Cosmology and particle physics

Transcription

1 Fedora GNU/Linux; LATEX 2ɛ; xfig Cosmology and particle physics Mark Alford Washington University Saint Louis, USA

2 Outline I Particle physics: What the universe is made of. quarks, leptons, and the forces they feel II Cosmology: How the universe developed. The big bang and its aftermath III How the universe will end. Not with a bang, probably...

3 The cosmos and the quark The little questions (Particle physics). What is matter made of? What are the smallest particles? Can we understand how they give rise to the world we see? What laws do they obey? The BIG questions (Cosmology). How big is the universe? Infinite or finite? How old is the universe? Eternal? Did it begin? How is the universe built? What is its anatomy? What laws does it obey?

4 I. Particle physics What is the universe made of? q m H

5 I. Particle physics What is the universe made of? water: molecules atom: nucleus electrons neutrons protons q m H molecule: atoms, H 2 O quarks give most of the mass electrons take up the space and determine chemical properties neutron/proton: up quarks down quarks ddu/uud

6 How do we find out about the smallest constituents of matter? The most powerful microscopes are Particle accelerators, (Fermilab near Chicago, LHC at CERN in Geneva, etc). We use them to bash particles together at high speed, and watch what comes out.

7 The CERN collider facility Centre européenne pour la recherche nucléaire LHC event read-out Large Hadron Collider

8 The standard model of particle physics Matter particles ( fundamental fermions ) Forces (carried by gauge bosons ) Antimatter: Every matter particle has a corresponding antiparticle with the same mass but opposite charge. A particle and its antiparticle can annihilate to pure energy. anti electron (positron) electron photon up quark anti up quark

9 Is antimatter real? This cloud chamber picture shows electron-positron pairs being produced from high-energy photons.

10 The four forces of nature Force boson Gravity graviton holds large lumps of matter together to make stars, planets, and galaxies Weak W +, W, Z 0 causes some radioactive decays of nuclei Electromagnetism photon binds electrons to nuclei to make atoms, and sticks atoms together to make molecules Strong gluon sticks quarks together to make protons and neutrons, and sticks protons and neutrons together to make nuclei Question: why don t we see quarks around us in nature?

11 Confinement We never see quarks in nature because the strong force is so powerful that it won t let them go free. It confines them into bound states, like atoms except that you cannot break them open. quarks anti quarks Baryon: eg proton (uud) neutron (udd) Meson: eg pions (ud, du, uu/dd)

12 Confinement in action What happens if you try to knock a single quark out of a proton? When there is enough energy stored in the flux string, it forms a quark-antiquark pair, breaking the string. So you end up making a proton and a pion.

13 Mysteries of the Weak interactions The weak interaction is mediated by the W and Z bosons. It is weak because they are heavy (mass about 100 GeV = kg). Question 1: What makes them so heavy, relative to the electron etc? I.e. what causes electroweak symmetry breaking? Search for the Higgs boson. Question 2: What makes them so light? Gravity becomes as strong as the other interactions at the scale of the Planck mass, GeV. Why is gravity so different? The hierarchy problem Energy (GeV) electron proton W, Z top quark Higgs? Planck mass

14 Unsolved problems in particle physics (a selection) 1. How exactly do the weak interaction (W and Z) bosons become so much heavier than the photon, proton, etc? The search for the Higgs boson. 2. Why is gravity so much weaker than the other forces? The hierarchy problem. 3. What is the quantum-mechanical theory of gravity? The black hole information paradox; String theory. 4. Why are the laws of physics almost symmetric between matter and antimatter? The CP problem. 5. What particle contributes most of the mass in the universe? The mystery of dark matter.

15 II. Cosmology: How the universe developed Executive summary: The big bang billion years ago the universe was very dense and hot, and has been expanding and cooling ever since. It now contains a mixture of 4% ordinary matter (atoms made of quarks and electrons) 22% dark matter (rather mysterious) 74% dark energy (completely mysterious) only about 3 times older than the oldest rocks on earth.

16 The history of the universe: our viewpoint

17 History of the universe

18 Outstanding features of the universe today 1. It is big, and full of galaxies. 2. It has structure: the galaxies are clumped in filaments and sheets The structure problem 3. It is expanding: the galaxies are moving apart, accelerating slightly The mystery of Dark Energy 4. It contains a uniform background glow of microwaves The smoothness ( horizon ) problem 5. It is almost flat: there is very little curvature. The flatness problem 6. There is more matter than antimatter. The antimatter problem 7. Galaxies are much heavier than the sum of their quarks and electrons. The mystery of Dark Matter

19 (1) The universe is big, and full of galaxies Powers of 10: the sizes of things in the universe Large-scale structure (walls, voids, etc in distribution of galaxies) 1 billion light-years Size of a Cluster of galaxies 50 million light-years Distance between galaxies in a cluster 5 million light-years Size of one galaxy m 100,000 light-years Star to star m 1 light-year Solar system m 1 light-day Planet Earth 10,000,000 (10 7 ) m Man 1 m

