Multiverse No More, a New Theory of Scale
August 25, 2014 11:39 AM   Subscribe

Perhaps the fundamental description of the universe does not include the concepts of “mass” and “length,” implying that at its core, nature lacks a sense of scale. 'Supersymmetry posits the existence of a missing twin particle for every particle found in nature.' But there's 'one big problem with supersymmetry: in the particle physics that is observed in today's accelerators, every boson most definitely does NOT have a matching fermion with the same mass and charge. So if supersymmetry is a symmetry of Nature, it must somehow be broken.' 'Scale symmetry[warning: slow-loading pdf], constitutes a radical departure from long-standing assumptions about how elementary particles acquire their properties. 'With their field stuck at a nasty impasse,' 'researchers have returned to the master equations that describe the known particles and their interactions, and are asking: What happens when you erase the terms in the equations having to do with mass and length?'

'A theory called “agravity” (for “adimensional gravity”) developed by Salvio and Strumia may be the most concrete realization of the scale symmetry idea thus far. Agravity weaves the laws of physics at all scales into a single, cohesive picture in which the Higgs mass and the Planck mass both arise through separate dynamical effects. As detailed in June in the Journal of High-Energy Physics, agravity also offers an explanation for why the universe inflated into existence in the first place. According to the theory, scale-symmetry breaking would have caused an exponential expansion in the size of space-time during the Big Bang.'
posted by VikingSword (25 comments total) 47 users marked this as a favorite
I was just reading about this in Wired. Excellent spotting, VikingSword!
posted by Kevin Street at 12:02 PM on August 25, 2014 [1 favorite]

Perhaps the fundamental description of the universe does not include the concepts of “mass” and “length”

I was trying to explain this to someone the other night, but she still seemed pretty disappointed.
posted by Strange Interlude at 12:06 PM on August 25, 2014 [29 favorites]

They're saying that "mass" may not be a real thing, in the sense that it's a fundamental attribute of objects like particles. (Particle X has mass M, spin S and so on.) Instead, it may be that mass arises as some kind of second-order effect created by the interaction of particles.
posted by Kevin Street at 12:13 PM on August 25, 2014 [1 favorite]

Does length also arise as some kind of second-order effect created by the interaction of particles?
posted by Samuel Farrow at 12:22 PM on August 25, 2014

Does length also arise as some kind of second-order effect created by the interaction of particles?

Is temperature a "second-order" effect of the movement of particles? In what sense? Cf. Boltzmann and Mach.
posted by phrontist at 12:28 PM on August 25, 2014 [1 favorite]

Particle physicists would tell you that mass (as we know it) is already not a real thing. Symmetry breaking by the Higgs boson (long-ago proposed and only recently seen) in the early universe led a bunch of previously massless particles to become the massive particles that we know and love.

The difference between this paper and the standard model is that they go a couple of steps further:
1 - They remove the mass term from the Higgs particle!
2 - They add in a gravitational interaction (which, confusingly, is totally different from having mass.)
3 - They then remove certain "massive" terms from the "normal" gravitational interaction, but then replace them with a few new "massless" terms that are normally not considered.

The claim of their work is that by these careful subtractions and additions, that you can reproduce the existing phenomena that we see _and_ you can solve in a single stroke the two huge problems of the Standard Model -- the "hierarchy" problem and the impossibility of incorporating gravity.

Bottom line: they have proposed a new(ish) equation that is slightly different from the existing equations that most particle physicists are true. They have waved their hands about what look like "obvious" problems with their theory (and the reasons why their theory was not more closely studied when it was first proposed long ago) and instead focused on what they believe are its strengths. Are they right? Who knows. It won't be anytime soon that this is put to rest.
posted by artichoke_enthusiast at 12:33 PM on August 25, 2014 [3 favorites]

Does length also arise as some kind of second-order effect created by the interaction of particles?

I guess so, if you go by this approach. There's a really nice little interactive chart in the first link that shows what they call the Hierarchy Problem (the problem that this new theory is attempting to solve), and the different approaches of Supersymmetry versus Scale Symmetry. Mass (and Length) is created on two different scales, leading to two different families of particles.

