Wednesday, 2 November 2011

Experimental success, theoretical debacle

(This post is an attempt to catch up with October subjects that were being trendy when I was on leave from blogging, even though I suppose no one cares anymore)

This year's Nobel prizes were by all means exceptional. In blatant disregard of noble traditions, the prize in physics was given for a groundbreaking(!) and recent(!!) discovery without omitting any of the key contributors(!!!). Indeed, the discovery of accelerated expansion is one of the greatest triumphs of modern science. The measurements of supernovae brightness in the 90s and subsequent experiments have demonstrated that the universe is currently dominated by a form of energy characterized by negative pressure. In fact, this "dark energy" has the properties of the vacuum energy aka the cosmological constant, first introduced by Einstein for completely wrong reasons. In science, experimental progress usually brings better theoretical understanding. And that's another exceptional thing about the recent Nobel: almost 15 years after, the understanding of the cosmological constant in the context of particle physics models is as good as non-existent.

The cosmological constant problem has been haunting particle physicists for nearly a century now. We know for a fact that all forms of energy gravitate, including the energy contributed by quantum corrections. Thus, we know that diagrams with a graviton coupled to matter loops, like the one in the upper picture, yield a non-vanishing contribution to scattering amplitudes. On the other hand, the sum of very similar diagrams with graviton coupled to matter loops in vacuum must be nearly zero, otherwise the approximate Minkowski vacuum in which we live in would be destabilized. The contribution of the electron loop alone (the lower picture) is about 50 orders of magnitude larger than the experimental limit. On top of that, there should be classical contributions to the vacuum energy, for example from the QCD condensate and from the Higgs potential, which are also naturally tens of orders of magnitude larger than the limit.

The usual attitude in theory is that when something is predicted infinite one assumes it must be zero, and that was a good enough approach before 1998. The discovery of accelerated expansion was a game-changer, because it experimentally proved that the vacuum energy is real and affects the cosmological evolution, therefore the problem can no longer be swiped under the carpet. In fact, the problems is now double. Not only we need to understand why the cosmological constant takes a highly unnatural value from the point of view of the effective low-energy theory (the old cosmological constant problem), but we need to understand why it is of the same order as the matter energy density today (the coincidence problem).

Neither the first nor the second problem has found a satisfactory solution to date. Not for a lack of trying. People have attacked the problem via IR and/or UV modifications of gravity, quintessence fields, self-tuning or attractor solutions, fancy brane configurations in extra dimensions, elephants standing on turtles, space-time wormholes, etc, see also the comment section for crazier examples. In vain, all these solutions either rely on theoretically uncontrollable assumptions, or they just shift the problem somewhere else. The situation remains so dramatic that there are 2 only solutions that are technically correct:
  1. The anthropic principle: the cosmological constant is an environmental quantity that takes different values in different patches of the universe, however more-or-less intelligent observers can see only those tiny patches where it is unnaturally small.
  2. The misanthropic principle: the cosmological constant is being adjusted manually by seven invisible dwarfs wearing red hats.
Both of these theories have a comparable predictive power. In the first case we currently have no way to know the fundamental theory that sets the statistical distribution of the cosmological constant. In the second case we don't know what the little bastards are really up to.

Maybe theory needs another clue that may be provide by one of the future experiments. The Planck satellite will publish an update on cosmological parameters in 2013, although the rumor is that there won't be any revolution. In the asymptotic future there is ESA's Euclid satellite who will precisely measure the distribution of dark matter and dark energy in the universe. Will I live to see the day when the problem is solved? My bet is that no, but I'd love to proven wrong...

For the best summary of the cc problem read Section 1 of Polchinki's review.

31 comments:

Robert L. Oldershaw said...

A novel and promising resolution of the vacuum energy density disparity was posted to arXiv.org.

http://arxiv.org/abs/0901.3381

Apparently, it was not the answer that theorists were looking for.

That is a shame because the theoretical reasoning also explained the physical meaning fine structure constant.

ThePeSla said...

I have speculations today from a third physics perspective that in some ways suggests things remotely similar to Oldershaw's link- for it concerns the nature of the vacuum and of the confusion of matters, and the fine structure constant(s).

I would not promote this as I am not making my blog widely circulated but just perhaps the answers we are looking for (for our era of physics) are or will be here in your lifetime. :-)

The PeSla

http://www.pesla.blogspot.com

Philip said...

I have read a news article pointing out this theoretical research:

http://arxiv.org/abs/1109.4315

I'm not qualified to judge it, but I'm deeply interested. It sounds very extraordinary and very simple at the same time. Are there definitive counter-arguements to it, or is it a valid option?

