The 5 greatest puzzles in fundamental physics

Does humanity finally understand the Universe?

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.

We’ve identified the particles, forces, and interactions underpinning reality.

On the right, the gauge bosons, which mediate the three fundamental quantum forces of our Universe, are illustrated. There is only one photon to mediate the electromagnetic force, there are three bosons mediating the weak force, and eight mediating the strong force. This suggests that the Standard Model is a combination of three groups: U(1), SU(2), and SU(3), whose interactions and particles combine to make up everything known in existence. Despite the success of this picture, many puzzles still remain.

Our cosmic history — past, present, and future — was finally determined.

The extent of the visible Universe now goes on for 46.1 billion light-years: the distance that light emitted at the instant of the Big Bang would be located from us today, after a 13.8 billion year journey. As time marches on, light that’s even farther away, that is still on its way to us, will eventually arrive: from slightly greater distances and with slightly greater redshifts. We see into the past when we look out to great distances because the light emitted from distant objects must traverse those great intergalactic distances at a finite speed: the speed of light.

However, numerous puzzles remain, including these five.

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.

1.) How did the Universe begin?

From a pre-existing state, inflation predicts that a series of universes will be spawned as inflation continues, with each one being completely disconnected from every other one, separated by more inflating space. One of these “bubbles,” where inflation ended, gave birth to our Universe some 13.8 billion years ago, with a very low entropy density, but without ever violating the 2nd law of thermodynamics.

Cosmic inflation set up and preceded the hot Big Bang.

Our entire cosmic history is theoretically well-understood, but difficult to depict in a static, 2D image. The Universe’s present expansion rate and energy composition are related, which is why most modern illustrations of our cosmic history have a tube-like shape: where they often (dubiously) depict an initial singularity, a period of inflation, and then a slower expansion that changes with time while our Universe evolves. No one diagram encodes all of these details correctly, including the one shown here, which seems to maintain a constant “size” for the Universe, disagreeing with reality.

The supporting observational evidence, however, leaves much undetermined.

The fluctuations in the cosmic microwave background, as measured by COBE (on large scales), WMAP (on intermediate scales), and Planck (on small scales), are all consistent with not only arising from a (slightly tilted, but almost-perfectly) scale-invariant set of quantum fluctuations, but of being so low in magnitude that they could not possibly have arisen from an arbitrarily hot, dense state. The horizontal line represents the initial spectrum of fluctuations (from inflation), while the wiggly one represents how gravity and radiation/matter interactions have shaped the expanding Universe in the early stages.

What “type” of inflation occurred? What preceded and/or caused inflation?

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.

Providing answers requires new, unprecedented data.

The contribution of gravitational waves left over from inflation to the B-mode polarization of the Cosmic Microwave background has a known shape, but its amplitude is dependent on the specific model of inflation. These B-modes from gravitational waves from inflation have not yet been observed, but detecting them would help us tremendously in pinning down precisely what type of inflation occurred. A false detection, from the BICEP2 team, famously occurred in the early 2010s. The upcoming generation of CMB experiments should be sensitive to tensor-to-scalar ratios (i.e., r-ratios) as low as 0.001, or possibly even less, but no B-modes from inflation have ever been robustly observed.

2.) What explains neutrino mass?

This diagram displays the structure of the standard model (in a way that displays the key relationships and patterns more completely, and less misleadingly, than in the more familiar image based on a 4×4 square of particles). In particular, this diagram depicts all of the particles in the Standard Model (including their letter names, masses, spins, handedness, charges, and interactions with the gauge bosons: i.e., with the strong and electroweak forces). It also depicts the role of the Higgs boson, and the structure of electroweak symmetry breaking, indicating how the Higgs vacuum expectation value breaks electroweak symmetry and how the properties of the remaining particles change as a consequence. Neutrino masses remain unexplained.

Neutrinos were originally massless within the Standard Model.

The neutrino is an intriguing and interesting particle. This infographic lays out some of the neutrino’s basic stats alongside fun facts.

Observations indicate non-zero masses: neutrinos oscillate while interacting with matter.

Vacuum oscillation probabilities for electron (black), muon (blue), and tau (red) neutrinos for a chosen set of mixing parameters, beginning from an initially produced electron neutrino. An accurate measurement of the mixing probabilities over different length baselines can help us understand the physics behind neutrino oscillations and could reveal the existence of any other types of particles that couple to the three known species of neutrino. For neutrinos to oscillate, they must have non-zero mass. If additional particles (such as dark matter particles) carry energy away, the overall neutrino flux will show a deficit.

