Ten Questions for the Philosophy of Cosmology

Last week I spent an enjoyable few days in Tenerife, one of the Canary Islands, for a conference on the Philosophy of Cosmology. The slides for all the talks are now online; videos aren’t up yet, but I understand they are forthcoming.

Stephen Hawking did not actually attend our meeting -- he was at the hotel for a different event. But he stopped by for an informal session on the arrow of time. Photo by Vishnya Maudlin.

Stephen Hawking did not actually attend our meeting — he was at the hotel for a different event. But he stopped by for an informal session on the arrow of time. Photo by Vishnya Maudlin.

It was a thought-provoking meeting, but one of my thoughts was: “We don’t really have a well-defined field called Philosophy of Cosmology.” At least, not yet. Talks were given by philosophers and by cosmologists; the philosophers generally gave good talks on the philosophy of physics, while some of the cosmologists gave solid-but-standard talks on cosmology. Some of the other cosmologists tried their hand at philosophy, and I thought those were generally less successful. Which is to be expected — it’s a sign that we need to do more work to set the foundations for this new subdiscipline.

A big part of defining an area of study is deciding on a set of questions that we all agree are worth thinking about. As a tiny step in that direction, here is my attempt to highlight ten questions — and various sub-questions — that naturally fall under the rubric of Philosophy of Cosmology. They fall under other rubrics as well, of course, as well as featuring significant overlap with each other. So there’s a certain amount of arbitrariness here — suggestions for improvements are welcome.

Here we go:

  1. In what sense, if any, is the universe fine-tuned? When can we say that physical parameters (cosmological constant, scale of electroweak symmetry breaking) or initial conditions are “unnatural”? What sets the appropriate measure with respect to which we judge naturalness of physical and cosmological parameters? Is there an explanation for cosmological coincidences such as the approximate equality between the density of matter and vacuum energy? Does inflation solve these problems, or exacerbate them? What conclusions should we draw from the existence of fine-tuning?
  2. How is the arrow of time related to the special state of the early universe? What is the best way to formulate the past hypothesis (the early universe was in a low entropy state) and the statistical postulate (uniform distribution within macrostates)? Can the early state be explained as a generic feature of dynamical processes, or is it associated with a specific quantum state of the universe, or should it be understood as a separate law of nature? In what way, if any, does the special early state help explain the temporal asymmetries of memory, causality, and quantum measurement?
  3. What is the proper role of the anthropic principle? Can anthropic reasoning be used to make reliable predictions? How do we define the appropriate reference class of observers? Given such a class, is there any reason to think of ourselves as “typical” within it? Does the prediction of freak observers (Boltzmann Brains) count as evidence against a cosmological scenario?
  4. What part should unobservable realms play in cosmological models? Does cosmic evolution naturally generate pocket universes, baby universes, or many branches of the wave function? Are other “universes” part of science if they can never be observed? How do we evaluate such models, and does the traditional process of scientific theory choice need to be adapted to account for non-falsifiable predictions? How confident can we ever be in early-universe scenarios such as inflation?
  5. What is the quantum state of the universe, and how does it evolve? Is there a unique prescription for calculating the wave function of the universe? Under what conditions are different parts of the quantum state “real,” in the sense that observers within them should be counted? What aspects of cosmology depend on competing formulations of quantum mechanics (Everett, dynamical collapse, hidden variables, etc.)? Do quantum fluctuations happen in equilibrium? What role does decoherence play in cosmic evolution? How does do quantum and classical probabilities arise in cosmological predictions? What defines classical histories within the quantum state?
  6. Are space and time emergent or fundamental? Is quantum gravity a theory of quantized spacetime, or is spacetime only an approximation valid in a certain regime? What are the fundamental degrees of freedom? Is there a well-defined Hilbert space for the universe, and what is its dimensionality? Is time evolution fundamental, or does time emerge from correlations within a static state?
  7. What is the role of infinity in cosmology? Can the universe be infinitely big? Are the fundamental laws ultimate discrete? Can there be an essential difference between “infinite” and “really big”? Can the arrow of time be explained if the universe has an infinite amount of room in which to evolve? Are there preferred ways to compare infinitely big subsets of an infinite space of states?
  8. Can the universe have a beginning, or can it be eternal? Does a universe with a first moment require a cause or deeper explanation? Are there reasons why there is something rather than nothing? Can the universe be cyclic, with a consistent arrow of time? Could it be eternal and statistically symmetric around some moment of lowest entropy?
  9. How do physical laws and causality apply to the universe as a whole? Can laws be said to change or evolve? Does the universe as a whole maximize some interesting quantity such as simplicity, goodness, interestingness, or fecundity? Should laws be understood as governing/generative entities, or are they just a convenient way to compactly represent a large number of facts? Is the universe complete in itself, or does it require external factors to sustain it? Do the laws of physics require ultimate explanations, or can they simply be?
  10. How do complex structures and order come into existence and evolve? Is complexity a transient phenomenon that depends on entropy generation? Are there general principles governing physical, biological, and psychological complexity? Is the appearance of life likely or inevitable? Does consciousness play a central role in accounting for the universe?

