Seven Misconceptions About Dark Matter

Dark matter seems able to attract the crazies like few other areas in astronomy. That's understandable, really. Sensible, even. Anyone half-rational ought to be a little bit suspicious about ivory-tower loons claiming that most of the Universe is made up of an invisible, massive substance lurking in outer space that makes galaxies spin too fast.

Original source.

But skepticism is one thing, denial is another. Many unqualified people insist that dark matter is somehow "obviously wrong", and sometimes they even get quite angry about it. They seem to think that astronomy is some kind of soft science that is somehow not based in the same understanding of physics that's given us things like, oh, I don't know... rockets. The internet. Nuclear weapons. Electricity. GPS satellites. Things that by and large work.


1) Dark matter is driving the universe apart.

It's pretty common to confuse dark matter and dark energy. In fact, they're completely different things. Dark energy refers to the observation that the expansion of the Universe appears to be accelerating. It's a catch-all term for various explanations as to why this is happening; no-one really know what's going on here.

Image credit : me.

Dark matter is a little less mysterious, but only a little. Observations of galaxies have revealed that they're rotating too fast and should all quickly fly apart according to accepted theories of gravity. Rather than driving things apart, like dark energy, dark matter is pulling them together. Astronomers like using the term "dark" as much as possible because a) it's super sexy and b) we're scared of daylight.

If dark matter disappeared...

2) Dark matter was discovered because of the movement of the stars.

This one is extremely common even in popular science articles. It's half-true. Jan Oort did postulate the existence of unseen matter back in 1932 based on observations of stars, but that evidence turned out to be pretty poor. Around the same time, the notoriously prickly Fritz Zwicky also decided that entire clusters of galaxies needed dark matter to hold them together, but he was such a complete jerk that no-one really cared*. A certain Horace Welcome Babcock (truly a wonderful name) did find evidence for dark matter in 1939 by looking at stars, but his evidence wasn't all that convincing either.

* Seriously, read the link. His colleague Walter Baade thought he was planning to murder him.

From left to right : Oort, Zwicky and Babcock.

The first really compelling evidence came in the 1970's when Vera Rubin found really good evidence that the gas in the outermost parts of galaxies was moving much too quickly - in fact it appeared to be moving just as quickly as the gas in the inner regions. Classical theories said it should be moving several times more slowly. But even though Rubin herself had uncovered evidence from stars that the rotation of the Milky Way remains flat, it wasn't until she (and many others) found the same evidence (by looking at the gas) in many tens and hundreds of galaxies, that people really started to become convinced. The astronomical community's initial skepticism was overcome only when the evidence was strong.

From wikipedia. The rotation speed of the gas stays high much
further from the centres of galaxies than predicted.

Gas in galaxies is usually more extended than the stars. Without dark matter, models predict that most of the time the stars at the edge of the disc should be just at the point where we should see their velocities start to drop. Gas makes it a lot easier to determine what's going on further away from the galaxy's center.


3) Dark matter is a fudge to make the data fit a model.

Yes, it is. But more accurately it's a fudge to make the data fit several models and many independent observations. The same dark matter postulated to explain galaxy rotation can also explain their motions in clusters (which should also quickly fly apart without dark matter), the large-scale structure of the Universe, gravitational lensing, and cosmological models predicting how much ordinary (i.e. visible) matter there should be (see point 4).

Simulation of the large-scale structure of the Universe, which
arises from the collapse of dark matter. Compare with observations
here.

Not so long ago, it would have been entirely reasonable to claim that dark mater is an invention designed solely to explain flat rotation curves in galaxies. Some other mechanism to spin-up the gas instead of the stars might have been invoked, but that's no longer the case. When you have multiple lines of inquiry pointing to the same result, it's not a fudge, it's a conclusion. It might not be the correct conclusion (and yes, we'll get to evidence against dark matter soon enough), but it's a perfectly legitimate interpretation of the data.

I've said it before and I'll say it again : believing what the evidence tells you is not somehow arrogant or dogmatic. It certainly isn't a fudge. It's science. Dismissing the evidence because you don't like the conclusion is arrogant and dogmatic - and in many cases, it most certainly is a fudge. Especially when that conclusion was determined by skeptical inquiry.


4) Dark matter could just be ordinary matter.

Obviously it can't be just normal stars or we'd see them. But perhaps it's just undetectable cold gas, or small compact objects like asteroids, dead stars or packing foam ? Small objects turn out not to be the case : observational limits (by looking for distortions in the light they'd produce in nearby stars) say that they can't account for more than a few percent of the missing mass.

Sorry Douglas.

Undetectable gas is much more intriguing. The idea pops up every so often* - and it's very tempting. Atomic hydrogen is relatively easy to detect - cold molecular hydrogen is nigh-on impossible. Instead, we look for another molecule (CO) which we think traces molecular hydrogen. But the relationship between CO and H2 is complicated, and thought to vary depending on the environment of the gas. So it's not at all beyond the bounds of possibility that we've got this wrong.

