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Starts With A Bang

5 revolutionary cosmic ideas that turned out to be wrong

No matter how beautiful, elegant, or compelling your idea is, if it disagrees with observation and experiment, it's wrong.
inflationary beginning big bang
The quantum fluctuations inherent to space, stretched across the Universe during cosmic inflation, gave rise to the density fluctuations imprinted in the cosmic microwave background, which in turn gave rise to the stars, galaxies, and other large-scale structures in the Universe today. This is the best picture we have of how the entire Universe behaves, where inflation precedes and sets up the Big Bang. Unfortunately, we can only access the information contained inside our cosmic horizon, which is all part of the same fraction of one region where inflation ended some 13.8 billion years ago.
Credit: E. Siegel; ESA/Planck and the DOE/NASA/NSF Interagency Task Force on CMB research
Key Takeaways
  • Coming up with novel, theoretical ideas that make concrete predictions is one step toward advancing our scientific understanding of the world.
  • But if we want to know whether or not these ideas are based in reality, we have to put them to experimental and observational tests.
  • These 5 ideas could have revolutionized our conception of the Universe, but since evidence paves the road to reality, we've had to abandon them.

In science, ideas require experimental or observational validation.

In this image, a massive set of galaxies at the center causes many strong lensing features to appear. Background galaxies have their light bent, stretched, and otherwise distorted into rings and arcs, where it gets magnified by the lens as well. This gravitational lens system is complex, but informative for learning more about Einstein’s relativity in action.
Credit: ESA, NASA, K. Sharon (Tel Aviv University) and E. Ofek (Caltech)

These five ideas, although brilliant, simply disagreed with reality.

dark matter
This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. The smallest scales collapse first, while larger scales require longer cosmic times to become overdense enough to form structure. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter; normal matter plays only a minor role.
Credit: Ralf Kaehler and Tom Abel (KIPAC)/Oliver Hahn

1.) The Steady-State Universe.

Over time, gravitational interactions will turn a mostly uniform, equal-density Universe into one with large concentrations of matter and huge voids separating them. For as long as radiation is still important, exerting an outward pressure even once the Universe becomes matter-dominated, the growth of matter imperfections is very small.
Credit: Volker Springel/MPE

Was the Universe not merely the same throughout space, but across time?

There is a tremendous scientific story about the Universe that humanity has revealed, from small, subatomic scales up to large, cosmic ones. We can understand this by evaluating the full suite of evidence in light of all we know, but it’s up to us to be honest and scrupulous with ourselves about our own ignorance and limitations.
Credit: NASA/COBE/DMR; NASA/WMAP science team; ESA and the Planck collaboration

The Cosmic Microwave Background’s discovery disproved it.

universe temperature
The Sun’s actual light (yellow curve, left) versus a perfect blackbody (in gray), showing that the Sun is more of a series of blackbodies due to the thickness of its photosphere; at right is the actual perfect blackbody of the CMB as measured by the COBE satellite. Note that the “error bars” on the right are an astounding 400 sigma. The agreement between theory and observation here is historic, and the peak of the observed spectrum determines the leftover temperature of the Cosmic Microwave Background: 2.73 K.
Credit: Sch/Wikimedia Commons (L); COBE/FIRAS, NASA/JPL-Caltech (R)

Its perfect blackbody spectrum proves its cosmic origin; it isn’t reflected starlight.

big crunch
In the far future, it’s conceivable that the quantum vacuum will decay from its current state to a lower-energy, still more stable state. If such an event were to occur, every proton, neutron, atom, and other composite structure in the Universe would spontaneously destroy itself in a remarkably destructive event, whose effects would propagate and ripple outward in a sphere at the speed of light. This “bubble of destruction” would be unnoticeable until it arrived.
(Credit: geralt/Pixabay)

2.) Our Universe will someday recollapse.

The expected fates of the Universe (top three illustrations) all correspond to a Universe where matter and energy fight against the initial expansion rate. In our observed Universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained. All of these Universes are governed by the Friedmann equations, which relate the expansion of the Universe to the various types of matter and energy present within it. Note how in a Universe with dark energy (bottom), the expansion rate makes a hard transition from decelerating to accelerating about 6 billion years ago.
Credit: E. Siegel/Beyond the Galaxy

Could gravitation defeat cosmic expansion, causing a Big Crunch?

