3New York Times: April, 2003Reports a debate among cosmologists about the Big Bang.lll1.html
4Dr. Tyson, who introduced himself as the Frederick P. Rose director of the Hayden Planetarium, had invited five"distinguished" cosmologists into his lair for a roastingdisguised as a debate about the Big Bang. It was part ofseries in honor of the late and prolific author IsaacAsimov (540 books written or edited). What turned out to beat issue was less the Big Bang than cosmologists'pretensions that they now know something about theuniverse, a subject about which "the public feels somesense of ownership," Dr. Tyson said."Imagine you're in a living room," he told the audience."You're eavesdropping on scientists as they argue aboutthings for which there is very little data."
5Dr. James Peebles, recently retired from Princeton, whom he called "the godfather"; Dr. Alan Guth from the MassachusettsInstitute of Technology, author of the leading theory ofthe Big Bang, known as inflation, which posits a spurt of akind of anti-gravity at the beginning of time; and Dr. PaulSteinhardt, also of Princeton, who has recently beenpushing an alternative genesis involving collidinguniverses.Rounding out the field were Dr. Lee Smolin, a gravitationaltheorist at the Perimeter Institute for Theoretical Physicsin Waterloo, Ontario, whom Dr. Tyson described as "alwaysgood for an idea completely out of left field - he's hereto stir the pot"; and Dr. David Spergel, a Princetonastrophysicist.
6But Dr. Smolin said the 20th-century revolution was not complete. His work involves trying to reconcile Einstein'sgeneral relativity, which explains gravity as the"curvature" of space-time, with quantum mechanics, thestrange laws that describe the behavior of atoms."Quantum mechanics and gravity don't talk to each other,"he said, and until they do in a theory of so-called quantumgravity, science lacks a fundamental theory of the world.The modern analog of Newton's Principia, which codified theprevious view of physics in 1687, "is still ahead of us,not behind us," he said.Although he is not a cosmologist, it was fitting for him tobe there, he said, because "all the problems those guysdon't solve wind up with us."
7Today, you are listening to someone seemingly more out in left field -- a particle physicist.Particle physics: seeks to determine the laws of nature at a``microscopic” – really submicroscopic, level.What does this have to do with the Big Bang?EVERYTHING!
8With due respect to the New York Times, articles like this give a very misleading impression.We know:There was a Big BangThis even occurred about 13 Billion Years AgoWe can describe the history of the universe,starting at t=3minutesThere is now a huge amount of data and a picturewith great detail.
9There are lots of things we don’t know. With due respect to Lee Smolin, the correct address for these questions isParticle Physics.We can’t answer any of these questions without resolvingmysteries of particle physics. These will be the subjectof this talk.What is the dark matter?Why does the universe contain matter at all?What is the dark energy?What is responsible for ``inflation”?What happened at t=0?
10Physical Law and the Universe Newton: F=ma FG= M1M1/R2Motion of PlanetsLaws of electricity and magnetism, nuclear physics:Understanding of StarsEinstein: General Relativity – Expansion of the Universe
11EINSTEIN1905: special relativity, photoelectric effect, Brownian Motion1916: General RelativityCulmination of a 9 year struggle to understand Newton’sGravity, starting with ``equivalence principle”Tests: Precession of Mercury’s Orbit, Bending of LightImplications for the universe: COSMOLOGY
12Einstein + CopernicusAssume the universe is homogeneous and isotropic –no special place or direction.Einstein’s equations have no Static solutions.The universe expands!Einstein was very troubled – remember that at that time (c. 1920)Astronomers didn’t know about galaxies!
13Edwin Hubble, who started out as a lazy, rich kid, became one of the most important of all astronomers.
14HUBBLE (1921) Galaxies move away from us at a speed proportional to their distance
15The Cosmic Microwave Background Still, limited evidence for aBig Bang. Gamow, Peebles: if true, thereshould be a ``glow” left over from thisHuge explosion (but of microwave radiation, not light).Objects give off a characteristic spectrum of electromagneticradiation depending on their temperature; ``blackbody.”The temperature then was 20,000 degrees; today it would beAbout 6 degreesDiscovered by Penzias and Wilson (1969).Today: thanks to COBE satellite,best measured black body spectrum in nature.
