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The Darkness of the Universe

The Darkness of the Universe. Eric Linder Lawrence Berkeley National Laboratory. The Night Sky is Dark. The dark night sky is a profound cosmological observation! The universe is filled with stars emitting photons.

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The Darkness of the Universe

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  1. The Darkness of the Universe Eric Linder Lawrence Berkeley National Laboratory

  2. The Night Sky is Dark The dark night sky is a profound cosmological observation! The universe is filled with stars emitting photons. Number of stars at distance r goes as r2; Intensity dies as 1/r2, so total flux received Ftot =  dr F(r) N(r) = 0 dr (L/4r2) (4nr2) = Ln 0 dr  Why aren’t we cooked?

  3. The Night Sky is Dark More rigorously: Energy density of light today E =tit0 dt L (1+z)-1 If constant comoving luminosity density L, then finite past gives finite limit E<Lt0.

  4. The Night Sky is Dark Liouville’s Theorem approach: Photon phase space density N = N/d3xd3p is conserved, so I = N/(At)dd = N 3 Therefore, I -3is conserved and the surface brightnessI0 = Ie (1+z)-4. Looking far enough, every line of sight should end on a star, and so the sky should be as bright as the surface of a star. The answer again is that we can’t look far enough, due to a finite past.

  5. Optical Depth of the Universe What is the probability of a line of sight intersecting a star? Consider a marble rolling on a tabletop.       •   Will the marble hit another marble before rolling off the table top?

  6. Optical Depth of the Universe Mean free path is L = 1/(n) Cross section to intersect a star is =  r2 =  (1011 cm)2 = 1022.5 cm2 Number density of stars is n = /m =(10-31 g cm-3)/1033 g =10-64 cm-3 Effective volume V = 1/n =1064 cm3 = (1021 cm)3 =[(1/3)kpc]3 MFP L =1/(n) = V/A = 1041.5 cm = 1017 Mpc = 1013.5 H0-1 Finite horizon solves Olbers’ paradox by 13 OOM!

  7. Expanding Universe? What role does the expanding universe play in Olbers’ Paradox? Almost none! I0 = Ie (1+z)-4 dimming arises from any frequency shift, not just expansion. E =tit0 dt L (1+z)-1 has little dependence on redshift. Expansion has factor of ~2 effect. So Newton could not have “discovered” the expanding universe. The solution is not expansion in Big Bang cosmology, but the Big Bang itself!

  8. Expanding Universe? Caveat 1 (CMB): Photons can be absorbed, and reradiated, e.g. IR degradation. For a diffuse glow like the CMB the sky is dark (E<< Mp4) because of the expansion. Caveat 2 (Inflation): In an inflationary epoch, 1+z ~ eH(t0-t), we are saved by the redshift, giving a finite energy density E=L/H even for an infinite past.

  9. What if Our Eyes Saw Dark (Energy)? The night sky is dark in photons, implying a finite past (Big Bang). The sky is bright in  (dark energy density dominates), implying an infinite future. Will this turn out to be as significant a discovery as the Big Bang?

  10. Cornerstones of Cosmology Another simple observation is that the universe appears rather isotropic. This is most notable in the cosmic microwave background (CMB) radiation. Isotropic to a part in 103, or part in 105 in comoving frame. Also holds (with less precision) in counts of galaxies, quasars, etc. Cosmological Principle (Copernican principle, principle of cosmic modesty) argues we are not in special location, so isotropy implies homogeneity.

  11. Cornerstones of Cosmology The global spatial geometry is isotropic and homogeneous. The theory of gravity characterizes the spacetime geometry, and relates the dynamics to the matter and energy contents. General relativity accurately describes gravity as far as it has been tested. Robertson-Walker metric: ds2 = dt2+a2(t)[dr2/(1-kr2)+r2d2] scale factora(t) ; spatial curvaturek

  12. Expansion of the Universe The universe is expanding: Photons received from distant galaxies exhibit a change to lower frequencies (redshift). This shift is at lowest order proportional to the distance: z = H0r Isotropic, radial: suggests vector relation v = H0r Linearity predicted by Weyl, before Hubble. Note v12 = v2-v1 = H0(r2-r1) = H0r12.        

  13. Expansion of the Universe Approximate timescale of the expansion is the Hubble timeH0-1. Since distances r ~ a, i.e. r = r0(a/a0), then r = r (a/a). So H = a/a Volumes behave as V ~ a3. So number densities n ~a-3. Photon frequencies (inverse wavelengths)  ~ a-1, so radiation energy density  ~ a-4 (T ~a-1). Early universe was hot and dense. . . .