* This is quite readable (to the non-specialist) article by my PhD supervisor. It's not peer-reviewed I don't think he intended it as anything more than speculation (he was, after all, one of the co-discovers of VIRGOHI21).

There are of course some problems with this. Massive amounts of extra gas would mean that our entire theory of star formation is just plain wrong, which is a little hard to swallow. And - if you'll forgive me for thinking on the keyboard - we ought to see this gas when it gets shock-heated by dwarf galaxies passing through it.

These problems might not be fatal, however. The Big Bang theory predicts more ordinary matter than we see - just enough, it turns out, to explain the flat rotation curves of galaxies if it was distributed correctly. But not enough to explain the formation of the large-scale structures given the age of the Universe, or (I think) the motions of galaxy clusters, or fluctuations in the Cosmic Microwave Backgound. Now that could just mean the whole Big Bang model is wrong, but it's also very difficult to see how ordinary matter could explain some specific cases like the Bullet Cluster.

Overlaid on a normal visible light image, hot gas (from X-ray observations) is shown in red. Dark matter (inferred in this case from gravitational lensing) is shown in blue.

Here two galaxy clusters have collided. Since the gas is distributed over a very wide area, the gas in the two clusters collides and gets stuck in the middle. The stars (and dark matter) keep going. If the dark matter was some form of gas, it should have got stuck in the middle along with the rest of the gas. But it didn't. Gravitational lensing measurements indicate that it carried on moving with the stars, exactly as conventional wisdom predicts.

EDIT : More accurately, gravitational lensing measurements are consistent with the predictions of dark matter. However, the amount of lensing predicted in alternative theories of gravity (more on them soon) doesn't necessarily depend on the sheer mass that's present but on the distribution and geometry of that mass. So, advocates tell me, that all the lensing follows the stars and not the gas is not necessarily inconsistent with the fact that most of the mass is in the gas. However, I haven't been able to find a paper to confirm if this is the case.

The Bullet Cluster may not be the silver bullet against other interpretations of dark matter that many people would like (we'll get back to those later), but it's tough to see how ordinary matter could explain this with conventional theories of gravity.

Dark matter, then, is very difficult to explain as ordinary matter. Normally, "dark matter" is taken to mean an as-yet unknown type of particle which doesn't interact with "normal" matter except through gravity. The consistency between observations of galaxies, galaxy clusters, and Big Bang cosmology is an intoxicating success of modern science. But before we get totally wasted on scientific glory, we should beware the hangover that comes when we discover problems with the models. I'll return to those a bit later.



5) Inventing huge amounts of an unknown substance is a little bit bonkers.

Sure, but what's more outrageous : postulating an unknown substance, saying that our theory of gravity (which is tested to an incredibly high level of precision) must be wrong, or saying that some other utterly mysterious process is at work ?

Now it's true, we know relativity cannot be the be-all and end-all description of gravity. Singularities, points of infinite density where models become nonsensical, indicate that we're missing something. But dark matter is generally only important on much larger scales, where there is as yet no reason to suppose relativity doesn't work (just as Newtonian gravity, even though we know it's wrong, is still an excellent approximation in most situations).

This overlaps with the idea that it's a "fudge". Well, learning new things is the whole point of the scientific process. In the late 19th century, it was thought that there might be a planet closer to the Sun than Mercury, which would explain some slight anomalies in Mercury's orbit. That turned out not to be the case - Newton's theory of gravity gave way to Einstein's - which allowed a raft of new possibilities, not least of which is time travel. Vulcan was disproved, and most people today know it only as the even more fictitious homeworld of Mr Spock.

Skeptical Spock is skeptical of your skepticism.

In the case of Vulcan, scientists proposed missing mass to explain their theories. On that occasion, it didn't work - scientists were forced to the truly startling conclusions of relativity.  Dark matter, however, is proving a lot harder to dislodge. It looks like this time the theories are correct, but it's not for want of trying to come up with a better theory (see below). Here we're forced to the startling conclusion that most of the Universe is made of completely different matter to what we see around us.

Philosophically, I can't see any reason you'd prefer modifying an established, well-tested theory over the idea that there's an unknown substance - there's simply no way to avoid making a "mad" choice here. Pragmatically, the existence of neutrinos - which are similar (but not identical) to the postulated dark matter particles - indicates that dark matter is not such a silly suggestion at all. Maybe dark matter doesn't exist, but I find it hard to see the alternatives as being any less radical.


6) But surely, if it disagrees with experiment, it is wrong ! Galaxy rotation curves are flat, therefore relativity is wrong and we need a better theory.