Pantheon+
Joint constraints from the Pantheon+ analysis, along with baryon acoustic oscillation (BAO) and cosmic microwave background (Planck) data, on the fraction of the Universe existing in the form of matter and in the form of dark energy, or Lambda. Our Universe is 33.8% total matter and 66.2% dark energy, to the best of our knowledge, with just a 1.8% uncertainty.
(Credit: D. Brout et al./Pantheon+, ApJ submitted, 2022)

No; dark energy exists, dominating the Universe’s expansion.

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. The Universe was decelerating for the first ~7.8 billion years of cosmic history, but transitioned to accelerating about ~6 billion years ago. If dark energy doesn’t remain constant but rather 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

Unless it decays away — an evidence-free assertion — space will expand forever.

space expanding
A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. The full suite of data, including the observations of the light elements and the cosmic microwave background, leaves only the Big Bang as a valid explanation for all we see. As the Universe expands, it also cools, enabling ions, neutral atoms, and eventually molecules, gas clouds, stars, and finally galaxies to form. Without the Higgs giving mass to the particles in the Universe at a very early, hot stage, none of this would have been possible.
Credit: NASA/CXC/M. Weiss

3.) The hot Big Bang began from a singularity.

The stars and galaxies we see today didn’t always exist, and the farther back we go, the closer to an apparent singularity the Universe gets, as we go to hotter, denser, and more uniform states. However, there is a limit to that extrapolation, as going all the way back to a singularity creates puzzles we cannot answer.
(Credit: NASA, ESA, and A. Feild (STScI))

An expanding, cooling Universe demands a smaller, hotter, denser past.

The density fluctuations in the cosmic microwave background (CMB) provide the seeds for modern cosmic structure to form, including stars, galaxies, clusters of galaxies, filaments, and large-scale cosmic voids. But the CMB itself cannot be seen until the Universe forms neutral atoms out of its ions and electrons, which takes hundreds of thousands of years, and the stars won’t form for even longer: 50-to-100 million years.
Credit: E.M. Huff, SDSS-III/South Pole Telescope, Zosia Rostomian

But arbitrary early temperatures are disallowed; the Cosmic Microwave Background sets stringent upper limits.

singularity
Blue and red lines represent a “traditional” Big Bang scenario, where everything starts at time t=0, including spacetime itself. But in an inflationary scenario (yellow), we never reach a singularity, where space goes to a singular state; instead, it can only get arbitrarily small in the past, while time continues to go backward forever. Only the last minuscule fraction of a second, from the end of inflation, imprints itself on our observable Universe today. The size of the now-observable Universe could’ve been no smaller than about 1 cubic meter in volume at the start of the hot Big Bang.
Credit: E. Siegel

They’re inconsistent with a singularity; an inflationary stage came first.

Any cosmic particle that travels through the Universe, regardless of speed or energy, must contend with the existence of the particles left over from the Big Bang. While we normally focus on the normal matter that exists, made of protons, neutrons, and electrons, they are outnumbered more than a billion-to-one by the remnant photons and neutrinos/antineutrinos. When a charged particle travels through the intergalactic medium, regardless of how it’s produced, it cannot ignore the “bath” of photons it will experience along its journey.
Credit: NASA/Sonoma State University/Aurore Simmonet

4.) The speed of gravity is infinitely fast.

When a gravitational microlensing event occurs, the background light from a star gets distorted and magnified as an intervening mass travels across or near the line-of-sight to the star. The effect of the intervening gravity bends the space between the light and our eyes, creating a specific signal that reveals the mass and speed of the object in question.
Credit: Jan Skowron/Astronomical Observatory, University of Warsaw

Do gravity and light propagate at identical speeds?

Artist’s illustration of two merging neutron stars. The rippling spacetime grid represents gravitational waves emitted from the collision, while the narrow beams are the jets of gamma rays that shoot out just seconds after the gravitational waves (detected as a gamma-ray burst by astronomers). Mass, in an event like this, gets converted into two types of radiation: electromagnetic and gravitational. About 5% of the total mass gets expelled in the form of heavy elements.
Credit: Robin Dienel/Carnegie Institution for Science

Gravitational wave and gamma-ray observations of 2017’s kilonova event settled the issue.

