Is Our Universe Decaying at an Astronomical Rate?

This post is just a copy of a thread over at Cosmic Variance. I’m copying it here because it’s an example of some of the mind-bending things that you come across in physics and mathematics, and one of the reasons that people do physics (and math). I’ve only read the post, so reading the thread should be interesting. Apparently Lee Smolin has written some comments on the thread, so it should be interesting. Lee, of course, is at the Perimeter Institute and is one of the high profile people looking at loop quantum gravity.

The thread starts here.

OO’s and BB’s
John at 3:02 am, February 21st, 2007
One nice thing about being a scientist, or at least an academic one, is that occaisionally you get your mind blown without any drugs or anything. Someone comes along and just pulls the rug completely out from under you – a total Denial of Reality Attack – and then you are left on your own to pick up the pieces.

Today at UC Davis we had a seminar from Don Page of the University of Alberta. The title and abstract of this talk sounded like science fiction, so I reproduce it here:

Don Page, University of Alberta

Title: Is Our Universe Decaying at an Astronomical Rate?

Abstract: Unless our universe is decaying at an astronomical rate (i.e., on the present cosmological timescale of Gigayears, rather than on the quantum recurrence timescale of googolplexes), it would apparently produce an infinite number of observers per comoving volume by thermal or vacuum fluctuations (Boltzmann brains). If the number of ordinary observers per comoving volume is finite, this scenario seems to imply zero likelihood for us to be ordinary observers and minuscule likelihoods for our actual observations. Hence, our observations suggest that this scenario is incorrect and that perhaps our universe is decaying at an astronomical rate.

Boltzmann brains? WTF? Intrigued, I went. This is a well-respected, highly-cited cosmologist after all. A former student of Stephen Hawking, no less. The jargon in the abstract, though bizarre, had a certain je ne sais quoi…

The idea Don put forward is this: there’s us, the ordinary observers (OO’s) in the world, who have achieved a certain stature after billions of years of evolution in the universe, and are now capable of making quite refined (or so we think) observations of the universe. Andre Linde called OO’s “just honest folk like us.” We’ve made it as a species, man- and womankind, and we’re figuring ou the really deep things that are going on like the Big Bang, genetics, and all the rest.

Then, though, there are the BB’s in the universe: Boltzmann Brains. Random fluctuations of the fabric of spacetime itself which, most of the time, are rather insignificant puffs which evaporate immediately. But sometimes they stick around. More rarely, they are complex. Sometimes (very very rarely) they are really quite as complex as us human types. (Actually, “very very rarely” does not quite convey just how rare we are talking now.) And sometimes these vacuum quantum fluctuations attain the status of actual observers in the world. But, the rarest of them all, the BB’s, are able to (however briefly) make actual observations in the universe which are, in fact, “not erroneous” as Don Page put it.

The man was a compelling speaker, and soon I realized there was an actual intellecutal debate underway in the high end of the cosmology/high energy community as to what the role of these BB’s might be in the universe, in the very far (or maybe not so far) future. We have a certain prejudice that, well, there just aren’t so many of them out there at this stage of the game, 14 billion years after the Big Bang. We’d like to think that we have the stage at the moment, we OO’s, um, assuming there are in fact more of us out there. (Any other non-human OO’s out there, could you let us know, please that you are listening? We have a few questions for you…)

The thing is, when you start talking about very very…very rare things like Boltzmann Brains, you are talking about REALLY long times. Much longer than we’ve had on earth (and I mean 4.5 billion years) by many orders of magnitude. Numbers like 10 to the 60th years were being batted around like it was next week in this talk. By those times, all the stars and all the galaxies have gone out, and gone cold, and space has continued to expand exponentially and things are long past looking pretty bleak for the OO’s still around, who (we presume) need heat and light and at least a little energy of some sort to survive, even if we are talking about very slow machine intelligence (even slower than humans for example).

So eventually, the mere fact that there is, at these long times, just oodles of space in the universe means that the BB’s become more and more common (even if they are rare) and eventually dominate the, uh, intellectual landscape of the universe. Of course this immediately raises all sorts of questions, such as mind/matter duality, the nature of reality and consciousness and multiple consciousnesses, perceived versus objective independent reality. Not to mention whether our “universe” is the only one. Okay, I’ll stop now…

Well, at this point in the talk, being new to this and my mind already quite blown, I had trouble keeping the thread. Somehow or other Don seemed to conclude that a BB-dominated universe was absurd (though are we sure we’re not in one already?) and then posited a radically different spacetime metric, an Anti-deSitter space, which he seemed to think might contain the problem. But then he hit another question which was the title of the talk: must the universe be decaying more rapidly than we expect? I am mangling this horribly, and of course before writing this I took just a glimpse at the already voluminous amount of literature on this topic, and realized that I have a lot of reading to do, both blog and academic. So it’s best I stop and let you all go look up Botzmann Brains, as I will, and do some more reading.

