As Eric Hedin has already noted (here and here), theoretical physicist Sean Carroll was recently interviewed by podcaster Alex O’Connor to defend his stance that one of the most thought-provoking scientific arguments for God’s existence, the argument from cosmological fine-tuning, “is the best argument for God, but it’s still a terrible argument.” Carroll is a prolific physicist and cosmologist who has been a prominent popularizer of science and, more recently, has been given a platform as a practitioner of the philosophy of science. He may not be E. F. Hutton, but when he talks, people do listen, so it’s important to respond to his confident pronouncement that the fine-tuning argument is “terrible.”
Display “The Fine-Tuning Argument is Terrible – Sean Carroll” from YouTube
Confidence aside, arguments by bold assertion are not refutations. In a series of posts, therefore, I will be evaluating the substance he tries to give to this claim and showing that it fails decisively. Though Carroll is the frontman for our purposes, this interview and his other critiques of the fine-tuning argument (FTA) are stand-ins for the whole intellectual coterie of the FTA’s “cultured despisers” (to borrow a phrase from Friedrich Schleiermacher) from Victor Stenger to Leonard Susskind to (astoundingly) theist Hans Halvorson. Properly formulated,1 the FTA is neither “God of the gaps” reasoning nor rhetorical flourish. It is a serious and substantial argument that can be given a precise Bayesian formulation and the objections to which have convincing responses. Carroll’s bold dismissiveness is little more than a rhetorical ploy whose lack of substance becomes apparent when the fine-tuning evidence and its significance is given our careful attention.
The Interview in Context
O’Connor’s interview of Carroll only lasts 12 minutes and change, but it covers a lot of ground. O’Connor begins with a concise but fair description of the fine-tuning evidence, noting that the physical constants of our universe are exquisitely balanced to permit the existence of the complex structures necessary for life. As physicist and cosmologist Luke Barnes (2012: 529) states more precisely, “[b]eginning with the classic papers of Carter (1974) and Carr and Rees (1979) , and the extensive discussion of Barrow and Tipler (1986), a number of authors have noticed that very small changes in the laws, parameters and initial conditions of physics would result in a universe unable to evolve and support intelligent life.” As Barnes documents, if you vary the strength of gravity with respect to the strengths of the other fundamental forces by a microscopic amount, the universe will either fly apart too fast for structures to form or recollapse before they have a chance to do so. Beyond gravity, there are different sensitivities associated with the strong force, the electromagnetic force, the weak force, the cosmological constant, and numerous other physical parameters, the slightest alteration in which would render the universe uninhabitable. The values these parameters actually have is fixed by measurement, not fundamental theory, but if they strayed ever so slightly from their actual values, we would not exist. The crucial question then becomes: What explains this extraordinarily serendipitous confluence of values?
O’Connor sets up three standard explanations for Carroll to discuss: (1) these lucky values for the physical constants came about by brute chance; (2) the parameters must have the values they do for reasons yet to be discovered, so they aren’t really fine-tuned; or (3) God created it to be that way, which O’Connor suggests is too simplistic and a bit of a cop out.
Carroll’s response to these alternatives takes the discussion in a different direction. He proposes a fourth way to explain the appearance of fine-tuning, namely, seeing our universe as one in a vast multiverse of universes with different values for the physical constants. It’s clear he takes multiverse explanations to be distinct from brute chance — perhaps because they involve mechanisms of a sort — while rejecting as untenable the idea that the values of the constants are somehow rendered necessary by the right background theory. Having laid claim to the explanatory resources of the multiverse, he goes on to argue that the fine-tuning of our universe, rather than supporting theism, counts against it, further defending the scientific credibility of the multiverse while attacking theistic explanations as contrived and unworthy of serious consideration.
