What is the world made of?

What is the world made of? Answers (and questions) from particle physics and cosmology Curtis Callan Physics Department Princeton University NISER Colloquium

Pre-particle physics version of the story Atoms are made of point-like Q=-e electrons (mass=.511MeV) bound to a very small nucleus of Q=+Ze that is also very heavy. Photons (visible or Xray) of a few eV energy can excite this compound system, or even eject its constituent parts. This picture, plus the Schroedinger equation, is all you need to understand chemistry, materials scienceand biology! But its just the first layer of the onion … By colliding atoms at MeV energies, the nuclei can be broken into simpler parts: the neutron (Q=0, m=940 MeV) and the proton (Q=+e, M=938 MeV). They attract each other in a complex way that leads to the existence of the stable nuclei, from Z=1 to Z=92. Some troubling facts suggest that this is not the core of the onion! The neutron decays (slowly): n→p + e--+ n emitting a mysterious m=0 particle (neutrino) that can’t be detected! The electron is not so simple either: It has a Q=+e, same mass partner, e+ . It has an identical copy at m=106 MeV(the muon) which also decays slowly: m-→e-- + n + n While this is what the world we see around us is made of, it is hard to believe that this random-looking set of constituents is the final answer to the big question. NISER Colloquium

We now know: nucleons are made of quarks! Structure inside the nucleons was sought by colliding them in accelerators of ever higher energies (from 3 GeV in the 50s to ≈3 TeV now). It soon became clear that nucleons can be excited to higher states, like atoms, and must have “stuff” inside. We are now sure that all “hadrons” are made of point-like constituents, paired up like (e m) .. in three “generations” : (ud), (cs), (tb). They have fractional charge (!), come in three identical “color” copies (?), and have their own weird masses: Q=+⅔ Q= -⅓ uct ds b uct ds b uct ds b Masses: (in GeV) The hadrons we know are made by putting quarks together in “colorless” combos₌: uu d dd u uu s p = n = L = …. However: the quarks are never seen in isolation! Nucleons cannot be torn apart to reveal their fractionally-charge constituents. This is pretty odd and completely new. NISER Colloquium

Deus ex machina: Confinement The heart of the matter is the way the quarks interact: through color gluons. Gluons are like the photon (m=0, spin1) … but couple to color not charge. They are colored themselves and self-interact … and this makes all the difference! 8 = 3x3-1 SU(3) gluons • ig • quark-gluon vertex • ig • gluon-gluon vertex    This is a new quantum field theory called QCD. It looks a lot like QED (electrons and photons), but has strange properties: force between charged objects never falls off! QCD : F = - const. Infinite ionization work QED: F = - Q/r2 Finite ionization work NISER Colloquium

What about the electron and friends (leptons)? It took some time to find them all, but it turns out that for each generation of quark, there is a corresponding generation of “leptons”. With odd masses … Masses: (in MeV) They interact with force carriers of the electromagnetic force (g) and the weak force (W, Z 0 ). They are SU(2)xU(1) gauge bosons and self-interact. The sym-metry is spontaneously “broken” (by the Higgs?) so that the W, Z 0have mass! ne e- nm m- nt t- Q= 0 Q= -1 The quarks also interact with W, Z in a very similar way (ignoring color). Neutron decay is “weak” because exchanged W is heavy. NISER Colloquium

The “Standard Model” of the world This appears to be a complete catalog of what’s needed to explain our world. The search for “constituents” could stop here: each particle type (quark, lepton, gluon, SU(3)xSU(2)xU(1) gauge bosons) is a point particle in the strict sense with no sub-structure. That’s the underlying dynamical concept of Quantum Field Theory. There are 21 numbers needed to specify the theory (quark masses and the like); within the theory there is no way to “explain” these parameters, or why there are three “generations” of quarks and leptons. Theory has been like this since Newton: his laws do not explain why g = 980 cm/sec2, the r-2 force law, initial conditions, … . Accelerator experiments have shown no evidence of “physics beyond the SM”, nor of the Higgs (in searches up to several hundred GeV). When we get a close look at electroweak symmetry breaking (LHC?), we fondly expect to see something new …. The SM encompasses, but not explain, two phenomena crucial to the creation of our world: time-reversal invariance violation and baryon decay (p→e+ne). Ask me later! There is another weirdness of the strong phenomenon of “confinement”: it depends on temperature and the quarks get loose in a hot enough plasma (T> 200 MeV?)! NISER Colloquium