Sigh. The Ultimate Fate of the Universe is of course a nice escape from our quotidian grind. But, as Lenny Susskind wrote in his inscription to us in his book The Cosmic Landscape, at a signing last fall in Davis, “Hey, things could be worse!”

(And, Lo! They were worse…)

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26 Comments on “OO’s and BB’s” rss feed
Domenic Denicola on Feb 21st, 2007 at 3:20 am
Please keep up updated on what you find in the literature! This is absolutely fascinating; I quite concur with your opening sentiments.

Also, Wikipedia seems to be in need of education about these Boltzmann brains… someone should write that article.

Sean on Feb 21st, 2007 at 3:25 am
You can start reading on this very blog! This is definitely good stuff, and well worth thinking about — I’m working on a paper about it right now, in fact. It can tend to sound like crazy metaphysical speculation, but I would argue that these questions are precisely as physical and important as the horizon and flatness problems that everyone is happy to use to justify inflation.

If you spend a lot of time in thermal equilibrium in a low-temperature universe, you will eventually make a lot of Boltzmann Brains. That’s why Don wants to appeal to a short lifetime, to avoid that problem. But we still have the problem of why there is any appreciable amount of matter at all — why isn’t the universe just empty flat spacetime?

The good news is, you needn’t worry — you are definitely not a Boltzmann Brain. But we’re not yet sure why.

Alex Nichols on Feb 21st, 2007 at 3:42 am
Lenny Spiegel? Shouldn’t that be Susskind?

We don’t experience intelligent observers popping into existence from quantum fluctuations though.

Richard Dawkin’s Chapter on Cumulative selection in “The Blind Watchmaker” looks at the odds of producing the phrase “Methinks it is like a Weasel” using a computer program.

Basically, longer than the time the universe has existed using single-step random mutation, versus a few seconds on a modern pc using cumulative selection.

Of course, once you introduce a timescale of infinity anything’s possible!

DIS on Feb 21st, 2007 at 8:11 am
Damn right Sean! Crazy metaphysical speculation it is. It might be amusing to talk about in a blog, but you should use your talent for real science not this gobblediguck!

The universe is not in equilibrium by any standard. It is even starting to accelerate. Sure looks like it is at an instability.

Robert the Red on Feb 21st, 2007 at 9:03 am
Isn’t this argument a kind of reverse anthropological teleology? “We don’t feel the domination of the late universe by Boltzmann brains to be plausible, so therefore the universe must cease to exist by then.” What kind of science argument is that? An OK one if you are sipping your fifth cup of soju, but otherwise pretty lameoid.

Stathis Papaioannou on Feb 21st, 2007 at 9:14 am
It occurred to me a long time ago that BB’s (although I didn’t call them that) were a means to immortality. All I had to do after I died was wait until the right configuration of atoms came about somewhere in the universe and I would continue to have experiences where I left off at the moment of death. I would have to wait a long time or the universe would have to be very large, but it’s not as if you notice time passing when you’re dead.

But wait, it’s even better than this because we have functionalist theories of mind! This means my mind could be multiply realisable: on a brain, on a laptop running Windows XP, on a Klingon computer based on the radioactive decay pattern of a sacred stone. In general, there is a countable infinity of abstract machines that could realise my brain when physically implemented (meaning, loosely, that there is an isomorphism between the states of the abstract machine and the states of the abstract system).

The implication of this is that I don’t have to wait zillions of years for the one “correct” configuration that will implement my brain, because at least one of the infinite possible configurations will come up somewhere every moment. You’re guaranteed of instant continuation after death, and in fact your next conscious moment will most likely be generated by this mechanism. If this conclusion seems wrong, then there is a problem with functionalism.

Douglas on Feb 21st, 2007 at 9:35 am
I’m sorry. Is this serious?

