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” .
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 .
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 . 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.
- 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.
- 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.)
- 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.
- 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.
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.
 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? 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.
 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.
TAGS: LARGE HADRON COLLIDER, BIG BANG, SCIENCE AND SOCIETY, PARTICLE PHYSICS