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Lecture 25: Dark Matter and Dark Energy

Objectives: examine the evidence for Dark Matter understand Ty Ia SNe and the evidence for Dark Energy. Lecture 25: Dark Matter and Dark Energy. most cosmologists believe >90% matter in the Universe is unseen or “dark”

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Lecture 25: Dark Matter and Dark Energy

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  1. Objectives: • examine the evidence for Dark Matter • understand Ty Ia SNe and the evidence for Dark Energy Lecture 25: Dark Matter and Dark Energy • most cosmologists believe >90% matter in the Universe is unseen or “dark” • moreover, “normal matter” (p, n, e-) is only a minor constituent! • there are now many lines of evidence to support this, here are 3 key ones: • (1) Dynamics of Galaxies: • disk of spiral galaxy is in circular motion: • then accelerations must balance at radius R • and so • N.B. M is the mass interior to R, not the total galaxy mass – why? M51 Additional reading: Kaufmann (chap. 28-29), Zeilik (chap. 25-26) PHYS1005 – 2003/4

  2. Rotation Curves of Galaxies: • Example: Sun is 8kpc from Galactic Centre and orbits at ~200 km/s. What is mass contained within orbit of Sun about Centre? • Answer: applying M = v2 R / G M = 7.5 x 1010 MO(verify!) PHYS1005 – 2003/4

  3. Thus, estimate gravitational masses of galaxies from their “rotation curves”: • measure spectra along long axis of spiral galaxy • use these to work out velocity shifts • correct for inclination of galaxy • calculate enclosed M versus R • N.B. need galaxy distance to convert angles to physical R • Main results: • rotation curves are flat • M visible in stars, gas, dust not enough to match gravitational M ! PHYS1005 – 2003/4

  4. PHYS1005 – 2003/4

  5. (2) Clusters of Galaxies: • random velocities of galaxies in clusters can be used to infer total M • apparently gravitationally bound, but requires much more M than is seen Hercules Cluster PHYS1005 – 2003/4

  6. Gravitational Lensing: - also allows direct inference of cluster’s gravitational M PHYS1005 – 2003/4

  7. (3) Cosmology: • consider sphere of radius R, density ρC, expanding according to Hubble’s Law • will expand forever if velocity at surface is the escape velocity • i.e. • which simplifies to = the critical density Open • - which for H0 = 72 km s-1 Mpc-1 ρC = 9.7 x 10-27 kg • - or about 6 H atoms m-3 • - evidence from CMB  Universe is exactly at ρC i.e. “flat” • - N.B. we only see a few % of ρC ! • - abundance of 2H  baryon (i.e. p/n) density ≈ 5% of ρC • - were it to be higher, we should see much less 2H (reminder of abundances from last lecture) Flat Closed PHYS1005 – 2003/4

  8. Dark Matter and Type Ia SNe: • 90% of matter in galaxies and clusters is not seen • Universe appears to be exactly at ρC • “normal” matter is only 5% of ρC 95% is “abnormal” ! • in 1990s, main alternates for this non-baryonic, or dark matter: • WIMPs (Weakly Interacting Massive Particles) • MACHOs (MAssive Compact Halo Objects) • best cosmological test comes from observations of standard candle out to largest distances! •  Type Ia SNe PHYS1005 – 2003/4

  9. What is a Type Ia Supernova? Normal star WD time • recognised by their light curves and spectra • not associated with young, massive stars •  product of older population of stars! •  accretion of matter onto a White Dwarf •  exceeds Chandrasekhar Limit  SN • (exact mechanism still hotly debated!) • Limit set by fundamental physics  Ty Ia SNe will be the same luminosity everywhere (1010 L) ! • N.B. this is the crucial assumption ! PHYS1005 – 2003/4

  10. the Supernova Cosmology Project spent much of the 1990s collecting observations of Ty Ia SNe • their efforts have paid off with spectacular results PHYS1005 – 2003/4

  11. Dark Energy: • >2000, most distant Ty Ia SNe observed •  implies that Universe is accelerating! •  need to re-insert Einstein’s Cosmological Constant Λ (his “greatest mistake”) into General Relativity! • Λcounteracts gravity and also contributes to making the Universe flat • but what is it? Energy of the vacuum? • composition of the Universe: current “best buy”: PHYS1005 – 2003/4

  12. see WMAP web site Formation of light elements: • Early phase: • photons, particles and anti-particles at first in equilibrium • photon energy drops, matter/anti-matter annihilation leaves small excess of matter (1 p or n per 109 photons) • t < 0.01s • equal protons and neutrons • as T↓ , equilibrium shifts 7:1 in favour of protons by T = 1.3 x 109 K • then stable, apart from n decays (1/2 life of 15 mins) • t > 300s • but could not happen until T < 8 x 108 K • then 2H  He via p-p chain (lecture 15) • almost every n ends up in He • 7:1 p:n  12:1 H:He  He abundance should be 25% (by mass), but too late to form heavier elements as T too low! • hence in first 10 mins, H, He + traces of D, 3He, 7Li are formed, but nothing more ! PHYS1005 – 2003/4

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