Partial credit ! It's extremely important to remember - and not stated nearly often enough - that dark matter is theoretical*. We don't know for certain that it exists, but you'd be forgiven for thinking from some articles that astronomers believe in dark matter in the same way that geologists believe in rocks. This isn't the case at all. A few do. Most simply accept its reality for convenience : it becomes impractical to give a long-winded explanation for dark matter's existence in every press release.

* Yes, you can say "only a theory", provided you really understand what this means.

It's entirely possible that if galaxy rotation curves had been found to be flat much earlier, when relativity was not firmly established, this might have caused severe problems. Instead, relativity was tested time and time again and found to be successful. That hasn't stopped some people from considering that maybe the flat rotation curves are the ugly factual flies in the ointment that may yet slay the beautiful Goliath theory that is relativity and/or modern cosmology.

And there are other anomalies with dark matter models. The number of dwarf galaxies predicted in simulations is much higher than what's observed in reality (some of these missing galaxies are worryingly large). Their distribution is all wrong. Dark matter particles weren't predicted by the Standard Model. There are lots of things in cosmology and galaxy evolution we don't understand, and anyone who says otherwise is quite, quite mad*.

* Many of these problems might be solved by better understanding of the physics of the ordinary matter involved. They do not automatically necessitate rejecting dark matter. "If it disagrees with experiment it is wrong" is a laudable principle, but you have to be sure it disagrees.

Planes of satellite galaxies (source). In normal cosmology smaller galaxies should be found in spherical distributions around their host galaxies. In reality this is not true, at least for the Milky Way and Andromeda (M31).

EDIT : Since writing this, a paper has been submitted which shows that such planes of satellites can form in standard dark matter simulations. In this case such planes are present around 15% of galaxies which formed particularly early (which the paper claims is the case for the Milky Way and Andromeda), possibly higher if those galaxies don't subsequently experience a major merger. At present, the paper does not give enough information to really say if planes around galaxies like ours are really unusual or quite common, but it does demonstrate that they can form.

The leading alternative idea to dark matter is MOND : MOdified Newtonian Dynamics (though I've always preferred the name MONG : MOdified Newtonian Gravity). This, as the name implies, avoids dark matter by suggesting a different theory of gravity, which is very similar to Newtonian gravity except when accelerations are very low.

MOND isn't pseudoscience - it offers some genuinely very interesting insights. For example, faint galaxies are known to follow similar scaling relations to bright galaxies, which conventional theories say they shouldn't. MOND explains this quite naturally. The distribution of dwarf galaxies is explained in MOND by most of them being formed in interactions between larger galaxies - that's why they're found in narrow planes around larger galaxies, rather than the spherical clouds predicted by standard models.

But MOND has its own problems. It's not so straightforward to change simulations to use MOND's law of gravity. This means we don't yet know whether a MONDian simulation of the Universe would do as good a job as dark matter does. And MOND does require some dark matter, though so little it could plausibly be ordinary matter rather than the more exotic kind of orthodox cosmology. It's also damn hard to find a theory of MOND that fits some of the known effects of general relativity.

The latest challenge on that front is the orbital decay of a double pulsar. GR describes this brilliantly, MOND fails. It appears that it may be difficult for MOND to simultaneously model both the Solar System, where gravity is weak, and pulsar systems, where it is strong - GR doesn't have this problem.

Double pulsar. GR has successfully predicted the orbital behaviour, MOND, allegedly, has not.

Yet pronouncing MOND as dead because of this is surely premature. I have not been able to find any response from MOND supporters regarding this. Until they do, MOND might be regarded as wounded  - perhaps fatally, perhaps not - but still very much alive; perhaps writing its last will and testament or perhaps rallying for a fresh assault. Pronouncing it dead now is like burying a man without checking if he's breathing : it runs of the risk of creating a terrible zombie-vampire MOND that will wreak havoc upon us all.

RAAAR ! I AM ZOMOND ! FEAR ME, FOR I SHALL
 CRUSH YOUR THEORIES WITH WELL-REASONED
 PHILOSOPHICAL ARGUMENTS !

EDIT : My MOND buddies tell me that indeed the best-studied version of MOND that's compatible with relativity is indeed disproved by this. However that doesn't rule out that there may be some other theory of gravity that could explain both solar system and pulsar measurements. In my view, that still means you're left with an uncomfortable choice : hoping that a new theory will come along and explain everything, or hoping that a dark matter particle will be detected.


7) Well, pish. Scientists dogmatically insist that dark matter must exist without any real evidence, because they haven't got any better ideas. So there.

This one is wrong every which way you look at it. For starters, there are alternative theories, but they have difficulties. They have yet to match the stunning self-consistency of modern cosmology, which successfully describes how quantum fluctuations can give rise to the galaxies and galaxy clusters we see today. Sure, there a ton of problems of detail. And it's been heavily modified along the way - maybe the day will come when we will have to declare it broken. But it is not this day.

