The Seasons of Space

by Adam Hadhazy

The changing of the seasons is a familiar experience here on Earth and, as it turns out, plenty of other elsewheres, too, with astrophysical and cosmological significance

Certain stars undergo a flash at the end of their lives, which astronomers can use as a measuring stick to estimate how fast the universe is expanding. Image courtesy of ESA/NASA

The Author

Adam Hadhazy

As summer draws to a close here in Earth's northern hemisphere, it’s a special time to ponder the comings and goings of the seasons of our planet. But a number of Kavli-affiliated researchers spend year-in, year-out on deciphering currents of seasonality and their consequences at the scales of worlds, galaxies, and the universe writ large. It's actually possible that our entire cosmos has a seasonality to it—a time to expand, and a time to contract. If so, we're roughly in year 13,800,000,000 of an expansion since the (most recent?) Big Bang. And while much of the scientific evidence we have to date suggests the universe will keep expanding forever, numerous, so-called cyclical cosmological models have called for an eventual contraction, back to a "Big Crunch," after which the whole shebang re-Bangs, so to speak, starting all over again. Alternatively, the universe might expand for trillions upon trillions of more years, thrown wide open and eventually rent apart by dark energy in the "Big Rip." That sundering could—who knows?—be the genesis of another universal go-around, serving as a dramatic end to what was precedingly a very, very long cosmic season.

Strongly tying supermassive black holes' heft to suppressed starmaking

A longstanding debate in astrophysics centers on how much supermassive black holes can suppress the formation of new stars in their host galaxies, versus other mechanisms. A new study led by researchers at the Kavli Institute for Cosmology, Cambridge (KICC) has now found that a supermassive black hole's mass is indeed a dominant factor in curbing star formation. The researchers created a simulation that churned out different versions of a representative segment of the universe, based on different astrophysical mechanisms theorized to impact star formation. The three mechanisms separately simulated were energy injection into the galaxy by supermassive black holes (which tends to go up with a black hole's mass), supernova explosions (known to suppress star formation locally), and shock heating of the gas around galaxies, called halos. The results of the simulations were then compared to observations by the long-running Sloan Digital Sky Survey. The sims with varying black hole mass strongly matched real life's variations in how hard galaxies pumped the brakes on starmaking.

The best of both cosmological modeling worlds

Nick Kokron, a PhD student studying at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University, describes in a Highlights post how he and KIPAC faculty are improving the models of the universe's evolution that will be used to understand the torrent of cosmic observational data gathered by next-generation instruments this decade. As Nick writes, both precision and accuracy must be achieved. In a wonderful analogy, Nick explains how a model ought to be precise, like a dart-thrower who always lands their darts near each other on a board. But the model must also be accurate; it's of little use if the area peppered by the darts is nowhere near the actual target. To ensure that models accurately describe reality with precision, researchers are combining paper-and-pencil calculations and numerical simulations by computers.

Ingredients for building a galaxy (from left to right): Particles of baryonic, or normal, matter; dark matter density; dark matter density squared; and tidal forces. (Credit: Kokron, et al., 2021.)

The former calculations, often relied on by particle physicists, deal first with the biggest factors contributing to the behavior of a system, and by doing so work down to surprisingly good approximations of, say, the universe's overall development. Computer simulations capture finer-grained details and are needed to fully fill in the model. The hybrid approach of pencil-and-paper and computers looks like it will help deliver key new insights into how the universe got from the Big Bang to now.

Getting the universe's expansion rate just right

So, 67 or 72? For cosmologists, these numbers are very important. The numbers reflect two well-supported, measured rates of the so-called Hubble constant, the universe's expansion rate. Nailing down the Hubble constant is integral for calculating the universe' age, understanding its evolution, and gauging its fate. One method based on observations of the universe's oldest, most distant light, the cosmic microwave background, yields the lower figure of an expansion rate of 67.4 kilometers per second per megaparsec (km/s/Mpc). Measurements of a special kind of flickering star in the local universe yield the higher 72 km/s/Mpc figure. Wendy Freedman, a professor of astrophysics at the Kavli Institute for Cosmological Physics (KICP) at the University of Chicago, has been a leader in the research efforts arriving at the higher rate. In recent years, though, she and her colleagues have begun analyzing giant red stars as an independent way of measuring the Hubble constant. Refining that new technique has led to a new figure of 69.8 km/s/Mpc, splitting the difference and suggesting that invoking novel physics to explain the rate discrepancies are likely not necessary, as some have thought. Future observations will bear out whether researchers have indeed come close to resolving the mystery.

Testing for life's possible origins off-Earth

Presumably, Earthly life started right here, planetside. But the tantalizing possibility remains that terrestrial life actually might be extraterrestrial in origin, having started somewhere else, perhaps on asteroids or even in the void of space itself, where we've learned that a surprising amount of complex chemistry can occur. A collaboration of researchers, including members at the Massachusetts Institute of Technology's Kavli Institute for Astrophysics and Space Research (MKI), has devised an experiment to further explore the off-world possibility. The experiment, dubbed Ø-scillation, is examining the effects of differing gravities on the rates of certain chemical reactions with relevance to prebiotic (prior-to-life) chemistry. The experiment flew on a parabolic flight this past May, which through quickly dropping an aircraft's altitude can provide 20-second periods of zero gravity. Stay tuned for results.

Second "TESS-apalooza" science conference held

Credit NASA/JPL-Caltech

In early August, close to 700 scientists worldwide virtually attended the second TESS Science Conference. TESS stands for the Transiting Exoplanet Survey Satellite, a seeker of worlds that launched back in 2018 and has already discovered dozens of alien planets and logged thousands of more candidates. Researchers at MKI spearheaded TESS's development and held the conference, which covered topics including stellar astrophysics, asteroseismology, cosmic geochronology, asteroids, and even the ongoing search for the hypothetical Planet 9 and extraterrestrial intelligences. The vast range of topics covered shows how the open access nature of TESS's data is empowering scientists, both professional and amateur, to advance multiple fields.