Chances are very small that anyone else interested in the field, forced at gunpoint to pick the ten biggest questions, would choose exactly these ten. Such are the wild and wooly early days of any field, when the frontier is unexplored and the conventional wisdom has yet to be settled. Feel free to make suggestions.

  1. A fascinating set of questions.

    My biggest single question thusfar, philosophically is:

    Suppose that in the early universe, symmetry breaking didn’t happen – what would be the result? The implied notion that symmetrical interactions might have continued to occur also implies that symmetry breaking in our universe is a “mistake” – an error in an otherwise symmetrical quantum system.

  2. I pretty much agree with these 10 questions.

    One might add “Is it possible to do something in this field without being tainted by Templeton money?” and “Is it possible for anyone outside the self-appointed circle of experts to be acknowledged by the Chosen Few?”

  3. Sean: Please take the time to real my unrelated email gmail to you!! About this, the questions are good, but invoking “philosophy” is a poor descriptive replacement for “science”. I would hope that we are working on the science of cosmology, slowly but surely. Thanks, MER

  4. Sean, you just couldn’t keep a secret … now every second-rate physicist will be conferencing in Tenerife.

    Besides, don’t two different Cosmology/Astrophysics events occurring at the same time and location somehow violate the Pauli exclusion principle?

  5. Others
    Does mathematics reflect reality or is it a convenient prediction method?
    Should life and physics obey the same fundamental principles?
    Is the speed of light the fastest changes may be transmitted (speed of gravity)?
    What is the mechanism of fine-tuning?
    Is the universe adiabatic? Related: Is the entropy of the universe constant, declining, or increasing?

  6. Concerning 2 of the “Ten Questions for the Philosophy of Cosmology”
    Is the elephant in the room wearing the emperor’s new robe?

    Re: Questions
    Q 6. Are space and time emergent or fundamental?, and,
    Q 7. Can the universe have a beginning, or can it be eternal?

    Irrespective of space, for “time” to be real or emergent, time would have to be shown to exist.
    It seems to me, our main reason for assuming time exists, seems to be because we can be forming patterns in our minds as we are watching the world around us, and we tend to “call” those patterns “memories” of “the past”.

    But surely, logically, and scientifically observably, all those patterns prove is that matter ( us, our brains, and the universe around us) all exist, and in areas can “be” forming intricate patterns in places.

    This in itself seems only to prove that matter exists, and is moving and changing. (Please correct me if I’m wrong), But we don’t actually seem to observe anything “coming out of a future”, or “going into a past”.

    Likewise, despite our thoughts, assumptions, guesses, and feelings, there doesn’t seem to be any scientific evidence to prove, that extra to observing the matter, in and around us, just existing and interacting, we are also actually “heading into a future”, or actually “leaving a temporal past behind us”. (or any variation of the above).

