Quasars

[sford]

Quasars have an interesting history. Originally, they just looked like stars. But when astronomers began using red-shifts to measure the distances to far-away objects, it was realized that these things were tremendiously far away! No single star could appear that bright from that distance, so the name “quasar” (quasi-stellar) was coined. They were considered the most mysterious objects in the universe.

Now, as Chronos said, they appear to be plain old boring black holes. Well ok, maybe black holes aren’t so boring (at least not to astronomers), but they aren’t nearly so mysterious. (I think that Gamma Ray Bursts now hold the record for “most mysterious objects”.)

A question - do Quasars show up in radio telescopes? I would think so, if for no other reason than the red shift would shift a lot of energy into the radio spectrum.

[dantheman]

Why are quasars always found at the edge of the universe?

[Chronos]

As a matter of fact, they mostly show up in radio telescopes. The afore-mentioned 3c273 is called that because it was the 273rd object discovered by the Third Cambridge Radio Survey. Since radio telescopes generally have very poor resolution, we didn’t even know that quasars were pointlike sources until we turned the optical telescopes on them.

Incidentally, even when astronomers started using redshifts, it still took quite a while to realize the implications for quasars, just because the redshifts involved are so extreme. Who in their right mind would think of looking for H-alpha lines in the radio band?

[sford]

DOH! And here I’ve been priding myself in being a faithful rule follower. :-/ Thanx dantheman.

Radio telescopes can give pretty darn good resolution now. Here’s an excerpt from http://antwrp.gsfc.nasa.gov/apod/ap010208.html

While individually even the largest radio telescopes have very blurry vision compared to their optical counterparts, networks of radio telescopes can combine signals to produce sharper pictures. In fact, using an NRAO supercomputer in New Mexico, USA and technique called VLBI (Very Long Baseline Interferometry), the European network of radio telescopes (EVN) has produced pictures of distant galaxies at a resolution some three times higher than the Hubble Space Telescope.

P.S. - although I’m sure Chronos knows, I hope everybody else with an interest in astronomy knows about “Astronomy Picture of the Day”, at:
http://antwrp.gsfc.nasa.gov/apod/astropix.html
It is the only “something of the day” web site that I actually do visit EVERY DAY.

[Eclipsee]

I have always wondered when reading about extremely distant objects in the universe, just how we can be so sure of the distance of the object and age of the light reaching us.

I have a basic understanding how this is calculated: essentially the redshift of the light is measured to determine distance, and that distance is then divided by the known speed of light. The distance to the object is typically described in “light years”, which are easily understood. If something is 12 billion light years away, the light has taken 12 billion years to reach us. Simple, right?

The problem I have stems from these objects that are up to 12 billion or so light years distant. Astronomers tell us that these give an indication to the age of the universe, or how long ago the “big bang” happened. The way that I understand the big bang (and if I’m wrong about this, that could be why I’m confused), it began with all pre-matter supercompressed into a point with no size and infinite mass, which spontaneously… well, erupted, I guess. Going from that point in time/space, the universe expanded outwards in four dimensions… la dee da dee da… to where we are today. The universe is still, from what I understand, expanding and will likely continue to do so. I think this means pretty much everything is getting further apart from everything else.

This brings me back to the distance thing. If the big bang happened 14 billion years ago, and we look at the light from something 12 billion light years distant, is it really that far away? Is there an accounting for the physical distance that object has traveled relative to us in the time it took for its light to reach us? It seems to me that in 12 billion years’ time, it would have traveled a considerable distance indeed. What we are seeing now is where it was then, but that was a VERY long time ago, and our own galaxy is probably nowhere near where it was then either.

So isn’t it possible that the objects we see are actually much farther away than what the redshift of the light indicates, if you take into account the expansion of the universe over the course of several billion years? And doesn’t that also make it possible for something to be further away in light years than the big bang happened years ago? In other words, can’t we be seeing something 12 billion light years away even if the big bang happened only 10 billion years ago?

