宇宙中的冷却
Cooling in the Universe
摘要：
宇宙学课程中的一个重要问题是根据热力学经典概念，讨论宇宙流体膨胀过程中的冷却机制。在这篇简短的教学论文中，我们引用了这些问题，并给出了一个自然的方法来处理这个问题，通过测量粒子在宇宙流体中的分散速度，由一个共同运动的观察者。我们表明，宇宙流体中粒子的热运动偏离哈勃流，并遵循由均匀宇宙引力支配的测地线。粒子的“热奇速”动力学导致在膨胀的宇宙中，粒子的动量以膨胀因子的倒数松弛，结果是失去粒子的能量，从而冷却宇宙。
英文摘要：
One of the questions in the cosmology courses is the cooling mechanism of cosmic fluid during it expansion according to classical concepts of the thermodynamics. In this short pedagogical paper, we quote the questions and give a natural approach dealing with this problem by measuring the dispersion velocity of the particles in the cosmic fluid by a comoving observer. We show that the thermal motion of the particles in the cosmic fluid deviates from the Hubble flow and follows the geodesics governed by the gravity of the homogeneous universe. The dynamics of the "thermal peculiar velocity" of the particles leads in an expanding universe, momentum of the particles relax by the inverse of the expansion factor and the result is losing the energy of particles, hence cooling the universe.
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