    (Only a vast amount of Argumentum ad populum to the contrary, which does not count as scientific proof).

    Likewise there seem to be no actual scientific experiments, as per the scientific method to prove the existence of a past, and or future, or time, or moments of time etc. Thus perhaps we should consider that the universe “may” be just as it appears, i.e. just full of matter, just existing, and just interacting.

    The agony and the Entropy…
    Professor Carroll suggests the constant (probably, or possibly irreversible) expansion, i.e. entropy of the universe prove “the arrow of time”, and thus time’s own existence.

    But I would suggest its worth considering that the (I assume) fact “all the matter in the universe is expanding outwards”, only in itself, proves that “all the matter in the universe is expanding outwards”.

    i.e. the one way expansion of the universe does not seem to prove matter leaves a “temporal past” behind it, or heads into a “temporal future”, or that a thing called “time” must exist, and “pass”, for things to be able to exist , and move. (it seems to me only the forcible overlaying (i.e. possible “confirmation bias”) of the theory of time onto what we only and actually observe that makes it seem to be credible.

    A timeless big bang?
    It may indeed be possible for an incredibly small speck of matter to materialise in empty space, and explode into a universe… but that doesn’t prove there is a past, or future or thing called time.
    (likewise matter may even be able to just disappear, but that prove neither a past, future or time).

    The naked elephant in the room?
    Therefore, in my opinion, the seemingly complacent willingness to ignore the complete lack of experiments to test the alleged components of “time”, and the seeming willingness to only observe everything is here , and interacting, but conclude an unobservable “past”, “future”, and “time” exist, makes me conclude that “time” may be neither fundamental or emergent. (i.e. this may be an misleading and invalid question from the outset)

    And, the question of the universe having a (I assume temporal) beginning may be moot, because, I wonder if, unless we can provide experimental proof that matter does not just exist, and interact, as directly, and only observed, perhaps the whole theory of time may just be the elephant in the room, wearing the emperor’s new robe?

    Matthew Marsden
    (auth: A Brief History of Tmelessness)

  7. Dr. Carroll, good questions. I highly recommend to you Lecture One in Professor James Hall’s Teaching Company course entitled “Philosophy of Religion” in which he begins by stating that, before you do philosophy to (of) religion (or cosmology for that matter), you must begin with a workable definition of philosophy. He distinguishes between four kinds of questions and notes that philosophy focuses on asking and answering only one of the four. I think some, but not all, of your questions fall within the scope of the other three. But, hey, I’m just a dumb sailor.

  8. ” Is the universe complete in itself, or does it require external factors to sustain it? Do the laws of physics require ultimate explanations, or can they simply be?”
    In other words, you’re asking if the supernatural exists or if God exists or is necessary to sustain or create it. That’s basically what you’re asking. I’m glad you’re starting to question your atheist beliefs, Sean.

  9. If time (or space) is infinite, everything that can happen has already happened, even down to me typing these words, and there is no change, only cycles of varying lengths. But how could time not be infinite? How can there be no state of being at all?

    The idea of infinite repetition seems also strange, however, and yet we can only get away from it by allowing for infinite possibilities, too — infinite time in infinite possibilities means repetitition is not required. But that requires we give up the idea of natural law, in some sense. I mean, it can certainly exist and govern the universe for uncounted bazillions of years, but some time or other it must vary.

    And is it even possible to consider an infinite range of possibilities for natural law? Perhaps logical consistency alone puts a finite cap on the possibilities for existence — which sends us back to cycles.

  10. A notable omission from your list:

    Is quantum mechanics fundamental or emergent?

    This question is especially relevant if spacetime is emergent. For example, if distances and time intervals are not well defined because they are emergent notions, then in what sense are energy and momentum (and thus the Hamiltonian) well defined notions? The Hamiltonian is pretty fundamental to QM…

    Emergent vs. fundamental QM also impacts some of your other questions.

