GLOWING GOBBLER: An artist's conception of a quasar ionizing the hydrogen gas surrounding it.
Image: Gemini Observatory
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The Best Science Writing Online 2012
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Peering far across space and time, astronomers have located a luminous beacon aglow when the universe was still in its infancy. That beacon, a bright astrophysical object known as a quasar, shines with the luminosity of 63 trillion suns as gas falling into a supermassive black holes compresses, heats up and radiates brightly. It is farther from Earth than any other known quasar—so distant that its light, emitted 13 billion years ago, is only now reaching Earth. Because of its extreme luminosity and record-setting distance, the quasar offers a unique opportunity to study the conditions of the universe as it underwent an important transition early in cosmic history.
By the time the universe was one billion years old, the once-neutral hydrogen gas atoms in between galaxies had been almost completely stripped of their electrons (ionized) by the glow of the first massive stars. But the full timeline of that process, known as re-ionization because it separated protons and electrons, as they had been in the first 380,000 years post–big bang, is somewhat uncertain. Quasars, with their tremendous intrinsic brightness, should make for excellent markers of the re-ionization process, acting as flashlights to illuminate the intergalactic medium. But quasar hunters working with optical telescopes had only been able to see back as far as 870 million years after the big bang, when the intergalactic medium's transition from neutral to ionized was almost complete. (The universe is now 13.75 billion years old.) Beyond that point, a quasar's light has been so stretched, or redshifted, by cosmic expansion that it no longer falls in the visible portion of the electromagnetic spectrum but rather in the longer-wavelength infrared.
Daniel Mortlock, an astrophysicist at Imperial College London, and his colleagues used that fact to their advantage. The researchers looked for objects that showed up in a large-area infrared sky survey but not in a visible-light survey covering the same area of sky, essentially isolating the high-redshift objects. They could thus discover a quasar, known as ULAS J1120+0641, at redshift 7.085, corresponding to a time just 770 million years after the big bang. That places the newfound quasar about 100 million years earlier in cosmic history than the previous record holder, which was at redshift 6.44. Mortlock and his colleagues report their finding in the June 30 issue of Nature. (Scientific American is part of Nature Publishing Group.)
The ancient quasar was spotted in the Infrared Deep Sky Survey at the U.K. Infrared Telescope, or UKIDSS, an ongoing seven-year project. The light from ULAS J1120+0641 shows a much greater imprint from neutral intergalactic hydrogen than its nearer, lower-redshift counterparts. "What this object tells us is that at least in front of this quasar, along this line of sight, back at that epoch the universe was about 10 percent, and maybe 50 percent, neutral hydrogen," Mortlock says. With more observations of ULAS J1120+0641, and perhaps the future discovery of more quasars at a comparable distance, astronomers and cosmologists will be better able to uncover the re-ionization history of the universe. "One of the reasons to look at these time-machine objects is to find out what was happening at that time," he says.
To glow so brightly at that early epoch in cosmic history, the newfound quasar would have to be powered by a black hole roughly two billion times as massive as the sun, or 500 times the mass of the black hole at the center of our galaxy. But such heft requires an explanation. "The quasar itself is a remarkable object in that no one really knows how to form a black hole that massive, two billion solar masses, in what in cosmological terms is a relatively short time," Mortlock says. In other words, the astrophysicists have found the cosmic equivalent of a newborn baby with the stature of a full-grown adult. "It's essentially the hardest object to make in the early universe that we know about," Mortlock adds. The gargantuan black hole's existence, discovered through exhaustive telescopic observations, now becomes a challenge for theorists to address. "Assuming that the universe makes sense," Mortlock says, "it has to form somehow."
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34 Comments
Add CommentWhy is this considered such a difficult object to have been created? During the earlier stages of the universe, enormous energy densities and masses were commonplace, and given the fragmentation of the universe over the course of the first few million years, I'd be surprised if there *weren't* many enormous black holes created while space was beginning its expansion.
Reply | Report Abuse | Link to thisIn fact, I would think the only time one could ever cook up a black hole of such magnitude would have to be during the early stages of the universe when mass and energy in those quantities was in such close proximity as to allow this.