In one family you've got the tiny little particles of the Standard Model, like quarks - and in the other family there's huge things like cosmic strings which can be as long as you like.
posted by Kevin Street at 12:39 PM on August 25, 2014

Today's SMBC seems pertinent.
posted by benito.strauss at 12:40 PM on August 25, 2014 [1 favorite]

There's a chestnut in philosophy that says "the more exciting a discovery is, the fewer people can understand it." I tend to believe that this applies to the disciplines involved here.

I know >>I<< don't understand it.
posted by DWRoelands at 12:44 PM on August 25, 2014

The claim of their work is that by these careful subtractions and additions, that you can reproduce the existing phenomena that we see _and_ you can solve in a single stroke the two huge problems of the Standard Model -- the "hierarchy" problem and the impossibility of incorporating gravity.

Bottom line: they have proposed a new(ish) equation that is slightly different from the existing equations that most particle physicists are true.

If they are right, what would the implications be?
posted by Sangermaine at 1:15 PM on August 25, 2014

Does this mean no nega-Strass in universe 8b or do we still get infinite realities?
posted by Strass at 1:16 PM on August 25, 2014 [1 favorite]

Just a quick note (he says. Let's see how long this comment turns out to be), about the "generation" of length and energy scales that is of issue here. First, for maximum sense-making, read my previous comments here, here, here, and here. I realize that's a lot, sorry. The Universe is complicated. Then, for what follows, recall that to a particle physicist such as myself, there is no difference between lengths and energies (or indeed, any other dimensional quantity). Length is inverse energy: very energetic phenomenon probe very small lengths.

Now, the question is not "why are there different energies or lengths." Even in a model of physics without massive particles, there would still be a background metric and distances would still be defined. You could still compare two objects and see if one is longer or shorter (or heavier or lighter), even if the Universe was scale-invariant in the way that these papers are wrestling with.

The question is why are particular scales *important*. If the laws of Nature were completely scale-invariant, one could talk about the physics observed at a particular length (or length scale), but one would not expect the laws of physics at a new energy scale to be different. Think of it like a spiraling conch shell, one that keeps spiraling down to infinity. Looking at the conch shell at the "human scale," we could determine the equations that govern the way the spiral grows in one direction and shrinks in the other. Then, if the shell were truly scale-invariant, we could imagine shrinking down to microscopic size and looking at the conch again. We'd still see the same equations governing the spiral there as well, though we could all agree that the absolute length scale has decreased. Incidentally, the equation governing the conch spiral are the Fibonacci sequence, and its just this sort of aside that keeps these comments at the length you've come to expect.

Now, in reality, at some point our conch shell spiral will end. Even though the spiral is scale invariant over a large range of lengths, the physics from which the shell is built (i.e. atoms in the Earth's gravity field) has fundamental scales (i.e. the size of the atoms and the physical constraints set by the interplay of gravitation and electromagnetic interactions). So the scale symmetry is broken, and the spiral doesn't get infinitely small or infinitely large.

The same thing appears to happen in physics.

First, let's consider the strong nuclear interaction, which gives much of the mass of a proton and a neutron. This interaction is scale invariant over many many orders of magnitude in energy. That doesn't mean that the strong interaction doesn't change with energy, just as with the conch spiral, not having a fundamental scale doesn't mean that something can't be different at different scales. Just that the way it changes must be predictable across all these scales. But in Nature, as the strong interaction is evolved down to lower and lower energies, eventually the interaction "goes nonlinear."

What happens is that, as the strong interaction is probed at smaller and smaller energies (larger distances), it gets more and more powerful. That's fine and dandy, and doesn't break scale invariance. Eventually though, the strong interaction coupling gets so powerful that the interaction starts feeding back on itself. The coupling between strongly interacting particles becomes infinitely strong, even though a small increase in energy would return the interaction to the nice finite scale-invariant result. Thus, there is a scale at which physics fundamentally changes: the scale at which the strong coupling runs off to infinity. This is around a GeV in energy, which not coincidentally, is the mass scale of a proton or a neutron. Thus, a scale invariant theory (high energy strong interactions have no fundamental mass terms) breaks scale invariance and selects a particular energy that will be important. We physicists have no problem with this type of scale breaking. It's nice and elegant, and doesn't need anyone fucking around with fundamental energy scales to get it to work.

