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New measurements of the sky's blackness

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Re: New measurements of the sky's blackness

#41

Post by iso1631 »

dclowd9901 wrote: post #33
pwagland wrote: post #5

Specifically, from the article: > New measurements of that weak background glow show that the unseen galaxies are less plentiful than some theoretical studies suggested, numbering only in the hundreds of billions rather than the previously reported two trillion galaxies. We used to think that we couldn't see 90% of the galaxies/stars, this study proposes that we can't say about ½ of the galaxies/stars.

One of my favorite statistics is the "we don't even know what we don't know" statistic. Every time, it's a session of imagination trying to figure out how they figured that out.

Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tends to be the difficult ones

That statement was ridiculed at the time, and not by people pointing out the unknown knowns

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

Re: New measurements of the sky's blackness

#42

Post by Cederfjard »

permille42 wrote: post #22
crazydoggers wrote: post #16

Earlier quoted context omitted.

That’s not known. This article refers to the observable universe which is bound by the speed of light. Whether the universe is infinite or not is an open question. It might be finite but unbounded. Spacetime geometry is complicated.

Neither the title nor the summary say anything about the "observable universe". They make broad claims beyond that. The detail of the article in no way mentions that there could very well be infinite duplicates of what we see out there beyond observable distance.

This is the first sentence in the lead paragraph:

>How dark is the sky, and what does that tell us about the number of galaxies in the visible universe?

Re: New measurements of the sky's blackness

#43

Post by pvorb »

tsimionescu wrote: post #30
pvorb wrote: post #11

Earlier quoted context omitted.

This depends on your definition of visible. If you think of visible as some form of light reaching a man-made object, then GPs statement is still true.

Well, stars whose light has never even reached us are likely out of our light hypercone, so they might not exist in the scientific sense (i.e. Their existence or non-existence is completely unprovable and has no consequences, even in principle).

Isn't this like saying that aliens do not exist, because we never received any signal from them? Of course you can see it like that, but does it really change the probability of their existence?

I think there's a difference between what we know and what actually exists.

Re: New measurements of the sky's blackness

#44

Post by XorNot »

dclowd9901 wrote: post #35
crazydoggers wrote: post #10

Important to note, that this number refers to the “observable universe”. The number might be much higher, especially given inflation theory, but those galaxies currently lay outside our observable universe due to speed of light constraints. Another interesting possibility is a finite but unbounded universe which means we might see duplicate galaxies from light circumnavigating (think of an old video game where you le…

> Another interesting possibility is a finite but unbounded universe which means we might see duplicate galaxies from light circumnavigating (think of an old video game where you leave the screen at the top and appear on the bottom). Would this mean the universe would be spherical?

Nope! Lots of possible finite but unbounded shapes - i.e. a torus shaped universe would have no observable edge, and also wouldn't be obviously wrapping around (you are at no risk of seeing light from yourself headed back at you from the other direction).

You can then twist those geometries up into all sorts of knots and strange shapes to get all manner of universes where there is no edge, but light paths don't obviously loop back around.

Re: New measurements of the sky's blackness

#45

Post by _0w8t »

sandgiant wrote: post #29
kkaranth wrote: post #9

Noob physics question: I assume this talks about stars close enough that we can receive their light. Can we say anything about the number of stars & galaxies that are further away than that?

Good question! As most results in extra galactic astrophysics this one also depends on the cosmological model used. The actual paper linked in the article describes it well: "The cosmic optical background (COB) is the average flux of visible light photons averaged over the volume of the observable Universe. It reflects, at least in part, an integral over the cosmological history of star formation occurring in recogni…

The article indirectly pointed out that the reason for previous overestimate was model error about solar system light background.

The biggest source of uncertainty with the new estimate is modeling of background light of our galaxy. Too bad that we cannot yet send a probe outside it to measure things directly.

