Monday, July 07, 2008

Recreating the Big Bang?

With the start of the Large Hadron Collider coming closer, the topic is present in the media more than ever. A commonly used motivation is the alleged recreation of the Big Bang (see illustration to the right).

Peter Woit recently mentioned that Martinus Veltman, winner of the '99 Nobelprize in physics, “described claims that the LHC will 'recreate the Big Bang' as 'idiotic', and as 'crap'. He said that this is 'not science', but 'blather', and that the field would come to regret this, arguing that if you start selling the LHC with pseudo-science, you will end up paying for it.”

I am totally with Veltman. But what is behind the story? What does the LHC have to do with the Big Bang?


The Making Of

It is interesting to trace back how this inaccurate description developed. In February 2000, BBC News wrote on CERN's SPS:
    'Little Bang' creates cosmic soup
    10 February, 2000

    Scientists have created what they describe as a "Little Bang" inside which are the conditions that existed a thousandth of a second after the birth of the Universe in the so-called Big Bang.

Two years later, CNN.com writes about RHIC:
    'Little' Big Bang stumps scientists
    November 20, 2002

    Smashing together atoms to produce conditions similar to those in the first cosmic moments, scientists came up with some startling results that could force them to reexamine their understanding of the universe.

Five more years later we can read on MSNBC about the LHC:
    Teams toil underground to re-create big bang
    March 2, 2007

    It is a $4 billion instrument that scientists at the European Center of Nuclear Research, or CERN, hope to use to re-create the big bang — believed to be the event that caused the beginning of the universe — by crashing protons together at high speed.

Within 7 years we have thus moved from a 'little bang' via a 'little Big Bang' to a complete recreation of the Big Bang, a story which catches on. The TimesOnline writes “The machine, the Large Hadron Collider (LHC), aims to recreate the conditions of the Big Bang, when the universe is thought to have exploded into existence about 14 billion years ago.”, the German magazine Stern titles "Large Hadron Collider" - Urknall im Labor (Big Bang in the Lab), and for the Telegraph the LHC turned into a “Big Bang Machine” that “could destroy the planet” [1].

True, the LHC speeds up particles to higher energies than SPS, but still this is far off from anything similar to the Big Bang.



The Big Bang

The Big Bang is believed to be the first moment of the universe. Technically seen, it takes place at arbitrarily high energy density. It is commonly expected however that in this regime quantum gravity becomes important, and the density is neither infinitely high nor is the volume arbitrarily small. But still the temperatures for this to happen would be somewhere in the Planckian regime, that is at average energies of about 1016 TeV.

To our best current understanding, the universe then undergoes a rapid phase of expansion during which all energy densities drop and all matter cools. With dropping temperature, we pass the scale above which we expect Grand Unification and the three forces of the standard model separate. This is believed to be somewhere at 1013 TeV. Then around a TeV there is the electroweak phase transition. At some hundred MeV, that is about 10-4TeV, quarks start to form bound states like protons and neutrons. This is commonly called hadronization. It is this transition that we can now hope to study in appropriately designed collider experiments [2].

After hadronization, at temperatures around one MeV (10-6TeV), atomic nuclei can form - a process that is called 'nucleosynthesis'. Around this temperature also the only weakly interacting neutrinos decouple [3]. At temperatures of the oder eV (10-12TeV) atoms form and photons decouple. These photons have been traveling freely since this so called `freeze-out'. We can observe them today in the cosmic microwave background with an average temperature of around 3K (10-3eV) because they have been further redshifted by a factor 1000 since the freeze-out. After freeze-out, structure formation sets in, first stars, galaxies, solar systems and planets form. Some of these planets might carry intelligent life, some might even have a blogosphere.

For a useful illustration of the universe's timeline, see here.



The Little Bang

There are several important differences between the conditions created at the LHC and the Big Bang.
  1. The LHC main program is proton-proton collisions. There is no sensible way in which one could understand the conditions created in these particle collisions as a thermal density distribution. These are scattering experiments. (Though some of the data obtained in these experiments can have thermal characteristics, this does not mean it was indeed similar to the early universe.) The LHC will also have a heavy ion program in which lead nuclei are collided which each other. In these circumstances it is more appropriate to speak of actually creating an intermediate state with a high density and energy density.


