5 revolutionary cosmic ideas that turned out to be wrong

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.

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

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.

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. Neutrinos and antineutrinos behave like radiation at early times in the Universe, but at late times, will fall into the gravitational wells of galaxies and galaxy clusters, as they lose speed owing to the expansion of space.

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.

The Cosmic Microwave Background’s discovery disproved it.

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; this demonstrates that the CMB cannot be due to reflected starlight.

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

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.

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.

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.

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

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.

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

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.

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.

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

The cold fluctuations (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 gravitational density of the regions the light passes through as it travels can show up in the CMB as well, teaching us what these regions are truly like.

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.

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.

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, from low-mass planets to high-mass black holes.

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.

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.

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

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 galactic scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Neutrinos are ubiquitous, but standard, light neutrinos cannot account for most (or even a significant fraction) of the dark matter.

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. 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.

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.

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, normal matter and all types of radiation, including neutrinos. As the Universe expands, this characteristic distance expands as well, allowing us to measure the Hubble constant, the dark matter density, and other cosmological parameters over time. The large-scale structure and Planck data must agree.

large-scale galaxy clustering,

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.

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.

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. Modified gravity theories, although they cannot explain many of these phenomena very well, do an outstanding job at detailing the dynamics of spiral galaxies.

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