1. H205 Cosmic Origins  Today: Dark Energy, Ch. 22  Complete EP5 APOD

2. Explaining the Origin of Structure The simple Big Bang model gives a Universe with no structure To explain why the Universe has structure, we need “inflation”

3. What is Inflation? In the first 10 -38 to 10 -36 seconds, regions of the Universe expanded from the size of an atomic nucleus to the size of the Solar System The rate of expansion then slowed Prior to inflation, matter on opposite sides of our Universe was in contact Quantum fluctuations prior to inflation became the seeds of structure

4. Structure in Our Universe  We see galaxies organized in a fluffy, sponge-like web in space  Galaxies are grouped in clusters, sheets, and filaments that surround great voids

5. Seeing Structure in the Hubble Deep Field Studying the locations of galaxies in the HDF shows the structure of the Universe

6. The Hubble Deep Field with Redshifts Redshift “z” is related to the velocity of recession When the velocity becomes large, the relativistic formula must be used instead

7. Describing Cosmology: Modeling the Universe with H 0 and Omega The Omega Parameter The density of the Universe determines its destiny Omega 0 is the “critical density” If Omega = Omega 0 the expansion of the Universe will slow to zero after infinite time If Omega < Omega 0 the universe will expand forever If Omega > Omega 0 the Universe will stop expanding and collapse due to its own gravity

8.  It depends on the density of the Universe  The critical density in the current epoch is 10 -29 g/cm 3, about one hydrogen atom per cubic meter  About 25 times more than the observed mass of stars and gas  Will dark matter recollapse the Universe?  There isn’t nearly enough to re-collapse the Universe Will the Universe Keep Expanding Or Re-Collapse???? But the universe isn’t even slowing down….

9. Exploring Cosmology Three factors control the expansion of the Universe: –the current expansion rate, known as the Hubble Constant, H0 –the amount of mass in the universe, measured by a parameter called OmegaM (M for mass) –the acceleration factor of the universe, measured by OmegaL (L for lambda)

10. Omega M Mass alters the expansion of the Universe through its gravitational effect on space. With enough mass in the Universe, gravity can halt the expansion and cause the Universe to recollapse on itself “Critical density" -- the density of mass which is just sufficient to halt the expansion Omega M, which is the density of the universe divided by the critical density: Omega M = rho/rho C Theoretical models predict that Omega = 1.

11. Basic Constraint! The Universe should not be younger than the objects in it Globular clusters have an age of 13 billion years (Gyr) The Universe must be at least 13 Gyr old!

12. The more we learn, the stranger it gets… The speed of a ball tossed up in the air slows down because of gravity Observations at the end of the 20 th century established that the Universe is not just coasting, or slowing down because of its own gravitational pull, but actually speeding up.

13. We observe Type Ia supernovae (exploding white dwarfs) to measure the distances of extremely distant galaxies. This tells us the Hubble Constant when the universe was younger. The speeds of very distant galaxies tell us the Universe is expanding faster today than in the past

14. WhyType Ia Supernovae ? All Type Ia supernovae are due to implosion of 1.4 Mo white dwarfs, so they all have the same amount of fuel to burn and produce the same luminosity They therefore act like “standard candles”

15. Distances to Type Ia Supernovae Decay time of light curve is correlated to absolute luminosity Luminosity comes from the decay of radioactive cobalt and nickel into iron Some Type Ia supernovae are in galaxies with Cepheid variables Good to 20% as a distance measure

16. Redshift measured tells us expansion factor (average distance between galaxies) Peak brightness tells us distance away (lookback time) Type Ia Supernova Lightcurves

17. Type Ia Supernovae and Cosmology Here is a typical supernova lightcurve and its spectrum Compare two distances to see if expansion rate has changed Measure shape of curve and peak to get distance Measure redshift  distance

18. Type Ia Supernovae at Redshift z=1 are too faint… Distant supernovae are further away than predicted by Hubble’s Law H 0 was smaller in the past

19. Type Ia Supernovae and Dark Energy Analyze lightcurves vs. redshifts for many Type Ia supernovae at redshifts z <2 Observations of over 100 SN show that SN are dimmer than would be expected if the Universe was expanding at a constant rate or slowing down (as was previously thought) Some unknown “dark energy” is causing the Universe to fly apart at ever-increasing speeds.

