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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Bang or whimper

I've always loved Robert Frost's razor-sharp poem, written in 1920, called "Fire and Ice":

Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.

How the world will end has fascinated people for as long as we've been able to think about the question.  Various mythologies created their own pictures of the universe's swan song -- the best-known of which is the Norse tale of Ragnarök, when the forces of good (the Æsir, Vanir, and their allies) teamed up against the forces of evil (the Jötnar, trolls, and various Bad Guys like Surtr, the trolls, Midgard's Serpent, Níðhöggr, and, of course, Loki).  Interestingly, in the Norse conception of things, good and evil were pretty evenly matched, and they more or less destroyed each other; only a few on either side survived, along with enough humans to repopulate the devastated world.

Once we started to take a more rational view of things, scientists naturally brought their knowledge to bear on the same question.  After figuring out about stellar mechanics, we've become fairly certain that the Earth will meet its end when the Sun runs out of hydrogen fuel, swells up into a red giant -- at which point it's likely the Earth's orbit will be inside the radius of the Sun -- then ultimately jettisons its outer atmosphere to become a white dwarf.  

But what about the universe as a whole?

When I was in school, just about everyone (well, just about everyone who understood science, anyhow) accepted that the universe had begun at the Big Bang.  The mechanism for what caused it, and what (if anything) had come before it, was unknown then and is still unknown now; but once it occurred, space expanded dramatically, carrying matter and energy with it, an outward motion that is still discernible in the red shift of distant galaxies.  But would that expansion go on forever?  I think the first time I ran into a considered answer to the question was in Carl Sagan's Cosmos, where he explained that the ultimate fate of the universe depended on its mass.  If the overall mass of the universe was above a particular quantity, its gravity would be sufficient to halt the expansion, ultimately sending everything hurtling backward into a "Big Crunch."  Below that critical quantity -- the expansion would slow continuously but would nevertheless keep going, spreading everything out until it was a uniform, thin, cold gas, a fate that goes by the cheery name "the Heat Death of the Universe."

But it turned out the picture wasn't even that simple.  In 1998, Adam Riess and others discovered the baffling fact that the universe wasn't slowing at all, so neither of the above scenarios seemed to be right.  Data from distant galaxies showed -- and it has since been confirmed over and over -- that the universe's expansion is accelerating.  The existence of a repulsive force powering the expansion was proposed, and nicknamed dark energy, but how that could possibly work was (and is) unknown.

Then they found out that dark energy comprises just shy of three-quarters of the universe's total mass-energy.  Physicists had a huge conundrum to explain.

[Image licensed under the Creative Commons NASA/ESA, SN1994D, CC BY 3.0]

It also led to another possibility for the universe's fate, and one that's even more dire than the Heat Death.  If the amount of dark energy per unit volume of space is constant -- which it appeared to be -- then the relative proportion of dark energy will increase over time, because conventional matter and energy is thinning out as space expands (and dark energy is not).  As this happens, the relative strength of the dark energy repulsion will eventually increase to the point that it overwhelms all other forces, including electromagnetism and the nuclear forces -- tearing matter up into a soup of fundamental particles.

The "Big Rip."

Confused yet?  Because the reason all this comes up is that there's just been another discovery, this one by DESI (the Dark Energy Spectroscopic Instrument) indicating fairly strongly that the force of dark energy has been decreasing over time.  I say "fairly strongly" because at the moment the data sets this is based on range from 2.8 to 4.2 sigma (this is an indicator of how strongly the data supports the claim; for reference, 3 sigma represents a 0.3% possibility that the data is a statistical fluke, and 5 sigma is considered the threshold for breaking out the champagne).  So it appears that although the quantity of dark energy per unit volume of space is constant, the strength of the dark energy force is less now than it was in the early universe.

So what does this mean about the fate of the universe?  Will it be, in Frost's terms, fire or ice?  A bang or a whimper?  We don't know.  The first thing is to figure out what the hell dark energy actually is, and how it works, and -- if the DESI results hold up -- why it seems to be diminishing.

All I can say is the cosmologists have a lot of explaining to do.

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Celestial smashup

Just about everyone with even a passing interest in astronomy knows that the universe is expanding.

