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Starts With A Bang

5 consensus ideas in astronomy that might soon be overturned

From black holes to dark energy to chances for life in the Universe, our cosmic journey to understand it all is just getting started.
quasar-galaxy hybrid
This tiny sliver of the GOODS-N deep field, imaged with many observatories including Hubble, Spitzer, Chandra, XMM-Newton, Herschel, the VLT, and more, contains a seemingly unremarkable red dot. That object, a quasar-galaxy hybrid from just 730 million years after the Big Bang, may be key to unlocking the mystery of galaxy-black hole evolution. Once speculative, the evidence for the physical existence and ubiquity of black holes is now overwhelming.
(Credit: NASA, ESA, G. Illingworth (UCSC), P. Oesch (UCSC, Yale), R. Bouwens (LEI), I. Labbe (LEI), Cosmic Dawn Center/Niels Bohr Institute/University of Copenhagen, Denmark)
Key Takeaways
  • With just a few ingredients, like the laws of physics, the contents of the Universe, and a set of initial conditions, we can make sense of almost all of the entire Universe.
  • But there are some aspects of the Universe that we think we've made sense of that might not quite pan out the way we've assumed they will.
  • Here are 5 ideas in astronomy, currently accepted by most astronomers, where the coming decades might rudely awaken us to their fundamental flaws.

Since 1920, we’ve determined the size, scope, and origin of the observable Universe.

dark energy
The farther away we look, the closer in time we’re seeing toward the Big Bang. As our observatories improve, we may yet reveal the very first stars and galaxies, and find the limits to which, beyond them, there are none. Even though the more distant objects recede from us incredibly rapidly, the expansion of the Universe obeys a very simple relationship between distance and apparent recession speed, with the relationship given by what we (dubiously?) call the Hubble constant.
(Credit: Robin Dienel/Carnegie Institution for Science)

Cosmic inflation preceded the Big Bang, forming atomic nuclei, atoms, stars, and galaxies successively.

The quantum fluctuations that occur during inflation get stretched across the Universe and when inflation ends, they become density fluctuations. This leads, over time, to the large-scale structure in the Universe today, as well as the fluctuations in temperature observed in the CMB. New predictions like these are essential for demonstrating the validity of a proposed fine-tuning mechanism and to test (and potentially rule out) alternatives.
(Credit: E. Siegel; ESA/Planck and the DOE/NASA/NSF Interagency Task Force on CMB research)

Still, many aspects of our standard picture remain uncertain.

quasar-galaxy hybrid
This tiny sliver of the GOODS-N deep field, imaged with many observatories including Hubble, Spitzer, Chandra, XMM-Newton, Herschel, the VLT, and more, contains a seemingly unremarkable red dot. That object, a quasar-galaxy hybrid from just 730 million years after the Big Bang, may be key to unlocking the mystery of galaxy-black hole evolution. Once speculative, the evidence for the physical existence and ubiquity of black holes is now overwhelming.
(Credit: NASA, ESA, G. Illingworth (UCSC), P. Oesch (UCSC, Yale), R. Bouwens (LEI), I. Labbe (LEI), Cosmic Dawn Center/Niels Bohr Institute/University of Copenhagen, Denmark)

Here are five potentially incorrect preliminary conclusions.

dark energy
Various components of and contributors to the Universe’s energy density, and when they might dominate. Note that radiation is dominant over matter for roughly the first 9,000 years, then matter dominates, and finally, a cosmological constant emerges. (The others do not exist in appreciable amounts.) Neutrinos first behave as radiation, and later, as matter. However, dark energy may not be a cosmological constant, exactly, and could evolve if we’ve incorrectly assumed its nature.
(Credit: E. Siegel / Beyond the Galaxy)

1.) Dark energy is a cosmological constant.

Measuring back in time and distance (to the left of “today”) can inform how the Universe will evolve and accelerate/decelerate far into the future. By linking the expansion rate to the matter-and-energy contents of the Universe and measuring the expansion rate, we can come up with a value for a Hubble time in the Universe, but that value isn’t a constant; it evolves as the Universe expands and time flows on.
(Credit: Saul Perlmutter/UC Berkeley)

Distant galaxies recede ever faster as time goes on: observationally demonstrated since 1998.

