cosmology - Looking for help in understanding how black holes can move

1. I saw Dr. Kip Thorne in a documentary on black holes state that there is no longer any matter in a black hole once it forms. He said something like, the matter was there, but it has been crushed out of existence and basically has been transformed into the energy that produces such a great curvature of spacetime.

Isolated black holes are indeed vacuum solutions to general relativity. Thus in particular, the mass and energy density is identically zero everywhere in spacetime. Because of some technical issues, this does not imply that black holes have no energy or mass; rather, it means that we can't directly think of them as an integral of mass or energy density. See also this question.

3. I also saw Dr. Andrew Hamilton in another documentary state that within the event horizon of a black hole (again paraphrasing), space is falling toward the singularity so fast that it effectively drags everything with it (including light).

For an isolated black hole in a particular frame field, this is a valid picture. For example, the Schwarzschild spacetime in Gullstrand–Painlevé coordinates can be interpreted as Euclidean space falling into the black hole. However, the spacetime is itself is stationary (even static) in the geometrical sense—there is a timelike field representing a direction in which spacetime geometry is left unchanged (this corresponds to Schwarzschild time, in fact). Similarly, rotating black hole spacetimes are stationary.

But generally speaking, you really can't think of black holes as some sort of suck-holes for space. It's just an analogy that applies to simple situations (isolated black holes, with nothing else in spacetime) and then only in a particular frame or coordinates. That's not to say it's useless--e.g., acoustic horizons in fluids are interesting analogies to event horizon--but don't take it too literally. See also this question.

4. Based on 1 (no matter remains in the black hole) and 2 (space is being pulled towards the gravitational singularity from all directions), I try to picture the black hole moving, and it seems to me that if, say, the black hole moves in one direction, I can see how its event horizon is also moving, bringing that bit of space into it.

So what's the problem?

But how can the other end of the black hole let space "escape" in order to complete the motion?

For realistic black holes, there is no "other end". If you really want to tie everything together, then interpret Dr. Thorne's comment about stuff falling into the singularity as crushed out of existence as also applying to space. It doesn't come out of any "other end". It stops existing.

But really, that's past the point where the fluid analogy make sense anyway. I think Dr. Hamilton might note that the full maximally-extended Schwarzschild spacetime is a black hole, so one may be able to think as it "coming out" of the corresponding white hole, but he would also tell you that this white hole is a mathematical artifact that doesn't have anything to do with actual astrophysical black holes.

But I want to understand why it is that space itself can cross the event horizon when other things can't.

Of course they can. In the infalling-fluid analogy, they're carried along by the space that's being sucked in. Perhaps you're thinking of the fact that it takes an infinite Schwarzschild time for an object to reach the event horizon. But that doesn't imply that things can't cross the horizon; rather, it's just a symptom of Schwarzschild coordinate chart not covering the horizon.

Intuitively, think of a coordinate chart as a "grid" drawn on a patch of spacetime. That patch may be the entire spacetime, or it might be just a piece of it. In the case of the usual Schwarzschild coordinates, it's just a piece... one that simply doesn't cover the horizon.

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Regarding the last part, I meant space crossing the event horizon the other way - out of the sphere demarcating the event horizon - whereas material objects go one way - in but not out, spacetime itself does not seem so restricted, at least not in the sense of "pinning" a black hole to one part of itself.

OK, your edited question provides some more context. But the answer is completely the same: what Dr. Hamilton is talking about in the video is the maximal analytic extensions of the rotating black hole solution, which does contain a passage into a completely different region (sometimes called different 'universe'). This is very much analogous to the maximally extended Schwarzschild spacetime, which contains a wormhole that comes back out of a white hole in another spacetime region, except that the extension of the rotating Kerr black hole spacetime contains an infinite chain of such connected regions.

I haven't watched the entire video, but it's clear that's what they're talking about when they say "in a ship propelled by pure mathematics", because the causal structure of such spacetimes is well-known. Once again, I direct your attention to Dr. Hamilton's own page, linked above, that explains that this dual waterfall picture is an artifact of mathematical idealization and not something that actually happens in reality.

However, even if you take the maximal analytic extension of such black hole spacetimes overly seriously, it's important to emphasize that you don't come back out into the same region, but rather a different one connected by a wormhole inside the horizon. If you're up to it, I also recommend Dr. Hamilton's conformal diagrams of said black hole spacetimes, which make it quite clear that's what going on is a whole "chain" of black holes and white holes.

age - Dating very old objects/events

I have wondered how they managed to make these assumptions about various events like when the first black holes form.