Ever since Edwin Hubble realized back in 1929 that almost everything outside of our own galaxy is redshifted (moving away from us), and that the degree of a galaxy's redshift is proportional to its distance from us -- something that has since been named Hubble's Law -- we've known that space is getting larger.  So, Hubble and others reasoned, if you run the clock backwards, there must have been a time when everything was collapsed together into one colossally dense point, that then for some reason that is still unknown, began to rush outward.

In other words, the Big Bang, which seems to have happened about 13.8 billion years or so ago, give or take a day or two.

However, that doesn't mean that everything is moving apart.  Within our own galaxy, there's enough mutual gravitational pull from all the massive objects therein to overcome the expansion, at least for now.  (Whether that'll continue forever remains to be seen; hold that thought, I'll get back to it.)  Even outside of our own galaxy, the members of the Local Group are gravitationally bound, and in fact, the nearest galaxy to us, Andromeda, is moving toward us at the impressive speed of 110 kilometers per second, so the Milky Way and Andromeda will eventually collide.

There are two reasons you shouldn't fret about this.  The first is that it's not going to happen for something like three billion years.  The other is that usually when two galaxies collide, shifts in the gravitational field fling stuff around, but very few collisions are expected to occur between individual stars.  Galaxies are, in fact, mostly empty space; if the Sun was the size of a typical orange and was sitting in the middle of downtown Washington D.C., the nearest star (Proxima Centauri) would be a slightly smaller orange... in San Francisco.

So while the alterations in mass distribution during a collision might throw stuff around a bit, and certainly change the shape of both galaxies, it's unlikely that any intelligent civilizations in the new combined Andromilkyway would be otherwise perturbed by it.

Note, however, I said that this is the case when two galaxies collide usually.

A paper last week in Monthly Notices of the Royal Astronomical Society describes a collision that occurred in a cluster of galaxies called "Stephan's Quintet," located (fortunately) about 290 million light years from here.  Recall my saying that the Andromeda Galaxy and Milky Way are moving toward each other at 110 kilometers per second; this enormous wreck happened eight times faster than that, with a speed that has generated a tremendous shock wave akin to a sonic boom in space.

Stephan's Quintet, showing the region affected by the collision [Image credit: Arnaudova et al., University of Hertfordshire]

"Since its discovery in 1877, Stephan's Quintet has captivated astronomers, because it represents a galactic crossroad where past collisions between galaxies have left behind a complex field of debris," said Marina Arnaudova of the University of Hertfordshire, who led the research.  "Dynamical activity in this galaxy group has now been reawakened by a galaxy smashing through it at an incredible speed of over 2 million mph (3.2 million km/h), leading to an immensely powerful shock, much like a sonic boom from a jet fighter.  As the shock moves through pockets of cold gas, it travels at hypersonic speeds – several times the speed of sound in the intergalactic medium of Stephan’s Quintet – powerful enough to rip apart electrons from atoms, leaving behind a glowing trail of charged gas, as seen with WEAVE [the William Herschel Telescope Enhanced Area Velocity Explorer]."

Which actually spells "WHTEAVE," but the discovery is cool enough that we'll let that slide.

The shock wave also compresses that interstellar gas and causes it to emit radio waves, which confirmed Arnaudova's team's discovery.

So locally, stuff can certainly move together, sometimes violently, even though the overall trend of the universe is to expand.

But.

According to a recent study by the Dark Energy Survey Project, there's a possibility that the amount of dark energy has changed over the life of the universe -- and is changing in such a way that it will affect the universe's ultimate fate.  If the amount of dark energy per unit volume of space were constant, it would mean that its effects on expansion would increase over time (since matter is thinning out, and the gravitational pull of matter is what's holding things together).  Thus, its outward pressure would proportionally increase, eventually overcoming all other attractive forces and ripping everything apart down to the constituent atoms.

This has always seemed to me to be a rather dismal prospect, not that I'll be around to see it.  Everything spread out in a thin soup of subatomic particles, and that's that.

But the new data suggests that the amount of dark energy is actually decreasing over time, meaning that its effects will gradually diminish -- and gravity will win, resulting in a "Big Crunch."  Everything turning around, falling inward, and ultimately colliding in a colossal smashup that might perhaps rebound in another Big Bang, and a new universe that resets the dials and starts it all over.