20. The universe is expanding faster today than it did in early times This expansion cannot be caused by ordinary matter or dark matter The acceleration suggests the possibility of a new type of repulsive force (anti-gravity) that acts on very large scales The Universe is speeding up!

21. WMAP’s observation of the “seeds” of structure inferred from the CMB confirm the existence of dark energy Overall geometry is flat – Total mass+energy has critical density Ordinary matter ~ 4.4% of total Total matter is ~ 27% of total – Dark matter is ~ 23% of total – Dark energy is ~ 73% of total Age of 13.7 billion years

22. A “fifth force?” …in addition to –strong force (holds nuclei together) –weak force (interactions of electrons) –electrical/magnetic force (holds atoms together) –gravity (works over large distances) Something else? Three models... What is Dark Energy?

23. When Einstein developed the theory of general relativity, astronomers thought the Universe was “static” Einstein included a repulsive force called the cosmological constant to balance gravity (anti-gravity!) The cosmological constant was rejected when we learned the Universe was expanding The cosmological constant is now needed to explain why the Universe is speeding up The New Force Is Called “Dark Energy” (=Omega L )

24. Properties of Dark Energy Einstein ’ s cosmological constant Omega L has the property that P = -  = -1 Significant quantities of matter-energy with “ negative pressure ” will cause the expansion of the universe to accelerate.

25. Quantum fluctuations? Virtual particle pairs continually emerge and disappear into the quantum vacuum If you observe the particles (hit them with a photon), you give them enough energy to become real The particles can also get energy from any nearby force field (like a BH)

26. Vacuum Energy = Dark Energy? The cosmological constant Omega L may be related to the “zero-point energy” of the Universe which comes from the quantum fluctuations of the vacuum. However, the vacuum energy density is 10 120 too high to allow structure formation to occur Something must cancel almost all of the vacuum energy in order for us to be here And that something left the ~73% of critical density at our current time, 13.7 billion years later

27. Quintessence  Quintessence is another theory for dark energy that involves a dynamic, time-evolving and spatially dependent form of energy.  It makes slightly different predictions for the acceleration  It’s name refers to a “fifth essence” or force

28. Today’s Cosmology – The “Concordance Model”  Omega TOT  = 1.0 from CMB measurements (WMAP). We live in a flat Universe.  Omega M <0.3 from extensive observations at various wavelengths. Includes dark matter as well as normal matter and light.  Omega L  ~ 0.7 derived from Type Ia SN data combined with WMAP and other measurements.  Hubble constant = 70 km/sec/Mpc from HST observations. Age of Universe = 13.7 billion years.  Universe accelerates and is open, even though it is geometrically flat.

29. Dark Energy History Dark Energy must have been insignificant at early times, otherwise its gravitational influence would have made it almost impossible for ordinary matter to form stars, galaxies and large-scale structure In the early Universe, gravity dominated and various structures formed. However as the Universe expanded, and self- gravitating structures (such as clusters of galaxies, etc.) grew further apart, the space between them expanded, and dark energy began to dominate gravity

30. Expansion History of the Universe Most distant Type 1a seen by HST occurred during the period when the expansion was still slowing down due to gravity from the galaxies in a smaller Universe Riess et al. 2001 Image: Ann Feild (STScI)

31. Old Cosmology New Cosmology

33. A brief History of the Universe Courtesy of Fred Adams University of Michigan

34. BIG BANG – 13.7 billion years ago, space, time, and energy burst into existence The part of the Universe that now comprises our “observable universe” was very small and very dense Why did the Universe suddenly appear????

35. INFLATION ERA – Regions of the universe rapidly expand from smaller than an atom to bigger than the Solar System. Because all of space was so compact, every part of the universe was in “contact” with every other part. Energy was uniformly distributed throughout the early universe

36. PHOTON ERA - energy in the form of electromagnetic radiation dominates the Universe - visible light, X rays, radio waves and ultraviolet rays. Energy transforms into matter: quarks the first nuclei: protons, neutrons,helium The density of energy was so great that matter could not exist. As the density was gradually reduced through expansion, matter began to form. Both matter and anti-matter formed, but for some reason, there was a slight excess of matter.

