1 / 12

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”

ady
Download Presentation

Lecture 25: Dark Matter and Dark Energy

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related