▲ 89 points
back
63 comments
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
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.
> I never unserstood that ridicule
Perhaps try the response with a different issue and respondent swapped in?
Q: There are reports there is no evidence of a direct link between [...]
Vaccination-autism policy advocate: Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; [...] 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.
Q: The real situation is worse than the facts show? (later NATO press event)
UFO-abduction policy advocate: The message is that there are no "knowns." [...] Absence of evidence is not evidence of absence.[1]
This is the money quote:
> 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.
The new data is based on observations of the luminosity of deep space images from the New Horizons program, which is outside the inner solar system. The older estimate was based on Hubble images along with mathematical models to try to account for the glow of space dust.
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 recognizable galaxies, proto-galaxies, and star clusters (Conselice et al. 2016), as well as mass accretion by black holes (BHs) associated with the systems." - https://arxiv.org/abs/2011.03052v2
A common problem in astronomy is the lack of ways to measure things (we can't travel to the stars, yet). The special thing about this result is that it is a very direct and independent measurement of the background optical light in the Universe. This means it can be used to put independent constraints on the star formation history, as well as black hole accretion, and perhaps things we haven't even thought about yet.
Edit: And all those results are model dependent as everything else.
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.
Good observation... just made another comment about that. You’re asking about the observable universe vs the universe, and it’s an open question.
Might be drastically more, or even less.
My understanding is that there isn’t much point theorizing about the “outside” universe beyond the horizon of our light cone, because there is no present or future interaction possible between it and us.
There’s good reason to theorize about it. Theories might be able to point to or estimate the size even if we can’t directly measure it. For instance if we find evidence of inflation in more fundamental physical processes, we can assume there’s more universe out there than we can see.
My main response to this article is essentially the same question. I think the article is presuming some things to come to this conclusion.
The main thing that is being presumed is the "big bang". If you proscribe to believing in the big bang, then the entirety of everything we see is an expansion from a single dense explosion of matter.
It is still a valid theory but I myself don't understand the physics of what is observed well enough to be convinced this is true. I believe that they are seeing something similar enough that it could be perhaps a "localized bang" or something like that.
The main evidence used as "proof" of the big bang is the observed data seeming to show that the everything in the universe is "dispersing" slowly. That is at least what I've heard claimed.
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 leave the screen at the top and appear on the bottom). In that case there might be fewer actual galaxies than what we observe.
> 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/
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.
There are far fewer *Visible stars and galaxies than we thought
This is more about the invisible stars and galaxies, the ones too far to actually count, but whose lights just gives a faint glow to the darkness.
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).
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.
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.
I would argue that any reasonable definition of visible includes being able to resolve features. In support of this, many dictionary definitions include words like 'accurate', 'clear' and 'obvious', while it's antonyms include 'unclear', 'indistinct' and 'obscured' and not just 'invisible'.
> So, what could be the source of this leftover glow? It’s possible that an abundance of dwarf galaxies in the relatively nearby universe lie just beyond detectability. Or the diffuse halos of stars that surround galaxies might be brighter than expected. There might be a population of rogue, intergalactic stars spread throughout the cosmos. Perhaps most intriguing, there may be many more faint, distant galaxies than theories suggest. This would mean that the smooth distribution of galaxy sizes measured to date rises steeply just beyond the faintest systems we can see – just as there are many more pebbles on a beach than rocks.
Does this mean there is "more" dark energy since we still have all the effects of extra matter but less visible matter?
No, because only about 10% of normal (baryonic) matter is actually in the form of stars. The rest is mostly in the form of hot, dilute gas in intergalactic space.
The idea that about 5% of the total mass-energy budget of the universe is in the form of baryonic matter is well-determined by cosmological observations (including the relative amounts of hydrogen and helium formed in the Big Bang). It if turns out we've overestimated the amount of baryonic matter in stars by a factor of two, that just means that 95% of the baryons are in the hot intergalactic medium instead of 90%.
Dark energy and dark matter are separate concepts. Dark energy refers to a factor that increases the expansion of the universe, ie a cosmological constant.
Dark matter on the other hand is matter we can’t detect currently, but we theorize is there from its gravitational effects.
There’s a lot of other data like galaxy rotation rates, CMB, gravitational lensing, etc that effect dark matter estimates. I’m not sure how much of the current estimate is based on larger observable universe structure, so it’s an interesting question.
I understand that, I mean the matter-energy balance in the universe at the moment accounts 5% to ordinary matter, 26% to dark matter and the rest to dark energy. Since we now assume there are less stars, hence, ordinary matter, but the galaxies are still accelerating at the same rate, doesn't that mean the dark energy accounts for more than the estimated 69%?
anyone else reminded of the Spinal Tap album "Smell the Glove" cover discussion?
> How much more black can it be? None! None more black
"It's just the same 20 galaxies, and the universe is just wraps around itself."
Um. What? Is this the new "the earth is flat"?
Space is infinite. I believe it goes on forever in every direction. Whether you can see whatever is out there from here in no way proves that it isn't...
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?
Because this is an article that’s using the layman terminology. The scientists involved surely understand the difference. In fact read the Wikipedia article and you’ll see the old estimate of 2 trillion for the observable universe.
Perhaps space is infinite, but that doesn’t necessarily mean there is infinite matter.
The set of natural numbers is also infinite. That doesn't mean you can't learn something by counting from 0 to 10. :)
We have no proof Universe is infinite, but we can also argue that the building blocks of Universe - atoms? - cannot come in unlimited quantity. At least it sounds more reasonable that way. Unless of course we are all in a symultation and our whole Universe takes up half a sq. inch of a silicone chip.
This is my point. We have no evidence either way, so it seems strange to me to blindly claim "this is everything".
Did you not blindly claim space is infinite? That seems the stranger assumption. If I’m standing in a forest counting all the trees I can see, I’d certainly take into account that there are some trees I can’t see, but I wouldn’t assume there exists an infinite forest of trees.
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." ?????
https://en.wikipedia.org/wiki/Expansion_of_the_universe
The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it. As the spatial part of the universe's spacetime metric increases in scale, objects move apart from one another at ever-increasing speeds. To any observer in the universe, it appears that all of space is expanding while all but the nearest galaxies recede at speeds that are proportional to their distance from the observer – at great enough distances the speeds exceed even the speed of light.
Red shift can be caused by Doppler shift of frequency and by loss of energy by photons. When Red shift is interpreted as 100% Doppler shift, then we have such absurd velocity. Of course, nobody saw a photonic boom when the whole galaxy crossed the speed of light barrier.
That’s not at all what I/others are talking about. This is about expansion of the universe and new space being created between us and distant locations. Locally the galaxy isn’t moving particularly fast (relative to CMB, or whatever).