Pantheon+
The latest constraints from the Pantheon+ analysis, involving 1550 type Ia supernovae, are entirely consistent with dark energy being nothing more than a “vanilla” cosmological constant. There is no evidence favoring its evolution across either time or space, but any deviation from w = -1 and w_a or w’ equaling 0 would totally alter the presumed fate of our Universe.
(Credit: D. Brout et al./Pantheon+, ApJ submitted, 2022)

But dark energy could either strengthen or weaken.

big crunch
The far distant fates of the Universe offer a number of possibilities, but if dark energy is truly a constant, as the data indicates, it will continue to follow the red curve, leading to the long-term scenario frequently described on Starts With A Bang: of the eventual heat death of the Universe. If dark energy evolves with time, a Big Rip or a Big Crunch are still admissible, but we don’t have any evidence indicating that this evolution is anything more than idle speculation.
(Credit: NASA/CXC/M. Weiss)

The forthcoming EUCLID and Nancy Roman telescopes could discover quintessence, instead.

dark energy
This illustration compares the relative sizes of the areas of sky covered by two surveys: the upcoming Nancy Roman Telescope’s High Latitude Wide Area Survey, outlined in blue, and the largest mosaic led by Hubble, the Cosmological Evolution Survey (COSMOS), shown in red. In current plans, the Roman survey will be more than 1,000 times broader than Hubble’s, revealing how galaxies cluster across time and space as never before, and enabling the tightest constraints on dark energy of all-time.
(Credit: NASA/GSFC)

2.) Stars predate black holes.

The anatomy of a very massive star throughout its life, culminating in a Type II Supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. If the core of this star is massive enough, it will produce a black hole when the core collapses.
(Credit: Nicolle Rager Fuller/NSF)

Theoretically, black holes first arise from stellar corpses.

The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time.
(Credit: NASA/ESA/C. Kochanek (OSU))

But the Big Bang could permit primordial black holes.

Primordial Black Holes
If the Universe was born with primordial black holes, a completely non-standard scenario, and if those black holes served as the seeds of the supermassive black holes that permeate our Universe, there will be signatures that future observatories, like the James Webb Space Telescope, will be sensitive to.
(Credit: European Space Agency)

Cold, massive gas streams could also birth black holes, predating stars.

supermassive black hole
This snippet from a supercomputer simulation shows just over 1 million years of cosmic evolution between two converging cold streams of gas. In this short interval, just a little over 100 million years after the Big Bang, clumps of matter grow to possess individual stars containing tens of thousands of solar masses each in the densest regions. This could provide the needed seeds for the Universe’s earliest, most massive black holes, as well as the earliest seeds for the growth of galactic structures.
(Credit: M.A. Latif et al., Nature, 2022)

3.) Jovian planets protect terrestrial ones.

During Voyager 1’s 1979 flyby encounter with Jupiter, a brief “point” of light was seen on Jupiter’s surface, representing the first observed bolide event in Jupiter’s atmosphere. Jupiter experiences several thousands of times as many such events as Earth does, at minimum, as its gravity draws large numbers of objects into it that wouldn’t strike it, despite its massive size, otherwise.
(Credit: NASA/JPL/Voyager 1)

Most potentially hazardous Solar System objects strike Jupiter, not Earth.

4 seconds of video, looped here, is sufficient to show the entirety of the September 13, 2021 impact event that occurred on Jupiter, as seen from Earth.
(Credit: José Luis Pereira (Brazil))

But simulations indicate Jupiter increases the terrestrial impact rate ~350%.

The animation depicts a mapping of the positions of known near-Earth objects (NEOs) at points in time over the past 20 years, and finishes with a map of all known asteroids as of January 2018. It’s vital that we recognize that the most hazardous asteroids of all, i.e., the ones that cross Earth’s orbit the most frequently, have largely not been characterized at all. Although Jupiter absorbs many asteroids and comets, it can also redirect them, potentially further endangering the Earth.
(Credit: NASA/JPL-Caltech)

Perhaps giant planets are foes, not friends.