Andrew Oh-Willeke said...

Here at the Dispatches From Turtle Island blog, we are strong proponents of the "elephants standing on turtles" theory. ;)

Kea said...

The DE density equals 9pi/4, which is three times the DM density of 3pi/4, which is precisely the upper limit of viscosity/entropy ratio for the quark gluon plasma. This factor of three (DE/DM) is due to the 1/3 ratio of dynamical quark mass to proton/neutron mass.

Anonymous said...

Jester, you seem surprisingly willing to let people use the comments section of your blog to promote their own fringe theories.

Jester said...

Rather too lazy to monitor

Anonymous said...

Well, why exactly do people find alternative ideas so upsetting, and need to quickly react with anger and/or ridicule?

Excuse me if I answer my own question, but it is simply that humans find it very disconcerting when their beliefs are challenged by observations or other ideas.

Ironically, when an aged paradigm is in imminent danger of collapse, and most needs an infusion of radical new ideas, that is when the backreaction against the new ideas is the strongest.

Curious!

matpitka@luukku.com said...

To the first Anonymous. Isn't the basic function of blogs to help new ideas to get communicated? At least this is what I have been thinking.

chris said...

"see also the comment section for crazier examples"

hehehe

did i ever say that i like your sense of humor? well, i do :-)

Anonymous said...

Dear Jester,

the misanthropic principle as conventionally understood is somewhat more subtle, see here.

Anonymous said...

re. fringe dwellers, one can always adopt the strategy suggested by xkcd...

http://xkcd.com/955/

Anonymous said...

Hi Jester,
out of curiousity, would contributions from negative mass particles come with minus sign also?

Jester said...

No, there is a quartic divergence which is independent of mass. You can get a cancellation in unbroken supersymmetry (between bosons and fermions) or using ghost symmetry (ghost partners with the opposite sign kinetic term).

Anonymous said...

Anon1 here, replying to Anon2 and matpitka. The thing that bothers me is blogs are read by lay people who might be mislead into thinking these fringe ideas have broad acceptance within the scientific community. Although I can completely understand why Jester would rather not spend his time policing this stuff. As for the best way to communicate new ideas, I would favor such conventional means as submitting to journals, presenting at conferences, etc. If someone can't find acceptance for their views from such venues, maybe it says more about the quality of the idea than the close-mindedness of the scientific community (which tends to be greatly exaggerated by the folks on the fringe).

Ervin Goldfain said...

Anon1,

I disagree with your viewpoint. Theoretical physics is currently riddled with many challenges and unsettled questions. Blogs are a valuable source for communicating novel ideas and often times being open-minded and bypassing the establishment is a likely recipe for scientific progress.

And, by the way, why do you choose to hide your identity if you hold such confident beliefs?

Cheers,

Ervin

Anonymous said...

"Anon1 here, replying to Anon2 and matpitka. The thing that bothers me is blogs are read by lay people who might be mislead into thinking these fringe ideas have broad acceptance within the scientific community."

Surely you jest, Mr. Anon1!

Who has misled the lay public more than the string theory community, which by a stretch of definitions might be called part of the scientific community?

Your professed concern for minds of the "lay people" is condescending and a blatant cover for your prejudice for conventional science and against attempts to improve the rapidly failing conventional paradigm.

Albert Zweistein

Kea said...

Perhaps Anon1 will accept a bet. If the Higgs boson is ruled out this month, he/she reveals their identity. If it is not, we refrain from insulting Anon1 in return for six months (I would offer a good bottle of wine, but my budget does not extend that far).

Anonymous said...

That'd be a winning bet for me, since analysis of the current LHC run won't be nearly complete in a month. But I don't particularly care if you insult me or not. The wine would have been tempting though.

Anonymous said...

Ok so the dark energy is a mysterious expansionary force.

And inflation in the early universe is a mysterious expansionary force.

So are they related to each other?

Anonymous said...

Kea, the Higgs boson cannot be ruled out this month, this year or the next one. Your misunderstandings are truly outrageous.

Anonymous said...

Of course not!

If porker Higgsy is not found in the remaining 120-145 GeV hiding place, then it will be claimed that he is hiding in some more arcane corner of the multiverse.

This was the gambit with "magnetic monopoles", and with the heroic search for "free quarks".

Theorists will say the porker's mass is much higher, or he resides in an unobservable extra-dimension, or there is one out there somewhere but we should not expect to be close enough to observe it.

Gotta love that postmodern pseudo-science!

