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 from the galaxies within it, and the gravitationally lensed galaxies magnified by the foreground cluster’s gravity. The variety of galaxy colors found within the central cluster highlight the enormous variation in intrinsic clusters present in the underlying stellar populations. (Credit: ESA, NASA, K. Sharon (Tel Aviv University) and E. Ofek (Caltech))
These five ideas, although brilliant, simply disagreed with reality.
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. Because simulations are limited in the number of particles they can handle at once, the largest-scale cosmic simulations are inherently limited in their ability to resolve individual, early galaxies. However, the great underdense regions, the cosmic voids within our Universe, are well-understood. (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.
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))
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)
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)
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. If dark energy isn’t a constant, more than 1 parameter will be required to describe it. (Credit: NASA/CXC/M. Weiss)
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. Early on, the highest temperature conditions of all-time were achieved; in the far future, everything will eventually cool off toward absolute zero. (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 Universe doesn’t just expand uniformly, but has tiny density imperfections within it, which enable us to form stars, galaxies, and clusters of galaxies as time goes on. Adding density inhomogeneities on top of a homogeneous background is the starting point for understanding what the Universe looks like today. (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. 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 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-or-galaxy 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. With enough technological advances, microlensing by rogue supermassive black holes could be measured. (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.
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. However, even state-of-the-art simulations like Illustris, shown here, struggle to reproduce the small-scale structure within the cosmic web; higher-resolution simulations, like Renaissance, are required for those aspects. (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,
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
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))
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)
