> 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.
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.
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.
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).
You introduced "will never" to this argument here. Neither me nor parent comments said that being visible today or in the past has anything to do with being visible in the future.
I would argue that science is about a core of what is considered proven and thus is true, as well as topics that are currently discussed and a large amount of knowledge that still has to be discovered. And I think these galaxies that are invisible today might be in the latter category - if they exist at all. But I wouldn't rule out their existence unnecessarily. In a few years, due to advances in technology, it could be possible to "discover" (or rather estimate) far more galaxies than what is proposed in this study.
Just because mankind in the middle ages had never seen Pluto doesn't mean it didn't exist back then.
The thing is, given the accelerating expansion of space in every point, it is likely, as I understand, that light from any star distant enough that it hasn't reached us yet will in fact never reach us.
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'.
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.
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).
> 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.
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]
The ridicule such as in the examples reveals epistemic immaturity.
The {known, unknown} X {knowns, unknowns} classification is a statement of existence. Just because any of the four combinations here exist, doesn't mean they're equally likely.
The error in reasoning such vaccination-autism policy advocate makes is: they correctly observe the possibility of unknown unknowns in vaccination, but then they incorrectly assume that a) they're likely, significant and impactful, and b) they have to do with body's reaction to the vaccine.
(Hell, arguably the primary unknown unknown in vaccination was the degree to which anti-vaccination memes will spread and prevent populations from being effectively protected.)
Additionally, the focus on this is itself a sign of immaturity. The adult answer here to note that various kinds of knowns/unknowns exist, and then talk about how we can make turn unknowns into knowns, how to estimate the importance and impact of the unknowns, and how to deal with them if they happen.
> UFO-abduction policy advocate: (...)Absence of evidence is not evidence of absence.
Actually, absence of evidence is evidence of absence. If your hypothesis predicts something should happen, and it does not happen, it does count as evidence against your hypothesis.
I agree with most of that. But those questionable replies? They're Rumsfeld's.
> Actually, absence of evidence is evidence of absence.
Agreed. And yet Rumsfeld says, in the linked press event transcript,
> Now what is the message there? The message is that there are no "knowns." There are thing we know that we know. There are known unknowns. That is to say there are things that we now know we don't know. But there are also unknown unknowns. There are things we don't know we don't know. So when we do the best we can and we pull all this information together, and we then say well that's basically what we see as the situation, that is really only the known knowns and the known unknowns. And each year, we discover a few more of those unknown unknowns.
> It sounds like a riddle. It isn't a riddle. It is a very serious, important matter.
> There's another way to phrase that and that is that the absence of evidence is not evidence of absence. It is basically saying the same thing in a different way. Simply because you do not have evidence that something exists does not mean that you have evidence that it doesn't exist. And yet almost always, when we make our threat assessments, when we look at the world, we end up basing it on the first two pieces of that puzzle, rather than all three.
[emphasis mine]
> The ridicule such as in the examples reveals epistemic immaturity.
Almost. The example replies, were Rumsfeld's, at the two events. Extracts, but I suggest fair ones. The example questions were paraphrased. Then I prefaced the alternate contexts, hoping to better illuminate the flaws in the replies.
The epistemic immaturity you observe, is a property of Rumsfeld's replies, and what I sought to illustrate, in support of GGP seeking to understand the historical ridicule. I'm sorry that ended up less than clear.
Thanks for clarifying. I didn't realize those were Rumsfeld's remarks - my fault, I didn't bother to re-read the transcript after recognizing the {known, unknown} X {knowns, unknowns} thing, which I studied in the past. That said, this doesn't change my response to it.
There's a pattern of confusing/faulty reasoning that I've noticed, which I highlighted in my previous comment. I don't have a name for it yet - but it boils down to noticing a thing or possibility exists and arguing just from it, without bothering to ask how much of it there is / how likely it is. It's reasoning in an unnecessary fog of war, kind of similar to arguing from point statistics (e.g. average and stdev) instead of looking at the actual distributions.
> I don't have a name for it yet - but it boils down to noticing a thing or possibility exists and arguing just from it, without bothering to ask how much of it there is / how likely it is.
That looks like "appeal to possibility"? Seemingly sometimes folded into "appeal to probability"?
> 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?
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.
Why in the fuck is sharing some knowledge of what I know about science that leads to people believing there is nothing outside the observable universe downvoted?
I can see having a dislike of my opinion and beliefs ( which is not a valid reason to downvote by the way according to what Hackers News itself says... ), but downvoting me when I am attempting to communicate useful information. Come on now.
It’s because opinions are not equal to science and suggesting your opinion without massive corroborating evidence isn’t going to fly here. It’s the same as suggesting climate change isn’t real and COVID is just like flu... these things are made up, not researched science.