Albert Z

Kea said...

Such certainty about fairies, even under anonymity! One might even suspect that Anon1 seriously believes what he was told at school.

Anonymous said...

I liked Princeton too.

Albert Z

Kea said...

Mark, your racist tone is not appreciated here. As for this story, I am sure that that is not at all unusual for a dude geek. As you say, they have different standards.

Anonymous said...

Hey Jester, there's some cleaning up to be done here.

"If porker Higgsy is not found in the remaining 120-145 GeV hiding place, then it will be claimed that he is hiding in some more arcane corner of the multiverse."

OK, so, if the Higgs turns out to deviate from its SM properties (which wouldn't be surprising but welcome) in such a way that it makes it harder to produce or detect then we'll need more time to find it. You really must have serious intellectual malfunctions to blame this on the theorists!

Postmodern pseudo-science? Take your medication before polluting the web, please.

Jester said...

...and yes, I agree I need to think of some stricter commenting rules, as the above discussion demonstrates :-)

Kea said...

BTW, it is sexist to demand femininity from a feminist.

David said...

The problem with any "exotic" solution to the cosmological constant problem, for example using modified gravity, is that it must also solve the basic problems that Jester alluded to: namely why is every other contribution zero. This is why it shifts the problem, doesn't solve it.

I prefer Bousso's solution, were we can define a probability measure on the landscape using the holographic principle. He has likened the measure problem to renormalisation in the early days of QFT. There is a horrible impasse, and its resolution unsettles people and seems non-physical, but may ultimately lead to new understanding.

In this case, string theory has helped us greatly. It has given a coherent framework in which to answer these questions, through giving us the landscape, and the AdS/CFT correspondence. In addition, we have gained negative classical contributions to the cosmological constant, that are sources of cancelation for the positive terms that Jester told us about. These are the AdS vacua in some moduli potentials.

As one commenter alluded to, in the standard landscape/eternal inflation scenario, inflation and the cosmological constant problem are intimately linked. So we have gained even more by understanding this, and in string theory have gained a coherent framework to ask systematic questions, which in the case of cosmology can make testable predictions.

Granted, however, there may still be entropy problems with populating the landscape via CdL instantons…

David said...

Also, the cyclic scenario of Steinhardt and Turok is a good one. Here the small cosmological constant is necessary for the mechanism to work, which is, I suppose, weakly anthropic. The scenario also intimately ties "inflation" (in this case the deflation dual, see Gasperini and Veneziano), to the current accelerated expansion. There is also the falsifiable prediction that no gravity waves will be observed by Planck.

Observation of primordial gravity waves by Planck would falsify many theories on inflation and the cosmological constant, but prove few without a precise measurement of the tensor to scalar ratio and tilt of the power spectrum.The problem with any "exotic" solution to the cosmological constant problem, for example using modified gravity, is that it must also solve the basic problems that Jester alluded to: namely why is every other contribution zero. This is why it shifts the problem, doesn't solve it.

I prefer Bousso's solution, were we can define a probability measure on the landscape using the holographic principle. He has likened the measure problem to renormalisation in the early days of QFT. There is a horrible impasse, and its resolution unsettles people and seems non-physical, but may ultimately lead to new understanding.

In this case, string theory has helped us greatly. It has given a coherent framework in which to answer these questions, through giving us the landscape, and the AdS/CFT correspondence. In addition, we have gained negative classical contributions to the cosmological constant, that are sources of cancelation for the positive terms that Jester told us about. These are the AdS vacua in some moduli potentials.

As one commenter alluded to, in the standard landscape/eternal inflation scenario, inflation and the cosmological constant problem are intimately linked. So we have gained even more by understanding this, and in string theory have gained a coherent framework to ask systematic questions, which in the case of cosmology can make testable predictions.

Granted, however, there may still be entropy problems with populating the landscape via CdL instantons…

Also, the cyclic scenario of Steinhardt and Turok is a good one. Here the small cosmological constant is necessary for the mechanism to work, which is, I suppose, weakly anthropic. The scenario also intimately ties "inflation" (in this case the deflation dual, see Gasperini and Veneziano), to the current accelerated expansion. There is also the falsifiable prediction that no gravity waves will be observed by Planck.

Observation of primordial gravity waves by Planck would falsify many theories on inflation and the cosmological constant, but prove few without a precise measurement of the tensor to scalar ratio and tilt of the power spectrum.

David said...

All of what I have said is covered in the article by Polchinski that Jester cited. This wasn't deliberate (I read it later), but isn't a coincidence.