It is entirely possible that in its earliest, nascent form, the physical forces were unified to one degree or another, and potentially posed less resistance to a large gathering of mass in one place. Which, when it occurred, would bend space into a black hole.
So I will repeat my question - why is this so unexpected?
It might help to refer to an artist's conceptual overview of the current consensus of universal development:
Reply | Report Abuse | Link to thishttp://upload.wikimedia.org/wikipedia/commons/6/6f/CMB_Timeline300_no_WMAP.jpg
It's included in:
http://en.wikipedia.org/wiki/Big_Bang
I had a similar initial reaction to the early black hole mystery...
As near as I can understand, it's thought that, by the time the first stars developed at about 400 million years after the big bang that the forces had all differentiated.
However, that left only the time from 400 million years to 770 million years for the enormous black hole thought to be powering the observed quasar to develop. Moreover, while the density of matter (mostly neutral hydrogen?) was much greater than it is now, I don't think there were as many stars as there are now and certainly weren't all the heavy elements that exist today.
I'm guessing that Mortlock is basically indicating that our current ideas about the development of supermasive black holes just don't seem to directly apply to the prevalent conditions 770 million years after the big bang...
How did we manage to get 13.75 billion light years from this object since the beginning if the universe since we do not travel at anywhere near lightspeed?
Reply | Report Abuse | Link to thisThis is the earliest object "observed" in the universe just 750,000 years after the Big Bang, and what does this mean for the continuing discoveries of the universe? For the light to reach us it took over 13 billion years, human years, to reach us, does this mean we are slowly reaching the point where we can witness the time span within the 750,000 years after the birth of the universe; maybe the Big Bang itself? AS Charles Darwin wrote the Evolution of a Species, so shall we write the Evolution of a Universe. Although, the question has to be raised on the possibility of seeing what existed prior to the Big Bang, since we do want to believe that the world is flat and if we travel far enough we would fall of the edge; or see the backs of our heads. We are the center of the universe and this is all that exist. Fortunately, the moment we see the actual event of the Big Bang, we will never know our true place in the fabric of life. Is time, matter and space infinite or is it finite? Only time will tell.
Reply | Report Abuse | Link to thisBlack hole growth is self-limiting -- these objects are as likely to blast stuff out as they are to swallow it up. We have an article in an upcoming issue of the magazine which will explore this in greater depth and suggest hypotheses for how these holes could have bulked up so fast.
Reply | Report Abuse | Link to thisI have no education or experience in physics, so these opinions do not necessarily represent anyone else. There may be many other, perhaps better, explanations based directly on general relativity.
Reply | Report Abuse | Link to thisIMO, we don't really know how far away we are from any distant object - we only estimate (based on cosmological models derived from the detected light's redshift) that the light traversed expanding spacetime for about 13 billion years.
While it is thought that spacetime is now "flat", I think that it was very likely highly curved in the much denser earlier universe. As a result, we could now be receiving light that not initially directed towards our relative location at the moment of emission; it could have traversed a highly curved path through expanding spacetime to reach us.
IMO, all we can determine is that we can detect photons from the ancient light emissions of specific very distant objects when we aim our telescopes in very specific directions in the sky, and that the detected photons indicate that they traversed a specific vast distance in expanding spacetime.
I do not think that we can reliably presume that distant light has reached us by a straight path through linearly expanding universal spacetime - only that distant light has reached us by traversing a temporally similar path through temporally varying expanding spacetime.
That brings up an interesting point. As I understand, the current view is that the relativistic jets of active galactic nuclei represent an overflow condition produced when a black hole ingests more matter than it can process.
Reply | Report Abuse | Link to thisHowever, I understand that some experts do not think that black holes contain any matter (sorry - no reference). In that case, the jets may represent the decomposition and expulsion of any and all ingested matter, with only the gravitational energy produced by the extracted particle mass-energy being locally retained, directed to the singularity.
That scenario very neatly avoids the problem of where to hide all of the presumedly retained dimensional material within the dimensionless singularity.