Carroll’s Contentions and Why They Merit a Response
We need to be clear about the nature of Carroll’s arguments and why they merit a response. His claims and objections fall into broad categories:
The Multiverse as a Preferred Alternative to Cosmological Design
Carroll argues that the multiverse explains the appearance of fine-tuning without recourse to God or any notion of cosmic design. Different regions of the multiverse have different values of the physical constants and an “observer selection effect” explains the rest — we naturally find ourselves in a region that is compatible with our existence. Furthermore, Carroll argues that the multiverse wasn’t invented as a response to fine-tuning; rather, it came about for independent reasons in the context of inflationary cosmology, string theory, and many-worlds (Everettian) quantum mechanics. In short, multiverses arise naturally from our best physical theories (Carr, 2007). But if the multiverse has a motivation that is independent of fine-tuning yet still relevant to its explanation, this would, it seems, undercut any inference to theism or design. Does it, though? We need to ask whether multiverse cosmology is as well motivated as Carroll claims, whether it succeeds as an explanation in a way that defeats the fine-tuning argument, and whether the multiverse itself, if it actually exists, might not also find a better explanation in theism than its own autonomous function.
The Likelihood Reversal and the Methodological Challenge
One of Carroll’s most interesting and unusual claims is that the fine-tuning of our universe is more supportive of naturalism than of theism. Yes, you read that right, though you probably did a double take. His argument, such as it is, starts by noting that the fine-tuned constants permit “physical configurations of matter that behave like living creatures — metabolizing, reproducing, processing information — while obeying physical laws.” He adds, strangely, that this is “exactly what you’d expect if God didn’t exist.” Why? Because life requires complex physical systems if naturalism is true, so if life exists, its taking this form is expected under naturalism. But “none of this is necessary” under theism because “God can make anything alive” without fine-tuning at all. In fact, since many theists believe in non-physical life after death, God doesn’t even need material reality at all. Carroll therefore concludes that “if anything, fine-tuning should increase your credence in naturalism, not theism.”
It might sound like Carroll is drinking some crazy juice here, but he’s actually articulating an interesting methodological challenge. He correctly notes that the FTA “plays by scientific rules: data, hypotheses, Bayesian likelihoods,” and that the strength of the argument derives from its amenability to rigorous probabilistic analysis (for examples of which, see Barnes 2018; Collins 2009). But, he argues, in light of the foregoing considerations, P(fine-tuning | naturalism) ≥ P(fine-tuning | theism), i.e., the probability of fine-tuning on naturalism exceeds its probability on theism, so the FTA collapses. This is a serious claim. It is also seriously wrong, but we need to show this.
The Neglected Evidence Gambit
Carroll also seeks to undermine the FTA by arguing that, even if life-permitting constants were likely under theism, there is other relevant data that should not be expected at all if theism is true. The vast size of the universe, with its hundreds of billions of galaxies, and its complete lack of human centrality pull against the idea that there is a God who has any regard for human beings at all. Such observations, he contends, cannot be ignored when assessing the evidence, and they tell against theism in the overall scoring.
Carroll is methodologically correct about this. Bayesian reasoning requires that we take all of the relevant evidence into account, not just the evidence that is favorable to the hypothesis we prefer (Howson and Urbach, 2006). Any features of the universe theists would find surprising — Carroll emphasizes its vast scale, its apparent indifference to human concerns, and his perception that God’s existence is (at best) obscured rather than revealed in nature — have to count against any supposed evidence for design in the fine-tuning data. All of the countervailing evidence must be included, not ignored, in Bayesian argumentation.
The Post Hoc Allegation
Carroll claims, especially with respect to the fine-tuning evidence, that theists are retrofitting their predictions to what science has discovered. With respect to the immense size of the cosmos, Carroll says to the theist “you can now say that God would want such a universe — but for centuries, no one predicted this from theism, so you don’t get to claim it was expected all along.” The same thing applies to the centrality of human beings — if God exists, we should be central to the universe’s purpose, but we live in its backwaters and it is manifestly indifferent to our existence, so Carroll concludes that any claim that “God wanted it this way” is “a post hoc cop-out.”
This is a serious charge. Carroll is quite right that scientific reasoning does and should penalize hypotheses that can always be adjusted after the fact to accommodate observations. If theism is infinitely malleable in light of the evidence, and adaptable to “predicting” anything after the fact, then it makes no substantive predictions and is explanatorily vacuous. In this respect too, we must show that theism is genuinely predictive and not merely subject to post hoc rationalizations.
