@ScienceClicEN

It's a very good video, however I think there is a mixup between the relativistic Doppler effect (due to different velocities) and the cosmological redshift (due to the expansion of the universe).

In what regards the expansion of the universe I would tend to argue that it's fine to say the expansion of space is what stretches the wavelength of light. It is not really the same effect as the Doppler effect. In fact, many far away galaxies move away from us faster than the speed of light (or, more rigorously, their distance increases faster than the speed of light), which wouldn't make sense in the Doppler effect formulas (the Lorentz factor is only defined for v < c).

The main difference is that the Doppler effect's formula only works locally, or in a flat spacetime, whereas the cosmological redshift only works for comobile objects (objects motionless relative to the expanding space). To have a full description one needs to combine the two.

Mathematically, the cosmological redshift is only described by the ratio of the expansion factor of space between the times of reception and emission. The relative distance between the two objects, or their apparent relative speed (which is not a true comobile speed) doesn't come into play in the calculation. Only the factor of how much has the expansion stretched space between emission and reception.

If you imagine that a galaxy "moving away from us" due to the expansion of the universe emits a light ray, but that before it reaches us space starts contracting back to its initial size, we will receive the light with exactly the same frequency as it had when emitted. Even though the galaxy was "moving away from us" when it emitted it, it was not a true motion, only an apparent one due to the expansion of the universe, and hence it did not produce any Doppler effect.

At the beginning of the video it is stated that energy must be conserved, but in what regards the cosmological redshift I think it is precisely because the expansion of the universe breaks the law of energy conservation that the effect exists. Because the expansion is not time symmetric, Noether's theorem states that the energy of light is not conserved over time, which is why it is measured to have different values over time (regardless of where it was emitted or received).

@akiraak2247

Your great channel should expand like the universe

@EricCatlin425

Cannot believe how few views these amazing videos have. Someday you'll hit the algorithm and make bank

@nero1612

I really don't get why you don't have more views. This is so high quality and interesting, plus you already have a viral video with almost 500k views. Anyway hope you keep doing what you're doing because it's really something great. 
Best of luck man! :D

@Roberto-REME

Outstanding video production and you explained the topic in an engaging and interesting manner.  Your narration is outstanding ....with the exception of your periodic "sighs" which makes viewer think you're getting exasperated.  Really well done!

@PabloRuan

Discovered your channel this weekend, amazing work! Really appreciate it, cheers from Brazil!

@sou9901

This channel is criminally underrated

@gabrielebassetto8598

Keep up, great work. By far my favourite yt channel on science and I stumbled upon many of them

@ssff6648

Really underrated Channel

@kalpeshchaware4491

Your video are great and very clear to understand! I just wanted to say that try to put reference or source in description in case viewers want to read that....

@elicallaway342

The doppler effect requires a medium to distort the sound. Relativity has no medium

@jgmeng88

Love the way you explain things! Makes so much more sense to me

@tadeuszbloki2971

You're channel becoming one of my favourite and I think it's one of the best popular science channel on youtube.

@nafeesaneelufer5023

At 8:10  check the formula please. Momentum should be Planck's Constant divided by wavelength. Anyhow from difference in timings of two successive crests impacting the wall at rest and in motion has really clarified the reason behind shifting of wavelength of light.

@MrSkid56

Thanks for the video-I now can understand the Doppler-Fizeau effect that I was reading about.

@Mohit-ir5xo

I wish I could be a teacher like this with so much indepth and no cram part

@EricLehner

Hello from Canada - Terrific graphics support in the narrative!

@jforkum3948

I suggest the red shift seen from distant light is not caused by expansion, but rather by the mass a light wave encounters on its journey to your red shift measuring device. Just as a buoy in the ocean stretches a wave that encounters it as the wave accommodates the energy released as it travels around the buoy, the light wave stretches around any mass it encounters and accommodates the loss of energy as it moves past the mass encountered.  There is no way of knowing how many mass encounters a light wave has had as it travels to your red shift measuring device, only that light from very distant sources must encounter more obstacles of mass during its journey.

@dexter8705

So light doesn't always travel the same speed relative to us, we just make up time dilation so that it looks the same in all reference frames.

@austincarter7358

Question! Say you have a gamma ray burst 6 billion light years away. Would that light not redshift down into a less energized form of itself. For instance, can a gamma ray red shift down into an x-ray? If so, would that gamma ray burst that occured 6 billion light years away be red shifted into x ray (or lower) by the time it reached us?