37. Originally, no stars The proton-electron gas was opaque to light Protons and electrons combined into atoms The Universe became transparent – We can “see” the first stars and galaxies The First Dark Era

38. STELLIFEROUS ERA – the current era Atoms condensed into the first generation of stars during the first 200 million years The first stars reheated the gas (reionization) The universe remains transparent because the density is so low Galaxies formed Sun, solar system formed 4.6 billion years ago Life appeared on Earth 3.8 billion years ago Modern humans show up just 100,000 years ago

39. The Universe in a Day Event When it happened Big Bang 12:00:00 midnight First Atoms form12:00:08 a.m. Stars and Galaxies form12:29 a.m. Our Sun, Earth, Moon are born4:00 – 4:48 p.m. Earliest life on Earth6:00 p.m. First multi-cellular life on Earth10:53 p.m. Dinosaurs appear11:40 p.m. Dinosaurs die11:54 p.m. Humans arise11:59:56 p.m. Present Day12:00 midnight tomorrow Sun becomes Red Giant8:00:00 a.m. tomorrow Sun becomes White Dwarf8:19:00 a.m. tomorrow

40. DEGENERATE ERA – 10 trillion trillion trillion years after the Big Bang Planets detach from stars Stars and planets evaporate from galaxies Most ordinary matter in the universe is locked up in degenerate stellar remnants Eventually, even the protons themselves decay

41. BLACK-HOLE ERA - 10,000 trillion trillion trillion trillion trillion trillion trillion trillion years after the Big Bang The only large objects remaining are black holes Eventually even the black holes evaporate into photons and other types of radiation.

42. The Final DARK ERA – Only photons, neutrinos, electrons and positrons remain, wandering through a universe bigger than the mind can conceive. Occasionally, electrons and positrons meet and form "atoms" larger than the visible universe is today. From here into the infinite future, the universe remains cold, dark and empty.

43. Multiverses Universe was originally defined to include everything However, the possibility exists that our “bubble universe” is only one of many such regions that could have formed, with the parameters arranged as in the concordance model so that we can be here having this discussion Other universes could have very different physical conditions and we will never see them – they may be on different “branes” or in other dimensions that we cannot measure

44. A Humbling Thought Not only do we not occupy a preferred place in our Universe, we may not occupy any preferred universe in the Multiverse!

45. For Next Week The Origin OF STARS!!!!!

46. Origin of the CMB – the thermal radiation of the first atoms Isotropic microwave radiation

47. Testing Inflation  Why is the Cosmic Microwave Background Radiation almost perfectly isotropic?  The CMB is isotropic because regions now on opposite sides of the sky were close together before inflation pushed them far apart

48. Testing Inflation  Patterns of structure observed by WMAP show the “seeds” of universe  Observed patterns of structure in the Universe agree (so far) with the “seeds” that inflation would produce

49. Inflation can make all the structure by stretching tiny quantum ripples to enormous size These ripples in density then become the seeds for all structures in the Universe

50. Recall the Uncertainty Principle The uncertainty principle states that you cannot know both the position x and the momentum p of a particle more precisely than Planck’s constant h/2    “h-bar” When dimensions are small, particles must therefore move in order to satisfy the uncertainty principle This motion creates a “zero point energy” > 0 Uncertainty Principle  x  p = h/2

51. The shape of the Universe The shape of the Universe is determined by a struggle between the momentum of expansion and the pull of gravity. The rate of expansion is determined by the Hubble Constant, H o The strength of gravity depends on the density and pressure of the matter in the Universe. –G is proportional to  + 3P –  is the density and P is the pressure For normal matter, P is negligible, so the fate of a universe filled with normal matter is governed by the density 

52. R(t) t scale factor time Big BangBig Crunchnow Dark Energy Causes the Universe to Accelerate Forever! (1) the Universe first slows down (decelerates) because of gravity (2) the Universe then speeds up forever (accelerates) because of Dark Energy