Lift your eyes up unto the heavens … Big Bang cosmology provides an alternate way of looking at the world at very high energy and short distance. It has revealed that there are two components of the “stuff” of the world that accelerator experiments have missed. • Review the arguments which lead us to believe that it all started with a BANG: • The galaxies are isotropically receding from us with redshiftprop’l to distance • We are not special … an observer on any other galaxy would see the same thing • The world is filled with homogenous, isotropic gravitating matter • Matter is described by energy density r(t) and pressure p(t) • Geometry is described by a scale factor R(t) (distance to some fixed object?) • Time variation of r, p, R is governed by energy conservation: a test particle at distance R from us is attracted by the 1/R potential of all the galaxies at r<R …. In GR, k=+1,0,-1 gives open,flat, or closed geometry to universe. We will stick to k=0. H is just the “Hubble constant”: the red-shift vs distance relation at a given cosmic moment (~70 km/s/Mpsc today) . H-1  age of universe !! NISER Colloquium

Equations of state for the universe To solve this equation, we need to know how r varies with R. That is governed by energy conservation and the equation of state r = f(p). Three basic cases will cover most of what we need to understand our world: Galaxies, etc. Photons, etc. Empty space! Very Cold Matter: p=0 No work done to squeeze it into a smaller volume. Thus Relativistic Matter: p=r/3 Pressure work is needed to squeeze it down. Find that Vacuum Energy: p = -r Negative pressure sounds weird, but it can happen. It leads to r = r0 = const (!) In short, the expanding universe we see must have emerged from an initial singularity (R=0) at some finite time in the past. As it expanded, it cooled .. and must still be at T>0 now. In fact, we know from the cosmic microwave background measurement that Tnow = 2.7 K! To trace the details of this history back in time, we need to know how the equation of state changes as things heat up. But, with our Standard Model of the constituents of the world we can in principle do it … we know all the particles that can be made and their masses (?). NISER Colloquium

Digression on Vacuum Energy In a world filled with fluid of energy densityrand pressure p, we define the energy-momentum tensor Tmn . Under a Lorentz transformation L , T → LT T L ≠ T : you can tell that the original frame was not the vacuum. The case p = - r an exception: the energy-momentum tensor is the Minkowski metric, it is unchanged by a Lorentz transformation: No principle is violated if the local “vacuum” is described this way! There is no aprioriway of setting the value of r : it could be non-zero and it could be anything. It sets the zero of the energy, and has no effect on anything but gravity/cosmology. More later! It is easy to understand from the first law why p = -r also implies r = const. Because of negative pressure, the external force has to do work W = F dV = rdVto expand V to V+dV. The increase energy in the expanded volume keeps the energy density fixed! E=rV E’=rV+W = rV+rdV = r(V+dV) NISER Colloquium

Two Other Crucial Cosmological Facts of Life Cosmic Microwave Background: Once the universal T falls below 1 eV, the hot plasma recombines, and photons stream freely. This hot gas of photons cools as the universe expands (R T  const). By now, it will have turned into thermal microwave radiation coming from space. By matching to the Planck spectrum, we measure current Tuniverse WMAP satellite The WMAP satellite (latest in a series of experiments going back to 1965) sees a precise Planckian blackbody spectrum with Tuniverse = 2.73K Non-Baryonic Dark Matter: Astronomers have for thirty years accumulated evidence (galaxy rotation curves, gravitational lensing, etc.) that there is a lot of matter out there that doesn’t “shine”, doesn’t interact with “baryonic” matter (except via gravity), and doesn’t have any pressure. It participates in the dynamics of the universe, but can’t get hot as we go back toward the Big Bang. How much of this “stuff” is out there? NISER Colloquium

