Moving past the swampland: a first philosophical look at the trans-Planckian censorship conjecture

by Mike D. Schneider

In the past several years in string theory, swampland arguments, first developed over a decade ago, have become prominent. This is the gist of the arguments: certain families of four-dimensional effective field theories (EFTs) may fail to reside in the string theoretic landscape, in which case they reside in the swampland. Plausibly, high confidence in the descriptive accuracy of an EFT that resides in the swampland is a knock against the viability of string theory as an adequate theory of quantum gravity.

The reason swampland arguments have become prominent in the past several years is primarily due to a conjecture, widely circulated since 2017, that places many models of inflationary dynamics in the swampland. Provided that the conjecture is true, on the interpretation of the swampland arguments sketched above, it falls to string theorists to ensure that nothing essential of the empirical story we tell of cosmic structure formation rests on our acceptance of those models. (This could take the form of providing alternatives to inflation that are compatible with string theory, or else by demonstrating that the particular inflationary model needed just so happens, surprisingly, to evade the scope of the conjecture.)

But the swampland is not what I want to focus on here. Instead, I would like to start a conversation about the potential value of the trans-Planckian censorship conjecture (TCC) in quantum gravity research. The TCC was first introduced onto the arXiV this past September. A few weeks ago, Brandenberger— an early advocate of the TCC— presented on the subject at the conference Foundations of Cosmology and Quantum Gravity, in Abu Dhabi.

Despite its roots in the same social circles that embrace swampland arguments, the TCC has surprisingly little to do with the details of string theory. Instead, the TCC is really best regarded as a “theory neutral” conjecture that inflationary dynamics will generally pose a challenge for candidate quantum gravity theories. The slogan form of the TCC is that sub-Planckian quantum fluctuations ought generally to stay quantum throughout inflation. In the context of string theory, this slogan is similar to a statement of which inflationary models would avoid the swampland conjecture already mentioned.

Indeed, one possible reason to believe the TCC is that it represents the conclusion of a bold inductive inference from the case of string theory, based on this similarity: just as the swampland conjecture already mentioned arises in the particular case of string theory and creates demands on string theorists, so too will analogous conjectures be relevant in other candidate quantum gravity theories. Another possible reason to believe the TCC is that it represents (something like) an inference to the best explanation: the best way to explain the success that has come from our persistently ignoring the sub-Planckian regime in our pursuit of explanations of cosmic structure formation is to assume that sub-Planckian physics simply does not play a role in any such explanations.

There is cause to argue about whether either of these possible reasons ought to be regarded as appropriate or compelling. The point I would like to conclude with is that there are real stakes involved in articulating some such reasons to believe the TCC, provided it is possible that good ones can be found. This is because an embrace of the TCC provides a means of leveraging our account of cosmic structure formation as an empirical constraint on theories of quantum gravity generally. In particular, it would fall on the theorist in favor of any such candidate theory to articulate reasons, according to that candidate theory, that sub-Planckian physics should be expected not to play any role in such an account.

The prospects of embracing the TCC therefore sound promising, particularly for a field of research often thought to be short on empirical data. This is reason enough to scrutinize in greater detail what good reasons there might be to believe it. 

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