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Seeing the Beginning: The Cosmic Microwave Background and What it Can Tell Us About Our Universe
- 1. Seeing the Beginning: ! The Cosmic Microwave Background ! and What it Can Tell Us About Our Universe! Blake Sherwin! Department of Mathematics and Theoretical Physics / Kavli Institute for Cosmology! University of Cambridge!
- 2. Outline! • Reminder: the expanding universe and the big bang! • The Cosmic Microwave Background (CMB): evidence for the big bang! • What we have learned about the universe from patterns in the CMB! • What we don’t understand and are working on now!! 2!
- 3. Where are we in the universe?! 3! 300 billion stars! >>100 billion galaxies!
- 4. We Have Observed an ! Enormous Number of Galaxies! 4! Image: Hubble Deep Field!
- 5. We Have Observed an ! Enormous Number of Galaxies! 5! Image: Hubble Deep Field!
- 6. • The further away galaxies are, the faster they seem to be moving away from us! All Galaxies Moving Away from Us! ! 6! speed distance
- 7. 7! galaxies! distance between galaxies! 1 billion years after big bang! Explanation: All distances are expanding with time! • Space itself is expanding. The more space in between two objects, the more it expands, so the faster they (seem to) move away. We are not at the center of the universe!! !
- 8. 8! galaxies! 2 billion years after big bang!
- 9. 9! galaxies! 3 billion years after big bang!
- 10. 10! galaxies! 4 billion years after big bang!
- 11. 11! galaxies! 5 billion years after big bang!
- 12. 12! galaxies! 6 billion years after big bang!
- 13. galaxies! 8 billion years after big bang!
- 14. galaxies! 10 billion years after big bang!
- 15. galaxies! 14 billion years after big bang!
- 16. • Space itself is expanding. The more space in between two objects, the more it expands, so the faster they (seem to) move away.! ! • Expansion suggests universe used to be denser and hotter, had a beginning in a fiery big bang! Explanation: Space is expanding! 16! +me
- 17. Outline! • Reminder: the expanding universe and the big bang! • The Cosmic Microwave Background (CMB): evidence for the big bang! • What we have learned about the universe from patterns in the CMB! • What we don’t understand and are working on now!! 17!
- 18. • Very early universe: so hot that heat holds apart electrons and protons (like interior of the sun)! • But: cools with expansion…atoms form after 400000 years! ! Predictions of The Big Bang ! (Universe Expanding out of a Fiery Primordial Origin)! 18! = light Hot early universe: opaque CMB Cool late universe: transparent
- 19. • Heat radiation (“light”) from the early fireball phase (like the surface of the sun) streams through the universe… to us ! ! Emission of the CMB from the Primordial Fireball! 19!
- 20. time (+ distance light travels!)! Early big-bang universe is dense and hot plasma (fire-like) ! History of the universe! ! Emission of the CMB from the Big Bang Fireball!
- 21. time (+ distance light travels!)! “afterglow” light should stream through the universe… to us:! ! Cosmic! Microwave! Background! Early big-bang universe is dense and hot plasma (fire-like) ! Emission of the CMB from the Big Bang Fireball! History of the universe!
- 22. time (+ distance light travels!)! “afterglow” light should stream through the universe… to us:! ! Cosmic! Microwave! Background! Early big-bang universe is dense and hot plasma (fire-like) ! Emission of the CMB from the Big Bang Fireball! We see faraway objects in the past because the light has taken long to reach us. So if I look out far enough into space I should see far enough back in time that I see the “primordial fireball”.!
- 23. time (+ distance light travels!)! “afterglow” light should stream through the universe… to us:! ! Cosmic! Microwave! Background! Early big-bang universe is dense and hot plasma (fire-like) ! Emission of the CMB from the Big Bang Fireball! Big bang theory makes a clear prediction: ! we should see light from the primordial “fireball” in the sky!