Standard cosmological model: first cut Look at the basic dynamical equation for a homogeneous, isotropic, flat, evolving universe, put in the known sources of energy/mass … does it all hang together? The big problem of cosmology is to assign numbers to these fractional energy contents today and project their variation back toward the Big Bang. Some facts: • baryon  .04 today. We can “see” it in our telescopes and there is a strong argument from primoridalnucleosynthesis (He/D/H ratios in primordial gas) • g,n .0001 today (negligible!). But grows relative to baryon as R -1and be-comes comparable when Tuniverse  3000K and the CMBR photons are set free. • dark matter  ??? The astronomical evidence suggests that dark matter > baryon but we really don’t know. These two components scale in much the same way. • dark energy  ??? There is astronomical evidence (nonlinear redshift/distance relation at large redshifts) which could be interpreted as dark energy  1. If it is in fact there, it is insignificant at early times and dominates at late times. Before the “first three minutes”, the equation of state gets complicated (quarks are liberated, T rises above quark masses, etc.) but the Standard Model predicts in detail what happens. NISER Colloquium

The Real NISER Student Colloquium, Bhubaneswar

DT “noise”: a new window on the Big Bang The cosmic microwave temperature fluctuations from the 5-year WMAP data seen over the full sky. The average temperature is 2.725 Kelvin, and the colors represent temp-erature fluctuations on a scale of 0.0002 K (blue to red): one part in ten thousand! Side remark: white circle shows the “horizon” at decoupling .. how far info could have traveled in the 300,000 years since t=0. But the T (not the DT!) in disconnected horizons is exactly the same. How can this be? This is the motivation for “inflation” (ask me later!). Fluctuations in T are the result of fluctuations in density. The latter are crucial for an astronomical understanding of the history of the universe: regions of overdensity undergo gravitational collapse to become stars, galaxies, clusters of galaxies, etc. The dr have to be of the right size and of the right scale for the universe of galaxies we see to have formed in the time available. So measuring this quantity is a very big deal … and it took an heroic experiment to do it right. NISER Colloquium

Precision cosmology from WMAP data dT is not just “white noise” on the celestial sphere: it has correlations on preferred angular scales. These correlations arise as primordial “white noise” fluctuations evolve during the first 300,000 years before decoupling. We understand this physics and we should be able to interpret the observed angular structure (I can only summarize this very long story!). • The primordial fluctuations arise at the moment of inflation and are probably nearly scale-invariant (dashed blue line). • The big enhancements at l = 100, 500, … are due to the physics of collapse. • Matter that couples to photons cannot collapse: radiation pressure smoothes it out. • Only dark matter density fluctuations can collapse at early times (they ignore the g’s) • The size of the first peak is set by darkmatter • The location of that peak has to do with the flatness of space (k=0), etc. etc. etc. One can fit the dT spectrum to the dozen or so parameters of a “standard cosmology”. The red curve is the (very good!) best fit. Pie charts show the values for x now, and at decoupling NISER Colloquium

What this means for particle physics • The existence of “dark matter” is now confirmed: neither Big Bang cosmology nor astronomy make any sense without it. Precision cosmology tells us that it constitutes 25% of the current energy density of the universe (five times as much as Standard Model matter). • What is this stuff? It consists of particles that interact weakly with ordinary matter and have m >> mproton. There is no such thing in the Standard Model … this is new. • There is a large cosmic flux of these particles (especially high in galactic gravity potential wells). With a big, quiet detector, rare collisions of dark matter with nuclei can be seen. • If the dark particle mass is not too big, it can be produced (as a rare event) at the LHC. If so, one could hope to determine its properties (mass, spin, interactions) • Both paths are being actively pursued: hope is to find the other 80% of the universe’s mass!! • The existence of “dark energy” is also confirmed. The particle energy content of the universe is consistent with flatness (confirmed) only if the “vacuum” has energy density rvac = (1.2 eV)4 • Our understanding of the vacuum in QFT permits any value for rvac, so this is not an embarrassment in itself, but it is hard to understand why it has the value it has. • It is possible that the vacuum energy is not constant, that it is some V(f) where f is a non-Standard Model field which varies with time … hard to attack this experimentally. • The (gentle) exponential expansion that will be our future may be related to the (violent) exponential expansion of “inflation” at the very beginning of the Big Bang. • Gaining a deeper understanding of the dark energy, if that is possible, is the most difficult challenge facing fundamental physics. It is an enigmatic messenger from outer space …. NISER Colloquium

NISER Student Colloquium, Bhubaneswar

What is the world made of?