  2. However, in such heavy ion collisions, the produced state of high density from the two nuclei expands much more rapidly than would be the case in the early universe. Everything is over within the time span needed for light to cross a few diameters of the colliding lead nuclei, or a few 10-22 seconds. In fact, the expansion is so rapid that it is not even clear from the outset if one can expect any thermalization. In contrast to this, in the early universe the hadronization transition happens after about the first microsecond, and the Hubble expansion is so slow compared to the back and forth of the quarks and gluons that it's granted the early universe is thermal. (Again, though some of the data obtained in heavy ion experiments has thermal characteristics, this does not mean it was indeed similar to the early universe.)


  3. Also, in the early universe the expansion of the matter is due to the expansion of space itself. In the laboratory, it is the matter that expands in an to very good approximation flat and static background. Though this might not make a difference for the cooling of the matter, it is conceptually very different.


  4. The typical temperature that is created in heavy ion collisions is some hundred MeV. That is about 19 orders of magnitude below the temperature we expect at the Big Bang.




Bottomline

The LHC is not a Big Bang machine. It is more accurate to say that with the heavy ion program at the LHC we will be able to create conditions closer to that in the early universe than ever before. This sounds more boring, but at least it isn't blatantly wrong. Aside from this, it is more useful to think of the LHC it as the world's largest microscope, that will help us to peer into the structure of elementary matter to a resolution better than ever before.



[1] For extensive explanation why it is implausible the LHC will cause the end of the world, see: Black Holes at the LHC - The CERN Safety report, Black Holes at the LHC - again, and Black Holes at the LHC - What can happen?
[2] Please note that we are here talking about temperatures. The energy scales usually quoted for the LHC (14 TeV for pp and about 1150 TeV for Pb-Pb) are total center-of-mass energies, not temperatures.
[3] Since neutrinos decouple considerably earlier than photons, measurement of the cosmic neutrino background could allow us to lock back further than the cosmic microwave background.


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24 comments:

plato said...

How about cosmic particle collisions?

coraifeartaigh said...

Hi Bee and Stefan,
I think there is a very simple explanation for this confusion, as somone who dabbles a little in science journalism.

The problem is that scientists frequently make statements like "such and such an accelerator will re-create energies not seen since the Big Bang". Not technically incorrect, but misleading..

It is not made clear to journalists that what is really meant is "energies not seen since shortly after the BB" (a different thing you'll agree) - so they therefore assume we are re-creating the Bang itself!

So it's a failure of science communcation!
Regards,
Cormac

Phil Warnell said...

Hi Bee and Stefan,

Nice piece, this recreating the early universe stuff is obviously what makes for good headlines and was never intended to be good science. Also it is probably considered by even those involved as what’s required in selling the public on the exspendager being justified. To describe it as simply a better microscope just isn’t sexy enough I’m afraid for those that have no idea about the frontiers of physics. On the other hand, it has had its backlash with many believing the scientists are playing god or some such silliness. This has a lot in common with the doomsday scenarios that proved to be so hard to dispel.

Best,

Phil

Phil Warnell said...

Hi Bee and Stefan,

Just as another side remark, which is how can one logically justify statements like ‘exploring energies not seen since the beginning of the universe’ with ‘not worrying about the energies created for they are far exceeded by everyday cosmic ray events’. Like the old saying goes, ”one cannot suck and blow at the same time”:-)

Best,

Phil

Bee said...

Hi Coraifeartaigh,

I understand that it must be difficult to be a science journalist and to find the right amount of details that can sensibly be communicated in popular writing. But I am really tired of hearing excuses like this. If a journalist doesn't know what energies 'not seen since the Big Bang' means, he or she should ask and clarify and not fantasize. Besides this, it isn't hard to find out. If you look at the three examples I have in the post, you will see that the first one is actually quite accurate, but then the explanation became increasingly fuzzy and eventually outright wrong. Every decent science journalist should have been able to figure that out. I suspect that there is just little motivation to actually get things right, but instead there is an emphasize on advertising and entertaining, and the more bang the better. Best,

B.