I first ran into this "oscillating universe" model when I took an astronomy class in college, and I thought it was a pretty cool idea; certainly better than the "Big Rip" that's predicted if the amount of dark energy per unit volume of space is a constant.  The point is still being debated, and (much) more data is needed to determine which is correct; but I, for one, would love it if the laws of nature were such that the universe might go through an unlimited number of bounces, and the whole game would begin again.

Maybe, just maybe, with any sentient life forms that evolve in Universe v. 2.0 getting a shot at doing it better next time.

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Quantum foams and tiny wormholes

One of the most frustrating things for insatiably curious laypeople like myself is to find that despite our deep and abiding interest in a topic, there's simply a limit to what we're capable of understanding.

I know that happened to me with mathematics.  All through grade school, and even into college, I found math to be one of my easiest subjects.  I never had to struggle to understand it, and got high grades without honestly trying all that hard.

Then I hit Calculus 3.

I use the word "hit" deliberately, because it felt like running into a brick wall.  I think the problem was that this was the point where I stopped being able to visualize what was going on, and without that concrete sense of why things worked the way they did, it turned into memorization and application of a set of what appeared to be randomly-applied rules, a technique that only worked when I remembered them accurately.  I lost the intuitiveness of my earlier experience.  It returned to some extent when I took Differential Equations (partly due to a stupendous teacher), but I went from there to Vector Calculus, and it was all over.

That was the moment I decided that I am a Bear Of Very Little Brain, and the effect of the experience (combined with a similar unfortunate roadblock in Classical Mechanics) convinced me that a career as a physicist was not in the cards.

That feeling came back to me full-force when I ran across a paper in the journal Physical Review D entitled "Dark Energy from Topology Change Induced by Microscopic Gauss-Bonnet Wormholes," by Stylianos A. Tsilioukas, Emmanuel N. Saridakis, and Charalampos Tzerefos, of the University of Thessaly.  Even reading the abstract left me with an expression rather like the one my puppy has when I try to explain a concept to him that is simply beyond his comprehension, like why he shouldn't eat my gym socks.  You can tell he's trying to understand, he clearly wants to understand, but it's just not getting through.

But as far as the paper goes, at least I can tell that the idea is really cool, so I'm going to attempt to tell you about it.  If there are any physics boffins in the studio audience who want to correct my misapprehensions or misstatements, please feel free to let me know in the comments.

About seventy percent of the mass/energy content of the universe is something called dark energy.  (It's entirely unrelated to dark matter; the potential confusion between the two has led to a push to rename it vacuum energy.)  Dark energy is a bit of a placeholder name anyhow, given that we don't really know what it is; all we see is its effect, which is the measured increasing expansion rate of the universe.

The current best guess about its nature is that dark energy is a property of space itself (i.e., not something that space contains, but an inherent characteristic of the fabric of spacetime).  This energy manifests as a repulsive force, but because it's intrinsic, it doesn't dilute as space expands, the way a cloud might dissipate into air; its content per unit volume remains constant, so as space expands, the total amount of dark energy in the universe increases, resulting in a steady acceleration of the expansion rate.  At the moment, at least on the local level, gravity is still stronger than the expansion, so we're safe enough; but eventually (we're talking a long way in the future) space will have expanded so much that dark energy will overwhelm all other forces, and matter itself will be torn to shreds.

But despite this, we still have no idea what causes it, or even what it really is.

The Tsilioukas et al. paper -- once again, as far as I can understand it -- proposes a solution to that.

On the smallest scales, spacetime seems to be a "quantum foam" -- a roiling, bubbling ferment of virtual particles and antiparticles, constantly being created and destroyed.  That these virtual particles are real has been demonstrated experimentally, despite their existing for such a short time that most physicists would question even using the word "existing" as a descriptor.  So these incredibly quick fluctuations in spacetime -- even in a complete vacuum -- can have a discernible effect despite the fact that detecting the particles themselves is theoretically impossible.