Is there anyone out there who can help me with this? I’ve been unable to wrap my mind around it well enough to come up with an answer so far, and it’s starting to bug me. Thanks in advance.

–Eclipsee

[BickByro]

sorry, eclipsee, can’t help you… i’m grappling with this:

Last I checked, the going theory was that there is a massive black hole at the center of our galaxy. And we know there isn’t a quasar at the center of our galaxy.

Yet there seems to be an abundance of “material to drop into the hole”—like, say, the whole rest of the galaxy. So why don’t we see a quasar? Is nothing falling into our galaxy’s black hole anymore? This seems unlikely…

Perhaps it’s simply the case that the Milky Way falls into the “active galactic nucleus” category Chronos speaks of, but that still leaves the question of what exactly is different about the matter (quantity? density?) that produces quasars versus the matter that produces an active galactic nucleus. I’m not clear on what we “ran out of,” if indeed we ever had it.

Chronos only defined “quasar fuel” as “stuff,” so perhaps it’s only a matter of specifics. And so I ask: what, specifically, determines whether the stuff falling into a black hole creates a quasar?

[Chronos]

Quasar fuel is, indeed, “stuff”. In actual practice, it’s mostly hydrogen, but that’s just because there’s so much hydrogen in the Universe. You could toss in old refrigerators or spent radioactive waste, or anything else with mass, and it’d work just as well. The deal with the Milky Way and other galaxies currently isn’t a lack of stuff, but a lack of stuff falling into the black hole. It’s a common misconception that a black hole will suck up everything in its vicinity. Until you get very close, the gravity of a black hole is essentially no different from the gravity of any other object of the same mass. If the Sun were to suddenly turn into a black hole, the Earth would go on circling it in the exact same orbit. Things will only fall into the Sun if their orbit brings them so close that the closest approach is less than the Sun’s radius. Similarly, to fall into a black hole, your closest approach has to bring you inside the black hole’s radius, and black holes are much smaller than anything else of the same mass, so they’re a small target. In actuality, it’s a little simpler than that, since you’ve also got friction with other material causing orbits to decay, and GR will have an effect when you get to within a few radii, but you’ve still got to get pretty close before you start “falling in”.

Most of the matter in our galaxy (including us, fortunately) is happily orbiting the center, without caring that that big ol’ mass is a hole. There’s still a trickle of hydrogen falling in, enough to make the galactic center (known to astronomers as Sag A*) the brightest astronomical radio source as seen from Earth, but it’s nowhere near quasar levels.

[BickByro]

Thank you, Chronos! Things make a lot more sense now, but, as usual, I have more questions:

1. I’m wondering if there is some terminology I don’t quite get when you talk about “closest approach.” As I read it, you’re saying that things will only fall into black holes if they are aimed directly at the black hole, just as a comet will only collide with the present-day sun if it is aimed at the sun. If I’m getting your point right here, black holes really are a LOT less scary than people make them out to be. (see point 4)

2. Or maybe I’m misunderstanding what you mean by “radius” here–not unlikely since I always thought a black hole was a singularity, which wouldn’t really have a radius. So please help me there.

3. So, supposing they’re right about the black hole at the center of our galaxy, would it have originally been a quasar until such time as all the particles that happened to be on a crash-course with it had done so? Then all the ones that were left (ie our galaxy as it stands now) were on “safe” orbits?

4. I caught a bit of the Steve Carell Daily Show Special on Comedy Central yesterday… there was a bit about how these scientists were hoping to create a micro black hole in NYC, I believe it was. Now, all joking aside, if someone were to create a black hole somewhere on earth, what would happen? Would it not suck anything in unless you aimed that something at a point within the black hole’s “radius” (whatever that would be at the scale we’d be talking about)? If so, is there a certain size at which the black hole would suddenly become dangerous?

[Chronos]

Point the first: Correct. To fall into the black hole, you need to be aimed right at it. There’s actually a small amount of slack here, because orbits can decay for various reasons mentioned above, but it’s essentially true. Yes, this does mean that the threat of black holes is generally overrated.