Now, there is another type of scale breaking: where you just add a mass term in by hand. This is what the Higgs theory seems to require, without something like supersymmetry, technicolor (which is just a retread of the strong interaction idea ported over to the Higgs, but appears not to be what the Universe decided to do), or more exotic ideas like Bill Bardeen's and the other authors here. Since the scale of the Higgs physics sets the mass of the electron, this sets the scale of physics at which atoms can be built and defines most of the Universe at the scales which we interact. Chemistry and thus us exist because the Higgs scale is only a few hundred GeV (well, that and the fact that the Yukawa couplings are small for the 1st generation. See, this is why these comments get really long).

The problem is that there is no apparent mechanism to pick the Higgs scale, and barring some active mechanism forcing the Higgs to be light, we might expect it to be the same as the other scale that's important to the Universe: the scale at which gravity gets strong. This is the Planck scale, and it's 10^19 GeV, which the electroweak breaking scale (and thus the mass of the Higgs) is at the 10^2 GeV scale. While nothing prevents these two scales from being separated, if you're willing to accept that the Universe is tuning two fundamental parameters to one part in ~10^32, it seems very unlikely. The problem is that the Higgs mass will want to be same as the heaviest particle to which it couples, and it's hard to prevent it from coupling to physics at the Planck scale. So you have to work to keep the Higgs light. The idea discussed in these links is that maybe the Higgs doesn't need to run up be as heavy as the new physics at the Planck scale, thus avoiding the problem in a way that other ideas (like supersymmetry) do not.

So, either there is some clever mechanism which we just haven't figured out to separate these two scales, or, as suggested here, the scales don't need to be kept separate, or the Universe just doesn't care that the Higgs should be much much heavier than we see it to be. In that last case, the generally attitude of the community is that this requires some ideas from inflationary physics to be true: namely that we are observing only a small patch of spacetime in a much much much .... much larger space. This larger space (called the Multiverse) can have wildly different laws of physics in different patches, including different Higgs scales. Those patches (different "universes" which are not our visible "Universe") are just so exponentially far away that we will never interact with them, and inflation gives us a mechanism to explain how this crazy scenario came to be, as well as doing the job it was initially thought up to do, which is explain why the initial conditions of our Universe (the Big Bang) were so uniform.

So in this scenario, there is no solution to why the Higgs is light. It should be heavy. But in our patch of multiverse, it is just small by complete happenstance. You can search for a reason until the end of time, but you'll never find a mechanism to keep it light, because it is just chance. Unfortunately, the very mechanism of the multiverse, inflation, wipes out evidence of the far-off other universes. So it's hard to prove that this is the right answer. I personally find it intellectually unsatisfying, even though I expect inflation is true. I still think there are viable mechanisms to stabilize the hierarchy of scales, and I spend my time looking for them in the data.

Careful observers will note I didn't spend much time talking about the Salvio and Strumia paper or Bill Bardeen's work in detail. I'm still thinking about what I think about the idea, and while I love commenting on science here, I reserve my right to not put half-thought ideas about active research topics out for posterity. Mostly because I don't like commenting my dumb thoughts to paper until I'm good and ready.
posted by physicsmatt at 1:21 PM on August 25, 2014 [64 favorites]

We love you physicsmatt
posted by Strass at 1:32 PM on August 25, 2014 [9 favorites]

If they are right, what would the implications be?

For you or me: nothing. As far as human-scale things go, we already know everything there is to know about how fundamental (or foundational) physics affects us. Literally nothing anyone ever does at CERN or SLAC or any of those places could ever (or has ever) really be useful. This is only about our understanding of the mathematical framework of the universe.

