The Cyclic Universe

1. The Universe's Unexplained Low Entropy

For some reason, when people enunciate the various problems of cosmology they don’t even ask, Why was the Big Bang not only such a state of low entropy, but a state of such low entropy in a very strange way—that it singles out gravity as the one thing that is not taking part in this thermal state?

A profound puzzle. Roger Penrose highlights a fundamental, often overlooked, mystery in cosmology: the incredibly low entropy state of the Big Bang. Entropy, a measure of disorder, is expected to increase over time according to the Second Law of Thermodynamics. Working backward, this implies the universe began in a state of extreme order.

Gravity's unique role. The early universe's low entropy is particularly strange because it appears that gravity was "turned off" or "not thermalized." While matter and radiation were in a high-entropy thermal equilibrium (evidenced by the uniform cosmic microwave background, CMB), the gravitational field remained in a highly ordered, low-entropy state, acting as a vast reservoir of potential disorder. This initial uniformity allowed structures like stars and galaxies to form later.

Contradictory evidence. The CMB, a "thermal equilibrium state" (maximum entropy for matter/radiation), seems to contradict the idea of a low-entropy Big Bang. Penrose argues that this apparent contradiction is resolved by recognizing that gravity was initially aloof, allowing for a low gravitational entropy despite the high matter/radiation entropy. This distinction is crucial for understanding the universe's evolution.

2. Inflation's Failure to Solve the Entropy Puzzle

My principal concern was that it didn’t actually explain what it was supposed to explain.

Inflation's purpose. Cosmic inflation, a widely accepted theory, proposes a period of rapid exponential expansion in the very early universe. It aims to solve problems like the universe's spatial uniformity and the uniform temperature of the CMB, which appear to be causally disconnected. Inflation suggests this rapid stretching smoothed out initial irregularities.

Aesthetic and scientific concerns. Penrose finds inflation "ad hoc" and "ugly" on aesthetic grounds, but his main critique is scientific: it doesn't address the deeper problem of the Big Bang's low entropy. He argues that inflation merely spreads out existing clumps; it doesn't explain why the gravitational degrees of freedom were initially in such an ordered state.

The "upside-down" universe. To illustrate inflation's inadequacy, Penrose imagines time running in reverse towards the Big Bang. If the universe were to collapse, gravity would cause massive clumping and black hole formation, leading to an "unbelievable messy situation" of enormously high gravitational entropy. If the Big Bang had started in such a high-entropy state, inflation would be useless, simply spreading out the mess rather than creating uniformity.

3. Conformal Cyclic Cosmology: Infinite Aeons

In essence, it proposes that cosmology should properly be regarded as an infinitely repeating series of “aeons,” with the far distant future of one aeon mapping on to the Big Bang of another one, thereby producing endlessly repeating cycles.

A radical proposal. Conformal Cyclic Cosmology (CCC) is Penrose's theory to resolve the Big Bang's low-entropy paradox. It posits that our universe, or "aeon," is just one in an infinite sequence of such aeons, each beginning with a Big Bang and ending in a vastly expanded, empty state.

Matching ends and beginnings. The core idea is that the infinitely remote future of one aeon, after all massive particles have decayed or evaporated into massless radiation, becomes conformally equivalent to the Big Bang of the next aeon. This allows for a seamless "gluing" of the end of one cycle to the beginning of the next, maintaining a consistent low-entropy initial state for gravity.

Avoiding the "bounce" mess. Unlike other cyclic models that involve a "bounce" from a collapsing universe, CCC avoids the problem of increasing entropy. A collapsing universe would become a "huge, horrendous mess" of black holes and singularities, making it impossible to smoothly transition to a new, ordered Big Bang. CCC requires the universe to expand indefinitely and for mass to disappear.

4. Conformal Geometry: Linking Cosmic Eras

If the angles are the same for two triangles, they’re similar triangles—they’re conformally the same.

Angles over distances. Conformal geometry is a mathematical framework that focuses on angles and shapes rather than absolute distances or sizes. Penrose uses this concept to bridge the vast scales between the end of one aeon and the beginning of the next. It allows for "squashing infinity" (the far future) and "stretching out" the Big Bang, making them mathematically similar.

The light cone's role. In relativity, light cones define the causal structure of spacetime, showing what light would do at any point. Crucially, the structure of light cones is preserved under conformal transformations. This means that even when distances are distorted, the fundamental causal relationships remain intact, allowing for a meaningful connection between conformally equivalent eras.

Physical implications. For a photon, which travels along the edges of light cones and experiences no passage of time, infinity is "just there." This means that in a universe dominated by massless particles (like photons), the distinction between finite and infinite scales, or between the Big Bang and the far future, becomes blurred from a conformal perspective. This mathematical trick gains physical relevance when mass is absent.

5. Mass Must Vanish for Aeons to Connect

The only time, as far we know, that you need the full metric geometry is when mass comes in.

The mass problem. For the conformal mapping between aeons to be physically meaningful, all particles must effectively become massless in the far future. This is because the full metric geometry, which includes information about distances and time (and thus mass), breaks conformal invariance. If massive particles persist, they act as "clocks" that would experience the passage of time and the vast distances, preventing the conformal identification.

