# 12. LIMITATIONS OF STANDARD MODEL

 Figure 12-1: MATHEMATICAL REPRESENTATION OF THE STANDARD MODEL.

Using a handful of particles of matter and a few fundamental forces, the Standard Model of particle physics provides a remarkably successful description of almost all presently known phenomena. Standard Model is really a collection of theories according to which quarks are the building blocks of matter, and forces act through carrier particles that are exchanged between the particles of matter. Even so, Standard Model is not complete–there are lingering questions in particle physics that have no answers in Standard Model:

• Why are there just four fundamental forces? Why are there only the particles that we see in the Standard Model, and no more?
• Standard Model does not offer a unified description of all the fundamental forces—it remains difficult to construct a theory of gravity similar to those for the other forces.
• The mass of the Higgs boson itself is not predicted by the Standard Model, although other particles acquire mass by interacting with the Higgs.
• Standard Model does not account for dark matter. Dark matter is thought to be made of invisible particles that do not interact with regular particles. Visible matter accounts for only 4% of the Universe while dark matter accounts for (27%).
• Standard Model does not offer an explanation for dark energy. Dark energy is a form of energy that appears to be associated with the vacuum in space, and makes up approximately 70% of the Universe. Dark energy is homogenously distributed throughout the Universe.

Physicists think there must be a larger, more elegant theory, a “theory of everything” and expect a new framework will be necessary near the Planck scale ($2.4\times 10^{18}$ GeV) where quantum gravitational effects become important.