Bounce

Today's post is about a pair of new scientific papers that have the potential to shake up the world of cosmology in a big way, but first, some background.

I'm sure you've all heard of dark energy, the mysterious energy that permeates the entire universe and acts as a repulsive force, propelling everything (including space itself) outward.  The most astonishing thing is that it appears to account for 68% of the matter/energy content of the universe.  (The equally mysterious, but entirely different, dark matter makes up another 27%, and all of the ordinary matter and energy -- the stuff we see and interact with on a daily basis -- only comprises 5%.)

Dark energy was proposed as an explanation for why the expansion of the universe appears to be speeding up.  Back when I took astronomy in college, I remember the professor explaining that the ultimate fate of the universe depended only on one thing -- the total amount of mass it contains.  Over a certain threshold, and its combined gravitational pull would be enough to compress it back into a "Big Crunch;" under that threshold, and it would continue to expand forever, albeit at a continuously slowing rate.  So it was a huge surprise when it was found out that (1) the universe's total mass seemed to be right around the balance point between those two scenarios, and yet (2) the expansion was dramatically speeding up.

So the cosmological constant -- the "fudge factor" Einstein threw in to his equations to generate a static universe, and which he later discarded -- seemed to be real, and positive.  In order to explain this, the cosmologists fell back on what amounts to a placeholder; "dark energy" ("dark" because it doesn't interact with ordinary matter at all, it just makes the space containing it expand).  So dark energy, they said, generates what appears to be a repulsive force.  Further, since the model seems to indicate that the quantity of dark energy is invariant -- however big space gets, there's the same amount of dark energy per cubic meter -- its relative effects (as compared to gravity and electromagnetism, for example) increase over time as the rest of matter and energy thins.  This resulted in the rather nightmarish scenario of our universe eventually ending when the repulsion from dark energy overwhelms every other force, ripping first chunks of matter apart, then molecules, then the atoms themselves.

The "Big Rip."

[Image is in the Public Domain courtesy of NASA]

I've always thought this sounded like a horrible fate, not that I'll be around to witness it.  This is not even a choice between T. S. Eliot's "bang" or "whimper;" it's like some third option that's the cosmological version of being run through a wood chipper.  But as I've observed before, the universe is under no compulsion to be so arranged as to make me happy, so I reluctantly accepted it.

Earlier this year, though, there was a bit of a shocker that may have given us some glimmer of hope that we're not headed to a "Big Rip."  DESI (the Dark Energy Spectroscopic Instrument) found evidence, which was later confirmed by two other observatories, that dark energy appears to be decreasing over time.  And now a pair of papers has come out showing that the decreasing strength of dark energy is consistent with a negative cosmological constant, and that value is exactly what's needed to make it jibe with a seemingly unrelated (and controversial) model from physics -- string theory.

(If you, like me, get lost in the first paragraph of an academic paper on physics, you'll get at least the gist of what's going on here from Sabine Hossenfelder's YouTube video on the topic.  If from there you want to jump to the papers themselves, have fun with that.)

The upshot is that dark energy might not be a cosmological constant at all; if it's changing, it's actually a field, and therefore associated with a particle.  And the particle that seems to align best with the data as we currently understand them is the axion, an ultra-light particle that is also a leading candidate for explaining dark matter!

So if these new papers are right -- and that's yet to be proven -- we may have a threefer going on here.  Weakening dark energy means that the cosmological constant isn't constant, and is actually negative, which bolsters string theory; and it suggests that axions are real, which may account for dark matter.

In science, the best ideas are always like this -- they bring together and explain lots of disparate pieces of evidence at the same time, often linking concepts no one even thought were related.  When Hess, Matthews, and Vine dreamed up plate tectonics in the 1960s, it explained not only why the continents seemed to fit together like puzzle pieces, but the presence and age of the Mid-Atlantic Ridge, the magnetometry readings on either side of it, the weird correspondences in the fossil record, and the configuration of the "Pacific Ring of Fire" (just to name a few).  Here, we have something that might simultaneously account for some of the biggest mysteries in cosmology and astrophysics.

A powerful claim, and like I said, yet to be conclusively supported.  But it does have that "wow, that explains a lot" characteristic that some of the boldest strokes of scientific genius have had.

And, as an added benefit, it seems to point to the effects of dark energy eventually going away entirely, meaning that the universe might well reverse course at some point and then collapse -- and, perhaps, bounce back in another Big Bang.  The cyclic universe idea, first described by the brilliant physicist Roger Penrose.  Which I find to be a much more congenial way for things to end.

So keep your eyes out for more on this topic.  Cosmologists will be working hard to find evidence to support this new contention -- and, of course, evidence that might discredit it.  It may be that it'll come to nothing.  But me?  I'm cheering for the bounce.

A fresh start might be just what this universe needs.

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The phantom whirlpool

The universe is a dangerous place.

I'm not talking about crazy stuff happening down here on Earth, although a lot of that certainly qualifies.  The violence we wreak upon each other (and by our careless actions, often upon ourselves) fades into insignificance by comparison to the purely natural violence out there in the cosmos.  Familiar phenomena like black holes and supernovas come near the top of the list, but there are others equally scary whose names are hardly common topics of conversation -- Wolf-Rayet stars, gamma-ray bursters, quasars, and Thorne-Zytkow objects come to mind, not to mention the truly terrifying possibility of a "false vacuum collapse" that I wrote about here at Skeptophilia a while back.

It's why I always find it odd when people talk about the how peaceful the night sky is, or that the glory of the cosmos supports the existence of a benevolent deity.  Impressive?  Sure.  Awe-inspiring?  Definitely.