In that case the AGN jets are not regulating devices...
I don't have the resources, but I'd certainly be interested if that future article could research and address this alternative view, thanks!
These are the seeds of black holes from earlier universe before big bounce as per cyclic theory of balloon inside balloon theory of matter and antimatter universe on opposite entropy path producing five god particles--four for four forces and one for mass creation by annihilation of matter and antimatter at common boundary and injected into our universe as dark matter and energy to give our laws and gravity --but when one universe approaches tends to zero entropy,then the re bounce occur to form again two universes and the black holes survive evaporation may now act as seeds of galaxy formation etc and it is obvious that these early objects are one of those very heavy seeds. THE PAPERS WERE PUBLISHED IN ASTRONOMY.NET AS--MISJUDGEMENTS BY NEWTON --AND--BALLOON INSIDE BALLOON THEORY OF MATTER AND ANTIMATTER UNIVERSE ON OPPOSITE ENTROPY PATH . THE PAPERS WERE PUBLISHED IN YEAR 2002 AND ONE COPY SENT TO DR.ROGER PENROSE.
Reply | Report Abuse | Link to thisI don't think anybody presumes that light reaching us from distant object travels in "straight lines", gravitational lensing experiments have pretty much confirmed that light passing near gravitationally large objects bends enough that we can observe it, and since all mass warps spacetime, their is no such thing as a straight euclidian line for light to travel along. Of course, the line it does follow is the shortest length possible, once you take into account this warpage.
Reply | Report Abuse | Link to thisAlso, distances to distant galaxies are not so much calculated by the redshift as they are by using standard candles (type 1a supernova) and the amount the light has dimmed as compared to how bright the supernova would be if viewed up close. Redshift tells us how fast and in which direction an object is moving in relation to us. However, it is true that their is a relationship between distance and redshift, farther objects are more redshifted than nearer ones.
The jets streaming out of black holes are not due to the black hole overflowing, or swallowing more than it can "process" as there is no "processing" that takes place, just as there is no processing taking place when a baseball falls downwards, other than its potential energy falls as it's kinetic energy increases. It is all just gravity. The jets shooting out of black holes are due to Hawking radiation, which is due to quantum effects taking place just above the horizon of the black hole. Interestingly, the larger the black hole, the less radiation it emits. As black holes shrink away through this radiative process, the rate of outflow increases more and more. The smallest black holes possible would have lifetimes of fractions of a second.
Your reasoning is sound, and is what I used to believe until somebody explained it to me differently. It is not that this distant object and ourselves are moving apart at extremely high speeds, rather, the separation between us is due to more space being created in between us. Dark energy causes "new" space to be created all the time. Just as a hypothetical example, lets pretend one inch is created every mile every year (this is a huge exaggeration from what is real but I don't know the actual figures). Then, for every x miles, we create x new inches. Now the space between us is greater, so the next year their will be x new inches created, plus whatever is created by the new inches added the year before. Repeat this over billions of miles over billions of years, and these extra inches per mile created every year add up to millions, or billions, or trillions of extra miles every year, to the point where the distance between us and far off objects approaches the speed of light. In fact, astronomers estimate that the known universe is over 40 billion light years across. Thus, there are actually parts of the universe moving apart from each other faster than the speed of light.
Reply | Report Abuse | Link to thisI know this sounds like it violates the speed of light speed limit, but it is not the objects themselves moving faster than the speed of light, it is that their is space being created between them that increases the separation between them at such a high rate. Thus, objects on opposite sides of the universe can never communicate with each other.
In fact, ever since inflation began after the big bang, these parts of the cosmos have been beyond each others reach - this is what caused the quantum variations in the initial ball of energy that was the big bang to not even out by communicating and evening out the energy densities in the early universe through radiation, but instead allowed these small differences to become slowly magnified by gravity and result in the galaxies, stars, and planets we have today.