Point the second: The center of a black hole is a singularity, a mathematical point or (if it’s rotating) a circle. When astrophysicists speak of the size of a black hole, though, they’re referring to the Schwartzschild radius, which is the size of the event horizon (the point of no return). Anything that passes the event horizon (including light) is lost and gone forever.

Point the third: Our galaxy was certainly once more active than it is now, but I’m not sure just how active it was. I’ll see if I can find any information on that, but I’m not sure if it’s possible to know, now.

Point the fourth: Ah, yes, the Relativistic Heavy Ion Collider. There was, indeed, a small probability that it would have created a microscopic black hole. That probability is comparable to the probability that every person on Earth would spontaneously die of a heart attack on the same day. Furthermore, even if they did create such a black hole, it would only last for an inconceivably short time. You’ve heard of Hawking radiation? It’s a totally insignificant process for a stellar-mass black hole (they radiate as though they had a temperature of a millionth of a degree above absolute zero), but the effective temperature is inversely proportional to the mass, so a microscopic black hole would be insanely hot, and would evaporate completely very quickly, converting all of its mass into energy. That sounds dangerous, until you remember that we’re talking about something created in a lab: The only energy it would have would be energy put into it by the accelerator, which might amount to a few joules at most, and probably a lot less. You wouldn’t even see the black hole itself; you’d just see what appeared to be a very weird decay process.

[sford]

Microscopic black holes - a long-time staple of science fiction now dashed forever by Hawking. sigh

As far as “closest approach” is concerned, remember that orbits are elipses, not circles. So there is a point in the eliptical orbit where it is closest (perigee) and another point where it is furthest (apogee). The “eccentricity” of an eliptical orbit tells how far the orbit deviates from a circle. Our solar system’s planets have low eccentricity, while our comets have high eccentricity.

Stellar (or black hole) systems will tend to evolve toward low eccentric orbits, at least for the material relatively close in, because highly eccentric orbits greatly increases the chances of collisions and gravitational interactions. (Comets are far enough away and spend little enough time near the Sun that they tend to stay in their highly eccentric orbits for long periods of time. But as SOHO has proven, lots and lots of comets do end up plopping into the Sun, so even our solar system is slowly continuing to evolve toward less eccentricity.)

So, during a galaxy’s youth, when lots of stuff is in highly eccentric orbit around the central black hole, there was enough friction, collision, and gravitational interaction that lots of that stuff ended up down the space-time drain, resulting in lots of energy being released. Mature galactic centers have evolved toward less eccentric orbits which, as Chronos said, are far more stable.

When two galaxies “collide” (a misnomer since there is very little literal collision going on), the gravitational interaction can disturb a lot of the matter in both galaxies, which can result in the centers becoming more active.

[sford]

We didn’t really talk about Eclipsee’s question about distance. One thing to say up-front is that the red-shift is only one way to measure distance. Astronomers have been using super nova brightness as an independant measure to make sure the red-shift method agrees. And it has indeed been forcing them to re-work the red-shift method a bit.

*Originally posted by Eclipsee *
If the big bang happened 14 billion years ago, and we look at the light from something 12 billion light years distant, is it really that far away? Is there an accounting for the physical distance that object has traveled relative to us in the time it took for its light to reach us? It seems to me that in 12 billion years’ time, it would have traveled a considerable distance indeed. What we are seeing now is where it was then, but that was a VERY long time ago, and our own galaxy is probably nowhere near where it was then either.

I believe that you are exactly correct. We can’t know where those distant galaxies are “right now” … we won’t know where it is today until another 12 billion years (actually a fair amount more since it is red-shifted and time-dialated).

Which leads me to an apparent paradox.

As you look 12 billion light years in all directions, you see galaxies as they existed when the universe was much younger … AND MUCH SMALLER. In other words, find a distant galaxy 12 billion light years due North, and another due South. They appear to be 24 billion light years apart from each other, right? But 12 billion years ago, they were much closer together to each other than that, right?