The incredibly wise Vera Rubin said is thusly :

"One hundred years ago, galaxies were an enigma. They still are. It is folly to believe that we know what a galaxy is, while the extent, the density distribution, and the composition of more than 90% of its mass are still a dark mystery...We have learned much about galaxies in the last 100 years. I think that we still have major surprises to uncover."

It's worth bearing in mind just how young modern astronomy is, and the scale of the challenges it faces. 80 years ago we didn't know that other galaxies were distant objects. 40 years ago we had no idea that dark matter might exist, except for a few vague hints. When inferring the existence of a mysterious substance that's more prevalent than "ordinary" matter, is it really so surprising that we don't understand everything ?

Galaxies like this one are incredibly complex systems. Hundreds of billions of stars would be enough, but they also contain a roughly equal mass of gas and varying amounts of dust. Nor do they exist in isolation - they're surrounded by many smaller satellite galaxies, and sometimes hundreds of giant companions. So when a world view is as successful as explaining them as modern cosmology, it makes no sense to declare it invalid as soon as the slightest problem is found.

Rubin again :

"In a spiral galaxy, the ratio of dark-to-light matter is about a factor of ten. That's probably a good number for the ratio of our ignorance-to-knowledge. We're out of kindergarten, but only in about third grade."

Moreover, experiments are underway to test the reality once and for all, to put it on the same level of certainty that geologists claim for rocks. Scientists are indeed putting their money where there theories are. The "dark matter is stupid" ilk might have a point if such experiments were not taking place.

I said earlier that it's not arrogant to believe the evidence. For most people, the evidence favours dark matter. For some it favours MOND. When there's evidence either way, it's a judgement call as to which side you want to pick. MONDers aren't necessarily arrogant, but those who unjustly claim that dark matter must/must not be real certainly are.

Even if MOND is falsified, it doesn't prove dark matter exists. Only a direct detection would constitute true certainty. But for me, and most astronomers, the spectacular self-consistency of dark matter cosmology makes it more probable that it does exist. Maybe I'm wrong. One day there might be a revolutionary discovery that overturns the whole notion of dark matter - if so, astronomers everywhere will not drown their sorrows : they will celebrate. Until that happens, dark matter cosmology is the best we've got.

Why Captain Kirk Was Chasing The Voth

A few days ago I received an email from a fellow Trekkie with a question about quasars. As one does.

"WHAT !?!? AN EMAIL THAT'S NOT FROM A PSEUDOSCIENTIST ????"

"If quasars weren't extragalactic, what might they have (otherwise have been found to have) been ? Lest that sound too much like something from Velikovsky, rest assured I'm asking in terms of SF…or space opera anyway, for strictly Trek-related reasons. Let me explain.

I breathed a sigh of relief at the Velikovsky dismissal, because I've had just about enough of dealing with wackos who think they understand science better than scientists. But I digress.

As you may or may not know, Trek's episode "Galileo 7" cited Kirk's "standing orders" to "investigate all quasars and quasar-like phenomena." This order was cutting-edge in 1967, when quasars' nature and location(s) were unclear. Today it's an anachronism…but my bent for "aired data is factual" Treknological speculation has me longing to be able to say more than, "If taken at face value, these orders imply the very COSMOS of [original series] Star Trek differs fundamentally from our own." "

Well, indeed, I am acutely aware of the episode in question. It's a great episode where we see some fundamental character conflicts that were barely seen in later Trek seasons. But the notion that Captain Kirk could take the Enterprise off on a study of quasars is, as everyone knows these days, just plain silly.

Of course, the rest of the show was a masterclass in subtle understatement.

What are quasars, anyway ?

Quasar, for those of you not in the know, is a short form of "quasi-stellar object" : things that look a bit like stars, but aren't stars at all. They were discovered in the early 1960's using radio telescopes (that they featured in Star Trek as early as 1967 is a credit to the writers). The name comes from the fact that they were bright sources of radio waves but with only a star-like object seen in visible light. At the time, no-one even knew how far away they were, let alone what they were.

Pretty soon though, someone measured the redshift of one of them - that is, how fast it's moving away from us. It was found to be a whopping 48,000 km/s (a hundred million miles per hour, or nearly three hundred billion furlongs per fortnight) - much, much faster than anything in our Galaxy. Later, galaxies were discovered at the same positions as the quasars. So it's now a certainty - yes, really, a certainty - that quasars are very distant, incredibly bright objects.

48,000 km/s is about 15% of the speed of light, which is jolly fast. Accelerating something as big as a star to that speed takes an enormous amount of energy - roughly speaking the entire energy output of our Galaxy for a year. So for quasars to be natural, nearby objects is basically a non-starter. But other galaxies are doing this quite naturally - pretty much all of them, really (much more on this later). The more distant a galaxy is, the faster it's moving away from us, a discovery known as Hubble's Law though the real credit should go to Georges Lemaître*.

