The pulsating theory, also called the oscillating universe theory, proposes that the cosmos does not have a single beginning and end but instead cycles through repeated phases of expansion and contraction, each “Big Bang” followed eventually by a “Big Crunch” that gives rise to a new bang. The idea has deep roots in both philosophy and physics, and while the simplest version ran into serious problems decades ago, modern physicists have developed several sophisticated cyclic models that keep the core intuition alive in very different forms.
What the Pulsating Model Actually Proposes
In the classic pulsating picture, gravity eventually halts the expansion of the universe, pulls everything back together, and compresses all matter and energy into an extremely hot, dense state. That state then rebounds into a fresh expansion, and the process repeats forever. The appeal is partly aesthetic: a universe that oscillates has no true beginning and no true end, sidestepping the uncomfortable question of what came “before” the Big Bang.
Some formulations go further and argue that the cycles should be identical. A recent theoretical paper, for instance, applies conservation of energy and gravitational equilibrium to argue that total energy and gravitational pull remain the same across cycles, enforcing identical starting conditions every time the universe bounces through a singularity.1Cambridge Open Engage. On the Constancy of Cyclic Universes: A Conservation of Energy and Gravitational Equilibrium Perspective Whether nature actually works this way is another matter entirely, but the logic captures the pulsating model in its purest form: an eternal, self-resetting cosmos.
Why the Simple Version Fell Apart
Two discoveries knocked the legs out from under the straightforward oscillating model. The first is thermodynamic. Each cycle of expansion and contraction generates entropy, the irreversible “disorder” that accumulates in any physical process. If entropy increases with every bounce, each new cycle should last longer and expand further than the one before it. Run the clock backward and the earlier cycles shrink to nothing, meaning the universe still has a finite past. The pulsating model was supposed to avoid a beginning, but entropy reintroduced one through the back door.
The second blow came from observations in the late 1990s showing that the expansion of the universe is not slowing down under gravity’s pull. It is speeding up. This accelerating expansion, driven by what physicists call dark energy, acts as a kind of cosmic repulsion that overcomes gravitational attraction between distant galaxies.2Physics-Uspekhi. Dark energy and universal antigravitation If the expansion is accelerating, the universe cannot simply coast to a halt and collapse back on itself. A Big Crunch, the essential pivot point of the pulsating model, appears to be off the table under standard cosmological conditions. These two problems together made most physicists abandon the naive oscillating picture by the early 2000s.
What Happens During a Collapse
Even setting aside dark energy, a collapsing universe turns out to be a messier affair than the simple “reverse the expansion” image suggests. Fluctuations in density, which are tiny during expansion, grow dramatically during contraction. Research extending classical work on collapsing cosmologies has shown that these fluctuations can come to dominate the smooth background, and scattering processes among the dense, compressed matter inevitably lead to a thick gas of black holes.3arXiv. Black Crunch The generic final state of a Big Crunch, in this picture, is not a tidy, uniform singularity ready for a clean rebound. It is a chaotic black-hole soup. This made it unclear whether a smooth bounce could ever occur through a classical singularity, pushing theorists to look for other mechanisms.
The Big Bounce from Quantum Gravity
Loop quantum cosmology offered one escape route. By applying principles from loop quantum gravity, where space itself has a granular structure at unimaginably small scales, researchers showed that the singularity at the heart of the Big Bang is not a true point of infinite density. Instead, quantum effects generate a repulsive force that kicks in when matter is compressed to extreme densities, causing the collapsing universe to bounce rather than crunch. The big bang singularity is robustly replaced by a big bounce.4arXiv. Loop Quantum Cosmology: Physics of Singularity Resolution and its Implications
This result does not depend on fine-tuning or special assumptions about what kind of matter fills the universe. The resolution of the singularity appears to be generic: quantum geometry at the smallest scales prevents the collapse from reaching infinite density, regardless of the matter content.5arXiv. Loop quantum cosmology and the fate of cosmological singularities The bounce gives the pulsating idea a new lease on life, because it provides a physical mechanism for the transition from crunch to bang without passing through a singularity where the laws of physics break down. The universe in this framework could, at least in principle, cycle through repeated bounces.
Colliding Branes and the Ekpyrotic Model
A completely different approach to cyclic cosmology emerged from string theory. In the ekpyrotic model, developed by Paul Steinhardt, Neil Turok, and collaborators, the Big Bang is not an explosion from a point but a collision between two higher-dimensional surfaces called branes. In this picture, the Big Bang is not the beginning of time. Before the collision, there is a slow contraction phase during which the universe’s equation of state takes on extreme values, and this phase alone can solve the same puzzles (flatness, uniformity) that inflationary cosmology was designed to address.6Physics Reports. Ekpyrotic and cyclic cosmology
The ekpyrotic model also predicts a nearly scale-invariant spectrum of density fluctuations, the tiny variations in density that eventually grow into galaxies and galaxy clusters. These fluctuations come with a twist: the model predicts a significant non-Gaussian component, meaning the statistical distribution of the fluctuations is slightly skewed compared to the perfectly Gaussian pattern predicted by simple inflation.7Physics Reports. Ekpyrotic and cyclic cosmology This is one of the few places where the cyclic and inflationary pictures make different observational predictions, and future measurements of the cosmic microwave background or the distribution of galaxies could, in principle, distinguish between them.
