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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.
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)
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.

The massive galaxy cluster SDSS J1004+4112, is an enormous clump of matter that allows us to probe the very early Universe. If any species of energy has decayed and/or transitioned into another, joint observations of the nearby and distant Universe will be the best avenue to reveal it.
(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. 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.

The growth of the cosmic web and the large-scale structure in the Universe, shown here with the expansion itself scaled out, results in the Universe becoming more clustered and clumpier as time goes on. Initially small density fluctuations will grow to form a cosmic web with great voids separating them, but what appear to be the largest wall-like and supercluster-like structures may not be true, bound structures after all, as late-time dark energy drives them apart.
(Credit: Volker Springel/MPE)

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

COBE, the first CMB satellite, measured fluctuations to scales of 7º only. WMAP was able to measure resolutions down to 0.3° in five different frequency bands, with Planck measuring all the way down to just 5 arcminutes (0.07°) in nine different frequency bands in total. All of these space-based observatories detected the Cosmic Microwave Background, confirming it was not an atmospheric phenomenon, and that it had a cosmic origin.
(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 grey), 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 all of the matter and energy presently contained within our expanding Universe will wind up in a single location owing to a reversal of the expansion. If this occurs, our Universe’s fate is that we’ll end in a Big Crunch: the opposite of the Big Bang. This, fortunately or unfortunately, dependent on your perspective, is not supported by any of the evidence we possess.
(Credit: geralt/Pixabay)

2.) Our Universe will someday recollapse.

The expected fates of the Universe (top three illustrations) all correspond to a Universe where the matter and energy combined 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. If your expansion rate continues to drop, as in the first three scenarios, you can eventually catch up to anything. But if your Universe contains dark energy, that’s no longer the case.
(Credit: E. Siegel/Beyond the Galaxy)

Could gravitation defeat cosmic expansion, causing a Big Crunch?

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 here: of the eventual heat death of the Universe. If dark energy evolves with time, a Big Rip or a Big Crunch are still admissible.
(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. However, the Big Bang was not an explosion, and cosmic expansion is very different from that idea.
(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 cold spots (shown in blue) in the CMB are not inherently colder, but rather represent regions where there is a greater gravitational pull due to a greater density of matter, while the hot spots (in red) are only hotter because the radiation in that region lives in a shallower gravitational well. Over time, the overdense regions will be much more likely to grow into stars, galaxies and clusters, while the underdense regions will be less likely to do so. The maximum temperature that our Universe ever could have achieved, as shown by the fluctuations in the cosmic microwave background, is only ~10^16 GeV, or a factor of 1,000 smaller than the Planck scale.
(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.

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.
(Credit: E. Siegel)

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

Any cosmic particle that travels through the Universe, regardless of energy, will move at the speed of light if it’s massless, and will move below the speed of light if it has a non-zero rest mass. Photons and gravitational waves, to an enormous precision, travel at exactly the same speed: speeds indistinguishable from the speed of light.
(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 intervening object in question. All masses are capable of bending light via gravitational lensing, and this method may become very successful at revealing the Milky Way’s population of rogue planets.
(Credit: Jan Skowron/Astronomical Observatory, University of Warsaw)

Do gravity and light propagate at identical speeds?

When two neutron stars collide, if their total mass is great enough, they won’t just result in a kilonova explosion and the ubiquitous creation of heavy elements, but will lead to the formation of a novel black hole from the post-merger remnant. Gravitational waves and gamma-rays from the merger appear to travel at indistinguishable speeds: the speed of all massless particles.
(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
The formation of cosmic structure, on both large scales and small scales, is highly dependent on how dark matter and normal matter interact. Despite the indirect evidence for dark matter, we’d love to be able to detect it directly, which is something that can only happen if there’s a non-zero cross-section between normal matter and dark matter. There’s no evidence for that, nor for a changing relative abundance between dark and normal matter.
(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 exceeding several hundreds of thousands of degrees.
(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 to explain what we observe. At both early times and late times, that same 5-to-1 dark matter-to-normal matter ratio is required.
(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 68 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. On smaller scales, however, baryons can interact with one another and with photons, leading to stellar structure but also leading to the emission of energy that can be absorbed by other objects. Neither dark matter nor dark energy can accomplish that task; our Universe must possess a mix of dark matter, dark energy, and normal matter.
(Credit: Ralf Kaehler/SLAC National Accelerator Laboratory)

and Big Bang nucleosynthesis

The lightest elements in the Universe were created in the early stages of the hot Big Bang, where raw protons and neutrons fused together to form isotopes of hydrogen, helium, lithium and beryllium. The beryllium was all unstable, leaving the Universe with only the first three elements prior to the formation of stars. The observed ratios of the elements allows us to quantify the degree of the matter-antimatter asymmetry in the Universe by comparing the baryon density to the photon number density, and leads us to the conclusion that only ~5% of the Universe’s total modern energy density is allowed to exist in the form of normal matter, and that the baryon-to-photon ratio, except for the burning of stars, remains largely unchanged at all times.
(Credit: E. Siegel/Beyond the Galaxy (L); NASA/WMAP Science Team (R))

all necessitate dark matter’s presence.

A spiral galaxy like the Milky Way rotates as shown at right, not at left, indicating the presence of dark matter. Not only all galaxies, but clusters of galaxies and even the large-scale cosmic web all require dark matter to be cold and gravitating from very early times 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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