“I would rather be ashes than dust!
I would rather that my spark should burn out in a brilliant blaze than it should be stifled by dry-rot.
I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet.
The function of man is to live, not to exist.
I shall not waste my days trying to prolong them.
I shall use my time.” -Jack London
When you think about the Universe all the time, from the smallest scales and the most fundamental particles to the largest cosmic structures and everything in between, the hard part isn’t choosing a topic to discuss; it’s choosing which small aspect to focus on! At Starts With A Bang this past week, here’s what we chose:
- Why doesn’t dark matter form black holes? (for Ask Ethan),
- The year in miniatures (for our Weekend Diversion),
- Curiosity’s greatest hits on its 3-year anniversary (for Mostly Mute Monday),
- Why are the Perseids always so good?,
- and The evolution of starlight (for Throwback Thursday).
And if you wanted another perspective on Meteor Showers, I had a contribution over at Forbes:
Our voting on which book chapter our Patreon supporters will receive is almost complete, and the two leading chapters are separated by just one vote! (Get in on this; join and donate today.) That said, it’s on to our Comments of the Week!
Image credit: ESO / L. Calçada.
From Chris Mannering on dark matter halos: “What I don’t understand is what sort of distribution of diffuse dark matter does it take for each next further away star from the core to speed up just enough in spirals that it’s as if the arms were solid. What is the formula for that?”
It’s not, as you say, that it’s as if the arms were solid; they aren’t. They don’t rotate around the galaxy like a record, where the outer portions spin at the same angular velocity as the inner parts. Nor do they rotate like the planets in our Solar System do, where the inner ones orbit much more quickly than the outer ones (below, left). Instead, they move at the same linear velocity as they orbit: the same speed, but because the orbital path is longer, they take longer to go around the galaxy.
Image credit: Chris B. Brook & Arianna Di Cintio, via http://ift.tt/1cyqG4X.
But make no mistake, the arms aren’t solid. Rather, they move according to density wave theory, the general takeaway being that these arms are not consistent features, but are waves of new star formation triggered by compression waves of greater density that collapse the gas present in the galaxies. There’s more going on than you might initially intuit here. Dark matter is part of the explanation (for the rotational speeds of the individual stars), but there’s a whole lot more physics at play than just dark matter!
Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).
From Anthony on the dark matter-black hole connection: “I assume that a black hole is still able to capture dark matter, even if one won’t form directly from dark matter.”
It not only can, it must. Once you have a black hole, it’s true that nothing that enters the event horizon can escape again. The vast majority of a black hole’s mass is going to come from the initial event that caused its formation: the collapse of a giant stellar object and/or mergers of many existing black holes. But if you’re a dark matter particle that happens to cross the event horizon, you’re in there for certain, too!
Image credit: NASA/ESA Hubble Space Telescope collaboration.
I ran the numbers about a year ago, and here were some important takeaways:
- Black holes form almost entirely (~100%) out of normal matter no matter where they form.
- The ones that form where the density of matter is low — like out where we are — will have a substantial portion of that growth (around 16%) come from dark matter, but that growth is (on average) negligible compared to the initial black hole’s mass.
- The ones that form where the density of matter is high — like near the galactic center — will experience significant growth, but at least 99.996% of that growth comes from normal matter and not dark matter.
So yes, it happens! But it’s not very important for the black hole’s mass… at least, not yet.
Image credit: Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC).
From Michael Kelsey on the dark sector: “Ethan, as a particle physicist I need to take exception to one of your introductory comments. You wrote, “Dark matter sure does have gravity, and it sure doesn’t form black holes, dark matter stars, planets, or dark atoms.”
Actually, we _don’t_ know this! In fact, some would argue that this assumption, in comparison to the part of the universe we do know about, is a horrible oversimplification. Why shouldn’t the various dark matter particles (why should there be only one?) have a whole suite of mutual interactions building up DM atoms, molecules, and larger structures?”