Benevolent?  Hardly.  The suggestion that the universe was created to be the perfectly hospitable home to humanity -- the "fine-tuning" argument, or "strong anthropic principle" -- conveniently ignores the fact that the vast majority of the universe is intrinsically deadly to terrestrial life forms, and even here on Earth, we're able to survive the conditions of less than a quarter of its surface area.

I'm not trying to scare anyone, here.  But I do think it's a good idea to keep in mind how small and fragile we are.  Especially if it makes us more cognizant of taking care of the congenial planet we're on.

In any case, back to astronomical phenomena that are big and scary and can kill you.  Even the ones we know about don't exhaust the catalog of violent space stuff.  Take, for example, the (thus far) unexplained invisible vortex that is tearing apart the Hyades.

The Hyades is a star cluster in the constellation Taurus, which gets its name from the five sisters of Hyas, a beautiful Greek youth who died tragically.  Which brings up the question of whether any beautiful Greek youths actually survived to adulthood.  When ancient Greeks had kids, if they had a really handsome son, did they look at him and shake their heads sadly, and say, "Well, I guess he's fucked"?

To read Greek mythology, you get the impression that the major cause of death in ancient Greek was being so beautiful it pissed the gods off.

Anyhow, Hyas's five sisters were so devastated by the loss of their beloved brother that they couldn't stop crying, so the gods took pity on them even though Zeus et al. were the ones who caused the whole problem in the first place, and turned them into stars.  Which I suppose is better than nothing.  But even so, the sisters' weeping wouldn't stop -- which is why the appearance of the Hyades in the sky in the spring is associated with the rainy season. (In fact, in England the cluster is called "the April rainers.")

The Hyades [Image licensed under the Creative Commons NASA, ESA, and STScI, Hyades cluster, CC BY-SA 4.0]

In reality, the Hyades have nothing to do with rain or tragically beautiful Greek youths.  They are a group of fairly young stars, on the order of 625 million years old (the Sun is about ten times older), and like most clusters was created from a collapsing clump of gas.  The Hyades are quite close to us -- 153 light years away -- and because of that have been intensively studied.  Like many clusters, the tidal forces generated by the relative motion of the stars is gradually pulling them away from each other, but here there seems to be something else, something far more violent, going on.

A press release from the European Space Agency describes a study of the motion of the stars in the Hyades indicating that their movements aren't the ordinary gentle dissipation most clusters undergo.  A team led by astrophysicist Tereza Jerabkova used data from the European Southern Observatory to map members of the cluster, and to identify other stars that once were part of the Hyades but since have been pulled away, and they found that the leading "tidal tail" -- the streamer of stars out ahead of the motion of the cluster as a whole -- has been ripped to shreds.

The only solution Jerabkova and her team found that made sense of the data is that the leading tail of the Hyades collided -- or is in the process of colliding -- with a huge blob of some sort, containing a mass ten million times that of the Sun.  The problem is, an object that big, only 153 light years away, should be visible, or at least detectable, and there seems to be nothing there.

"There must have been a close interaction with this really massive clump, and the Hyades just got smashed," Jerabkova said.

So what is this "really massive clump" made of?  Given the absence of anything made of ordinary matter that is anywhere nearby, the team suggests that it might be something more exotic -- a "dark matter sub-halo."  These hypothesized objects could be scattered across the universe, and might provide the energetic kick to objects whose trajectories can't be explained any other way. But what exactly they are other than a bizarre phantom gravitational whirlpool, no one knows.

Nor what the risk is if we're close to one.

So add "dark matter sub-halos" to our list of scary astronomical phenomena.  I find the whole thing fascinating, and a little humbling.  I'll still find the beauty of a clear night sky soothing, but that's only if I can get my scientific mind to shut the hell up long enough to enjoy it.  Because the truth is, a lot of those twinkling lights are anything but peaceful.

But I suppose it's still better than the gods killing you if you're too handsome.  That would just suck, not that I personally am in any danger.

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In the dark

Dear Readers:

After today's post, I'm going to be taking a long-overdue break from Skeptophilia.  My intent -- lord willin' an' the creek don't rise, as my grandma used to say -- is that my next post will be Monday, May 13.  See you then!

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To further investigate our general topic of people giving woo-woo explanations to damn near everything, today we investigate: The Dark.

First, a brief physics lesson.

Things are generally called "dark" for one of two reasons.  First, there are objects whose chemical makeup results in their absorbing most of the light that falls on them.  Second, there are things that don't interact with light much at all, so they neither absorb nor reflect light -- light passes right through them.  An example of the first would be a charcoal briquet.  An example of the second would be interstellar space, which is sort of dark-by-default.

This whole thing comes up because of an extrasolar planet with the mellifluous name TrES-2b.  TrES-2b orbits the even more charmingly named GSC 03549-02811, a star about 718 light years away.  More interestingly, it has the distinction of being the darkest extrasolar planet yet discovered.  David Kipping, of the Harvard-Smithsonian Center for Astrophysics, stated, "TrES-2b is considerably less reflective than black acrylic paint, so it is truly an alien world."

Artist's conception of TrES-2b  [Image is in the Public Domain courtesy of NASA/JPL-Caltech]

That was all it took.  Whereas my reaction was, "Huh!  A Jupiter-sized charcoal briquet!  That's kinda cool," the woo-woos just couldn't resist wooing all over this story.  We now have the following speculations, all from websites owned by people who probably shouldn't be allowed outside unsupervised:

• TrES-2b is made of antimatter, and we shouldn't go there because it (and we) would blow up.  We know it's antimatter because antimatter has the opposite properties to matter, so it's dark.
• TrES-2b is made of "dark matter," and yes, they're not just talking about stuff that's black, they're talking about the physicists' "dark matter," about which I'll have more to say in a moment.
• TrES-2b is dark because it's being hidden by aliens who are currently on their way to Earth to take over.  Lucky for us we spotted it in time!
• TrES-2b is hell.  No, I'm not making this up.