Higgs and anti-Higgs. In the early universe, the Higgs mechanism gives mass to particles. Penrose hypothesizes an "anti-Higgs mechanism" in the extremely late universe, long after black holes evaporate. This mechanism would cause all particles, including electrons, to gradually lose their rest mass, becoming effectively massless.

A necessary hypothesis. Penrose acknowledges this "fade-out of mass" as potentially the "weakest point" of CCC, as there's no current experimental evidence for it. However, he argues it's a necessary hypothesis for the theory's consistency, allowing the conformal picture to hold and enabling the transition of massless entities across aeon boundaries.

6. Dark Energy and Dark Matter: CCC's Core Pillars

The theory that I’m proposing, Conformal Cyclic Cosmology (CCC), wouldn’t work without dark matter and so-called dark energy—the cosmological constant.

Dark energy's crucial role. A positive cosmological constant, which drives the universe's accelerating expansion (dark energy), is not an incidental feature but a necessary condition for CCC. It ensures that the far future boundary of an aeon is "space-like," a geometric property essential for it to be conformally glued to the Big Bang of the next aeon. Without it, the picture simply "can't make this picture work."

Dark matter's emergence. The mathematical process of transitioning between aeons in CCC, specifically "turning the conformal factor upside down," inherently generates a new scalar field. Penrose identifies this field as dark matter.

• It's a "phantom field" in the previous aeon.
• It becomes a "major contribution to the matter in the new full Einsteinian metric."
• It's initially massless but acquires mass later.
• It must also decay over the course of an aeon to prevent accumulation.

Rest mass's return. The equations of CCC also demand that rest mass "creeps back in again" in the new aeon. This re-introduction of mass, tied to the Higgs idea, along with dark energy and dark matter, are three fundamental requirements for the theory's consistency.

7. Black Hole Information Loss: Entropy's Reset Button

I think that information is lost in black holes.

The information paradox. A contentious but vital aspect of CCC is Penrose's insistence that information is lost in black holes, contrary to Stephen Hawking's later view and mainstream quantum mechanics. This loss is crucial for preventing entropy from accumulating infinitely across successive aeons.

Entropy's transcendence. Penrose argues that the Second Law of Thermodynamics is not violated, but "transcended." When information falls into a black hole and is lost, those degrees of freedom are no longer considered in the calculation of entropy.

• Black holes have immense entropy, far exceeding the CMB.
• Their evaporation via Hawking radiation removes this entropy from the observable universe.
• By "forgetting" the lost information, the effective entropy of the universe can reset for the next aeon.

A necessary re-evaluation. This stance reopens a debate many physicists consider settled, but Penrose believes it's essential to resolve the cosmological entropy problem. The destruction of information provides the mechanism for the universe to "cleanse" itself of accumulated entropy, allowing for a fresh, low-entropy start in each new aeon.

8. Cosmic Microwave Background Rings: Echoes of a Prior Aeon

Because one of the main predictions of the theory is a specific ring-shaped pattern in today’s cosmic microwave background (CMB) caused by giant black-hole collisions from the previous aeon.

Observable imprints. CCC makes a specific, testable prediction: the existence of concentric ring patterns in the Cosmic Microwave Background (CMB). These rings would be the observable remnants of violent events, specifically the collisions of supermassive black holes, from the aeon preceding ours.

The mechanism.

• Supermassive black hole collisions in the previous aeon emit powerful gravitational waves.
• At the aeon crossover, these gravitational waves convert into waves in the nascent dark matter of our aeon.
• As dark matter acquires mass, these waves manifest as slight movements or "ripples" in the cold dark matter fluid.
• These ripples leave an imprint on the CMB, appearing as slightly warmer or cooler concentric rings, depending on their direction of motion relative to us.

Controversial detection. Penrose and his colleague Vahe Gurzadyan claim to have found evidence of these uniform rings in CMB data, but their findings are highly controversial. The debate centers on the statistical methods used, particularly the definition of a "random sky" for comparison. Penrose's "twisting technique," which shows CMB data is highly sensitive to circularity but not elliptical distortions, offers an alternative approach to validation.

9. The Independent Thinker's Battle Against Orthodoxy

But then, most of us aren’t Roger Penrose.

A unique voice. Roger Penrose stands out in the scientific community for his willingness to challenge established paradigms and his unique approach to communicating complex ideas, even including equations in popular books. His work on CCC is a testament to his independent thinking, driven by a deep-seated concern for fundamental cosmological puzzles.

Facing skepticism. Penrose acknowledges significant pushback on several fronts:

• The "artificial" nature of the mass fade-out hypothesis.
• The statistical validity of the CMB ring detections.
• His minority view on information loss in black holes, which contradicts mainstream quantum mechanics.
• The general inertia of the scientific community, deeply invested in theories like inflation.

Prioritizing fundamental problems. Despite the opposition, Penrose remains steadfast, arguing that the fundamental problem of the Big Bang's low entropy is often ignored or inadequately addressed by current theories. He believes that even if CCC is not entirely correct, his long-standing cosmological concerns are valid and demand serious attention.

Last updated: March 20, 2026