Welcome back.
So in the last lecture, we saw how
cosmology has established itself as
a science in its own right
in just over a century.
In this class,
we look at the current cosmological model,
the so-called concordance model,
or lambda CDM.
This model builds on Einstein's
general relativity and
the so-called
Friedmann-Lemaitre-Robertson-Walker model,
and asserts that our universe is
infinite and consists of 5%
ordinary matter, 25% cold dark matter and
70% dark energy.
So according to this picture, the vast
majority of our universe consists of
two exotic entities: dark energy and
dark matter.
While the search for those two
entities is at the very forefront of
contemporary research in cosmology,
from a philosophical point of view,
we should ask, what are dark energy and
dark matter, and
how justified are we in believing in them?
The philosophical debate
behind dark energy and
dark matter concerns
the rationality of theory choice.
What reasons do scientists have for
choosing one theory over another?
How do we go about making those
rational decisions in the light of
the available evidence?
So, in this class we look at
the history of dark energy and
dark matter and some of
the experimental evidence for them,
as well as examining some of the possible
rival theories that have been proposed
within the broader philosophical debate
about the rationality of theory choice.
To get us started with the debate
about the rationality of
theory choice in science,
lets consider for
example, the discovery of
the planet Neptune back in 1846.
The anomalous perihelion of the planet
Uranus was known for some time, and
two astronomers, Urbain Le Verrier and
John Couch Adams, independently of
each other tried to reconcile
the anomaly with Newtonian theory
by postulating the existence of a new
planet called Neptune, whose orbital mass
was supposed to interfere with and
explain the anomalous orbit of Uranus.
The new planet was indeed observed
on the 23rd September, 1846.
A very similar phenomenon was also
observed for the planet Mercury,
and also in this case, Le Verrier
postulated the existence of a new planet,
Vulcan, to explain the observed anomaly.
But this time, the predicted planet was
not found, and a final explanation of
the phenomenon came only with
the advent of general relativity.
This historical example illustrates
a phenomenon that we have
already encountered in
the introductory lecture, namely,
the problem of underdetermination
of theory by evidence.
Whenever we have more than
one scientific theory or
hypothesis, the available evidence may not
be sufficient to determine the choice for
one theory over a rival one.
For example, in the case of Uranus,
the anomalous perihelion was evidence that
there was something wrong with
the set of assumptions, including both
main theoretical hypotheses about
Newtonian mechanics, as well as auxiliary
hypothesis about the number of planets in the
solar system, their masses, and orbits.
But the anomalous perihelion by
itself didn't tell scientists whether
the culprit for the anomaly was one of the
main theoretical assumptions, as opposed
to one of the auxiliary hypothesis about
number of planets, masses and so forth.
As is seen in our case of the anomalous
perihelion of Mercury shows,
finding the right answer to these
questions may well be far from obvious.
The physicist and
philosopher Pierre Duhem at the beginning
of the 20th century concluded that
scientists often followed their good sense
in making decisions in such situations.
But Duhem's solution in terms of
good sense is not satisfactory.
For one thing,
it's not clear what good sense is.
Second, it's not clear why scientist
X's good sense should agree with
scientist Y's good sense.
And worse, Duhem's solution delegates
the rationality of theory choice to
whatever a scientific community deems
as the most sensible choice to make,
even if that choice may
well be the wrong one.
Let's then take a closer look at the argument
from underdetermination and
how it challenges the rationality
of theory choice.
The argument proceeds from
three premises to a conclusion.
So, premise one of the argument
says that scientists' belief in
theory T1 is justified.
They have good reasons for
believing that theory T1 is true, or
corresponds the way things are in nature.
Premise two says that scientific
theory T1 has to be read literally.
In other words, if the theory
talks about planetary motion,
we must take what the theory says
about planetary motions at face value.
Premise three says that theory T1 is
empirically equivalent to another
theory T2, whenever T1 and T2 have
exactly the same empirical consequences.
And given those three premises, one,
two, three, we draw the conclusion
that premises two and three jointly
imply that premise one must be false.
In other words, it is not the case
that scientists have good reasons for
believing that theory T1 is true.
In other words, the scientists are not
justified in believing that a theory T1
is true, or corresponds to the way
things indeed are in nature,
if there is another rival theory, T2,
which is empirically equivalent to T1.
These considerations resonate in the work
of the influential historian and
philosopher of science, Thomas Kuhn.
In his 1977 book,
The Essential Tension, Kuhn argued that
theory choice seems to be governed by
five seemingly objective criteria.
The five seemingly objective criteria for
theory choice, according to Kuhn,
are the following ones.
Number one, accuracy.
The theory we go for
has to be accurate, has to agree with
the available experimental evidence.
Number two, consistency.
The theory has to be consistent with
other theories accepted at the time.
Number three, broad scope.
The theory has to be able to go
beyond the original realm of phenomena it was
designed to explain.
Number four, simplicity.
We want our theory to be simple.
Number five, fruitfulness.
Our theory should also be able to
predict novel, undreamt of phenomena.
However, Kuhn continued, those five
criteria are either imprecise, for
example, we don't know how
to define simplicity, or
they conflict with one another.
Take the example of Copernican astronomy.
Copernican astronomy seems better than
Ptolemaic astronomy on the basis of
accuracy, but
it fared worse than Ptolemaic astronomy
on the basis of consistency with other
well accepted theories at the time
such as Aristotle's physics.
Therefore Kuhn concluded that the five
criteria are not sufficient to
determine theory choice.
And external sociological
considerations are decisive in
gathering scientists'
consensus around one theory.
Going back to our topic,
we should ask what the evidence is for
the concordance model in cosmology, and
whether in this case too there might
be empirical equivalent rivals.
These questions are the more pressing
if we consider that the search for
dark energy and
dark matter is still ongoing, with large
galaxy surveys currently underway.
In the next section we review the evidence
for dark energy, and dark matter.
And we return to
the underdetermination problem and
the rationality of theory choice
at the very end of today's class,
when we assess the prospects and promises
of the concordance model in cosmology.
