Bw

RR Lyr Statistical parallax

Globular cluster RR Lyr

Parallax subdwarfs

Dn-s ay 10 kLY

Figure 9.9. Distance "ladder" showing how distance determinations of close objects (e.g., through parallax) are the basis of distance determinations out to greater and greater distances, e.g., through cepheid and RR Lyrae variables to extragalac-tic indicators such as line broadening in the spectra of galaxies. Abbreviations: B-W - Baade-Wesselink; GCLF - globular-cluster luminosity function; Dn-a -Diameter-line width extension of Faber-Jackson method; LG - local group (of galaxies); LMC/SMC - Large (Small) Magellanic Cloud; LSC - local superclus-ter (of galaxies); PNLF - planetary-nebula luminosity function; SBF - surface-brightness fluctuations; SG - supergiant; SN - supernova. [From G. Jacoby et al., PASP 104, 599 (1992)]

due to gravitational attractions between galaxies and between clusters of galaxies. Studies of this tell us how much gravitational mass there is in the universe.

Systematic deviations from the linear Hubble law at large distances and in all directions in principle can reveal whether the expansion rate is decreasing, constant or increasing. Light from distant galaxies left those galaxies long ago so observations probe the expansion at early times. Slowing of the expansion is expected from mutual gravitational attraction; it is a natural outcome of Einstein's general theory of relativity. It is the same as a ball thrown upward; it will decelerate as it moves upward due to the gravitational attraction between it and the earth.

A major complicating factor in using distant galaxies (at large look-back times) as standard candles is that they evolve with time. Their colors and luminosities change as their constituent stars form, age, and die. Thus measurements have not been able to resolve whether or not the universe is actually slowing as gravity would suggest.

Recent studies that use distant Type Ia supernovae as standard candles are now indicating that the expansion might, in fact, be accelerating, exactly counter to our expectation. If so, this could be due to a weak repulsive force that acts preferentially over very large distances. Such a force is implied by anon-zero value of the cosmo-logical constant in Einstein's theory of general relativity. The mysterious energy source for this acceleration has been called dark energy. Generally, the nature of the expansion is of great interest to astrophysicists because it is fundamental to the theory behind the evolution of the universe.

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