13 ms·

Astronomers have created the largest ever map of dark matter

77 points by chriskanan 5y ago

DudeInBasement 5y ago
Lol, they haven't even found any yet and they made a map? I guess you got to do something with that grant money
- Bancakes 5y ago
  Not to sound hateful but who are the neo-scientists? You go through school, high school, then you get a bachelor's. Great. Then you also go for a master's and a PhD. Maybe even a post-doc?! So you haven't taken part in the economy for 10 years of your adulthood, you don't have serious job prospects, and you need mortgage/family money. What do you do? Maybe that explains the abundance of cheap, unverifiable, irreproducible studies.
  - anticristi 5y ago
    Not sure who invented the academia-industry dichotomy, but he/she deserves a society-wide slap. Society loses, the individual loses. My career is just a big middle finger pointed at this dichotomy.
    - alexeldeib 5y ago
      > My career is just a big middle finger pointed at this dichotomy. I only saw your comment after the parent it replied to was flagged/removed. This section of your reply jumped out to me, care to elaborate? My experience (undergrad, minimal research -> industry) has shown me that the gap is quite real, but both sides are important. They simply align to different goals. The most extreme trope of this in tech is industry engineers making fun of researchers who produce systems which "only work on paper" (paraphrased, some meaning is lost in that phrasing). I find that criticism too harsh (and rude), but it does highlight the differing goals: industry frequently needs to build systems which 1) work, and 2) scale, among other things. Academia can afford to push the cutting edge by compromising on constraints real world systems have to obey. It's the classic exploration vs. exploitation problem. Industry thinks the exploration isn't important because they need to optimize the local maxima/minima now. Academia cares about the global maxima/minima, pushing the state of the art, exploring new solutions in the hope of advancing knowledge and understanding. In the largest companies, the cost/benefit analysis shifts back toward exploration. Google, Microsoft, Xerox PARC, etc. And the natural extension of academics elsewhere pushing the state of the art is finding new optimums and bringing that back to industry (e.g., as a startup). So I feel there gap is real, with lots of symbiotic back and forth. Broad generalizations, but not unfair I think.
      - anticristi 5y ago
        I don't 100% agree, but let's play with "academia is exploration, industry is exploitation". How would you ensure you explore something useful? How would you ensure to transfer what you explored, so you can exploit that knowledge? What if you exploitation is stuck in a local minimum, how would you ensure you send feedback back to exploitation? I feel a more useful model is Technological Readiness Level (TRLs). Academia deals with lower TRLs, while industry deals with higher TRLs. Somewhere in between there is -- there needs to be -- a huge overlap to ensure a proper continuum. This can also be phrased in a risk perspective. Lower TRLs are extremely high risk, almost like throwing money out the window, but have extremely high societal reward. Think curing cancer or terraforming Mars. These are best funded via grants. High TRLs are low risk, low rewards; issue some bonds. Somewhere in the middle is VC territory. But how? PhD students doing industrial internships; PIs doing industrial sabaticals; engineering managers becoming adjunct lecturers or giving guest lectures; academia and industry working on common projects; making patents could as much as papers on an academic CV. Is anyone in the world either exploring or exploiting? Then why don't we create a system that allows people to quickly switch hats, as required to achieve larger goals?
        alexeldeib 5y ago
        I like your comparison to TRLs. I think we’re saying the same thing, actually! I don’t mean that one is exploitation and one is exploration exclusively. Re-reading my post, the phrase “gap” might be a misnomer — it’s more like a balancing act of priorities. > How would you ensure you explore something useful? How would you ensure to transfer what you explored, so you can exploit that knowledge? What if you exploitation is stuck in a local minimum, how would you ensure you send feedback back to exploitation? I’d argue you can’t necessarily be sure. Not all research works out. But we need to ask questions and get answers to move forward, even if what we discover may not be worth exploiting (getting tired of my own phrasing). Getting stuck in local min/maxima is part of why we need both angles. This is the flip side of what I said about industry needing to build systems that work — lower budget for risk tolerance, but also increased likelihood of getting stuck in a local optimum. It sounds like I’m using those terms to describe the same spectrum you are with TRLs. At least, I wholeheartedly agree with everything you said — the “but how” portion sounds exactly like the symbiotic overlap/crossover I had in mind. I guess to answer my own original question, it sounds like you’ve swung back and forth a few times? > Then why don't we create a system that allows people to quickly switch hats, as required to achieve larger goals? Preaching to the choir ;) and appreciate your perspective
  - Blikkentrekker 5y ago
    At least where I live, many start earning some salary from academic contribution of some minor sort during the bachelor's trajectory, typically by assisting in lessons to lower classes, most art to do so during master's, and Ph.D. students are employed by the institute that promotes them and earn a full salary for their research. The real issue isn't that, the real issue to me is quite a few research fields are purely taking part in œconomy and entertainment, and essentially funded of sponsorships generate by spectacular news reports and nothing more. A very large quantity of scientific research is purely infotainment and doesn't actually generate knowledge that is used for anything but merely interesting to read, which is what could cause the replication crisis to go unnoticed for so long among other things: no one found out, as no one was using it in any way that relied upon it's veracity, and many even made up data for decades and went unnoticed. I recently read a paper that studied some social parts of the acquisition of language in Japanese, — quite interesting to read, and as usual it was quite spectacular because the findings went against the established ideas, but this knowledge merely exists to be “fascinating” and it won't ever be used for anything that assumes it's veracity, and as a consequence the data could have been pulled from the æther and no one would ever find out and replication will surely never occur,and even if it would, it would be too late as some years would have passed and the original auctor can always argue that these social matters changed within the interval.
  - Mike86534 5y ago
    That's one reason why they sell their souls to the Climate UN / NWO scam and pump out trash papers based on fudged data/models that backs up the fearmongering narrative. https://www.thegwpf.org/content/uploads/2018/02/Groupthink.pdf https://www.thegwpf.org/content/uploads/2018/02/Groupthink.p...
  - ad404b8a372f2b9 5y ago
    You have a bit of a caricatural take on research. From a societal perspective, PhDs and postdocs take part in the economy, they have a job, make money, spend money, produce research sometimes in partnership with the industry, sometimes with immediate applications, sometimes theoretical with applications that will only come decades down the line. There is no reason to set the line at Bachelors, as a society we've found it's best to have a subset of the population go into higher studies. We used to have children participate in the economy 10 to 15 years earlier in fields and factories and it wasn't so good. From the perspective of the individual, getting a PhD isn't always much longer than getting a Masters, and teaches you skill you will not acquire anywhere else. It also opens the door to research positions that would not be accessible to you without it. And you can have greater expected monetary returns on the medium and long term. I've known a lot of people doing PhDs, a good amount doing it for prestige, because it was the expected next step after a masters. Many others were doing it out of passion, for the freedom to study things they were interested in an amazing environment. So on an individual level many people find it worth it whether their priority is money or intellectual pursuits.
  - deleted 5y ago
    [deleted]
- gus_massa 5y ago
  Alternative title: "Group of scientists assuming that the current model of gravity is correct look at the discrepancies in the observations and calculate a map of where and how much matter would be necessary to fix this difference. Some of them believe this additional mater is real, and some of them doubt, but meanwhile to make the titles shorter they use the catchy name of dark matter."
- Xorlev 5y ago