The Hidden God Problem
Finally, Carroll thinks that modern theists are “pushed toward a God who wants to be invisible” and who lets the universe run “entirely by physical laws” so we have “no evidence of divine action.” He regards it as deeply suspicious and warranting of skepticism that God, if he existed, would hide himself in such a way, leaving, he asserts, no discernible fingerprints. He argues that “it would be trivially easy for God to let us know God exists,” and for this reason “there are many universes that would fit theism better than one governed entirely by impersonal physical laws.” Thus he concludes that our universe looks exactly like we’d expect it to look if no God exists, and nothing like what we’d expect if he does.
This too is a serious allegation, and while it largely reveals Carroll’s ignorance of religious texts, discussions, and traditions going back the better part of four millennia, it is nonetheless an issue very worthy of examination and response.
A Forward-Looking Conclusion
Some may be tempted to dismiss Carroll’s observations and objections as run-of-the-mill atheist talking points, but that would be a mistake. As I noted, his voice carries considerable influence, and he raises issues that some will find compelling. For the sake of such people, a cogent and comprehensive response is needed — a task I will begin in my next post.
Annotated Selected References
On the Fine-Tuning Argument Generally:
Barnes, Luke A. “The Fine-Tuning of the Universe for Intelligent Life.” Publications of the Astronomical Society of Australia 29 (2012): 529–564. [Comprehensive scientific review of the fine-tuning evidence across multiple parameters.]
Barnes, Luke A. “Fine-Tuning in the Context of Bayesian Theory Testing.” European Journal for the Philosophy of Science 8 (2018): 253–269. [Technical treatment of Bayesian approaches to fine-tuning.]
Barnes, Luke A. “A Reasonable Little Question: A Formulation of the Fine-Tuning Argument.” Ergo 6 (2020): 1220–1257. [Rigorous Bayesian formulation addressing common objections.]
Barnes, Luke A. “The Fine-Tuning of the Universe for Life.” In the Routledge Companion to the Philosophy of Physics, edited by Eleanor Knox and Alastair Wilson. New York: Routledge (2022): 719-730. [Rigorous scientific presentation of fine-tuning evidence.]
Barrow, John D., and Frank J. Tipler. The Anthropic Cosmological Principle. Oxford: Clarendon Press, 1986. [The classic tome on fine-tuning and the anthropic principle.]
Carr, Bernard J., and Martin J. Rees. “The Anthropic Principle and the Structure of the Physical World.” Nature 278 (1979): 605–612. [A seminar work on fine-tuning and the anthropic principle.]
Carter, Brandon. “Large Number Coincidences and the Anthropic Principle in Cosmology.” In Confrontation of Cosmological Theories with Observational Data, edited by M.S. Longair, 291–298. Dordrecht: D. Reidel, 1974. [The scientific article that got the whole fine-tuning discussion going.]
Collins, Robin. “Evidence for Fine-Tuning.” In God and Design: The Teleological Argument and Modern Science, edited by Neil A. Manson, 178–199. London: Routledge, 2003. [Early systematic philosophical treatment of the fine-tuning argument.]
Collins, Robin. “The Multiverse Hypothesis: A Theistic Perspective.” In Universe or Multiverse?, edited by Bernard J. Carr, 459–480. Cambridge: Cambridge University Press, 2007. [Analysis of multiverse alternatives to theistic explanation.]
Collins, Robin. “The Teleological Argument: An Exploration of the Fine-Tuning of the Universe.” In The Blackwell Companion to Natural Theology, edited by William Lane Craig and J. P. Moreland. Oxford: Wiley-Blackwell, 2009: 202–281.. [Detailed philosophical defense with extensive treatment of objections.]
Collins, Robin. “The Fine-Tuning Evidence is Convincing.” In Debating Christian Theism, edited by J.P. Moreland, Chad Meister, and Khaldoun A. Sweis. Oxford: Oxford University Press, 2013: 35-46. [A response to Victor Stenger’s argument against fine-tuning.]
Collins, Robin. “The Argument from Physical Constants: The Fine-Tuning for Discoverability.” In Two Dozen (or So) Arguments for God, edited by Jerry L. Walls and Trent Dougherty. Oxford: Oxford University Press, 2018: 89–107. [An argument that the constants of nature are not just fine-tuned for life, they are fine-tuned for the purpose of scientific discovery.]
Davies, Paul. The Accidental Universe. Cambridge: Cambridge University Press, 1982. [Early popular treatment of cosmic fine-tuning and anthropic reasoning.]
