Katherine Freese on Dark Matter Stars in the Early Universe

Astronomers have long hypothesised that the first stars that formed after the Big Bang were a type they rather confusingly named “Population III” (with the “III” indicating only that they were the third stellar population named). Since the Big Bang produced only hydrogen and helium (plus trace quantities of deuterium and lithium), these first stars would have been composed only of these elements, and the dynamics of stellar formation in the absence of heavy elements (in particular, carbon) would have allowed them to be extremely massive, hundreds of solar masses, with correspondingly short lives (2 to 5 million years), after which they would explode as supernovæ and enrich the interstellar medium with heavy elements fused in their interiors, from which the next generation (“Population II”) stars would form. So far, there has not been an unambiguous observation of a Population III star—that is one of the goals of the James Webb Space Telescope’s (JWST) observations of the early universe. See the 2023-02-06 post, “Has the James Webb Space Telescope Spotted Population III Stars?” for a possible detection of these objects.

The theory of Population III stars was developed before observational evidence suggested the universe contained some form of dark matter with mass much larger than the conventional matter from which stars and everything else visible are composed. In 2007, Katherine Freese and co-authors published a paper, “Dark matter and the first stars: a new phase of stellar evolution” (full text at link), which considered the effect dark matter may have had on formation of the first stars in the early universe. Here is the abstract.

A mechanism is identified whereby dark matter (DM) in protostellar halos dramatically alters the current theoretical framework for the formation of the first stars. Heat from neutralino DM annihilation is shown to overwhelm any cooling mechanism, consequently impeding the star formation process and possibly leading to a new stellar phase. A "dark star’’ may result: a giant (≳1 AU) hydrogen-helium star powered by DM annihilation instead of nuclear fusion. Observational consequences are discussed.

This argued that in a hot and dense early universe dominated by dark matter, if it was in a form where energy could be released by particle-antiparticle annihilation, it would allow the formation of enormous “dark stars” with radii as large as 960 astronomical units, masses up to a million times that of the Sun, and the luminosity (in deep infrared) of billions of Suns. The heat generated by annihilation of dark matter would be sufficient to keep these “stars” from gravitationally collapsing to a density where nuclear fusion could begin.

The dark star hypothesis was intriguing, but with no observational evidence for the existence of such objects at hand, remained one of many speculations about the nature of the early universe. In April 2023, Freese and two co-authors published, “Supermassive Dark Star candidates seen by JWST?” (full text at link), which suggested that three luminous objects observed by the JWST at extreme redshifts are consistent with the expectations for dark stars. This is the abstract:

The first generation of stars in the Universe is yet to be observed. There are two leading theories for those objects that mark the beginning of the cosmic dawn: hydrogen burning Population~III stars and Dark Stars, made of hydrogen and helium but powered by Dark Matter heating. The latter can grow to become supermassive (M_\star\sim 10^6M_\odot) and extremely bright (L\sim10^9L_\odot). We show that each of the following three objects: JADES-GS-z13-0, JADES-GS-z12-0, and JADES-GS-z11-0 (at redshifts z∈[11,14]) are consistent with a Supermassive Dark Star interpretation, thus identifying, for the first time, Dark Star candidates.

The discussion of dark stars and the observational evidence begins at the 29 minute mark in the video.


Excellent episode. I often find myself distracted in Brian’s interviews because he can’t always carry his guests through the dry spots in their discussions.

Definitely not a problem with Katherine. The irony that she was holding my attention during the discussion of teaching communication skills to scientists was not lost on me. :grin:


At this time code they gloss over the primary question that I wanted to watch this video for: Since dark matter is nearly 90% of the mass of the universe even today, why aren’t there dark stars all over the place in the later universe? They just kind of patted me on the head and said, “Don’t worry your pretty little head, Jimmy. Trust us.”


I think the main argument is that the era during which dark stars could form was a relatively short period in the very early history of the universe. Before long, cosmic expansion would have reduced the density of dark matter and ordinary matter below that at which dark matter annihilation could release sufficient energy to prevent gravitational contraction of primordial gas and formation of stars. The latter process is vastly more efficient in producing radiating bodies than dark star formation—while a galactic mass of primordial gas may produce only one dark star (albeit huge and luminous), it may yield hundreds of billions of ordinary stars.

In the 2007 paper, they discuss whether dark stars could have survived to the present day. If the annihilation rate of the dark matter is sufficiently low, this is possible, but they would occur only within intergalactic space and be detectable by anomalous emission of energetic radiation. They would also be extremely rare compared to ordinary stars, so easily overlooked.

One intriguing possibility is that dark stars might have provided the seeds for the supermassive black holes found in the cores of many galaxies today. How that much mass could have accreted so early in the universe is an unsolved puzzle today. In the 2007 paper they note, “Alternatively the initial protostellar object may be larger, and dark stars might accrete enough material to form large black holes en route to building the (as yet unexplained) 10^9M_⊙ black holes observed at z ∼ 6.”


Why do people think I went through ALL of Wolfram’s nearly ONE THOUSAND particles to see which of them resulted in a dimensionless gravitational scaling constant closest to 2^127 – in expectation that the proton would be the best fit – which it was?

Does ANYONE have an explanation for this? I ask in this context because it seems to me it points toward protons and neutrons as comprising most of the gravitational mass of the universe, dark or light.


Her answer may have been related to that but she said nothing about density of dark matter in the early (~200Myr) universe when she said, “As soon as you have different (nuclear) chemistry going, it’s different.”

In other words “formation of stars” is “different (nuclear) chemistry” – but it begs the original question:

What is it about “different (nuclear) chemistry” (ie: “formation of stars”) that prevents the formation of dark stars?

Is it the higher density of dark matter in the early universe that enabled the formation of dark stars ~200Myr but the expansion since then that reduced it to insignificance? If so, has the density of matter in the universe changed that much since the ~200My mark? Or is it that somehow regular star formation disrupts dark star formation? If so what is the interaction that does so in, say, galactic halos of dark matter where there is more than enough space for them to form?


That is how I understand it. Here is a Cosmology Calculator where you can plug in a redshift and get the age of the universe, lookback time, and other factors including critical density (average density assuming a flat spacetime). If I enter redshift 20, around where they discuss dark stars forming, it gives an age of the universe around 180 million years and critical density 2.4897e-26 g/cm³, while for redshift 0 (today), the density is 8.4441e-30 g/cm³, or almost a factor of 3000 times less. That should be far more than needed to space out the dark matter so annihilation rarely happens except in regions like close to supermassive black holes where gravity concentrates it. In the early universe, the idea is that the gravity of an overdense concentration of molecular gas was enough, and once annihilation started, it would cause the dark matter, which usually does not interact except through gravity, to lose energy and become further concentrated within the dark star.


In that case, galactic centers should provide neutralino annihilation signatures as additional evidence of supersymmetry. But I don’t think this addresses the core cusp tension which empirically would require about 1.7cm^2/gm collisional cross section for neutralinos.

Using a neutrino interaction cross section estimate of 10e-40m^2 to 10e-45m^2, and the dark star paper’s estimate of neutralino mass of 50GeV to 2TeV, the collisional cross section per mass ratio (relevant for core cusp tension comparisons) range from 2.8e-20cm^2/gm to 1.1e-13cm^2/gm.