Are neutrinos Dirac or Majorana particles? Are there heavy, sterile neutrino species?

A neutrino event, identifiable by the rings of Cherenkov radiation that show up along the photomultiplier tubes lining the detector walls, showcase the successful methodology of neutrino astronomy. This image shows multiple events, and is part of the suite of experiments paving our way to a greater understanding of neutrinos.

Their nature could break the Standard Model.

This cutaway illustration shows the path of neutrinos in the Deep Underground Neutrino Experiment. A proton beam is produced in Fermilab’s accelerator complex (improved by the PIP-II project). The beam hits a target, producing a neutrino beam that travels through a particle detector at Fermilab, then through 800 miles (1,300 km) of earth, and finally reaches the far detectors at Sanford Underground Research Facility.

3.) Why is our Universe matter-dominated?

The colliding galaxy cluster “El Gordo,” the largest one known in the observable Universe, shows the same evidence of dark matter and normal matter separating when galaxy clusters collide, as seen in other colliding clusters. If normal matter alone is to explain gravity, its effects must be non-local: where gravity is found where the mass/matter isn’t.

More matter than antimatter permeates the Universe.

Through the examination of colliding galaxy clusters, we can constrain the presence of antimatter from the emissions at the interfaces between them. In all cases, there is less than 1-part-in-100,000 antimatter in these galaxies, consistent with its creation from supermassive black holes and other high-energy sources. There is no evidence for cosmically abundant antimatter.

However, known physics cannot explain the observed matter-antimatter asymmetry.

In the very early Universe, there were tremendous numbers of quarks, leptons, antiquarks, and antileptons of all species. After only a tiny fraction-of-a-second has elapsed since the hot Big Bang, most of these matter-antimatter pairs annihilate away, leaving a very tiny excess of matter over antimatter. How that excess came about is a puzzle known as baryogenesis, and it is one of the greatest unsolved problems in modern physics.

Fundamental symmetry violations — and LHCb experiments — could explain baryogenesis.

Parity, or mirror-symmetry, is one of the three fundamental symmetries in the Universe, along with time-reversal and charge-conjugation symmetry. If particles spin in one direction and decay along a particular axis, then flipping them in the mirror should mean they can spin in the opposite direction and decay along the same axis. This was observed not to be the case for the weak decays, which are the only interactions known to violate charge-conjugation (C) symmetry, parity (P) symmetry, and the combination (CP) of those two symmetries as well.

4.) What is dark matter?

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, and also explain the shapes and behavior of matter located within a galaxy’s spiral arms.

It clumps and gravitates but passes through atoms and light.

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. Without dark matter, these observations (along with many others) cannot be sufficiently explained.

Its indirect evidence is overwhelming; direct searches remain fruitless.

Hall B of LNGS with XENON installations, with the detector installed inside the large water shield. If there’s any non-zero cross section between dark matter and normal matter, not only will an experiment like this have a chance at detecting dark matter directly, but there’s a chance that dark matter will eventually interact with your human body.

Its effects are understood, not its underlying cause.

The dark matter structures which form in the Universe (left) and the visible galactic structures that result (right) are shown from top-down in a cold, warm, and hot dark matter Universe. From the observations we have, at least 98%+ of the dark matter must be either cold or warm; hot is ruled out. Observations of many different aspects of the Universe on a variety of different scales all point, indirectly, to the existence of dark matter, but direct detection experiments haven’t found the particle responsible for it.

5.) What is dark energy?

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. 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.

The Universe’s expansion is accelerating.

While matter and radiation become less dense as the Universe expands owing to its increasing volume, dark energy is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant.

Its properties indicate a constant, positive spatial energy density.

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 can strengthen, weaken, or reverse sign over time, however, all bets are off, and alternative possibilities suddenly abound.

To advance, understanding the quantum vacuum is mandatory.

A visualization of QCD illustrates how particle-antiparticle pairs pop out of the quantum vacuum for very small amounts of time as a consequence of Heisenberg uncertainty. The quantum vacuum is interesting because it demands that empty space itself isn’t so empty, but is filled with all the particles, antiparticles, and fields in various states that are demanded by the quantum field theory that describes our Universe, even though this tool is a visualization only. If the vacuum becomes polarized, such as by having a charged particle nearby, then positive and negative charges will respond differently, effectively “screening” the space nearest to the charge from the charge itself.

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