  11. Dr. Carroll,
    It is an intriguing idea that we could have a “philosophy of cosmology” discipline. As is usual in such matters, there is a hierarchy in the list of relevant questions to think about; some questions tend to be more FUNDAMENTAL than others and must, therefore, be prioritized in the answering process. Here are an additional three questions which I think are critical in the answering of, for instance, your question number 8: “Can the universe have a beginning or can it be eternal?… Are there reasons why there is something rather than nothing?”
    These additional questions are really fundamental to ALL of physics and not just the cosmology discipline:

    11. What actually IS a physical law?
    In what sense can a physical law be said to exist? Are physical laws information? Is all information physical? This leads to the question:
    12. What defines a physical state?
    Any definition here should DIFFERENTIATE a physical state from a non-physical (metaphysical) state; additionally,
    13. What is energy?
    This is NOT to be confused with a mathematical description of how energy BEHAVES, which is well understood, but just what, itself IS energy? Richard Feynman famously emphasized this in his Lectures on Physics: “It is important to realize that in physics today we have no knowledge of what energy IS.” (“The Feynman Lectures on Physics,” Feynman, Layton, and Sands, 1963, 4-2). Furthermore: Is energy conserved? Is the universe time-translationally invariant?
    It seems to me that an accurate answering of the above additional questions would dispel the “mystery” that Dr. Alex Vilenkin expresses at the end of his book “Many Worlds In One,” when he states:

    “The [quantum] tunneling process is governed by the same fundamental laws that describe the subsequent evolution of the universe. It follows that the laws should be ‘there’ even prior to the universe itself. Does this mean that the laws are not mere descriptions of reality and can have an independent existence of their own? In the absence of space, time, and matter, what tablets could they be written upon? The laws are expressed in the form of mathematical equations. If the medium of mathematics is the mind, does this mean that the mind should pre-date the universe?
    This takes us far into the unknown, all the way to the abyss a great mystery.” (Alex Vilenkin: “Many Worlds In One: The Search for Other Universes,” Hill and Wang, New York, 2006, p.205).

    T.E. Oakley

    Sent from my iPhone

  12. Here’s another candidate question:

    Are spacetime symmetries fundamental or emergent?

    If spacetime is emergent, is it automatically true that Lorentz symmetry and translation invariance (i.e., Poincare symmetry) must “govern” that emergent spacetime? If Poincare symmetry is emergent, would that not constrain what emergence scenarios are possible (in that the scenario must generate the symmetry)?

  13. “Feel free to make suggestions.”- Sean Carroll

    You asked for it!

    1. I don’t believe the fine tuning argument will ever lead to anything substantive about the nature of physics with our current knowledge of Big Bang Cosmology. There are just too many drastically different models that would generate completely different conclusions about fine tuning depending on what model you use. For instance, a multiverse theory would say something completely different than a model that used the Copenhagen Interpretation. An oscillating universe would yield different results than a one time single universe. The list could go on and on…

    2. I believe the early universe could be defined as having a specific quantum state. Then the most rudimentary and fundamental processes would take place there. I think it would take incorporating a theory of gravity to quantum theory in order to appropriately define it with time. I don’t think it should be pursued to be a separate law of nature, because a Big Bang singularity would seem to have problems expanding due to an infinite force of gravity.

    3. Anthropic reasoning cannot generate any reliable result because of the same reasons I listed in number 1.

    4. Unobservable realms shouldn’t play any role in cosmological models. If the mathematics that describes our observable realm is exactly the same, then the results of which would be of little or no consequence to the results of the observable data.

    5. Personally, I would prescribe the Copenhagen Interpretation, and since the mass of the Higgs boson is right in the middle of what it should be to be either a multiverse or supersymmetry, I would suggest that the many worlds interpretation (MWI) is wrong. Then if you really needed other worlds then they could exist from breaking “temporal symmetry”. A different definition of the MWI could lead to something.