Well.  You just opened the floodgates, now didn't you, Dr. Kipping?

The first two explanations left me with a giant bruise on my forehead from doing a faceplant while reading.  At the risk of insulting my readers' intelligence, let me just say quickly that (1) antimatter's "opposite properties" have nothing to do with regular matter being light and antimatter being dark, because if it did, the next time a kindergartner pulled a black crayon out of the box, he would explode in a burst of gamma rays; and (2) "dark matter" is called "dark" because of the second reason, that it doesn't interact with much of anything, including light, so the idea of a planet made of it is a little ridiculous, and in any case physicists haven't even proved that it exists, so if some astrophysicist found a whole freakin' planet made of it it would KIND OF MAKE HEADLINES ALL OVER THE FUCKING WORLD, YOU KNOW?

*brief pause to do some nice, slow deep breathing*

Sorry for getting carried away, there.  But I will reiterate something I have said more than once, in this blog; if you're going to start blathering on about science, for cryin' in the sink at least get the science right.  Even the least scientific woo-woo out there can read the Wikipedia page for "Dark Matter," for example, wherein we find in the first paragraph the sentence, "The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum." (Italics mine, and put in so that any of the aforementioned woo-woos who are reading this post will focus on the important part.)

And I won't even address the "secret alien base" and "hell" theories regarding TrES-2b, except to say that it should come as a relief that the evil aliens or Satan (depending on which version you went for) are safely 718 light years away.  To put this in perspective, this means that if they were heading here in the fastest spacecraft humans have ever created, Voyager 1, which travels at about 16 kilometers per second, it would still take them eleven million years to get here.

In any case, I guess it's all a matter of how you view what's around you.  I find the universe, and therefore science, endlessly fascinating, because what scientists have uncovered is weird, wonderful, and counterintuitive.  I don't need to start attaching all sorts of anti-scientific bunk to their discoveries -- nature is cool enough as it is.

Okay, thus endeth today's rant.  I will simply end with an admonishment to be careful next time you barbecue.  I hear those charcoal briquets can be made of antimatter, which could make your next cook-out a dicey affair.  You might want to wear gloves while you handle them.  Better safe than sorry!

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Cosmological conundrums

Three of the most vexing problems in physics -- and ones I've hit on a number of times here at Skeptophilia -- are:

1. dark matter -- the stuff that (by its gravitational influence) seems to make up 26% of the mass/energy of the universe, and yet has resisted every effort at detection or inquiry into what other properties it might have.
2. dark energy -- a mysterious "something" that is said to be responsible for the apparent runaway expansion of the universe, and which (like dark matter) has defied detection or explanation in any other way.  This makes up 69% of the universe's mass/energy -- meaning the ordinary matter we're made of comprises only 5% of the apparent content of the universe.
3. the conflict between the general theory of relativity (i.e. the theory of gravitation) and quantum physics.  In the realm of the very small (or at high energies), the theory of relativity falls apart -- it's irreconcilable with the nondeterministic model of quantum mechanics.  Despite over a century of the best minds in theoretical physics trying to find a quantum theory of gravity, the two most fundamental underpinnings of our understanding of the universe just don't play well together.

A while back I was discussing this with the fiddler in my band, who also happened to be a Cornell physics lecturer.  Her comment was that the mess physics is currently in suggests we're missing something major -- the same way that the apparent constancy of the speed of light in a vacuum, regardless of reference frame, created an intractable nightmare for physicists at the end of the nineteenth century.  It took Einstein coming up with the Theories of Relativity to show that the problem wasn't a problem at all, but a fundamental reality about how space and time work, to resolve it all.

"We're still waiting for this century's Einstein," Kathy said.

[Image licensed under the Creative Commons ESA/Hubble, Collage of six cluster collisions with dark matter maps, CC BY 4.0]

There's no shortage of physicists working on stepping into those shoes -- and just last week, two papers came out suggesting possible solutions for the first two problems.

One claims to solve all three simultaneously.

Both of them start with a similar take on dark matter and dark energy as Einstein did about the luminiferous aether, the mysterious substance that nineteenth-century physicists thought was the medium through which light propagated; they simply don't exist.  

The first one, from Rajendra Gupta of the University of Ottawa, proposes that the need for both dark matter and dark energy in the model comes from a misconception about how the laws of physics change on a cosmological time scale.  The prevailing wisdom has been "they don't;" the laws now are the same as the laws thirteen billion years ago, not long after the Big Bang.  Gupta suggests that making two modifications to the model -- assuming that the strength of the four fundamental forces of nature (gravity, electromagnetism, and the weak and strong nuclear forces) have decreased over time, and that light loses energy as it travels over long distances, explain all the astrophysical observations we've made, and obviates the need for dark matter and dark energy.

"The study's findings confirm that our previous work -- JWST early-universe observations and ΛCDM cosmology -- about the age of the universe being 26.7 billion years [rather than the usually accepted value of 13.8 billion years] has allowed us to discover that the universe does not require dark matter to exist," Gupta said.  "In standard cosmology, the accelerated expansion of the universe is said to be caused by dark energy but is in fact due to the weakening forces of nature as it expands, not due to dark energy."

The second, by Jonathan Oppenheim and Andrea Russo of University College London, suggests a different solution that (if correct) not only gets rid of dark matter and dark energy, but in one fell swoop resolves the conflict between relativity and quantum physics.  They propose that the problem is the deterministic nature of gravity; if a quantum-like uncertainty is introduced into gravitational models, the whole shebang works without the need for some mysterious dark matter and dark energy that no one has ever been able to find experimentally.

