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12.2 Cosmological kinematics: observing the expanding universe |
at a distance d can be inferred if we know the proper diameter D of the object transverse to the line of sight, θ = D/d. This leads to the definition of the angular diameter distance dA to an object anywhere in the universe:
dA = D/θ . |
(12.43) |
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The dependence of dA on redshift z is explored in Exer. 12, § 12.6. The result is |
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dA = Rer = (1 + z)−2dL, |
(12.44) |
where Re is the scale factor of the universe when the photon was emitted. The analogous expression to Eq. (12.42) is
dA = R0r/(1 + z) = |
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%1 + −1 + |
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z& + · · · . |
(12.45) |
H0 |
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H˙ 0 |
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There are situations where we have in fact an estimate of the comoving diameter D of an emitter. In particular, the temperature irregularities in maps of the cosmic microwave background radiation (see below) have a length scale that is determined by the physics of the early universe.
Although we have provided small-z expansions for many interesting measures, it is important to bear in mind that astronomers today can observe objects out to very high redshifts. Some galaxies and quasars are known at redshifts greater than z = 6. The cosmological microwave background, which we will discuss below, originated at redshift z 1000, and is our best tool for understanding the Big Bang. Even so, the universe was already some 300 000 years old at that redshift. Sometime in the future, gravitational wave detectors may detect random radiation from the Big Bang itself, originating when the universe was only a fraction of a second old.
The derivation of Eq. (12.42) illustrates a point which we have encountered before: in the attempt to translate the nonrelativistic formula v = Hd into relativistic language, we were forced to re-think the meaning of all the terms in the equations and to go back to the quantities we can directly measure. If the study of GR teaches us only one thing, it should be that physics rests ultimately on measurements: concepts like distance, time, velocity, energy, and mass are derived from measurements, but they are often not the quantities directly measured, and our assumptions about their global properties must be guided by a careful understanding of how they are related to measurements.
The universe is accelerating!
The most remarkable cosmographic result since Hubble’s original work was the discovery that the expansion of the universe is not slowing down, but rather speeding up. This was done by essentially making a plot of the luminosity distance against redshift, but where luminosities are given in magnitudes. This is called the magnitude–redshift diagram, and we derive its low-z expansion in Exer. 13, § 12.6. Two teams of astronomers, called the
