Bounded from the Unbounded

by Adam Hadhazy

Research highlights from Kavli Astrophysics Institutes

An MIT study finds that, for now, the catalog of known black hole binaries does not reveal anything fundamental about how black holes form. Pictured is a simulation of the light emitted by a supermassive black hole binary system where the surrounding gas is optically thin (transparent). Credits: Credit: NASA’s Goddard Space Flight Center

The Author

Adam Hadhazy

The mind staggers over the depth and breadth of everything out there. Just starting right here in our own solar system, we have a star, eight planets, five dwarf planets, and more than 200 moons. That's a lot to take in. Going out from there, our home galaxy contains more than we, in our biological brains, can ever process. Estimates hold that the Milky Way possesses upwards of 300 billion stars. Assuming our solar system is fairly run-of-the-mill, that equates to on the order of a trillion planets, easy, and something like 60 trillion moons, in our galaxy alone. (Pause here to let the brain catch up.)

Now if that weren't enough to boggle, consider that the universe is estimated to contain roughly, oh, a trillion galaxies. Lest making sense of such multitudes seem more than humans are capable of, though, all those commas and zeroes actually boil down to a comparatively very manageable set of physical principles—the laws of nature, which number less than a handful: electromagnetism, gravity, and the strong and weak nuclear forces. These four laws, acting across all of the universe from the very smallest to the very largest scales, determine how it all works. The power of mathematics—for instance via statistics, where a representative sample can tell us about the unassessable total—further enables researchers to corral the vastness of existence. These approaches of physics and mathematics—fleshed out further in fields such as astrophysics and cosmology, and exercised by researchers at Kavli Astrophysics Institutes, with ever-greater instruments and theoretical frameworks—are continuing to make the once-unknowable, knowable.

Recently, Kavli Institute-affiliated researchers have put out results and discussion on a rich array of subjects, from the primordial periods of the universe to considering the odds of intelligent life here in our Milky Way. Piecemeal, these researchers and their colleagues worldwide are making the depth and the breadth accessible and comprehendible.

A lot to be desired in deciphering black holes' spins

A key way that researchers expect to learn about the origin of black hole binaries—pairs of black holes closely orbiting each other before eventually merging—is by measuring the spin of the extreme objects. That spin speaks to the formation of the black hole duo. The favored formation pathways are either from a pair of massive stars that orderly orbited each other before collapsing into black holes, or two separate black holes that crossed paths in a jumbled star cluster. Spin rates of nearly 70 of these binary systems have been catalogued to date, suggesting ample examples for this determination of origin. Alas, a new study by members of the Massachusetts Institute of Technology's Kavli Institute for Astrophysics and Space Research (MKI) has demonstrated that the current data is insufficient. The MKI scientists showed that plugging in the known data yields different results based on models and assumptions used; accordingly, there is not a reliable way to infer the constituent objects' spins. In time, though, accumulating observations will help refine the models and should lead to a firmer understanding.

Bringing the missing matter in the universe to light

A recent study coauthored by a researcher at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at Tokyo University is helping to solve the so-called missing baryon problem. Baryons, such as protons and neutrons, are the "normal" matter in the universe. Although theoretically making up only about 5% of the universe's total mass-energy, a significant portion of these baryons still remain unaccounted for. Much of the missing matter is thought to be in the form of a hot plasma that filamentously connects galaxies; however, observing this speculated matter has proven difficult. Using old and new data from x-ray telescopes in space, researchers are starting to stack up observations of x-rays emitted by these filaments, helping to fill in the missing baryon gap. More than a mere clerical cleanup, accounting for this missing matter will help further refine galactic evolution and formation models.

Challenging prevailing cosmological theories with JWST

JWST, the most powerful telescope of its kind to ever ply the skies, has already gathered some surprising data over just its first year of service. JWST has, for instance, let us peer back deeper than ever before into the early universe, where the telescope has seen unexpected oodles of bright, well-developed, large galaxies merely a few hundred million years after the Big Bang. A story at Quanta Magazine covers this exciting development, quoting researchers at MKI and the Kavli Institute for Cosmological Physics (KICP) at the University of Chicago. At first glance, these gobsmacking galaxies would seem to severely challenge cosmological theories about the how and when galaxies first formed. Some of the galaxies have turned out though, upon further calibration and examination, to be not quite as distant and therefore theoretical envelope-pushing. Others may instead turn out to be new kinds of dusty galaxies, masquerading as more distant, developed ones. That said, though, at least four galaxies have had their extreme distances strongly confirmed, putting theorists in a pickle. One possibility is that the relation between stars' brightnesses and their masses—gathered from studies of the more modern universe—may be significantly different in the earliest cosmic times. Another x-factor is that early developing black holes could be gobbling up matter and making early galaxies appear brighter than expected. Only time, and more observations with JWST, will sort this brewing mystery out.

Identifying the responsible parties for a major cosmic sea change

Via its deepest-ever dives into cosmic history, JWST is also further helping explain a profound change the universe underwent known as reionization. After the cosmos initially cooled down from the Big Bang, neutral (non-ionized) atoms of mostly hydrogen could at last form. These atoms then collected in billions upon billions of batches to form the first generation of stars and galaxies. These stars, in turn, emitted high-energy radiation that re-ionized the vast medium of remaining ambient gas in the universe. Researchers at Kavli IPMU are studying early galaxies and their constituent stars to assess how this reionization unfolded. The work involves comparing galaxy size to galaxy luminosity in those primordial epochs. The efforts are offering a better sense of the kinds and numbers of the galactic sources necessary and responsible for reionization.

Gauging the likelihood of alien life (and not via blurry UFOs)

In a new Research Highlight, Jack Singal, a former member of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University, tackles the profound question of whether we are alone in the universe. Singal—who gave a KIPAC public lecture on the topic last year—picks up on the headline-generating releases in 2021 of footage declassified by the Pentagon allegedly depicting UFOs. The grainy, ambiguous footage, however, stands in contrast to genuine, verifiable knowledge we have gathered in recent years, Singal points out, about the actual likelihood of alien life existing in our galaxy. Continuing discoveries of exoplanets—worlds around other stars—and our increasing understanding of them is moving the state of the science forward at a rapid pace. Given all we already know, it is safe to say that worlds hosting plausible habitats for life as we know it should exist by the billions in the Milky Way Galaxy alone, Singal says. Just how common intelligent life is, though, remains harder to gauge. But given the evidence, for instance, that biological brains have evolved at least three different times on Earth alone suggests intelligence may not be rare, either.