For particle physicists: if their work is popular and deemed interesting, it will mean lots of new predictions about the universe. Most of these predictions will be fairly obscure (mainly having to do with what to expect or not expect as they crank up the power at LHC or measure the cosmic microwave background more carefully or learn more about dark matter), but given that there aren't any good theories that successfully combine particle physics with gravity, there is definitely a possibility of something more exciting.
posted by artichoke_enthusiast at 1:50 PM on August 25, 2014

Dang, I need to revisit this thread later on when I'm not busy at work.
posted by surazal at 2:05 PM on August 25, 2014

Physicists prove size doesn't matter!
posted by Zed at 2:06 PM on August 25, 2014 [1 favorite]

For you or me: nothing. As far as human-scale things go, we already know everything there is to know about how fundamental (or foundational) physics affects us. Literally nothing anyone ever does at CERN or SLAC or any of those places could ever (or has ever) really be useful. This is only about our understanding of the mathematical framework of the universe.


this is the kind of thinking that got the SSC canceled.

forgetting about the physics for a minute… i think the WWW is pretty useful, don't you?
posted by joeblough at 3:46 PM on August 25, 2014 [4 favorites]

All I want to know is:

if they create a Higgs something or other, could that create a new universe/big bang that destroys our current universe?

(j/k only kinda not, since I'm science illiterate?)
posted by subversiveasset at 3:59 PM on August 25, 2014

subsersiveasset. Short answer: no.

Physicsmatt longanswer (TM): The Higgs field in our visible Universe has a particular vacuum expectation value (vev), which in a minimal Higgs model (and we do not yet know if this is the correct model) is set by a combination of couplings and mass terms. It is conceivable that the Higgs vev is not the true vacuum, meaning that a different configuration of the field would have lower energy than the present situation.

If that is the case, then our Universe would be metastable: with an expected lifetime which is long, but not infinite. At some point in space and time, if the present vev is not the true vacuum, the Higgs field will quantum tunnel to the true vacuum (quantum tunneling is a real thing. Your computer would not work without it). This would be energetically favorable, and this lower vacuum state would act as a bubble nucleation site (like the defects in the wall of a glass of soda which cause CO2 bubbles to nucleate), and a bubble of true vacuum would start expending out from that point, at nearly the speed of light. It is also possible that this has already happened: we wouldn't know until the bubble wall hits us, since we can't get a warning of something moving at light speed. Interesting, if the Standard Model + simplest Higgs is the only physics, then the expected lifetime of the present vev may not be infinite. It depends critically on the mass of the top quark, due to the large coupling between the Higgs and the top (which is why the top is the heaviest fermion). We know the top mass well, but not perfectly. With a top mass of ~173 GeV, the uncertainty is about a GeV. Within the experimental uncertainties, the lifetime of the present vacuum configuration goes from infinite to "about a billion years." The paper I'm getting that from has an author list with a large overlap with the people working on the articles in the FPP. This is not a coincidence.

So, as I have joked before, it is a matter of national security that we measure the top mass precisely.

Keep in mind that this assumes nothing but the Standard Model up to the Planck Scale. Also, the low end of the lifetime is pretty much experimentally ruled out. The Universe is still here, and its been 13.7 billion years.

Joking aside, please don't lose sleep over this. This is *theoretical* physics. We're extrapolating over 17 orders of magnitude here, and minor changes in the input parameters can move around our results greatly. It's a possibility, but there are many FAR FAR FAR more probable and critical issues to worry about. Spontaneous vacuum collapse is not one of them.

Now, does the LHC increase this theoretical danger? Absolutely, completely, unambiguously no. One could be concerned that, by dumping a lot of energy into a small region, we could initiate vacuum decay by kicking the Higgs field over the potential barrier, rather than waiting for quantum tunneling. That's a fair concern, so we physicists worried about it. The answer is that there are significant number of cosmic ray events hitting the Earth all the time with a high center-of-mass energy than the LHC. If high energies were all that it took to destroy the visible Universe, the Universe would have been wiped out billions of years ago. (The create-a-black-hole-kill-the-Earth concern was addressed in a similar way; there the arguments are a bit more subtle, but lead to the same conclusion: if the LHC was going to kill us through unknown physics, cosmic rays would have gotten us before we evolved out of the oceans).