20 Hubble ultra-deep field: Galaxies going on for ever Direction: Fornax Exposure time: 11 days Area covered: a dime, 75 feet away ( 2.5 meter soda straw ) Depth: about 10 billion light years

21 (2) Structure: How are galaxies arranged in space? Sloan Digital Sky Survey 2-degree Field Galaxy Redshift Survey We find voids and filaments. Why are they there? Nature 440, (27 April 2006)

22 (3) The expanding universe Edwin Hubble (1929): Other galaxies seem to be moving away from us. This just means the whole universe is uniformly expanding. There is no center to the universe. We now know that the expansion is accelerating We guess that this is due to a background of dark energy We have no idea what the dark energy is. Are galaxies, stars etc individually getting bigger? No, not yet

23 (4) The cosmic microwave background An amazingly uniform and perfect blackbody thermal spectrum, with a temperature of 3 K. Why is the universe so uniform?

24 (4) The cosmic microwave background, deviations from uniformity Wilkinson Microwave Anisotropy Probe These tiny deviations help to confirm that the universe is flat.

25 (5) The geometry of the universe: space is flat What do we mean by flat? What are the alternatives? The geometry of the universe depends on the overall energy density ρ relative to a critical density ρ c. ρ/ρ c = Ω 0 > 1 ρ/ρ c = Ω 0 < 1 ρ/ρ c = Ω 0 = 1 More energy: closed universe Less energy: open universe Critical energy: flat universe How does the Cosmic Microwave Background tell us that Ω 0 = 1?

26 How the universe ends up flat Matter and dark energy both contribute, Ω 0 = Ω m + Ω Λ Current observations indicate that they add up to exactly the critical energy density, giving a flat universe: Ω m 0.26 = 0.22 Ω Λ 0.74 Ω 0 = 1 }{{} dark matter }{{} normal matter The dark energy causes the expansion of the universe to accelerate.

27 (6) Matter vs antimatter The laws of physics have symmetry with respect to matter and antimatter. But our universe consists entirely of matter. How did the asymmetry arise? We don t know. Perhaps somehow from the slight matter-antimatter ( CP ) asymmetry of the particle interactions in the standard model? How do we know there isn t any antimatter? Even empty space contains Hydrogen, a few atoms per m 3. If there were antimatter anywhere in the universe it would annihilate with the matter around it, creating distinctive gamma rays that we would see on earth. The presence of antimatter would lead to a characteristic distortion of the cosmic microwave background.

28 (7) Dark Matter Dark matter is some additional type of matter, not made of quarks and electrons. The evidence for it comes from rotation curves of galaxies and gravitational lensing by galaxy clusters. We know it is not ordinary atoms, i.e. quarks and electrons, from our knowledge of nucleosynthesis combined with measurements of deuterium abundance in interstellar clouds.

29 Cold Dark Matter explains large scale structure (the voids and filaments) How would the evolution of the universe look if you could see dark matter? The best explanation of the observed distribution of galaxies comes from assuming cold dark matter, which is made of some type of heavy particle, something that is not included in the standard model of particle physics. The VIRGO consortium has run huge computer simulations of how a universe with cold dark matter would evolve.

30 Inflation: a solution to cosmological puzzles Inflation is the hypothesis that the very early universe, between and seconds old, underwent a sudden massive expansion, much faster than it is expanding now. Inflation has the potential to solve the flatness, structure, and smoothness problems. Inflation naturally leaves the universe very close to being flat. Inflation provides the seeds of structure by stretching quantum fluctuations out to galactic sizes. Inflation explains why the universe (specifically, the CMB) looks the same in all directions: before inflation it was a tiny thermally-equilibrated region. But that still leaves the mysteries of dark matter, dark energy, and (absense of) antimatter

31 Inflation and the structure problem Inflation stretches out tiny quantum fluctuations in the matter density Inflation Gravitational clumping After inflation they are big enough to lead to inhomogeneities in the cosmic microwave background, and ultimately to large scale structure and galaxies.

32 III. The fate of the universe Current observations indicate that the universe is open, and will expand for ever (or until a big rip ) years (now) stars burn H and He to heavier elements years years years years the bright stars burn out: galaxies consist of dark matter and dim stars, brown dwarfs or white dwarfs. the galaxies collapse into black holes, throwing out some of their dim stars into the intergalactic void. if GUTs are correct, protons decay: atoms fall apart into electrons, positrons, and neutrinos. the black holes evaporate into photons; the universe becomes a cold, rarefying gas of electrons, positrons, photons, and neutrinos.

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34 Inflation and the Smoothness problem time now (15 billion yrs) decoupling (300,000 yrs) A A B B } inflation ( sec) big bang A and B seem to be far apart, but really the universe expanded very rapidly at an early stage (inflation), and before that A and B had plenty of time to influence each other and come to the same temperature.