When we learnt about the Hubble Constant at school we were taught that Hubble had observed Red Shift due to the Doppler Effect, and thus the universe (and space time itself?) must be expanding. However, the Cosmic Microwave background (CMB) exhibits the same behaviour, but is explained through Cosmological Red Shift where the space itself is actually expanding.

Was Hubble observing Cosmological Red Shift (the same principle for the CMB)? If he perceived this Red Shift as the Doppler Effect then surely he didn't conclude that space itself was expanding?


1 Answer 1


The cosmological redshift can be also thought of as a Doppler shift. We cannot distinguish the difference between cosmological redshift and Doppler redshift, by just looking at the redshift data.

For example, let us imagine that you are looking at a car on the road. And you see that car is going with 50 km/h. You can say, the car is moving with 50km/h naturally. Or, I can claim that "the road is expanding" in such a way that car seems to move away with 50km/h. And by just looking at the light coming from the car, you cannot say which one is true.

Simply, the redshift of the galaxies can be thought of as a Doppler shift or cosmological redshift, it just depends on which frame you want to choose.

The CMBR data actually can contain the doppler effect since the COBE satellite (A satellite that measured the CMBR) rotates around the earth, the earth rotates around the sun, son rotates around the center of the milky way etc.

So the original CMBR image also contains this Doppler shift affect but we can remove these effects from it. enter image description here

CMB dipole distortion

Reference for the image

After removing these affects we get the normal CMBR image.

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    $\begingroup$ Thank you for the fantastic answer! Can we assume that Hubble did not conclude that the actual fabric of space time was expanding itself, instead the universe was just "growing"? $\endgroup$ Commented Feb 2, 2019 at 23:53
  • $\begingroup$ The only "growing" type in a homogeneous and isotropic universe is an expansion of the space in such a way that the distances between every point increase with time (the balloon analogy). Otherwise, it would violate the cosmological principle. I think that when he observed the redshifted galaxies he must have been thought that this cannot be caused by the local motions of the galaxies since all galaxies are moving away from us. So he concluded that space is expanding. Also, when you talk about the expansion you should not include time into it. Only space expands. $\endgroup$
    – seVenVo1d
    Commented Feb 3, 2019 at 6:24

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