* Interestingly, it seems that Hubble didn't believe redshift was really equivalent to speed, but we'll get back to that later.

Anyway, quasars are tremendously far away (many hundreds of millions of light years), so they have to be extremely bright. But they can't just be unusual galaxies - as we've seen, their host galaxies don't look all that weird - because their brightness can vary on a timescale of days. Galaxies don't, indeed, can't, do that.

Whatever quasars are, they have to be small - no more than the distance light travels in a day or so (about 26 billion kilometres, several times the distance to Pluto). Since nothing can travel faster than light, if they were any larger than that there's no way the change in brightness would vary consistently across the object : some parts would be bright while others were dim, and vice-versa. It would all cancel out so it would look like there was never any variation at all.

Long story short, our current best guess for what quasars are is something like this :

The idea is that quasars look different from different viewing angles. So if we happen to be looking straight down the jet, they look very bright; if we're edge on and looking at the torus, they're dimmer. Image author unknown.

A huge black hole (by the standards of black holes, which is still tiny compared to galaxies) gobbling up material which swirls around it in an accretion disc. Before it falls in to the hole itself, from which nothing can escape, it gets hot and radiates energy. It turns out that accretion is one of the most efficient ways to release energy, second only, perhaps, to a matter-antimatter explosion. This is not so much because of the mass of the black hole, but more because that mass is very concentrated.

Imagine you dug a hole right down to the center of the Earth, and, feeling reckless, you jumped in. Obviously, you'd get faster and faster. But while your speed would always increase right until you met a miserable fate at the bottom of the hole*, your acceleration would steadily decrease. The further down you went, the less mass there'd be pulling you down. For a solid sphere, it turns out that you only feel the pull of gravity from the mass beneath you - all the mass above you cancels out.

* I said you dug a hole to the center. I never said it went right out to the other side.

This Wikipedia illustration is too sensible. It even puts
the little dude inside an elevator so the tunnel can be
a vaccum.

Not so with a black hole. To squish the Earth into a black hole we'd have to make it about the same size as a pea. Let's suppose we were clever and stupid enough to do this. Well, we'd suddenly find ourselves floating in space with a black hole now around 6,000 km below our feet. We'd start falling... but this time our acceleration wouldn't decrease. It would get higher the closer we were to the hole. All of the mass of the Earth would still be beneath us right until the bitter end - which means we end up with a much higher final speed (as in near light speed, though things get horrendously complicated with such strong gravity), releasing more energy.

Funny thing though - our initial acceleration wouldn't have been any greater than if we'd dug a hole. Turn the Sun into a black hole and it wouldn't start sucking everything in, in fact it wouldn't pull on us any more strongly than it does now. Even at where the surface of the Sun is now, the gravity wouldn't be any stronger - but get closer, and things get much, much worse. It's the concentration of mass that makes things go haywire, not the amount of mass.


What about Kirk ?

That's enough about science (said no-one, ever*). How can we reconcile what we know about quasars today with Kirk's standing orders ? Well... we can't. The Enterprise was limited to exploring our own Galaxy, it couldn't go flying off to study distant quasars. Unless... what if we got that whole redshift thing wrong ?

* No-one worth knowing, at any rate.

Bad news, we didn't. We know quasars are extragalactic because we've seen their host galaxies, remember ? But maybe we can make this work with what we knew about quasars in 1967.

The "quasar" as it originally appeared in the show. In the remastered version it looks like this.

The strictest definition of redshift is that the frequency (wavelength) of light is altered. It doesn't actually mean things are very far away or even moving very fast. There are actually three ways we know of creating redshift : 1) Expand space between us and the object; 2) Move it very fast; 3) Put it in a strong gravitational field.

Expanding space is basically the real-world answer (that's why galaxies can have high redshifts), sheer speed is something we'll come back to in a moment, as is strong gravity. Suffice for now that the latter two have problems.

Some astronomers were so startled by how frickin' bright they would have to be if they were as far away as redshifts would conventionally suggest, that they proposed a fourth mechanism : intrinsic redshift. The idea was that the light was emitted at at different frequency to begin with, but as to how this was supposed to happen was anyone's guess. It wasn't an outrageous idea, it just didn't work.

So that leaves us with gravity and speed. If we have incredibly strong gravity, we can create the same redshift we'd normally mistake for extreme speed. From this formula, all we need is the size of the object, its distance and redshift, and we can work out its mass.

We've got the redshift from the observations, so that's easy. We can place an upper limit on the distance given the size of the Federation (about 8,000 light years across) and the highest resolution observations available in 1967 (about 1" or 0.0003 degrees) - any larger than that and quasars would have looked like diffuse objects rather than stars, even with telescopes of the 1960's. Allowing the Murasaki quasar to be around 5,000 light years from Earth, this gives us an upper size limit of around 0.024 light years or 1500 AU.