When the first black holes formed ( and many other things ) are things we predict from theoretical models of the development of the universe. They are no proven facts, but better classed as speculative predictions based on theories. Different theories give different predictions.

The way it works ( broadly ) is this : We try and develop theoretical models of the universe's creation and early development that will develop into a universe that matches what ours looks like. Doing this has led to such controversial concepts as string theory, which are not proven facts in a scientific sense, but theories subject to ongoing investigation.

We try and verify ( or disprove ) theories by comparing what they predicts with what we see.

So these "histories" should be considered as "best guesses" to some extent.

The approximate date of the Sun's formation, for example, is based on our current best models for the evolution of stellar bodies like the sun, which of course are based on our current best measurements of whatever we can measure in relation to it.

For history related to objects we can examine in more detail, like stuff on Earth, some of which came from space, we can perform detailed analysis and look for patterns in related objects and areas. This makes estimates dating from the start of life on Earth on reasonably good.

So the far distant history beyond that is concerned we're relying primarily on models driven by astronomical observations and the law of physics we regard as "safe".

telescope - What makes small interferometers useful? Like NIRISS on JWST

NIRISS is an instrument on the James Webb Space Telescope. It has a "non-redundant aperture mask" which obviously covers most of the area of the sensor. It seems to be advantageous for high contrast imaging (like finding an exoplanet next to a star) and an alternative to coronagraphs. But however does that work? Why is it good to cover most of a sensor?

I have associated interferometers with creating as large as possible baselines for higher resolution, like the Very Large Baseline Array and the Spectr-R radio space telescope which gives up to a 390,000 km long baseline. So what is the magic with sacrificing sensor area to turn a single small telescope into an interferometer? Aren't all photons welcome? Would such an instrument do as well with a correspondingly smaller main mirror (maybe in separate fragments)?

galaxy - Is there a strong galactic magnetic field?

My main question is: Is there a strong galactic magnetic field, perhaps driven by the supermassive black hole at the center of our galaxy? I am also wondering if this field would be strong enough to make it so that the galaxy rotates in the way it does (with the outer stars moving faster than would be expected), and if this would be an alternate explanation for dark matter.

The thing that led me to ask this question is reading about Jupiter's magnetic field interactions with the plasma emitted by IO. Jupiter's magnetic field forces the plasma to orbit Jupiter about as fast as Jupiter spins, and I am wondering if likewise, the supermassive black hole at the center of our galaxy "herds" the rest of the galaxy in a similar manner as per the article and image below.

http://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter#Role_of_Io

How Much Overlap Will the Andromeda Galaxy and the Milky Way Have When They Collide?

Measurements of Andromeda's blue shift let us conclude that the distance between the Andromeda galaxy and the Milky Way is decreasing and in a few billion years they will "collide".

The blue shift only yields the radial component of Andromeda's velocity vector. It is my understanding that measuring the tangential component is crucial in determining whether a "collision" will actually happen (in a gravitationally bound two body system, for point-like bodies to collide, the relative velocity component must point exactly towards the other body, i.e. the tangential component must vanish).

Now, galaxies are not point-like, so some small nonzero tangential component might lead to a collision where at least some galaxy arms intersect.

Has the tangential velocity been measured? If so, how? How central is the collision (bulge into bulge, bulge into arms, arms into arms)?

molecular genetics - How does the stem-loop cause intrinsic transcription termination?

In this animation, towards the end (about three quarters) the process of transcription termination is shown. It states that the transcribed RNA forms a hairpin loop (or stem-loop), which halts the transcription process.

My question is, why does it halt the transcription process, considering that the RNA polymerase moves in the opposite direction of where the stem loop is created? Does it somehow change the RNA conformation such that no further bases can be added? Interpreting it as a physical obstacle doesn't seem to make any sense here.

redshift - How do we know that light is redshifted/blueshifted and not the original light of a star/galaxy?

If you had a simple slit spectroscope, and looked at an incandescent light, you'd see a smear of light with red on one end and blue on the other. This is because the filament is producing light by glowing from being heated.

If you looked at one of those orange colored sodium vapor street lamps, instead of a smear of color, you'd see a group of lines. This light is produced by ionizing the gas.

