became increasingly pressing. At the Royal Astronomical Society in 1930,
Eddington went for the jugular: “why (should there be) only two solutions
[i.e. Einstein’s and de Sitter’s models, both of which Eddington regarded as
static](?) I suppose the trouble is that people look for static solutions”
(p.39). Seconded by de Sitter (likewise present at the conference), he
enjoined that dynamical solutions be looked into in earnest, as a solution of
the redshift conundrum.

Reading the reports of this London meeting, Lemaître instantly apprehended
that he had _solved_ the problem three years earlier in his paper. He
immediately wrote to Eddington. Not only was Eddington the leading authority
on relativity and astronomy, but he also happened to have been Lemaître’s
supervisor during his postdoctoral studies in Cambridge. In fact, a few years
before, Lemaître had shared with Eddington a copy of his 1927 paper—which
Eddington simply seemed to have forgotten about.

In his letter, Lemaître reminded Eddington of his paper. He also attached
copies, requesting that they be circulated. Eddington—embarrassed by the
oversight—obliged, enthusiastically endorsing Lemaître’s results: not before
long would he and de Sitter fully acknowledge that Lemaître had hit on “a
brilliant solution” (Eddington, as cited in Kragh, 2007, p.147) of the
redshift enigma—as a manifestation of an _expanding_ universe. Eddington made
sure that Lemaître’s work would be brought to the attention of the world at
the next meeting of the Royal Society, and moreover, he sponsored an English
translation.

Having studied—this time carefully—Lemaître’s (1927) paper, Eddington re-
analysed Einstein’s 1917 model. He showed that Einstein’s corresponding
solution of his field equations was in fact unstable, “like a pen balanced on
its point” (Luminet, 2011, p.2914): a puny deviation from the delicately
balanced matter distribution that Einstein had postulated would undo the
model’s staticity; any arbitrarily small perturbation would cause the
resulting universe to either contract or expand. This instability argument was
the coffin nail for Einstein’s model: it exposed his static universe to
require _unphysical_ fine-tuning. By the beginning of the next year, 1931,
even Einstein converted to the dynamical universe (see Nussbaumer, 2014;
McCoy, 2020, esp. sect. 2 for details). “By 1933, the theory of the expanding
universe was accepted by a majority of astronomers and subjected to detailed
reviews […]. It was also disseminated to the public through a number of
popular works […]” (Kragh, 2007, p. 148). While the expanding universe had
become a reality, it provoked new questions: what drives the expansion? How to
improve our cosmological—from now on: typically dynamical—models? And how to
test them?464646 This question is of special importance since alternatives to
models of an expanding universe were studied (e.g. Zwicky’s (1929) tired light
hypothesis (expressly intended as an alternative explanation of the redshift-
distance relation), Milne’s (1935) special-relativistic cosmological model, or
the Steady State Cosmology of Bondi and Gold (1948))—even if only as
_minority_ research programmes. O’Raifeartagh & O’Keeffe (2020, p.222) rightly
point out that “the redshifts of the nebulae are merely one manifestation of
cosmic expansion; other manifestations exist, notably the frequency range of
the cosmic microwave background […].  These would define the agenda of
cosmology for the rest of the century until now (see e.g. Kragh, 2007, Ch.
4&5; MacCallum, 2015; or the contributions in Kragh & Longair, 2019).

Who, then, discovered the expanding universe? In 2018, the International
Astronomical Union voted to rename the redshift-distance relation, commonly
known as “Hubble’s Law”. The resolution recommends “that from now on the
expansion of the universe be referred to as the Hubble-Lemaitre law”. While
the resolution passed with 78% of voters in support, there was some pushback
(Kragh, 2018b; Grøn, 2018; Elizalde, 2019, 2020, Ch.5; O’Raifeartaigh &
O’Keeffe, 2020). In part, it excoriated historical omissions and distortions
(grievances that the above presentation, we hope, evades); in part, the
criticism was based on different construals of the very notion of a
discovery—without explicit attention to those construals themselves. The
following subsections will fill this lacuna. To that end, we’ll first (§4.2)
apply the philosophical accounts discussed in §2 to the expanding universe
case, and then turn to the change-driver model (§4.3).