Well, there are some things we know. We know that you can’t have incredibly dense, compact, bound clumps or we would have detected them due to microlensing. We know that the self-interaction cross-section can’t be too large, or dark matter would be “sticky” and wouldn’t form the diffuse network of large-scale structure that we require to reproduce the observed Universe. And we know that dark matter can’t build up atoms and molecules the same way normal matter does because when two big “clumps” of dark matter collide, they don’t emit heating signatures the way normal matter does.
Image credit: Gerard Lemson & the Virgo Consortium, via http://ift.tt/1l8cv5n.
But with that said, we have only constraints on what dark matter cannot do. There’s plenty of potential for it to be made of multiple components, for those components to self-assemble in some way(s), for self-interactions or dark matter-normal matter interactions to be non-zero, and for a slew of diffuse, low-interaction dark matter substructures to exist.
It’s important to both keep an open mind, but also to be aware of what the constraints are. If Michael Kelsey is lucky, the dark matter-normal matter cross section will net SuperCDMS a Nobel Prize! If he’s (and all of us are) unlucky, it may be so small that there will be no experiment we can design to detect that interaction at all.
The search continues.
Image credit: Tatsuya Tanaka of Miniature Calendar; original via http://ift.tt/1hTHcrf.
From PJ on miniature repairmen: “Love the back of the PC motherboard above! Makes me think about how small components are becoming. Tweezers & magnifying lenses are too common for repair of these boards. I could definitely do with a handful of these little guys to help.
“
I’m pretty sure these farmers are planting rice-fields, but the detail on figurines this small is definitely impressive. This seems to be a uniquely perfected-by-the-Japanese phenomenon — of miniaturization without the loss of quality — but I’m glad we’re equally impressed.
Image credit: NASA / JPL-Caltech / MSSS.
From Denier on Mars Curiosity: “The Wile E Coyote landing was amazing. Kudos to ACME, but to this day the design decision on the wheels is hard for me to wrap my head around. Before Curiosity was ever packaged up for launch, they knew the wheel design was not good. Even Scarecrow, a testing version of Curiosity stripped down to only the drivetrain, was shredding these wheels.”
To say that Scarecrow was stripped down is being completely unfair to the testing team: Scarecrow was configured exactly so that the forces on its wheels on Earth would be the same forces on Curiosity’s wheels on Mars. They also have deliberately driven Scarecrow over harsher terrain and rougher conditions than they expected to find on Mars. Finally, the damage you show to Scarecrow’s wheels (below) is severe, but you left something out.
Image credit: NASA’s “Scarecrow” test team, MSSS.
The rover still drove fine! Even with that damage, there’s no problem in traversing sandy, rocky terrain. One of the tricks the Mars Curiosity rover uses to deal with its damaged front-and-middle wheels is very clever: it spends a good deal of time driving backwards, so that the undamaged rear wheels are in the lead. As Matt Heverly, the lead driver for Mars Curiosity, says:
“We have driven Scarecrow about 12 kilometers (7.5 miles) in the Marsyard over rocks and slopes much harsher than we expect for Curiosity. There are some dents and holes in these wheels, but the rover is still performing well. We will continue to characterize the wheels both on Mars and in the Marsyard, but we don’t expect the wear to impact our ability to get to Mt. Sharp.”
I think it’s okay to talk about limitations, but three years in, Curiosity is doing just fine, even with the damage!
Image credit: David Kingham 2013 | The National Maritime Museum.
From Steve Waclo on Perseid meteor viewing: “After pondering potential dark viewing sites, I finally took an old mat to the back of our community and just after midnight, reclined on a sidewalk with a view to the NE and minimal street lighting in my line of sight.
Within a few minutes, I was rewarded with an impressive streak of light that made my efforts worthwhile! 10 minutes later, two more in rapid succession, then a fourth about 15 minutes later. Five seemed to be a nice round number, but it was not to be and after another 15 minutes, I went home.
While I convinced myself I had seen other, fainter streaks, I believe the highly visible, and of course, much more spectacular displays that presented, would justify a half hour of post-midnight watching from even a bright, urban viewpoint.”
Just a fun story that I thought I’d enjoy sharing with you from the comments. I’ve been “blessed” with clouds during Wednesday, Thursday and Friday nights, but I did get to see a big bright streak on Monday. There are other great meteor showers out there, but the Perseids really shine for the enhanced brightness that comes from the great speed of the meteors hitting us. Tonight is probably the last decent night for viewing, so if you have a chance, go out and take a look!