The lines represent specific frequencies of light coming from the lamp. You could add a horizontal scale and find that the lines represent a specific frequency in the light spectrum.

If the street lamp was coming at you at a significantly high speed, the individual lines would be shifted in frequency towards blue, but would still have the same pattern. Conversely, if the light was moving away from you, you'd again see the same pattern of light lines, but their frequencies would be shifted toward red.

This is what's being measured when the spectra of stars and galaxies are measured: not just what the color looks like, but whether the spectra of things like hydrogen, helium and iron are shifted in frequency towards red or blue.

So, it's the specific line patterns produced by these elements when they are ionized in stars that helps us identify them. Comparing the light frequencies of locally ionized elements to the frequencies coming from distant stars that tell us if the stars themselves are approaching or receding.

black hole - Is antimatter also attracted by gravitational field?

The consensus opinion of physicists is that antimatter has mass, and it is attracted to other massive objects by gravity in exactly the same way as matter: Antimatter isn't anti-gravity.

Proving this is difficult. It is hard to obtain enough antimatter in one place to observe any gravitational interactions. The best observations aren't even able to conclusively show that antimatter "falls" in a gravitational field. However for theoretical reasons it is considered extremely likely. If antimatter were repulsed by matter, it would allow for violations of the conservation of energy.

A black hole is a region of extreme gravity, and a black-hole would attract matter in just the same way as it attracts antimatter. It would even be possible for antimatter to form a black hole. In fact there are only 11 numbers that define a black hole: mass (made of either matter, antimatter, or energy), position, velocity, spin rate and direction, and electrical charge.

orbit - Best planets profile for a tattoo of the solar system

[Not sure if I should answer this, but I will try to answer something while trying hard to not go off-topic.]

Mercury surface is essentially a collection of small random craters with no discernible pattern at all, so you might not consider which side is presented. The only distinguished feature is a set of dark craters in its north pole.

Venus features few discernible aspects in visible light to the human eyes. There are only a few and faint distinguishable cloud bands, so you might also not consider which side is presented.

Earth is the most important, because it have continents and oceans with distinct designs. It also features a lot of clouds.

Mars has polar caps and a system of canyons. It's northern hemisphere is also much less cratered and has a lower altitude than its southern hemisphere, except that the greatest crater is in the southern hemisphere.

Jupiter has a banded structure of clouds covering the entire planet. It features a large red spot with some nearby fainter and smaller whiter spots.

Saturn also has a banded structure, which is faintier to Jupiter's structure but still clearly visible. It also features a curious hexagon on its north pole. But it is barely noticeable.

Uranus have very homogenous atmosphere (as seen in visible colors by human eyes), so it have almost no visible features to be drawn in your tatoo. Their presented side do not matters, because it is essentially a bland featureless ball.

Neptune has also few visible features. There are no more than a few cloud bands with low variation on color or hue. However there is a dark spot.

Saturn has an extensive system of rings, which also contains some defined gaps.

Jupiter, Uranus and Neptune also feature faint sets of rings (1, 2, 3). Uranus has a weird orientation, so its rings are not aligned with the orbital plane, but roughly perpendicular to it.

All the planets rotates at different velocities, so any side of them would do. No planet is showing always the same face to any other planet or to the Sun.

All of the planets, except Uranus, are rotating roughly in the same plane. So, their north is all pointing to a side and their south to another side (lets call those north/south axis Y). Their north-south axis is perpedicular to their planet-Sun axis (which we will call X). Uranus is special because at their summer/winter, its north/south axis points roughly to the Sun (i.e., in the X axis) and in its autumn/spring it is in a direction that is perpendicular to both X and Y (so it's the Z).

Also, don't forget the Sun. It may also feature flares and sunspots sometimes. The Sun rotates in the same way as most of the planets, with its north/south axis along the Y direction.

You might also be tempted to include the planets moons. Those are:

• Earth's Moon, which always presents the same face to Earth.




• Mars moons Phobos and Deimos.




• Jupiter moons Io, Europa, Ganymede, Callisto, Amalthea and lots of smaller moons.




• Saturn moons Titan, Rhea, Iapetus, Dione, Tethys, Enceladus, Mimas, Hyperion, Phoebe and lots of smaller moons.




• Uranus moons Ariel, Umbriel, Titania, Oberon, Miranda and several smaller moons.




