11 Modern Cosmology
223
three-dimensional maps of the Universe, and contains spectra for more than
three million astronomical objects. It measures red-shifts using a dedicated
2.5-m wide-angle optical telescope at Apache Point Observatory in New
Mexico, U.S.A.
We see from Fig. 11.2, in which every little dot is a galaxy, that the
observed distribution of galaxies is anything but homogeneous. Close inspection reveals a spongy or foamy structure. The galaxies are located on walls
and filaments that border large voids where nothing is visible. The underlying
cause of this structure is unknown. It may be related to events that arose in
the pre-recombination era, or to the prevalence of dark matter. Speculations
abound, but we really don’t know.
11.5 The Accelerated Expansion
In Chap. 10, we discussed the understanding of the cosmos at the stage that
it had reached towards the close of the 20th Century. However, at the end
of 1998 and at the beginning of 1999, the complacency that was becoming
common among cosmologists was shattered when two independent groups
of astrophysicists, guided respectively by Adam Riess and Saul Perlmutter,
published the results of extended surveys of type Ia supernovas located in
other galaxies.
The measurement of distance in astronomy by the “standard candle”
approach is described in Appendix 10.1. Type Ia supernovas are suitable
standard candles; they all have the same intrinsic brightness, which can be
estimated by measuring their apparent brightness at known distances. Further
measurements of the apparent brightness of these supernovas in more distant
galaxies can then provide an estimate of the distance to these galaxies.
A second independent measurement of this distance can be obtained by
measuring the red shift from one of the spectral lines visible in the light
emitted by a supernova, and applying the Hubble constant to calculate the
distance to the supernova, and thus to the galaxy that contains it. Unfortunately, as is sometimes the case in physics when two independent approaches
are available to measure the same quantity, the results from the two sets
of measurement do not agree with each other. It is found that for type Ia
supernovas at distances larger than approximately 1 billion light years, the
supernovas appear to be fainter than they should be. Since both types of data
come from the same sources, the explanation cannot be in terms of different
distances.
223
three-dimensional maps of the Universe, and contains spectra for more than
three million astronomical objects. It measures red-shifts using a dedicated
2.5-m wide-angle optical telescope at Apache Point Observatory in New
Mexico, U.S.A.
We see from Fig. 11.2, in which every little dot is a galaxy, that the
observed distribution of galaxies is anything but homogeneous. Close inspection reveals a spongy or foamy structure. The galaxies are located on walls
and filaments that border large voids where nothing is visible. The underlying
cause of this structure is unknown. It may be related to events that arose in
the pre-recombination era, or to the prevalence of dark matter. Speculations
abound, but we really don’t know.
11.5 The Accelerated Expansion
In Chap. 10, we discussed the understanding of the cosmos at the stage that
it had reached towards the close of the 20th Century. However, at the end
of 1998 and at the beginning of 1999, the complacency that was becoming
common among cosmologists was shattered when two independent groups
of astrophysicists, guided respectively by Adam Riess and Saul Perlmutter,
published the results of extended surveys of type Ia supernovas located in
other galaxies.
The measurement of distance in astronomy by the “standard candle”
approach is described in Appendix 10.1. Type Ia supernovas are suitable
standard candles; they all have the same intrinsic brightness, which can be
estimated by measuring their apparent brightness at known distances. Further
measurements of the apparent brightness of these supernovas in more distant
galaxies can then provide an estimate of the distance to these galaxies.
A second independent measurement of this distance can be obtained by
measuring the red shift from one of the spectral lines visible in the light
emitted by a supernova, and applying the Hubble constant to calculate the
distance to the supernova, and thus to the galaxy that contains it. Unfortunately, as is sometimes the case in physics when two independent approaches
are available to measure the same quantity, the results from the two sets
of measurement do not agree with each other. It is found that for type Ia
supernovas at distances larger than approximately 1 billion light years, the
supernovas appear to be fainter than they should be. Since both types of data
come from the same sources, the explanation cannot be in terms of different
distances.