The cosmological distance scale
stages:
1. 视差( parallax ):
最直接,根据对称视觉位移来计算距离,仅用于近地星
2. Cepheid variable:
3. supernovae:
白矮星爆炸产生巨大的能量,通过同一个星系的超新星来确定cepheid star的位置
Bonus:红移
哈勃定律:
通过爆炸物质光谱到达地球红移的比例来计算距离。这也是宇宙膨胀论的证据之一。(https://zh.wikipedia.org/wiki/%E5%93%88%E5%8B%83%E2%80%93%E5%8B%92%E6%A2%85%E7%89%B9%E5%AE%9A%E5%BE%8B)
如何判断估计红移:
斯隆数字巡天计划使用5个300到1100纳米的光学和红外滤光片测量每个物体的通量。
ugriz是光片名,数据中表示不同波长光线的通量。
The five Sloan filters linked to four colors. u-g, g-r, r-i and i-z.差值表示颜色。
困难之处:如何判断原来位置接收到的变化通量是红移造成的还是另一个东西(object)
用决策树,使用已确认红移的星系数据作为训练集
two different populations: regular galaxies and quasi-stellar objects (QSOs). QSOs are a type of galaxy that contain an actively (and intensly) accreting supermassive black hole. This is often referred to as an Active Galactic Nucleus (AGN). The light emitted from the AGN is significantly brighter than the rest of the galaxy and we are able to detect these QSOs out to much higher redshifts. In fact, most of the normal galaxies we have been using to create our models have redshifts less than z≈0.4, while the QSOs have redshifts all the way out to z≈6.
QSO的红移比一般的星系大得多,且分布很随机
(其实做这个之前应该先看一下自变量跟因变量的分布关系的)