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Ask HN: How do astronomers find the oldest sections of the sky to look at?

61 points by thatbat ↗ HN
Obviously a lot of space related questions with the JWST doing it's thing, but i wanted to know how they find the ancient sections of space to analyze? Do they scan the entirety of space looking for super redshifted space, or are there areas of the sky that are known to be particularly old?

15 comments

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mytailorisrich
I don't think that there is any section older than any other section. It's rather a question of looking 'far' enough, which probably means targeting a very narrow section to detect very faint, 'reddish' signal.
dredmorbius
"Young" sections of the sky are near. They typically have bright stuff in them --- stars within our own galaxies, bright nearby galaxies otherwise.

In general, any part of the sky in which there are known objects ... is likely younger than parts in which there are none.

Because of the dynamics of the Big Bang, that is, the fact that the known universe was compressed to a singularity of infinite density, if you look back far enough, you'll find something, if only the cosmic background. To date, the background of every deep field image take has had fainter and remoter background objects.

Aim away from the galactic plane and away from the galactic centre. Look away from known galaxies and galactic clusters. Those are the emptiest parts of the sky, and anything you can see there will likely be both distant and old. Look for a long time (use long exposures) to gather as much light as possible. Those are deep fields, and that is old space.

You can also choose your observation frequency/wavelength based on the anticipated redshift, which itself is a function of distance.

TL;DR: Find the emptyiest part of the sky you can, and stare at it for as long as you can, at the right red for the distance.

https://en.wikipedia.org/wiki/List_of_deep_fields

Disclaimer: Though I've looked through a few 'scopes and blew up a planet once, I'm no astronomer.

mlsu
What I'm not quite sure of exactly is: (1) why infrared? Why doesn't the redshift extend further, into microwave or radio? Is that an instrument limitation (we can't build a radio telescope big enough to look at 14 billion years ago so we settle on an infrared telescope that can look 13 bya?

And, (2) is there a limit to this? I understand that the Hubble constant is where the "amount" of redshift comes from (because, the it's the rate of expansion of the universe?) -- but is this linear? If we can see things from 6bya in the visible spectrum, and things from 13bya in the infrared, does that mean that we could see the beginning of the universe itself to 14bya if only we looked just past infrared? Or does it ramp up exponentially and blast through radio because it's a singularity?

And -- if we managed to look "in between" every star/galaxy so that we were truly looking at a 0% deep field with no "objects" in the way, would we be looking at the beginning of the universe itself?

I know that these are all hypotheticals and we can't actually do this, just wanted to clarify that I'm on the right track about the way this works, since I'm the resident JWST geek of the group.

x86x87
It does goes into microwave etc. We sort of can see the "beginning of the universe" See: https://en.m.wikipedia.org/wiki/Cosmic_microwave_background

The other thing to consider is that since space is expanding, we can only see the observable universe. Given enough time, the only thing that we're going to see is out own local part of the universe (that is not expanding - so there are parts of the universe that are expanding and parts that are somewhat steady state - think bubbles in liquid)

dredmorbius
The degree of redshift is determined by the Hubble Constant, which describes how much the expansion of the Universe increases with distance.

The maximum distance (time) is ~13.8 billion years. The very earliest period of the Universe cannot be directly observed as matter was too dense for light to be transmitted through it (about 375,000 years after the Big Bang). The first stars are thought to date from 400 million years.

Knowing when you want to look to, and how far that is (time is distance at light speed), the red-shift amount is calculable. Peak stellar emission (~3000K for a Class G star such as our Sun) would be redshifted by that amount, and the detection frequency(ies) / wavelength(s) could be chosen correspondingly.

Detection in the infrared or below also requires chilling reflector and sensor surfaces, as was the case for the JWST, which raises challenges.

My understanding is that, yes, looking back and observing nothing is looking toward the primordeal Big Bang radiation itself. My understanding is that that itself would be emitting at the spectrum of the Cosmic Background Radiation, in the microwave range, at a blackbody temperature of about 3K. That would have a range of wavelengths, though I believe the peak would be about 0.2cm.

https://map.gsfc.nasa.gov/universe/bb_tests_cmb.html

That's much longer (lower frequency) than the JWST can detect, and it would simply see blackness (no detectable radiation).

potamic
Not an expert. My layman understanding is that we are at the center of the observable universe. There are no sections of the sky that are older than others. I think what the deep field image shows is that, no matter where we look we are surrounded by ancient galaxies going as far back in space and time as we are able to.
drdeca
I think one just, looks at the parts that look emptier, so that there is less nearby stuff to get in the way of seeing the stuff further away (and this from a longer time ago)