Penrose’s Conformal Cyclic Cosmology
Roger Penrose proposed yet another way to make the universe cyclic, and it is perhaps the most conceptually radical. In conformal cyclic cosmology, the universe does not contract at all. Instead, it expands forever, but the infinitely distant future of one cosmic era smoothly transitions into the Big Bang of the next. Penrose calls each era an “aeon,” and the full history of the cosmos is an unending sequence of such aeons, with the far future of each one joining to the beginning of the next across a mathematical surface.8arXiv. The Physics of Conformal Cyclic Cosmology
The trick relies on a deep feature of physics called conformal symmetry. In the very far future, when all massive particles have decayed and only massless radiation remains, the universe loses its sense of scale. A universe that has expanded to incomprehensible size becomes geometrically indistinguishable from a very small, very hot one. This allows Penrose to “paste” the end of one aeon directly onto the beginning of the next without any contraction or singularity in between. In this framework, our universe is directly connected to both an earlier aeon and a later one, with the beginning and ending of our cosmos linked to those neighboring eras.9PubMed Central. The Great Loop: From Conformal Cyclic Cosmology to Aeon Monism
Conformal cyclic cosmology avoids the entropy problem in a clever way: it does not require entropy to reset. Instead, it redefines what counts as “low entropy” at the start of each aeon by tying entropy to gravitational degrees of freedom. Penrose argues that the gravitational entropy, measured by something called the Weyl curvature, is naturally very low at the transition even if the total entropy in radiation and matter is high. The model also dispenses with inflationary cosmology, which Penrose has long been skeptical of, replacing it with the smoothing effects of the aeon transition itself.
How Modern Cyclic Models Handle Entropy
The entropy objection that sank the original pulsating model remains the sharpest challenge for any cyclic cosmology, so modern versions have to address it head-on. Different models take different strategies, and some of the solutions are genuinely inventive.
In the Steinhardt-Turok cyclic model, which involves smooth (non-singular) bounces, the entropy after each bounce is naturally split: it is close to maximal in the matter and radiation sectors but close to minimal in the gravitational sector. This satisfies the conditions that Penrose conjectured are necessary for a cosmology consistent with what we observe, where the gravitational field starts out smooth and clumps up over time as structures form.10Physics Letters B. Entropy, black holes, and the new cyclic universe The trick is that the contraction phase and bounce manage to reset the gravitational entropy without violating the second law of thermodynamics for the total system.
Another approach invokes dark energy with an equation of state more extreme than a cosmological constant. In one model, a turnaround occurs extremely shortly before what would otherwise be a “Big Rip” (where dark energy tears everything apart), at which point both the volume and entropy of the universe decrease by an enormous factor. The entropy drops almost to zero at the turnaround, then builds back up during the next cycle’s inflationary expansion.11PubMed. Turnaround in cyclic cosmology A related proposal describes a cyclic universe where the average expansion over many cycles looks like a slowly expanding universe with a small effective expansion rate. Because the universe is on average expanding, entropy and compact objects from earlier cycles get diluted to irrelevance before the next cycle begins.12Physics Letters B. A new kind of cyclic universe
None of these entropy solutions is universally accepted. Each relies on specific physical assumptions that are difficult to test directly. But together they show that the entropy problem, while serious, is not the conversation-stopper it once appeared to be. Theorists have found at least three independent frameworks in which a cyclic universe can be made thermodynamically consistent.
Searching for Echoes from Before the Big Bang
If the universe has been through previous cycles, those earlier eras might have left faint imprints in the cosmic microwave background, the relic radiation from the early universe. Penrose and his collaborator Vahe Gurzadyan claimed to have found such imprints in the form of concentric circles with unusually low temperature variance in the microwave background sky, which they interpreted as the remnants of colliding black holes in the previous aeon.
Independent teams investigated the claim and found the circles, but at a much lower significance than Penrose and Gurzadyan had reported. When compared against simulated skies generated from standard cosmological models with random Gaussian fluctuations and realistic detector noise, the circles were indistinguishable from what you would expect by chance. None of the low-variance circles exceeded the three-sigma threshold that would indicate a genuine anomaly.13The Astrophysical Journal. Are There Echoes from the Pre-Big-Bang Universe? A Search for Low-Variance Circles in the Cosmic Microwave Background Sky The result did not rule out conformal cyclic cosmology, but it removed what would have been a striking piece of direct observational support.
