Scope of Outstanding Problems

1It seems reasonable to start off this section with a brief overview of some of the open problems in science, which may require a speculative solution that exist outside of what might be described as today’s accepted  science. As such, the following summary is not meant to be exhaustive, only reflective of the scope of problems now facing 21st century science, which Lord Kelvin’s optimistic quote at the start of the 20th century appears to have misjudged:

"There is nothing new to be discovered in physics now.
All that remains is more and more precise measurement."

General Problems

Entropy and the Arrow of Time:
Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics? Why are CP violations observed in certain weak force decays, but not elsewhere? Are CP violations somehow a product of the Second Law of Thermodynamics or are they a separate arrow of time? Are there exceptions to the principle of causality? Is there a single possible past? Is the present moment physically distinct from the past and future or is it merely an emergent property of consciousness?

Quantum Mechanics:
Is there a preferred interpretation of quantum mechanics? How does the quantum description of reality that includes the superposition of states, the wave function collapse and/or quantum decoherence give rise to the reality as perceived? In terms of the measurement problem, what constitutes a measurement that would caused the wave function to collapse into a definite state?

Physical Information:
Are there physical phenomena, such as black holes or wave function collapse that actually destroy information about their prior states?

The Theory of Everything:
Is there a theory which explains the values of all fundamental physical constants? Is there a theory which explains why the gauge groups of the standard model are as such? Why does observed space-time have 3+1 dimensions and why are the laws of physics as such? Do fundamental physical constants vary over time? Are any of the particles in the standard model of particle physics actually composite particles too tightly bound to observe as such at current experimental energies? Are there fundamental particles that have not yet been observed and if so which ones and what are their properties? What are fundamental particles made of? Are there unobserved fundamental forces implied by a theory that explains other unsolved problems in physics?

Particle Physics

2Higgs Mechanism:
Is the nature of the massive boson consistent with the Higgs boson? Are the branching ratios consistent with the standard model? Is there only one type of Higgs boson?

Hierarchy Problems:
Why is gravity such a weak force? Why are the force scales so different from each other? What prevents quantities at the electroweak scale, such as the Higgs boson mass, from getting quantum corrections on the order of the Planck scale? Is the solution super-symmetry and extra dimensions just anthropic fine-tuning?

Magnetic Monopoles:
Did particles that carry ‘magnetic charge’ exist in a past, higher-energy epoch? If so, do any remain today?

Proton Decay and Spin:
Is the proton a fundamentally stable particle or does it have a finite lifetime as predicted by some extensions to the standard model? What explains the anomalous spin of protons?

Is spacetime super-symmetry realized in nature? If so, what is the mechanism of super-symmetry breaking? Does super-symmetry stabilize the electroweak scale, preventing high quantum corrections? Does the lightest super-symmetric particle comprise dark matter?

Generations of Matter:
Are there more than three generations of quarks and leptons? Why are there generations at all? Is there a theory that can explain the masses of particular quarks and leptons in particular generations from first principles? Are they just different energy configurations?

Electroweak Symmetry Breaking:
What is the mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the W-Z bosons? Is it the simple Higgs mechanism of the Standard Model or does nature make use of strong dynamics in breaking electroweak symmetry?

Neutrino Mass:
What is the mass of a neutrino? Is mass hierarchy normal or inverted?

Why has there never been a measurement of a free quark or gluon, but only objects that are built out of them, like mesons and baryons? How does this phenomenon emerge from QCD?

Strong CP Problem:
Why is the strong nuclear interaction invariant to parity and charge conjugation?

What is the heaviest possible stable or meta-stable nucleus?

Quantum Physics

Quantum Chromo-Dynamics:
What are the phases of strongly interacting matter and what roles do they play in the cosmos? What is the internal landscape of the nucleons? What does QCD predict for the properties of strongly interacting matter? What governs the transition of quarks and gluons into pions and nucleons? What is the role of gluons and gluon self-interactions in nucleons and nuclei? What determines the key features of QCD, and what is their relation to the nature of gravity and spacetime?

Vacuum Catastrophe:
Why does the predicted energy-mass of the quantum vacuum have little effect on the expansion of the universe?

Quantum Gravity:
Can quantum mechanics and general relativity be realized as a fully consistent theory? Is spacetime fundamentally continuous or discrete? Would a consistent theory involve a force mediated by a hypothetical graviton or be a product of a discrete structure of spacetime itself, as in loop quantum gravity? Are there deviations from the predictions of general relativity at very small or very large scales or in other extreme circumstances that flow from a quantum gravity theory?

3Black Holes:
Can we really understand black holes without understanding the structure of matter? Do black holes produce thermal radiation, as expected on theoretical grounds? Does this radiation contain information about their inner structure, as suggested by gauge-gravity duality, or not, as implied by Hawking's original calculation? If not, and black holes can evaporate away, what happens to the information stored in them as quantum mechanics does not appear to allow the destruction of information? Does the radiation stop at some point leaving black hole remnants? Is there another way to probe the internal structure of a black hole, if such a structure even exists?

Cosmic Censorship Hypothesis:
Can singularities, not hidden behind an event horizon, known as ‘naked singularities’, arise from realistic initial conditions or is it possible to prove some version of the ‘cosmic censorship hypothesis’ which proposes that this is impossible? Will the closed time-like curves, which arise in some solutions of general relativity, be ruled out by a theory of quantum gravity, which unites general relativity with quantum mechanics, as suggested by the ‘chronology protection conjecture’.

Extra Dimensions:
Does nature have more than four spacetime dimensions? If so, what is their size? Are dimensions a fundamental property of the universe or an emergent result of other physical laws? Can we experimentally observe evidence of higher spatial dimensions?