• Neptune moons Triton, Proteus, Nereid and several smaller moons.

You can get more data (and also some images) about them here and also here.

Also, you might want to include the dwarf planets. The confirmed dwarf planets are Ceres, Pluto, Charon, Eris and Makemake. Ceres is located in the main asteroid belt. Although technically being a dwarf planet, for practical purposes Charon is Pluto's main satellite and they always shows the same face one to the other. Makemake and Eris are beyond Pluto.

To complete the Solar Sytem, you could also add other transneptunian objects (likely to also be dwarf planets) like Quaoar, Sedna, Haumea, Orcus, Salacia, 2002 MS₄, 2007 OR₁₀, 2012 VP₁₁₃, 2010 GB₁₇₄, 2004 XR₁₉₀, 2000 CR₁₀₅ and 2004 VN₁₁₂. Those 12 transneptunian suspected dwarf planets, along with Eris and Makemake, are no more than singular dots of light on the telescopes, their appearance is unknown and even their size is known only crudely, with very large error margins in their size estimatives (see more about that below).

If you want also specific asteroids, you might see this page.

• The Sun's diameter is roughly 10 times Jupiter's diameter.


• Jupiter's diameter is almost 11 times Earth's diameter.


• Saturn has almost the size of Jupiter, having 9.5 Earth's diameters. Also, it is visibly oblate (oval).


• Uranus and Neptune are about the same size, with 4 times the Earth's diameter.


• Venus is almost the same size of Earth's (95% of Earth's diameter).


• Mars is a bit larger than a half of Earth's diameter (53%).


• Mercury's diameter is 38% of Earth's diameter.


• Moon has 27% of Earth's diameter.


• Pluto and Eris were determined to be essentially of the same size, which is 18.6% of Earth's diameter.


• Ceres has 14.8% of Earth's diameter.


• Charon has 9.5% Earth's diameter.


• All the others known transneptunian objects, although surely smaller than Pluto, have uncertain sizes.


• All the dwarf planets and asteroids are significantly smaller than the Moon. Check the links for more details.


• Some satellites of Jupiter, Saturn, Uranus and Neptune are pretty large. Titan and Ganymede are even larger than Mercury.

For more details about sizes, check this page.

About the transeptunian objects, only Pluto and Charon have a known appearance (due to being photographed by New Horizons). This way, all that you would need to draw in your tatoo beyond Pluto and Charon is a group of little spheres smaller than Pluto (I named 14 of them above), with only the one representing Eris with the same size as Pluto.

You will find images and further detailed information in the previous links, all of them points to wikipedia. I hope that you can get a perfect tatoo with that information!

cosmology - Why is dark energy preferred to the cosmological constant?

A cosmological constant should be considered a special case of dark energy. The effective stress-energy tensor for a cosmological constant is proportional to the metric $g_{munu}$, so in a local inertial frame will be proportional $mathrm{diag}(-1,+1,+1,+1)$. This is equivalent to perfect fluid with energy density and pressure directly opposite one another, but more importantly, it is the only possible form for the stress-energy that would give the exact same energy density and pressure in all local inertial frames.

If by 'dark energy', we understand it to mean all the contributions to stress-energy in the above form, then there is no reason for this to be constant, and plenty of reasons why it might not be, as this situation is not exceptional in fundamental physics. For example, there could be false vacua with various different energy densities, and they must be invariant across inertial frames.

In particular, the basic idea of inflation considers a flat FRW universe with expansion driven by a scalar field $phi$ at a local extremum of its potential, $V'(phi_0) = 0$, which yields an exponential expansion with constant energy density $T^0{}_0 = V(phi_0)$. More refined models, such as slow-roll inflation, could therefore be directly interpreted as a time-varying dark energy density, while eternal inflation would also include spatial variability. There's plenty of other inflationary models besides.

One the interpretational flip-side, one could always have a $Lambda$ that corresponds to the energy density of the true vacuum, and the rest as separate contributions on top of that. It's just not as useful in a cosmological context compared to grouping all 'dark energies' together, as all stress-energy gravitates equally.

Does Mars contain more iron than the Earth?