Re: New measurements of the sky's blackness

#46

Post by antonvs »

dclowd9901 wrote: post #35
crazydoggers wrote: post #10

Important to note, that this number refers to the “observable universe”. The number might be much higher, especially given inflation theory, but those galaxies currently lay outside our observable universe due to speed of light constraints. Another interesting possibility is a finite but unbounded universe which means we might see duplicate galaxies from light circumnavigating (think of an old video game where you le…

> Another interesting possibility is a finite but unbounded universe which means we might see duplicate galaxies from light circumnavigating (think of an old video game where you leave the screen at the top and appear on the bottom). Would this mean the universe would be spherical?

The standard FLRW metric[1] that's used to model the universe is "not necessarily simply connected," which means it allows for shapes such as toruses, multiple connected toruses, and so on. There's one proposal for something called the Poincaré dodecahedral space - see "Is the universe a dodecahedron?"[2]

But a sphere, or rather, a higher-dimensional hyperspherical equivalent, is the simplest possibility for that kind of scenario, and the FLRW metric allows that.

This article, "The Shapes of Space", has some nice explanations: http://www.ic.uff.br/~aconci/poincare.pdf

[1] https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtr... [2] https://physicsworld.com/a/is-the-universe-a-dodecahedron/

Re: New measurements of the sky's blackness

#47

Post by ljlolel »

ivalm wrote: post #8
ipnon wrote: post #4

This paper would seem to put many others estimating the prevalence of astrobiological activity into question.

I don’t think so. For astrobiology we are mostly looking for life in our galaxy, this study looks at presence of very far away galaxies. For purposes of communication those galaxies are forever out of reach as they are at such high red shifts they are escaping from us faster than speed of light.

"escaping from us faster than speed of light." ?????

Re: New measurements of the sky's blackness

#48

Post by tsimionescu »

pvorb wrote: post #43
tsimionescu wrote: post #30

Earlier quoted context omitted.

Well, stars whose light has never even reached us are likely out of our light hypercone, so they might not exist in the scientific sense (i.e. Their existence or non-existence is completely unprovable and has no consequences, even in principle).

Isn't this like saying that aliens do not exist, because we never received any signal from them? Of course you can see it like that, but does it really change the probability of their existence? I think there's a difference between what we know and what actually exists.

Well, if the aliens exist outside our light hyper-cone (that is, by definitio, any kind of information from them has never reached and will never reach any piece of information we create) then it is the same.

However, if any aliens have lived or will live anywhere in the visible universe, then that is something we could at least in theory confirm our deny at some point, so it is not the same.

It's also important to remember that time is relative - that is, that events in far away places don't happen at a definite time compared to those happening here. That makes their existence different to me than what we normally think of as existence.

Existence has two senses anyway - one is the empirically verifiable bottom of existence that science can work with: this is the one I was referring to.

The other is a philosophical notion of existence that is harder to pin down. In this sense, God may well exist, the nultiverse with many big bangs may be real, infinity may be real etc. But it is impossible right now to make any kind of scientific arguments about these things, so they can only exist outside the realm of science (as we know it today).

Re: New measurements of the sky's blackness

#49

Post by tsimionescu »

dclowd9901 wrote: post #34
tsimionescu wrote: post #30

Earlier quoted context omitted.

Well, stars whose light has never even reached us are likely out of our light hypercone, so they might not exist in the scientific sense (i.e. Their existence or non-existence is completely unprovable and has no consequences, even in principle).

I know this is goofy, but it would certainly have consequences if this realm of space contained an advanced civilization capable of reaching us before the light did.

If the theory of relativity is wrong, than yes, absolutely, the notion of a light hypercone is wrong as well, and the notion of what is within the realm of science changes.

Re: New measurements of the sky's blackness

#50

Post by detritus »

iso1631 wrote: post #41
dclowd9901 wrote: post #33

Earlier quoted context omitted.

One of my favorite statistics is the "we don't even know what we don't know" statistic. Every time, it's a session of imagination trying to figure out how they figured that out.

Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the…

I never unserstood that ridicule - I always thought it was an apt-description of the levels of ignorance we can honestly presume for ourselves. Refreshingly honest of Rumsfeld, if anything.
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