###  4.2 The view from extant accounts

With the historical material in hand, let’s now apply the models of discovery
(§2.1-§2.4) to the case of the expanding universe. Whom amongst our dramatis
personæ of §4.1 should we proclaim its discoverer? We’ll argue: none! Those
models rule out the most plausible candidates, or are simply inapplicable—for
reasons _inherent_ to those models.

A Kuhnian analysis is quickly dealt with.474747We’ll set aside a fundamental
line of attack, criticising the applicability of Kuhn’s model of theory
dynamics to cosmology (see Marx & Bornmann, 2010; and O’Raifeartagh, 2013 for
negative assessments, and Kragh & Smith, 2003, pp.143; Kragh, 2007, pp.243 for
more positive ones). The single most important candidate “what-that”-discovery
of the expanding universe—a discovery where a phenomenon’s anticipated
existence and conceptualisation predate its empirical confirmation—was
Lemaître’s prediction of a linear redshift-distance relation. But since for
Kuhn, discovery requires both a theoretical and an empirical component, the
applicability of Kuhn’s model falters: while Lemaître lacked the empirical
data confirming his prediction, the person who provided the confirmation,
Hubble (together with Humason), rejected the interpretation as a confirmation
of the expanding universe (an epistemologically not unjustified reaction, as
we’ll elaborate below). Arguably the most plausible reaction, within Kuhn’s
model, is to regard the episode as a discovery with joint (or collective)
discoverers, none of whom can individually be credited with the
discovery.484848Verbatim the same conclusion is reached for other “what-
that”-discoveries concerning the expanding universe, such the predictions of
the Alpher-Beta-Gamow theory. The same conclusion is reached for the “that-
what”-discovery of the expanding universe: while the researchers involved in
the “anomalous redshift data”—chiefly: Slipher, Lundmark, and Hubble and
Humason—lacked the theoretical framework for their findings; the person who
provided that, Lemaître, lacked the empirical findings.

The Kuhnian perspective is overtly disappointing in its reduction of the rich
nexus of discoveries, reviewed in §4.2, to the above black and white
conclusion; it belies the complexity of historical science.

Let’s move on to Schindler’s model. For Schindler, to discover means both to
observe an object X or its direct effects, and to conceptualise the essential
properties needed to identify it.

A first stumbling stone to applying Schindler’s model to the expanding
universe concerns quibbles regarding the requisite ontological category of the
discovered entity. Cosmic expansion itself isn’t an object: it’s a _global
property_ of the universe (as the largest possible physical system in its
large-scale structure).

On the one hand, although Witz, Lundmark, Hubble and Humason have plausible
claims to first observing the redshift-distance relation, this falls short of
having discovered the expanding universe. None held the right
conceptualisation—the interpretation of the phenomenological redshift-distance
correlations in terms of an expanding universe. On the other hand, Schindler’s
model also disqualifies Friedmann and Lemaître: even though they possessed the
right conceptualisation, neither had at his disposal a convincing material
demonstration of the universe’s actual expansion.

Perhaps one might reconsider Lemaître: after all, he understood the
significance of Hubble’s results as evidence for his previously identified
conceptualisation. But also this option runs afoul of another problem:
Schindler’s insistence on essence-identification. We’d be hard-pressed to say
how this condition would be satisfied here. Is cosmic expansion, or does it
constitute, a natural kind? Does it have an essence? In which sense is
expansion an essential property of the universe? As far as standard cosmology
is concerned, any such invocation of essentialism seems a non-starter:
universes _needn’t_ expand; in no sense is expansion necessary! Even as
associated with a natural kind, expansion doesn’t fit the bill. Relativistic
cosmological models are arguably more naturally classified according to the
spacetime (or spatial) curvature, i.e. geometrically. But there is _no_ one-
to-one link between a cosmological model’s curvature and whether it represents
an expanding universe.

