Time, Cosmology, and the Beginning

The thermodynamic arrow traced the direction of all time to a single cosmological fact — the Past Hypothesis, that the universe began in an extraordinarily low-entropy state. This pushes the deepest questions about time to the beginning of the universe. Did time have a beginning? If the Big Bang is the first moment, what does it mean for time itself to have an edge — and can there be a "before" the first moment? Why was the initial state so improbably ordered? And does a temporal beginning fit better with a relational or a substantival view of time? These questions sit at the intersection of physics, metaphysics, and — historically — theology, and they are the natural culmination of the temporal question.

This page examines the beginning of time, the low-entropy past, and the metaphysics of a first moment; it presupposes the thermodynamic arrow and connects to the problem of time and the philosophy of religion.

Did time begin?

Standard Big Bang cosmology, extrapolating general relativity backward, reaches an initial singularity roughly 13.8 billion years ago: a state of diverging curvature and density where the smooth spacetime manifold breaks down. The Penrose–Hawking singularity theorems show that, under reasonable energy conditions, such a singularity is generic, not an artifact of idealized symmetry. If this singularity is a genuine temporal boundary, then time itself began: there is a first moment, with no earlier one, and the question "what happened before the Big Bang?" is not unanswered but ill-formed — there is no "before," as there is no point on Earth's surface north of the North Pole.

This is precisely the answer Augustine gave, sixteen centuries early, to the taunt "what was God doing before He made the world?": God created with time, not in it, so there was no "before" for anything to happen in — time is a feature of the created order (see the religion, time, and cosmology page). Modern cosmology recovers the structure of Augustine's point in physical dress: a temporal beginning need not be a beginning in a prior time, but a boundary of time.

But the singularity is where classical general relativity fails, so whether time really began there depends on the unknown physics of quantum gravity. Several possibilities are live:

Time began at the singularity — the boundary is real; there is a first moment.
The singularity is smoothed away. Quantum-gravitational effects may replace the singularity with a bounce (a prior contracting phase), making the Big Bang a transition rather than a beginning, with time extending indefinitely into the past.
Time emerges, with no sharp "first moment." In the Hartle–Hawking "no-boundary" proposal, the early universe has no temporal boundary because time itself becomes space-like near the origin (a "rounding off," using imaginary time), so the universe is finite in the past yet without a first moment or an edge — finite but unbounded, like the surface of a sphere. Time does not so much begin as fade in. This connects directly to the problem of time: if fundamental physics is timeless, "the beginning of time" is a feature of the emergent description, not of the underlying reality.

The low-entropy past and its explanation

Whatever the status of the first moment, cosmology must confront the Past Hypothesis: the early universe had staggeringly low entropy — Penrose's oft-cited estimate is a fine-tuning of one part in $1 0^{1 0^{123}}$ . This is the ultimate source of the arrow of time, so explaining it is explaining why time has a direction at all. The candidate explanations:

A brute law-like boundary condition. The low-entropy start is simply a fundamental fact about the universe, needing no further explanation — perhaps elevated to a law (Callender: demanding an explanation for the boundary condition may be a category mistake, since laws and boundary conditions are different things).
A dynamical/cosmological mechanism. Inflation — a brief epoch of exponential expansion — is often invoked to explain the smoothness and flatness of the early universe, but critics (Penrose, Price) argue it presupposes rather than explains a low-entropy start, since inflation itself requires special initial conditions. A genuine explanation would derive the low-entropy past from deeper principles, still lacking.
Multiverse and selection effects. If our universe is one of many with varying initial conditions, observers can only arise in low-entropy regions capable of supporting complex structure — an anthropic selection (parallel to the dimensionality and fine-tuning arguments). But this faces the Boltzmann brain problem: in most such scenarios it is overwhelmingly more probable for a low-entropy region just large enough to contain a single momentarily-fluctuated observer (a "Boltzmann brain") to arise than for an entire orderly universe — so anthropic/fluctuation explanations threaten to predict that our apparently ordered past is an illusion, which is self-undermining. This is a serious constraint on any fluctuation-based account of the arrow.

The metaphysics of a first moment

Suppose time did begin. Several classic puzzles follow, and they bear on the substantivalism–relationism debate:

The Leibnizian question. Leibniz argued that absolute time is unreal partly because God could have had no sufficient reason to create the world at one moment rather than another in a pre-existing absolute time. A relational view of time defuses the puzzle elegantly: if time is just the order of events, there is no antecedent time in which the beginning could have been "placed," so no arbitrary choice and no violation of the Principle of Sufficient Reason. A substantival absolute time, by contrast, seems to demand a first moment sitting in an infinite prior temporal void — exactly what Leibniz found objectionable. The beginning of time is thus a point for relationism about time.
Kant's first antinomy. Kant argued that reason falls into contradiction over whether the world has a beginning in time: the thesis (it must, else an infinite past would have had to be completed to reach now — an actual infinity traversed) and the antithesis (it cannot, since a first moment would be preceded by an empty time with no reason for the world to begin then) seem equally provable. Kant took this to show that space and time are forms of appearance, not features of things-in-themselves. The antinomy still frames debates over whether an infinite past is possible — the same issue that drives the kalām cosmological argument in the philosophy of religion, which infers a beginning (and a cause) from the alleged impossibility of a completed infinite past.
Can time exist without change? If there could be a stretch of time with no events (Shoemaker's thought experiment of a world with periodic universal "freezes" imagines empirical evidence for empty time), then time is more than the order of change, favouring substantivalism. If not — if time is nothing but the order of change (Aristotle, Leibniz, Barbour) — then a "first moment" is just the first change, and relationism about time is vindicated. The problem of time's emergent, change-based time sides firmly with the relationist.

Where physics and religion meet — and part

The beginning of time is the classic contact point between cosmology and the philosophy of religion. The kalām cosmological argument — the universe began to exist; whatever begins to exist has a cause; therefore the universe has a cause — takes the Big Bang as empirical support for a temporal beginning and hence a transcendent cause. The philosophy-of-space-and-time pages keep the physics and metaphysics here (whether time began, what the singularity means, whether an infinite past is coherent) and leave the theological inference — from a beginning to a creator — to the religion section, exactly as the Augustinian treatment of creation-with-time is developed there. The physics is common ground; the metaphysical conclusions drawn from it are not.

Where this sits

Cosmology and the beginning of time are the culmination of the temporal question: the arrow of time and its thermodynamic ground trace back to the low-entropy initial condition, and the status of the "beginning" depends on the problem of time and the unknown physics of quantum gravity. The metaphysics of a first moment feeds directly back into the substantivalism–relationism debate (relational time dissolves the Leibnizian puzzle) and into the philosophy of religion (the kalām argument and Augustine on creation). With this the physics subfolder — and the section's engagement with modern science — is complete; the context pages step back to survey the history, method, and figures of the whole field.