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.

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

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.

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

A new field

I was fortunate enough that the day-job of my bandmate of many years, Kathy Selby, was working as a physicist at Cornell University.

As you might suspect, our conversations while traveling to gigs were pretty interesting.

One time we were on our way to play for a dance in Rochester, and I asked her what she thought about dark matter and dark energy -- which according to current models make up, respectively, 27% and 68% of the mass-energy content of the universe.  [Nota bene: the use of the word "dark" in both names does not mean that they are in any sense the same thing.  Dark matter is a name for the observation that the gravitational attraction of conventional matter is insufficient to account for the measured velocities of galaxies and galaxy clusters; there must be some other, unseen matter there that does not interact with ordinary matter electromagnetically, or else our model for gravity is incorrect.  Dark energy, on the other hand, is a theoretical energy inherent in space itself that might explain the accelerating expansion of the universe.]

So yes, only five percent of the universe is the regular stuff we see around us on a daily basis.  The other 95% is largely unexplained, and is yet to be detected directly.

In any case, I asked Kathy what her opinion was about the rather uncomfortable situation of having the vast majority of the universe thus far inaccessible to scientific study.

"In my opinion," she said, "we're in a situation a bit like physicists were in the late nineteenth century.  They knew light had strange properties.  It acted like a wave much of the time, so they'd postulated a medium -- the luminiferous aether -- through which the wave was propagating.  The problem was, every attempt to detect the aether failed.  Then Michelson and Morley came along and showed that the prediction of an 'aether drag' caused by the motion of the Earth through space didn't exist, suggesting very much that the aether didn't either.  The speed of light in a vacuum seemed to be the same in all reference frames, which was unlike any other wave ever studied.  Then Einstein said, 'Well, let's start by assuming that the speed of light in a vacuum is the same regardless of your reference frame, and see what happens,' and the aether became unnecessary.  Of course, what came out of that shift in perspective was the Theories of Relativity.

"What I think," she concluded, "is that we're waiting for this century's Einstein to tell us that we've been looking at everything the wrong way -- and suddenly the problems of dark matter and dark energy will evaporate, just like the aether did."

Well, we may have just gotten a glimpse at one possibility for that shift in perspective, courtesy of physicist Lucas Lombriser of the Université de Genève.

A paper published two weeks ago in the journal Classical and Quantum Gravity started by looking at what has been called "the worst prediction in physics" -- the value of the cosmological constant, which sets the expansion rate of the universe.  The prediction by theoretical physicists of what the cosmological constant should be given what we know about matter, and what we actually measure it to be, differ by 120 orders of magnitude -- that's 1 followed by 120 zeroes.

Oops.  Major oops.  This is what gave rise to the mysterious dark energy, some peculiar property of space itself that solves the mismatch.  But as far as what exactly this dark energy might be, physicists have come up empty-handed, so more and more it's seemed like a placeholder to cover up for the fact that we don't really understand what's going on.

This, Lombriser says, is because -- like with Einstein's solution to the aether -- we're starting out with the wrong assumption.

Maybe the universe is flat and static, as Einstein himself believed (after the discovery of red shift and the expansion of the universe, Einstein was forced unwillingly to accept an expanding universe and a cosmological constant -- which he later called "the greatest blunder of my career").  Perhaps space isn't expanding; it's the masses of particles that have changed over time.  The altered masses change the gravitational field that permeates space, and that's what generates red shift and the appearance of expansion.  So there is a cosmological constant, but it comes from the particles themselves, and the field in which they reside, evolving.

[Image licensed under the Creative Commons Original image by User:Vlad2i, slightly modified by User:mapos., Gravitational red-shifting2, CC BY-SA 3.0]

This new take solves three problems at once.  It does away with the cosmological constant mismatch; dark energy pretty much disappears completely; and the field itself that's responsible for the mass change could account for dark matter, as it shares many properties with an axion field, and axions are one of the leading candidates for the constituents of dark matter.  

This simultaneous solution of three vexing problems is certainly intriguing.  But the question is, is Lombriser right?  "The paper is pretty interesting, and it provides an unusual outcome for multiple problems in cosmology," said physicist Luz Ángela García, of the Universidad ECCI Bogotá, who was not involved in the research.  "The theory provides an outlet for the current tensions in cosmology.  However, we must be cautious.  Lombriser's solution contains elements in its theoretical model that likely can't be tested observationally, at least in the near future."

Which, of course, is the issue, and is all too common in this branch of science.  Even though Einstein's Theories of Relativity did a good job of accounting for various anomalies in the properties of light, the first precise confirmation of his predictions didn't occur until 39 years after he wrote his seminal paper in 1915.  How to detect the fluctuating field Lombriser postulates -- and, more importantly, how to distinguish its effects from the current model of expanding space -- is currently beyond us.

So maybe Lombriser is what my bandmate Kathy called "this century's Einstein."  Or maybe his ideas will prove to be just another unverified or (worse) unverifiable hypothesis.  But I have to say, when I read about what he's proposing, my ears did perk up.  It has the feel of a paradigm shift -- just what we've been waiting for.

And you can bet that the physicists are going to be all over this, looking for ways either to confirm or refute what he's saying.

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