- 24. • Look out at sky… most of it is dark. Why don’t I see the primordial fireball wherever I look?! ! The CMB in the Sky?! 24! No fireball L just see Milky Way stars L
- 25. • Answer: expanding universe stretches wavelength of light (by 1000 times) from visible to microwave wavelength! • Our eyes can’t see this stretched radiation.! ! The CMB in the Microwave Sky! 25! 0.0005 mm 0.5 mm
- 26. • But can build “microwave eyes” / telescopes!! ! ! !“A Measurement of Excess Antenna Temperature at 4080 Mc/s”" ! The CMB in the Microwave Sky! 26! Penzias/Wilson! 1968!
- 27. • But can build “microwave eyes” / telescopes!! ! ! !“A Measurement of Excess Antenna Temperature at 4080 Mc/s”" ! The CMB in the Microwave Sky! 27! Penzias/Wilson! 1968!
- 28. • But can build “microwave eyes” to see this radiation!! ! ! !“A Measurement of Excess Antenna Temperature at 4080 Mc/s”" ! The CMB in the Microwave Sky! 28! Penzias/Wilson! 1968! "To get rid of them, we finally found the most humane thing was to get a shot gun…and at very close range we just killed them instantly. It’s not something I’m happy about, but that seemed like the only way out of our dilemma.”! Arno Penzias!
- 29. • Radiation that looks like the heat radiation of a 2.7 degree K object without color – i.e. blackbody radiation! Cosmic Microwave Background Light!! 29!
- 30. • Compare CMB spectrum measurement with predicted blackbody spectrum: great confirmation of big bang! ! Cosmic Microwave Background Light!! 30! xkcd! Frequency! CMBBrightness!
- 31. Outline! • Reminder: the expanding universe and the big bang! • The Cosmic Microwave Background (CMB): evidence for the big bang! • What we have learned about the universe from patterns in the CMB! • What we don’t understand and are working on now!! 31!
- 32. • Radiation that looks like a blackbody / heat radiation of a 2.7 degree K object! • Increase contrast by 1000: still uniform…! ! Is the CMB Brightness Uniform?! 32!
- 33. • NO! Turn up “contrast” by 100000! • See fluctuations in brightness across the sky:! ! Is the CMB Brightness Uniform?! 33! COBE Satellite Data ! (Nobel Prize for Smoot/Mather)! brighter! less bright!
- 34. • Enhance: send up better satellites or use better ground based telescopes...! ! CMB Fluctuations – Learning More! 34!
- 35. ! CMB Fluctuations – ! Best Picture From Planck Satellite ! 35! CMB Brightness Fluctuations!
- 36. ! CMB Fluctuations – ! Best Picture From Planck Satellite ! 36! Because we are observing light that has traveled to us for such a long time from so far away, we are seeing the past. This CMB image is a picture of the universe as it looked 13.8 billion years ago.!
- 37. ! CMB Fluctuations – ! Best Picture From Planck Satellite! 37!CMB Brightness Fluctuations (an actual picture of the Big Bang fireball)!
- 38. • Why are there these fluctuations in brightness?! • Very simplified explanation: some regions have more matter and so appear brighter.! ! ! CMB Fluctuations! 38!
- 39. • Regions with more matter è pull in even more matter through gravity è collapse to form galaxies, stars etc.! • The CMB is the universe’s “baby picture” – from 13.8 billion years ago, almost the beginning (99.997% there) ! ! Why is this Important – ! Understand Cosmic Evolution! 39! A bit more matter than average at start (“density fluctuation”)! Over time: a galaxy!
- 40. • Galaxies today grow from runaway gravitational collapse of small initial density fluctuations as seen in the CMB! Small Fluctuations in CMB turn into Galaxies! 40! [Video: A. Kravtsov++]
- 41. • Regions with more matter è pull in even more matter through gravity è collapse to form galaxies, stars etc.! • The CMB is the universe’s “baby picture” – from 13.8 billion years ago, almost the beginning (99.997% there) ! ! Why is this Important – ! Understand Cosmic Evolution! 41! A bit more matter than average at start (“density fluctuation”)! Over time: a galaxy!