Bee said...

Plato, please read point 1. A collision of elementary particles doesn't create a hot and dense plasma for the same reason a grain of sand doesn't make a beach.

nige said...

"I suspect that there is just little motivation to actually get things right, but instead there is an emphasize on advertising and entertaining, and the more bang the better."

It's about making money. The exact reason why so-called cosmological "implications" of high energy physics research are eagerly hyped by the media was explained by Jeremy Webb, former BBC sound engineer and now Editor of New Scientist, in Roger Highfield's article "So good she could have won twice" in the Daily Telegraph, 24 Aug. 2005:

"Prof Heinz Wolff complained that cosmology is 'religion, not science.' Jeremy Webb [Editor] of New Scientist responded that it is not religion but magic. ... 'If I want to sell more copies of New Scientist, I put cosmology on the cover,' said Jeremy."

Since Jeremy's job includes the requirement to make sure that the magazine sells well, it's obvious why any alleged development in particle physics which has a cosmological connection is likely to end up as the story spin on the front cover.

The easiest way for a science journalist to begin an article about the LHC in a way that will grab an editor's attention and the typical reader's attention, is to write something like:

"The purpose of the LHC is to recreate exactly the conditions which occurred in the big bang, allowing us to see precisely what occurred during the creation of the universe."

I'm glad that Martinus Veltman and bloggers such as Peter Woit and yourselves decided to be honest about what the LHC really is up to. It's interesting enough to investigate the electroweak symmetry breaking mechanism, without the really interesting physics of the LHC being misrepresented by the BBC News and other purveyors of nonsense.

Plato said...

That's not what I was asking Bee:)

Does the LHC "simulate" cosmic particle collisions?

Bee said...

depends on what you mean with 'cosmic particle collisions' or 'simulate'. The LHC is supposed to collide pp at 14 TeV, and PbPb at 1150 TeV, period.

Plato said...

Simulate

These are aspects of the standard model are they not?

What's with the "recent comment" section on the right hand side?

Bee said...

Plato: It would really help if you could be more precise, I neither have the time nor the patience to guess around. This picture you link to with an incredibly bad resolution could

a) Show an incoming cosmic ray that creates a shower by scattering on something in the Earth's atmosphere. It neither says what the incoming particle is in this case, nor what it hits in the primary collision, nor what the energy is you are looking at. It is commonly believed that cosmic rays are caused by protons, but they hit atoms in the atmosphere, which contain, but are not just protons. The energy scales of cosmic ray collisions range from energies far below to somewhat above what the LHC can reach, the signatures are very different because the created particles don't just hit a detector in some meters distances but they travel some more thousand meters through the Earth's atmosphere.

b) Show a proton scattering on the CMB background. The LHC doesn't scatter protons on photons.

Best,

B.

Bee said...

I don't know what's wrong with the recent comments applet.

Plato said...

Layman apologies:) You don't have to answer any more if you don't like too.:)

Cosmic rays are caused by protons from outer space. When a proton (shown in yellow) hits the air in the earth's upper atmosphere it produces many particles. Most of these decay or are absorbed in the atmosphere. One type of particle, called muons (shown in red), lives long enough that some reach the earth's surface See: Introduction to Cosmic Rays

Does LHC create muons(Gran Sasso)?

Bee said...

Hi Plato,

You don't have to apologize for being a layman, but I can't answer questions if I don't know what the question is. Yes, the LHC will produce muons. See these funny looking caps of ATLAS? They are part of the muon detector. Best,

B.

Plato said...

The Pierre Auger Observatory in Malargue, Argentina, is a multinational collaboration of physicists trying to detect powerful cosmic rays from outer space. The energy of the particles here is above 1019eV, or over a million times more powerful than the most energetic particles in any human-made accelerator. No-one knows where these rays come from.

This is why John Ellis was instrumental in my comprehension of what is taking place.

To deny astrophysics in correlation to the work being done in the LHC is how you might say "a nutty response" to what is actually happening in reality.:)

Uncle Al said...

A black hole has a finite diameter measured from outside its event horizon but a[near(?])n infinite diameter measured inside. The LHC is therefore a tardis creator. CERN secretly hopes to sell sonic screw drivers.

