21 August 2025
Joan Najita (NOIRLab)
DESI recently reported compelling evidence that dark energy is dynamic and evolves with time. What drives this evolution? In a recent paper, a group of DESI scientists propose that the evolution is driven by the creation of non-singular black holes, which converts ordinary (baryonic) matter into dark energy. The black holes form as a consequence of the evolution of massive stars and grow with the expansion of the Universe over cosmic time. The scientists’ model for this process not only provides a good fit to the existing constraints on dark energy, it also relaxes the cosmological constraints on neutrino masses, allowing for positive neutrino masses. We sat down with a few of the coauthors — Kevin Croker, Greg Tarlé, and Steve Ahlen — to learn more about this intriguing result.
Q: How is baryonic matter converted into dark energy in this scenario?
Greg: We don’t yet know how this works exactly. We do know that in the very early universe, inflationary vacuum energy was eventually converted into the matter of our universe. If you ask yourself the simple question, “Where in the current universe are conditions of high density, intense gravity, and curvature comparable to those in the very early Universe,” there is only one place, near the center of a forming black hole. Something must happen to stop an unphysical singularity from forming. We believe that it is the formation of dark energy from matter, like a little Big Bang played in reverse.
Steve: The standard form of our model involves the conversion of matter to dark energy inside black holes during gravitational collapse. If I allow myself to speculate, one possible mechanism for this is the reverse of the electro-weak phase transition whereby the Higgs scalar field acquires energy from the collisions of high energy massive particles (the inverse of the “Higgs mechanism”).
Q: What is a “non-singular” black hole? And why is it the right kind of black hole to create dark energy?
Kevin: Technically, it’s a solution to Einstein’s equations that is identical to the Schwarzschild black hole solution outside the Schwarzschild radius, but contains dark energy inside the Schwarzschild radius. The Schwarzschild black hole solution is empty on the inside, and the mathematical description of the inside breaks where you’d expect the object’s center to be. That’s the singularity part. But if you have dark energy inside your black holes, the object is stabilized and there are no singularities. Some solutions even lack one-way layers (horizons), so the dark energy on the inside can influence, and be influenced by, the Universe outside.
Q: Earlier DESI papers seemed to push us toward negative (unphysical) neutrino masses. What is the physical argument for this interpretation? And how does your new model ease the tension and allow for positive neutrino masses?
Kevin: In interrogating the universe’s energy density budget, the DESI data (from the late-time Universe) prefer a smaller total matter contribution than do data from the early universe (from cosmic microwave background experiments). Because neutrinos contribute to the total matter budget, when other types of matter are assumed to remain unchanged as the universe grows, the data push the neutrino contribution to negative values in order to reconcile the two distinct measurements.
Under the Cosmologically Coupled Black Hole (CCBH) hypothesis, matter is converted into dark energy because stars (made of matter) collapse to non-singular black holes (made of dark energy). This naturally decreases the amount of matter today, relative to the early universe. It turns out that this decreases the amount of matter in the late universe by just the right amount for neutrinos to contribute as we know they must from terrestrial neutrino experiments.
Q: Fascinating! A couple more questions about how the CCBH picture works. As a human, the idea that dark energy continues to increase as the Universe expands seems to violate some kind of conservation law, like we’re somehow getting a free lunch: dark energy causes the Universe to expand, which creates yet more dark energy. How is this explained in your picture?
Greg: In the CCBH model, the dark energy interiors of black holes are coupled to the external expanding universe. Dark energy is characterized by negative pressure, so the dark energy interiors of black holes do negative pressure-volume work on the universe as it expands. To balance this negative work, the interior dark energy must grow as the universe expands. To an outside observer, the mass of the black hole will therefore grow over cosmic time. This is all consistent with Einstein’s General Theory of Relativity, and remarkably, there are enough black holes formed from heavy stars in the Universe for CCBHs to account for the amount of dark energy we observe. So there is no free lunch.
Kevin: Regarding the apparent free lunch, the catch here is that, when your universe isn’t empty, energy is no longer conserved in Einstein gravity. You can lose energy to the gravitational field, like photons do (that’s why they redshift). But you can just as easily gain energy from the gravitational field too. In the Cosmologically Coupled Black Hole (CCBH) hypothesis, non-singular black holes “tap into” the gravitational field and gain energy in this way.
Q. How does the dark energy created in the black hole lead to the acceleration of the expansion of the universe? In the usual picture in which dark energy pervades all of space, it seems easy to imagine it pushing the Universe everywhere to expand. But here, I’m guessing that the dark energy created inside a CCBH doesn’t escape out into the Universe (because nothing escapes a black hole). If that’s the case, how does the concentration of dark energy in small regions (the insides of black holes) cause the Universe’s expansion to accelerate in a uniform way?
Kevin: You’re right that dark energy doesn’t escape from black holes. It sits inside, stabilizing them. As to how it influences the expansion of the Universe: Friedmann’s equation, the one that determines the cosmological behavior, does not care where the dark energy is. My thesis work with the differential geometer Joel Weiner established this back in 2018. This is the counter-intuitive result, and seven years later, it is still not widely understood. The result is technical, but it can be understood from symmetry as follows: the only gravitational degree of freedom in the Robertson-Walker cosmological model is the scale factor. The scale factor is defined to have only dependence in time. This means that its dynamics cannot have any spatial awareness. But without spatial awareness, the scale factor cannot discriminate between the inside or outside of anything. So cosmological dynamics are determined only by what is in the universe, not where it is.