The mathematics of the model -- which, I must admit up front, are beyond me -- introduce new terms to explain the behavior of gravity at low accelerations, which are (not coincidentally) the regime where the effects of dark matter become apparent.  It's a striking approach; physicist Sabine Hossenfelder, who is generally reluctant to hop on the latest Grand Unified Theory bandwagon (and whose pessimism has been, unfortunately, justified in the past) writes in an essay on the new theory, "Reading Oppenheim’s new papers—published in the journals Nature Communications and Physical Review X—about what he dubs 'Post-Quantum Gravity,' I have been impressed by how far he has pushed the approach.  He has developed a full-blown framework that combines quantum physics with classical physics, and he tells me that he has another paper in preparation which shows that he can solve the problem of infinites that plague the Big Bang and black holes."

Despite this, Hossenfelder is still dubious about Post-Quantum Gravity.  "I don’t want to withhold from you that I think Oppenheim’s theory is wrong, because it remains incompatible with Einstein’s cherished principle of locality, which says that causes should only travel from one place to its nearest neighbours and not jump over distances," she writes.  "I suspect that this is going to cause problems sooner or later, for example with energy conservation.  Still, I might be wrong...  If Oppenheim’s right, it would mean Einstein was both right and wrong: right in that gravity remained a classical, non-quantum theory, and wrong in that God did play dice indeed.  And I guess for the good Lord, we would have to be both sorry and not sorry."

So we'll just have to wait and see.  If either of these theories is right, we're talking Nobel Prize material.  If the second one is right, it'd be the physics discovery of the century.  Like Sabine Hossenfelder, I'm not holding my breath; attempts to solve definitively the three problems I started this post with are, thus far, batting zero.  And I'm hardly qualified to make a judgment about what the chances are for these two.  But like many interested laypeople, I'll be fascinated to see which way it goes -- and to see if we might, in the words of my bandmate/physicist friend, be "looking at the twenty-first century's Einstein."

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MoND denied

Almost exactly a year ago, I wrote a post about MoND -- modified Newtonian dynamics -- a new(ish) model of gravitation that purported to explain some of odd measurements of stellar and galactic motion without the necessity of dark matter.

Here's the situation.

In the 1970s and 1980s, the brilliant astronomer Vera Rubin discovered something peculiar.  The project she was working on involved mapping the speed of revolution of stars around galactic centers.  According to Newton's Law of Gravitation and Kepler's Laws of Planetary Motion (which, after all, have the same mathematical underpinning), stars farther away from the center should be moving more slowly.  This principle works fine, for example, in our own Solar System; Neptune moves more slowly than Mercury does.

This, Rubin found, turned out not to be true on larger scales.  The velocities of stars in the farther reaches of galaxies were moving just as fast as the ones closer to the center.  Nicknamed the flat rotation curve problem, it seemed like the only possible explanation was that there was more mass in the galaxy than had been detected -- something appeared to be causing the outer stars to orbit faster than Newtonian dynamics said they should.

Rubin and others called this mysterious something dark matter. 

And you probably know the amount of this stuff is significant.  If you add up all the detectable mass/energy in the universe, only 5% of it is ordinary matter.  26.8% of it is dark matter, and 68.2% is dark energy, an unrelated type of mass/energy that is thought to be responsible for the runaway expansion of the universe, and which is even less understood than dark matter is.

Dark matter interacts with regular matter via gravity, but -- as far as we can tell -- in no other way.  It seems to be completely unaffected by the other forces that act on the ordinary stuff we see on a day-to-day basis.  There have been various experiments set up to try to detect dark matter particles, but as of the time of this writing, every single one of them has come up empty-handed.  It's bizarre to think about; a substance that makes up five times more of the mass of the universe than all the regular matter put together, and thus far, we haven't the slightest idea what it's made of.

There's also the problem that the Standard Model -- the framework that accounts for all the ordinary matter particles, and how they interact -- is one of the most rigorously-tested theories in science, and performs to a level of precision that beggars belief.  And nothing in the Standard Model appears to admit of some strange extra particle(s) that might account for dark matter.

Well, along came Mordehai Milgrom, who in 1983 tried something a little like what Einstein did with respect to the luminiferous aether -- he said, "I have a new theory that eliminates the need for dark matter entirely."  It's called modified Newtonian dynamics, MoND for short, and proposes that the problem is that Newton's Law of Gravitation doesn't work for objects experiencing really low accelerations (like the stars in the outer reaches of galaxies).  Like the Theories of Relativity, it leaves the model relatively unchanged at the velocities and accelerations we encounter on a daily basis; here on the surface of the Earth, Newton still works just fine.  But what Einstein did for systems in extreme gravitational fields or high velocities, Milgrom did for systems experiencing really low accelerations -- tweaking the mathematics to make it match the observations.

And those tweaks, in one stroke, eliminated the need for some hypothetical and undetectable form of matter.

The Whirlpool Galaxy [Image licensed under the Creative Commons NASA/ESA/JPL/Hubble Heritage Team & C. Violette, M51 (2), CC BY-SA 4.0]

Well, the race was on to try to figure out which model was correct -- Newton (with dark matter) or MoND (without it).  And earlier this year, Korean astrophysicist Kyu-Hyun Chae seemed to have settled it once and for all, showing that observations of wide binaries -- pairs of stars orbiting their common centers of gravity at large distances -- matched the predictions of MoND brilliantly.