  14. Big Bang This is the hot Big Bang theory: Expansion of the universe from a hot, dense state over a finite time. Tested by Expansion redshift Cosmic microwave background radiation Primordial nucleosynthesis Cosmic ages General relativity Dark night sky

  15. Our Expanding Universe Bertschinger & Ma ; courtesy Ma

  16. Our Cosmic Address Our Sun is one of 400 billion stars in the Milky Way galaxy, which is one of more than 100 billion galaxies in the visible universe. Earth 107 meters Solar system 1013 m Milky Way galaxy 1021 m Local Group of galaxies 3x1022 m Local Supercluster of galaxies 1024 m The Visible Universe 1026 m

  17. The Cosmic Calendar Inflation1016 GeV Quarks  Hadrons1 GeV Nuclei form1 MeV Atoms form1 eV [Room temperature 1/40 eV] Stars and galaxies first form:1/40 eV Today:1/4000 eV

  18. Characteristic Scales Physics today: Cosmic size (Hubble scale) -- H0-1= 1028 cm Cosmic time (Hubble age) -- H0-1= 1010 yr Mass scale -- H0 = 10-33 eV Energy density scale -- H02MP2 = (10-3 eV)4

  19. Mapping Our History The subtle slowing down and speeding up of the expansion, of distances with time: a(t), maps out cosmic history like tree rings map out the Earth’s climate history. STScI

  20. Discovery! Acceleration

  21. cf. Tonry et al. (2003) accelerating accelerating decelerating decelerating Cosmic Concordance • Supernovae aloneAccelerating expansion •  > 0 • CMB (plus LSS) •  Flat universe •   > 0 • Any two of SN, CMB, LSS •  Dark energy ~75%

  22. Frontiers of Cosmology Us STScI 95% of the universe is unknown!

  23. Dark Energy Is... Dark Energy Is!!! • 70-75% of the energy density of the universe • Accelerating the expansion, like inflation at 10-35s • Determining the fate of the universe ! 70-75% of the energy density of the universe 95% of the universe unknown! ! Accelerating the expansion, like inflation at 10-35s Repulsive gravity! ! Determining the fate of the universe Fate of the universe! Is this mysterious dark energy the original cosmological constant , a quantum zeropoint sea?

  24. : Ugly Duckling Field Theorist: Vacuum – Lorentz invariant Tab ~ab = diag { -1, 1, 1, 1} p = - Naturally, Evac ~ 1019 GeV E ~ (meV)4 =0? Astrophysicist: Einstein equations – gab  p = - Naturally,  =const= PL = 10120 Today M • Fine Tuning Puzzle – why so small? • Coincidence Puzzle – why now?

  25. What’s the Matter with Energy? Why not just bring back the cosmological constant ()? When physicists calculate how big  should be, they don’t quite get it right. They are off by a factor of 1,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000. This is modestly called thefine tuningproblem.

  26. Cosmic Coincidence Size=2 Size=1/4 Size=1/2 Size=4 But it gets worse: Think of the energy in  as the level of the quantum “sea”. At most times in history, matter is either drowned or dry. Dark energy Matter Today

  27. On Beyond ! We need to explore further frontiers in high energy physics, gravitation, and cosmology. New quantum physics?Energy of the vacuum (nothing weighs something)? New gravitational physics? Quantum gravity, supergravity, extra dimensions? We need new, highly precise data

  28. Cosmic Archaeology Supernovae: direct probe of cosmic expansion Time: 30-100% of present age of universe (When you were 12-40 years old) Cosmic matter structures: less direct probes of expansion Pattern of ripples, clumping in space, growing in time. 3D survey of galaxies and clusters - Lensing. CMB: direct probe of quantum fluctuations Time: 0.003% of the present age of the universe. (When you were 0.003% of your present age, you were 2 cells big!)

  29. The Universe: Early and Late Relic imprints of quantum particle creation in inflation - epoch of acceleration at 10-35 s and energies near the Planck scale (a trillion times higher than in any particle acclerator). These ripples in energy density also occur in matter, as denser and less dense regions. Denser regions get a “head start” and form into galaxies and clusters of galaxies. How quickly they grow depends on the expansion rate of the universe (traced by SN). It’s all connected!

  30. Cosmic Archaeology Inflationsets seeds of structure, patterning both radiation (CMB) and matter (galaxies) CMB } Large scale structure: Baryon acoustic oscillations Lensing (weak and strong) Galaxy clustering Sunyaev-Zel’dovich effect NASA GSFC/COBE

  31. Fundamental Physics Astrophysics  Cosmology  Field Theory a(t)  Equation of state w(z)  V() V ( ( a(t) ) ) SN CMB LSS The subtle slowing and growth of scales with time – a(t) – map out the cosmic history like tree rings map out the Earth’s climate history. STScI Map the expansion history of the universe

  32. What if Our Eyes Saw Dark (Energy)? The night sky is dark in photons, implying a finite past (Big Bang). The sky is bright in  (dark energy density dominates), implying an infinite future. Will this turn out to be as significant a discovery as the Big Bang? Next:The Darkness of the Universe 2:Acceleration and Deceleration

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