- 42. • Regions with more matter è pull in even more matter through gravity è collapse to form galaxies, stars etc.! • The CMB is the universe’s “baby picture” – from 13.8 billion years ago, almost the beginning (99.997% there) ! ! Why is this Important – ! Understand Cosmic Evolution! A bit more matter than average at start (“density fluctuation”)! Over time: a galaxy! Where do fluctuations in matter density come from? [See later]!
- 43. • Our measurements of the CMB are now so precise that we can use it as a tool to learn about the universe! • Make PERCENT-level measurements of its composition, age, geometry…! ! Why is this Important II – ! Understand Cosmic Properties / Composition! 43!
- 44. • Determine the distance to the CMB fireball by looking at size of large bright spots; gives size of the universe and age.! ! Example: Size of the Universe! 44! too close! too far!correct! distance! what we observe!
- 45. • We don’t actually compare the fluctuations in the CMB brightness (~T) by eye with our models to measure things! • Instead, we use the power spectrum – tells us the magnitude of fluctuations as a function of angular size! Power Spectrum! 45! Size of ! Fluctuations! Angle spanned on Sky! best fit theory (with 6 free parameters)! Planck! data!
- 46. • Best described in Fourier / Spherical Harmonic space.! • Spherical harmonic transform:! • Measure power spectrum (variance)! Details 1: Measure Power Spectrum of ! CMB Temperature (T)! 46! Size of ! Fluctuations! Cl! Angle spanned on Sky! (multipole l) ! best fit theory (with 6 free parameters)! Planck! data! alm = Z dˆnT(ˆn)Ylm(ˆn) Cl = X m 1 2l + 1 |alm|2
- 47. • Calculate Cl(parameters – for example age of universe)! • How? Codes that rapidly solve differential equations for evolution / propagation of linear photon perturbations! ! Details 2: Theory Calculation! 47! Size of ! Fluctuations! Angle spanned on Sky! best fit theory (with 6 free parameters)! Planck! data!
- 48. • Can determine: P(data|parameters) –" " " " "Gaussian likelihood ! ! • But want: P(parameters|data)! Get via Bayes theorem: ! P(parameters|data) P(data|parameters) x P(parameters)! ! ! • Example: want to know about P(size of universe|data)! ! Details 3: Bayesian Parameter Estimation! / prior (choose!)!likelihood!posterior! / e [Cl Ctheory l (parameters)]2 /(2 2 l )
- 49. • Problem: P(many parameters), i.e. P(size,θ1,θ2,...|data),! • To get size distribution, need to integrate /“marginalize”: """! "- hard as high dimensional. Fast: draw samples from P! Details 4: Deriving Parameter Constraints! P(size) = Z d✓1d✓2 · · · P(size, ✓i)
- 50. • Problem: P(many parameters), i.e. P(size,θ1,θ2,...|data),! • To get size distribution, need to integrate /“marginalize”: """! "- hard as high dimensional. Fast: draw samples from P! • Great algorithm: MCMC = Markov-Chain Monte Carlo! Details 4: Deriving Parameter Constraints! Age of universe! Θ1 (Matter density)! ! samples! P(size) = Z d✓1d✓2 · · · P(size, ✓i)
- 51. – Origin and evolution of structure! – Age: 13.798±0.037 billion years! – Geometry: flat! – Composition: see following…! What have we learned from the CMB! 51! Size of ! Fluctuations! Angle spanned on Sky! best fit theory (with 6 free parameters)! Planck! data!
- 52. But Galaxies Only Trace ! Even More Underlying Dark Matter! 52! Galaxies! Dark Matter! <20% of the mass! Composition of the universe (in mass/energy): ! 5% normal matter in galaxies and gas! Galaxies, Gas: <20% of the mass!
- 53. 53! Composition of the universe: ! 24% dark matter! ! • Dark matter: an invisible, poorly understood substance! • We know it is there because it still has gravitational pull! • Know not just from CMB, but also many other methods!