    6. I believe spacetime is an emergent quantity. I think the one exception to my reasoning in 1 and 3 is that it can rule out certain models. I think a gravitational singularity of infinite mass can be ruled out, because there would be no bang for us to be here from. Then if the early universe consisted of mostly energy, spacetime would be dilated to zero for any observer. Then with the creation of mass, spacetime could then have values larger than zero.

    7. If spacetime had a value of zero when the universe had no mass, then it would seem like space would have to be infinite if the appearance of mass made it non-zero due to General Relativity. Other than that it is just to make us wonder why Newton was so good working and solving problems with it in Calculus, but we are so horrible at using it in physics.

    8. I don’t believe the universe could be eternal for much of the same reasons why people think there couldn’t be a God that always existed that created everything.

    9. I think the laws of physics could change. The universe is incredibly lazy. If there would be a way for it to use less energy to prevent spacetime paradox’s it would would defiantly go for it any chance it got. Even Hawking predicted that the properties of the universe could change from the Higgs Boson settling to a different energy level or something from breaking symmetries.

    10. I think the likely hood of life coming into existence in a suitable environment would be just as likely as life or fungus growing in something that you had left out accidentally for days or weeks.

  14. If we find that the universe is literally infinitely large or literally has an eternal past, and not just really really large, or potentially infinite, then I’m headed upstairs to my bed planning to pull a blanket over my head and silently scream.

  15. With no scientific education, though if snags had not intervened I might have studied physics, I instead philosophise about spiders, cosmos, psyche, life as it unfolds …
    And I enjoy coming here to read.

    The idea of a ‘Philosophy of the Cosmology’ interests me.

  16. This is essentially the same question asked by John Hodge but it merits repeating. Is the description of physical laws through modern mathematics an elaborate curve-fitting scheme intended to minimize the number of free parameters? Or, as has apparently been the underlying assumption of theoretical physics for at least a century, is the picture something more like Nature=Mathematics? The astounding success of theoretical physics has led to a widespread tacit view that the latter possibility could actually reflect reality, but perhaps we are entering a time when it will be seriously called into question.

  17. In what sense, if any, is the universe fine-tuned? When can we say that physical parameters (cosmological constant, scale of electroweak symmetry breaking) or initial conditions are “unnatural”? What sets the appropriate measure with respect to which we judge naturalness of physical and cosmological parameters? Is there an explanation for cosmological coincidences such as the approximate equality between the density of matter and vacuum energy? Does inflation solve these problems, or exacerbate them? What conclusions should we draw from the existence of fine-tuning?


    I really doubt this question would have made the cut without the influence of the creationists and the so called intelligent design crowd. This one is not worth wasting time on.

