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I have a follow up question for What do the stars really look like, if you could travel at almost lightspeed?

If we are on a spaceship, travelling at a constant speed of 99.999% the speed of light, most of the stars I see on one side will be blue-shifted. And most of the stars on the other side will be red-shifted. If I can detect and count those red and blue shifted photons and plot a distribution graph of it, can't I detect that I am in motion relative to the universe? Is it the same as being in absolute motion? But that is not allowed in the Relativity. So I won't be able to do such detections.

Please help me understand where I'm wrong.

Consider both these scenarios, if possible:

  • passing through a galaxy at relativistic speeds.
  • transversing the inter-galactic space at relativistic speeds.

Also, on Earth, expansion of the universe means that all distant galaxies appear to be receding away from us. Will this observation still hold true on the relativistic spaceship?

AbduSami bin Khurram
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[...] can't I detect that I am in motion relative to the universe? Is it the same as being in absolute motion? But that is not allowed in the Relativity. So I won't be able to do such detections.

Why are you saying this? The question that you have linked to, have answers talking about the Cosmic Microwave Background radiation. We know that we are moving relative to the CMBR, and we also know how fast we are moving relative to it.

Also, on Earth, expansion of the universe means that all distant galaxies appear to be receding away from us. Will this observation still hold true on the relativistic spaceship?

Yes. The "velocity of receeding" at extremely large distances will exceed the speed of light, and so obviously the relativistic spaceship will conclude that the universe is expanding too.

naturallyInconsistent
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