Vũ trụ quan sát được chứa nhiều hơn 80 tỷ thiên hà, được nhóm lại trong quần thiên hà và siêu quần thiên hà. [7]. Những thiên hà điển hình bao gồm từ những thiên hà lùn với chỉ khoảng 10 triệu (107) sao[8] tới những thiên hà kềnh với một nghìn tỷ (1012) sao [ | QUARK SOUP ô ions .AV 0Ạt ữf iìga world I N THE LATE 1980s physicists at CERN Europe s particle physics lab in Geneva began a long series of experiments aimed at simulating the Big Bang in little bangs hot and dense enough to set quarks free. These are the fundamental entities that constitute the heavy matter in the atomic nucleus. No one doubted by then that each of the protons and neutrons in a nucleus consists of three fundamental entities called quarks. Various experiments with particle accelerators had indirectly confirmed the presence of the quarks in the nuclear material. But no one had seen any free quarks. If you try to liberate a quark in ordinary reactions between particles you unavoidably create a new quark and an antiquark. One of them immediately replaces the extracted entity. The new antiquark handcuffs the would-be escaper in a particle called a meson. This is the trick by which Mother Nature has kept quarks in purdah since the world began. To be more precise the confinement of quarks began about 10 millionths of a second after the start of the Big Bang at the supposed origin of the Universe. Before then in unimaginably hot conditions each quark could whizz about independently. Technically speaking it was allowed to show its colour in public. By the colour of a quark physicists mean a quality similar to an electric charge. But instead of just plus and minus the colour charge comes in three forms labelled red green and blue. The quarks are not really coloured but it s a convenient way of thinking about the conditions of their confinement in ordinary matter. In a TV screen a red green and blue dot together make white and the rule nowadays is that nuclear matter too must be white. That s why protons and neutrons consist of three quarks apiece and not two or four. One red one green and one blue quark within each proton or neutron are held loosely together by particles called gluons. The colour force carried by the gluons operates only over very short .
