1 / 49

Anisotropies in the CMB

Anisotropies in the CMB. Current Topics 2010 Katy Lancaster http://www.star.bris.ac.uk/katy. The course. Today (12pm, 4pm): The Cosmic Microwave Background (CMB) This Thursday: NO LECTURE Next Monday (12pm, 4pm): The Sunyaev Zel’dovich (SZ) Effect Next Thursday (5pm)

dora
Télécharger la présentation

Anisotropies in the CMB

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. Anisotropies in the CMB Current Topics 2010 Katy Lancaster http://www.star.bris.ac.uk/katy

  2. The course • Today (12pm, 4pm): • The Cosmic Microwave Background (CMB) • This Thursday: • NO LECTURE • Next Monday (12pm, 4pm): • The Sunyaev Zel’dovich (SZ) Effect • Next Thursday (5pm) • Journal workshopwith many hints for the assessment

  3. General Resources • CMB temperature anisotropies • Wayne Hu’s website and associated articles: http://background.uchicago.edu/~whu/ • Particularly ‘Ringing in the new cosmology’ • CMB polarisation • Angelica Oliviera-Costa’s website and links therein: http://space.mit.edu/~angelica/polarization.html • Particularly her review article: http://xxx.lanl.gov/abs/astro-ph/0406358 • And movies! • WMAP / Planck websites, wikipedia….

  4. Assessment • Case study of a CMB experiment: • Relevant scientific background • How it works and any unique features • Key science achieved / promised • Comparison with competitors (esp WMAP) • Essay Format • No strict word limit, ~1500 words • Hard copies to me by 5pm Thursday 18th March • Essay Format • Lecture 5: an interactive case-study of WMAP

  5. Assessment • You could choose via topic: • CMB temperature anisotropies • CMB polarisation • Thermal SZ effect • Kinetic SZ effect • Brain storm of possible experiments: Some expts look at a combination CBI ACT DASI OVRO/BIMA ACBAR MAXIMA BOOMERANG SPT EBEX Ryle Telescope VSA SuZIE II

  6. Today’s lectures The Cosmic Microwave Background Lecture 1: Production of the CMB and associated temperature anisotropies

  7. Why are we interested? • The CMB is the oldest and most distant ‘object’ we can observe • It provided definitive proof of the proposed Big Bang model • Its intrinsic features allow us to place tight constraints on the cosmological model • Opened up the era of ‘precision cosmology’

  8. Discovery Penzias & Wilson

  9. Primordial Universe • Primordial (early) Universe hot and dense • Plasma of photons, electrons, baryons • T > 4000K • Hot, dense, devoid of structure, too hot for atoms to form • most photons had energies greater than the binding energy of Hydrogen • Photons and baryons tightly ‘coupled’ via Thomson scattering • Unable to propagate freely (opaque, like ‘fog’) • Perfect thermal equilibrium

  10. Recombination and decoupling • Universe expands, cools • 380,000 years after the big bang, T~4000K • Very few photons have E > 13.6 eV, binding energy of hydrogren (despite large photon-baryon ratio) • Electrons and protons combine: H • Very few charged particles (eg free electrons), Universe largely neutral • Photons no longer scattered, no longer coupled to the baryons • Escape and stream freely across the Universe We observe these photons today: the CMB

  11. Thermal spectrum COBE Perfect black body Proof that Universe Was once in thermal equilibrium as required By big bang models

  12. Thermal spectrum • COBE: CMB has perfect blackbody spectrum • As required by the big bang model • ie, at some time, the Universe was in thermal equilibrium • How? Two processes: • Thermal Bremstrahlung: e+pe+p+ • Double Compton scattering: e+  e+2 • Effective while collision rate > expansion rate • No process since has been capable of destroying the spectrum

  13. Last scattering surface • CMB photons have (mostly) not interacted with anything since they last scattered off electrons immediately before recombination • We are viewing the ‘surface of last scattering’ • All photons have travelled the same distance since recombination • We can think of the CMB as being emitted from a spherical surface, we are at the centre • Behind the surface (ie further back in time) the universe was opaque like a dense fog: we can’t see into it • Strictly speaking, the surface has a thickness as recombination was not instantaneous • This is important for polarisation…..coming later

  14. Last scattering surface

  15. Last scattering surface

  16. Observing the CMB today:Frequency spectrum COBE

  17. Observing the CMB today:Uniform glow across sky

  18. Observing the CMB today:Uniform glow across sky • This presents us with the ‘Horizon problem’ • Universe isotropic at z~1000? Must have been in causal contact! • Impossible! • Sound horizon size = speed of light x age of Universe @ z=1000 • We know this is ~1 degree • Universe was NOT in causal contact • Invoke inflationary theory to solve this • Universe in causal contact and thermal equilbrium, then experienced a period of rapid growth

  19. Observing the CMB: Blackbody Temperature

  20. Observing the CMB today • Photons released at recombination have travelled unimpeded to us today • Blackbody spectrum, T=2.73K • Much cooled via expansion of Universe • Observe at microwave frequencies • Highly isotropic (at low contrast) • Fills all of observeable space, makes up majority of Universe’s energy density • ~5x10-5 of total density