Aaar ! Thaat quasaaar be one aaarcsecond across, arrr !

That means we now have size, distance, and redshift, so we can work out mass - which turns out to be 9 billion times the mass of the Sun, or 15% of the stellar mass of the Milky Way. The density wouldn't be that great by everyday standards (about 300x less than water, about the same as ordinary clouds), but it would still be staggeringly huge compared to most of the matter between the stars. The free-fall time (for it to collapse to a point ignoring everything except gravity) would be about two weeks. This could be prevented if the gas was hot enough, but the temperature needed to prevent collapse would be around six hundred billion Kelvin. Or Celsius. Or Fahrenheit. At that temperature it really doesn't make any difference.

An object that size with that temperature would have the energy output of five tredecdillion Suns. That's a number far larger than all the stars in the Universe. The energy received on Earth in one square metre would be the same as the total output of 200 Suns. This monster wouldn't be a planet killer, it would be a galaxy killer.

We'd die.

Admittedly, I'm pushing the equations well beyond their breaking points, but the point is that a giant nearby quasar with strong gravity is a bloody stupid idea.

But remember, the size limit was imposed only because of telescope resolution. Maybe we could still get equivalent gravitational redshifts from smaller objects ? Alas, not really. It turns out that to get a redshift of 0.15 (the lowest known), the light would have to be emitted from a radius only about four times the Schwarzchild radius, the size below which an object of a given mass becomes a black hole. As we've seen above, it's nigh-on impossible to get a large, stable object like a star that's so massive for its size.

But couldn't we just do this by having material emitting light when it's close to the surface of a different object, like a neutron star or a black hole ? In my email response, I made a mistake and said that to do this would require emitting material to be below the Schwarzchild radius, which is impossible. But that's not the case at all. Theoretically, you can get any redshift you like if the material is close enough to the event horizon.

Stellar mass black holes have event horizons ~10 km across, far smaller than the maximum 1500 AU size allowed by observations, so that's good. Emission that looks like it's at a redshift of 0.15 would have to come from around 350 km from the singularity. The problem is there's no obvious reason why the emission should peak at this distance - you'd have to somehow keep the material stable far above the hole itself, otherwise you'd see a much higher redshift as the material fell further in to the gravitational well. Also, apparently it just isn't possible to have material creating redshifts greater than 0.62, which are observed for many quasars.



What about a rocket ?

Good thinking, Batman ! The one remaining option to generating such high redshifts is sheer speed. But, how do we get material up to such a tremendously high speed ? Easy :

In the Trek universe, "aliens" are not only a perfectly valid explanation, but they're probably the most likely explanation. There's one alternative : white holes, a.k.a wormholes. Material falling into a black hole might be spat out of its opposite somewhere else in the Universe (but almost certainly isn't). Fortunately this one is easy to dismiss : in Trek cosmology, wormholes are very rare and very unstable (with one notable exception), nor do they ever eject huge quantities of matter. So these aren't a very likely explanation at all*.

* Naturally occurring jets from black holes can also reach tremendous speeds. The problem is that each black hole has two jets, pointing in opposite directions. So if quasars were actually black holes in our own galaxy, we'd see this second jet.

So natural explanations are out. Which leaves the question : what is it the aliens are accelerating up to such high velocities, and why ? Obviously, nothing larger than 1500 AU in diameter, but things which are bright enough to look like stars. So, stars then ? Well, as I mentioned, this would take a tremendous amount of energy, equivalent to the mass of a small asteroid in anti-matter. That's an awful lot in the Trek universe, but not out of the question. Also, of course, Trek science allows for faster than light travel - in reality this requires infinite energy. So maybe with Trek physics there could be a way to use far smaller quantities of anti-matter to achieve the same result.

There are a couple of other interesting observations about quasars that are also relevant in working out what the aliens are up to. One is that there are no quasars with blueshifts - they're all moving away from us, unlike some galaxies. That means that - accepting that quasars are not extragalactic, for the purposes of Kirk being able to visit them - they have to be a purely local phenomena. If other galaxies were spewing out there own quasars by some natural process, we'd see some of them heading toward us*. Which makes "aliens" a very plausible explanation indeed.

* In the real world, we don't see this because quasars are almost exclusively found in very distant galaxies, where the expansion of the Universe is much greater than their peculiar velocities.

The other interesting nugget is that quasars are found more or less evenly across the whole sky. Stars, of course, are not distributed like this - because we live in a spiral galaxy we see a band of them across the sky. Those outside the band are nearby.

With Earth at the center of the circle.

In reality, that means that quasars - if they are basically weird stars - have to be very close, within 2,000 light years or so. But since we've constrained quasars to be a local phenomenon distinct from stars, with redshift indicating speed, that's no longer the case. They can't be too far away though, or their density will be so low that Kirk is never going to be able to visit any. Say a cloud of about the same diameter as the Milky Way.

