APOD: A Map of the Observable Universe (2023 Jul 05)

[APOD Robot]

A Map of the Observable Universe

Explanation: What if you could see out to the edge of the observable universe? You would see galaxies, galaxies, galaxies, and then, well, quasars, which are the bright centers of distant galaxies. To expand understanding of the very largest scales that humanity can see, a map of the galaxies and quasars found by the Sloan Digital Sky Survey from 2000 to 2020 -- out to near the edge of the observable universe -- has been composed. Featured here, one wedge from this survey encompasses about 200,000 galaxies and quasars out beyond a look-back time of 12 billion years and cosmological redshift 5. Almost every dot in the nearby lower part of the illustration represents a galaxy, with redness indicating increasing redshift and distance. Similarly, almost every dot on the upper part represents a distant quasar, with blue-shaded dots being closer than red. Clearly shown among many discoveries, gravity between galaxies has caused the nearby universe to condense and become increasingly more filamentary than the distant universe.

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[Ann]

A Map of Galaxies and Quasars in the Observable Universe. Image Credit & Copyright: B. Ménard & N. Shtarkman; Data: SDSS, Planck, JHU, Sloan, NASA, ESA

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Understanding redshift when it comes to galaxies is not so hard. The more distant a galaxy is, the more redshift-reddened it will be. Of course, the optical color that we observe from a redshift-reddened galaxy also depends (sometimes heavily depends) on the intrinsic color of the galaxy. Nevertheless, it's a good rule of thumb to think of galaxies as more distant the more optically red-looking they are.

So in the APOD, the lower half of the slice mostly represents galaxies. The most nearby ones are mapped as blue because they are not very redshift-reddened at all. The yellow galaxies are moderately redshifted, and the red galaxies are very redshifted.

Quasars are different. They remain blue-looking out to large distances, and in their case we are not talking about mapped color, but about actual optical colors.

Optical colors and spectra of quasars of different redshifts.
Credit: SDSS.

Optical colors and spectra of quasars of different redshifts.
Credit: SDSS.

The spectra of the redshift-reddened quasars are quite different, but the quasars appear optically blue out to redshifts way past 3. But after redshift 4, the quasars start becoming optically red.

The color depends on what part of the quasar's total spectrum becomes optically visible because of its redshift. Quasars are intrinsically very blue - or rather, they produce copious amounts of hard shortwave radiation. At higher and higher redshifts, various parts of their total spectra become redshifted into the optical part of the electromagnetic spectrum. We keep seeing their optical colors as blue or bluish, until their redshift becomes larger than 4.

I highly recommend this page from Sloan Digital Sky Survey, where you can see the the redshifted spectra and optical colors of quasars at different redshifts.

That is why the upper half of the "slice of the Universe" in today's APOD is so blue: It's because quasars remain optically blue up to quite high redshifts. Eventually, though, redshift does make their optical light red, which is why most of the uppermost dots in today's APOD are in fact red, from very highly redshifted quasars.

Ann

[Eclectic Man]

I love the 'nod' to the Hitch-Hiker's Guide to the Galaxy 'Total Perspective Vortex' with the notice "You are here"

Thanks folks, you put a smile on my face.

[Conrad12n]

This is an interesting approach to vitalizing the universe, however I am confused about the slice in the third dimension. What is thickness, or wedged angle? It seems to me if the slice were significantly deep the filamentary nature of the near region would be blurred and lost. If it was thin the Quasar count would be small.

Conrad

[Guest]

Why does every depiction of the universe always seem to imply that we are at the center? Would all this look the same to someone living out near the "outer reaches?" Where, exactly are "we"?

[Chris Peterson]

Why does every depiction of the universe always seem to imply that we are at the center? Would all this look the same to someone living out near the "outer reaches?" Where, exactly are "we"?

Every observer is at the center of their own observable universe, and this would look structurally the same to an observer anywhere in the Universe (including from outside our observable universe).

[VictorBorun]

Why does every depiction of the universe always seem to imply that we are at the center? Would all this look the same to someone living out near the "outer reaches?" Where, exactly are "we"?

Every observer is at the center of their own observable universe, and this would look structurally the same to an observer anywhere in the Universe (including from outside our observable universe).

IMHO universe means oikoumene, or home space, or reachable zone. The tools we have reach as far as the cosmic microwave background in every direction that is not obscured by the clouds. It makes our universe similar to an octopus with straight tentacles, the longest ones reaching the CMB

[Chris Peterson]

Why does every depiction of the universe always seem to imply that we are at the center? Would all this look the same to someone living out near the "outer reaches?" Where, exactly are "we"?

Every observer is at the center of their own observable universe, and this would look structurally the same to an observer anywhere in the Universe (including from outside our observable universe).

IMHO universe means oikoumene, or home space, or reachable zone. The tools we have reach as far as the cosmic microwave background in every direction that is not obscured by the clouds. It makes our universe similar to an octopus with straight tentacles, the longest ones reaching the CMB

Conventionally, "the Universe" refers to the entire thing, including the likely vastly larger section outside our observable universe. The observable universe is a sphere. The shape of the entire universe is uncertain.

[VictorBorun]

Every observer is at the center of their own observable universe, and this would look structurally the same to an observer anywhere in the Universe (including from outside our observable universe).

