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希格斯宇宙学

希格斯相变

我们知道现在的希格斯势能,希格斯的场的期望值是不为零的,在早期宇宙,由于温度效应,希格斯场的真空期望值为零

我们期待,这中间会有相变的过程,其序参量为希格斯的真空期望值。

我们来看一下相变,最常见的相变过程是水的相变: 固液气三态。随着温度的变化,存在三相,随着压强的变化,也有改变。气体到液体的相变是一阶相变,随着压强的增加,逐渐过度到二阶相变,压强再增加,相变变成了平滑过渡。

同样的,希格斯的相变也是类似的。只不过压强变成了希格斯的质量,随着希格斯质量的变化,相变也发生变化。当希格斯质量很小的时候,我们得到一阶相变,随着希格斯质量的增加,逐渐变为二阶相变,当希格斯质量更大时,相变变成了平滑过渡。根据标准模型,希格斯质量为125 GeV,标准模型的相变因此是平滑过渡。

为了实现电弱相变,需要加入新的标量自由度,随着标量自由度的增加,从平滑过渡逐渐过渡到一阶相变。

让我们关注于液气相变,当我们煮开水的时候,我们发现有气泡逐渐长大,气泡内是气体相。类似的,希格斯的相变,也会有泡泡生成,泡泡内是破缺相,泡泡外是对称相,泡泡不断长大,最终充满整个宇宙。

CP破坏

重子生成

泡泡的扩张过程。

宇宙中充满着由基本粒子组成的等离子体,因为一阶相变的存在,泡泡会形成。泡泡形成以后,不断扩张的过程中,泡泡的边界不断碰撞各种等离子体粒子。一般情况下,等离子体粒子和泡泡壁的碰撞,在泡泡壁的静止坐标系中,根据量子力学原理,会存在入射和反射的几率,这一几率一般对于正粒子和反粒子都是一样的。但是如果泡泡和泡泡壁的碰撞过程是破坏CP守恒的,粒子的入射和反射几率,和,反粒子的入射和反射几率会有所不同。对于正粒子,一般入射比反射的多,对于反粒子,反射比入射的多。如果只看泡泡壁内的粒子,透射的粒子比反粒子多。可是泡泡也在扩张,所以反射的粒子也进入泡泡中了。这样的话,粒子反粒子不对称并不存在。可是,由于泡泡壁以外,由于Sphaleron过程,反粒子有一部分转化为粒子,所以泡泡里的粒子比反粒子多。

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