Unless 100+ years of studying solar system formation didn't get us anywhere then you'd actually predict that mars would have less iron than earth (in relative terms). This is because the further out in the solar system you go, the lighter the materials that make up the celestial bodies become. You start off with the inner planets for example, which are made of rock and metal. At its most extreme is mercury, which has a metallic core that makes up the vast majority of its volume. As you start to move to the outer parts, there's much less metal. Even further out rocky material starts to peter until you're just left with frozen ices and covalent compounds. This pattern exists because when the solar system formed the heavier materials condensed near the centre while the lighter gases were blown away to the edges. Mars, being the last terrestrial planet, should have formed in an orbit that had less iron and heavy materials than earth and all the other rocky bodies.

However, its good to bet that on average there is more relative iron on mars surface than there is on earth. This would be because despite it's lower abundance of iron overall, it's much more spread out among the lithosphere rather than being entirely concentrated deep in its core like on earth. Mars being smaller planet might not have ever built up enough heat during its formation to completely liquefy its surface, which a planet needs in order for the heavier materials to sink to the centre, making differentiated layers such as the ones we see on earth like the crust, core and mantle etc. Don't get me wrong, mars does have these differentiated layers, but they probably aren't as pronounced as they are here on earth. This is what I think we can safely say is thus the reason for mar's red surface, despite it's location in the solar system.

solar system - Angular momentum in planetary disk formation

Reading about the formation of planetary disks, one of the major problems, it seems like, is the evacuation of angular momentum. Aparently planets can't form with the amount o angular momentum the system has in its early stages. I think I understand where that excess comes from, the collapse of the nebula onto itself and provoking a spin. Then there are many hypothesis on how it's evacuated, which are mostly pretty logical.

Now my question, as a beginner in the study of physics, is this: Why does the angular momentum even need to be evacuated? If the angular momentum is too big, why can't planets still form? Does this have something to do with too much kinetic energy in the system?
Thank you!

galaxy - Is there a strict chronological order that determines the shape of galaxies?

Galaxies evolve from spiral to elliptical. The spirals are formed by patterns of new star formation in the disk surrounding the bulge, which contains mostly old stars.

As galaxies run out of hydrogen gas clouds, which is the raw material from which stars are formed, then no new stars form, and with no new star formation, the complex structures of the arms are lost. Elliptical galaxies are made of old stars and have little new star formations. They were formed when spiral galaxies merged, causing a burst of star formation, and leaving a bulge of old stars and little gas to make new ones. The ultimate fate of the milky way and the andromeda galaxy is to merge and form a giant elliptical galaxy.

What is the direction of a comet's dust tail before and after perihelion?

Comet tail always aims away from the sun, as it's torn from the comet's gas cloud by solar wind. It trails slightly behind, as the comet moves along its orbit while the gas travels directly away, but that's relatively minor - the speed at which the gas and particles are pulled away forming the tail is much higher than the comet's orbital velocity.

Your visualisation shows it trailing behind the comet as if it was moving in some gaseous/liquid medium that stops the lighter particles while letting the heavy comet head travel ahead. This is not the case - as the comet is far away from the sun, the gas cloud just forms its atmosphere and travels with the comet. As it approaches though, the intensity of solar wind increases and it pulls away the atmosphere which can't be protected against it by magnetic field or strong gravity, as the comet has neither.

the sun - Could our Sun be a companion star of a massive black hole?

There is no indication nor any astrophysical reason for such a scenario. The most relevant constraints are

1. The Solar velocity is typical for stars in our immediate Galactic neighbourhood.


2. Soft binaries (those with orbital velocity smaller than the local velocity dispersion $sigma$) dissolve (Heggie's law: soft binaries become softer and hard binaries harder).

These are in contradiction with a Solar binary nature: a viable binary has a large orbital velocity that would put the Sun outside of the typical velocity for local stars. This contradiction can only be temporarily avoided if the current (but not in $sim1000$ years) orbital velocity and the velocity of the massive binary companion add up to a typical stellar velocity. This is an unlikely chance.

Binaries with a stellar-mass black hole always form from an ordinary stellar binary, with one of the stars going supernova. Often, such binaries are quite compact. All this is ruled out for the Sun.

Finally, black holes of intermediate masses (100 to $10^5$ M$_odot$) haven't (yet?) been detected unambigously (though there are several objects that have been claimed to be such intermediate-mass black holes also IMBHs).

cosmology - How could we tell if the Universe is infinite?

We don't know for sure, but it certainly fits into our theories. There is, of course, no way to actually test if the Universe is infinite, but right now we think it is likely.