Sorry, I am mistaken, when you talked about the super relativistic jets, I thought you were talking about Hawking radiation. However, black holes do not "overflow", and can swallow as much matter as can be directed towards them. The high velocities achieved by matter falling or streaming towards a black hole can cause gases to heat up to extreme temperatures, and various processes may cause some matter to be ejected away from the black hole, but all this occurs outside of the horizon of the black hole, it is not processing being done by the black hole, but rather the interaction between atoms in clouds of gases surrounding and falling towards the hole, with huge amounts of kinetic energy being added by the huge gravitational pull of the hole. Once something falls beyond the horizon, there is no processing, at least not that we know of, and their is no overflow. The only thing leaving a black hole (it is believed) is Hawking radiation.
Reply | Report Abuse | Link to thisI think black hole formation may have been kept done to an extent as a result of the inflationary expansion of the early universe.
Reply | Report Abuse | Link to thisThanks, but as I understand the distances to most distant galaxies can only be derived from cosmological models applied to redshift, with type Ia SNe observations being used to calibrate the models. As I understand, there have been little more than 100 type Ia SNe galaxies surveyed: they are relatively rare events for a given galaxy and must be carefully monitored to determine their period of peak emission luminosity, crucial for use in estimating distance.
Reply | Report Abuse | Link to thisPlease see: http://en.wikipedia.org/wiki/Cosmic_distance_ladder
which concludes:
"Hubble's Law is the primary means we have for estimating the distances of quasars and distant galaxies in which individual distance indicators cannot be seen."
http://en.wikipedia.org/wiki/Relativistic_jets seems to contradict your attribution of the jets to Hawking radiation:
"The mechanics behind both the creation of the jets and the composition of the jets are still a matter of much debate in the scientific community; it is hypothesized that the jets are composed of an electrically neutral mixture of electrons, positrons, and protons in some proportion."
However, one of the competing theories attributing the jet energy to black hole rotation is the unrelated Penrose mechanism... Wiki's entry includes a number of freely available research report references.
I coined the term 'overflow' to characterize a process I think gmusser was referring to: "Black hole growth is self-limiting -- these objects are as likely to blast stuff out as they are to swallow it up."
Reply | Report Abuse | Link to thisPerhaps I'm misinterpreting, but I think he was referring to "self-limiting" growth as an ingestion overflow process.
Also, please see my preceding comment.
Your statement:
Reply | Report Abuse | Link to this"It is not that this distant object and ourselves are moving apart at extremely high speeds, rather, the separation between us is due to more space being created in between us."
- is a very good description of the effects of expansion (its a travesty that even physicists refer to expansion as 'distant objects receding away from us' at increasing velocity).
However, your next statement:
"Dark energy causes "new" space to be created all the time."
- goes a step too far. Any expansion of spacetime, even prior to the proposed acceleration of expansion that is thought to have begun about 5 billion years ago, "causes "new" space to be created all the time."
The proposed dark energy is merely a proxy for some unknown cause of the relatively recent apparent acceleration of expansion. It is required to somehow reverse the previously decelerating rate of expansion over the prior some 8 billion years.
IMO, the recent acceleration of expansion might be the result of continued expansion of intervening spacetime as gravitation has continuously localized mass, increasing its 'clumpiness'. As bubbles of voided spacetime increasingly dominate the large scale structure of the universe, the rate of expansion may have begun to increase as it is increasingly less restricted by localized gravitation. In this case, expansion may accelerate with no 'dark energy' required.
I fail to see how "clumpiness" would alter the characteristics of gravity. I have read many of you comments over the previous months and have picked up many hints and references to this idea you have that the clumping together of matter somehow changes the overall character of gravitational properties. I am not sure what to make of this as I have never heard you explain this fully. From what I have learnt, the gravitational effects of a collection of mass does not change depending on how it is configured. Clumpy or not, it can be dealt with by treating it as if all the mass where centered at an idealized point (the center of mass).
Reply | Report Abuse | Link to thisSecondly, dark energy does not only begin when expansion accelerates, it is there all along. Early on, gravitational effects keep expansion in check, but as dark energy is a scalar field that is the same at every point in space, as more space is created, more dark energy is present, negative pressure builds, and expansion begins to inflate as it overcomes the opposing pull of gravity. It reverses the previous deceleration because of this additive effect - the more space is created, the more the energy driving the creation of more space there is.