I think this misses something fundamental, and I think it has to do with time dialation. Lets see … 14 billion years ago (very shortly after Big Bang), that distant galaxy to the North was very close to us. It emitted a photon. Let’s say that our point in space gets that photon in one year.

But our two galaxies are moving away from each other at near the speed of light. So, within that galaxy’s time frame, it might emit another photon a minute later. But now it is much farther away, so maybe it takes two years for it to travel to our spot. By our point of view, that galaxy only aged one minute during the year between our getting the first and second photons, and we see it in the position it was in one minute after the first photon.

Fast-forward to today. From our frame of reference, 14 billion years have passed. But the distant galaxy appears only to have aged 2 billion years. I.e., we are seeing its position 2 billion years after the big-bang, which is less than 2 billion light years from it’s “original” position when those first few photons drifted our way.

It is US that seen to have left home!

The same discussion holds true for the galaxy due South of us. It is less than 2 billion light years from it’s original position. So, those two distant galaxies were less than 4 billion light years apart when they emitted the light that we’re seeing today.

So the paradox is that we seem to have traveled some 10 billion light years away from the Northern galaxy, toward the South. And we’ve traveled some 10 billion light years away from the Sothern galaxy, toward the North. We’re still half-way between the two galaxies, but they’re only 4 billion light years apart.

The solution to the paradox?

We haven’t traveled at all. And neither have the two distant galaxies. All three of us are exactly where we started (or close to it).

It is space itself that has expanded.

Without our having moved at all, the distance between two points has grown. So when we look at the distant galaxy, we’re seeing a time when space itself was much more compact.

I’m still not sure I understand it, and I suspect I have butchered the explanation. But it’s the best I can do…

[Chronos]

Thanks, sford, I think you’ve mostly nailed that one. It’s a slippery concept, and some days, I find myself able to explain it, and some days, I can’t.

[Eclipsee]

Wow. I was going to thank you for actually tackling my weird little question, but now my head just hurts. Ouch.

Seriously, though, doesn’t that mean that the methods we use to measure distance are inherently flawed? I mean, I’m sure it works great to gauge the distance to Andromeda, but at distances like what we’re talking about, the data appears to be seriously compromised by what you mentioned.

What I’m envisioning based on your explanation is that a photon leaves a galaxy at one point traveling toward our own galaxy, which would have reached us in… say 1 billion years, had the two galaxies been stationary relative to each other. Because they were NOT stationary, the photon takes a hell of a lot longer to get here because it has to basically run us down, which adds 10 billion years onto its trip.

Now doesn’t this make the whole attempt to quantify the distance to that galaxy pointless? How can we know anything useful without the other part of the information, ie. exactly how the universe has expanded.

The part about the objects remaining stationary and the space expanding doesn’t actually make much difference in a practical sense… at least I don’t think it does. Whether things move away from each other or the space between them grows seems a bit academic if they both have exactly the same effect with exactly the same end result. I’m sure I’m oversimplifying here, and if I’m wrong about this, just let me know, because I certainly can’t claim to fully understand that concept.

I guess what it comes down to is that I’ve always felt in my gut that the numbers thrown around for public consumtion about the age of the universe based upon the distance of visible objects was suspect. The reason for this is that I don’t feel that they have made allowances for the expansion of the universe in its youth, which would mean that the calculations have to be way off. Is this a valid concern or no? Based on what you said, I think I had something of a point, but I’m not sure.

Thanks

–Eclipsee

[Chronos]

Depends on what you mean by “for public consumption”. If you see something by a practicing cosmologist or reputable astronomer (one of the popular books by Steven Hawking, perhaps, or Carl Sagan), then he’s used a variety of measurements, together with some sort of model of the expansion, to come up with the number, and everything’s taken into account. Of course, the model could be wrong, or the data could be misinterpreted, or any of a host of things could go wrong to give the wrong answer, but it’s a scientific answer, at least.

On the other hand, if you see something in the New York Times thrown together at the last minute by some philosophy-major copyboy, then you have no clue what sort of inaccurate methods might have been used.