ZP on Feb 21st, 2007 at 9:37 am
Slathis, the only way for your brain to have the same configuration is if the universe you observe outside your brain also had the same configuration, which would be improbable to the point of impossibility, except in a cyclical universe. In addition, duplicating the same brain state you had at the moment of death may not be desirable. The only way you can continue where you left off, is if this is all just a dream, and you wake up into a greater reality.

Peter Woit on Feb 21st, 2007 at 10:14 am
I would argue that these questions are precisely as physical and important as the horizon and flatness problems that everyone is happy to use to justify inflation.

You’re serious about this claim?

Vince on Feb 21st, 2007 at 10:25 am
“Then, though, there are the BB’s in the universe: Boltzmann Brains. Random fluctuations of the fabric of spacetime itself which, most of the time, are rather insignificant puffs which evaporate immediately. But sometimes they stick around. More rarely, they are complex. Sometimes (very very rarely) they are really quite as complex as us human types. (Actually, “very very rarely” does not quite convey just how rare we are talking now.) And sometimes these vacuum quantum fluctuations attain the status of actual observers in the world. But, the rarest of them all, the BB’s, are able to (however briefly) make actual observations in the universe which are, in fact, “not erroneous” as Don Page put it.”

I’m sorry if I’m not making a huge effort to read about this on my own (I have to go pretty soon!), but I hope there’s a more rigorous way of saying all this (with mathematics, perhaps) and some rigorous evidence for this. I mean, our abilities to make observations definitely depends on a properly-working brain, but isn’t there more to “an observer” than “complexity”? How can fluctuations of the spacetime fabric ever be “complex” enough to produce an observer, making observations? And what is meant by “observer” in this context anyway? Finally, I just don’t see how these spacetime fluctuations can be “complex” enough to produce something so ordered as the human brain. What is meant by “complex”? Through evolution, brains have evolved to be the main organ of the nervous system, and the origin of sentience (spelling?).

I don’t know, I’m completely missing something here.

Hmmm….isn’t there speculation in LQG that standard model particles are the twisting, etc. of the spacetime fabric? Must go…

B on Feb 21st, 2007 at 10:52 am
reg. literature, he has a paper on that

Is Our Universe Decaying at an Astronomical Rate?
http://arxiv.org/abs/hep-th/0612137

I read that a while ago, meant to write something about it, but then I took just a glimpse at the already voluminous amount of literature on this topic, and realized that I have a lot of reading to do, both blog and academic Sean’s post was quite enlightening in this regard. (But what I eventually wrote was too silly even for my blog )

The only thing that I can remember two months later (this reflects the importance I give to the topic) was that the paper features some sentences you really shouldn’t miss. E.g.

Consider imaginary humans who have a ‘youthful’ phase of 100 years of life with frequent and mostly ordered observations, followed by a ‘senile’ phase of trillions of years of infrequent and mostly disordered observations. Assume that […]

or

First of all, the destruction of the universe would likely occur by a very thin bubble wall traveling extremely close to the speed of light, so no one would be able to see it coming to dread the imminent destruction. Furthermore, the destruction of all we know […] would happen so fast that there is not likely to be nearly enough time for any signals of pain to reach our brains. And no grieving survivors will be left behind. So in this way it would be the most humanely possible execution.

Elliot on Feb 21st, 2007 at 10:54 am
This leads me to conclude, as others have (Kaufmann), that the emergence of complex structures is a fundamental property of our universe as opposed to being an accidental feature.

If so then it would put a very different perspective on this discussion.

Elliot

Aaron Bergman on Feb 21st, 2007 at 11:03 am
The good news is, you needn’t worry — you are definitely not a Boltzmann Brain. But we’re not yet sure why.

But you could be a Boltzmann brain in a vat!

(Solipsism in the landscape: the final frontier)

Sean on Feb 21st, 2007 at 11:10 am
Sure I’m serious. The BB problem and the horizon/flatness problems are just different manifestations of the fact that the hot Big Bang model has very finely-tuned low-entropy initial conditions. You can either impose such conditions by hand (in which case none of these problems should worry you) or you can seek a dynamical explanation (in which case all of them should).

Plato on Feb 21st, 2007 at 11:53 am
Hmmm… layman was wondering. A dynamical explanation?

Boltzmann distribution. Binomial series. Ever heard of the marble drop/bean machine? Pascal’s triangle?

What was the “initial condition” that allowed each “probabilistic future?” How were you to explain it? New numbered systems?