Davies, Paul. The Goldilocks Enigma: Why Is the Universe Just Right for Life? London: Allen Lane, 2006. [Comprehensive exploration of fine-tuning for general audiences.]
Hogan, Craig J. “Why the Universe Is Just So.” Reviews of Modern Physics 72 (2000): 1149–1161. [Scientific review of fine-tuning across cosmological parameters.]
Leslie, John. Universes. London: Routledge, 1989. [Philosophical examination of fine-tuning and multiverse hypotheses.]
Lewis, Geraint F., and Luke A. Barnes. A Fortunate Universe: Life in a Finely Tuned Cosmos. Cambridge: Cambridge University Press, 2016. [Accessible presentation combining scientific evidence with philosophical analysis.]
Loke, Andrew. The Teleological and Kalam Cosmological Arguments Revisited. Cham: Palgrave Macmillan, 2022. [Systematic treatment of both arguments with thorough engagement of contemporary physics and philosophy.]
Manson, Neil A. ed. God and Design: The Teleological Argument and Modern Science. New York: Routledge, 2003. [An examination of the teleological argument across the sciences from disparate perspectives, with particular attention to fine-tuning.]
Rees, Martin. Just Six Numbers: The Deep Forces That Shape the Universe. New York: Basic Books, 2000. [Popular treatment of fundamental constants and their fine-tuning.]
Stenger, Victor. The Fallacy of Fine-Tuning: Why the Universe is Not Designed for Us. Amherst: Prometheus Books, 2011. [Argues that fine-tuning arguments are fallacious and the universe shows no evidence of God’s existence.]
Swinburne, Richard. The Existence of God. 2nd ed. Oxford: Oxford University Press, 2004. [Classic Bayesian approach to theistic arguments including fine-tuning.]
Waller, Jason W. Cosmological Fine-Tuning Arguments: What (if Anything) Should We Infer from the Fine-Tuning of Our Universe for Life? New York: Routledge, 2020. [Argues that the fine-tuning of the universe is best explained by some form of theism.]
On Bayesian Epistemology and Probability Interpretation:
Climenhaga, Nevin. “Epistemic Probabilities are Degrees of Support, Not Degrees of Belief.” Philosophy and Phenomenological Research (2024). [Defense of objective interpretation of epistemic probability; central to Part 2.]
Howson, Colin, and Peter Urbach. Scientific Reasoning: The Bayesian Approach. 3rd ed. Chicago: Open Court, 2006. [Standard treatment of Bayesian confirmation theory.]
Keynes, John Maynard. A Treatise on Probability. London: Macmillan, 1921. [Classic defense of logical interpretation of probability.]
On the Multiverse and Its Problems:
Boyce, Kenneth, and Philip Swenson. “The Fine-Tuning Argument Against the Multiverse.” The Philosophical Quarterly, vol. 76, no. 1 (2026): 39-50. [Argues that fine-tuning may provide evidence against multiverse hypotheses.]
Carr, Bernard, ed. Universe or Multiverse? Cambridge: Cambridge University Press, 2007. [Collection of essays from multiple perspectives on multiverse hypotheses.]
Dyson, Lisa, Matthew Kleban, and Leonard Susskind. “Disturbing Implications of a Cosmological Constant.” Journal of High Energy Physics 2002, no. 10 (2002): 011. [Seminal paper on the Boltzmann brain problem.]
Ellis, George F. R. “Does the Multiverse Really Exist?” Scientific American 305 (2), 2011: 38–43. [Skeptical assessment from a leading cosmologist.]
Gibbons, Gary W., and Neil Turok. “The Measure Problem in Cosmology.” Physical Review D 77 (6), 2008: 063516. [Technical treatment of the measure problem.]
Gordon, Bruce L. “Balloons on a String: A Critique of Multiverse Cosmology.” In The Nature of Nature, edited by Bruce L. Gordon and William A. Dembski, 558–601. Wilmington, DE: ISI Books, 2011. [Detailed critique covering BGV theorem, measure problem, Boltzmann brains, and the reification of mathematics.]
Gordon, Bruce L. “Does the Multiverse Refute Cosmic Design?” in William Dembski, Casey Luskin, and Joseph Holden, eds. The Comprehensive Guide to Science and Faith: Exploring the Ultimate Questions about Life and the Cosmos. Eugene, OR: Harvest House (2021): 457-470.