Also, if you read my updated answer on your other post, the Universe has always been infinite in size. I explain over there how it actually works: space is created in between everything, and thus one could say the Universe is expanding.

These objects can actually drift away from each other faster than the speed of light. That is, light from them eventually won't make it to us, since they'll be drifting away too quickly.

Now, this doesn't actually go against Einstein's theory that the speed of light is the fastest thing in the Universe. Einstein said that nothing can travel through space faster than light — but here, space itself is actually being created between the objects. Distances are increasing because space itself is dilating, and thus we can drift apart from other objects faster than light.

Really, there is no limit (as far as I know) to how fast we can drift away. Farther objects will keep drifting faster and faster away, since our gravity has a much weaker effect on them.

bioinformatics - Difference between strand-specific and not strand-specific RNA-seq data

I would like to ask the difference between strand-specific and not strand-specific dataset.

As far as I know, strand-specific data means that we know which strand the transcript is from.

I do not have biological background. Please confirm whether it is correct. If we have a transcript, which is from sense strand, when RNA-seq is producing reads, is that first the cDNA is synthesised. Then this cDNA is used for PCR to amplify the sample? Then the reads generated could be from both strands of the original DNA?

For strand-specific protocols, what is different?

========================================================

Follow up.

Please correct me if I am wrong. There are multiple protocols to produce strand-specific RNA-seq libraries. The basic process is like:

1. Get the RNA;


2. Get its cDNA;


3. Somehow mark the cDNA as sense or antisense when amplifying (PCR?) (here comes the differences between different protocols);


4. Then REMOVE all antisense (or sense) cDNAs;


5. Read the reads from clean cDNA library.

The result is that, the reads from this RNA can be used to assemble the sense cDNA. And for not strand-specific libraries, using the reads would be able to assemble both the anti-sense and sense cDNA.

Am i right about this problem? Thanks.

Star like light moving in the sky, what could it be?

Most likely a satellite. They look exactly like stars, but they glide across the sky smoothly. Airplanes may have multiple light sources, some blinking lights, and you can definitely perceive how low it seems to be: An airplane somewhat seems to come from the horizon and disappear the same way, while a satellite "seems to always be at the same distance from you", like gliding on an imaginary half-sphere or dome of a night sky covering you. Last summer I went for a night bike ride, just riding a bike, sometimes sitting down to have a snack and look around. The trip lasted for 6 hours and during this time, I spotted 12 satellites just by observing. They're a very common thing. This time I got very lucky though, because one of the 12 satellites unexpectedly was something called an Iridium flare and I had never seen one of those in real life before. There's a lot of man-made space junk orbiting the Earth, some satellites still operated, some not. There's a bunch of satellites known as Iridium, which happen to have a design that includes large, reflective solar panels. I live in the North so in the summer, the sun only barely goes below the horizon and the summer nights are short. When I'm just in the dark side but an Iridium satellite happens to fly approximately above me, if everything is in a specific angle, the sun from behind the horizon hits the satellite's solar panels, and like a mirror, reflects those rays to the dark side, into a viewer's eyes. It was my first time seeing that and I must say, it's extremely impressive when you're merely looking at a "moving star", then unexpectedly it grows very bright, brighter than any star in the night sky, and finally reverts back to the normal looking gliding satellite. Because people are very much aware of all of the space junk that's up there, there are plenty of websites and mobile apps that you can use to see where the most common satellites are at the moment, and, because we know the route of each satellite, as well as the year and location specific data regarding the Earth and the Sun, Iridium flares can also be predicted, like you would predict things like Solar eclipses.

supermassive black hole - Mass distribution in the early universe

It is unfortunate that the usual poor journalism labels the growth of the black hole as "inexplicable" and then further down in the article refers to some possible explanations.

The basic problem is a growth timescale one. Radiation pressure introduces a negative feedback, such that there is a "theoretical" maximum for spherical accretion called the Eddington limit, which occurs when the quasar is radiating at its Eddington luminosity. The shortest growth timescale is achieved if the efficiency with which mass is converted to luminosity is low; but if it's too low we wouldn't see the quasar at all. This is the crux of the problem. You can look at this Physics SE answer for some more of the details.

