intrinsically more luminous galaxies, the recessional velocity is also correlated with apparent brightness; the greater the recessional velocity, the fainter a galaxy appears and the more distant it is
  • Hubble's velocity-distance law: farther away a galaxy is from our Galaxy, faster that galaxy is receding from us; i.e., recessional velocity equals a constant times distance
    • Interpretation: redshifts of distant galaxies represents amount the Universe has expanded since time galaxy's light was emitted
    • Redshifts are result of expansion of the Universe
      • Cosmological redshifts - an expansion of space
      • Not same as Doppler or gravitational redshifts
    • Galaxies also exhibit a small peculiar velocity superimposed on expansion velocity
      • For nearby galaxies, peculiar velocity larger than recessional velocity; thus observe some blueshifted galaxies
      • For very distant galaxies, recessional velocity much larger than peculiar velocity; thus may neglect peculiar velocity
    • Mathematically, Hubble's law is v = Hr, where constant of proportionality H is called Hubble's constant
    • Estimates of Hubble's constant lie between 15 and 30 km/s/Mly or 50 to 100 km/s/Mpc
    • Example; largest redshifts supposedly normal galaxies equal about 1.2, which corresponds to a recessional velocity of about 200,000 km/s
      • If Hubble constant 17 km/s/Mly, galaxies about 11 x 109 ly; observe them as they were 11 billion years ago
      • Some quasars redshifts over 3 and a few over 4; distances equal almost 15 x 109 ly
• Establishing a Cosmic Distance Scale
  • A cosmic distance scale has been established by building a chain of overlapping distances proceeding from the nearest objects to farthest
  • Cosmic distance scale begins with the distances of relatively close stars determined from parallax
  • Second link in the chain comes from distances of variable stars, chiefly Cepheids, and distances from spectroscopic and intrinsic brightness of stars in our Galaxy
  • Third link comes from distances of neighboring galaxies of the Local Group, which are determined from characteristics of their brightest stars, Cepheid variables, and other stellar data
  • Fourth link uses distances of what astronomers refer to as "Nearby Galactic Groups" taking as distance indicators their brightest stars, surface brightness of galaxy, and apparent size of their bright gaseous nebulae
  • Fifth link in the chain is clusters of galaxies, such as the Virgo cluster, using the cluster's brightest galaxy or its luminosity type as a standard of comparison
  • Final link in the cosmic distance scale is distances of more remote clusters of galaxies by means of the Hubble constant derived from expansion of the Universe
• Large scale distribution of matter
  • Levels of cosmic clumping
    • 1st level; largest galaxies are a few Mly in diameter
    • 2nd level; clusters of galaxies are a few 10s Mly in diameter
    • 3rd level; superclusters are a few 100s Mly in diameter
    • Galaxies are only a small percentage of the size of superclusters
    • Evidence for 4th level; superclusters may be part of an even larger organization of interlocking chains that begins to approach a billion light years in size; if real, could be called clusters of superclusters
  • Cosmological scale is one of billions to tens of billions of light years, where superclusters participate in a general cosmic flow known as the expansion of the Universe
    • Superclusters are only a few percent of cosmological scale
    • Clusters a few tenths of a percent of cosmological scale
    • Largest galaxies a few hundredths of a percent of the cosmological scale
    • Analogy; if size of visible Universe were to shrink to that of this page, a supercluster would be about the size of a capital letter and any galaxy would be smaller than a period

20.3. Mental Models for Supercluster Structure

• Three models used for mental picture of supercluster structure of Universe (scales < 109 ly)
  • Island model: matter is isolated islands in great void of space
  • Sea-sponge model: matter and voids form one interconnected structure like sponge
  • Swiss-cheese model: voids are isolated pockets in an otherwise continuous structure
• Which model most nearly fits what astronomers think they are finding?
  • At present, structure seems clearly to be more sponge-like than island- or swiss-cheese-like
  • Having this model in mind, one wonders whether such structure is just local or is there reason to believe that throughout all space and over all cosmic time such structure has existed?
• Sizes and separations of clusters of galaxies
  • Object, Typical size, Typical separation, Percent of largest
  • galaxies, 104-5 ly, 105-6 ly, 0.01%
  • small groups, 105-6 ly, 106 ly, 0.1%
  • rich clusters, 106 ly, 107 ly, 1%
  • superclusters, 108 ly, 108 ly, 10%
  • clusters of superclusters, 109 ly, 109 ly, 100%

20.4. The "Missing Mass" Problem

• 1933, Fritz Zwicky (1898-1974) noted that there did not appear to be enough mass in the form of galaxies to bind galaxies gravitationally into a cluster; "missing mass problem"
• Attempts to estimate the mass of an entire cluster of galaxies have led to conflicting figures
• Dynamical mass, found by analyzing observed radial velocity differences, is many times greater than the luminous mass, obtained from light emitted; luminous mass of all galaxies in a cluster amounts to only 3-5% of mass needed to provide gravitational stability
• In what form might this dark matter be? Suggestions offered
  • Intergalactic material lying between galaxies
    • If it is cool atomic hydrogen, it should emit 21-cm photons; none observed
    • If it is molecular hydrogen, it should ultraviolet light detectable with orbiting observatories; none observed
    • If the dark matter is a very hot gas, it should emit X-ray photons but no 21-cm radiation; dozens of clusters are powerful X-ray sources, but estimates are that the mass of hot gas is about equal to the mass of all the cluster's galaxies; therefore, hot intergalactic gas is not sufficiently massive to prevent the cluster from eventually coming apart
  • Subluminous galaxies
    • If missing mass is in the form of subluminous galaxies, there would have to be millions to billions of them for each bright galaxy
    • They should be visible collectively as a faint glow spread over the entire cluster; no such effect is observed
  • Extended halos of galaxies
    • If missing mass is to be found in an extended halo, such as is suspected for our Galaxy, the large central galaxies of the cluster should have rapidly cannibalized the other galaxies earlier in the life of the cluster; hence clusters ought to appear very different from the way they do now
    • Although evidence grows that dark matter does exist in extended halos in some galaxies, extended galactic halos raise many perplexing questions if they are the location of the missing mass
  • The unanswered problem of the missing mass in clusters of galaxies is as important, if not more so, to our understanding of the Universe as any problem in astronomy today

20.5. Cosmic Background Radiation (CBR)

• 1934, George Gamow (1904-1968) predict that following the big bang the Universe was filled with high-energy photons permeating all space and subsequent expansion cooled this radiation so that today most of its energy lies in microwave region and in a background sea of neutrinos
• 1965, CBR discovered accidentally by physicists Arno Penzias (b. 1933) and Robert Wilson (b. 1936) at the Bell Telephone Laboratory (Penzias and Wilson shared the Nobel Prize in physics in 1978)
• Observed spectral distribution of CBR (its photons have a typical wavelength of 1 mm) is to a high degree a 3o K blackbody radiation curve
• CBR appears to be coming from all directions in space (isotropic) and to have same intensity in all directions (homogeneous)
• Extremely small departures in CBR uniformity may have been discovered in spring of 1992

20.6. Structure of the Universe

• Distribution of matter on sizes of several hundred million light years resembles a sea sponge
• This is apparently how matter is distributed within 1 x 109 ly, and is apparently same in all direction
• On largest scales in Universe, CBR and X-ray background radiation suggest Universe is surprisingly isotropic and homogeneous and that no structure exists on scale of billions to tens of billions of light years
• If Universe is as uniform on largest scales as it appears, any large-scale curvature of space as a consequence of general relativity must be uniform throughout space and also Hubble's velocity-distance law is applicable everywhere in the Universe