Are there non-local phenomena in quantum physics? If they exist, are non-local phenomena limited to the entanglement revealed in the violations of the Bell Inequalities or can information and conserved quantities also move in a non-local way? Under what circumstances are non-local phenomena observed? What does the existence or absence of non-local phenomena imply about the fundamental structure of spacetime? How does this relate to quantum entanglement? How does this elucidate the proper interpretation of the fundamental nature of quantum physics?


4Cosmic Inflation:
Is the theory of cosmic inflation correct? What is the hypothetical inflaton field? If inflation happened, at what point does it become self-sustaining as an inflation of a quantum-mechanical fluctuation?

Horizon Problem:
Why is the distant universe so homogeneous, when the Big Bang theory seems to predict larger measurable anisotropies of the night sky than those observed? Although cosmic inflation is generally accepted as a solution, are other possible explanations such as the variable speed of light hypothesis valid alternatives?

Future of the Universe:
Is there any clear consensus that the universe is heading towards a Big Freeze, Big Rip, Big Crunch or Big Bounce? Is our universe part of an infinitely recurring cyclic model?

Gravitational Waves:
Can gravitational waves be detected experimentally?

Baryon Asymmetry:
Why is there far more matter than antimatter in the observable universe?

Cosmological Constant Problem:
Why does the zero-point energy of the vacuum not cause a large cosmological constant? What cancels it out?

Energy/Mass Distribution:
Can we really estimate the distribution of dark matter and dark energy in the universe that we do not really understand?

Dark Matter:
What is dark matter? Are there alternative theories? Is it related to super-symmetry? Is the mass that makes up the dark matter halos around galaxies made up of the lightest form of super-symmetric particles? Do the phenomena attributed to dark matter point, not to some form of matter, but to an extension of gravity?

Dark Energy:
What is the cause of the observed accelerated expansion of the universe? Why is the energy density of the dark energy component of the same magnitude as the density of matter at present when the two evolve quite differently over time? Could it simply be that we are observing at exactly the right time? Is dark energy a pure cosmological constant or are models of quintessence, such as phantom energy applicable?

Dark Flow:
What is the cause of a large swath of galaxy clusters all moving towards one part of the universe?

Ecliptic Alignment of CMB Anisotropy:
Some large features of the microwave sky, at distances of over 13 billion light years, appear to be aligned with both the motion and orientation of the Solar System. Is this due to systematic errors in processing, contamination of results by local effects or an unexplained violation of the Copernican principle?

Shape of the Universe:
What is the 3-manifold of co-moving space? It appears that neither the curvature nor the topology of the universe is really known, even though the curvature appears to be ‘close’ to zero, as makes no difference, on observable scales. The cosmic inflation hypothesis suggests that the shape of the Universe may not be measurable.

Astronomy and Astrophysics

5Nuclear Astrophysics:
What is the nature of the nuclear force that binds protons and neutrons into stable nuclei and rare isotopes? What is the origin of simple patterns in complex nuclei? What is the nature of exotic excitations in nuclei at the frontiers of stability and their role in stellar processes? What is the nature of neutron stars and dense nuclear matter? What is the origin of the elements in the cosmos? What are the nuclear reactions that drive stars and stellar explosions?

Accretion Disc Jets:
Why do the accretion discs surrounding certain astronomical objects, such as the nuclei of active galaxies, emit relativistic jets along their polar axes? Why are there quasi-periodic oscillations in many accretion discs? Why does the period of these oscillations scale as the inverse of the mass of the central object? Why are there sometimes overtones, and why do these appear at different frequency ratios in different objects?

Coronal Heating Problem:
Why is the Sun's corona so much hotter than the Sun's surface? Why is the magnetic reconnection effect many orders of magnitude faster than predicted by standard models?

Diffuse Interstellar Bands:
What is responsible for the numerous interstellar absorption lines detected in astronomical spectra? Are they molecular in origin and, if so, which molecules are responsible for them? How do they form?

Gamma Ray Bursts :
How do these short-duration high-intensity bursts originate?

Super-Massive Black Holes:
What is the origin of the M-sigma relation between super-massive black hole mass and galaxy velocity dispersion?

Observational Anomalies:
Why do the rotation curve of a typical spiral galaxy predicted one thing and observation other? Can the discrepancy between the curves be attributed to dark matter?  The Hipparcos anomaly questions the actual distance to the Pleiades? Flyby anomaly question the observed energy of satellites flying by Earth, albeit  by a minute amount from the value predicted by theory?

Galaxy Rotation Problem:
Is dark matter responsible for differences in observed and theoretical speed of stars revolving around the center of galaxies or is it something else?

What is the exact mechanism by which an implosion of a dying star becomes an explosion?

Ultra High Energy Cosmic Rays:
Why is it that some cosmic rays appear to possess energies that are impossibly high given that there are no sufficiently energetic cosmic ray sources near the Earth? Why is it that some cosmic rays emitted by distant sources have energies above the Greisen-Zatsepin-Kuzmin limit?

Rotation Rate of Saturn:
Why does the magnetosphere of Saturn exhibit a slowly changing periodicity close to that at which the planet's clouds rotate? What is the true rotation rate of Saturn's deep interior?

Origin of Magnetar Magnetic Field:
What is the origin of magnetar magnetic field

Closing Remarks

While the previous summary raises lot of questions, it is not clear how much scope is allowed when it comes to any speculative answers. Therefore, the next section will take a look at this issue and possibly try to reduce the complexity of all these questions to a more basic set of issues.