A huge cloud of star-like things rushing away from Earth is a plain ridiculous explanation in the real world, but totally satisfactory if you're allowed to say, "because aliens". Making this cloud about the size of the Milky Way also means that only a very few quasars will be within range of a Federation starship, so Kirk's orders then make a lot of sense. Not too close - or the Feds would already have investigated hundreds of the buggers - and not too far away, otherwise there'd be none to visit at all.


Enter the Voth

I mean, come on. A bunch of mysterious objects rushing away from Earth ? It could hardly be more sci-fi. A mega-engineering project to hurl stars out of our Galaxy is one possibility, a more obvious explanation is that quasars are in some way alien spaceships themselves.

Initially, I rejected the idea of star-hurling for several reasons. The energy requirements are huge, and their isotropic distribution implies something like an explosive event. But would the alien spaceship interpretation be any better ?

Given the brightness of the brightest quasar, and assuming it to be at a distance of 5,000 light years, it would be about 400 times as luminous (that is, its energy output) as the Sun. That's equivalent to combining 4 million tonnes of matter with 4 million tonnes of anti-matter every second. Which, for comparison, is roughly a cube of basalt (the densest rock) 100 metres on a side. With something as dense as the material inside a neutron star, things are a bit better - maybe 1 cubic metre per second or even much less, though storing such material is hazardous.

Hazardous here meaning, "mistakes will briefly make your starship shine as brightly as the Sun".

One wonders why aliens capable of such stupendous feats of engineering haven't, in the Trek universe, just invented warp drive like everybody else.

But let's assume they didn't. My initial thought would be that alien ships moving away from Earth that have been travelling for millennia sounds very much like the Voth. In Voyager, we learn that a bunch of dinosaurs managed to escape Earth before the asteroid hit (presumably before they even knew it was coming, otherwise deflecting an asteroid would certainly have been easier than launching a fleet of starships), eventually settling in the Delta Quadrant.

You know it makes sense.

The Voth are pretty advanced by Federation standards - they have transwarp drive and transporters which can beam aboard whole starships. But they're not nearly as advanced as one might imagine for a species that's been space-faring for 65 million years, and their politics is positively medieval. That suggests a generally very slow pace of development. They also command vast resources, with at least one spaceship that's tens of kilometres long.

They sound ideal. A species that's left Earth and has been travelling across the Galaxy, probably initially in very primitive craft. Presumably they would have travelled in many separate spacecraft travelling in different directions, to increase the chances that at least one of them would survive. So the Voth Voyager encounters need not be the only ones. Given Voth politics, it doesn't seem much of a stretch to suggest that maybe some of those others never even invented warp drive, and that's what 1960's scientists were observing as quasars.


Exit the Voth

But there are problems. The average speed to cross the galaxy in 65 million years is about 400 km/s. That's way faster than anything we have access to today, but it would still take over 3,000 years to reach the nearest star. Which is so long as to make the whole venture pointless, and given the Voth's stupendous feats of engineering, they ought to do rather better than that. We know they can manipulate energies equivalent to that of stars, which for a 10 km spaceship (assuming it to have a mass as though it were made of solid steel) should allow them to reach such a speed in a couple of seconds.

There's just no way the Voth - even the primitive ones - can be travelling so slowly. Especially given the redshift of what are supposed to be their ships, which are more than 100 times as great as their supposed initial speed.Worse, if quasars are spaceships, what exactly are we observing ? Their engine exhaust ? If so, we're seeing the engine exhaust rushing away from Earth...which means the ships themselves are heading right for us.

At this point I faffed an explanation, "maybe it's the bow-shock of their ships moving through the interstellar medium". Such features do exist around real-life high velocity stars (which are moving much more slowly than real quasars, < 100 km/s), but this is a stupid explanation. Only a tiny fraction of the material directly ahead of the ship would be forced up to the speed of the ship itself; the vast majority will be pushed aside into a tail.

Unnamed stars seen with Hubble.

The only realistic explanation of quasars for the Trek universe is that they are small, bright objects moving very rapidly away from us. That brings us back to the star-hurlers again.

What about Murasaki ?

OK, so by invoking some treknological solution, an ancient race of super-advanced beings started hurling stars out of the Galaxy several tens of millennia ago. One thing that worried me is that since no quasars are blueshifted, all of those stars would have to have been close to our own Solar System, suggesting a sort of explosion. That means not only having the mass of an asteroid in anti-matter, but also inertial dampers powerful enough to stop a star from being ripped to shreds.

However, as the inquirer pointed out, this isn't necessarily the case. The explosion could have been far away from Earth, if it occurred long enough ago that the stars flung towards us have now moved past us. Or, perhaps the stars were selected from completely random locations in the Galaxy but always flung away from the Galaxy, rather than specifically away from Earth. Redshift tells us about motion along our line of sight, but it doesn't mean the stars aren't also moving - from our perspective - across the sky.