Just hours after the gravitational wave and gamma-ray signals arrived, optical telescopes were able to hone in on the galaxy home to the merger, watching the site of the blast brighten and fade in practically real-time. This 2017 event allowed us to place tremendous constraints on alternative scenarios for both gravitation and electromagnetism, especially considering that the first light signals, in gamma-rays, arrived just 1.7 seconds after the gravitational wave signal completed, across a distance of some ~130,000,000 light-years.
(Credit: P. S. Cowperthwaite/E. Berger/DECAm/CTIO)

They mutually travel at indistinguishable speeds to ~1-part-in-1015; infinite speeds are disallowed.

how much dark matter
While the web of dark matter (purple, left) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red, at right) can severely impact the formation of structure on galactic and smaller scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Structure formation is hierarchical within the Universe, with small star clusters forming first, early protogalaxies and galaxies forming next, followed by galaxy groups and clusters, and lastly by the large-scale cosmic web.
Credit: Illustris Collaboraiton/Illustris Simulation

5.) Dark matter is simply “normal matter” that’s invisible.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures ranging from several hundreds of thousands of degrees up to ~100 million K. Meanwhile, the fact that the gravitational effects (in blue) are displaced from the location of the mass from the normal matter (pink) shows that dark matter must be present.
(Credit: NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland; University of Edinburgh, UK), R. Massey (Durham University, UK), T. Kitching (University College London, UK), and A. Taylor and E. Tittley (University of Edinburgh, UK))

Gravitational properties of colliding galaxy clusters,

The largest-scale observations in the Universe, from the cosmic microwave background to the cosmic web to galaxy clusters to individual galaxies, all require dark matter and dark energy to explain what we observe. While the equations that govern the evolution are well known, as are the magnitudes of the initially overdense regions in our Universe, obtaining the necessary small-scale resolution to tease out the masses and properties of the smallest, earliest galaxies remains difficult.
Credit: Chris Blake and Sam Moorfield

oscillatory features in the Cosmic Microwave Background,

An illustration of clustering patterns due to Baryon Acoustic Oscillations, where the likelihood of finding a galaxy at a certain distance from any other galaxy is governed by the relationship between dark matter and normal matter, as well as the effects of normal matter as it interacts with radiation. As the Universe expands, this characteristic distance expands as well, allowing us to measure the Hubble constant, the dark matter density, and even the scalar spectral index. The results agree with the CMB data, and a Universe made up of ~25% dark matter, as opposed to 5% normal matter, with an expansion rate of around 67 km/s/Mpc.
Credit: Zosia Rostomian, LBNL

large-scale galaxy clustering,

dark matter-free
The cosmic web that we see, the largest-scale structure in the entire Universe, is dominated by dark matter. Simulations of the large-scale structure of the Universe must include both dark matter and normal matter, including the effects of star-formation, feedback, and gas infall, as all of them are needed in order to predict the emergence of visible structures. Identifying which regions are dense and massive enough to correspond to star clusters, galaxies, galaxy clusters, and filaments, as well as determining when and under which conditions they form, is one of the great achievements of modern cosmology.
Credit: Ralf Kaehler/SLAC National Accelerator Laboratory

and Big Bang nucleosynthesis

elements
From beginning with just protons and neutrons, the Universe builds up helium-4 rapidly, with small but calculable amounts of deuterium, helium-3, and lithium-7 left over as well. Until the latest results from the LUNA collaboration, step 2a, where deuterium and a proton fuse into helium-3, had the largest uncertainty. That uncertainty has now dropped to just 1.6%, allowing for incredibly strong conclusions.
Credit: E. Siegel/Beyond the Galaxy (L); NASA/WMAP Science Team (R)

all necessitate dark matter’s presence.

A galaxy that was governed by normal matter alone (left) would display much lower rotational speeds in the outskirts than toward the center, similar to how planets in the Solar System move. However, observations indicate that rotational speeds are largely independent of radius (right) from the galactic center, leading to the inference that a large amount of invisible, or dark, matter must be present. These types of observations were revolutionary in helping astronomers understand the necessity for dark matter in the Universe.
Credit: Ingo Berg/Wikimedia Commons; Acknowledgement: E. Siegel

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


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