However, there were astrophysicists who immediately had objections.  Chae, they said, had not done a good job of eliminating data points that were problematic.  The difficulty is that if you're observing a binary pair from Earth, to figure out the velocities and accelerations of the stars in the pair, you have to take into account a variety of complicating factors, including:

• the speed the entire system is moving toward or away from Earth
• the eccentricity (elliptical-ness) of the orbit
• the inclination of the orbit -- how much it's tilted toward or away from us

So a group of researchers, led by astrophysicist Indranil Banik of the University of St. Andrews, has developed a technique for sifting through the data points and using the ones for which there is the best confidence in the velocity measurements (significantly, Banik's team only eliminated about twenty percent of Chae's data points).  And in their paper, which came out five days ago, they found when they do that, the agreement of the data with MoND vanishes completely.

Without the wonky data points, the measurements from wide binaries that seemed to settle the argument in favor of MoND actually agree with Newtonian dynamics...

... to a confidence of 19 σ.  To us non-scientists -- as the wonderful YouTuber Dr. Becky Smethurst explains it -- a 19 σ confidence level means there is only a one in one hundred thousand trillion trillion trillion chance that their result is a statistical fluke.  (And if you want to know more, I highly recommend watching Dr. Becky's video on the new paper, which is awesome.)

So with regards to theories of gravity, it appears that Newton is the only game in town, meaning we're stuck with dark matter.  MoND is dead in the water, so unless someone comes up with some sort of different model entirely that matches the data better than Newton does, we'll have to keep looking for this ghostly matter whose only fingerprint is its gravity.

It's an exciting time to be an astrophysicist... or just a deeply curious science nerd.

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Newton modified

Back in the 1970s and 1980s, astrophysicist Vera Rubin discovered something odd about the rates at which stars were revolving around their home galaxies; the stars in the outer reaches of the galaxy were orbiting as quickly as the ones nearer to the center.

Called the "flat rotation curve problem," this observation flies in the face of an astronomical principle that's been known since the seventeenth century, which is Kepler's Third Law.  Kepler's Third Law states that for bodies orbiting the same center of gravity, the square of the orbital period (time taken for the object to make a single orbit) is proportional to the cube of the average distance between the object and the center of gravity.  Put more simply, the farther out an orbiting object is, the slower it should be moving.  This law holds beautifully for the planets, asteroids, and comets in the Solar System.

Unfortunately, when Rubin looked at galactic rotation rates, she found that Kepler's Third Law appeared not to hold.  What it looked like was that there was a great deal more mass in the galaxy than could be seen, and that mass was spread out in some kind of invisible halo surrounding it.  That additional mass would account for the flatness of the rotation curves.

It was forthwith nicknamed dark matter.

The calculations of Rubin and others showed that the amount of dark matter was not insignificant.  Current estimates place it at around 27% of the total mass of the universe.  Only 5% is baryonic (ordinary) matter, so the matter we can't see outweighs ordinary matter by over a factor of five.  (The other 68% is the even weirder and more elusive dark energy, about which we know next to nothing.)

The problem is, every experiment designed to directly detect dark matter has resulted in zero success.  Whatever it is, it seems not to interact with ordinary matter at all other than via its gravitational pull.  These repeated failures drew rueful comparisons between dark matter and the luminiferous aether, the mysterious substance through which light waves were alleged to propagate.  The aether was proposed back in the nineteenth century because it was hard to imagine how light waves moved through a vacuum unless it had a medium -- what, exactly, was waving?  The existence of aether was conclusively disproven by the elegant Michelson-Morley experiment, which showed that unlike any other kind of wave, the speed of light waves seemed to be invariant regardless of the direction of motion of the observer.  It remained for Albert Einstein to explain how that could possibly be -- and to figure out all the strange and counterintuitive outcomes of this phenomenon, with his Special and General Theories of Relativity.

More than one modern physicist has surmised that dark matter might similarly be the result of a fundamental misunderstanding of how gravity works -- and that we are just waiting for this century's Einstein to turn physics on its head by pointing out what we've missed.

Enter Israeli physicist Mordehai Milgrom.

Milgrom is the inventor of MoND (Modified Newtonian Dynamics), a model which -- like the Theories of Relativity -- proposes that the explanation for the anomalous observations is not that there's some unseen and undetectable substance causing the effect, but that our understanding of how physics works is incomplete.  In particular, Milgrom says, there needs to be a modification to the equations of motion at very small accelerations, such as the ones experienced by stars orbiting in the outer reaches of galaxies.

With those modifications, the orbital rates make perfect sense.  No dark matter needed.

The Whirlpool Galaxy [Image licensed under the Creative Commons NASA/ESA/JPL/Hubble Heritage Team & C. Violette, M51 (2), CC BY-SA 4.0]

As with relativity -- and any other time someone has claimed to overturn a long-established paradigm -- MoND hasn't achieved anywhere near universal acclaim.  The Wikipedia article on it (linked above) states, gloomily, "no satisfactory cosmological model has been constructed from the hypothesis."  And it does lack the blindingly bright insight of Einstein's models, where taking the "problem of the seeming invariance of the speed of light" and turning it into the "axiom of the actual invariance of the speed of light" triggered a shift in our understanding that has since passed every empirical test ever designed.  Compared to Einstein's model, MoND almost seems like "Newton + an add-on," with no particularly good explanation as to why high accelerations obey Newton's laws but low ones don't.  (Of course, there's a parallel here to Einstein, as well -- at low speeds, Newton's laws are accurate, while at near-light speeds, Einsteinian effects take over.  So maybe Milgrom is on to something after all.)

After all, it's not like the other option -- dark matter -- has much going for it experimentally.

And MoND just got a significant leg up with an observation of the behavior of star clusters that was the subject of a paper in Monthly Notices of the Royal Astronomical Society last week.  In open star clusters, as new stars ignite it produces an outward push that can blow away material (including other stars), creating two "tidal tails" that precede and trail the cluster as it moves through space.  According to Newtonian dynamics (with or without dark matter), the two tails should have about the same mass.