On Hudson’s account, discovering an object X requires being the first person
to have an appropriate base description, and the latter’s material
demonstration; here, the base description is supposed to ensure—as far as we
know—that whenever it’s satisfied, X is present. Setting aside the limitation
that Hudson’s account shares with Schindler’s (due to the account’s focus on
“objects”), let’s parse out Hudson’s criteria for the expanding universe. A
natural base description of the expansion of the universe would seem:

> The distance between any two objects that are not gravitationally bound (and
> interact only negligibly in all other ways) increases over time.

Now what would have to be “materially demonstrated”?494949 One may also
wonder: since the expansion of the universe is a global claim about the
universe at large, does one also need to materially demonstrate it globally?
If so, given our necessarily limited observational access to a fragment of the
universe, material demonstrations don’t seem possible in cosmology. For the
sake of the argument, we set aside this difficulty. Recall that for him
(paraphrasing Hudson, 2001, p.78), “to have discovered the expanding universe
one need only possess enough conceptual resources to recognize its presence in
_a fairly reliable manner_.” The material demonstration should confer
sufficient empirical-evidential credentials upon the base description: when
it’s satisfied one should have sufficient reason to epistemically prefer X’s
presence _over alternative hypotheses_.

For two historical reasons this condition rules out the standard candidate
discoverers: none of them had sufficient epistemic warrant for the base
description’s adequacy over alternatives.

First, the methodological status of general relativity, and a fortiori
cosmological models based on it, until the 1950s was flimsy (Eisenstaedt,
1989): the theory could hardly count as well-tested, even on terrestrial or
astronomical scales (with the available tests probing only minor deviations
from Newtonian theory). The empirical evidence in its favour was, at best,
inconclusive. On cosmic scales, general relativity was simply untested: “[…]
(i)ndeed, it could be said that cosmology did not truly constitute a test for
the general theory of relativity in these years [1940-1955]” (O’Raifeartaigh,
2022, p.13, original emphasis). With the age problem one had a puissant reason
to be leery of models of an expanding universe: until the 1950s, the main ones
entailed an age of the universe below that of the Earth and the oldest known
stars! Distrust in an expanding universe, insofar as it rested on general
relativity’s validity at cosmic scales, was thus _not irrational_!

Secondly, and relatedly, alternative explanations for the redshift-distance
relation—the main empirical clue for an expanding universe until the 1950s or
even 1960s—existed (see Kragh, 2017). Promptly following the publication of
Hubble’s paper, Zwicky (1929) “advocated thinking of the redshift as the
result of an interaction between photons and intervening matter rather than
cosmic expansion […]” (Kirshner, 2003, p.11). In a gravitational analogue of
the Compton effect, light loses its energy (“gets fatigued”) through
scattering on its journey from the source to the receiver. Zwicky indeed
calculated a linear redshift-distance relation—effectively reproducing
Hubble’s phenomenological law—but construed now as an energy dissipation.
Several variants of the “tired light”-hypothesis have been suggested (see e.g.
Kragh, 2019, sect. 4.7). Admittedly, after WWII its popularity dwindled.
Nonetheless, Kirshner (2003, p.11) concedes: “(t)he reality of cosmic
expansion and the end of the ‘tired light’ has only recently [in the 90s and
early 2000s!] been verified in a convincing way.”505050For an assessment of an
earlier verification—but still significantly after an expanding universe had
already been elevated to the standard model in cosmology—see Norton (2023).

Lastly—McArthur’s account, a liberalised version of Hudson’s, amalgamated with
structural realism: McArthur allowed, as will be recalled, for discoveries
even prior to material demonstrations—provided the discoverer harnesses
_already well-confirmed theoretical_ relations in deriving a base description
of something not yet observed.

