3
$\begingroup$

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?

Improve this question
$\endgroup$

2 Answers 2

Reset to default
6
$\begingroup$

Yes.

When light moves through the expanding space, it loses energy and becomes redshifted. As galaxies form, they usually undergo intense starbursts which emit lots of visible and ultraviolet light. In fact, the most distant galaxies — and hence the ones we see farthest back in time — are often detected from their 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 directions. 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.

Improve this answer
$\endgroup$
2
$\begingroup$

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.

Look at the Star that is moving away

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

Improve this answer
$\endgroup$
6
  • 1
    $\begingroup$ 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. $\endgroup$
    – pela
    Commented Mar 31, 2015 at 21:49
  • 1
    $\begingroup$ So there is a difference between something like cosmological redshift and nearby objects that exhibit a local Doppler-effect redshift? $\endgroup$
    – theVerma
    Commented Mar 31, 2015 at 21:54
  • $\begingroup$ @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? $\endgroup$
    – aloha
    Commented Mar 31, 2015 at 21:54
  • 1
    $\begingroup$ @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. $\endgroup$
    – pela
    Commented Mar 31, 2015 at 22:05
  • 2
    $\begingroup$ I agree with pela here. Cosmological redshift and Doppler shift are quite distinct from one another. Special relativity says that redshift tends to infinity as the relative velocity between two objects approaches the speed of light. A finite redshift means a recession velocity less than that of the speed of light. In general relativity, that's not the case. In the ΛCDM model, all objects we see with a redshift of 1.5 or more not only are currently receding from us at a superluminal velocity, they were doing so when the light we see now was original emitted. $\endgroup$
    – David Hammen
    Commented Apr 1, 2015 at 23:18

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .