We show that the same physical mechanism is fundamental for two seemingly different phenomena such as the formation of two-level systems in glasses and the boson peak in the reduced density of low-frequency vibrational states

g (ω) ∕ ω^{2}

. This mechanism is the vibrational instability of weakly interacting harmonic modes. Below some frequency

ω_{c} ⪡ ω_{0}

(where

ω_{0}

is of the order of Debye frequency), the instability, controlled by the anharmonicity, creates a new stable universal spectrum of harmonic vibrations with a boson peak feature as well as double-well potentials with a wide distribution of barrier heights. Both are determined by the strength of the interaction

I \propto ω_{c}

between the oscillators. Our theory predicts in a natural way a small value for the important dimensionless parameter

C = \bar{P} γ^{2} ∕ ρ v^{2} \approx 10^{- 4}

for two-level systems in glasses. We show that

C \approx {(W ∕ ℏ ω_{c})}^{3} \propto I^{- 3}

and decreases with increasing interaction strength

I

. The energy

W

is an important characteristic energy in glasses and is of the order of a few Kelvin. This formula relates the two-level system’s parameter

C

with the width of the vibration instability region

ω_{c}

, which is typically larger or of the order of the boson peak frequency

ω_{b}

. Since

ℏ ω_{c} ≳ ℏ ω_{b} ⪢ W

, the typical value of

C

and, therefore, the number of active two-level systems is very small, less than 1 per

1 \times 10^{7}

of oscillators, in good agreement with experiment. Within the unified approach developed in the present paper, the density of the tunneling states and the density of vibrational states at the boson peak frequency are interrelated.

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