I didn’t downvote, but you demonstrated a very poor understanding of the big bang, and put forward equally ill-informed conclusions/suspicions based on that. It is not useful.
I think some read some of paragraph 2 and thought it was heading into religious beliefs or far away from the topic.
The last consensus I remember is a Big Bang, rapid expansion, and then slower expansion. There are some other theories but not as popular.
It’s a fun topic to dive into. The horizon problem, variable speeds of light, and of course string theory.
> ... that leads to people believing there is nothing outside the observable universe
No mainstream scientific source says this. The observable universe is purely a limit on the distance we can see - a horizon - due to the speed of light and the age of the universe.
> 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.
> 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.
This is misleading. The Big Bang is not an "explosion" of the kind we see on Earth, where some matter is pushed apart from a central point by an explosive force. If that were the case, then what we observe would be different.
Instead, what we observe has so far only been well explained by the idea that space itself is expanding. In this model, there is no center - there's no location you can point to as where the explosion happened, the expansion happened everywhere equally, it didn't radiate outward from a central point.
We have evidence of this omnidirectional expansion, in the form of the cosmic microwave background (CMB) radiation, which is radiation from the early universe that comes from all directions in the sky, because the Big Bang similarly occurred in all directions. Again, this is not what we would observe if it was similar to a traditional explosion.
> I believe that they are seeing something similar enough that it could be perhaps a "localized bang" or something like that.
If you're thinking of something more like a traditional explosion, no, that is ruled out by the observational evidence.
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.
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).
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.
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.
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...
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.
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.
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.
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.
You're right. The earth is clearly flat and that is a sensible assumption if you haven't left your local area or been in space or seen a picture from space.
Or not. Flat earthers are quacks.
It is the same principle essentially. It is illogical to think "hey if I keep walking that way I'll hit a wall eventually". That is why flat earthers are quacks from the beginning.
The same principle applies to space. What quack seriously thinks that eventually there is just a wall in space or that it wraps around?
Wrapping around makes zero sense in 3-dimensional space.
If you ever played the classic game "asteroids", you might remember how everything wrapped around at the edges of the screen. A 3-torus would be like that, only in 3D.
On the other hand, one could imagine the universe as a 3-sphere, which I believe would mean it would be both locally curved and finite in volume. https://en.wikipedia.org/wiki/3-sphere
There are also other possibilities, like something analogous to a Klein bottle with an extra dimension.
There are a bunch of possibilities for "flat and finite" as well as "positively curved and finite" that I can't begin to visualize.
There is even supposedly such a (hypothetical) thing as a negatively curved, (commonly described as saddle shaped) yet finite universe.
But regardless, the current belief about the real universe is that it is at least very close to flat, but not known to be finite or infinite.
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%?
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%.
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[ 2.6 ms ] story [ 123 ms ] threadI think there's a difference between what we know and what actually exists.
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).
I would argue that science is about a core of what is considered proven and thus is true, as well as topics that are currently discussed and a large amount of knowledge that still has to be discovered. And I think these galaxies that are invisible today might be in the latter category - if they exist at all. But I wouldn't rule out their existence unnecessarily. In a few years, due to advances in technology, it could be possible to "discover" (or rather estimate) far more galaxies than what is proposed in this study.
Just because mankind in the middle ages had never seen Pluto doesn't mean it didn't exist back then.
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.
> 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.
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
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]
[1] https://www.nato.int/docu/speech/2002/s020606g.htm
The {known, unknown} X {knowns, unknowns} classification is a statement of existence. Just because any of the four combinations here exist, doesn't mean they're equally likely.
The error in reasoning such vaccination-autism policy advocate makes is: they correctly observe the possibility of unknown unknowns in vaccination, but then they incorrectly assume that a) they're likely, significant and impactful, and b) they have to do with body's reaction to the vaccine.
(Hell, arguably the primary unknown unknown in vaccination was the degree to which anti-vaccination memes will spread and prevent populations from being effectively protected.)
Additionally, the focus on this is itself a sign of immaturity. The adult answer here to note that various kinds of knowns/unknowns exist, and then talk about how we can make turn unknowns into knowns, how to estimate the importance and impact of the unknowns, and how to deal with them if they happen.
> UFO-abduction policy advocate: (...)Absence of evidence is not evidence of absence.
Actually, absence of evidence is evidence of absence. If your hypothesis predicts something should happen, and it does not happen, it does count as evidence against your hypothesis.
> Actually, absence of evidence is evidence of absence.
Agreed. And yet Rumsfeld says, in the linked press event transcript,
> Now what is the message there? The message is that there are no "knowns." There are thing we know that we know. There are known unknowns. That is to say there are things that we now know we don't know. But there are also unknown unknowns. There are things we don't know we don't know. So when we do the best we can and we pull all this information together, and we then say well that's basically what we see as the situation, that is really only the known knowns and the known unknowns. And each year, we discover a few more of those unknown unknowns.