So the BB had to be explicable in how that condition could be arrived at, and thus “any consequence” of that initial condition, as our universe?

Sierpinski triangle. An outcome of “one possible universe?”

John on Feb 21st, 2007 at 11:57 am
I did indeed mean Lenny Susskind…a quantum fluctuation in my brain caused me to write Spiegel. Thanks for pointing ou tthe typo!

– Joh

Vince on Feb 21st, 2007 at 12:07 pm
“You can either impose such conditions by hand (in which case none of these problems should worry you)”

The hand….of God, maybe?

Eugene on Feb 21st, 2007 at 1:49 pm
And to think I am planning to give a talk on this stuff at Columbia next month .

I do think that the objection of #4 about universe is not in thermal equilibrium is valid. And “Page’s Paradox” (as Don’s proposal is kinda known as) is bogus; one can’t really play the BB game simply by considering the “local expanding universe” (I can’t find a better word to describe it).

This is not the same as saying the BB problem is not there of course, but that’s different from Don’s setup.

Eugene

Alex Nichols on Feb 21st, 2007 at 1:59 pm
“I did indeed mean Lenny Susskind…a quantum fluctuation in my brain caused me to write Spiegel. Thanks for pointing ou tthe typo!”

Or perhaps Lenny Spiegel is the mirror-image Boltzman Brain version of Lenny Susskind….

Quasar9 on Feb 21st, 2007 at 3:34 pm
Well, we do accuse governments of being short termist. Anything more than short term political gain – ie the next election means ‘little’ to a politician.

But indeed in the cosmological scale of things, once we start looking 60 Trillion years into the future, has man become little more than an ‘anecdote’ or a blot on the landscape – after all if there’s 13.7 or 40 billion years behind us (before man puts in an appearance) only man could presume to speculate what may or may not be in 60 Trillion years time.

If it is any consolation it is perhaps interesting to know that so much ‘history’ will take place – and though we may not be here to observe or record it – one could speculate that that is good, not worse. lol!

But hopefully by then we (humans) will have evolved into beings composed entirely of photons – and be able to experience all (or at least see it) – without suffering from the decay. Or will photons seize to exist?
I say, Who turned the light out?

Lee Smolin on Feb 21st, 2007 at 8:12 pm
With all due respect to Don and Alex, who I’ve respected and liked for 30 years, this stuff is nuts. It is leading neither to predictions nor to explanations, and I would argue it can’t. But what is useful to understand is why it is nuts and why, nonetheless, it appears to many good scientists to be science.

I think the first thing to say is that given the assumptions that go into most work on eternal inflation, the Boltzman’s Brain puzzle is a real puzzle. This is why good people who buy the scenario of eternal inflation are taking it seriously. But the conclusion I draw is that this is a reducto ad absurdum of the main assumptions of that approach to cosmology. In particular, the key assumption which I believe is wrong is that there is a timeless probability distribution on the landscape. While many good people are attempting to compute that distribution, the fact that it is hard to get out of the BB paradox suggests that there may be no such thing as a timeless, static probability distribution on the landscape.

What is the alternative? In the case of Boltzman, the alternative was real evolutionary biology. Notice that Darwinian biology manages to be predictive and highly falsifiable in spite of there being no timeless probability distribution on the fitness landscape (which is btw where the term came from). instead, in biology, living creatures are relativerly fitter than non-viable creatures, but there is no extremization of fitness at any one time. This is partly because novel properties that influence fitness emerge from time to time from exaptation, in a way that is very diffiuclt, if not impossible, to specify in advance. The emergent properties that govern sexual selection in mammals could not be expressed in terms of the properties that goven fitness of bacteria. As a result, one can try to compute whether a given small mutation increases or decreases fitness of a real organism-and that is good enough to derive lots of predictions-but it would be useless to try to extremize fitness over the whole landscape.

Similarly in cosmology, if we have to live with a landscape, but we want to be predictive, we should learn from what worked in biology and give up the notion that there is a single timeless probability distribution to compute, and instead start asking what makes a region or a baby universe more or less relatively fit than its near cousins. (The scenario of cosmological natural selection shows that taking this route does lead to falsifiable predictions for the same reasons that Darwinian biology is falsifiable.)