Saying dark energy is a proxy for an "unknown force" is sort of redundant as nobody claims to knows what it is..
I did not understand what he meant when he said that black hole growth is self limiting. I have never heard this before. I have also never heard of a black hole exploding either, nor can I think of a possible mechanism for this. Then again, I am neither a particle physicist nor cosmologist by trade, merely by recreation. Can you explain what is meant by these ideas?
Reply | Report Abuse | Link to thisI can't reverse your education (I'm not qualified, anyway), but consider that if gravitation "can [universally] be dealt with by treating it as if all the mass where centered at an idealized point (the center of mass)" then how can the planar discs of spiral galaxies exist? Why aren't we all standing (or sitting, etc.) on the surface of the Sun's core right now?
Reply | Report Abuse | Link to thisThe effects of gravitation are proportionate to mass and diminish with distance. Some collective effects can be successfully approximated by treating distributions of mass as if it were contained within the center of mass, but this is never more than an approximation and is subject to various inescapable error. Right now I can only suggest that you read some of the research reports that I've repeatedly provided links to in the past.
You've described specific effects of dark energy as a scalar field, then told me that no on claims to know what it is. The only evidence for the existence of any dark energy is the conclusion that universal expansion began accelerating based on the discrepancies between galactic distance estimates derived from type Ia SNe peak emission luminosities and those derived from cosmological models applied to the redshift of galactic light. Those observations seem to indicate that 'negative deceleration' of spacetime expansion began around 5 billion years ago. I simply suggested a potential alternative explanation that does not require any new force or type of energy. Apply Occam's razor here...
I'm not sure what gmusser is referring to - he said an upcoming article will address it. I've already generally described my guess.
gmusser stated:
Reply | Report Abuse | Link to this"Black hole growth is self-limiting -- these objects are as likely to blast stuff out as they are to swallow it up."
I think by "blast stuff out" he meant that matter is expelled in relativistic polar jets, not that the BH explodes. Hang in there...
Its a known fact that massive stars tend to be brighter and have shorter life spans as the nuclear furnace will fuse all of its hydrogen extremly quick. So wouldn't it makes sense that Ultra massive stars (billions of solar masses) would have formed in the early universe with all of that hydrogen in close proximity. As Steven Hawkins has predicted; nothing in the universe is perfect. The slight irregulaties in hydrogen density created after the big bang allowed gravity to pull massive quantities of hydrogen together and form the first stars. It just makes sense that ultra massive stars would form. These stars would use up its hydrogen fuel extremly fast and eventually collapse under its own gravity to form supermassive black holes. These early supermassive black holes would no doubt begin pulling all the gas towards it thus creating early galaxies.
Reply | Report Abuse | Link to thisYou could be right - it would seem that the increased density of matter most likely played a role in the early formation of supermasive black holes and galaxies. There are many theories of supermassive black hole formation, none of which have become accepted - many of which do not involve any stellar collapse.
Reply | Report Abuse | Link to thisThe maximum size a star can attain is likely determined by the inception of the fusion reaction: once that occurs the outward pressure of its stellar wind would seemingly prevent any further accretion.
There now seems to be a great disparity between the maximum attainable stellar mass and black holes produced by stellar collapse and supermassive black holes. http://en.wikipedia.org/wiki/Supermassive_black_holes#Formation
states:
"There are stellar-mass black holes, generated from collapsing stars, which range up to perhaps 33 solar masses. The minimal supermassive black hole is in the range of a hundred thousand solar masses. Between these regimes there appears to be a dearth of intermediate-mass black holes. Such a gap would suggest qualitatively different formation processes."
I understand there was once some conjecture about whether supermassive black holes were necessary for the formation of galaxies or whether galaxies were necessary for the formation of supermassive black holes. I don't know if that question has been satisfactorily resolved or not...
