1. 26. Cosmology Significance of a dark night sky The Universe is expanding The Big Bang initiated the expanding Universe Microwave radiation evidence of the Big Bang The Universe was initially hot & opaque The importance of the shape of the Universe The Universe seems filled with dark energy The Universe’s expansion rate is accelerating Dark matter & dark energy determine the future

2. Significance of a Dark Night Sky Physical cosmology –Science of the structure & evolution of the Universe One fundamental expectation –Universe is large enough to fill the sky with stars Night skyshould befilled with light Night skyis notfilled with light ⇒ Olber’s paradox The model of an infinite static Universe cannot be correct One possible solution –Einstein’s special theory of relativity Distance, mass & time are all relative to all observers –Einstein’s general theory of relativity Space is warped by massive objects Einstein introduced the cosmological constant  –Introduced to produce the preconceived static Universe

3. The Dark Sky Anomaly

4. The Universe Is Expanding The Hubble law1920 –Galaxy clusters are moving away from each other Pervasive galaxy cluster redshift is compelling evidence The Hubble law is a linear relationship –Tenfold distance increase ⇒ Tenfold recessional speed increase –Expansion of space, not velocity through space Cosmological redshift rather than Doppler redshift Space does not expand in objects tightly bound by gravity –Model of solid objects on an inflating balloon One fundamental conclusion –The center of expansion cannot be identified Everything is moving away from everything else Every viewing location observes the same expansion

5. The Expanding Balloon Analogy

6. The Hubble Law & Expansion

7. Fundamental Assumptions Assumptions inscientific method –Any reasonable assumptions can be made Assumptions are presumed unproveable for some reason Future observations may prove some assumptions invalid Assumptions incosmology –Observations can verify only one Universe Cosmologists imagine but cannot produce other universes –The cosmological principle The Universe is assumed to behomogeneous –At the largest scale, allregions of space areidentical The Universe is assumed to beisotropic –At the largest scale, alldirections in space lookidentical

8. Big Bang & the Expanding Universe Observational evidence –Inverse of the Hubble law In the distant past, the Universe was very small Far enough back, all matter & energy were in one point 1/H 0 ⇒ Age of the Universe –The Universe began 13.8 Bya if H 0 = 75 km. sec –1. Mpc –This assumes that H 0 is now & has always been constant –One major problem Stars cannot be older than the Universe –Observational evidence suggests this may not be the case Recent evidence may resolve this issue The Big Bang –“Cosmic singularity” may be a better term Remarkably similar to the singularity in a black hole

9. The Observable Universe Expected size of the Universe –Almost certainly far larger than what we can see Observational limits –We only see as far back as the Universe is old This results from the finite speed of EMR in vacuum –Presume the Universe is 13.8 billion years old We can only see objects < 13.8 Bly away in all directions This is our cosmic particle horizon –As the Universe ages, we see back even farther

10. The Changing Observable Universe Entire Universe

11. Microwave Radiation As Evidence The overabundance of He –Theoretically, there is too much He in the Universe Can be accounted if the early Universe was extremely hot If so, there should be evidence –This would be severely redshifted to ~ 1.1 mm Cosmic background microwave radiation –Penzias & Wilson at Bell LabsEarly 1960s Observed microwave background radiation from space –This radiation is about 1% the strength of analog TV noise –Earth’s atmosphere is largely opaque at 1.1 mm Satellites are a better option –Cosmic Background Explorer (CoBE)1989

12. Tri-Color Noise Noise

13. Bell Labs Horn Antenna

14. Microwave Background Spectrum Blackbody curve T = 2.725 K

15. Microwave Background Temperature

16. Our Motion Through the Universe

17. Density Dominance in the Universe Radiation- dominated Universe –Prevailed in the earliest stages of the Universe This corresponds to a cosmic redshift of z = 25,000 –Since then, ’s have been stretched by a factor of 25,000 This occurred when the Universe was ~ 2,500 years old –EMR had = 40 nm in the UV part of the spectrum Matter- dominated Universe –Prevailed since the Universe was ~ 2,500 years old This may prevail for all time in the future

