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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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!]