[Eclipsee]

I would say that’s something of an overstatement. I haven’t kept up on the subject recently, but I have in the past, and I have never seen anything mentioned about the expansion of the universe when calculating distance to very distant objects. That’s really what led me to start wondering about it in the first place.

[sford]

Actually, the red shift method for measuring distance doesn’t apply at all to Andromida, or any of the other “nearby” galaxies. Our local group of galaxies are gravitationally bound and have been “orbiting” each other since they first formed. I over-simplified when I said that none of the galaxies are moving. They are certainly moving in response to near-by graviational influences. Given a few zillion years, most of the galaxies in our local group (Milky Way included) will probably end up merged into one huge eliptical galaxy.

The red-shift measurement tool only applies to galaxies (or galaxy clusters) that are MUCH farther away, and not significatly effected by our cluster’s gravitation.

As for these issues seriously comprimising the validity of the data, you have to understand what the data are being asked to explain. Nobody is suggesting that the galaxies in question are 13 billion light years away “right now”. If you ask any astronomer/cosmologist, they will freely admit that we are seeing these galaxies in their youth, and that we don’t have any idea what their nature is “today”. Chances are that many of them have merged with other nearby galaxies, and gone through other forms of evoltion that we’re familliear with. We can make approximations of where they are today just by knowing where they were 10 billion years ago, knowning their velosity, and extrapolating using the Big Bang model. But I don’t think astronomers bother doing that because the results wouldn’t be particularly useful, enlightening, or interesting.

I’ll tell you the thing that throws all this discussion into question. The whole Big Bang model of the universe is just that - a model. It is a model based primarily on mathematics.

If I’m not mistaken, the origins of today’s Big Bang theory were in Einstein’s General Relativity. The theory gave a mathematical model of the universe’s origins. Over the years since then, thousands of experiments and observations have been made that basically support the theory, and have also led to refinements of the theory. What we have today is a huge number of astronomical observations and data that fit very will with the current Big Bang model, and do NOT fit will with any other model that anybody has been able to come up with.

But never never never never confuse the model with reality.

Here’s a favorite analogy used in physics texts. Suppose you are given an old pocket watch. You are not allowed to open it, but you are allowed to observe it and manipulate its normal controls. After listening to its ticking, playing with the stem, winding it and timing how long it takes for it to unwind, etc, you come up with a model of what is inside. Your model might be based on pullies and string, compression springs, and weights. You might even build a prototype and demonstrate that it is virtually impossible to differentiate between the two. In that respect, it is a very accurate model, not because you’re confident that the watch really has pullies and string, but because it accurately predicts the behavior of the watch under the conditions you observed it.

Then some experimenters come along to test your model. They expose the watch to high humidity to see if the strings stretch out and allows slippage. They see that no slippage happens. You improve your model by suggesting the case forms an air-tight seal. The experimenters then expose the watch to a variety of atmospheric pressures and carefully weigh the watch, and conclude that a sealed container “should” weigh slightly less than they measured under high pressure. You improve your model again by assuming a small balloon inside that allows the air pressure to equalize without allowing humidity to affect the strings. The experimenters then plunge the watch into water and measure that the watch slows drastically down (and never works quite right after that). You hypothisize a semi-permiable membrain that kept humidity out but allowed water through.

One trait of a poor model is that every time new data is taken, you have to add more spit and glue to make the model explain the data. A good model is one which accurately predicts future data. (And a good scientist spends most of his efforts trying to find weaknesses in his own models.)

The Big Bang theory has done a pretty good job of keeping up with experimenters and new data. It has required a few bandaids, but as such things go, it has a good track record.

But good scientists still resist assuming that the universe actually happened the way Big Bang suggest it happened. They don’t want to be disappointed, like they were when they finally pried the back off the watch and saw that it used gears, not pullies and string. So, they use the model for what it is good at - measuring distances - and they constantly look for alternate methods of measuring the same things to see if the results agree (like supernova brightness).