Q. As another angle on that question: most CCBHs will reside in galaxies (i.e., where stars have formed). Does the higher density of CCBH in galaxies affect galaxy dynamics, e.g., cause them to expand? Or if not, why do CCBHs drive the expansion of the Universe more than their local environment?
Kevin: Great question! How CCBHs act on scales below galaxy cluster scales is not yet computed. We’ve only done the calculations through linear order. Observationally, they cannot cluster as strongly as cold dark matter, otherwise certain observed dwarf galaxies like Eridanus II would likely have been shredded. Local mass growth (CCBH mass proportional to scale factor cubed), if present, helps to evade these constraints, so we’ve not had cause to visit a detailed calculation yet. Above cluster scales, we demonstrated a scenario (published in ApJ back in 2020) where the CCBHs disperse toward genuine uniformity. This was more a proof of concept that you could build the model so that galaxies didn’t get destroyed. We’re currently working on a more comprehensive picture, including continual input from star formation. Whether or not CCBH will pass this important first-order test remains to be seen!
As far as local vs. global dynamics, I feel it important to emphasize my position that Friedmann’s equations and the linear perturbation equations, at late-times, only make sense as effective theories. In the early universe, everything was so hot and dense, that it really was a uniform soup with small perturbations. But 13 billion years later, the universe is most certainly not isotropic and homogeneous (look out the window). Modeling the late-time universe as a Robertson-Walker cosmology in a mathematically consistent way then requires that a stress-energy inventory determine the global expansion rate. Not “where,” only “what.”
Q: What drew you to this problem?
Kevin: The connection to neutrino masses was serendipitous. We were working with the DESI public year-one data, and saw that the CCBH hypothesis gave dark energy which tracked the best-fit DESI models using the star formation rate alone. In passing, Greg mentioned that there was some sort of trouble with the neutrinos. I forgot about this for a few days and then things just sort of clicked: we’re consuming more matter to make things work at later times, so that might open up space for the neutrinos. So providing a way to fix the neutrino mass problem just dropped out of the wash.
Greg: My connection to this line of research was also serendipitous. In 2018, when I took a sabbatical at the University of Hawaii Institute for Astronomy, this passionate young theorist named Kevin Croker came into my office, handed me some dense and obscure General Relativity papers he wrote and wanted to know whether DESI (which hadn’t started taking data yet) could measure the effect he predicted. I hadn’t touched a General Relativity calculation in over 44 years since graduate school. Because he was so enthusiastic, I didn’t have the heart to tell him I didn’t have the time to read his papers. It took me five full passes through the papers before I could understand them and follow the equations. I couldn’t find anything wrong with the theory but, at the time, I thought the effect would be too small to see with DESI. Nevertheless, I joined Kevin and a small group of physicists and astronomers who searched for places where the impact of CCBHs could be measured. In the last three years we found three independent observational signatures; SMBH growth without accretion or merger in passively evolving elliptical galaxies, DESI DR1 dark energy density tracking star formation history and now, the easing of the DESI neutrino mass tension with DESI DR2 data. It took the amazing data that only DESI can now provide to observe the effects of CCBHs on cosmology.
Q: Was the result what you expected to find? Were there any surprises?
Steve: The result that neutrino masses could be constrained and found to be consistent with the experimental particle physics results — this is the result I hoped for. What was surprising and unexpected was how well the cosmology neutrino mass agreed with the particle physics mass if we allowed for the possibility that some matter is lost in being converted to dark energy.
Kevin: While the result was consistent with my expectations, it was a bit surprising how “flexible” the standard model, Lambda CDM, is. The neutrino masses push up against zero in LCDM when you allow them to do so. But if you only allow them to float down to physically measured lower-bounds, the LCDM model will adjust itself in various ways so that there is very little statistical penalty. So it’s really going to take a confluence of observational constraints to definitively pin things down.
Q: What’s next for you?
Steve: I am personally interested in applying some of the same principles of CCBH/Dark Energy to study the early Universe. In particular, it seems possible to me that “Cosmologically Coupled Topological Defects” may provide a mechanism for the accelerated expansion (known as “Inflation”) in the first fraction of a second after the beginning of the Universe during the so-called Grand Unified Theory phase transition. I find it appealing to contemplate that the very early and the very recent Universe could be explained by similar physical mechanisms.
Kevin: The success of such a simple model has drawn excellent critical attention. While things work well at cosmological scales, investigations by other groups at the scale of individual black holes paint an observational picture far less coherent. For example, if you know a black hole’s birthday and mass, you can figure the smallest its mass could be today. Some clever studies figured out ways to get birthdays, but made typical assumptions about birth masses. If you do that, black holes don’t grow fast enough to contain dark energy. But we don’t know the smallest birth mass for black holes without horizons, and if you set it to 25% of the typically assumed value of 2 suns, then things stay consistent. There are many assumptions that have to be carefully revisited when studying non-singular black holes. This is my primary focus these days.
Greg: I plan to continue developing this picture through a more detailed analysis of its cosmological implications. I am thrilled that many DESI collaborators have decided to join me on this exciting adventure.
Read more in the University of Michigan press release.