Like Hudson’s model, the application of McArthur’s model scuppers on the
historical standing of relativistic cosmology: GR wasn’t yet sufficiently
well-tested. Accordingly, Friedmann’s or Lemaître’s derivations of the base
description for cosmic expansion (i.e. the redshift-distance relation as a
manifestation of the universe’s expansion) couldn’t rely on relations—GR’s
structural content—that would qualify as well-confirmed. NB: not even the
third of the so-called “classical tests” of GR that is phenomenologically and
explanatorily closest to the redshift-distance relation, gravitational
redshift (i.e. energy loss of electromagnetic radiation in the presence of a
gravitational field, manifesting itself in redshift) had been satisfactorily
performed;515151While Adams’ measurements of the star Sirius B, a white dwarf,
in 1925 were viewed as (tentative) confirmatory evidence, a critical re-
evaluation has deflated such a view (Hetherington, 1980; Holberg, 2010; see
also Earman & Glymour, 1980 for fallacies and subtleties in the derivation and
interpretation of gravitational redshift). a convincing test of gravitational
redshift would have to wait until the famous Pound and Rebka experiment in
1960.525252A similar conclusion is reached, with a curious twist, for a
slightly different base description for an expanding universe (for which we
take our cue from O’Raifeartaigh et al.‘s (2014) characterisation of
Einstein’s cosmological bias in terms of his preference for an
unchanging—rather than static—universe. This is compatible with stationarity.
Equating “expansion” with “evolution” or “variation with time”, one may adopt
the alternative base description: “The universe changes over the course of
time, with its evolution being time-dependent; cosmic structures aren’t
stationary.” In this case, a discovery of the expanding universe, on
McArthur’s model, wouldn’t have been possible before the Big Bang Theory’s
main rival, the Steady-State Theory, had become empirically discredited, again
in the 1950s or 1960s (see Kragh, 1996, Ch. 6&7).

In conclusion, the three main models of discovery don’t apply to the case of
the expanding universe. Per se, we don’t deem such a negative
result—difficulties in identifying an individual discoverer—problematic. What
_does_ seem problematic is rather the kind of reason for those model’s
inapplicability: general _philosophical_ assumptions about what science can
and does achieve, i.e. something inherent to the model. Unproblematically, by
contrast, applying models of discovery may fail through _historiographical
complexities_ : say, because many researchers contributed important elements
or because the relevant theoretical and empirical developments took a long
time, it may not be possible to single out one or several individuals.

###  4.3 Applying the change-driver account

The change-driver model yields a nuanced verdict on the discovery of the
expanding universe. It identifies different plausible individuals for
_distinct_ tokens and types of discovery under that umbrella term. The two key
advantages of the model become manifest: it not only does justice to the
complexity of the science and its historical evolution but also aligns with
the judgements of the main historical analysts.

####  4.3.1 Empirical-material discoveries of the expanding universe

The empirical-material discovery of the expansion of the universe begins with
Slipher in the 1910s. His measurements suggested a curious empirical trend
amongst faint spiral nebulae. In a list of velocities measurements (inferred
from spectral displacements) published in 1914, “all but a few of the
velocities were velocities of recession”, i.e. moving away from Earth/our
galaxy. “In addition, the radial velocities of the spirals were generally much
larger than the radial velocities of the stars or gaseous nebulae. The very
size of the spectral shifts prompted some astronomers to query the Doppler, or
velocity, interpretation of the shifts in the spectrum of the spirals” (Smith,
1979, p.136). Slipher, in other words, empirically-materially discovered the
phenomenon that (i) the distances between us and other galaxies systematically
tend to increase, and (ii) the velocities corresponding to this apparent
“recession” are _unusually_ high (vis-$\grave{a}$-vis the proper motion of
stars and nebulae).535353Recall that Slipher’s pioneering velocity
measurements were indispensable for Hubble’s (and Lemaître’s) own later work;
Hubble’s ground-breaking achievement resulted from the combination of his own
distance measurements and Slipher’s velocity measurements. Interestingly,
“(i)t is one of the great ironies of science that Hubble’s measurements of
distance were later substantially revised due to significant systematic
errors. Due to an error in the classification of Cepheid variables, Hubble’s
cosmological distance ladder was later substantially revised by Walter Baade
(1956) and Allan Sandage (1958)” (O’Raifeartaigh, 2013, p.8).