  It's clear you didn't even try to read the article. They mapped the distortion effects on visible light thought to be caused by dark matter. They don't know what it is or how it works, but they know _something_ is causing apparent distortion to the light.
  - WesleyHale 5y ago
    That _something_ might as well be interdemensional pixies.
    - ben_w 5y ago
      Sure, just so long as the pixies don’t interact electromagnetically, or strongly with each other by anything except gravity.
    - bdamm 5y ago
      Won’t it be interesting when our model of physics is updated to explain these cosmological phenomena? Who knows what new technology will be discovered as a result!
      - ben_w 5y ago
        My brother once jokingly suggested that Dark Energy is the waste product of the power sources of alien civilisations. (Neither of us is a physicist, but it’s an interesting idea for a short story).
        macintux 5y ago
        There's a very old short story I've never been able to find again that beat you to it. Every civilization that discovers FTL travel contributes to a more rapid expansion of the universe. The older civilizations beg them to stop, but inevitably they fail.
        irrational 5y ago
- _Microft 5y ago
  What's surprising about that if I may ask? We have observations that match quite well what would be expected if there was additional matter in the universe. We cannot detect this matter though, so the conjecture is that there must be something that interacts gravitationally with the rest of the universe but not in other ways. We call it dark matter for now. We do not yet know what it consists of, there are even competing theories but mapping the effects of it? That we can do. This approach is not at all unusual. If you want to know about another example from a different field, look no further than genetics. Gregor Mendel observed rules for inheritance of certain traits in plants and was able to make predictions from that. He discovered genetics. In the 1850-60ies. This was a long time before Crick, Watson and Franklin discovered the structure of the underlying mechanism for inheritance (DNA).
  - galaxyLogic 5y ago
    What if Dark Matter is just that? Dark matter. If there were lots and lots of something like planets out there, which for one reason or another don't reflect light, that would be something dark right? Or what if Dark Matter was lots of Black Holes? We (more or less) know what Black Holes are so why don't we assume there could be lots of them having a gravitational effect but no visibility? In other words why do we need to assume that Dark Matter is made of some kind of exotic unknown matter?
    - _Microft 5y ago
      That’s indeed a possible explanation. See also: https://en.m.wikipedia.org/wiki/Massive_compact_halo_object https://en.m.wikipedia.org/wiki/Massive_compact_halo_object
    - kl4m 5y ago
      Here's an expanded explanation of the "lots of black holes" hypothesis, posted this week by PBS Space Time. "What If (Tiny) Black Holes Are Everywhere?" https://www.youtube.com/watch?v=srVKjWn26AQ https://www.youtube.com/watch?v=srVKjWn26AQ Dark Matter is just a placeholder name, don't read too much into the "Matter" part. There are no assumptions there and plenty of alternate explanations are proposed, like Modified Gravity (MOND)
deleted 5y ago
[deleted]
naveen99 5y ago
In an infinite universe with random local density of matter, the gravitational constant would vary by location. Anyone know what the consensus is among astonomers if the universe is infinite or not ? Is there even a way to theoretically know ?
- redis_mlc 5y ago
  > In an infinite universe It's expanding now according to red-shift measurements. Whether that means it expands to infinity or collapses is unknown. > Is there even a way to theoretically know? Not directly, but if our red-shift, etc. measurements are only 13.8 billion years old, then either we're in the first universe, or it cycles. > the gravitational constant would vary by location Your gravitational constant question is a common one for non-physicists. The gravitational constant, G (aka "big G"), is a universal constant, which means it's a property of the gravitational force itself, not of density. Little g is the local gravitational acceleration, 9.81 m/s2 at the earth's surface. On the moon, it's 1.62 m/s2. You can read more here: https://en.wikipedia.org/wiki/Gravitational_constant https://en.wikipedia.org/wiki/Gravitational_constant To completely understand why G is needed, it's a scaling factor for our human-convenient measuring units for mass and distance. You could use alternate units that would make G = 1.0 if you wanted - called using "natural units" or "Planck units": https://en.wikipedia.org/wiki/Planck_units https://en.wikipedia.org/wiki/Planck_units I hope you enjoyed this physics lesson. :)
