8
A. Balbi and M. M. ´
Cirkovi´ c
to some specified reference class (for example, the class of earth-like planets in the
habitable zone of their star).
In [9] we argued that a similar appeal to typicality is not possible with respect to
time, and, in fact, typicality in time cannot be a well-defined concept in the standard
cosmological model. Here we summarize our reasoning and the implications for the
search for life in the universe, and in particular for SETI.
2.2 Is Temporal Copernicanism Well-Defined?
The now falsified classical steady-state theory [6] was based on a perfect cosmological principle (henceforth PCP) that postulated homogeneity in both space and time,
thus being a special case of CP with the highest level of symmetry. Average in time
was a well-defined operation, similar to average in space: any epoch was equal to
any other. With the advent of the hot big bang model, PCP was clearly disproved.
The now accepted standard cosmological model has a strong evolutionary component
(see, e.g., [2, 7]), meaning that the physical average state of the universe has changed
radically in time. This has long been recognized as one of the main motivations for
astrobiology (see e.g. [12]): while in a steady state universe there would have been
an infinite time to produce life, the universe described by the big bang model cannot
have been equally habitable at all epochs in the past (and will probably not be in the
far future). Thus, there is only a limited window in time for abiogenesis, and the fact
that there is at least one planet with life (the Earth) requires an explanation that fits
into the available temporal interval. Life requires, for example, the buildup of heavy
elements in stars, and possibly even of organic prebiotic molecules in interstellar
space and in protoplanetary discs. Typical observers should thus find themselves
living in an epoch when the age of the universe is of the same order of magnitude
of the lifetime of a main sequence star—a coincidence of time scales that would be
unexplainable in a steady-state model [19]. Increasing attention has been devoted, in
recent times, to the issue of when the universe started to be habitable and how long it
will remain so in the future [14, 15], as well as to the existence of optimal locations
and epochs within our galaxy where complex life could preferentially evolve [17].
The evolutionary framework introduced by the big bang model implies that we
cannot regard at different epochs as representative of some ‘average’ state, and that
the idea of being ‘typical’ in time has to be discarded. In short, there are at least
three ways that make temporal Copernicanism an ill-defined concept (we refer to
[9] for further details). First, some of the objects (e.g. stars and planets, and more
generally any bound object) existing in the present universe have not existed in the
past and will not exist in the future [1]. Second, the value of any physical quantity
fluctuates over long time-scales. Take, for example, a simple (and, at present, welldefined) astrophysical quantity such as the star formation rate (SFR). In what sense
can we define an average SFR in time? There is no way of doing this over arbitrarily
long scales (for example, the rate would be exactly 0 if averaged over the entire
history of the universe, including the future), but only on narrowly specified intervals.
A. Balbi and M. M. ´
Cirkovi´ c
to some specified reference class (for example, the class of earth-like planets in the
habitable zone of their star).
In [9] we argued that a similar appeal to typicality is not possible with respect to
time, and, in fact, typicality in time cannot be a well-defined concept in the standard
cosmological model. Here we summarize our reasoning and the implications for the
search for life in the universe, and in particular for SETI.
2.2 Is Temporal Copernicanism Well-Defined?
The now falsified classical steady-state theory [6] was based on a perfect cosmological principle (henceforth PCP) that postulated homogeneity in both space and time,
thus being a special case of CP with the highest level of symmetry. Average in time
was a well-defined operation, similar to average in space: any epoch was equal to
any other. With the advent of the hot big bang model, PCP was clearly disproved.
The now accepted standard cosmological model has a strong evolutionary component
(see, e.g., [2, 7]), meaning that the physical average state of the universe has changed
radically in time. This has long been recognized as one of the main motivations for
astrobiology (see e.g. [12]): while in a steady state universe there would have been
an infinite time to produce life, the universe described by the big bang model cannot
have been equally habitable at all epochs in the past (and will probably not be in the
far future). Thus, there is only a limited window in time for abiogenesis, and the fact
that there is at least one planet with life (the Earth) requires an explanation that fits
into the available temporal interval. Life requires, for example, the buildup of heavy
elements in stars, and possibly even of organic prebiotic molecules in interstellar
space and in protoplanetary discs. Typical observers should thus find themselves
living in an epoch when the age of the universe is of the same order of magnitude
of the lifetime of a main sequence star—a coincidence of time scales that would be
unexplainable in a steady-state model [19]. Increasing attention has been devoted, in
recent times, to the issue of when the universe started to be habitable and how long it
will remain so in the future [14, 15], as well as to the existence of optimal locations
and epochs within our galaxy where complex life could preferentially evolve [17].
The evolutionary framework introduced by the big bang model implies that we
cannot regard at different epochs as representative of some ‘average’ state, and that
the idea of being ‘typical’ in time has to be discarded. In short, there are at least
three ways that make temporal Copernicanism an ill-defined concept (we refer to
[9] for further details). First, some of the objects (e.g. stars and planets, and more
generally any bound object) existing in the present universe have not existed in the
past and will not exist in the future [1]. Second, the value of any physical quantity
fluctuates over long time-scales. Take, for example, a simple (and, at present, welldefined) astrophysical quantity such as the star formation rate (SFR). In what sense
can we define an average SFR in time? There is no way of doing this over arbitrarily
long scales (for example, the rate would be exactly 0 if averaged over the entire
history of the universe, including the future), but only on narrowly specified intervals.