Going a bit deeper, the governing assumption of eternal inflation is that there is a static and eternal population of universes that the fundamental laws of nature apply to more transparently than they do to our single, evolving, timebound universe, which is governed by only one theory at a time. Given that there can be no direct evidence for such an infinite and eternal multiverse, the interest in it must be driven by something other than observation. I am beginning to suspect it is because physicists would prefer to invent and think about something imagined to be eternal and timeless rather than try to explain what we observe in our universe entirely in terms of the information that comes to us from our past light cone.

I suspect this reflects the expectation many people have that time is not fundamental, but rather emerges only at a semiclassical approximation in quantum cosmology. If you believe this then you believe that the fundamental quantities a quantum cosmology should compute are timeless. This in turn reflects a very old and ultimately religious prejudice that deeper truths are timeless. This has been traced by scholars to the theology of Newton and contemporaries who saw space as “the sensorium” of an eternal and all seeing god. Perhaps the BB paradox is telling us it is time to give up the search for timeless probability distributions, and recognize that since Darwin the deep truths about nature cannot be divorced from time.

The alternative is to disbelieve the arguments that time is emergent-which were never very convincing- and instead formulate quantum cosmology in such a way that time is always real. I would suggest that the Boltzman Brain’s paradox is the reducto ad absurdum of the notion that time is emergent and that rather than play with little fixes to it we should try to take seriously the opposite idea: that time is real.

Thanks,

Lee

ps some of the arguments aluded to are developed in hep-th/0612185, but they also are in my 1997 book on the landscape. I am also working on something new on these issues with the philosopher Roberto Unger.

pps Before someone suggests that if we believe the evidence for inflation we have to believe in eternal inflation, let me mention that there is no convincing argument to that conclusion.

Aaron Bergman on Feb 21st, 2007 at 8:27 pm
(The scenario of cosmological natural selection shows that taking this route does lead to falsifiable predictions for the same reasons that Darwinian biology is falsifiable.)

No, it doesn’t. Cosmological Natural Selection is just a different choice of measure on the space of vacua, neither more nor less absolutely falsifiable than any other.

Matt on Feb 21st, 2007 at 8:48 pm
So if I’m to understand all this, then at some point in the future, there WILL be a teacup in orbit around Mars? Not to mention a flying spaghetti monster? Invisible pink unicorns would still be difficult, I suppose, which is comforting.

Elliot on Feb 21st, 2007 at 9:13 pm
Lee,

You have argued that black hole production is the “trait” that gives provides “survival” value and the ability to produce offspring. Have you or others considered other “traits” that a universe might have apart from black hole production under the CNS paradigm that might be more amenable to proof or falsifiablilty?

I am suggesting that perhaps the CNS model may be correct but the specific identification of black hole production as the trait leading to the production of offspring may be either incorrect or limiting in some manner.

Elliot

Plato on Feb 22nd, 2007 at 12:29 am
Layman scratching head while faceless expression of Boltzmann puzzlement takes hold?

How is one suppose to find “a equilibrium” in such a “low entropic state?”

If we were to experimentally challenging any thinking with “relativistic processes” how could they have ever emerged out of the BB? Maybe, it was a “highly symmetric event” for any asymmetry to show itself as “discrete measures” defined in relation to the “energy of probable outcomes?”

Where did such reductionism begin for us to ask about the “cross over?”

We needed high energy perspective to realize that we were still talking about the universe. Are there any other processes within the cosmos that can be taken down to such rejuvenated qualities to new universes being born that while the arrow of time is pointed one way, that the universe itself allowed such expression to continue in the expansion rate, and the speed up?

A Higg’s fluid? Something had to be “happening now” that would dictate?

Forgive me here for my ignorance in face of those better equipped.

Alex Nichols on Feb 22nd, 2007 at 5:32 am
I disagree with Lee Smolin and agree with Sean. In fact the “Boltzmann Brain” paradox actually disproves an absolute space-time.
Occam’s razor suggests that intelligent observers only arise from cumulative selection in a biological framework which is dependent on the physical evolution of the universe.

I don’t think Julian Barbour got very far with his attempt to dispense with time from quantum physics. At best it’s a valid insight, inasmuch as time can only be the measure of relational changes between particles (or wavefronts) But this insight does not lead to any predicitions about the future development of the system.

So whether time is ontologically “real” or not, to all intents and puroposes we have to treat it as a measure of the entropy gradient we exist in.

The question of what produced this gradient, whether a quantum fluctuation, a cyclic process or the multiverse is what cosmology is all about.
I feel that some conclusions on these questions will be forthcoming from future observations.