Image credit: Boots Hart, of http://ift.tt/1Mq63pZ.
From Denier on concern about what we don’t know about cometary orbits: “Comet Swift-Tuttle has been documented all the way back to 69 BC. It is in a nice, stable 11:1 orbital resonance with Jupiter. If you know the last time it passed Earth, and the current position of Jupiter then you can deduce the location of Comet Swift-Tuttle. The physics of orbital mechanics is also pretty well ironed out at this point. Astrophysicists are routinely projecting orbital paths of objects in space dozens of orbits into the future.
Given all that, when Comet Swift-Tuttle last came by the Earth in 1992 it was 17 days from where it should have been. With an object moving 133,000 miles per hour, 17 days is a long way off. With the current awesomeness of telescopes, computers, an object 26 miles across with the Perseids serving as a yearly reminder of its existence, how was the Astrophysics community in 1991 so wrong?”
There are two issues here, both of which are very important to understand:
- The outer Solar System, beyond Neptune, where Comet Swift-Tuttle spends most of its time, is very poorly mapped. Gravitational interactions happening out there — even small ones — are completely unaccounted for. A slight tug could change its speed by just micrometers per second, which would change its arrival time by days.
- The comet is named Swift-Tuttle because it was first identified and discovered independently by Swift and Tuttle in the mid-1800s. The observations we had of its position then: 1860s era measurements (peaking in 1862), were the only precise observations we had to base its orbit off of.
Image credit: NASA, of Comet Swift-Tuttle, via http://ift.tt/1N2XD9P.
We’ll do much better the next time around, since we did much better in the 1990s when the comet returned. But if you’re getting at the important point that if we’re off in our estimates by too much, the “cone of uncertainty” could indicate that the comet’s path might include… our planet. Well, the next pass looks like this: “It is now known that the comet will pass 0.153 AU (22,900,000 km; 14,200,000 mi) from Earth on August 5, 2126.”
Every orbit carries an estimated threat of around ~2 x 10^-8, but comets don’t live for hundreds of millions of years before evaporating completely or getting gravitationally perturbed into a hyperbolic (escape) orbit. I wouldn’t worry too much about it, but I wouldn’t worry zero about it, either. If you’re concerned with the long-term fate of life on Earth — over the next many thousands of years — pay attention to this guy, just a little, every 133 years or so.
Image credit: NASA, ESA, and the Hubble SM4 ERO Team.
And finally, from See Noevo on starlight (kind of): “What do think about the light the universe appears to be missing?”
It’s very easy to grab a headline and draw an “OMG!” conclusion from it. Like in this case that light is missing from the Universe! But what does this actually mean? Not the headline, but the science.
What this study talks about is that intergalactic clouds and filaments of gas are ionized in a certain amount that indicates that there’s this presence of ultraviolet (ionizing) radiation that must exist. The problem is, we only know of two sources for this radiation: hot, young, blue stars and quasars/active galaxies. And in the nearby Universe, we don’t have enough of either one of those things to explain the ionization we see.
Image credit: ESA & NASA; Acknowledgement: E. Olszewski (U. Arizona).
The funny thing is, farther away, we do see enough to explain it! So what do I think of it?
I think there’s very likely a mundane explanation out there. Perhaps the last big burst of star formation was sufficient to cause the ionization, which has persisted; perhaps we’re just going through a period of inactive galactic nuclei that’s transient nearby; perhaps the UV photons from young star clusters do escape into the IGM at higher rates than we expect; perhaps there’s something happening at the outskirts of our galaxy absorbing the UV photons that try to re-enter our galaxy from the IGM.
But I don’t think it poses a crisis of any type, save for astronomers that thought their picture of this phenomenon was complete. There’s more to learn about a whole slew of things in the Universe, and that’s a good thing. The process of investigation never ends, and I don’t think we’ll ever run out of interesting questions to ask! Thanks for sharing a great week with me, and know I’m looking forward to the next one.
from ScienceBlogs http://ift.tt/1Mq665i
“I would rather be ashes than dust!
