Hubble的发驰 ■Hubble's Discovery Z=(s-元e)/。≈(H人C)D H=100ho km/(s.Mpc) h =0.71±0.07 v≈HD Doppler Effect 仅当v<<c时才成立 e
n Hubble’s Discovery n Doppler Effect Z (obs e ) / e (H0 / c)D 100 km/(s.Mpc) 0 0 H h h0 0.71 0.07 c v Z e obs e 仅当v c时才成立 v H D 0
Why is there a cosmic redshift? X WHY IS THERE A COSMIC REDSHIFT? 星系基本 不在空间 WRONG:Because receding galaxies are moving R/GHT:Because expanding space stretches all light waves as through space and exhibit a Doppler shift. they propagate. 因为退行 中运动， In the Doppler Galaxies hardl 星系在空 effect,a galaxy's move through 它们向四 movement away space,so they from the observer emit light with 面八方发 间中运动， stretches the nearly the san light waves, wavelength in 它会产生 making them directions (to 射的光子 redder (top).The The wavelengt Doppler wavelength of gets longer du 有同样的 light then stays the journey. because spac 红移。测 the same during 波长。它 its journey expanding.Th through space the light gradu 定红移可 们因空间 (middle].The reddens (mid observer detects and bottom). 膨胀而增 算出星系 the light, amount of measures its redshift differ Doppler redshift from what a 长，造成 运动速度。 and computes the Doppler shift galaxy velocity would produc 红移。此 (bottom]. 红移与 Doppler 红移量值 不同
因为退行 星系在空 间中运动， 它会产生 Doppler 红移。测 定红移可 算出星系 运动速度。 星系基本 不在空间 中运动， 它们向四 面八方发 射的光子 有同样的 波长。它 们因空间 膨胀而增 长，造成 红移。此 红移与 Doppler 红移量值 不同。 × √
Can galaxies recede faster than light? X Speed of Light Distance Distance Hubble distance (Hubbleagexc)D=c/Ho 28
28 0 Hubble distance (Hubble agec)：DH c / H × √
Two kinds of motions Motion in space,can not be superluminal. Expansion of space itself can be superluminal
n Motion in space, can not be superluminal. n Expansion of space itself can be superluminal
Can we see galaxies receding faster than light? WRONG:Of course not.Light from those galaxies never R/GHT:Sure we can,because the expansion rate changes reaches us. overtime. A galaxy farther than the Hubble The photon initially is unable distance (sphere]recedes from to approach us.But the Hubble us faster than light.It emits a distance is not constant;it photon (yellow squiggle).As is increasing and can grow to space expands,the photon is encompass the photon.Once dragged away like someone that happens,the photon trying to swim against the approaches us and eventually current.The photon never reaches us. reaches us. 超过哈勃距离的星系退行速度超过光 起初光子不能接近我们。但哈勃距离 速。正如逆流游泳的人，当空间膨胀 不是常数：它在增大且会包裹光子进来， 时，光子会被拖住。光子到不了我们。 这时，光子会接近并最终到达我们
× √ 超过哈勃距离的星系退行速度超过光 速。正如逆流游泳的人，当空间膨胀 时，光子会被拖住。光子到不了我们。 起初光子不能接近我们。但哈勃距离 不是常数：它在增大且会包裹光子进来， 这时，光子会接近并最终到达我们