Accelerating polyisotopic lead is not clever (plus Enviro-whiner jabber about toxic vacua). Naturally monoisotopic gold or bismuth would be clever.

coraifeartaigh said...

Hey Bee,
I'm not a science journalist either!Re making the minimum effort at understanding, I think you're broadly right, but have defnitely come across this issue as a genuine misunderstanding more than once.

I guess like most teachers, I tend to blame myself if those I'm trying to inform misunderstand - it's easy to make statements that are ambiguous / misleading.

It's certainly an important issue - I suspect the confusion of LHC energy with BB is partly reponsible for the BH scaremongering..
Cormac

Bee said...

Hi Coraifeartaigh,

Yes, I guess the problem is on both sides of the communication. But instead of blaming each other, we should try to improve the situation.

I suspect the confusion of LHC energy with BB is partly reponsible for the BH scaremongering..

It didn't cross my mind there could be a relation between the two, do you really think so?

Best,

B.

Plato said...

It is necessary unfortunately to have to go even further from this post by you Bee:)An attempt by a layman for sure.:)

While one might think they are exempt from such "ism's" unfortunately "by association in post topic," such a view should be extended beyond the current value of this post?

I do not believe you would like to be held in such a regard as you have attested.

Anonymous said...

The following link:
http://arxiv.org/abs/astro-ph/9903300
on pg. 11 has an interesting figure representing the BBNS time/temperature evolution for light nuclei. The highest
temperature is roughly equivalent to 10 MeV (LHS of the figure) .
Question: you see anything wrong in extrapolating the p,n,d and A=3 curves to 150 MeV? The physics shouldn't change much. The nucleon-nucleon x-section decreases with energy, but the baryonic density increases, so the (say) deuteron fraction actually increases with temperature. Is this ok?

stefan said...

Hi anonymous,

anything wrong in extrapolating the p,n,d and A=3 curves to 150 MeV?

... well, at around 150 MeV at latest, the quark-gluon substructure comes into play, but even before, there should be thermal pions around in non-negligible numbers.

I really doubt how bound nuclei can be stable in these conditions - especially deuterons, with a binding energy of only about 2 MeV (1 MeV per nucleon). But even for He-4, binding energy per nucleon is about 7 MeV, so I am not sure if one can do this extrapolation.

Best, Stefan

Anonymous said...

Dear Stefan,
At the T = 10 MeV limit (LHS of the referred BBNS figure) photon disintegration is said to be responsible for the small amount of d, and A=3 nuclei. My argument is that their abundance IS small, but definitively non-zero, and seems to increase with energy. The pion-pair production channel opens up at E = 240 MeV, although (sure) at T = 150 MeV the thermal-tail contribution should be important. My point is: although individual light nuclei would (indeed) have a small survival rate, their overall abundance at those temperatures would not be zero, and may even increase with energy. Can you refer me to someone working in this field today. I wrote to those authors but haven't heard from them, yet. Thanks for your prompt response.

stefan said...

Hi anonymous,

My argument is that their abundance IS small, but definitively non-zero, and seems to increase with energy. ... My point is: although individual light nuclei would (indeed) have a small survival rate, their overall abundance at those temperatures would not be zero, and may even increase with energy.

True, besides just the binding energy, there are other factors in play - most importantly, I would guess, the energy-dependence of the cross sections and density in phase space. After all, there should be an equilibrium state changing adiabatically with time/dropping temperature (if I recall correctly, Hubble rate at this period is already much lower than typical collision/reaction rates...)

Now, I am not an expert at all at primordial nucleosynthesis, but naively, I would not expect the fraction of bound nuclei to increase very much further with temperature/energy.

Sorry, I haven't worked on this topic and don't know anyone whom I could ask directly. Maybe you can try to mail some textbook authors who show the plot, or the old-fashioned sci.phys.research, or a physics forum?

Cheers, and good luck,
Stefan

Anonymous said...

Dear Stefan,
I did try contacting the authors of that paper, without luck. It's perhaps understandable, since the work is 10 years old.
Thanks for responding, I'll follow your advice.