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Looking at the dark side

Looking at the dark side. Henk Hoekstra Department of Physics and Astronomy University of Victoria. What dark side?. It is clear that part of the contents of the universe consist of “ordinary” baryonic matter and photons .

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Looking at the dark side

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  1. Looking at the dark side Henk Hoekstra Department of Physics and Astronomy University of Victoria

  2. What dark side? It is clear that part of the contents of the universe consist of “ordinary” baryonic matter and photons. Observations show, however, that the matter we are made of is not the most important ingredient in the universe. These “dark” components do not interact through electro-magnetic interactions and therefore do not produce detectable radiation. In this talk I will outline why we have to take such a crazy idea serious!

  3. Cosmology The study of the contents of the universe is part of cosmology. Cosmology is the study of the global properties of the Universe. Hence we need to measure quantities on large scales. This is easier said than done…

  4. Major cosmological questions Some fundamental numbers/questions: • Mean density of the Universe • Geometry of the Universe • Age of the Universe • Future of the Universe • What is the Universe made of?

  5. Changing Cosmology Our view of the universe has changed a lot since Galilei... Cosmological Principle: • We are not in a special place in the universe • Universe is isotropic (looks the same in all directions) • Universe is homogeneous (same stuff everywhere)

  6. Changing Cosmology Our view of the universe has changed much in the last 100 years... • Small, island universe (Milky Way) • Static Universe (Einstein’s early model) • Expanding Universe (Hubble, Big Bang) • Inflationary Universe • Accelerating Universe • something completely different…?

  7. What do we need to measure? • To understand the universe and answer some of the fundamental questions, it is important to measure masses and distances. • Unfortunately this is very difficult: • We can only measure direct distances to the nearest objects; other methods are indirect measures. • Most of the matter in the universe is invisible: the “dark matter”. In addition there is “dark energy”

  8. What can we measure? The universe expands and therefore more distant objects appear redder:redshift We can measure redshifts very well!

  9. Using 1037 W “lightbulbs” Measuring large distances using “standard candles” The more distant an object, the dimmer it appears. If we would happen to know the total energy output of the object, we can infer the distance! This technique has been used for nearby variable stars and very distant supernovae.

  10. Using 1037 W “lightbulbs” • discover supernova • get spectrum • is it type Ia? • follow multicolor lightcurve • model lightcurve • go to 1 Eventually……

  11. A runaway universe? The 1998 supernova results were surprising: the rate of expansion is accelerating! This implies that the dynamics of the Universe is dominated by the “dark energy”. This measurement has profound implications for our understanding of particle physics: the observed (small) amount of “dark energy” is not easily explained.

  12. What is the Universe made of? (and how much is out there….) • To answer this question we would like to measure masses of astronomical objects. This can tell us about: • Formation of structure • Evolution of galaxies • Properties of dark matter • Cosmological parameters

  13. How to measure masses? Comparison of masses for objects in the universe to the amount of light they emit suggests there is much more matter than meets the eye. Dark matter Could the emission be missed? (unobserved wavelength) NO!

  14. How to measure masses? Comparison of masses for objects in the universe to the amount of light they emit suggests there is much more matter than meets the eye. Dark matter Could the masses be overestimated? NO!

  15. How to measure masses? • Masses can be determined through various techniques • “direct”: • Measure the strength of the gravitational force • theory of gravity • way of measuring the force • “indirect”: • Infer the mass from the dynamics of the system • theory of gravity • tracer of the potential • assumptions about dynamical state (equilibrium)

  16. Clusters of galaxies • 1000s of galaxies • many elliptical galaxies • ~1 Mpc radius Optical image of the Coma cluster

  17. First evidence for dark matter “On the masses of nebulae and of clusters of nebulae” Fritz Zwicky (1937) “The Coma cluster contains about one thousand nebulae. The average mass of one of these nebulae is therefore M > 4.5x1010 solar masses. … This result is somewhat unexpected, in view of the fact that the luminosity of an average nebula is equal to that of about 8.5x107 suns. The conversion factor from luminosity and mass for nebulae in the Coma cluster would be of the order 500 as compared with about 3 for the local Kapteyn stellar system.”