- 54. 54! Composition of the universe: ! 71% dark energy! ! • Form of energy that is accelerating the universe apart!! • Implies that universe will expand faster and faster, leading to cold dilute universe!
- 55. 55! 71% of the universe: dark energy! ! • Form of energy that is accelerating the universe apart!! • Implies that universe will expand faster and faster, leading to cold dilute universe!
- 56. We have learned an ! enormous amount about the universe!! ! ! ! ! ! ! We live in a simple universe, evolved out of small density fluctuations and described by just six numbers! 56!
- 57. We know a lot!!!! ! ! ! ! ! ! ! But it seems the more we know, ! the more we don’t know… ! 57! ?? ? ??? fluctuations!
- 58. Outline! • Reminder: the expanding universe and the big bang! • The Cosmic Microwave Background (CMB) and what we have learned from it: evidence for the big bang and the standard cosmological model! • What we still don’t understand and are working on now!! 58!
- 59. time (+ distance light travels!)! Many Big Questions Left – Ongoing Research!! • What happened at the very beginning? ! " • What and where is dark matter?" ![ask! – topic of my research]" " " • What is dark energy?" !
- 60. What happened at the very beginning? ! 60! • What blew the universe up in the first second of the big bang and made it nearly smooth?! ! ! ! ! ! ! ! ! CMB!
- 61. 61! • Where do the tiny fluctuations in density come from?! ! ! ! ! ! ! ! ! CMB CMB! What happened at the very beginning? !
- 62. Inflation • Inflation: initial accelerated cosmic expansion, REALLY fast! ! • Many models: typically assume a single “inflaton” substance (a scalar field)! ! CMB!
- 63. Inflation • Inflation: initial accelerated cosmic expansion, REALLY fast! ! ! • Enormous expansion inflates the microscopic world of quantum mechanics to the macroscopic.! ! CMB!
- 64. Theory of Inflation! 64! ! • Inflation: period of REALLY fast initial universe expansion (1040 in just 1/1035 seconds) –! ! • Enormous expansion inflates the microscopic to the macroscopic. Random quantum fluctuations get magnified to big density fluctuations.! Quantum uncertainty: empty space is not really empty!
- 65. 65! Inflation blows quantum fluctuations up, generates small differences in density! ! Cosmic Inflation: Expanded! Quantum Weirdness Makes Everything! We can see these density differences in the CMB, a picture of (nearly) the beginning! Over time, they turn into the stars and galaxies we see around us today!!
- 66. 66! Inflation blows quantum fluctuations up, generates small differences in density! ! Cosmic Inflation: Expanded! Quantum Weirdness Makes Everything! We can see these density differences in the CMB, a picture of (nearly) the beginning! Over time, they turn into the stars and galaxies we see around us today!! Inflation: we (and everything) exist because of random quantum vacuum fluctuations in the early universe!
- 67. Inflation – How to Test Experimentally?! 67! ! • Lots of confirmed predictions already: ! – properties of CMB fluctuations (bell-curve distribution) ! – properties of CMB power spectrum! – flatness of the universe…! ! ! ! ! ! ! consistent with inflationary predictions! Size of ! Fluctuations! Angle spanned on Sky! best fit theory (with 6 free parameters)! Planck data!
- 68. • Prediction not yet seen: leaves a characteristic swirly pattern in the CMB polarization, “B- modes”:! ! • Find this: confirm inflation, understand the beginning of the universe!! CMB! Smoking Gun Evidence for Inflation: ! CMB B-mode Pattern
- 69. Understanding the Physics of Inflation with B-modes! • CMB B-modes with different strengths correspond to different kinds of inflation physics! Strength! of! signal! ~size of typical fluctuations! different kinds of inflation!
- 71. Problems with the BICEP B-modes! 71! • Initially claimed to see swirly B-mode patterns! • It turns out they were instead “boring” B-modes from dust in our galaxy mimicking the inflation signal! ! ! ! ! ! ! ! ! But: solvable problem, search continues at high precision!! ! distribution of ! dust in our galaxy!