  18. For a moment lets jump the chasm of a pure abstract, successful in prediction, mathematical formulation that has been derived through observation and experimentation and begin defining a dynamic three dimensional means for achieving that consistent, comparable, directional, physical change.
    In the early 1900’s the demise of the Aether catapulted Relativity to the forefront of concepts but the ringer was the needed acceptance of a fourth physical dimension called time. The world of mathematics would never be the same and the world of physics became forever entwined in predictions of isolated conditions that to this day have not been reconciled.
    For nearly a century advances in mathematics and theoretical physics have catapulted advances in all of the sciences to levels not even considered just a few short years ago. I am at once awed and amazed by the new and then the New and then the NEW that seems to happen every day but I am also saddened by the acceptance of the physical being defined by pure, self-isolating, abstract mathematical formulations that, by design, have had correlative references to a physical object or condition removed. Gravity became a Field and no longer a Fundamental Force, for a Force required an exchange of something as opposed to a field that can exist as a condition of space and time that surrounds a body with mass. That is to say if you are not considering the exchange of gravitons. Advances in theory seems to require extra dimensions and multi-verses but again predictions of conditions found through observations as in Astronomy and the conditions defined as Dark Energy and Dark Matter that are observed but exist without a base in theory that would speak to the three dimensions of reality. Traveling up the scale of complexity and not paying any attention to the lack of support of theory to the conditions of Fundamental forces, Constants of Nature, and Time, you find consistent form and structure, consistent comparable directional change, identified limits both Global and Local, and an overriding condition of Uncertainty as well as a gap associated with the transitions from Physics, to Chemistry, to Biology, to Classical Physics, to Astronomy, to Cosmology and a host of Engineering and Technical disciplines in between.
    To develop a philosophy of Cosmology it would seem that a must have basis would be an overview of what we have vs. what we think we have. First we have the ability to create predictive mathematics that may or may not have an intuitively developed and imposed definition…after all we do not have a viable definition of the physical process that is inclusive and can be applied at all levels of application. We must consider the number of Cornerstone Concepts and Theories that have not been reconciled and thus lead into a possible false premise that have been proven to exist but exist in isolation as a problem with the pure, self-isolating, mathematical formulations or a miss interpretation of the events of the day. Another component that must be considered, may be over the top simple, evolves from the question of which came first, a means to a consistent comparable physical process or an ability to predict an outcome no matter the what or how. Another thought to consider is how close are we to a complete understanding of the physical…on a scale of 1 to 10, are we at a 1 or a 2 or with all of the predictions do you believe we are at an 8 or 9?
    I too have used the lessen of “The Emperor’s new Suit” by Hans Christion Anderson in 1837 as I consider the monastic, self-supporting, peer reviewed, members of the Guild of the Golden Cloth that receive their gold in research funding while weaving virtual golden threads into a robe that only they can see. As harsh a statement that this may seem to be it is also a commendation for tenacity and focus for few without acceptance can speak to or better yet interrupt the findings associated with this greatest Black Box experiment of all time that we find ourselves within.
    With due consideration being given to the question at hand of a Philosophy of Cosmology, I would like to suggest that a clear understanding of the physical process that supports the ability to predict is needed and when achieved, as physicist John Archibald Wheeler of Princeton once wrote an unpublished poem, published and reprinted here with the thankful allowance of K. Ferguson’s book Bantam Books,1991, pg 21 “STEPHEN HAWKING: Quest for a Theory of Everything” ,

    “Behind it all
    Is surly an idea to simple,
    so beautiful,
    so compelling that when—
    in a decade, a century,
    or a millennium—
    we grasp it,
    we will all say to each other,
    how could it have been otherwise?
    How could we have been so stupid
    for so long?”

    For a philosophy to have any chance at succeeding and carrying on through time, it must begin with the truth, the whole truth, and an applied perspective that is not exclusive but rather inclusive of all that we know and will know.
    R. D. Shriner

  19. The line separating metaphysics and science is not always clear cut and stable. What seems to be philosophical today may be scientifically tested or observed in the future. Sometimes it’s hard to determine whether something should be scientific theory or philosophical speculation. But one should beware of those who give up and promote the supernatural as the only explanation.

  20. I think this is an excellent list of philosophical questions. Many of them are questions that each of us will have knee jerk reactions to, but most will be reactions that can’t necessarily be backed up scientifically.

    Along those lines, I think 4 is crucial and is also more a broader philosophy of science question.

  21. @John Barrett

    A cosmological singularity MUST exist. It can be the Big Bang, Big Crunch, Big Rip or Black Hole singularity but it had to exist.
    In any model (like LQG) where classical cosmological singularity is avoided, you get an exotic cosmological singularity wich is even worse.
    I will not ever mention the weird sudden or spontaneous singularity.
    Today we have observational evidence only for Black Hole singularity.
    The cosmological singularity is very interesting: a dot with infinite density, temperature, information and where all laws of physics breakdown and pure indeterminism rules.

    Also if you don’t believe in an eternal universe then your only option is the eternal initial singularity (the singularity can exist “outside” of space-time).