> It sounds like a riddle. It isn't a riddle. It is a very serious, important matter.
> There's another way to phrase that and that is that the absence of evidence is not evidence of absence. It is basically saying the same thing in a different way. Simply because you do not have evidence that something exists does not mean that you have evidence that it doesn't exist. And yet almost always, when we make our threat assessments, when we look at the world, we end up basing it on the first two pieces of that puzzle, rather than all three.
[emphasis mine]
> The ridicule such as in the examples reveals epistemic immaturity.
Almost. The example replies, were Rumsfeld's, at the two events. Extracts, but I suggest fair ones. The example questions were paraphrased. Then I prefaced the alternate contexts, hoping to better illuminate the flaws in the replies.
The epistemic immaturity you observe, is a property of Rumsfeld's replies, and what I sought to illustrate, in support of GGP seeking to understand the historical ridicule. I'm sorry that ended up less than clear.
There's a pattern of confusing/faulty reasoning that I've noticed, which I highlighted in my previous comment. I don't have a name for it yet - but it boils down to noticing a thing or possibility exists and arguing just from it, without bothering to ask how much of it there is / how likely it is. It's reasoning in an unnecessary fog of war, kind of similar to arguing from point statistics (e.g. average and stdev) instead of looking at the actual distributions.
That looks like "appeal to possibility"? Seemingly sometimes folded into "appeal to probability"?
> 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.
Might be drastically more, or even less.
https://en.m.wikipedia.org/wiki/Observable_universe
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.
I can see having a dislike of my opinion and beliefs ( which is not a valid reason to downvote by the way according to what Hackers News itself says... ), but downvoting me when I am attempting to communicate useful information. Come on now.
The last consensus I remember is a Big Bang, rapid expansion, and then slower expansion. There are some other theories but not as popular. It’s a fun topic to dive into. The horizon problem, variable speeds of light, and of course string theory.
No mainstream scientific source says this. The observable universe is purely a limit on the distance we can see - a horizon - due to the speed of light and the age of the universe.
> 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.
There is much, much more evidence than this. Wikipedia lists observational evidence in 7 categories: https://en.wikipedia.org/wiki/Big_Bang#Observational_evidenc...
> 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.
This is misleading. The Big Bang is not an "explosion" of the kind we see on Earth, where some matter is pushed apart from a central point by an explosive force. If that were the case, then what we observe would be different.
Instead, what we observe has so far only been well explained by the idea that space itself is expanding. In this model, there is no center - there's no location you can point to as where the explosion happened, the expansion happened everywhere equally, it didn't radiate outward from a central point.
We have evidence of this omnidirectional expansion, in the form of the cosmic microwave background (CMB) radiation, which is radiation from the early universe that comes from all directions in the sky, because the Big Bang similarly occurred in all directions. Again, this is not what we would observe if it was similar to a traditional explosion.
> I believe that they are seeing something similar enough that it could be perhaps a "localized bang" or something like that.
If you're thinking of something more like a traditional explosion, no, that is ruled out by the observational evidence.
https://news.ycombinator.com/newsguidelines.html
"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 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.
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.
https://en.m.wikipedia.org/wiki/Observable_universe
Would this mean the universe would be spherical?
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.
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/
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...
Whether the universe is infinite or not is an open question. It might be finite but unbounded. Spacetime geometry is complicated.
https://en.m.wikipedia.org/wiki/Observable_universe
>How dark is the sky, and what does that tell us about the number of galaxies in the visible universe?
Or not. Flat earthers are quacks.
It is the same principle essentially. It is illogical to think "hey if I keep walking that way I'll hit a wall eventually". That is why flat earthers are quacks from the beginning.
The same principle applies to space. What quack seriously thinks that eventually there is just a wall in space or that it wraps around?
Wrapping around makes zero sense in 3-dimensional space.
...if the universe was like a "3-torus", it could be both flat and finite: https://en.wikipedia.org/wiki/Three-torus
If you ever played the classic game "asteroids", you might remember how everything wrapped around at the edges of the screen. A 3-torus would be like that, only in 3D.
On the other hand, one could imagine the universe as a 3-sphere, which I believe would mean it would be both locally curved and finite in volume. https://en.wikipedia.org/wiki/3-sphere
There are also other possibilities, like something analogous to a Klein bottle with an extra dimension.
There are a bunch of possibilities for "flat and finite" as well as "positively curved and finite" that I can't begin to visualize.
There is even supposedly such a (hypothetical) thing as a negatively curved, (commonly described as saddle shaped) yet finite universe.
But regardless, the current belief about the real universe is that it is at least very close to flat, but not known to be finite or infinite.
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.
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%.
> How much more black can it be? None! None more black