183 minutes: Synthesis of the Light Elements CMBR: A fossil from t=100,000 years.He,Li,De: Produced at t=3 minutespe+Neutrino reactions stop;neutrons decay.nn
19Results of Detailed Nucleosynthesis Calculations: The fraction of the universe made of ``baryons”=protons + neutrons:During last two years, an independent measurement from studies of CMBR:Very impressive agreement!
21The CMBR and the Copernican Principle Just how homogeneous and isotropic is the universe? Reasonably so for galaxies on scales of 100’s of millions of light years.What about the CMBR?-- the temperature is the same to a part in 10,000 in every direction in the sky!Small variations only observed in Now studied with great precision.
23Is this reasonable?Remember, the CMBR is light from the time that the universe was 100,000 years old. At that time, 1,000 times smaller than it is today. When we look at photons separated by 3^o in the sky, those photons came from points that were separated by far greater distance than light any signal could have traveled between at that time in the history of the universe – they are ``causally disconnected.”gg
24So it’s not reasonable!The solution: ``Inflation”– the universe underwent a period of very rapid expansion, probably at about t=10-24 seconds.Is there evidence for this?
25Yes!The very rapid expansion of the universe produces small variations in the energy density (energy in different places), which lead, eventually, to the formation of galaxies and stars.The same small variations appear in the temperature of the CMBR!
26Observational Confirmation From satellites and earth based (balloon) experiments. Most recently the WMAP satellite.
29COMPOSITION OF THE UNIVERSE If 5% of the Universe is Baryons, What is the Rest?From studies of CMBR, of distant Supernova explosions, and from Hubble and Ground-Based observations we know:5% Baryons (protons, neutrons)35% Dark Matter (zero pressure)65% Dark Energy (negative pressure)
30A Confusing Picture: Where Do We Stand? We have a good understanding of the history of the universe, both from observations and well understood physical theory, from t=180 seconds.BUT:We don’t know why there are baryons at all!We don’t know what constitutes 95% of the energy of the universe.We know that the universe underwent a period of inflation. But we have little idea what inflation is.
31What’s the Problem?To answer these questions, we need to know how the universe behaved when the temperature was extremely high. Temperature=energy. So we need to know about high energies.In quantum mechanics, high energies=short distances. We need to know about the laws of physics which operate at very short distances.
32What do we know?Particle physicists know the laws of nature on scales down to one-thousandth the size of an atomic nucleus. The ``Standard Model”. This corresponds to temperatures about 1,000,000 times those of nucleosynthesis.BUT THIS IS NOT ENOUGH TO ANSWER OUR QUESTIONSExperiments at higher energy accelerators at CERN (Geneva) and Fermilab (Chicago) are testing our understanding at even shorter distances. Expect to discover new phenomena.
34One possible new phenomenon: Supersymmetry A new symmetry among the elementary particles. ``Fermions ! bosons; bosons ! fermions.There’s not time to explain why here, but if this idea is right, then it explains what the dark matter is!
39This symmetry proposed to solve puzzles of particle physics, but it turns out that the photino is – automatically – a naturalcandidate for the dark matter. If it exists with the conjecturedproperties, it is produced in just the right quantity to be the darkmatter.Supersymmetry also provides a natural way to understand whythere are baryons in the universe at all (a puzzle first posed byAndrei Sakharov).
40So a better understanding of the laws of nature – in the not too distant future – might answer two of the puzzles in our list.What about the others?Harder: But over time, we may have answers. All require, asSmolin says, an understanding of quantum mechanics andgravity (general relativity). Particle physicists do have a theorywhich reconciles both: String Theory. This is the subject ofanother talk. But String Theory does haveSupersymmetry (dark matter, baryogenesis)Candidate mechanisms for inflationA possible explanation of the dark energy.
41April 10, 2003ThursdayPrinceton Physics Department Colloquium, 4:30 p.m. - Jadwin A-10Speaker: Michael Dine UCSCTitle: "Bringing String Theory into Contact With Experiment"Abstract:String theory bears a striking resemblance to the real world. Butmaking precise predictions for future experiments is surprisinglydifficult. In this talk, I will explain the difficulties, and outlinethe approaches which are being pursued to developing a stringphenomenology. I will also describe some of the insights which stringtheory has already provided into long-standing puzzles of particlephysics.Host: Chiara NappiTea in Room 218 Jadwin Hall, at 4 p.m.