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 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.

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).

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

Artist’s logarithmic scale conception of the observable universe. The Solar System gives way to the Milky Way, which gives way to nearby galaxies which then give way to the large-scale structure and the hot, dense plasma of the Big Bang at the outskirts. Each line-of-sight that we can observe contains all of these epochs, but the quest for the most distant observed object will not be complete until we’ve mapped out the entire Universe.

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 only because we understand the theory of gravitation that underlies it, and because we know the Universe’s present expansion rate and energy composition. Light will always continue to propagate through this expanding Universe, and we will continue to receive that light arbitrarily far into the future, but it will be limited in time as far as what reaches us. We still have unanswered questions about our cosmic origins, but the age of the Universe is known.

The supporting observational evidence, however, leaves much undetermined.

The fluctuations in the CMB are based on primordial fluctuations produced by inflation. In particular, the ‘flat part’ on large scales (at left) have no explanation without inflation. The flat line represents the seeds from which the peak-and-valley pattern will emerge over the first 380,000 years of the Universe, and is just a few percent lower on the right (small-scale) side than the (large-scale) left side.

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

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. Additionally, gravitational wave imperfections and angular momentum fluctuations are created as well, but the latter decay as the Universe expands.

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.

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.

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

A neutrino event, identifiable by the rings of Cerenkov radiation that show up along the photomultiplier tubes lining the detector walls, showcase the successful methodology of neutrino astronomy and leveraging the use of Cherenkov radiation. 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, showing the same evidence of dark matter and normal matter as other colliding clusters. There is practically no room for antimatter in this or at the interface of any known galaxies or galaxy clusters, severely constraining its possible presence in our Universe.

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.

The Big Bang produces matter, antimatter, and radiation, with slightly more matter being created at some point, leading to our Universe today. How that asymmetry came about, or arose from where there was no asymmetry to start, is still an open question, but we can be confident that the excess of up-and-down quarks over their antimatter counterparts is what enabled protons and neutrons to form in the early Universe in the first place.

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 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.

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 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.

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.

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. 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.

The Universe’s expansion is accelerating.

While matter (both normal and dark) and radiation become less dense as the Universe expands owing to its increasing volume, dark energy, and also the field energy during inflation, 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 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.

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. Put this all together, and you find that empty space has a zero-point energy that’s actually greater than zero.

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