The thing is there are ways and means by which this limit can be exceeded - non spherical accretion for one - so there are lots of ideas about how this can be achieved. Another possibility is that you start off with a seed black hole that is pretty big to begin with, perhaps as a result of a merger. Or the quasar could have been less efficient in the past and is more efficient now, which is why we can see it.

Is there enough matter? Well, yes, galaxies have masses that can be much bigger than the mass of this black hole. They are rare, but of course so are > 1 billion solar-mass black holes, and these tend to be the only ones that we can see at distances of >10 billion light years.

One way of assessing the feasibility would be just to ask what a freefall timescale would be. If you have say $10^{11} M_{odot}$ in a sphere of radius 10 kpc (I am just using typical sorts of numbers for a big galaxy), then the average density is $5times 10^{-22} kg/m^{3}$ and has a freefall time $sim (Grho)^{-1/2}$ of 200 million years. Of course there are other problems, like shedding angular momentum, but it looks like this timescale is short enough for gravity to do its thing (in the absence of radiation pressure).

Of course the short answer to your question is that yes, there must be enough time, because this is just the latest in a population of such objects. We know that quasars with supermassive black holes have formed within a billion years after the big bang.

python - Plot an AltAz grid over a square grid of RADec points

first post here. As I'm new, StackExchange won't let create or use the wcsaxes tag. wcsaxes looks like the most appropriate tool for the job, but astropy is closely related.

I think the title says it all, but I'll give a little more detail. I have a bunch of sources in (RA, Dec) and want to plot them in the simplest possible projection (i.e. square, but if this is not possible we can make allowances). I want to see the geometry of the Earth over my region of interest, mostly to identify the Earth's magnetic field lines.

The following code gets me close, but I get this error:

AttributeError: 'NoneType' object has no attribute 'to_geodetic'

If I change "altaz" to "galactic", I get a Galactic coordinates grid over the points, which is what I want, but obviously in the wrong coordinate frame.

#!/usr/bin/env python2

import numpy as np
from astropy.wcs import WCS
from astropy.time import Time
import matplotlib.pyplot as plt


# time = Time(2606629, format="jd", location=("116.670810d", "-26.756528d")).iso
w = WCS(naxis=2)
w.wcs.ctype = ["RA---MER", "DEC--MER"]
# w.wcs.dateavg = time

ra_min = 0
ra_max = 15
dec_min = -45
dec_max = -15
N = 1000

sim_ra  = np.random.uniform(ra_min, ra_max, size=N)
sim_dec = np.random.uniform(dec_min, dec_max, size=N)

fig = plt.figure()
ax = fig.add_axes([0.1, 0.1, 0.9, 0.9], projection=w)
overlay = ax.get_coords_overlay('altaz')

overlay[0].set_ticks(color='white')
overlay[1].set_ticks(color='white')
overlay[0].set_axislabel('Longitude')
overlay[1].set_axislabel('Latitude')
overlay.grid(color='black', linestyle='solid', alpha=0.5)

plt.scatter(sim_ra, sim_dec)
plt.xlabel('RA')
plt.ylabel('Dec')
plt.show()

I played a little with trying to get the observation time into the WCS header (note that the actual time is artificial, but should work regardless), without success. Any ideas?

human anatomy - Does red light preserve your night vision?

This is a very good question. Red light is routinely used by scientific laboratories to do low light dissections of retinas, and of course it is used in other low light contexts such as printing plate development.

In both of the above contexts, you have a clear subject: the retina being dissected or the printing plate being developed. In the case of the printing plate the film has been designed to be specifically non-reactive to red light, so red light is used because your eyes can see it, but the film doesn't react to it. Similarly in some scientific settings it makes sense to use red light during dissections. Mice lack a long wavelength opsin, and therefore using a dim red light allows the experimenter to have a relative sight advantage compared to the mouse when keeping the mouse dark adapted.

But in the case you're asking about, there is no film or animal to serve as a second party. So is there any intrinsic advantage to using red light? As it turns out, there is. The fovea, which is in the center of our eye and used for high acuity vision, has no rods and primarily L- or red sensitive cones. Note the high density center area which lacks blue sensitive cones and has 2:1 red to green cones.

