Explore the Endless Universe

In terms of the scale alone， the earth on which human beings live is just a drop in the ocean of universe. Indeed， the Earth is not easy to find in the solar system， where the sun is in a galaxy of 200 billion stars， and the Milky Way is a member of the Virgo cluster with more than 1000 galaxies， and the Virgo cluster is just a common member of the universe. Human beings， who are not at the center of the universe， and whose existence， will not affect the fact that universe runs in accordance with laws of nature. From this point of speaking， human is tiny and miserable. However， the tiny human beings on earth who cannot influence or change the laws of the universe， can observe from the fireball of the cosmic explosion 13.8 billion years ago to the stars in the beautiful universe today. The comprehension of laws of cosmic evolution is something human beings should be proud of given that the history of human civilization is only 5000 years， negligible compared with the 13.8 billion-year-old universe.

Human beings first discovered gravitational waves two years ago， opened the fourth window of cosmic exploration， allowing us to understand the extreme physical state and process in the universe， to reveal the revolution and ultimate fate of the dense objects such as neutron stars and black holes， and even to hear the "sound" of the primordial explosion of the universe.In 20 years， with the completion of a large number of exploration equipment on the ground and in the space， gravitational wave astronomy will prosper and reveal the secrets that the universe once hid from a new perspective.

Compared with gravitational wave astronomy which is new and strange， the most familiar means of exploring the universe is the beginning of optical astronomy. However， optics is just a very narrow window of electromagnetic waves. Astronomers today have opened all the windows of the electromagnetic wave， collecting various radiation information from the universe： radio astronomy （including microwave）， optical astronomy （including infrared and ultraviolet）， X-ray astronomy， gamma ray astronomy， etc.， with wavelengths ranging from dozens of meters to several picometers， spanning up to 12 magnitudes. For example， the remnants of the Big Bang （cosmic microwave background） are currently in the microwave band. Electromagnetic wave is widely used and is the most informative window with the most mature technology.

The third and fourth kind of messengers holding the secrets of the universe are called cosmic rays and neutrinos. Both fall into the category of high-energy astrophysics. They dont come in the form of waves， but physical particles used to detect remote celestial bodies. High-energy cosmic rays are charged particles， consisting mainly of protons and helium nuclei. Its energy is far beyond the range of land accelerator， just like a natural high energy physics laboratory. As cosmic rays are affected by the interstellar magnetic field， the receiving direction has deviated far from the original celestial body position， so that it is very difficult to trace the source of rays， but the neutrino can compensate for the disadvantage of the high energy cosmic ray localization as it can travel freely in the universe free from any constraint， so we can find its source easily. However， due to the same reason， neutrinos are hard to capture， and building a neutrino detector or telescope is even more difficult. So far， people are using giant glaciers in Antarctica or large water containers （including directly using the sea as a container） as a target in order to intercepting neutrinos from the universe. The “top secret” of many celestial bodies and the sun will finally be revealed one day.

There are infinite secrets concealed in the vastness of the universe. People with true curiosity will go further and farther on the journey of exploring the universe. Besides the four detecting methods in effective use today， there may be a fifth or even more new ways to reveal the evolution patterns and mysteries of stars， galaxies and even the entire universe. The greatness of ours lies in our exploration of the infinite universe with our finite life， and yet we will not rest， for its our greatest joy.endprint