Beyond the
Standard Model

The Standard Model is the most successful physical model of all time — and at the same time, obviously incomplete. It doesn't explain dark matter, doesn't explain gravity, doesn't explain why the universe is dominated by matter over antimatter.

What's missing

The open problems of the Standard Model

The Standard Model describes all known elementary particles and three of the four fundamental forces with astonishing precision. The anomalous magnetic moment of the muon was predicted to parts-per-million accuracy — even though a still-unresolved tension between different theoretical calculation methods persists to this day. The mass of the Higgs boson was theoretically bounded roughly 35 years before its discovery, since the late 1970s. It is one of the best-tested theories in science.

And yet physicists know it's incomplete. Not because experiments have falsified it — but because the model simply provides no answers to central questions in cosmology and physics. At least six fundamental gaps are known:

Gravity is missing

The Standard Model does not incorporate quantum gravity. General relativity and quantum field theory are incompatible.

Dark Matter

27% of the universe consists of matter that interacts gravitationally but emits no light. No Standard Model particle fits.

Dark Energy

68% of the universe consists of a mysterious energy that accelerates its expansion. The Standard Model has nothing to say about it.

Matter-Antimatter

The known CP violation is orders of magnitude too weak to explain the matter dominance of the universe.

Neutrino Mass

Neutrinos have mass — oscillations prove it. The original Standard Model made no provision for neutrino masses.

Hierarchy Problem

The Higgs mass is quantum-mechanically extremely unstable against corrections. Why isn't it astronomically large?

Candidates

Theories beyond the Standard Model

S̃

Supersymmetry (SUSY)

SUSY postulates a symmetry between bosons and fermions: every Standard Model particle gets a "superpartner" with opposite spin. Squarks, sleptons, gluinos. The lightest supersymmetric particles would be stable and could explain dark matter. SUSY also elegantly solves the hierarchy problem. So far, no superpartners have been found.

∞

String Theory

Instead of point-like particles, string theory treats one-dimensional vibrating strings as the fundamental objects. Different vibrational modes of the string correspond to different particles. String theory naturally contains a massless spin-2 particle — the graviton — and could describe quantum gravity. It requires 10 or 11 spatial dimensions, of which the remaining ones are "compactified".

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Grand Unified Theories (GUTs)

GUTs unify the three forces of the Standard Model (strong, weak, electromagnetic) into a single gauge group such as SU(5) or SO(10). At energies around 10¹⁶ GeV, all coupling constants should converge. GUTs predict proton decay — not yet observed. They could also explain the matter-antimatter asymmetry.

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Loop Quantum Gravity (LQG)

An alternative approach to quantizing gravity without strings. In LQG, space itself is quantized — it consists of discrete "spin networks", whose evolution over time is described as a "spinfoam". No continuous space at the Planck scale (10⁻³⁵ m). LQG makes different predictions than string theory for quantum gravity effects and could be tested through gamma-ray bursts or CMB polarization.

Dark Matter

What the universe is really made of

Only 5% of the universe consists of the visible matter that the Standard Model describes. 27% is dark matter — a form of matter that interacts gravitationally but neither emits, absorbs, nor reflects light. 68% is dark energy — a mysterious anti-gravity effect that accelerates the expansion of the universe.

Dark Energy: 68%

Cosmological constant Λ · origin unknown

Dark Matter: 27%

WIMPs · axions · sterile neutrinos · primordial black holes

Baryonic Matter: 5%

Everything the Standard Model describes

The strongest candidate for dark matter is WIMPs (Weakly Interacting Massive Particles) — hypothetical particles that interact only via the weak force and gravity. Despite decades of searching with direct-detection experiments (LZ/LUX-ZEPLIN, XENON, PandaX), no WIMPs have yet been found. This rules out large regions of parameter space, but still leaves many possibilities open.

Outlook

The future of particle physics

The next generation of experiments promises new insights: the High-Luminosity LHC (HL-LHC) is expected to drastically increase the collision rate from 2030 onward, making rare processes accessible. Proposals for new accelerators could produce new physics directly: the FCC (Future Circular Collider, a planned tunnel circumference of ~91 km) is intended to reach energy scales of up to 100 TeV in its final stage, FCC-hh; the ILC (International Linear Collider), by contrast, is planned as a precision "Higgs factory" at 250 GeV up to roughly 500 GeV to 1 TeV at most, not in the 100 TeV range.

Gravitational-wave detectors (LIGO, LISA), neutrino telescopes (IceCube), cosmic microwave background missions, and direct dark matter detectors open up new windows. The next great discovery in particle physics could come just as easily from an accelerator as from a cosmic observation instrument.

The Standard Model will remain the framework — but it is a framework waiting to be extended. The question is not whether, but when and which new physics will be found. Every measurement at the LHC, every glimpse into the depths of space, every detector deep underground is another piece in the puzzle of a deeper theory of nature.

The Goal

Physicists dream of a "theory of everything" — a single mathematical framework that describes all four fundamental forces, all particles, and the structure of space and time in one coherent theory. Whether that is ever achievable, no one knows. But the search for it has already delivered the deepest insights into the nature of reality that humanity has ever had.