So star-hurling it is then. Where does that leave the Murasaki quasar ? Err... floating around in space. That's about all, really. If we're saying that quasars are just ordinary but fast-moving stars, they wouldn't necessarily seem all that weird up close. I have absolutely no idea what the bow shock from a relativistic star would look like though. That Murasaki doesn't seem to have one could just a viewing angle effect - since we don't see a bright central source, perhaps the Enterprise was directly behind the quasar.

We're given precious little information about Murasaki. It's described as a "quasar-like formation", with "negative ionic concentration 1.64 times ten to the ninth power metres". I don't know what this means. "Ionic concentration" is an odd term, more suited to chemistry. Astronomers would probably talk of "ionization fraction" or just "ion fraction", for convenience - what fraction of the gas is ionized. Why it matters to specify that these are negatively charged ions, I don't know. As for the incredibly clumsy, "times ten to the ninth power", they could have just said "billion" or "giga". I guess they wanted to sound more sciency. And "metres" as a unit of concentration is like saying, "I'd like seven Pascals of sausages, please."

Quite. The second piece of information is that the radiation wavelength is 370 angstroms. That puts it in the UV band (almost X-rays), which will indeed ionise interstellar gas. So that one is totally accurate.

Finally, we learn that "Harmonics - upward along the entire spectrum". This could mean anything. A harmonic might refer to a multiple of some frequency of radiation, with interference being successively weaker at higher harmonics. Maybe.

The entire sector has been ionised : "four entire solar systems" - presumably referring to all of the interstellar gas. Well I suppose as it's travelling through space, it could be shock-heating the surrounding gas, ionizing it. Maybe.

In summary, Murasaki itself tells us nothing that prohibits it from being a relativistic star, though nothing that indicates it definitely is one either.


What if quasars really were high velocity stars ?

Or rather, what if quasars weren't giant, distant black holes ? Much more difficult to answer. Our Universe would require different physical laws, but just how different is impossible to say. No-one had predicted the existence of quasars prior to their discovery - so you might think that implies they're not all that important. The problem with that is that we barely had any understanding our galaxy evolution at all in the 1960's - after all, it was at that time only about 30-40 years since galaxies were proved to be distant objects.

There are two ways we could go about preventing quasars : change the laws of physics...

Oh, right, yes. Of course. Silly of me. Guess that option isn't available then. Anyway, I have no idea how much we'd need to alter gravity and/or themodynamics, let alone what would happen if we did. Probably nothing good.

The second option is to be semi-magical and say that pretty much everything in the Universe proceeded from the Big Bang just as it did in reality, until the point where quasars first formed. At that point, for some reason, they didn't. Exactly how important quasars are in galaxy evolution isn't clear. Oddly enough, the mass of the black hole correlates with the mass of a galaxy's bulge and even the angle of its spiral arms. There's probably some underlying common connection between the two, so preventing quasars might necessitate dramatic differences in galaxy structure.

Would this prevent the Universe from being habitable ? My guess is probably not. Many simulations have reproduced the basic features of the Universe without invoking quasars at all. That doesn't mean there might not be massive differences in detail though : totally different star formation histories of galaxies, different structures, maybe the wrong numbers of dwarf galaxies... all kinds of things. But I can't see any obvious reason why lack of quasars would lead directly to a lack of a Federation.


Why are the aliens doing this ?

What, you mean hurling random stars out of our galaxy at tremendous speed ?  The questioner suggested that it might relate, somehow, to the creation of the Galactic Barrier that (in Trek) surrounds the Milky Way. Possible, but it would have to be an indirect connection since quasars are distributed isotropically. Though I suppose this barrier might surround the entire galaxy like a shell, in which case its depictions in the show shouldn't be taken literally - for one thing, it couldn't be so visible at optical wavelengths, otherwise we wouldn't be able to see other galaxies.

A.k.a. the Very Pink Barrier, the Barrier of Slight Inconvenience,
or the Barrier of Thinking Two-Dimensionally.

Of course, there's always Q, if we're allowing other Trek series. Q is the sort of entity who would choose to randomly hurl stars into extragalactic space for the sole purpose of confusing bloggers. Which seems an entirely sensible possibility if you ask me.

Conclusion

Kirk's standing orders are a bloody nightmare to reconcile with real-world physics, bordering on impossible. It's barely possible to make them fit what was known in the 1960's, but, if we must, relativistic stars seem a strong possibility. No other natural explanation (that we know of) seems to fit - and the power requirements for alien spaceships don't seem plausible, especially given their absurdly slow speeds by Trek standards.

As for how the aliens have gone about hurling stars out of the galaxy, I have no really no idea. Their reasons for doing so, I think, are best left as an exercise for the reader.