Why Wasn’t the Early Universe at Maximum Entropy?
No. The second law of thermodynamics states that entropy can only increase over time. If the early universe had begun in a state of maximum entropy, the cosmos would have reached heat death immediately after the Big Bang. Instead, the present universe contains burning stars, functioning heat engines, and life, which are only possible because the early universe started in a highly ordered, low-entropy state.
Table of Contents
Key Takeaways
- The second law of thermodynamics requires entropy to increase, so a universe born at maximum entropy could not later produce stars or life.
- Physicist Roger Penrose argued in a 2005 talk at the Isaac Newton Institute that no known explanation exists in general relativity, the standard model, or inflation theory for why the early universe had such low entropy.
- The cosmic microwave background radiation shows a blackbody spectrum, indicating that matter’s degrees of freedom reached thermal equilibrium, but this says nothing about the gravitational degrees of freedom.
- Physicist Charles W. Misner’s 1969 “mixmaster universe” model describes what a universe with fully equilibrated, strong gravitational modes would look like, a state the observed universe does not match.
- In Newtonian gravitational systems that can radiate away energy, particles tend to collapse into a single clump rather than spread out evenly, the opposite of how a gas fills a box.
Why a Low-Entropy Beginning Is Required
The existence of burning stars, working heat engines, and living organisms in the present-day universe implies that the early universe existed in a very low-entropy state. This observation shows that the universe’s initial conditions were extremely finely tuned toward order rather than disorder.
Roger Penrose, in a 2005 talk at the Isaac Newton Institute, noted that there is no known explanation for this fine-tuning within general relativity, the standard model of particle physics, or inflationary theory. This remains an open problem in theoretical cosmology.
Why the Early Universe Doesn’t Look Low-Entropy at First Glance
These ideas run counter to common intuition. Most people picture the early universe as an undifferentiated soup of hot gas, which sounds similar to how a heat-dead universe might appear. The natural question is: in what sense was the early universe not already equilibrated?
What the Cosmic Microwave Background Actually Shows
The cosmic microwave background radiation displays a blackbody spectrum, a curve that is normally interpreted as evidence of thermal equilibrium. However, this observation only demonstrates that the universe’s matter degrees of freedom, meaning the particles and radiation, had reached thermal equilibrium with each other.
The gravitational degrees of freedom were not in equilibrium. Standard cosmological models are constructed to be as simple as possible and contain no gravitational waves. The real universe presumably does contain gravitational waves, but they are very weak.
Charles W. Misner’s 1969 mixmaster universe model illustrates what a maximum-entropy universe would look like: the gravitational modes would be equilibrated with the matter degrees of freedom and would be very strong, unlike what astronomers actually observe.
How Gravity Breaks Ordinary Thermodynamic Intuition
Even in Newtonian mechanics, gravitating systems violate most people’s intuitive expectations about entropy and equilibrium, as physicist John Baez has discussed. If helium atoms are passed into a sealed box through an inlet valve, they quickly reach a maximum-entropy state in which their density stays nearly constant throughout the box.
A box of Newtonian gravitating particles behaves in the opposite way, provided the particles have some mechanism, such as radiation, for releasing energy into their surroundings. The particles eventually collapse into a single dense clump rather than spreading out evenly. The clump’s own entropy decreases during this collapse, but this decrease is more than offset by the increase in entropy of the surrounding environment, so the second law is not violated.
Frequently Asked Questions
Did the early universe start at maximum entropy?
No. If it had, the second law of thermodynamics would have prevented any further increase in entropy, meaning the universe would have experienced heat death immediately. The presence of stars, heat engines, and life today shows the early universe instead began in a highly ordered, low-entropy state.
What evidence shows the early universe had low entropy?
The existence of burning stars, functioning heat engines, and life in the present universe is evidence that entropy has been increasing since a low starting point. According to Roger Penrose’s 2005 Isaac Newton Institute talk, physics currently offers no explanation for why this initial state was so finely tuned.
Why doesn’t the cosmic microwave background’s blackbody spectrum indicate a high-entropy origin?
The blackbody spectrum of the cosmic microwave background radiation only confirms that matter’s degrees of freedom reached thermal equilibrium. It says nothing about the gravitational degrees of freedom, which remained far from equilibrium, unlike what would be expected in a true maximum-entropy universe.
What is the mixmaster universe model?
The mixmaster universe is a cosmological model proposed by Charles W. Misner in a 1969 Physical Review Letters paper. It describes a universe in which gravitational modes are fully equilibrated with matter and are correspondingly very strong, a scenario that does not match the gravitational waves observed in the real universe.
Why do gravitating particles behave differently from gas in a box?
Ordinary gas particles in a sealed box spread out to reach a state of nearly constant density, which is their maximum-entropy configuration. Gravitating particles that can radiate away energy instead collapse into a single clump, because that configuration, combined with the entropy released into the environment, represents the true maximum-entropy outcome for a gravitational system.
Sources
- Charles W. Misner, “Mixmaster Universe,” Physical Review Letters 22 (1969): 1071. Read the original paper
- Roger Penrose, 2005 talk at the Isaac Newton Institute. Watch the seminar recording
- John Baez, on gravitational entropy. Read John Baez’s explanation
PhD in physics. I teach physics at Fullerton College, a community college in Southern California. I enjoy writing, playing viola, brewing beer, climbing and mountaineering.
