I’m a little hazy about the difference between dark matter and missing matter. Edit: some googling suggests that there is an amount of matter we expect to exist because of the BBN (big bang...
I’m a little hazy about the difference between dark matter and missing matter.
Edit: some googling suggests that there is an amount of matter we expect to exist because of the BBN (big bang nucleosynthesis) and CMB (cosmic microwave background) calculations, but we don’t seem to see all of it. That’s the missing matter. There’s also an amount of matter we expect to need due to gravitational behaviours of galaxies, which we also don’t see. That’s the dark matter. Not an astrophysicist though, would appreciate anyone weighing in on this.
I can explain in more detail (I am a cosmologist). As you identify, there are two different terms involved here, "dark matter" and simply "missing matter". Dark matter is a term that refers to the...
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I can explain in more detail (I am a cosmologist).
As you identify, there are two different terms involved here, "dark matter" and simply "missing matter".
Dark matter is a term that refers to the large amount of matter whose existence we infer based on many different lines of evidence, but which does not seem to interact with other matter other than through gravitation. Namely, it does not emit light or respond to magnetic fields (electromagnetic interaction), hence the terminology "dark". We also believe that it is "cold", meaning that the dark matter particles don't have appreciable velocities. This means that when dark matter falls into gravitational potentials it tends to stay there — not enough velocity to zip back out the other way. Technically, not all dark matter is unknown — cosmic neutrinos are an example of a type of dark matter (which is not cold). The issue is that we infer there is much more dark matter than cosmic neutrinos, and we don't know what it is.
How do we infer its existence? There are multiple lines of evidence. The most compelling, in my opinion, come from measurements of the CMB. The statistics of the hot and cold spots we see in the CMB (e.g. the distribution of the size of spots) depend on things like how much overall matter there is when the CMB forms, and depend in a different way on things like how much matter there is that interacts electromagnetically (i.e. normal matter, or in the field we say "baryonic" matter). So by measuring the CMB we can work out the density type of matter there must be, on average, in the universe. There are other lines of evidence supporting dark matter, as you mention, like measurements of the rotation curves of galaxies, or estimates of the matter distribution from measuring how the matter gravitationally bends light as it passes by, known as lensing. In short, dark matter is matter which only interacts gravitationally, and we think there is a lot of it (4/5 of all matter), but we don't know what it is.
On the other hand, there is this "missing matter", which is a subset of "normal" or "baryonic" matter. Like I wrote above, we can estimate the average density of baryonic matter through measurements of the CMB or BBN, as you point out. So these measurements give us a sense of of much overall baryonic matter there must be. We can then go out and try to do a sort of census, by estimating the total mass of all of the visible matter we can observe (stars, galaxies, clouds of very hot gas around galaxies, the mass in supermassive black holes, etc). When we do that, we came up short, and this discrepancy was termed "missing matter". Where is this extra, normal matter that we think must be there from measurements of the CMB and BBN, but which doesnt show up in stars or galaxies, etc?
These researcher think they have found it, with the conclusion being that between clusters of galaxies there are long filaments of hot gas that contain a large portion of the baryonic matter.
Quick question on specifics to try and wrap my head around this: Is their an idea about what a filament like this even looks like? Because that word makes me imagine a continuous strand of matter...
Quick question on specifics to try and wrap my head around this:
Is their an idea about what a filament like this even looks like? Because that word makes me imagine a continuous strand of matter streched out like pulled toffee or a long gas blanket a few atoms thick. But I'm well aware that normal expectations don't really translate when a "short trip" is several lifetimes.
If you’ve ever seen images from large cosmological simulations, you can see these filaments as part of the “cosmic web”. This gives a sense of what they would look like if they emitted visible...
If you’ve ever seen images from large cosmological simulations, you can see these filaments as part of the “cosmic web”. This gives a sense of what they would look like if they emitted visible light, but of course in reality they do not.
From the article: … This is not about “dark matter.” It’s about finding evidence of the “ordinary” matter that was missing according to cosmological models.
From the article:
Clocking in at over 10 million degrees, the filament contains around 10 times the mass of the Milky Way and connects four galaxy clusters: two on one end, two on the other. All are part of the Shapley Supercluster, a collection of more than 8000 galaxies that forms one of the most massive structures in the nearby Universe.
The filament stretches diagonally away from us through the supercluster for 23 million light-years, the equivalent of traversing the Milky Way end to end around 230 times.
Konstantinos and colleagues characterised the filament by combining X-ray observations from XMM-Newton and Suzaku, and digging into optical data from several others.
…
“This research is a great example of collaboration between telescopes, and creates a new benchmark for how to spot the light coming from the faint filaments of the cosmic web,” adds Norbert Schartel, ESA XMM-Newton Project Scientist.
“More fundamentally, it reinforces our standard model of the cosmos and validates decades of simulations: it seems that the ‘missing’ matter may truly be lurking in hard-to-see threads woven across the Universe.”
This is not about “dark matter.” It’s about finding evidence of the “ordinary” matter that was missing according to cosmological models.
I'm very confused, this article claims we had already found the last of the ordinary matter... In 2018. Is the new article following up on and confirming the results from back then? One of the...