So we don't know what the LHC might find, but we know that whatever it is, it poses no risk to Earth or the Universe. Simply put, the Universe runs the equivalent of the LHC many times in our own atmosphere with cosmic rays. The Universe just neglected to have clear beam parameters and didn't build us the CMS or ATLAS detectors. The Universe is a cheap bastard like that.
posted by physicsmatt at 4:38 PM on August 25, 2014 [12 favorites]

Thanks for the tutelage, physicsmatt. I have a question about inflation I've been confused about that may not even parse, but here goes:

The Hubble has shown us images of galaxies traveling 13.2 billion light-years to reach it -- which means what they depict is from 13.2 billion years ago. But here’s what I don’t get: the light only took that long to get here if the starting point for it was in fact 13.2 billion light years away. Since the universe is expanding, if one rewinds time, it shrinks. Indeed, I've thought the Big Bang to mean that at one point the Universe was a singularity, both meaning in a condition before which our laws of physics can’t say anything, and that it was essentially compressed into a single point.

But if it was compressed into a single point — apparently about 5-600 million years further back from the 13.2 billion we’re now seeing — that means that 14 billion years ago everything was, to say the least, extremely close to everything else. So unless the universe is expanding faster than the speed of light, how could anything be 13.2 billion light years away from us, 13.2 billion years ago? Maybe something is that far now, but if so its light would only just be starting its journey to us. The whole light year calculation presumes that something was that far away from us then – a time when the whole universe was much, much smaller in diameter. Maybe it has something to do with the universe’s expansion as a matter of dark energy, e.g., the fabric of the universe itself expanding, vs. the expansion found as all the galaxies speed away from one another (countered by the actions of gravity)? Or is this exactly what inflation is meant to answer: the “inflationary period,” independent of dark energy, catapulted everything really far away from everything else in one swoop, and without anyone needing to worry about the limits of the speed of light? Just seems a bit convenient.
posted by zittrain at 5:12 PM on August 25, 2014

zittrain, the Universe is about 13.7 billion years old. We can see the Cosmic Microwave Background, which is the decoupling of photons from the rest of the cosmic bath, which occurred once the Universe cooled enough for electrons to bind with protons into neutral atoms. This occurred about 380,000 years after the "Big Bang." You can take a look here for my attempt to distinguish what we know about the early Universe (it was hot and dense and we understand physics back to a temperature of about 1 GeV), versus what we suspect (anything before that, which includes things like inflation).

Once you start the Universe up in as a flat, uniform, hot bath, you don't need inflation or dark energy to get the Universe we see today. When we look back in time with telescopes, we see things as they were at the moment the light was emitted, though the photons have been redshifted (lost energy due to cosmic expansion), and of course the observed objects are very very dim. We usually refer to the redshift (or z) of distance objects, rather than the actual distance, for reasons which will likely be clear in a moment.

z tells you the ratio of energy of photons at emission to the energy of photons now. It also tells you the size of the (visible) Universe today compared to the size at emission. z=1 is about 5.9 billion years after the Big Bang. z=2 is 3.3 billion years after the Big Bang. The CMB decoupled at z=1100 or so. You can see that z and age don't scale linearly.

Now, there's the lookback time, the age of the Universe at the time of emission. There's the distance light had to travel (which is just the lookback time times the speed of light, so the lookback time in lightyears). There is also the comoving distance of the object that emitted the light at this moment. For z=1, the comoving distance is 11 billion light years, though the light itself only travelled 13.7-5.9=7.8 billion light years. z=2 has a comoving distance of 17.2 billion light years. Something at z=1100 (the CMB) has a comoving distance today of 45.5 billion light years. Thus, the point in space which emitted the CMB photons that make up about 2% of the "snow" on a old-school CRT TV set (tuned to a dead channel) is now 45.5 billion light years away from us.

So that seems to imply superluminal (velocities > c) travel, right? Not at all, though it is confusing, I realize.

The thing that changed wasn't that a galaxy or a atom emitting the CMB ran away from us at v>c, it is that, as the photons travel towards us, the distance between objects, space itself increased in length. That is what causes the redshift, by the way. A photon's energy goes like the inverse wavelength, but if space stretches, the wavelength goes up, and so the energy goes down. Similarly, the distance between us and another object is free to stretch at a rate which would increase the distance between us at a rate greater than c, but nothing need (or indeed can) move faster than light. Light emitted today from a point in space that we saw in the CMB (now all grown up into a nice big galaxy with stars and planets and such) will take longer than 45.5 billion years to reach us: it will get here eventually (unless dark energy takes over), but the expansion of the Universe means that the distance the photons needs to cover will be larger.