What Tsilikouas et al. suggest is that there's a feature of the quantum foam that, described mathematically, is basically a network of tiny wormholes -- tunnels through spacetime connecting two separate points.  They're (1) as quick to appear and vanish as the aforementioned virtual particles, and (2) extremely submicroscopic, so don't get your hopes up about visiting Deep Space Nine any time soon.

The mathematics of these wormholes is described by a principle from topology called the Gauss-Bonnet theorem, named after mathematicians Carl Friederich Gauss and Pierre Ossian Bonnet (no relation), and when you include a Gauss-Bonnet term in the equations of General Relativity, you get something that seems to act just like the observed effects of dark energy.

So the runaway expansion of the universe might be due to tiny wormholes forming from the quantum foam of the vacuum -- and those minuscule fluctuations in spacetime add up to seventy percent of the total mass/energy content of the universe.

Like I said, it's not like I'm any more qualified to analyze whether they're on to something than Jethro is to explain why chewing up my gym socks makes him a Very Bad Puppy.  And it must be said that these theoretical models sometimes run into the sad truth from Thomas Henry Huxley, that "the great tragedy of science is the slaying of a beautiful hypothesis by an ugly fact."

But given that up till now, dark energy has been nothing more than a mysterious, undetectable, unanalyzable something that nevertheless outweighs all other kinds of matter and energy put together -- a rather embarrassing situation for physicists to find themselves in -- the new explanation seems to be a significant step in the right direction.

At least to a Bear Of Very Little Brain.

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Mirror image

One of the hallmarks of science is its falsifiability.  Models should generate predictions that are testable, allowing you to see if they conform to what we observe and measure of the real universe.  It's why science works as well as it does; ultimately, nature has the last word.

The problem is that there are certain realms of science that don't lend themselves all that well to experiment.  Paleontology, for example -- we're dependent on the fossils that happen to have survived and that we happen to find, and the genetic evidence from the descendants of those long-gone species, to piece together what the ancient world was like.  It's a little difficult to run an experiment on a triceratops.

An even more difficult one is cosmology -- the study of the origins and evolution of the universe as a whole.  After all, we only have the one universe to study, and are limited to the bits of it we can observe from here.  Not only that, but the farther out in space we look, the less clear it becomes,  By the time light gets here from a source ten billion light years away, it's attenuated by the inverse-square law and dramatically red-shifted by all the expanding space it traveled through to get here, which is why it takes the light-collecting capacity of the world's most powerful telescopes even to see it.

None of this is meant as a criticism of cosmology, nor of cosmologists.  But the fact remains that they're trying to piece together the whole universe from a data set that makes what the paleontologists have look like an embarrassment of riches.

The result is that we're left with some massive mysteries, one of the most vexing of which is dark energy.  This is a placeholder name for the root cause of the runaway expansion of the universe, which (according to current models) accounts for 68% of the mass/energy content of the universe.  (Baryonic, or ordinary, matter is a mere 5%.)  And presently, we have no idea what it is.  Attempts either to detect dark energy directly, or to create a model that will account for observations without invoking its existence have, by and large, been unsuccessful. 

But that hasn't stopped the theorists from trying.  And the latest attempt to solve the puzzle is a curious one; that dark energy isn't necessary if you assume our universe has a partner universe that is a reflection of our own.  In that universe, three properties would all be mirror images of the corresponding properties in ours; positive and negative charges would flip, spatial "handedness" (what physicists call parity) would be reversed, and time would run backwards.

Couldn't help but think of this, of course.

The idea is intriguing.  Naman Kumar, who authored the paper on the model, is enthusiastic about its potential for explaining the expansion of the universe.  "The results indicate that accelerated expansion is natural for a universe created in pairs," Kumar writes.  "Moreover, studying causal horizons can deepen our understanding of the universe.  The beauty of this idea lies in its simplicity and naturalness, setting it apart from existing explanations."

Which may well be true.  The difficulty, however, is that the partner universe isn't reachable (or even directly detectable) from our own, Lost in Space notwithstanding.  It makes me wonder how this will ever be more than just an interesting possibility -- an idea that, in Wolfgang Pauli's often-quoted words, "isn't even wrong" because there's no way to test whether it accounts for the data any better than the other, less "natural" models do.