I would rather that my spark should burn out in a brilliant blaze than it should be stifled by dry-rot.
I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet.
The function of man is to live, not to exist.
I shall not waste my days trying to prolong them.
I shall use my time.” -Jack London
When you think about the Universe all the time, from the smallest scales and the most fundamental particles to the largest cosmic structures and everything in between, the hard part isn’t choosing a topic to discuss; it’s choosing which small aspect to focus on! At Starts With A Bang this past week, here’s what we chose:
- Why doesn’t dark matter form black holes? (for Ask Ethan),
- The year in miniatures (for our Weekend Diversion),
- Curiosity’s greatest hits on its 3-year anniversary (for Mostly Mute Monday),
- Why are the Perseids always so good?,
- and The evolution of starlight (for Throwback Thursday).
And if you wanted another perspective on Meteor Showers, I had a contribution over at Forbes:
Our voting on which book chapter our Patreon supporters will receive is almost complete, and the two leading chapters are separated by just one vote! (Get in on this; join and donate today.) That said, it’s on to our Comments of the Week!
Image credit: ESO / L. Calçada.
From Chris Mannering on dark matter halos: “What I don’t understand is what sort of distribution of diffuse dark matter does it take for each next further away star from the core to speed up just enough in spirals that it’s as if the arms were solid. What is the formula for that?”
It’s not, as you say, that it’s as if the arms were solid; they aren’t. They don’t rotate around the galaxy like a record, where the outer portions spin at the same angular velocity as the inner parts. Nor do they rotate like the planets in our Solar System do, where the inner ones orbit much more quickly than the outer ones (below, left). Instead, they move at the same linear velocity as they orbit: the same speed, but because the orbital path is longer, they take longer to go around the galaxy.
Image credit: Chris B. Brook & Arianna Di Cintio, via http://ift.tt/1cyqG4X.
But make no mistake, the arms aren’t solid. Rather, they move according to density wave theory, the general takeaway being that these arms are not consistent features, but are waves of new star formation triggered by compression waves of greater density that collapse the gas present in the galaxies. There’s more going on than you might initially intuit here. Dark matter is part of the explanation (for the rotational speeds of the individual stars), but there’s a whole lot more physics at play than just dark matter!
Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).
From Anthony on the dark matter-black hole connection: “I assume that a black hole is still able to capture dark matter, even if one won’t form directly from dark matter.”
It not only can, it must. Once you have a black hole, it’s true that nothing that enters the event horizon can escape again. The vast majority of a black hole’s mass is going to come from the initial event that caused its formation: the collapse of a giant stellar object and/or mergers of many existing black holes. But if you’re a dark matter particle that happens to cross the event horizon, you’re in there for certain, too!
Image credit: NASA/ESA Hubble Space Telescope collaboration.
I ran the numbers about a year ago, and here were some important takeaways:
- Black holes form almost entirely (~100%) out of normal matter no matter where they form.
- The ones that form where the density of matter is low — like out where we are — will have a substantial portion of that growth (around 16%) come from dark matter, but that growth is (on average) negligible compared to the initial black hole’s mass.
- The ones that form where the density of matter is high — like near the galactic center — will experience significant growth, but at least 99.996% of that growth comes from normal matter and not dark matter.
So yes, it happens! But it’s not very important for the black hole’s mass… at least, not yet.
Image credit: Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC).
From Michael Kelsey on the dark sector: “Ethan, as a particle physicist I need to take exception to one of your introductory comments. You wrote, “Dark matter sure does have gravity, and it sure doesn’t form black holes, dark matter stars, planets, or dark atoms.”
Actually, we _don’t_ know this! In fact, some would argue that this assumption, in comparison to the part of the universe we do know about, is a horrible oversimplification. Why shouldn’t the various dark matter particles (why should there be only one?) have a whole suite of mutual interactions building up DM atoms, molecules, and larger structures?”