I'm very confused, this article claims we had already found the last of the ordinary matter... In 2018. Is the new article following up on and confirming the results from back then? One of the quotes in the new article reads:
For the first time, our results closely match what we see in our leading model of the cosmos – something that’s not happened before
Which kind of suggest the old article is either invalid or does not exist.
I think the headlines are hyping it up a bit. This article explains what’s new a little better: 'The models were right!' Astronomers locate universe's 'missing' matter in the largest cosmic...
I think the headlines are hyping it up a bit. This article explains what’s new a little better:
Though astronomers have seen these filaments before, the fact that they are faint has meant that their light has been washed out by other sources like galaxies and supermassive black hole-powered quasars. That means the characteristics of these filaments have remained elusive.
But now, a team of astronomers has for the first time been able to determine the properties of one of these filaments, which links four galactic clusters in the local universe. These four clusters are all part of the Shapley Supercluster, a gathering of over 8,000 galaxies forming one of the most massive structures in the nearby cosmos.
"For the first time, our results closely match what we see in our leading model of the cosmos – something that's not happened before," team leader Konstantinos Migkas of Leiden Observatory in the Netherlands said in a statement. "It seems that the simulations were right all along."
It seems to be about getting increasingly detailed measurements of something they already had some evidence of.
I’m a little hazy about the difference between dark matter and missing matter.
Edit: some googling suggests that there is an amount of matter we expect to exist because of the BBN (big bang nucleosynthesis) and CMB (cosmic microwave background) calculations, but we don’t seem to see all of it. That’s the missing matter. There’s also an amount of matter we expect to need due to gravitational behaviours of galaxies, which we also don’t see. That’s the dark matter. Not an astrophysicist though, would appreciate anyone weighing in on this.
I can explain in more detail (I am a cosmologist).
As you identify, there are two different terms involved here, "dark matter" and simply "missing matter".
Dark matter is a term that refers to the large amount of matter whose existence we infer based on many different lines of evidence, but which does not seem to interact with other matter other than through gravitation. Namely, it does not emit light or respond to magnetic fields (electromagnetic interaction), hence the terminology "dark". We also believe that it is "cold", meaning that the dark matter particles don't have appreciable velocities. This means that when dark matter falls into gravitational potentials it tends to stay there — not enough velocity to zip back out the other way. Technically, not all dark matter is unknown — cosmic neutrinos are an example of a type of dark matter (which is not cold). The issue is that we infer there is much more dark matter than cosmic neutrinos, and we don't know what it is.
How do we infer its existence? There are multiple lines of evidence. The most compelling, in my opinion, come from measurements of the CMB. The statistics of the hot and cold spots we see in the CMB (e.g. the distribution of the size of spots) depend on things like how much overall matter there is when the CMB forms, and depend in a different way on things like how much matter there is that interacts electromagnetically (i.e. normal matter, or in the field we say "baryonic" matter). So by measuring the CMB we can work out the density type of matter there must be, on average, in the universe. There are other lines of evidence supporting dark matter, as you mention, like measurements of the rotation curves of galaxies, or estimates of the matter distribution from measuring how the matter gravitationally bends light as it passes by, known as lensing. In short, dark matter is matter which only interacts gravitationally, and we think there is a lot of it (4/5 of all matter), but we don't know what it is.
On the other hand, there is this "missing matter", which is a subset of "normal" or "baryonic" matter. Like I wrote above, we can estimate the average density of baryonic matter through measurements of the CMB or BBN, as you point out. So these measurements give us a sense of of much overall baryonic matter there must be. We can then go out and try to do a sort of census, by estimating the total mass of all of the visible matter we can observe (stars, galaxies, clouds of very hot gas around galaxies, the mass in supermassive black holes, etc). When we do that, we came up short, and this discrepancy was termed "missing matter". Where is this extra, normal matter that we think must be there from measurements of the CMB and BBN, but which doesnt show up in stars or galaxies, etc?
These researcher think they have found it, with the conclusion being that between clusters of galaxies there are long filaments of hot gas that contain a large portion of the baryonic matter.
Quick question on specifics to try and wrap my head around this:
Is their an idea about what a filament like this even looks like? Because that word makes me imagine a continuous strand of matter streched out like pulled toffee or a long gas blanket a few atoms thick. But I'm well aware that normal expectations don't really translate when a "short trip" is several lifetimes.
If you’ve ever seen images from large cosmological simulations, you can see these filaments as part of the “cosmic web”. This gives a sense of what they would look like if they emitted visible light, but of course in reality they do not.
Very interesting and informative-thank you.
From the article:
…
This is not about “dark matter.” It’s about finding evidence of the “ordinary” matter that was missing according to cosmological models.
I'm very confused, this article claims we had already found the last of the ordinary matter... In 2018. Is the new article following up on and confirming the results from back then? One of the quotes in the new article reads:
Which kind of suggest the old article is either invalid or does not exist.
I think the headlines are hyping it up a bit. This article explains what’s new a little better:
'The models were right!' Astronomers locate universe's 'missing' matter in the largest cosmic structures
It seems to be about getting increasingly detailed measurements of something they already had some evidence of.