In any case, that's the latest from the cosmologists.  Mirror-image universes created in pairs may obviate the need for dark energy.  We'll see what smarter people than myself have to say about whether the claim holds water; or, maybe, just wait for Evil Major West With A Beard to show up and settle the matter once and for all.

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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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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.

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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!]

The swing of a pendulum

 Physicists have a serious problem.

Back in the mid-1970s, astrophysicist Vera Rubin made an interesting discovery.  She had initially been interested in quasars, but moved away from that because the subject was "too controversial" -- and landed herself in the midst of one of the biggest scientific controversies to hit the field since the discovery of the quantum nature of reality back in the 1920s and 1930s.

She was looking at the rotation rates of galaxies, and found something curious; based on what was known about gravitational interactions between massive objects, the outer fringes of every galaxy she studied were moving at the "wrong" velocity.  The outermost stars were moving far faster than the model predicted, suggesting there was some unseen mass increasing the gravitational field and whirling the edges of the galaxy around faster than the visible matter could have.

And it wasn't by a small margin, either.  Rubin's calculations suggested that there was five times the unseen stuff as there was all of the visible matter in the galaxy put together.  This was way too much to be accounted for by something like diffuse dust clouds or other agglomerations of non-luminous, but completely ordinary, matter.  Rubin nicknamed the invisible stuff dark matter, more or less as a placeholder name until the physicists could figure out what the stuff was, something most researchers figured would be accomplished in short order.

Almost fifty years later, we still are hardly any further along.  Better measurements have confirmed that there is way more dark matter than ordinary matter; Rubin's estimate was spot-on, and current data indicates that 27% of the universe's total mass is dark matter, as compared to only 5% ordinary matter.  (The other 68% is an even more mysterious thing called dark energy, about which the astrophysicists are even more completely, um, in the dark.)

Every attempt to figure out the nature of dark matter -- or even to detect it by anything else but its gravitational effects on the galactic scale -- has resulted in failure.  The leading candidate, called weakly interacting massive particles (WIMPs), has been the subject of repeated detection attempts, and every single one of them has generated "null results."

Which is science-speak for "bupkis."

At some point, you have to wonder if the scientists are going to give the whole thing up as a bad job.  The problem is, if that happens you have 95% of the universe made of stuff we can't account for, which isn't a state of affairs anyone is happy with.

So a team at the National Institute of Standards and Technology is giving dark matter one more chance to show itself, using the only way in which we're certain it interacts with ordinary matter -- gravity.

The trouble is, gravity is a really weak force.  It's only a big player in our lives because we live on a massive chunk of rock, big enough to have a significant gravitational field.  Of the four fundamental forces -- gravity, electromagnetism, and the weak and strong nuclear forces -- gravity is weaker than the next in line (electromagnetism) by a factor of 10 to the 36th power.

So gravity is 1,000,000,000,000,000,000,000,000,000,000,000,000 times weaker than the electromagnetic force that holds molecules together, generates static electricity, and toasts your bread in the morning.

How on earth could you detect something that small, when even a trace of a stray electrical field could overwhelm it by many orders of magnitude?  The NIST scientists think they have the answer: an array of over a billion tiny, incredibly sensitive pendulums, each only a millimeter long, shielded and then cooled to near absolute zero to minimize interference from other forces.

[Image licensed under the Creative Commons Ben Ostrowsky, Foucault's Pendulum, CC BY 2.0]

There are four possibilities of what could happen to the array:

• Nothing.  Then we're back to the drawing board.
• Motion of one or two pendulums only.  This is probably due to interaction with an ordinary matter particle, which would hit a pendulum and stick, causing it to swing but leaving the ones around it unaffected.
• Chaotic or random movement in a number of the pendulums.  This "noise" would most likely be caused by a fluctuation in an electric field -- i.e. the array wasn't well enough shielded.
• A coordinated "ripple" passing through the detector, setting more or less a straight line of the pendulums swinging.  This, the researchers say, would be the signal of a dark matter particle zooming through the array, and its gravitational ripple streaking across in a specific direction.