- magicalhippo 5y ago
  Theory can say what shape the universe should have, for example inflation predicts a flat universe[2]. Of course you then have to check if the theory matches what we observe. As far as I've gathered, the current measurements are consistent with a flat universe. Here's[1] a post about a recent, improved measurement. [1]: https://astrobites.org/2020/12/03/cosmic-clocks/ https://astrobites.org/2020/12/03/cosmic-clocks/ [2]: https://en.wikipedia.org/wiki/Flatness_problem#Inflation https://en.wikipedia.org/wiki/Flatness_problem#Inflation
- naveen99 5y ago
  Edit: cosmological constant, not gravitational constant
geokon 5y ago
Maybe someone could help me understand why a simple explanation for dark matter isn't possible. I get how you can measure the mass of stars, and how you could also maybe measure the mass of interstellar gasses. You add it all up and there is missing mass. But how could you estimate the mass of larger objects? Couldn't the interstellar medium be full of moon sized cold rocks? You'd have no way to observe them on a galactic scale since they're not illuminated, they have effectively no absorption spectrum b/c of their small surface area and yet I'd think they could make up the difference you need for this "dark matter" The explanation seems too simplistic, so I'm clearly missing something :)
- cedilla 5y ago
  On the mass scales we talk about, even small rocks would add up and lead to an observable dimming. Nebulae usually are usually even less dense, just a few molecules per cubic metre, but they are still easily observable.
  - magicalhippo 5y ago
    I ran some numbers, for the Milky Way[1], the visible matter is roughly 1e11 solar masses, while the dark matter halo contains[2] about 1-2e12 solar masses (depends on your cut-off), I'll use 1e12. The moon is about 3.7e-8 solar masses[3], so you' need about 2.7e19 of these "rocks" to make up the Milky Way dark matter halo. For comparison the Milky Way has about 250e9 stars[4]. So we're talking over 100 million "cold rocks" per star in our galaxy. [1]: https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dark_matter_halo https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dar... [2]: https://arxiv.org/abs/1510.06409 https://arxiv.org/abs/1510.06409 [3]: https://www.google.com/search?q=mass+of+the+moon+in+solar+mass https://www.google.com/search?q=mass+of+the+moon+in+solar+ma... [4]: https://public.nrao.edu/ask/how-many-stars-do-the-andromeda-and-milky-way-galaxies-have/ https://public.nrao.edu/ask/how-many-stars-do-the-andromeda-...
  - geokon 5y ago
    Nebulae are an argument in favor. They are observable b/c they're not dense. They have a very high surface area to mass ratio. Light passes through them and they have absorption spectrum. They also emit a lot of black body radiation. By contrast a moon size object has a very low surface area to mass ration. It has no absorption spectrum b/c light can't pass through - and any black body radiation would be negligible b/c of relatively small surface area
state_less 5y ago
Seems like they observed distortions of space time, but not seeing any (dark)matter? Just for fun, what if matter actually has coordinates (x,y,z,h1,...,hn). Where hn is some number of hyper dimensions. Visible matter is stuck in our 3 space (x,y,z,0,...,0) and gravity is the only force that permeates across hyperspace. We can’t see the hypermatter except by the gravitational imprint it leaves on 3 space material. If we had the ability, it’d be neat to see if our visible matter can alter the configuration of dark matter, just as dark matter seems to affect our visible matter.
- paulrouget 5y ago
  Then gravity would “leak” and the inverse square law would not perfectly apply. This has been measured and an extra dimension has been ruled out.
Juntu 5y ago
Al made dark matter so the Big Bang spaced out and time was created by tick if I say the time and planet started at monday would their be a such answer of dark matter. Matter is complete opposite of air the more you consume it in a tight space it will explode what theorem is looking at the telescope to see map of it so a baby is borned with its eyes closed the child can only tell and his future of knowing that an astronomer can tell him god created all things but religious belief is out the door. Scientist began and leaked the whats when’s where why. Who am I the names juntu in my maximum level. Hax//((::!!

