If we’re to explore a hypothesis that could upend our understanding of black holes, I should first explain why I once asked if the earth expands with the universe. It doesn’t. Expansion is a large-scale phenomenon and gravitationally bound objects are thought to remain stable. Gravity may be weak compared to electromagnetism or the strong and weak nuclear forces, but it’s stronger than the pull of expansion. Even so, I had reason to ask. An object in freefall is stationary in its frame of reference. According to General Relativity, rather than falling to the ground, the earth is accelerating to meet us. Einstein compared gravity to standing in a rising elevator; the earth is always moving outward in every direction. I asked my question because, despite Einstein’s point, the earth isn’t blowing up like a balloon. It’s in a state of hydrostatic equilibrium where the pull of its own gravity is equal to the outward pressure of its matter and the geothermal processes inside. If the earth is accelerating in every direction without getting bigger, the idea that it stretches with the expansion of the universe was wrong but not absurd.
The mass of the earth curves spacetime, and it’s the time aspect I had forgotten. The earth accelerates at a constant rate across this curved geometry of the universe, but not in the three spatial dimensions. Our experience of gravity is a physical manifestation of the earth traveling through time.
By contrast, the dark energy of “empty” space creates a negative pressure that pushes space out, smoothly, in all directions. Mass has a center of gravity toward which it curves spacetime. Dark energy has no such center, and so the push that moves galaxies apart brings with it no curve, as long as we’re talking about empty space.
This is where black holes and the Cosmological Coupling Hypothesis comes into play. Standard cosmology sees the center of a black hole as a singularity. That’s a problem because information is destroyed in the singularity, and quantum mechanics says information can never be destroyed. Physicists have offered theories for how the universe might get around this, but the Cosmological Coupling Hypothesis does so by getting rid of singularities altogether. Behind the event horizon, a black hole is filled with dark energy.
The Cosmological Coupling Hypothesis gets its name from the fact that unlike the earth, a black hole doesn’t have a surface area of matter. Its surface area is the event horizon which itself is curved spacetime. According to the hypothesis, that couples the black hole to the fabric of the universe, and as the universe expands, so does the black hole. Meanwhile, the dark energy within the black hole has its own negative pressure pressing out against the event horizon, expanding the black hole in volume and in turn adding to the pressure that manifests as the expanding universe.
As a storyteller, I want to talk about the number of ways that observational cosmology has supported this hypothesis in recent months. Black holes in cold-gas galaxies are expanding despite a lack of mass-intake to trigger that expansion, and they are doing so at a rate that equals the predicted constant given by the Cosmological Coupling Hypothesis. Additionally, the Dark Energy Spectroscopic Instrument (DESI) has shown that dark energy might not be a constant but rather is evolving at a rate consistent with the birth and death of stars, and thus with the birth rate of black holes. However, physicists are more cautious than storytellers. The apparent proof could be a case of confirmation bias with the expansion being drawn from some unknown variable. The idea still needs time to prove itself.
Einstein’s insight that local gravity is identical to a uniformly accelerating frame of reference is known as the Equivalence Principle. The term dark energy was coined by Michael Turner, but it was discovered earlier, in 1998, through the Nobel prize-winning work of Saul Perlmutter, Brian Schmidt, and Adam Riess. They were the first to realize that the universe isn’t just expanding but that its expansion is accelerating. Kevin Croker is the theoretical physicist behind the Cosmological Coupling Hypothesis along with astrophysicist Duncan Farrah who also led the efforts to produce the first real-world observations supporting the concept.
Baryon Acoustic Oscillations (BAO) are patterns of galaxies across the universe, marking the death of great waves that once tore through the early universe. They act like a cosmic clock that DESI focused upon and which data Duncan Farrah studied to demonstrate the evolving nature of dark energy.
In the first 400,000 years after the Big Bang, the universe was much smaller and filled with a hot, dense plasma, too hot for electrons to attach to baryons (protons and neutrons). These free-moving electrons constantly collided with photons, keeping them coupled to the plasma. Roughly 380,000 years after the Big Bang, the temperature of the universe cooled to 3000 Kelvin, allowing baryons to trap electrons. For the first time photons were decoupled from the plasma, and over a few thousand years, space went from opaque to transparent. This escape of light gave us the Cosmic Microwave Background (CMB).
In the days of that hot plasma, particles would attempt to cluster due to gravity, only to be thrust apart by their own radiation. This created acoustic waves, spherical pressure ripples that ripped through the baryonic plasma, expanding at half the speed of light. When the universe cooled enough and light decoupled from the plasma and escaped, those waves froze as patterns of baryonic matter, influencing the formation of galaxies along these ancient rims.
On today’s scale, the most massive of those rings consistently have a radius of 150 megaparsecs or 490 million light-years. That consistency allows them to act as a standard candle, and if distance makes them appear smaller than is predicted by the redshift of their light, that tells us the intervening space has expanded since that light was emitted.
The standard model assumes dark energy is a constant, but the readings from DESI suggest dark energy is evolving; the expansion of space is accelerating, but that acceleration was stronger in the past and has recently begun to weaken. That weakening dark-energy curve matches a universe where dark energy is tied to cosmologically coupled black holes: star formation slows down; fewer coupled black holes are created; and the acceleration of impact from the universe’s dark energy weakens.
On a final note, the hypothesis also rescues small black holes, formed early in our universe, as a contender for dark matter. In the standard model, that doesn’t work because the black holes would have lost so much mass to Hawking radiation in that time that they would have since evaporated or exploded. A cosmologically coupled black hole, however, can achieve a stable survival boundary where the mass lost to Hawking radiation is equivalent to the mass gained from cosmic expansion.
— Thaddeus Thomas
P.S. All corrections are welcome.
This was the second essay in the series. You can find the first one here:
Once again, I’m reminded of when I asked my early elementary school teacher if it’s possible to divide a small number by a larger one. She lied and said no because I wouldn’t be ready for that possibility for a couple more years. The way I’m learning physics, there’s no determination made whether I’m ready for the answers my questions produce. I simply get my answers. Moreover, imagine if fractions and decimals were only hypothetical and not accepted math. In this analogy, I’m new to basic arithmetic, but if I ask the question, I’ll get my answer with very little concern over what’s settled and what’s theoretical.
That’s how I’m learning, and I try to share what interests me using much of the same structure I use to pursue my answers. I highlight connections between ideas, and these will regularly fall in a few areas: quantum mechanics, gravity and spacetime, deep space and dark energy, and black holes.
As for what’s theoretical and what’s settled, everything I touched upon today should be settled physics until we reach the Cosmological Coupling Hypothesis. I’ve noted that the standard model considers dark energy a constant, but I didn’t mention that it also states that gravity once held expansion back and only recently did acceleration of expansion begin. That’s very different from Farrah’s interpretations of the DESI information.
Baryon Acoustic Oscillations (BAO) and their birth in the early years of the universe seem settled. DESI is simply the instrumentation by which dark energy is studied. Farrah’s interpretation of those results makes certain assumptions that are being rigorously challenged because they support a hypothesis that would upend our understanding of black holes and dark energy, as well as potentially undermine the standard candles and rulers scientists have relied upon in their studies of the universe.