Accelerating universe

What is the source of the observed accelerated expansion of the universe? What is the nature of the dark energy driving this acceleration?

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?

Amorphous solids

What is the nature of the transition between a fluid or regular solid and a glassy phase? What are the physical processes giving rise to the general properties of glasses?

Ball lightning

Are these glowing, floating objects real? How can they be explained? ****

Baryon asymmetry

Why is there far more matter than antimatter in the universe?

Black hole information

Hawking radiation allows black holes to evaporate to (perhaps) nothing, and yet carries no information. Quantum mechanics does not allow information to be destroyed. Where does it go? Is Hawking's formula incorrect, and the radiation carries information? Or does it stop at some point leaving black hole remnants?

Black hole structure

Does a black hole have an internal structure, and if so, what is its nature?

Cold fusion

What is the theoretical explanation for the apparent production of excess heat and helium in palladium metal when it is saturated with deuterium? ****

Corona heating problem

Why is the Sun's Corona (atmosphere layer) so much hotter than the Sun's surface?

Cosmological constant

Why doesn't the zero-point energy of vacuum cause a large cosmological constant? What cancels it out? Is a non-total cancellation of the cosmological constant responsible for the observed accelerated expansion (deSitter phase) of the Universe? If it is, why is the energy density of the cosmological constant of the same magnitude as the density of matter at present when the two evolve quite differently over time; could it be simply that we are observing at exactly the right time?

Dark matter

What is dark matter?^[1] Is it related to supersymmetry? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?

Electroweak symmetry breaking

What is the mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the W and Z? Is it the simple Higgs mechanism of the Standard Model?^[2]

Entropy (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 does time flow in one direction at all, on macroscopic scales, when there does not seem to be an arrow of time on the scale of fundamental interactions?^[3] ***

Fundamental physical constants

Can fundamental constants be predicted by a more complete theory? ^[4] Do "fundamental physical constants" vary over time?

Gamma ray bursts (short duration)

What is the nature of these extraordinarily energetic astronomical objects that last less than two seconds?^[5]

High-temperature superconductors

What is the responsible mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 50 kelvins?^[6]

Hipparcos Anomaly

How far away are the Pleiades, exactly?

Island of stability

What is the largest theoretically possible stable atom?

Minkowski space

Why does the Minkowski metric describe the vacuum?

Neutrino mass

What is the mechanism responsible for generating neutrino masses? Is the neutrino its own antiparticle?

Pioneer anomaly

What causes the apparent residual sunward acceleration of the Pioneer spacecraft?^[7][8] ****

Proton Spin Crisis

As initially measured by the European Muon Collaboration, the quarks of the proton account for about 12% of its total spin. What accounts for the rest of it?

Sonoluminescence

What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?

Turbulence

Is it possible to make a theoretical model to describe the statistics of a turbulent flow (in particular, its internal structures)?^[9]

Ultra-high-energy cosmic ray

Why is it that some cosmic rays appear to possess energies that are impossibly high (the so called Oh-My-God particle), given that there are no sufficiently energetic cosmic ray sources near the Earth? Why is it that (apparently) some cosmic rays emitted by distant sources have energies above the Greisen-Zatsepin-Kuzmin limit?^[10][11]

Theoretical ideas in search of experimental evidence

Axions

Is the Peccei-Quinn theory (i.e. mechanism) the solution to the strong CP problem? What are the properties of the predicted axion?

Cosmic inflation

Is the theory of cosmic inflation correct, and if so, what are the details of this epoch? What is the hypothetical inflaton field giving rise to inflation? If inflation happened at one point, is it self-sustaining through inflation of quantum-mechanical fluctuations, and thus ongoing in some impossibly distant place?

Emergent phenomena

Is a complete understanding of particle physics sufficient to fully understand all physical phenomena, or are there emergent phenomena in physics whose existence cannot be definitively predicted from a complete understanding of the fundamental particles and forces that govern the universe?

Extra dimensions

Does nature have more than four spacetime dimensions? If so, why are they not easily observed? Are dimensions a fundamental property of the universe or an emergent result of other physical laws?

Faster-than-light

Is it possible to go faster than the speed of light (create path that is locally timelike but globally spacelike)? Is it possible to transmit information faster than the speed of light? If so, what is the consequence for the principle of causality (i.e., does it make time machines possible), or why not? **

Magnetic monopoles

Do particles that carry "magnetic charge" exist?

Multiple universes

Are there physical reasons to believe in other universes that are fundamentally non-observable? For instance: Are there quantum mechanical "alternate histories"? Are there "other" universes with physical laws resulting from alternate ways of breaking the apparent symmetries of physical forces at high energies, possibly incredibly far away due to cosmic inflation? Is the use of the anthropic principle to resolve global cosmological dilemmas justified? ***

Physical information

Are there physical phenomena which irrevocably destroy information about their prior states?

Proton decay

As the lightest baryon, are protons absolutely stable? If current theoretical ideas are correct, quarks and leptons are ultimately unified and thus nothing in principle forbids proton decay. If so, then what is the proton's half-life? (This question is related to extracting experimental information about physics near the Planck scale.)

Quantum chromodynamics (QCD) in the non-perturbative regime

The equations of QCD remain unsolved at energy scales relevant for describing atomic nuclei. How does QCD give rise to the physics of nuclei and nuclear constituents?

Quantum gravity

How can gravity be realized as a fully consistent quantum theory? Is string theory (M-theory) the correct approach? More pressing, how much experimental information can be extracted about physics near Planck scale?

Quantum mechanics in the correspondence limit

Is there a preferred interpretation of quantum mechanics? How does the quantum description of reality, which includes elements such as the superposition of states and wavefunction collapse, give rise to the reality we perceive? ***

Standard Model Higgs mechanism

Does the Standard Model Higgs particle exist with its predicted Standard Model interactions?

Supersymmetry

Is spacetime supersymmetry realized in nature? If so, what is the mechanism of supersymmetry breaking? Does supersymmetry stabilize electroweak symmetry breaking? Does the lightest supersymmetric particle make up the dark matter?

Tachyons

Do these hypothetical particles exist? If so, how do they travel FTL ignoring the laws of special relativity?

Technicolor

Does nature make use of strong dynamics in breaking electroweak symmetry?