[bryananderson]

Seeing through dust is part of it. Another part is redshift: because of relativity, things moving away from us appear redder (longer wavelength) the faster they’re moving away. That’s the same principle (Doppler effect) as the lower-pitched siren sound as the ambulance drives away. Because the universe itself is expanding, the farther something is from us, the faster it’s moving away relative to us, and the more redshifted it appears to us. And again because of relativity, the oldest objects we can see are the ones that appear farthest away (i.e. their light is just now getting here, after 13 billion years, from 13 billion light years away). Thus, if we want to study the earliest times of the universe, we must study the most redshifted objects—which have shifted all the way out of the visible spectrum and into the infrared. Hence, Webb is an infrared telescope.

[dotancohen]

Your statements about redshift and distance are correct, but I do not see where relativity comes into it. Even in Newtonian physics redshift occurs. If you could elaborate I would love to learn. Thank you!

[sushibowl]

Newtonian physics does have the Doppler effect which can cause red shift, but it doesn't accurately explain the red shift that occurs when observing distant galaxies. That's because this red shift occurs due to three factors:

1) The galaxy is moving away from us. This is most like the classic Doppler effect, but because of the high relative velocities involved, time dilation needs to be taken into account to model the red shift accurately, thus at least special relativity should be used.

2) The light travels through space with different curvature. For example, light originating near a very massive star will red shift when moving away from that star because it moves into less curved space. General relativity is needed to explain this effect.

3) The light travels through expanding space. For very distant galaxies this becomes the dominating factor of red shift, as we see an amount of red shift directly corresponding to their distance from earth. General relativity also explains this effect.

[dotancohen]

I see, thank you.

[bryananderson]

Yeah, you’re correct, you don’t need to invoke relativity to explain the Doppler effect. Sibling comment did a good job explaining the ways relativity does impact redshift, but my initial statement (“because of relativity…”) was not correct.