So by having red light present, you stimulate this area. But red light is present in white light, too, why not just use that? Leonardo's answer comes the closest, but it's a little off. Red light is used because it preferentially stimulates L cones more than rods, but you are definitely not able to preserve night vision by using red light. Why not? Well it may look like it is possible to exclusively stimulate cones from the chromatic sensitivity figure

But that figure is 1) normalized and 2) not indicative of synaptic signal processing. 1000's of rods can converge onto a single ganglion cell, where cone convergence in the fovea can be on the order of a single cone per ganglion cell. When it comes to perception, in order to compare the black rod line above with the red L-cone line you'd have to magnify it dramatically in size. Practically speaking, it is nearly impossible to stimulate cone pathways without stimulating rod pathways when using a relatively broad spectrum LED that you're powering with a battery. Maybe with a high power infrared laser.

So the purpose of using red light is to attempt to balance the activation of high sensitivity (red insensitive) rods with that of the low sensitivity (but red sensitive) cones in the fovea. While using a similar level of rod activation with blue light, you would perceive a "blind spot" where your fovea is.

Finally, instrinsically photosensitive cells (the melanopsin cells brought up) do not factor into this processing. These cells are activated only with extraordinarily bright levels of light, and the therefore do not enter into conversations dealing with night vision.

human biology - Mechanism of syndesmophyte growth in AS

Ankylosing Spondylitis (AS) causes inflammation around joints and the growth of syndesmophytes that may eventually fuse vertebrae. I'm familiar with the genetics (HLA-B27, IL1A) related to the condition, but I can't find any information about the mechanism that causes the actual growths to occur.

My current assumption is that AS causes the over-production or under-production of a particular compound or enzyme at the growth site, but I can't find any studies or papers that explain this. Is the mechanism known? Is it directly related to abnormal levels of a particular substance?

What are the differences between matter, dark matter and antimatter?

Matter is the stuff you are made of.

Antimatter is the same as matter in every way, looks the same, behaves the same, except its particles have electrical charges opposite to matter. E.g., our electrons are negatively charged, whereas a positron (an antimatter "electron") is positively charged. The positron is the "anti-particle" of the electron.

When a particle meets its anti-particle, they "annihilate": the two particles disappear, and gamma photons are released carrying off their energy. For this reason, should a lump of matter touch a lump of antimatter, they would annihilate, and a giant explosion would result because of the huge energy released (E=mc^2).

Matter and antimatter are definitely related: same thing, but with opposite signs. Twins, but opposites.

It is not clear why, but it seems like there isn't that much antimatter out there, more like trace amounts. Definitely not as much as regular matter as far as we can tell. This is puzzling to physicists and cosmologists, because you'd expect the Big Bang to make roughly equal amounts of matter and antimatter. Scientists agree that the paradox of "excess matter" will advance physics even further once it's solved.

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Dark matter - we don't really know what it is. It's not even sure it's "matter" in a conventional sense, or related to it in any way. We just know that galaxies are rotating in such a way that indicates there's a lot more mass out there, but it is mass that we cannot see and cannot be accounted for in the usual ways. Hence the name "dark" (as in invisible) matter.

Dark matter doesn't seem to interact much with regular matter, except gravitationally. Right now dark matter could be passing through you and you wouldn't notice. Dark matter also does not interact with light, so you can't see it. It doesn't seem to interact much with itself either, so for this reason dark matter cannot form "clumps" such as planets or stars. Instead, it probably exists in a diffuse form. Bottom line, dark matter interacts pretty much only via gravity.

The shape of galaxies is a proof of the existence of dark matter, and is a result of the interaction between matter and dark matter. Without dark matter, galaxies would be much less massive, and the outer parts would rotate much more slowly compared to the center. Due to dark matter, galaxies are quite massive, and they rotate almost as solid objects - the outer parts rotate approximately as fast as the central parts.

Estimates vary, but it seems like there's something like 5x to 6x more dark matter out there compared to regular matter.

proteins - What are the most important differences between HSP70 and HSP90?

Often cells have multiple types of the same protein — this redundancy can have different effects for different requirements such as having proteins function under different physiological conditions, or providing specificity to a certain class of ligand proteins or so on.

But here, it seems like the two have some synergistic interaction, a tag team if you will.

Wegele H, Müller L, Buchner J. 2004. Hsp70 and Hsp90 — a relay team for protein folding. Reviews of physiology, biochemistry and pharmacology 151: 1–44.

Unfortunately this article's full version can only be accessed if you're at a university or somewhere that has a subscription to some of the large research databases, but the abstract is free and it may provide more clarification.