A to-scale size comparison of Earth and Jupiter. If we look at these two worlds in terms of cross-sectional area alone, Jupiter’s is 125 times as great, which should lead to a collision rate with asteroids and comets 125 times as large as Earth’s. But the actual rate is much, much larger, owing to Jupiter outmassing Earth by a factor of ~317. Jupiter’s gravitational attraction, combined with its size, results in a collision rate that’s 10,000+ greater than Earth’s collision rate with interplanetary objects.
(Credit: NASA; Brian0918 at English Wikipedia)

4.) Most of the galaxy is uninhabitable.

Among its many discoveries, the ESA’s Gaia mission has found that the Milky Way galaxy not only has a warp to its galactic disk, but that the warp in the disk precesses and wobbles, completing a full rotation for roughly every three revolutions of the Sun (in yellow) around the galactic center. Most astronomers assume that regions with too many stellar cataclysms in them, like the centers of galaxies, may be completely uninhabitable. But this picture is far from certain.
(Credit: Stefan Payne-Wardenaar)

Are galactic centers too energetically variable for life?

Most galaxies contain only a few regions of star-formation: where gas is collapsing, new stars are forming, and ionized hydrogen is found in a bubble surrounding that region. In a starburst galaxy, pretty much the entire galaxy itself is a star-forming region, with M82, the Cigar Galaxy, being the closest one with those properties. The radiation from hot, young stars ionizes a variety of atomic and molecular gases, particularly in the galaxy’s central region. Flares, supernovae, and radiation will be common in these environments, but not necessarily so ubiquitous that life thriving and sustaining on a world will be impossible.
(Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (National Science Foundation))

The “galactic habitable zone” remains dubious.

Although research from the early-2000s professed that habitability should only be possible in an annular ring surrounding most Milky Way-like galaxies, with low metallicity and frequent stellar cataclysms and/or dense gravitational interactions disfavoring life in the outermore or innermore regions, that research has been called into question, particularly concerning the inner galactic regions.
(Credit: NASA/Caltech)

Common cataclysms might not forbid planetary habitability.

This color-coded map shows the heavy element abundances of more than 6 million stars within the Milky Way. Stars in red, orange, and yellow are all rich enough in heavy elements that they should have planets; green and cyan-coded stars should only rarely have planets, and stars coded blue or violet should have absolutely no planets at all around them. Note that the central plane of the galactic disk, extending all the way into the galactic core, has the potential for habitable, rocky planets.
(Credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO)

5.) Globular clusters are planet-free.

Here in the heart of Omega Centauri, one of the largest, richest globular clusters visible from Earth’s location within the Milky Way, lots of stars of various colors have been imaged. Despite the long exposure times devoted to Omega Centauri and the millions of stars inside, no transit events have been observed. Is this because no stars in globular clusters are allowed to maintain their planets? Or because the imaged stars are preferentially too low in metallicity to form them?
(Credit: NASA, ESA, and the Hubble SM4 ERO Team)

Transit surveys haven’t discovered any globular cluster planets.

5000 exoplanets
This diagram shows the discovery of the first 5000+ exoplanets we know of and where they’re located on the sky. Circles show location and size of orbit, while their color indicates the detection method. Note that the clustering features are dependent on where we’ve been looking, not necessarily on where planets are preferentially found. No planets have been found within globular clusters, including the long-imaged 47 Tucanae and Omega Centauri.
(Credit: NASA/JPL-Caltech)

But gravitational interactions might not forbid them.

how many planets
In dense environments with many stars, such as young star clusters, the galactic center, or the centers of globular clusters, gravitational interactions could perturb the orbits of exoplanets, rendering them unstable. However, this may not be the explanation as to why no planets have been found in globular clusters; perhaps the metal-poor nature of the clusters examined is why no planets are present.
(Credit: ESO/M. Kornmesser)

Heavy element-rich globulars might contain planets; the search continues.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

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