"According to Newton's laws of gravity, it's a matter of chance in which of the tails a lost star ends up," explains Dr. Jan Pflamm-Altenburg of the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn.  "So both tails should contain about the same number of stars.  However, in our work we were able to prove for the first time that this is not true: In the clusters we studied, the front tail always contains significantly more stars nearby to the cluster than the rear tail."

This peculiar observation fits the predictions of MoND much better than it does the predictions of the Newtonian model.

"The results [of simulations using MoND] correspond surprisingly well with the observations," said Ingo Thies, co-author of last week's paper.  "However, we had to resort to relatively simple computational methods to do this.  We currently lack the mathematical tools for more detailed analyses of modified Newtonian dynamics."

So the matter is very far from settled.  What's certain is that, similar to the physicists' situation in the late nineteenth century with regards to the behavior of light, there's something significant we're missing.  Whether that's some odd form of matter that doesn't interact with anything except via gravity, or because we've got the equations for the laws of motion wrong, remains to be seen.

And of course, after that, we still have dark energy to explain.  I think the physicists are going to be busy over the next few decades, don't you?

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Supernothing

The things that keeps astrophysicists up at night are the irritating little questions about the universe that are simple to ask, and wildly difficult to answer.

Of course, they probably like being kept up at night.  Part of the job, really.

In any case, one of the most curious is why the universe is almost isotropic, but not quite.  "Isotropic" means, basically, "the same everywhere you look."  You can pick out any point in the night sky, and the amount of matter and energy within that region should be the same as if you picked out somewhere else.  Now, there are local conglomerations of matter -- you're residing on one, and working your way up the size ladder, the Solar System and the Milky Way are both clumps with higher matter density than the surrounding regions -- but on the largest scales, you'd expect things to be evenly spread out.

When I first ran into the idea of the Big Bang as a teenager, this was one of the hardest things for me to grasp.  If there really was a giant explosion at the beginning of the universe, why can't we find out where that explosion occurred?  You'd expect high matter density in that direction, and low density at the antipodal spot in the sky.  In fact, you see no such thing.  But far from being an argument against the Big Bang, it's an argument in its favor.  I didn't understand why until I took an astronomy class in college, and the professor, Dr. Whitmire, explained it as follows:

Imagine you're on the surface of an enormous balloon, and the surface is covered with dots.  You're standing on one of the dots.  Then, someone inflates the balloon.  What do you see?  You see all the other dots moving away from you, and in every direction, there are just about equal numbers of dots.  It's isotropic -- similar densities and recession speeds no matter where you look.  It doesn't depend on your perspective; you didn't just happen to choose the one dot that was at the center of the expansion.  It would look the same if you were standing on any other dot.  The reason is that the dots aren't moving through space; the space itself -- the surface of the balloon -- is expanding, carrying the dots with it.

"So there is no center of the universe," Dr. Whitmire said.  "Or everywhere is the center.  It amounts to the same thing."

In the first milliseconds after the Big Bang, the expansion rate was so fast that it smoothed everything out, spreading matter and energy fairly uniformly (again, allowing for localized clumps to form, but even the clumps would be expected to have a uniform distribution, like chocolate chips in cookie dough).  When the cosmic microwave background radiation was discovered in 1965 by Arno Penzias and Robert Wilson, it was powerful evidence for the Big Bang Model, especially when they found that -- like matter -- the CMBR was isotropic: the same no matter where you looked.

Well, almost.  One of the annoying little questions I mentioned in the first paragraph is that the CMBR is nearly isotropic -- but there are "cold spots," which have a lower temperature than the surrounding regions.  I'm not talking about a big difference, here; the average temperature in interstellar space is 2.7 K (-270.5 C), and the largest of these cold spots -- the Eridanus Supervoid -- is 0.00007 K lower.  The difference was small enough that at first it was thought to  be a glitch in the equipment or some sort of error in the data, but repeated measurements by the Wilkinson Microwave Anisotropy Probe (WMAP) has found that it is, in fact, a real phenomenon.

[Image licensed under the Creative Commons Piquito veloz, Eridanus supervoid in celestial sphere, CC BY-SA 4.0]

The "Eridanus Supervoid" is a name for the universe's largest collection of nothing.  It's a region on the order of between 500 million and one billion light years in diameter, in which there is so little matter that if the Earth sat in the center of it, you wouldn't be able to see a single star in the night sky.  It wouldn't have been until the 1960s that we would have found out about the existence of stars and galaxies, at the point that there were telescopes powerful enough to see something that distant.

This empty spot is a bit of a bother to cosmologists.  During the "inflationary period" -- thought to be between 10 ^-36 and 10 ^-33 seconds after the Big Bang -- space was stretching so unimaginably fast that it smoothed out most of the local variability, rather like taking a crumpled-up bedsheet and having four people pull on the corners; most of the wrinkles and folds disappear.

So what caused the Eridanus Supervoid?  Are we left with, "Well, it just happened because it happened?"

A new study hasn't exactly answered the question, but has generated another piece of data -- and a partial explanation.  A paper in Monthly Notices of the Royal Astronomical Society describes research that uses information from WMAP and from the Dark Energy Survey to see what's different about that region of space, and they found something curious.  The mysterious and elusive "dark matter" -- a component of the universe that amounts to 27% of its detectable mass, and six times more than all the ordinary matter put together -- has as its sole observable characteristic its gravitational effects on the matter and space around it, and that's measurable even if you can't see it, because it bends the path of light passing through it.  (The "gravitational lensing effect.")  And the recent study found that the Eridanus Supervoid has way less dark matter than is normal for other regions in the universe.  As it expands, it becomes a sink for energy -- a photon crossing it is moving through successively more stretched-out space, and its energy drops, as does its frequency.  The photon, therefore, is red-shifted, not because its source is moving away from us, but because it's traveling through expanding space.