This anomaly was quickly recognised. According to Eddington (1923, p.161), it
constituted “one of the most perplexing problems of cosmogony”. Its absorption
by extragalactic-astronomical background knowledge enabled new computational
techniques (primarily for calculating solar motion), crucial for further
developments. Soon attempts followed to correlate the radial velocities of
those spiral galaxies with other of their observable parameters, in
particular, their magnitudes and diameters (in turn correlated with distance).

The next milestone in the empirical-material discovery of the expanding
universe is the fruit of such endeavours: a positive correlation between the
radial velocities and apparent magnitudes of the nebulae. Here it’s difficult
to attribute this result definitively to an individual. “As early as 1918
Harlow Shapley […] had suggested that ‘the speed of spiral nebulae is
dependent to some extent upon apparent brightness, indicating a relation of
speed to distance, or possibly, to mass’, but he did not follow up this
speculation” (Smith, 1979, p.142). Wirtz’s announcement that “he had
established a well-defined observational relation between the radial
velocities and apparent magnitudes of the spiral nebulae […] were not taken
seriously by the principal students of the spiral nebulae” (ibid.); hence, not
impacting the knowledge of the scientific community, the change-driver model
disqualifies him, too, as a discoverer. Only once less crude distance
indicators were available—Hubble’s observations of Cepheids variables in
nearby spiral nebulae in 1924—could progress be made. Lundmark may be credited
with priority in establishing—albeit only tentatively—a relation between
redshift and distance in his 1924 study. We therefore identify him as its
empirical-material discoverer. In his 1925 paper on “the motions and distances
of the spiral nebulae” he even determined the relation to be dominantly
linear.

The establishment of this linear relationship concerns the third major
empirical-material discovery relevant for our purposes. As Smith (1979,
pp.147) writes, during the 1920s several anticipations of such a linear
redshift-distance relation were made. “Hubble did not claim that he had found
a linear velocity-distance relation […]. Also Hubble, as he himself
acknowledged, was working within a well-defined problem area, and before 1929
other astronomers had considered the existence of a redshift-distance
relation. Hubble’s success was not to ‘discover’ a relation; rather, it was to
_convince_ his colleagues that the relation was linear” (op.cit., p.133,
emphasis in the original). On the change-driver model, the superior quality of
the data, and the impact Hubble’s (and Hubble and Humason’s) work had for the
body of astronomical knowledge—in particular the quickly recognised relevance
for the theoretical models of the universe—Hubble should be credited with this
third discovery, the firm consolidation of a linear empirical distance-
velocity relation. It “ushered in a new area in cosmology by presenting a
novel set of problems”, producing “a dialectic between theory and data in
studies of the large-scale properties that was undreamt of only a few years
before” (op.cit., p.157).

This leads us to the other motor of that dialectic, theory—and the associated
theoretical discoveries, i.e. theory-internal, formal results related to the
expanding universe.

####  4.3.2 Theoretical discoveries concerning expanding universes

The first milestone is best jointly attributed to Einstein and de Sitter,
right at the cradle of relativistic cosmology in 1917. Einstein may be said to
have formally discovered that his original (1915) field equations, sans
cosmological constant, didn’t allow for a static solution—even if he
_rejected_ that conclusion. To forestall it, he resorted to an ad-hoc
modification of his original theory via the cosmological constant. Einstein’s
(indirectly expressed) insight is complemented by de Sitter’s discovery of his
“solution B”, the first non-static solution (even though its non-staticity was
fully grasped only later). The object of Einstein’s and de Sitter’s twin
formal discoveries were non-static cosmologies as theoretical possibilities
within GR.