18. The Universe Began Hot & Opaque Basic physical processes –High temperatures tend to ionize atoms & molecules Hydrogen has only 1 proton in its nucleus –The bond with the 1 electron is therefore relatively weak Hydrogen ionizes at temperatures > 3,000 K –This was true for ~ 300,000 years after the Big Bang –An ionized gas is called a plasma Plasma interacts very strongly with EMR –This makes a plasma opaque –Cosmic redshift reduces energy content of EMR Energy is inversely proportional to wavelength Recombination –Protons & electrons bind to form neutral hydrogen Misnomer because they were never previously combined

19. Radiation & Matter In the Universe Two basic possibilities in the Universe –Something is either energy or matter Special relativity: They are two forms of the same entity Two important concepts –Universe’s averagemass density of radiationtoday  rad = 4.6. 10 –31 kg. m –3 Equivalent to 550,000,000 photons per cubic meter Extremely low energy per severely redshifted photon –Universe’s averagedensity of mattertoday  m = 2 to 11. 10 –27 kg. m –3 Equivalent to 1 to 6 hydrogen atoms per cubic meter Extremely high energy per hydrogen atom

20. Evolution of Density

21. Evolution of Radiation Temperature

22. Before & After Recombination Free protonsBound protons& electrons

23. Microwave Temperature Variations

24. Importance of the Universe’s Shape Three basic possibilities –Positivecurvature of spaceSpherical Two parallel beams of EMR will converge Three-dimensional analog of a sphere The Universe’s expansion rate will decrease –The Universe will eventually collapse upon itself –Zerocurvature of spaceFlat Two parallel beams of EMR will remain parallel Three-dimensional analog of a plane The Universe’s expansion rate will remain constant –The Universe will continue expanding forever –Negativecurvature of spaceHyperbolic Two parallel beams of EMR will diverge Three-dimensional analog of a saddle The Universe’s expansion rate will increase –The Universe will continue expanding forever

25. Measuring the Universe’s Shape Hypothetical –Draw a huge triangle –Measure the three angles Practical –Count the number of extremely distant galaxies SphericalConcentration ishighest nearby FlatConcentration isuniform HyperbolicConcentration ishighest far away

26. The Geometry of the Universe

27. Microwave Background & Curved Space

28. A Universe Filled With Dark Energy The observational evidence –Temperature variations ⇒ A nearly flat Universe –Galaxy clusters ⇒ Matter density  m = 0.2 to 0.4 This clearly suggests a non-flat Universe The tentative conclusion –There is substantial dark energy in our Universe –Necessarily   = 0.6 to 0.8 Presumed dark energy must be 60 – 80% of the Universe Dark Energy Dark Matter

29. Proportions of Mass & Energy http://upload.wikimedia.org/wikipedia/commons/4/4b/DarkMatterPie.jpg

30. Universe’s Variable Expansion Rate http://www.nasa.gov/images/content/56200main_dark_expansion-lg.jpg

31. Universe’s Matter & Energy Distribution http://upload.wikimedia.org/wikipedia/commons/b/b9/Cosmological_composition.jpg

32. Universe’s Expansion is Accelerating The observational evidence –Type Ia supernovae in distant galaxies A highly reliable standard candle The tentative conclusion –The expansion rate of the Universe is increasing Einstein’s cosmological constant may be correct after all ! ! !

33. Varying Rates of Cosmic Expansion

34. Distant Supernovae & Hubble Diagram

35. Limits on the Nature of the Universe

36. The Evolution of Density Revisited

37. The Growth of the Universe

38. The expanding Universe –How can the sky be so dark? Stars should be literally everywhere –The Hubble Law (1920) Space itself is expanding Concept of the Big Bang –Matter & energy in a very small space –Explosion created space & time Space expanded faster than light –Expansion of space has slowed down Evidence for the Big Bang –The cosmological red shift –The cosmic background radiation Remnants of heat from the Big Bang –Big Bang produced excess helium Required extremely high temperatures –Milky Way moves amidst CBR The Great Attractor Characteristics of the Big Bang – Extremely hot & opaque plasma The primordial fireball – Hydrogen atoms eventually formed Temperature < 3,000 K Universe became transparent The shape of the Universe – Three major possibilities Zerocurvature=Flat Positivecurvature=Closed Negativecurvature=Open – Two remarkable things ~90% of all matter is “dark” ~80% of all energy is “dark” The strangest thing yet – Universe’s expansion is accelerating Chapter 28: Important Concepts