When such a massive black hole is formed already 1 billion years after the "how shall we call it" this means that massive stars have to be formed in this only 1 billion years (minus 400.000 years) in order to collapse to this kind of black hole, could this mean prehaps that these massive black holes from so long ago are the residues of a huge explosion of another even more huge black hole (holographic theory : all information is on the exteriour of the black hole and we are IN it), so holographic bubble in bubble in bubble in bubble in bubble...
Reply | Report Abuse | Link to thisit is just a thought...
keep on bubbling
Wilhelmus
Hmm interesting way to think about the relationship between the origins of super massive black holes and galaxies. Which one was first?
Reply | Report Abuse | Link to thisIt could be that since space was expanding at a slower rate in the early universe, the distances between the hypothetical ultramassive stars would be small enough for large clusters of them to collide into each other from strong gravitational attraction. This would create an extremly active region of space that might provide the makings for supermassive blackholes. It might be a process similar to white dwarfs colliding into supernovea.
There is at least on hypothesis in which extremely dense hydrogen collapses to directly form a supermassive BH. Perhaps galaxies were produced from an external accretion disc during that process - somewhat analogous to star and protoplanetary disc formation. Or, perhaps some other process is responsible...
Reply | Report Abuse | Link to thisAs I understand, in the currently accelerating universe scenario, spacetime expansion was decelerating until about 5 billion years ago when it first began accelerating. I don't think it's been proposed that the current expansion rate exceeds that of the early universe: only that it exceeds the prevailing rate of expansion around 5 billion years ago. Since the initial rate of expansion was decelerating for about 8 billion years before it began accelerating, the initial rate of expansion quite likely exceeded the current rate, depending on the rates of deceleration and subsequent acceleration. I don't think the current distance sampling of type Ia supernovae peak emission luminosity provides sufficient resolution for such precise determinations.
Of course not all situations can be dealt with by using the center of mass of an object or collections of objects, but for the orbit of the earth around the sun it sure can. I think you are totally confused about what I mean when you use the center of mass to solve a gravitational problem such as this one. We don't need to concern ourselves with the difference in gravity between mass on the far side of the sun versus the near side, instead we can pretend that all the mass is concentrated in the centre. In no way would this result in the earth being on the surface to the sun. The earth's orbit is maintained it's forward velocity in a direction perpendicular to gravities pull. The distance used to determine the gravitational force between the two is the distance from the centre's of each body, it's centre of mass.
Reply | Report Abuse | Link to thisThe gravitational effects prevailing within the Solar system represent a special condition: 99.86% of the total system's mass is contained within the Sun. The Solar system's center of mass never strays far from the Sun. In relation to the Sun, planetary mass is negligible. Planets are effectively isolated from each other, as they rarely perturb one another's orbit. It was Newton that proved these assumptions and limitations in Kepler's equations in his 'Principia' and improved on them using his dynamics in conjunction the law of universal gravitation.
Reply | Report Abuse | Link to thisUnlike the Solar system, spiral galaxies contain no singular dominating mass. The 'central' supermassive black holes that have been identified represent only aa tiny fraction of total system mass. There are even very large, well formed spiral galaxies that do not have a massive 'central' galactic bulge.
Even in the Milky way, the Sun is surrounded by neighboring comparably massive stars and clouds of gas, while the center of the galaxy (it's axis of rotation) is many tens of thousands of light years away. In fact the galactic center is not even visible in the night sky, only the massive clouds of gas within the nearest spiral arm of the galactic disc.
The empirically derived gravitational evaluation of the sparse objects within our Sun dominated Solar system cannot be directly applied to spiral galaxies. Please refer to:
"Need for context-aware computing in astrophysics",
http://arxiv.org/abs/0805.4163
"Rotating thin-disk galaxies through the eyes of Newton",
http://www.arxiv.org/abs/1007.3778
You are totally misunderstanding what I am trying to say. That is okay, what you are saying in respect to spiral galaxies is correct. What you are misunderstanding is not that I think you can treat the entire galaxy or the solar system as a point of mass at the centre, but rather, that each body is treated as a point of mass. Of course, all these points of mass affect one another, I never said otherwise. You treat each star as a singular point of mass, not as a body with mass spread out over the entirety of the star/planet. I never meant to imply (and I don't think I did, if you understood the way the term is usually applied) that we can treat the solar system as a singular point of mass. We obviously need the masses of the planets (or of stars, in galaxies) located at a distance in order to figure out how they will rotate and effect each other
Reply | Report Abuse | Link to thisSorry for misunderstanding.