That they are generic possibilities is the object of the second group of
formal discoveries. Friedmann (1922) not only derived the equations governing
cosmological dynamics from general relativity, but also explicitly showed that
cosmological models typically describe expanding universes. But given the
delayed attention to his work, which—when it finally came—was mentioned in one
breath with Lemaître’s independent re-discovery, this discovery is, on the
change-driver model, most plausibly attributed to _both_ Lemaître and
Friedmann, as independent co-discoverers. A second, impactful discovery in
this second group is Eddington’s (1930) explicit demonstration of the
instability of Einstein’s cosmological model. This exposed Einstein’s static
universe to be fine-tuned (as a mathematical property)—or extraordinarily
special (and thus the opposite of generic).

A third group of formal discoveries revolves around the linear
redshift/distance relation as a _consequence_ of cosmological models. Here,
simultaneously and independently of each other, Weyl (1923) and Eddington
(1923) were the first to obtain the result in a solid mathematical way, even
if it was limited to the de Sitter solution. Lemaître (1927) derived a more
general result, for a generically expanding universe. A further refinement
came a year later by Robertson: he derived the linear redshift/distance
relation from the most general solution to Einstein’s field equations, where
matter is subject to the observational constraints of homogeneity and
isotropy.

####  4.3.3 Synthetic discoveries of the expanding universe

What synthetic discoveries does the change-driver model highlight—i.e. results
that reveal an evidential relationship between empirical phenomena—already
detected or still-to-be-performed—and the notion of an expanding universe?

We can discriminate between two distinct types, negative and positive
synthetic discoveries. Members of the former pertain to evidential or
explanatory deficiencies, members of the latter, to positive explanatory or
evidential achievements.

Negative synthetic discoveries comprise two clusters. The first is composed of
the waxing realisation that redshift data pose growing and intensifying
anomalies for the existing cosmological models. It begins with Slipher’s
redshift data, and continues with their solidification and the final emergence
of a (linear) redshift-distance relation. The redshift data posed a perplexing
challenge for static models of the universe: how to explain the trend towards
redshift amongst galaxies? What sense to make of the high velocities (if one
decided to translate, via an interpretation in terms of a Doppler effect, the
redshift into velocities)? The redshifts weren’t predicted or easily explained
within Einstein’s static model. The explanatory resources of de Sitter’s model
seemed more promising (based on minor positive synthetic discoveries, such as
the various vague theoretical anticipations of a redshift-distance
relation)—but the latter was patently unrealistic. Significant research
efforts were invested into connecting de Sitter’s model with observed
redshifts. Neither Einstein’s nor de Sitter’s model had the resources to
explain the redshifts without additional suppositions. This, in the main,
drove Lemaître to explore an expanding universe. It’s inherently difficult to
identify individual discoverers for the insight that anomalies aren’t
naturally accommodated or explained within the existing cosmological models:
different researchers must typically try out different ansätze—each of which
turns out to be unsatisfactory. Plausibly, such negative synthetic discoveries
are a collective achievement: the explanatory inadequacy of the extant models
gradually dawns on the scientific community as a whole.

A second family of negative discoveries, often overlooked, concern—even after
Hubble’s establishment of a linear redshift/distance relation, and after the
widespread acceptance of the expanding universe—the evidential shortcomings of
cosmological models representing an expanding universe: that they were
evidentially underdetermined, with alternatives (_later_ to be refuted),
plausibly counts a as significant insight that incentivised improved testing
and the development of (unsuccessful) rivals (cf. for instance, Bschir,
2015).545454Kragh (2019, p.120, also for a discussion of such rival theories)
rightly stresses: “(t)hey belong to the history of the field as much as do the
more successful theories that in a more direct way paved the way to the modern
view of the universe.” Again, several researchers can be given credit for such
discoveries.