Gravitational waves offer another potential window. Different cyclic models predict different signatures in the primordial gravitational wave background, the faint ripple of spacetime distortions left over from the earliest moments. Inflationary models generally predict a detectable level of these gravitational waves, while ekpyrotic models predict a much weaker signal. If next-generation gravitational wave observatories can measure (or set tight upper limits on) the primordial gravitational wave background, the result would be a powerful discriminator between inflation and cyclic alternatives.
Dark Energy Evolution and the Fate of Expansion
The discovery that the universe’s expansion is accelerating seemed to kill the original pulsating model, but dark energy itself might turn out to be more dynamic than the simplest models assume. If dark energy’s strength changes over time, or if its equation of state evolves, the long-term fate of the universe could be very different from the eternal expansion that a constant cosmological constant predicts.
One class of models involves “phantom” dark energy, where the energy density actually increases over time. In the most extreme version, this leads to a Big Rip, where the expansion eventually tears apart galaxies, stars, planets, and even atoms. A milder variant, called the “Little Rip,” avoids the formal singularity but still gradually disintegrates bound structures. One analysis found that in a Little Rip scenario, a system like the Sun and Earth would be pulled apart on a timescale of roughly 8,500 billion years, vastly longer than the current age of the universe.14Physics Letters B. Phantom cosmology without Big Rip singularity – Section: Scalar Little Rip cosmology These phantom energy scenarios are relevant to cyclic cosmology because some cyclic models use the approach to a Big Rip as the trigger for a turnaround into the next cycle, as described in the entropy section above.
Recent observations from the DESI collaboration have hinted that dark energy may indeed be evolving rather than constant, though the evidence is still preliminary. If confirmed, dynamical dark energy would reopen questions about the universe’s ultimate fate that most cosmologists had considered settled, and it would make certain cyclic models more physically plausible.
Cosmological Natural Selection
Lee Smolin proposed a speculative but fascinating twist on the pulsating idea. In his cosmological natural selection hypothesis, every black hole that forms in our universe gives rise to a new universe on the other side of its singularity. At the moment of this “bounce,” the fundamental physical constants undergo small random changes, analogous to mutations in biological evolution. Universes whose constants happen to favor the production of many black holes will, by definition, produce more “offspring” universes, and over countless generations the population of universes will be dominated by those with parameters tuned for maximum black hole production.15Classical and Quantum Gravity. Did the Universe evolve?
This is not the traditional pulsating model, since it involves branching rather than a single universe cycling through time. But it shares the core premise that singularities are not endpoints but transitions, and it offers an intriguing explanation for why the constants of nature seem finely tuned for complexity. If Smolin is right, our universe’s physical laws are not arbitrary; they are the product of a selection process that favors universes rich in stellar collapse. The hypothesis is extremely difficult to test, though Smolin has pointed out that it makes at least one falsifiable prediction: no neutron star should exist above a certain mass, because if it did, that would mean the constants of nature are not optimized for black hole production. Observations of neutron star masses have so far been roughly consistent with this, though the prediction is not tight enough to be decisive.
Stellar Pulsation and the Other Meaning of “Pulsating Theory”
Outside of cosmology, “pulsating theory” also refers to the physics of pulsating variable stars, and the two uses get confused more often than you might expect. Stars like Cepheid variables and RR Lyrae stars rhythmically brighten and dim because their outer layers expand and contract in a self-sustaining cycle. The driving mechanism behind most of these stellar pulsations is an opacity effect: when a layer of the star’s atmosphere compresses inward, it becomes denser and more opaque, trapping heat. The trapped heat builds pressure, pushing the layer back out. As it expands, the layer becomes more transparent, releases the heat, cools, and falls inward again. The cycle repeats indefinitely.
This process is deeply important to astronomy for a practical reason. Cepheid variables have a tight relationship between their pulsation period and their intrinsic brightness. By measuring how quickly a Cepheid pulses, astronomers can determine how bright it truly is, and by comparing that to how bright it appears from Earth, they can calculate its distance. Cepheids are one of the key rungs on the cosmic distance ladder, which is how astronomers measure the size and expansion rate of the universe. So stellar pulsation theory is not just a cousin of cosmological pulsation theory in name; it is one of the observational tools that produced the measurements that challenged and reshaped the cosmological version.
The physics of stellar pulsation is well understood and well confirmed by observation, in contrast to cosmological cyclic models, which remain speculative. But both involve the same intuitive pattern: a system oscillates because a restoring force pushes it back whenever it overshoots in either direction. In a pulsating star, that restoring force comes from radiation pressure and gravity. In a pulsating universe, it would have to come from something powerful enough to reverse the expansion or contraction of all of spacetime, which is why the cosmological version has always been the harder sell.