Of course, even if the best possible outcome -- option #4 -- occurs, it still doesn't tell us what dark matter is.  After all, Vera Rubin's research in the 1970s showed that it interacts gravitationally with ordinary matter (i.e., we already knew that).  But at least we'll have a demonstration that it exists, that we're not looking at something like the nineteenth century's luminiferous aether, the mysterious substance that supposedly was the medium through which light waves propagated, and was shown not to exist by the Michelson-Morley interferometer experiment (and the nature of light propagation ultimately explained by Einstein and others, decades later).

So I'll be eagerly awaiting the outcome.  Right now, the array is still in development, so it will be a while before we can expect results.  But if it generates positive results, it'll be the first conclusive demonstration that we're talking about something detectable right here on Earth, not just by its effects on distant galaxies.

Of course, that still leaves us with the other 68% unknown stuff to explain.

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Have any scientifically-minded friends who like to cook?  Or maybe, you've wondered why some recipes are so flexible, and others have to be followed to the letter?

Do I have the book for you.

In Science and Cooking: Physics Meets Food, from Homemade to Haute Cuisine, by Michael Brenner, Pia Sörensen, and David Weitz, you find out why recipes work the way they do -- and not only how altering them (such as using oil versus margarine versus butter in cookies) will affect the outcome, but what's going on that makes it happen that way.

Along the way, you get to read interviews with today's top chefs, and to find out some of their favorite recipes for you to try out in your own kitchen.  Full-color (and mouth-watering) illustrations are an added filigree, but the text by itself makes this book a must-have for anyone who enjoys cooking -- and wants to learn more about why it works the way it does.

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

The unbalanced universe

In her brilliant 2011 TED Talk "On Being Wrong," journalist Kathryn Schulz said, "For good and for ill, we generate these incredible stories about the world around us, and then the world turns around and astonishes us."

That astonishment is at the root of scientific discovery.  Many laypeople have the sense that scientists do what they do by patiently adding data bit by bit, assembling a theory from the results of experiment -- less understood is the fact that much of the time, the experiments themselves happened because of something unexpected that the old model couldn't explain.  It's those moments of, "Hey, now, wait a moment..." that have generated some of the most fundamental theories we have -- universal gravitation, relativity (special and general), evolution, genetics, plate tectonics.

Or, as another luminary of the philosophy of science put it -- James Burke, in his brilliant documentary series The Day the Universe Changed -- "The so-called voyage of discovery has, as often as not, made landfall for reasons little to do with the search for knowledge...  As far as one discovery following another along the way as part of some grand plan, what way?  Going where?"

Now, this is not to say that the lazy student's complaint, "why should we learn science, since it could all be proven wrong tomorrow?", has much merit.  The big ideas, like the ones I listed before, have been so extensively tested that it's unlikely they'll change much.  Any refinements will most probably be on the level of details.  Still... those head-scratching moments do occur, and sometimes they result in an overturning of what we thought we understood -- like the observation that was announced this week from NASA's Chandra X-ray Observatory.

[Image courtesy of NASA/Harvard University/Chandra X-ray Observatory]

One of the basic pieces of the Big Bang theory is that it resulted in a universe that is isotropic -- it basically looks the same no matter which direction you're looking.  The idea here is that when the universe began to expand, the fabric of space/time stretched out so much in the first tiny fraction of a second (something called cosmological inflation) that it resulted in a uniform, isotropic universe.

The analogy that's been around a long time to explain this -- I remember my college astronomy teacher using it, back in the early 1980s -- is to picture yourself as a tiny person, standing on one dot of a deflated polka-dotted balloon.  If the balloon is inflated, you see all the other dots moving away from you, regardless of which dot you're standing on; and in every direction, the dot-density is pretty much the same.  "There is no center of the universe," I recall our professor, Dr. Daniel Whitmire, saying.  "Or, perhaps, everywhere is the center.  It means essentially the same thing."

So the idea of isotropy is pretty deeply built into the Big Bang cosmology.  So the observation from Chandra announced this week that the universe seems to be anisotropic was a little startling, to say the least.

"Based on our cluster observations we may have found differences in how fast the universe is expanding depending on which way we looked," said study co-author Gerrit Schellenberger of the Center for Astrophysics of Harvard University.  "This would contradict one of the most basic underlying assumptions we use in cosmology today."