Hacking, Ian. “The Inverse Gambler’s Fallacy: The Argument from Design. The Anthropic Principle Applied to Wheeler Universes.” Mind 96, No. 383 (1987): 331–340. [Original statement of the inverse gambler’s fallacy objection.]
Olum, Ken D. “Is There Any Coherent Measure for Eternal Inflation?” Physical Review D 86 (2012): 063509. See: https://arxiv.org/pdf/1202.3376. [Argues that probabilities cannot be defined coherently in eternal inflation.]
Susskind, Leonard. The Cosmic Landscape String Theory and the Illusion of Intelligent Design. New York: Little, Brown, and Company, 2005. [Argues that the inflationary string landscape eliminates fine-tuning and shows the universe lacks design.]
Wall, Aaron C. “Proof of the generalized second law for rapidly changing fields and arbitrary horizon slices.” Physical Review D 85 (2012): 104049; “Erratum.” Physical Review D 87 (2013): 069904. [Provides a rigorous general proof of the GSL in quantum gravity.]
White, Roger. “Fine-Tuning and Multiple Universes.” Noûs 34 (2000): 260–276. [Develops the inverse gambler’s fallacy objection to multiverse reasoning and gives an extended treatment of multiverse objections.]
On the BGV Theorem and Cosmic Beginnings:
Ashtekar, Abhay, and Parampreet Singh. “Loop Quantum Cosmology: A Status Report.” Classical and Quantum Gravity 28 (21), 2011: 213001. [Technical review of LQC, including discussion of the bounce and its fine-tuning issues.]
Borde, Arvind, Alan H. Guth, and Alexander Vilenkin. “Inflationary Spacetimes Are Not Past-Complete.” Physical Review Letters 90 (2003): 151301. [The original BGV theorem paper.]
Mithani, Audrey, and Alexander Vilenkin. “Did the Universe Have a Beginning?” arXiv:1204.4658 (2012). [Extension of singularity arguments to Wheeler-DeWitt quantum cosmology.]
On Divine Hiddenness and the Scale of the Universe:
Howard-Snyder, Daniel, and Paul Moser, eds. Divine Hiddenness: New Essays. Cambridge: Cambridge University Press, 2002. [Collection of responses to Schellenberg.]
Lovejoy, Arthur O. The Great Chain of Being. Cambridge, MA: Harvard University Press, 1936. [Classic study of the principle of plenitude in Western thought.]
Schellenberg, J. L. Divine Hiddenness and Human Reason. Ithaca: Cornell University Press, 1993. [Influential statement of the hiddenness argument against theism.]
On the History of Science and Theistic Expectations:
Carroll, Sean. The Big Picture: On the Origins of Life, Meaning, and the Universe Itself. New York: Dutton, 2016. [Carroll’s comprehensive statement of his naturalistic worldview.]
Gordon, Bruce L. “The Rise of Naturalism and Its Problematic Role in Science and Culture,” in The Nature of Nature, edited by Bruce L. Gordon and William A. Dembski. Wilmington: ISI Books, 2011, pp.3-61. [Argues that theism birthed science and that metaphysical naturalism is inimical to the foundations and growth of science.]
Gordon, Bruce L., and Stephen C. Meyer. “There and Back Again: How Theism Grounds Science and Science Abductively Supports Theism.” In The Blackwell Companion to Christian Apologetics, edited by Timothy McGrew and Robert B. Stewart. Oxford: Wiley-Blackwell, forthcoming 2026. [Argues that science requires theism to ground it, and science in turn evidentially supports the truth of theism.]
Harrison, Peter. The Fall of Man and the Foundations of Science. Cambridge: Cambridge University Press, 2007. [On how theological commitments shaped early modern scientific method.]
Harrison, Peter. The Territories of Science and Religion. Chicago: University of Chicago Press, 2015. [Historical study of the complex relationship between science and religion.]
Klaaren, Eugene M. Religious Origins of Modern Science. Grand Rapids: Eerdmans, 1977. [On the theological roots of the scientific revolution.]
Notes
Please see the list of references above for a preliminary guide to the literature on fine-tuning evidence in cosmology and the fine-tuning argument (FTA) based upon it.