77 points by chriskanan 5y ago

DudeInBasement 5y ago

Lol, they haven't even found any yet and they made a map? I guess you got to do something with that grant money

Bancakes 5y ago
Not to sound hateful but who are the neo-scientists? You go through school, high school, then you get a bachelor's. Great. Then you also go for a master's and a PhD. Maybe even a post-doc?! So you haven't taken part in the economy for 10 years of your adulthood, you don't have serious job prospects, and you need mortgage/family money. What do you do? Maybe that explains the abundance of cheap, unverifiable, irreproducible studies.
- anticristi 5y ago
  Not sure who invented the academia-industry dichotomy, but he/she deserves a society-wide slap. Society loses, the individual loses. My career is just a big middle finger pointed at this dichotomy.
  - alexeldeib 5y ago
    > My career is just a big middle finger pointed at this dichotomy. I only saw your comment after the parent it replied to was flagged/removed. This section of your reply jumped out to me, care to elaborate? My experience (undergrad, minimal research -> industry) has shown me that the gap is quite real, but both sides are important. They simply align to different goals. The most extreme trope of this in tech is industry engineers making fun of researchers who produce systems which "only work on paper" (paraphrased, some meaning is lost in that phrasing). I find that criticism too harsh (and rude), but it does highlight the differing goals: industry frequently needs to build systems which 1) work, and 2) scale, among other things. Academia can afford to push the cutting edge by compromising on constraints real world systems have to obey. It's the classic exploration vs. exploitation problem. Industry thinks the exploration isn't important because they need to optimize the local maxima/minima now. Academia cares about the global maxima/minima, pushing the state of the art, exploring new solutions in the hope of advancing knowledge and understanding. In the largest companies, the cost/benefit analysis shifts back toward exploration. Google, Microsoft, Xerox PARC, etc. And the natural extension of academics elsewhere pushing the state of the art is finding new optimums and bringing that back to industry (e.g., as a startup). So I feel there gap is real, with lots of symbiotic back and forth. Broad generalizations, but not unfair I think.
    - anticristi 5y ago
      I don't 100% agree, but let's play with "academia is exploration, industry is exploitation". How would you ensure you explore something useful? How would you ensure to transfer what you explored, so you can exploit that knowledge? What if you exploitation is stuck in a local minimum, how would you ensure you send feedback back to exploitation? I feel a more useful model is Technological Readiness Level (TRLs). Academia deals with lower TRLs, while industry deals with higher TRLs. Somewhere in between there is -- there needs to be -- a huge overlap to ensure a proper continuum. This can also be phrased in a risk perspective. Lower TRLs are extremely high risk, almost like throwing money out the window, but have extremely high societal reward. Think curing cancer or terraforming Mars. These are best funded via grants. High TRLs are low risk, low rewards; issue some bonds. Somewhere in the middle is VC territory. But how? PhD students doing industrial internships; PIs doing industrial sabaticals; engineering managers becoming adjunct lecturers or giving guest lectures; academia and industry working on common projects; making patents could as much as papers on an academic CV. Is anyone in the world either exploring or exploiting? Then why don't we create a system that allows people to quickly switch hats, as required to achieve larger goals?
      - alexeldeib 5y ago
        I like your comparison to TRLs. I think we’re saying the same thing, actually! I don’t mean that one is exploitation and one is exploration exclusively. Re-reading my post, the phrase “gap” might be a misnomer — it’s more like a balancing act of priorities. > How would you ensure you explore something useful? How would you ensure to transfer what you explored, so you can exploit that knowledge? What if you exploitation is stuck in a local minimum, how would you ensure you send feedback back to exploitation? I’d argue you can’t necessarily be sure. Not all research works out. But we need to ask questions and get answers to move forward, even if what we discover may not be worth exploiting (getting tired of my own phrasing). Getting stuck in local min/maxima is part of why we need both angles. This is the flip side of what I said about industry needing to build systems that work — lower budget for risk tolerance, but also increased likelihood of getting stuck in a local optimum. It sounds like I’m using those terms to describe the same spectrum you are with TRLs. At least, I wholeheartedly agree with everything you said — the “but how” portion sounds exactly like the symbiotic overlap/crossover I had in mind. I guess to answer my own original question, it sounds like you’ve swung back and forth a few times? > Then why don't we create a system that allows people to quickly switch hats, as required to achieve larger goals? Preaching to the choir ;) and appreciate your perspective
- Blikkentrekker 5y ago
  At least where I live, many start earning some salary from academic contribution of some minor sort during the bachelor's trajectory, typically by assisting in lessons to lower classes, most art to do so during master's, and Ph.D. students are employed by the institute that promotes them and earn a full salary for their research. The real issue isn't that, the real issue to me is quite a few research fields are purely taking part in œconomy and entertainment, and essentially funded of sponsorships generate by spectacular news reports and nothing more. A very large quantity of scientific research is purely infotainment and doesn't actually generate knowledge that is used for anything but merely interesting to read, which is what could cause the replication crisis to go unnoticed for so long among other things: no one found out, as no one was using it in any way that relied upon it's veracity, and many even made up data for decades and went unnoticed. I recently read a paper that studied some social parts of the acquisition of language in Japanese, — quite interesting to read, and as usual it was quite spectacular because the findings went against the established ideas, but this knowledge merely exists to be “fascinating” and it won't ever be used for anything that assumes it's veracity, and as a consequence the data could have been pulled from the æther and no one would ever find out and replication will surely never occur,and even if it would, it would be too late as some years would have passed and the original auctor can always argue that these social matters changed within the interval.