Reply | Report Abuse | Link to thisGoing back a few comments, you stated:
"From what I have learnt, the gravitational effects of a collection of mass does not change depending on how it is configured. Clumpy or not, it can be dealt with by treating it as if all the mass where centered at an idealized point (the center of mass)."
Many might agree with you. I do know that the perceived requirement for galactic dark matter was predicated on the expected gravitational force affecting for any star (exhibited as rotational velocity) being determined by its distance from the galactic center. This would only be correct if the total galactic gravitational force applied to any star physically emanated directly from the galactic center.
Newton's law of universal gravitation correctly applies only to valid point-masses. As explained in:
http://en.wikipedia.org/wiki/Inverse_square_law#Gravitation
"If the distribution of matter in each body is spherically symmetric, then the objects can be treated as point masses without approximation, as shown in the shell theorem. Otherwise, if we want to calculate the attraction between massive bodies, we need to add all the point-point attraction forces vectorially and the net attraction might not be exact inverse square. However, if the separation between the massive bodies is much larger compared to their sizes, then to a good approximation, it is reasonable to treat the masses as point mass while calculating the gravitational force."
For example, a distant galaxy that appears to be a point of light in the sky could effectively be treated as a point-mass, while the visible portion of the Milky way galaxy filling the night sky could not be treated as a single point mass.
As wikipedia states, Newton proved with his Shell theorem that a truly spherically symmetrical distribution of mass could be treated as a singular point mass, depending on distance. While the moon can effectively be treated as a singular point-mass in determining the force of gravitation from the Earth, the Lunar landers required pilot control to gently land on the moon, since the proximal distribution of mass being traversed was variable. Likewise, gravitational anomalies can be measured from near the Earth's surface.
There are conditions in which gravitation can be determined from a single idealized point determined by a collective center of gravity. In many other conditions, IMO, this generalized approach will produce significant errors.
Correction - the preceding comment was intended to be a response to kebil. Sorry again for any confusion.
Reply | Report Abuse | Link to thisWell, I am glad to see that we now seem to understand that we were both not totally understanding exactly what the other was trying to say, and instead we were forming arguments against points the other had not raised, but that we had misinterpreted. I do think that for most astronomical cases, the approximations of point mass are valid, that is, for astronomical distances. Close distances are a different case.
Reply | Report Abuse | Link to thisCheers
As gravity bends space and in case of a black hole, space around it even closes because of all that pent up gravity so no information can escape the black hole beyond it's horizon. How then can a quasar be visible if it were powered by the energy of a black hole? Heike.
Reply | Report Abuse | Link to thisAs I understand, the validity of using a point mass to represent and object is not determined by absolute distance: it is a matter of proportionality. Consider the case of the Milky Way, or even the Andromeda galaxy: they do not appear as a point in the sky: their attraction vectors cannot be represented by a single point mass. Please refer to the wikipedia entry referenced in my preceding comment.
Reply | Report Abuse | Link to thisAs this pertains to large scale structure in the universe, even very distant exceedingly large scale objects would present a broad spectrum of directional 'attractions' even though faint, that could influence the continued localization of large scale material structure, producing equally large scale voids in which any opposition to spacetime expansion provided by large scale gravitational effects continuously diminish.
I suggest that this large scale diminishment of opposition to expansion could produce the large scale acceleration of universal expansion without any requirement for some unknown dark energy.
As the unimaginable strong gravity of a black hole will close the space around it so that no information can escape it, (gravity bends space) how then can we see the light of a quasar powered by it as assumed?
Reply | Report Abuse | Link to thisThere's a pretty good overview available on wikipedia:
Reply | Report Abuse | Link to thishttp://en.wikipedia.org/wiki/Black_hole#Accretion_of_matter
This section contains reference links to three freely available research reports, if you're interested.