So that's just "standard" cosmology. No tricks with dark energy (though nowadays the "Standard Cosmology" includes dark energy. This model is usually denoted as \Lamba-CDM - \Lambda Cold Dark Matter). Dark energy is the observation that, rather than the rate of increase in the Universe slowing, as would be expected with just matter and radiation, it is in fact speeding up. This must be due to some new, unknown form of energy-density, which we dubbed "dark energy." It might be a cosmological constant (energy which doesn't dilute as the Universe expands), or it might be something similar to that, but not quite the same. We don't have enough experimental knowledge yet, but it does pretty clearly exist, unless numerous experiments are misleading us in separate ways.

Inflation is something like dark energy, but occurring at the start of the Universe, bringing the "visible Universe," the point which we can see in the CMB right now, from the size of an atom to the size of a football, in a fraction of a fraction of a fraction... of a second. We suspect that this is the case because the CMB photons hitting us right now from opposite directions are the same to one part in 10^5, even though there was never enough time without inflation for those two points to have ever been in causal contact. So without inflation, how did those two points, 180 degrees apart on the sky, and now 90 billion light years apart, "know" to be the same temperature?

This doesn't mean inflation is right, but it does solve a lot of problems from the first moments of the Big Bang cosmology. My previous comment explains more about it.
posted by physicsmatt at 5:45 PM on August 25, 2014 [9 favorites]


Thank you so much for your contributions, Matt. You're very good at making this all somewhat approachable.
posted by flippant at 6:54 PM on August 25, 2014

What's the current consensus on whether the universe will expand infinitely, reach a stopping point, or start collapsing?

If this happens, how will this affect redshift/z value? Presumably if expansion slows, redshift decelerates, z starts to slow down too? Shrink? If that happens, the 13.7billion year sphere of known universe around us will start to expand faster than the actual universe is expanding, and eventually they'll match. When that happens, how will our perception of time compensate? If the sphere is expanding faster than the universe is expanding, doesn't that mean that the sphere is playing back from the edges in fast-forward?

Physics makes my head spin.
posted by Strass at 12:40 PM on August 26, 2014

Before the discovery of dark energy in 1998, the fate of the Universe was closely tied to the curvature of the space-time metric.

If the Universe has exactly the right energy density (the critical density), then space-time would be "flat": geometry would be Euclidean. For example, angles of triangles drawn in space well away from local gravity wells would add up to 180 degrees. If the energy density was any larger than this, the curvature would be positive, the Universe would have spherical geometry (equivalent to a sphere, but in 4 dimensions), and angles of triangles drawn in empty space would add up to 180 degrees (draw a triangle on the surface of a globe to see how this works). If the energy density is subcritical, the curvature is negative, and the Universe would be hyperbolic (sort of a saddle-shape, if we were in 3 dimensions). Separate from this the Universe could have topologies that would be compact or non-compact. That is, I think it is possible in all curvatures for the Universe to possibly be such that, if you went off in one direction, you'd eventually end up at the point you started from. Sort of like the "universe" of the game Asteroids: the space is flat, but the manifold is a torus (donut shaped).

Before the discovery of dark energy, we assumed that the Universe was made up of matter (energy that dilutes as the volume of the Universe, length^3), and radiation (energy that dilutes as length^4, the extra power of length coming from the redshift expansion). In that case, a Universe with positive curvature would necessarily have enough energy density for gravity to stop the expansion of spacetime and collapse back in a reverse Big Bang. A Gnag Gib or Big Crunch. Such a Universe is called "closed."

In such scenarios, in time we would see objects stop redshifting and start blueshifting, their apparent z would decrease, and things that were too far away to see would appear as they seem to start moving in towards us (the light from them would, of course, reach us before they themselves did). Eventually, it'd start getting somewhat toasty, and the Universe would start getting hot enough to rip electrons from atoms, then nucleons away from nuclei, and eventually protons and neutrons themselves would dissolve into quarks and gluons. What happens then? No idea.