- Mike86534 5y ago
  That's one reason why they sell their souls to the Climate UN / NWO scam and pump out trash papers based on fudged data/models that backs up the fearmongering narrative. https://www.thegwpf.org/content/uploads/2018/02/Groupthink.pdf https://www.thegwpf.org/content/uploads/2018/02/Groupthink.p...
- ad404b8a372f2b9 5y ago
  You have a bit of a caricatural take on research. From a societal perspective, PhDs and postdocs take part in the economy, they have a job, make money, spend money, produce research sometimes in partnership with the industry, sometimes with immediate applications, sometimes theoretical with applications that will only come decades down the line. There is no reason to set the line at Bachelors, as a society we've found it's best to have a subset of the population go into higher studies. We used to have children participate in the economy 10 to 15 years earlier in fields and factories and it wasn't so good. From the perspective of the individual, getting a PhD isn't always much longer than getting a Masters, and teaches you skill you will not acquire anywhere else. It also opens the door to research positions that would not be accessible to you without it. And you can have greater expected monetary returns on the medium and long term. I've known a lot of people doing PhDs, a good amount doing it for prestige, because it was the expected next step after a masters. Many others were doing it out of passion, for the freedom to study things they were interested in an amazing environment. So on an individual level many people find it worth it whether their priority is money or intellectual pursuits.
- deleted 5y ago
  [deleted]
gus_massa 5y ago
Alternative title: "Group of scientists assuming that the current model of gravity is correct look at the discrepancies in the observations and calculate a map of where and how much matter would be necessary to fix this difference. Some of them believe this additional mater is real, and some of them doubt, but meanwhile to make the titles shorter they use the catchy name of dark matter."
Xorlev 5y ago
It's clear you didn't even try to read the article. They mapped the distortion effects on visible light thought to be caused by dark matter. They don't know what it is or how it works, but they know _something_ is causing apparent distortion to the light.
- WesleyHale 5y ago
  That _something_ might as well be interdemensional pixies.
  - ben_w 5y ago
    Sure, just so long as the pixies don’t interact electromagnetically, or strongly with each other by anything except gravity.
  - bdamm 5y ago
    Won’t it be interesting when our model of physics is updated to explain these cosmological phenomena? Who knows what new technology will be discovered as a result!
    - ben_w 5y ago
      My brother once jokingly suggested that Dark Energy is the waste product of the power sources of alien civilisations. (Neither of us is a physicist, but it’s an interesting idea for a short story).
      - macintux 5y ago
        There's a very old short story I've never been able to find again that beat you to it. Every civilization that discovers FTL travel contributes to a more rapid expansion of the universe. The older civilizations beg them to stop, but inevitably they fail.
        irrational 5y ago
_Microft 5y ago
What's surprising about that if I may ask? We have observations that match quite well what would be expected if there was additional matter in the universe. We cannot detect this matter though, so the conjecture is that there must be something that interacts gravitationally with the rest of the universe but not in other ways. We call it dark matter for now. We do not yet know what it consists of, there are even competing theories but mapping the effects of it? That we can do. This approach is not at all unusual. If you want to know about another example from a different field, look no further than genetics. Gregor Mendel observed rules for inheritance of certain traits in plants and was able to make predictions from that. He discovered genetics. In the 1850-60ies. This was a long time before Crick, Watson and Franklin discovered the structure of the underlying mechanism for inheritance (DNA).
- galaxyLogic 5y ago
  What if Dark Matter is just that? Dark matter. If there were lots and lots of something like planets out there, which for one reason or another don't reflect light, that would be something dark right? Or what if Dark Matter was lots of Black Holes? We (more or less) know what Black Holes are so why don't we assume there could be lots of them having a gravitational effect but no visibility? In other words why do we need to assume that Dark Matter is made of some kind of exotic unknown matter?
  - _Microft 5y ago
    That’s indeed a possible explanation. See also: https://en.m.wikipedia.org/wiki/Massive_compact_halo_object https://en.m.wikipedia.org/wiki/Massive_compact_halo_object
  - kl4m 5y ago
    Here's an expanded explanation of the "lots of black holes" hypothesis, posted this week by PBS Space Time. "What If (Tiny) Black Holes Are Everywhere?" https://www.youtube.com/watch?v=srVKjWn26AQ https://www.youtube.com/watch?v=srVKjWn26AQ Dark Matter is just a placeholder name, don't read too much into the "Matter" part. There are no assumptions there and plenty of alternate explanations are proposed, like Modified Gravity (MOND)

