# Why are distant objects observed in the near infrared?

I was reading an article that explains why JWST is a successor to Hubble and not a replacement for Hubble. They explained that Hubble's science pushed astronomers to look at longer wavelength. And then they said:

In particular, more distant objects are more highly redshifted, and their light is pushed from the UV and optical into the near-infrared.

So basically to observe the first galaxies, astronomers have to observe in infrared. My question is why distant objects require observations in the infrared?

Is it because they are at a very large distance from us, so the light has lost a lot of energy on its way so it's detectable in the infrared?

Yes.

When light moves through the expanding space, it loses energy and becomes redshifted. As galaxies form, they a emit a lot of UV light, and are often detected from there emission at the so-called Lyman $\alpha$ light, which has a wavelength of 1216 Å. Galaxies began forming just a few hundred million years after the Big Bang. Since then, the Universe has a expanded by more than a factor of 10 in all direction. The wavelength of the light expands by the same factor, and thus a Lyman $\alpha$ photon emitted in this epoch today has a wavelength of $\sim12,000$ Å, or 1.2 $\mu$m, which is in the infrared.

This is because of something called the Doppler effect. As something moves away from us, the Electromagnetic waves it releases will have a longer wavelength.

Also, because of Hubble's law, galaxies that are further away have an increased velocity, making the red shift even more pronounced.

• Actually, it's not really the Doppler effect, as the galaxies lie approximately still in space. But you're right about Hubble's law, which describes the expansion of space. – pela Mar 31 '15 at 21:49
• So there is a difference between something like cosmological redshift and nearby objects that exhibit a local Doppler-effect redshift? – theVerma Mar 31 '15 at 21:54
• @pela, why it's not the Doppler effect? After all, all the galaxies are moving away from us. So why can't you account for Doppler effect in this case? – aloha Mar 31 '15 at 21:54
• @po6 I think this animation will explain the difference. webbtelescope.org/webb_telescope/science_on_the_edge/… – theVerma Mar 31 '15 at 21:59
• @po6: It's because the Doppler effect arises when observing light emitted from an object moving though space away from or towards us. But the galaxies don't move through space (well, they do move a little, but that's a minor effect). If you calculate the wavelength of observed photons emitted from a distant galaxy in a hypothetical universe that is static when they are emitted, then at some point expands for a while, and then stops before we observe, you will also see that they are redshifted, even though the emitting galaxy were stationary wrt. us both at the time of emission and observation. – pela Mar 31 '15 at 22:05