Or, as Konstantinos Migkas of the University of Bonn in Germany, who led the new study, put it, "One of the pillars of cosmology... is that the universe is 'isotropic....'   Our work shows there may be cracks in that pillar."

It's possible that the measurement doesn't mean what it seems to mean.  One possibility the researchers came up with that would be less-than-earthshattering is that some of the distant clusters might be moving together because of the gravitational pull of an unseen massive object or objects, throwing off the data enough to make it look like an anisotropy.  Another possibility -- which in my mind raises more questions than it solves -- is that the hypothesized "dark energy" that makes up three-quarters of the energy density of the universe is unevenly distributed, meaning its repulsive force is greater in some places than in others.  "This would be like if the yeast in the bread isn't evenly mixed, causing it to expand faster in some places than in others," said study co-author Thomas Reiprich, also of the University of Bonn, adding, "It would be remarkable if dark energy were found to have different strengths in different parts of the universe."

Remarkable especially since we still basically have no idea what dark energy is.  Going from there to any kind of cogent explanation of why there's more of it here than there seems to me to be a significant leap.

Or, perhaps, none of those is correct, and the anisotropy was built-in at the moment of the Big Bang by some process we haven't even dreamed of.

Whatever it turns out to be, this seems to me to be one of those "wait a moment..." discoveries that could potentially lead to a major revision of what we thought we knew.  What's certain is that it demonstrates how far we have to go in science -- and despite our progress, to paraphrase Kathryn Schulz, the universe will time after time turn around and astonish us.

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This week's Skeptophilia book recommendation of the week is brand new -- only published three weeks ago.  Neil Shubin, who became famous for his wonderful book on human evolution Your Inner Fish, has a fantastic new book out -- Some Assembly Required: Decoding Four Billion Years of Life, from Ancient Fossils to DNA.

Shubin's lucid prose makes for fascinating reading, as he takes you down the four-billion-year path from the first simple cells to the biodiversity of the modern Earth, wrapping in not only what we've discovered from the fossil record but the most recent innovations in DNA analysis that demonstrate our common ancestry with every other life form on the planet.  It's a wonderful survey of our current state of knowledge of evolutionary science, and will engage both scientist and layperson alike.  Get Shubin's latest -- and fasten your seatbelts for a wild ride through time.

In the dark

You've all heard of dark matter, the strange stuff that comprises 85% of the total matter in the universe and about a quarter of its overall mass-energy, and the nature of which -- although its presence has been shown in a variety of ways -- we're no nearer to understanding than we were when investigations of galactic rotation rates demonstrated its existence to astronomer Vera Rubin in 1978 (as I mentioned in yesterday's post).

Less well-known, and even more mysterious, is dark energy.  It's a little unfortunate the monikers of these two strange phenomena sound so similar, because dark energy is entirely different from dark matter (both obtained the sobriquet "dark" mainly because they've resisted all methods for direct detection, so we still have not a damn clue what they are).  Dark energy is a peculiar (hypothesized) form of energy that permeates all of space, and is responsible for the observation that the rate of expansion of the universe is accelerating.  Dark energy, whatever it is, acts on matter as if something were pushing it, working opposite to the pull of gravity that otherwise would cause the expansion to reverse eventually, ending the universe in a "Big Crunch."

Oh, and whatever it is, looks like it's common.  Measurements based on the expansion rate of the universe put estimates in the range of 68% of the total mass-energy of the universe.  So that places ordinary matter and energy -- the kind we are made of and interact with on a daily basis -- at a mere 7% of the stuff in the universe.

Kind of humbling, isn't it?  If the data are correct, 93% of the mass-energy of the universe is made up of stuff we can't detect and don't understand.

[Image licensed under the Creative Commons Design Alex Mittelmann, Coldcreation, Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation, CC BY-SA 3.0]

Well, maybe.  According to a press release two days ago from Yonsei University (Seoul, South Korea), scientists at the Center for Galaxy Evolution and Research are suggesting that the foundational assumption that led to the "discovery" of dark energy may simply be wrong.