deleted 5y ago

[deleted]

naveen99 5y ago

In an infinite universe with random local density of matter, the gravitational constant would vary by location. Anyone know what the consensus is among astonomers if the universe is infinite or not ? Is there even a way to theoretically know ?

redis_mlc 5y ago
> In an infinite universe It's expanding now according to red-shift measurements. Whether that means it expands to infinity or collapses is unknown. > Is there even a way to theoretically know? Not directly, but if our red-shift, etc. measurements are only 13.8 billion years old, then either we're in the first universe, or it cycles. > the gravitational constant would vary by location Your gravitational constant question is a common one for non-physicists. The gravitational constant, G (aka "big G"), is a universal constant, which means it's a property of the gravitational force itself, not of density. Little g is the local gravitational acceleration, 9.81 m/s2 at the earth's surface. On the moon, it's 1.62 m/s2. You can read more here: https://en.wikipedia.org/wiki/Gravitational_constant https://en.wikipedia.org/wiki/Gravitational_constant To completely understand why G is needed, it's a scaling factor for our human-convenient measuring units for mass and distance. You could use alternate units that would make G = 1.0 if you wanted - called using "natural units" or "Planck units": https://en.wikipedia.org/wiki/Planck_units https://en.wikipedia.org/wiki/Planck_units I hope you enjoyed this physics lesson. :)
magicalhippo 5y ago
Theory can say what shape the universe should have, for example inflation predicts a flat universe[2]. Of course you then have to check if the theory matches what we observe. As far as I've gathered, the current measurements are consistent with a flat universe. Here's[1] a post about a recent, improved measurement. [1]: https://astrobites.org/2020/12/03/cosmic-clocks/ https://astrobites.org/2020/12/03/cosmic-clocks/ [2]: https://en.wikipedia.org/wiki/Flatness_problem#Inflation https://en.wikipedia.org/wiki/Flatness_problem#Inflation
naveen99 5y ago
Edit: cosmological constant, not gravitational constant

geokon 5y ago

Maybe someone could help me understand why a simple explanation for dark matter isn't possible. I get how you can measure the mass of stars, and how you could also maybe measure the mass of interstellar gasses. You add it all up and there is missing mass. But how could you estimate the mass of larger objects? Couldn't the interstellar medium be full of moon sized cold rocks? You'd have no way to observe them on a galactic scale since they're not illuminated, they have effectively no absorption spectrum b/c of their small surface area and yet I'd think they could make up the difference you need for this "dark matter" The explanation seems too simplistic, so I'm clearly missing something :)

cedilla 5y ago
On the mass scales we talk about, even small rocks would add up and lead to an observable dimming. Nebulae usually are usually even less dense, just a few molecules per cubic metre, but they are still easily observable.
- magicalhippo 5y ago
  I ran some numbers, for the Milky Way[1], the visible matter is roughly 1e11 solar masses, while the dark matter halo contains[2] about 1-2e12 solar masses (depends on your cut-off), I'll use 1e12. The moon is about 3.7e-8 solar masses[3], so you' need about 2.7e19 of these "rocks" to make up the Milky Way dark matter halo. For comparison the Milky Way has about 250e9 stars[4]. So we're talking over 100 million "cold rocks" per star in our galaxy. [1]: https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dark_matter_halo https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dar... [2]: https://arxiv.org/abs/1510.06409 https://arxiv.org/abs/1510.06409 [3]: https://www.google.com/search?q=mass+of+the+moon+in+solar+mass https://www.google.com/search?q=mass+of+the+moon+in+solar+ma... [4]: https://public.nrao.edu/ask/how-many-stars-do-the-andromeda-and-milky-way-galaxies-have/ https://public.nrao.edu/ask/how-many-stars-do-the-andromeda-...
- geokon 5y ago
  Nebulae are an argument in favor. They are observable b/c they're not dense. They have a very high surface area to mass ratio. Light passes through them and they have absorption spectrum. They also emit a lot of black body radiation. By contrast a moon size object has a very low surface area to mass ration. It has no absorption spectrum b/c light can't pass through - and any black body radiation would be negligible b/c of relatively small surface area

state_less 5y ago

Seems like they observed distortions of space time, but not seeing any (dark)matter? Just for fun, what if matter actually has coordinates (x,y,z,h1,...,hn). Where hn is some number of hyper dimensions. Visible matter is stuck in our 3 space (x,y,z,0,...,0) and gravity is the only force that permeates across hyperspace. We can’t see the hypermatter except by the gravitational imprint it leaves on 3 space material. If we had the ability, it’d be neat to see if our visible matter can alter the configuration of dark matter, just as dark matter seems to affect our visible matter.

paulrouget 5y ago
Then gravity would “leak” and the inverse square law would not perfectly apply. This has been measured and an extra dimension has been ruled out.

Juntu 5y ago

Al made dark matter so the Big Bang spaced out and time was created by tick if I say the time and planet started at monday would their be a such answer of dark matter. Matter is complete opposite of air the more you consume it in a tight space it will explode what theorem is looking at the telescope to see map of it so a baby is borned with its eyes closed the child can only tell and his future of knowing that an astronomer can tell him god created all things but religious belief is out the door. Scientist began and leaked the whats when’s where why. Who am I the names juntu in my maximum level. Hax//((::!!