  18. Hot cluster gas

  19. More evidence for dark matter

  20. Where is dark matter relevant? log(M/Msun) 8-12 12-13.5 13.5-15 >15 • galaxies (in particular outer parts) • groups of galaxies • clusters of galaxies • super clusters of galaxies But where/what is the dark matter?

  21. Gravitational lensing or …Nature’s own weighing scales Zwicky (1937): “… The gravitational fields of a number of “foreground” nebulae may therefore be expected to deflect light coming to us from certain background nebulae. The observations of such gravitational lens effects promises to furnish us with the simplest and most accurate determination of nebular masses. No thorough search for these effects has as yet been undertaken.” The first gravitational lens was discovered in 1979

  22. Gravitational lensing

  23. Gravitational lensing Nowadays it is seen everywhere!

  24. Gravitational lensing or …Nature’s own weighing scales • Gravitational lensing provides a powerful tool to study the dark matter distribution in the universe. • It does not require assumptions about the dynamical state of the system under investigation. • It can probe the dark matter on scales where other methods fail, as it does not require visible tracers of the gravitational potential.

  25. Strong lensing Image splitting by massive galaxies

  26. Weak lensing We observe that the images of distant galaxies are aligned.

  27. Weak gravitational lensing Example of “everyday lensing”.

  28. Weak lensing A measurement of the shape of a galaxy provides an unbiased but noisy measurement of the lensing signal.

  29. Mass distribution in clusters

  30. Visualizing the “invisible” mass distribution

  31. Visualizing the “invisible” mass distribution

  32. Cosmological weak lensing • To measure the weak lensing signal we need to measure the shapes of large numbers of galaxies • We need to observe a large area on the sky • Measure shapes accurately • Compare the results to numerical simulations Cosmological parameters

  33. CFHT Legacy Survey Megacam: FOV 1 square degree

  34. CFHT Legacy Survey MegaCam has ~ 350 Megapixels! We will observe 140 square degrees on the sky multiple exposures in 5 different filters ~5500 images, ~ 1.5GB each… More than 8TB of data!

  35. Comparison with simulations

  36. Comparison with simulations

  37. The ultimate probe: CMB The Universe started with a Big Bang For about 300,000 years the universe was ionized and opaque. Then protons and electrons combined and the “fog” lifted. The surface of last scattering gives rise to the Cosmic Microwave Background (CMB), which is now observed to have a temperature of 2.7K. Small seeds of structure give rise to small temperature fluctuations, which allow us to do cosmology.

  38. The ultimate probe: CMB

  39. The ultimate probe: CMB The temperature fluctuations are tiny: one part in 100,000 and hence very accurate measurements are needed. After subtracting the mean temperature, the motion of Earth through the Universe and removing the emission of the galaxy, one obtains a map of the “ripples” in the CMB

  40. The ultimate probe: CMB WMAP image of the CMB

  41. The Dark Universe • Cosmic Microwave Background • Type Ia supernovae • Large Scale Structure • all provide strong evidence for the existence of • Dark Energy • in addition to the dark matter…

  42. The Dark Universe

  43. The result? • The progress made in recent years is amazing! • The result, however, is embarrassing: • The more we measure, the less we understand! • ~70% is dark energy, which we do not understand • ~25% is dark matter, which we do not understand

  44. The Future? • Understanding the nature of dark matter and dark energy are among the most important questions of this decade (and coming ones…) • Ongoing Canadian-French effort: • Canada-France-Hawaii-Telescope Legacy Survey • largest weak lensing survey (cosmic shear) • largest type Ia survey • Next step: space based missions • Dark UNiverse Explorer (DUNE) • SuperNova Acceleration Probe (SNAP) • Joint Dark Energy Mission (JDEM)

  45. Conclusions • The evidence for dark matter from observations of various objects (galaxies, clusters) in the universe is convincing. • These results are supported by studies of the global properties of the universe (CMB) • Alternative theories of gravity cannot explain the results Dark matter exists! Now we (only) need to detect it directly

  46. Conclusions The accuracy with which we can measure cosmological parameters is increasing rapidly, thanks to new, wellunderstood techniques. The improved measurements lead to new puzzles that need to be solved before we understand the Universe we live in! It is a good time to be a cosmologist!

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