On the side of _positive_ synthetic discoveries, one outshines all others: the
insight that a linear redshift-distance relation receives a natural
explanation—or is in fact predicted—by an expanding universe. This insight can
be attributed to Lemaître; he was the first to determine this empirical
prediction of an expanding universe model, and identified the body of evidence
that would, eventually, be used to confirm that prediction. Of course, only
Hubble’s result would corroborate it, and thereby actualise Lemaître’s
heretofore merely _potential_ synthetic discovery. As likewise pointed out,
only thanks to Eddington’s interposition did Lemaître’s discovery become a
scientific discovery proper and advanced scientific knowledge within the
physics community. Nonetheless, it was Lemaître’s interpretation of the
redshift data that drove the acceptance—and further development of
cosmology—of an expanding universe.

Hubble was well-aware of that interpretation, but didn’t commit to it. On the
change-driver model, however, this—Hubble’s personal convictions—isn’t
decisive for a synthetic discovery. To be sure, Hubble’s work, as an
observational finding, had a huge influence on the acceptance of an expanding
universe in virtue of Lemaître’s interpretation. On its own, it was an
impactful empirical-material discovery—not a synthetic one: as a
phenomenological law it didn’t per se link the notion of an expanding universe
and observational phenomena. Hubble may, to some extent, be credited with a
negative synthetic discovery, however: the impactful insistence on the
underdetermination of the expanding universe interpretation of the linear
redshift-distance relation.

The importance of this negative discovery—perhaps even a matter of
intellectual honesty—for the status of cosmology as an empirical science can’t
be overstated. Still in 1963, “there (were) only 2.5 facts in cosmology”
(Longair, 1993, p.160): the darkness of the night sky (peripheral to
developments in cosmology at issue here), Hubble’s empirical redshift-distance
law, and the half-fact, in the twilight since the first reliable source counts
in the mid-1950s, “a matter of considerable controversy” (ibid.), concerning
the universe’s evolution, i.e. the idea that “the contents of the universe
have probably changed as the universe grows older” (ibid.). Within a few
years, cosmology multiplied the number of facts it could confidently point to.
Important tests were invented and performed that confirmed the model of an
expanding universe (see e.g. Longair, 2019). These developments in turn amount
to (positive) synthetic discoveries.

##  5 Summary and conclusion

The present paper undertook a comprehensive review of extant philosophical
models/explications of scientific discovery: how to construe the notion, and
its link to other facets of science, especially its dynamics?

While Kuhn’s account is compromised by its entanglement with his wider, and
problematic, views on science (especially incommensurability), we discern the
principal weaknesses of the more philosophical accounts in their unduly strong
metaphysical and epistemological commitments (especially to realism), and
their disconnect from both scientific change and the growth of scientific
knowledge, as well as from normative-social aspects of discoveries (especially
the valuation of discoveries as scientific goods and the concomitant reward
system).

As an alternative that avoids these defects, we proposed the change-driver
model. It conceptualises scientific discoveries as cognitive scientific
results that have significantly impacted science, and advanced communal
knowledge. In tandem with a more fine-grained taxonomy of forms of
discovery—dependent on the kind of interaction between the discovery and
existing knowledge—the change-driver model was shown to exhibit superior
intensional and extensional adequacy over other models.

The case of the expanding universe further illustrated this, underscoring the
model’s fertility and relevance for a recent science-political debate. Our
model yields a nuanced verdict on whom to identify as a discoverer of what—a
verdict that does greater justice to the complexity of science and its history
than rivalling models. Chiming with other historical analyses, the change-
driver model _opposes_ the IAU’s (2018) recommendation: while the empirical
law in question—the correlation between redshift and distance of
galaxies—ought to be attributed to Slipher, Lundmark, Hubble and Humason
(rather than _just_ Hubble, or Hubble _and Lemaître_), Lemaître (but not
Hubble) deserves privileged credit for discovering its interpretation as
evidence for the expanding universe. The discovery of the expanding universe
as a broader claim—its wider theoretical understanding and epistemic
backing—is best viewed as a more collective accomplishment, involving several
researchers, an extended process that is on-going.

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