As study co-author Juan Garcia-Bellido, of the Institute for Theoretical Physics at the University of Madrid, explained:

Photons or particles of light enter into a void at a time before the void starts deepening, and leave after the void has become deeper.  This process means that there is a net energy loss in that journey; that’s called the Integrated Sachs-Wolfe effect.  When photons fall into a potential well, they gain energy, and when they come out of a potential well, they lose energy.  This is the gravitational redshift effect.

Then once the region became a little less dense than the surrounding areas, every photon that crossed through it dropped its temperature and energy density a little more.

This still doesn't explain where the original anisotropy came from; the current thought is that it was caused by random fluctuations on the quantum level when the universe was still smaller than a grain of sand.  At that scale and energy, quantum effects loom large, and any minor unevenness might get "locked in" to the pattern of the universe; after that the process described by Garcia-Bellido takes over and makes it bigger.

And 13.7 billion years later, we have a huge blob of space that is just about completely empty, and ridiculously cold.  The Eridanus Supernothing.

So that's our excursion into deep space for the day.  And some more data on one of those mysterious questions that have, thus far, defied all attempts to answer them.  I'm nowhere near an expert, but I'm still endlessly fascinated with these sorts of things -- even if all we've got at the moment are unsatisfying partial solutions.

*******************************

It's obvious to regular readers of Skeptophilia that I'm fascinated with geology and paleontology.  That's why this week's book-of-the-week is brand new: Thomas Halliday's Otherlands: A Journey Through Extinct Worlds.

Halliday takes us to sixteen different bygone worlds -- each one represented by a fossil site, from our ancestral australopithecenes in what is now Tanzania to the Precambrian Ediacaran seas, filled with animals that are nothing short of bizarre.  (One, in fact, is so weird-looking it was christened Hallucigenia.)  Halliday doesn't just tell us about the fossils, though; he recreates in words what the place would have looked like back when those animals and plants were alive, giving a rich perspective on just how much the Earth has changed over its history -- and how fragile the web of life is.

It's a beautiful and eye-opening book -- if you love thinking about prehistory, you need a copy of Otherlands.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

The cosmic whirligig

It seems like whenever I look at the realm of the very large or the very small, I quickly get overwhelmed by scale.

I remember, for example, when a teacher in high school was trying to impress upon us kids how small atoms were, and asked us the following question: if you counted up the number of atoms in a typical raindrop, then someone gave you that many grains of sand, how much sand would you have?

A bucket?  A swimming pool full?  A whole beach full?  All of those, it would seem, constitute a crapload of sand grains.  Surely there can't be more atoms in a raindrop than there are sand grains on a typical beach.

But there are.  By several orders of magnitude.  Her answer was that you'd have enough sand to fill a trench a meter deep and a kilometer across, stretching from New York to San Francisco.  (I've never checked her math, but from other similar analogies, it seems pretty spot-on.)

The same happens when I'm considering things that are very large; as much as I've studied astronomy, I never fail to be blown away simply by how enormous the universe is.  In fact, this is why the topic comes up -- a paper in Nature Astronomy last week by astrophysicists Peng Wang and Noam Liebeskind (of the University of Potsdam), Elmo Tempel (of the University of Tartu, Estonia), Xi Kang (of Zhejiang University, and Quan Guo (of Shanghai Astronomical Observatory) has demonstrated that there are filaments spanning entire galactic superclusters, and possibly longer than that.

[Image licensed under the Creative Commons The cosmic web, CC BY-SA 4.0]

The presence of these filaments, which seem to be composed largely of dark matter, comes from their effects on the galaxies they pass near.  As if they were the axle of an enormous whirligig, the filaments cause the galaxies to circle around them, drawn in by the gravitational pull.  The existence of the filaments was demonstrated by the fact that the galaxies on one side exhibit a lower than expected red shift and the ones on the other side a higher than expected red shift, meaning one side is moving away from us and the other side toward us -- just as you'd expect if the galaxies were circling some invisible center of gravity.

As with any groundbreaking discovery, it's opened up as many questions as it's answered.  "It's a major finding,” said study co-author Noam Libeskind, in an interview with Vice.  "It's a pretty big deal that we've discovered angular momentum, or vorticity, on such a huge scale.  I think it will help people understand cosmic flows and how galaxies are moving throughout the cosmic web and through the universe... [and] to understand the important scales for galaxy formation and ultimately, why everything in the universe is spinning and how spin is generated.  That is a really, really hard question to solve.  It's an unsolved question in cosmology."

That was my first reaction; what on earth (or off it, in this case) could generate that kind of angular momentum?  Think of the mass of a typical galaxy, and imaging that you tie that amount of mass at the end of a long rope and try to swing it in circles.

That's the quantity of energy we're talking about, here.  Multiplied by the number of galaxies in the universe.

But the upshot is that the universe on the largest scales seems to have an intrinsic spin, and no one knows why.  All I know is that it makes me feel very, very small.

Of course, I'm way larger than the atoms in a raindrop.  So there's that.  Now that my mind is sufficiently blown, I think I need to go huddle under my blanket for a while, because the universe is sometimes a really overwhelming place to live.

*************************************

One of the most devastating psychological diagnoses is schizophrenia.  United by the common characteristic of "loss of touch with reality," this phrase belies how horrible the various kinds of schizophrenia are, both for the sufferers and their families.  Immersed in a pseudo-reality where the voices, hallucinations, and perceptions created by their minds seem as vivid as the actual reality around them, schizophrenics live in a terrifying world where they literally can't tell their own imaginings from what they're really seeing and hearing.

