Density Fluctuations in Our Universe (IMAGE) Hong Kong University of Science and Technology Caption It is well accepted that sinusoidal waves are produced in the primordial universe, through an event called "horizon crossing". At "horizon crossing", those waves got frozen and no longer propagating. In the figure, the horizon is represented by the gray vertical lines. Once the wavelength of the fluctuation become greater than the size of the horizon (the distance between the two vertical lines), the mode is frozen, which eventually become the seed of the large scale structure. Credit Department of Physics, HKUST Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.