- 72. 72! Search ongoing, e.g., with current Atacama Cosmology Telescope (ACT)! • Ground based CMB telescope high in the Chilean Atacama desert!
- 73. ALMA! ACT! CLASS! POLARBEAR!Existing! Notional Pads for Simons Observatory Phase 2 and CMB S4! And Upcoming Simons Observatory ! – 100 times more sensitive! Notional Simons Observatory Phase 1 ! Power! Control !Vehicle s!
- 74. time (+ distance light travels!)! We have learned a lot from the CMB, ! but there is much we still need to understand! • What happened at the very beginning? ! " • What is dark matter?" " • What is dark energy?" !
- 75. One thing is certain:! The CMB will continue to give us insight into the beginning ! of the universe and the composition, evolution and ultimate fate of the cosmos! ! ! Thank you!! ! ! ! ! 75!
- 76. Summary! • The CMB is a picture of the afterglow of the fiery early universe: our best proof of the hot big bang.! • From the CMB picture: we live a simple yet strange universe, with primordial density fluctuations, 24% dark matter, and 71% dark energy! • We are working on understanding the details of what happened at the very beginning (and more)! 76!
- 77. CMB Gravitational Lensing! • Distribution of dark matter (80% of the mass!) deflects CMB light that passes through! 77!
- 78. “Light” Source for Lensing: ! The Cosmic Microwave Background (CMB)! • CMB: leftover radiation from the hot primordial plasma – most distant observable source of light! • Distribution of dark matter deflects light that passes through! 78!
- 79. 79! ! • Look for characteristic lensing magnification feature to map distribution of dark matter! Mapping Dark Matter with ! Gravitational Lensing of the CMB! CMB with lensing feature! Map of dark matter!
- 80. ACTPol CMB Lensing Dark Matter Map (51 degs. long)! Mapping all the Dark Matter! ! • Precise lensing map of dark matter from ACTPol:! Light spot: a real blob of dark matter!
- 81. ACTPol CMB Lensing Dark Matter Map (51 degs. long)! Mapping all the Dark Matter! ! • Precise lensing map of dark matter from ACTPol:! Does the dark matter look as we expect?! By removing lensing “blurring”: what happened at the beginning of the universe?!
- 82. Outline! • The expanding universe and the big bang! • Why the big bang leads to the Cosmic Microwave Background (CMB)! • Patterns in the CMB! • Ongoing research II: seeing dark matter with CMB lensing! 82!
- 83. CMB Gravitational Lensing! • Distribution of dark matter (most of the mass!) deflects CMB light that passes through! 83!
- 84. “Light” Source for Lensing: ! The Cosmic Microwave Background (CMB)! • CMB: leftover radiation from the hot primordial plasma – most distant observable source of light! • Distribution of dark matter deflects light that passes through! 84!
- 85. 85! • Clump of dark matter in front…! Lensing: Effect on the CMB!
- 86. 86! ! • Dark matter causes lensing magnification feature in the CMB! Lensing: Effect on the CMB!
- 87. 87! ! • Dark matter causes lensing magnification feature in the CMB! Lensing: Effect on the CMB! d! d! d! described by lensing deflection field: d! ! (very small: here exaggerated by x ~100)!
- 88. CMB Lensing: How to Measure! d: lensing deflection field! CMB!CMB! Measure lensing by looking for magnification of characteristic scales in CMB correlations
- 89. CMB Lensing: How to Measure! CMB! [Seljak 1997; Hu 2001] d: lensing deflection field! Measure lensing by looking for magnification of characteristic scales in CMB correlations
- 90. ACTPol CMB Lensing Dark Matter Map (51 degs. long)! Mapping all the Dark Matter! ! • From first detections in temperature (ACT) and polarization (POLARBEAR) to precise lensing maps of dark matter:! Does the dark matter look as we expect?! By removing lensing “blurring”: what happened at the beginning of the universe?!