  21. Observing the CMB today:Turn up the contrast….. • Dipole pattern due to motion of Earth/Sun relative to CMB • Indicates a velocity of 400 km/s WMAP

  22. Observing the CMB today:Subtract dipole WMAP • Snapshot of the Universe aged 380,000 years! • Very beginnings of structure formation

  23. ‘Seeds’ of structure formation • At recombination, when the CMB was released, structures had started to form • This created ‘hot’ and ‘cold spots’ in the CMB • K in the presence of 3K background: difficult to see! • These were the seeds of the structures we see today

  24. Characterising the CMB:Statistical properties • Other astronomy: observe individual star / galaxy / cluster in some direction • CMB astronomy: concerned with overall properties • Quantify the fluctuation amplitude on different scales • Qualitatively: • Measure temperature difference on sky on some angular separation…..many times….find mean • Plot as a function of angular scale • Higher resolution doesn’t mean better in this context • ‘Power spectrum’

  25. < 20 > 9° 2 < < 1000 0.02° < < 90°

  26. Characterising the CMB:Statistical properties Amplitude of fluctuations as function of angular scale

  27. More rigorously • Measure temperature of CMB in a given direction on sky, • Subtract mean temperature and normalise to give dimensionless anisotropy: • Expand anisotropies in spherical harmonics (analogue of Fourier series for surface of sphere):

  28. Analogy: Fourier series • Sum sine waves of different frequencies to approximate any function • Each has a coefficient, or amplitude

  29. Back to the CMB… • Use spherical harmonics in the place of sine waves • Calculate coefficients, and then the statistical average: Amplitude of fluctations on each scale. This is what we plot!

  30. Visualising the components Multipoles

  31. In practice • Design experiment to measure • Find component amplitudes • Plot against • is inverse of angular scale,

  32. Plotting the power spectrum Doublebinned Note third peak Very small array (VSA), 2002

  33. ?? Generating theoretical INPUT Favorite cosmological Model: t0, , b, z* PHYSICS Via powerful Computer code CMBFAST Or CAMB OUTPUT Fit to data

  34. Primordial Anisotropies • As we have seen, the CMB exhibits fluctuations in brightness temperature (hot and cold spots) • Quantum density fluctuations in the dark matter were amplified by inflation • Gravitational potential wells (and ‘hills’) develop, baryons fall in (or away) • Various related physical processes which affect the CMB photons: • Sachs-Wolfe effect, acoustic oscillations, Doppler shifts, Silk damping • Signatures observeable on different scales

  35. Sachs-Wolfe Effect • Gravitational potential well • Photon falls in, gains energy • Climbs out, loses energy • No net energy change • UNLESS the potential increases / decreases while the photon is inside it • Additional effect of time dilation as potential evolves • Most important at low multipoles • Probes initial conditions • Also: integrated Sachs-Wolfe

  36. Acoustic Oscillations • Baryons fall into dark matter potential wells, • Photon baryon fluid heats up • Radation pressure from photons resists collapse, overcomes gravity, expands • Photon-baryon fluid cools down • Oscillating cycle on all scales Springs: Photon pressure Balls: Baryon mass

  37. Acoustic peaks • Oscillations took place on all scales • We see temperature features from modes which had reached the extrema • Maximally compressed regions were hotter than the average • Recombination happened later than average, corresponding photons experience less red-shifting by Hubble expansion: HOT SPOT • Maximally rarified regions were cooler than the average • Recombination happened earlier than average, corresponding photons experience more red-shifting by Hubble expansion: COLD SPOT

  38. First peak ~200 ~1º Characteristic scale ~1º

  39. Other peaks • Harmonic sequence, just like waves in pipes / on strings: ‘overtones’ • Same physics, 2nd, 3rd, 4th peaks…. • 2nd harmonic: mode compresses and rarifies by recombination • 3rd harmonic: mode compresses, rarifies, compresses • 4th harmonic: 2 complete cycles • Peaks are equally spaced in

  40. Harmonic sequence Sound waves in a pipe Sound waves in the early Universe

  41. Harmonic sequence Modes with half the wavelength oscillate twice as fast, =c/

  42. Peaks equally spaced 1 3 2

  43. Doppler shifts • Times inbetween maximum compression / rarefaction, modes reached maximum velocity • Produced temperature enhancements via the Doppler effect • Power contributed inbetween the peaks • Power spectrum does not go to zero

  44. Doppler shifts

  45. Silk Damping • On the smallest scales, easier for photons to escape from oscillating regions • This ‘damps’ the power at high multipoles • Referred to as the ‘damping tail’ Power falls off

  46. Power spectrum summary Acoustic Peaks Sachs-Wolfe Plateau Damping tail

  47. Many experiments…

  48. Many experiments… • Broadly fall into three categories: • Ground based: • VSA, CBI, DASI, ACBAR • Balloons • Boomerang, MAXIMA, Archeops • Satellites • COBE, WMAP, Planck • Listen out for mentions of these and their most significant results

  49. Summary • The cosmic microwave background (CMB) radiation is left over from the big bang • It was released at ‘recombination’, when the Universe became neutral and Thomson scattering ceased • Structure formation processes were already underway, and are imprinted on the CMB as temperature anisotropies • Next lecture: what we can learn from the anisotropies, and polarisation in the CMB

More Related