Well, there are some things we know. We know that you can’t have incredibly dense, compact, bound clumps or we would have detected them due to microlensing. We know that the self-interaction cross-section can’t be too large, or dark matter would be “sticky” and wouldn’t form the diffuse network of large-scale structure that we require to reproduce the observed Universe. And we know that dark matter can’t build up atoms and molecules the same way normal matter does because when two big “clumps” of dark matter collide, they don’t emit heating signatures the way normal matter does.
Image credit: Gerard Lemson & the Virgo Consortium, via http://ift.tt/1l8cv5n.
But with that said, we have only constraints on what dark matter cannot do. There’s plenty of potential for it to be made of multiple components, for those components to self-assemble in some way(s), for self-interactions or dark matter-normal matter interactions to be non-zero, and for a slew of diffuse, low-interaction dark matter substructures to exist.
It’s important to both keep an open mind, but also to be aware of what the constraints are. If Michael Kelsey is lucky, the dark matter-normal matter cross section will net SuperCDMS a Nobel Prize! If he’s (and all of us are) unlucky, it may be so small that there will be no experiment we can design to detect that interaction at all.
The search continues.
Image credit: Tatsuya Tanaka of Miniature Calendar; original via http://ift.tt/1hTHcrf.
From PJ on miniature repairmen: “Love the back of the PC motherboard above! Makes me think about how small components are becoming. Tweezers & magnifying lenses are too common for repair of these boards. I could definitely do with a handful of these little guys to help.
I’m pretty sure these farmers are planting rice-fields, but the detail on figurines this small is definitely impressive. This seems to be a uniquely perfected-by-the-Japanese phenomenon — of miniaturization without the loss of quality — but I’m glad we’re equally impressed.
Image credit: NASA / JPL-Caltech / MSSS.
From Denier on Mars Curiosity: “The Wile E Coyote landing was amazing. Kudos to ACME, but to this day the design decision on the wheels is hard for me to wrap my head around. Before Curiosity was ever packaged up for launch, they knew the wheel design was not good. Even Scarecrow, a testing version of Curiosity stripped down to only the drivetrain, was shredding these wheels.”
To say that Scarecrow was stripped down is being completely unfair to the testing team: Scarecrow was configured exactly so that the forces on its wheels on Earth would be the same forces on Curiosity’s wheels on Mars. They also have deliberately driven Scarecrow over harsher terrain and rougher conditions than they expected to find on Mars. Finally, the damage you show to Scarecrow’s wheels (below) is severe, but you left something out.
Image credit: NASA’s “Scarecrow” test team, MSSS.
The rover still drove fine! Even with that damage, there’s no problem in traversing sandy, rocky terrain. One of the tricks the Mars Curiosity rover uses to deal with its damaged front-and-middle wheels is very clever: it spends a good deal of time driving backwards, so that the undamaged rear wheels are in the lead. As Matt Heverly, the lead driver for Mars Curiosity, says:
“We have driven Scarecrow about 12 kilometers (7.5 miles) in the Marsyard over rocks and slopes much harsher than we expect for Curiosity. There are some dents and holes in these wheels, but the rover is still performing well. We will continue to characterize the wheels both on Mars and in the Marsyard, but we don’t expect the wear to impact our ability to get to Mt. Sharp.”
I think it’s okay to talk about limitations, but three years in, Curiosity is doing just fine, even with the damage!
Image credit: David Kingham 2013 | The National Maritime Museum.
From Steve Waclo on Perseid meteor viewing: “After pondering potential dark viewing sites, I finally took an old mat to the back of our community and just after midnight, reclined on a sidewalk with a view to the NE and minimal street lighting in my line of sight.
Within a few minutes, I was rewarded with an impressive streak of light that made my efforts worthwhile! 10 minutes later, two more in rapid succession, then a fourth about 15 minutes later. Five seemed to be a nice round number, but it was not to be and after another 15 minutes, I went home.
While I convinced myself I had seen other, fainter streaks, I believe the highly visible, and of course, much more spectacular displays that presented, would justify a half hour of post-midnight watching from even a bright, urban viewpoint.”
Just a fun story that I thought I’d enjoy sharing with you from the comments. I’ve been “blessed” with clouds during Wednesday, Thursday and Friday nights, but I did get to see a big bright streak on Monday. There are other great meteor showers out there, but the Perseids really shine for the enhanced brightness that comes from the great speed of the meteors hitting us. Tonight is probably the last decent night for viewing, so if you have a chance, go out and take a look!