IMHO universe means oikoumene, or home space, or reachable zone. The tools we have reach as far as the cosmic microwave background in every direction that is not obscured by the clouds. It makes our universe similar to an octopus with straight tentacles, the longest ones reaching the CMB

Conventionally, "the Universe" refers to the entire thing, including the likely vastly larger section outside our observable universe. The observable universe is a sphere. The shape of the entire universe is uncertain.

I just checked what wiki says.
They do say that the universe is Pythagoras' pan, meaning all there is (even beyond oikoumene, currently inhabited or reachable by us).
Well you cannot possibly learn and map the part that is so far not reachable by us. So this APOD properly added the word Observable.

[johnnydeep]

Ann wrote above:

The spectra of the redshift-reddened quasars are quite different, but the quasars appear optically blue out to redshifts way past 3. But after redshift 4, the quasars start becoming optically red.

The color depends on what part of the quasar's total spectrum becomes optically visible because of its redshift. Quasars are intrinsically very blue - or rather, they produce copious amounts of hard shortwave radiation. At higher and higher redshifts, various parts of their total spectra become redshifted into the optical part of the electromagnetic spectrum. We keep seeing their optical colors as blue or bluish, until their redshift becomes larger than 4.

Thanks for this explanation, Ann! Why there were two sections of this image, both with color gradations from blue to read, rather than one long gradation, was confusing the Beelzebub out of me!

[Chris Peterson]

IMHO universe means oikoumene, or home space, or reachable zone. The tools we have reach as far as the cosmic microwave background in every direction that is not obscured by the clouds. It makes our universe similar to an octopus with straight tentacles, the longest ones reaching the CMB

Conventionally, "the Universe" refers to the entire thing, including the likely vastly larger section outside our observable universe. The observable universe is a sphere. The shape of the entire universe is uncertain.

I just checked what wiki says.
They do say that the universe is Pythagoras' pan, meaning all there is (even beyond oikoumene, currently inhabited or reachable by us).
Well you cannot possibly learn and map the part that is so far not reachable by us. So this APOD properly added the word Observable.

The observable universe is the part of the universe where the expansion rate is less than c. While we cannot observe the rest of the Universe, we may well be able to understand its nature.

[Avalon]

Is there any possible way to estimate, to know, or have any inkling at all of where we are in the universe, observable or not?

[Chris Peterson]

Is there any possible way to estimate, to know, or have any inkling at all of where we are in the universe, observable or not?

Most likely, the center. Because in three dimensions, any point can be seen as the center.

[VictorBorun]

Is there any possible way to estimate, to know, or have any inkling at all of where we are in the universe, observable or not?

Most likely, the center. Because in three dimensions, any point can be seen as the center.

and if there are some hints in the foam of voids and filaments that we are in fact off some kind of a centre or an axis or a plane then the Euclid space telescope may well notice and the James Webb space telescope's follow-ups may well confirm this kind of news in a couple of years

[Ann]

Is there any possible way to estimate, to know, or have any inkling at all of where we are in the universe, observable or not?

Each of us is at the center of our own personal universes. That is because the observable universe looks more of less the same in all directions. Astronomers believe that no matter where you are in the observable universe, your telescopes would reveal more of less the same galactic skyscape around you. Not exactly though, because your local neighborhood would look different if you found yourself in a densely populated cosmic filament than if you were located in a cosmic void.

Clusters of galaxies line up in filaments. Between them are cosmic voids. Image credit: TNG COLLABORATION.

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However, the observable universe does not look the same everywhere in time. Closer to the Big Bang, or farther away from the Big Bang, the observable universe would look different than it does today. If we could travel back in time, to an era closer to the Big Bang, we would find that the observable universe was more dominated by star formation, spiral and irregular galaxies and quasars than it is today. And the universe itself would be smoother, with less pronounced filaments and voids.

Evolution of the cosmic web. The filaments and voids grow more pronounced
over time. Image credit: smcalpine.com/Durham University

Chris once told me that the center of the Universe is the Big Bang. Thanks, Chris, that's a good way of thinking about it.

So as long as we are at the same distance in time from the Big Bang, the observable universe looks more or less the same in every direction.

Ann

[beryllium732]

So the blue part over the red are full of quasars? How come they are blue instead of red? Shouldn't it be the other way around? Or is it because they are giving out so much blue light gamma, x-ray and uv light that they are more blue than the nearer galaxies?

[Ann]

So the blue part over the red are full of quasars? How come they are blue instead of red? Shouldn't it be the other way around? Or is it because they are giving out so much blue light gamma, x-ray and uv light that they are more blue than the nearer galaxies?

You may want to read my post here.

But yes, the long and short of it is that quasars produce so much hard shortwave radiation that when different part of their spectra become redshifted into the visible part of the spectrum, their optical light remains blue or bluish up until very high redshifts. That is why only the most distant quasars have their spectra so redshifted that they become "optically red".

I highly recommend this page from Sloan Digital Sky Survey, where you can see the the redshifted spectra and optical colors of quasars at different redshifts.

Ann

[righttotwobarearms]

The biggest and brightest things in the known universe are also the very youngest, Sorry, I ain't fallin' for it!