4
M. V. John
1 Introduction
A cosmological solution to Einstein’s general theory of relativity [1–4] was obtained
by A. Friedmann in the early 1920s. This first suggested an expanding universe,
though the solution was not widely noticed at that time. It is interesting to note that
even Einstein himself believed in the prevailing paradigm of ‘static universe’ during
that period and did not heed to the prediction of general relativity that the universe
is either expanding or contracting. Hubble’s observation in 1929 that the redshifts
of galaxies and their apparent magnitudes obey certain relation was interpreted as
evidence to an expanding universe. This model was based on only two basic principles
in addition to general relativity: (1) the universe is homogeneous and isotropic at large
scales and (2) it contains only ordinary matter. But in the 1930s itself, astronomical
observations such as that of galaxy rotation curves suggested that the visible matter is
not the entire component of the universe. It may contain some invisible matter, which
is even now called ‘dark matter’. Such a model of the universe that contains ordinary
matter, possibly along with some dark matter, provided a satisfactory picture of the
universe with an initial singularity and a very hot early phase. This model is now
famous as the hot big bang model and had several successes like the prediction of
cosmic microwave background radiation (CMBR), which was actually observed in
1964. The two scientists, A. A. Penzias and R. W. Wilson, who made this milestone
observation, were awarded Nobel prize in 1978.
The Friedmann model of the universe reigned till the end of last century, though
not without opposition. An unmistakable prediction of this model was that the cosmic
expansion gradually slows down (decelerated expansion) with time. A paradigm shift
occurred in cosmology with the 1998 observation that standard candles such as Type
Ia supernovae show exceptional dimming at very high redshifts and this turned the
graph upside down and led to the notion that the universe is at present undergoing
an accelerated expansion. The reason for this was speculated to be the presence of
large amount of some unobservable, repulsive dark energy and this culminated in
the currently popular Lambda-CDM model. Adam Riess, Brian Schmidt and Saul
Perlmutter, who led the teams that made this observation were awarded Nobel prize
in 2011 for their discovery.
Now, after two more decades, when the dust settles, is there evidence laid bare
for a universe that is neither accelerating nor decelerating? Was the universe expanding linearly throughout its history, with a constant expansion rate? A universe that
expands with a constant expansion rate is said to be ‘coasting’. Such a straightening of the graph may mark yet another milestone in the development of cosmology.
This is now discussed in a large number of papers published in leading journals of
astronomy. In the first part of this paper, we discuss various coasting cosmological
models, starting from the first such model by the British astronomer A. Milne. The
Milne model is still considered in the literature as an interesting, albeit unphysical
model. We also review briefly the first realistic eternal cosmological model with
M. V. John
1 Introduction
A cosmological solution to Einstein’s general theory of relativity [1–4] was obtained
by A. Friedmann in the early 1920s. This first suggested an expanding universe,
though the solution was not widely noticed at that time. It is interesting to note that
even Einstein himself believed in the prevailing paradigm of ‘static universe’ during
that period and did not heed to the prediction of general relativity that the universe
is either expanding or contracting. Hubble’s observation in 1929 that the redshifts
of galaxies and their apparent magnitudes obey certain relation was interpreted as
evidence to an expanding universe. This model was based on only two basic principles
in addition to general relativity: (1) the universe is homogeneous and isotropic at large
scales and (2) it contains only ordinary matter. But in the 1930s itself, astronomical
observations such as that of galaxy rotation curves suggested that the visible matter is
not the entire component of the universe. It may contain some invisible matter, which
is even now called ‘dark matter’. Such a model of the universe that contains ordinary
matter, possibly along with some dark matter, provided a satisfactory picture of the
universe with an initial singularity and a very hot early phase. This model is now
famous as the hot big bang model and had several successes like the prediction of
cosmic microwave background radiation (CMBR), which was actually observed in
1964. The two scientists, A. A. Penzias and R. W. Wilson, who made this milestone
observation, were awarded Nobel prize in 1978.
The Friedmann model of the universe reigned till the end of last century, though
not without opposition. An unmistakable prediction of this model was that the cosmic
expansion gradually slows down (decelerated expansion) with time. A paradigm shift
occurred in cosmology with the 1998 observation that standard candles such as Type
Ia supernovae show exceptional dimming at very high redshifts and this turned the
graph upside down and led to the notion that the universe is at present undergoing
an accelerated expansion. The reason for this was speculated to be the presence of
large amount of some unobservable, repulsive dark energy and this culminated in
the currently popular Lambda-CDM model. Adam Riess, Brian Schmidt and Saul
Perlmutter, who led the teams that made this observation were awarded Nobel prize
in 2011 for their discovery.
Now, after two more decades, when the dust settles, is there evidence laid bare
for a universe that is neither accelerating nor decelerating? Was the universe expanding linearly throughout its history, with a constant expansion rate? A universe that
expands with a constant expansion rate is said to be ‘coasting’. Such a straightening of the graph may mark yet another milestone in the development of cosmology.
This is now discussed in a large number of papers published in leading journals of
astronomy. In the first part of this paper, we discuss various coasting cosmological
models, starting from the first such model by the British astronomer A. Milne. The
Milne model is still considered in the literature as an interesting, albeit unphysical
model. We also review briefly the first realistic eternal cosmological model with