Image credit: Boots Hart, of http://ift.tt/1Mq63pZ.
From Denier on concern about what we don’t know about cometary orbits: “Comet Swift-Tuttle has been documented all the way back to 69 BC. It is in a nice, stable 11:1 orbital resonance with Jupiter. If you know the last time it passed Earth, and the current position of Jupiter then you can deduce the location of Comet Swift-Tuttle. The physics of orbital mechanics is also pretty well ironed out at this point. Astrophysicists are routinely projecting orbital paths of objects in space dozens of orbits into the future.
Given all that, when Comet Swift-Tuttle last came by the Earth in 1992 it was 17 days from where it should have been. With an object moving 133,000 miles per hour, 17 days is a long way off. With the current awesomeness of telescopes, computers, an object 26 miles across with the Perseids serving as a yearly reminder of its existence, how was the Astrophysics community in 1991 so wrong?”
There are two issues here, both of which are very important to understand:
- The outer Solar System, beyond Neptune, where Comet Swift-Tuttle spends most of its time, is very poorly mapped. Gravitational interactions happening out there — even small ones — are completely unaccounted for. A slight tug could change its speed by just micrometers per second, which would change its arrival time by days.
- The comet is named Swift-Tuttle because it was first identified and discovered independently by Swift and Tuttle in the mid-1800s. The observations we had of its position then: 1860s era measurements (peaking in 1862), were the only precise observations we had to base its orbit off of.
Image credit: NASA, of Comet Swift-Tuttle, via http://ift.tt/1N2XD9P.
We’ll do much better the next time around, since we did much better in the 1990s when the comet returned. But if you’re getting at the important point that if we’re off in our estimates by too much, the “cone of uncertainty” could indicate that the comet’s path might include… our planet. Well, the next pass looks like this: “It is now known that the comet will pass 0.153 AU (22,900,000 km; 14,200,000 mi) from Earth on August 5, 2126.”
Every orbit carries an estimated threat of around ~2 x 10^-8, but comets don’t live for hundreds of millions of years before evaporating completely or getting gravitationally perturbed into a hyperbolic (escape) orbit. I wouldn’t worry too much about it, but I wouldn’t worry zero about it, either. If you’re concerned with the long-term fate of life on Earth — over the next many thousands of years — pay attention to this guy, just a little, every 133 years or so.
Image credit: NASA, ESA, and the Hubble SM4 ERO Team.
And finally, from See Noevo on starlight (kind of): “What do think about the light the universe appears to be missing?”
It’s very easy to grab a headline and draw an “OMG!” conclusion from it. Like in this case that light is missing from the Universe! But what does this actually mean? Not the headline, but the science.
What this study talks about is that intergalactic clouds and filaments of gas are ionized in a certain amount that indicates that there’s this presence of ultraviolet (ionizing) radiation that must exist. The problem is, we only know of two sources for this radiation: hot, young, blue stars and quasars/active galaxies. And in the nearby Universe, we don’t have enough of either one of those things to explain the ionization we see.
Image credit: ESA & NASA; Acknowledgement: E. Olszewski (U. Arizona).
The funny thing is, farther away, we do see enough to explain it! So what do I think of it?
I think there’s very likely a mundane explanation out there. Perhaps the last big burst of star formation was sufficient to cause the ionization, which has persisted; perhaps we’re just going through a period of inactive galactic nuclei that’s transient nearby; perhaps the UV photons from young star clusters do escape into the IGM at higher rates than we expect; perhaps there’s something happening at the outskirts of our galaxy absorbing the UV photons that try to re-enter our galaxy from the IGM.
But I don’t think it poses a crisis of any type, save for astronomers that thought their picture of this phenomenon was complete. There’s more to learn about a whole slew of things in the Universe, and that’s a good thing. The process of investigation never ends, and I don’t think we’ll ever run out of interesting questions to ask! Thanks for sharing a great week with me, and know I’m looking forward to the next one.
from ScienceBlogs http://ift.tt/1Mq665i
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