A negative curvature Universe would not have enough energy density and so while the expansion would slow, it would not ever stop and the Universe would just get colder and colder as it got more and more empty, and the far future would be a Fimbulwinter of lonely burned out galaxies as all available energy sources were slowly used up. So, the expansion just keeps on keepin' on. This is an "open" Universe.

So your choice of fates was fire (closed) and ice (open).

A flat Universe has the same fate as an open Universe: the expansion here will slow and get infinitely close to stopping, but never quite get there. Before the discovery of dark energy, it was known that we were, by all observations, in a Universe with energy density remarkably close to critical, and so we were consistent with a flat Universe. Though it is what I would have picked if I was building a Universe (it has a certain je nais se quoi), this fact is surprising. A Universe that is even slightly away from perfectly flat in its infancy will get more and more un-flat as time goes on. Gravity perturbations build on themselves, so if you are even slightly "closed," the contraction of that extra energy density will pull you more and more "closed" as the Universe evolved. Same with openness. So the Universe must have been remarkably flat after the Big Bang for us to see it as flat as it is today. This is another reason we like the idea of inflation. A brief inflationary epoch in the early Universe drives you to near-perfect flatness. Thus explaining the present observations without appealing to some jackass like myself's sense of artistic style to explain the near criticality.

Now, the big change in 1998 was we discovered dark energy. We now know that the Universe has energy density very close to critical: the space-time is flat. We don't know the global topology (as far as I know. It is really possible that I'm messing this bit up. However, from observation we know that we are at least consistent with a Universe that is unbounded).

However, we know that the density of the Universe is critical (within errors) and about 32% matter (4% baryons, the rest dark matter), and the remaining 68% is something called dark energy. Dark energy, as far as we can tell, is consistent with not diluting at all as the Universe expands (last I heard, we are planning some space-based telescopes designed to test this to higher accuracy in the relatively near future). That is, if you double the size of the room, then you double the amount of dark energy in the room. Yes, this means that energy is not conserved in our Universe. That is fine, as I've mentioned before, in an expanding Universe, there is no time-invariance (the future and the past are obviously different), and without time invariance, Emmy Noether's amazing work proved that there is no energy conservation (even without dark energy, a redshifting photon loses energy and nothing "gains" the lost energy).

Your magic perpetual motion machine however, is still bullshit, unless you are tying the thing to two galaxy clusters not gravitational bound to each other and waiting 100 billion years.

Because dark energy (and we really have no idea what the hell it is) has this particular property, it causes the expansion of the Universe to accelerate. It acts as 'negative pressure' pushing the Universe's metric expansion to accelerate. So the rate at which distances are increasing isn't slowing down over time, the rate at which the metric is expanding (and thus the rate at which we see distant objects recceed) is increasing. This decouples curvature from the fate of the Universe. We still happen to be a flat Universe, but regardless, the end state of the Universe is one of ice: in some quintillions of quintillions of years from now, after all available light elements have been fused into stars and all decaying elements have died away (and maybe after even protons decay sometime after 10^34 years), the Universe will be a very cold and very dark place.

Of course, if you wait long enough, and we will have plenty of time, quantum fluctuations will bring a bit of heat and light to some small region of space, and you can have some wacky fun again. And if you wait a really really really long time, you might even get to see another set of initial conditions that look a lot like the Big Bang. Is that how we started? It seems unlikely (our Universe is way too big for this to be a viable solution), but at least it will keep eternity interesting.
posted by physicsmatt at 4:44 PM on August 26, 2014 [3 favorites]

Ugh. In the 2nd paragraph above, the line:

"If the energy density was any larger than this, the curvature would be positive, the Universe would have spherical geometry (equivalent to a sphere, but in 4 dimensions), and angles of triangles drawn in empty space would add up to 180 degrees (draw a triangle on the surface of a globe to see how this works)."

should be

""If the energy density was any larger than this, the curvature would be positive, the Universe would have spherical geometry (equivalent to a sphere, but in 4 dimensions), and angles of triangles drawn in empty space would add up to more than 180 degrees (draw a triangle on the surface of a globe to see how this works)."

Apparently I need an editor. Every time I read one of my old comments I find the most ridiculous typos.
posted by physicsmatt at 7:19 AM on August 27, 2014

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