I'm no astrophysicist, so I won't try to summarize the press release, but simply quote the salient paragraphs:

The most direct and strongest evidence for the accelerating universe with dark energy is provided by the distance measurements using type Ia supernovae (SN Ia) for the galaxies at high redshift.  This result is based on the assumption that the corrected luminosity of SN Ia through the empirical standardization would not evolve with redshift.

New observations and analysis made by a team of astronomers at Yonsei University (Seoul, South Korea), together with their collaborators at Lyon University and KASI, show, however, that this key assumption is most likely in error.  The team has performed very high-quality (signal-to-noise ratio ~175) spectroscopic observations to cover most of the reported nearby early-type host galaxies of SN Ia, from which they obtained the most direct and reliable measurements of population ages for these host galaxies.  They find a significant correlation between SN luminosity and stellar population age at a 99.5% confidence level.  As such, this is the most direct and stringent test ever made for the luminosity evolution of SN Ia.  Since SN progenitors in host galaxies are getting younger with redshift (look-back time), this result inevitably indicates a serious systematic bias with redshift in SN cosmology.  Taken at face values, the luminosity evolution of SN is significant enough to question the very existence of dark energy.  When the luminosity evolution of SN is properly taken into account, the team found that the evidence for the existence of dark energy simply goes away.

I don't know about you, but I read this with my mouth hanging open.  The idea that 68% of the mass-energy density of the universe could disappear if you alter the assumptions came as a bit of a shock.

It probably shouldn't have, of course, because this sort of thing has happened before.  There was phlogiston (the mysterious substance inherent in combustible matter) and the luminiferous aether (the mysterious substance through which light propagates in the vacuum of space), both of which turned out to be not so much mysterious as nonexistent.  Both of these vanished when the baseline assumptions changed -- in the first case, when a good theory of chemical energy was developed, and in the second when Einstein showed that light didn't act like an ordinary wave.

And honestly, even if I'm shocked by the way the dark energy scenario is playing out, I've been half expecting something like this to happen.  A physicist friend of mine was chatting with me one day about dark matter and dark energy (as one does), and she said that just like the aether stuck around until Einstein came and blew away the need for it by changing the perspective, the same would happen with the strange and undetectable dark matter and dark energy.

"We're just waiting for this century's Einstein," she said.

But it seems like it might not even require something as groundbreaking as a Theory of Relativity, here, at least in the case of dark energy.  All it might take is reevaluating the data on supernova luminosity to remove the need for the hypothesis.

Also would explain why we haven't detected it.

But this, like any scientific claim, is bound to be challenged, especially consider that it's nixing 68% of the universe in one fell swoop.  So keep your eyes on the physics journals -- I'm sure you haven't heard the last of this.

And you can count on the new research casting some light on the darkness -- whatever the ultimate outcome.

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This week's Skeptophilia book of the week is simultaneously one of the most dismal books I've ever read, and one of the funniest; Tom Phillips's wonderful Humans: A Brief History of How We Fucked It All Up.

I picked up a copy of it at the wonderful book store The Strand when I was in Manhattan last week, and finished it in three days flat (and I'm not a fast reader).  To illustrate why, here's a quick passage that'll give you a flavor of it:

Humans see patterns in the world, we can communicate this to other humans and we have the capacity to imagine futures that don't yet exist: how if we just changed this thing, then that thing would happen, and the world would be a slightly better place. 

The only trouble is... well, we're not terribly good at any of those things.  Any honest assessment of humanity's previous performance on those fronts reads like a particularly brutal annual review from a boss who hates you.  We imagine patterns where they don't exist.  Our communication skills are, uh, sometimes lacking.  And we have an extraordinarily poor track record of failing to realize that changing this thing will also lead to the other thing, and that even worse thing, and oh God no now this thing is happening how do we stop it.

Phillips's clear-eyed look at our own unfortunate history is kept from sinking under its own weight by a sparkling wit, calling our foibles into humorous focus but simultaneously sounding the call that "Okay, guys, it's time to pay attention."  Stupidity, they say, consists of doing the same thing over and over and expecting different results; Phillips's wonderful book points out how crucial that realization is -- and how we need to get up off our asses and, for god's sake, do something.

And you -- and everyone else -- should start by reading this book.

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