The origins of schizophrenia are still poorly understood, and largely because of a lack of knowledge of its causes, treatment and prognosis are iffy at best.  But much of what we know about this horrible disorder comes from families where it seems to be common -- where, apparently, there is a genetic predisposition for the psychosis that is schizophrenia's most frightening characteristic.

One of the first studies of this kind was of the Galvin family of Colorado, who had ten children born between 1945 and 1965 of whom six eventually were diagnosed as schizophrenic.  This tragic situation is the subject of the riveting book Hidden Valley Road: Inside the Mind of an American Family, by Robert Kolker.  Kolker looks at the study done by the National Institute of Health of the Galvin family, which provided the first insight into the genetic basis of schizophrenia, but along the way gives us a touching and compassionate view of a family devastated by this mysterious disease.  It's brilliant reading, and leaves you with a greater understanding of the impact of psychiatric illness -- and hope for a future where this diagnosis has better options for treatment.

[Note: if you purchase this book from the image/link below, part of the proceeds goes to support Skeptophilia!]

 

The cosmic spiderweb

I've posted here before about dark matter, the mysterious substance which -- if current models are even close to correct -- makes up 27% of the matter/energy density of the universe.  Even more mysterious is dark energy, which is thought to be responsible for the acceleration of the universe's expansion; that's another 68% percent.

Yes, the implication is what it sounds like.  Ordinary matter -- like you, me, the Earth, all the planets and stars and galaxies -- accounts for a measly 5% of the matter/energy in the universe.

Most frustrating of all is how little we know for sure about dark matter and dark energy.  To date, every experiment to detect a particle of either one has failed.  In fact, we don't even know if either one is made of particles.  (If not, what it might be made of is an open question.)  Some scientists have compared it to the luminiferous aether, which according to nineteenth century physicists was the substance through which light waves allegedly propagated.  Every wave they knew about traveled in some sort of medium; water waves, sound waves in the air, vibrations in a fiddle string.  That light might travel in a vacuum -- that it might not need a medium -- was incomprehensible.

In the words of one of my college physics professors: "If light waves don't need a medium, then what, exactly, is waving?"

It took Einstein to come up with the answer to this, and in the process proved that the luminiferous aether didn't exist.  The result revolutionized physics.  I've heard physicists say that dark matter and dark energy are this century's aether -- artifacts of measurement created by a fundamental piece of our model being misunderstood, missing, or flat-out wrong.

Be that as it may, if dark matter is an error, it's a pretty persistent one.  We haven't been able to detect it other than by its gravitational signature, but that signature is a bold flourish.  The discovery of it, by astronomer Vera Rubin and others, came about because measurements of the spin rate of galaxies indicated there had to be some extra mass holding them together; at the measured rotation rates, they should fly apart.  That extra mass turned out to be huge.  The best estimates were that there had to be over five times as much of this invisible matter as there was ordinary matter -- and that estimate held for every galaxy studied, so it wasn't a local phenomenon.

A paper last week in The Astrophysical Journal adds a new layer to dark matter not being local.  Astrophysicists Sungwook Hong, Donghui Jeong, Ho Seong Hwang, and Juhan Kim, of Pennsylvania State University, have created the most detailed map yet of the dark matter in the universe, using the known motion of seventeen thousand galaxies.  Strangest of all is that whatever this weird, invisible -- but extremely common -- substance is, it's not distributed uniformly.

Not only are there lumps of it within galaxies, holding them together, there are long filaments of dark matter between them -- threading the galaxies together like dewdrops clinging to an enormous spiderweb.

Another feature that makes the spiderweb analogy even more apt is that there are huge voids, nearly devoid of... just about everything, including dark matter.  One of them, the Boötes Void, is 330 million light years across.

[Image licensed under the Creative Commons El C at English Wikipedia., Boovoid, CC BY-SA 2.5]

To put that number in some perspective, if you took the Sun and the rest of the Solar System and put it in the middle of the Boötes Void, the night sky would be completely dark.  No stars at all.  In fact, it wouldn't have been until the 1960s that we would have had telescopes powerful enough to see the nearest galaxies; until that time, we would have thought the Sun was the only star in the entire universe.

So whatever dark matter is, we're gradually closing in on it.  We know how it affects ordinary matter gravitationally, and now we have a map of how it's distributed in the universe.  Maybe soon we'll have an idea of what it actually is.

Of course, then we still have dark energy to tackle, and there's over twice as much of that stuff as there is dark matter.  So, as is usual in science, we're not going to run out of mysteries to investigate any time soon.

*************************************

Astronomer Michio Kaku has a new book out, and he's tackled a doozy of a topic.

One of the thorniest problems in physics over the last hundred years, one which has stymied some of the greatest minds humanity has ever produced, is the quest for finding a Grand Unified Theory.  There are four fundamental forces in nature that we know about; the strong and weak nuclear forces, electromagnetism, and gravity.  The first three can now be modeled by a single set of equations -- called the electroweak theory -- but gravity has staunchly resisted incorporation.

The problem is, the other three forces can be explained by quantum effects, while gravity seems to have little to no effect on the realm of the very small -- and likewise, quantum effects have virtually no impact on the large scales where gravity rules.  Trying to combine the two results in self-contradictions and impossibilities, and even models that seem to eliminate some of the problems -- such as the highly-publicized string theory -- face their own sent of deep issues, such as generating so many possible solutions that an experimental test is practically impossible.

Kaku's new book, The God Equation: The Quest for a Theory of Everything describes the history and current status of this seemingly intractable problem, and does so with his characteristic flair and humor.  If you're interesting in finding out about the cutting edge of physic lies, in terms that an intelligent layperson can understand, you'll really enjoy Kaku's book -- and come away with a deeper appreciation for how weird the universe actually is.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]