Galaxies 626. Lecture 3: From the CMBR to the first star

Transcription

1 Galaxies 626 Lecture 3: From the CMBR to the first star

2 Galaxies 626 Firstly, some very brief cosmology for background and notation:

3 Summary: Foundations of Cosmology 1. Universe is homogenous and isotropic - need to consider large volumes - very good observational evidence from CMB 2. Universe is expanding - Hubble law compatible with homogenous / isotropic assumptions 3. Universe was once hot - existence of the microwave background with a thermal spectrum 4. Evolution described by General Relativity

4 Homogeneity + isotropy Expansion of the Universe Expansion of Universe cannot alter the relative orientations of galaxies expanding with the Universe Means that if the present separation between two galaxies is d 0, then the separation at time t can be written as: d = d 0 a(t) a(t) is the scale factor - it is dimensionless and depends upon time but not on position. Relative velocity of the two galaxies is: v = d a = d a (t) = 0 a d

5 Definition of the Hubble parameter is v = H x d, so: a H = a H is a function of time, present value is denoted H 0 Sometime useful to define comoving coordinates. If we divide distances by a(t), then two galaxies which simply recede from each other due to the Hubble expansion always have the same separation in comoving coordinates. We will usually express densities as per comoving volume equivalent to the density they would have after expansion to the present time.

6 Can derive the evolution of a(t) using mostly Newtonian mechanics, provided we accept two results from General Relativity: 1) Birkhoff s theorem: this states (in part) that for a spherically symmetric system, the force due to gravity at radius r is determined only by the mass interior to that radius. 2) Energy contributes to the gravitating mass density, which equals: " m + u energy density c 2 (ergs cm -3 ) of radiation and relativistic particles density of matter

7 Consider the evolution of a spherical volume of the Universe, radius L: L Sphere expands with the Universe, so L = L 0 a(t) Since expansion is described entirely by a(t), can consider any size sphere we want - if L is small `reasonable to assume that space is approximately Euclidean. Expansion of the sphere will slow due to the gravitational force of the matter (+energy) inside: d 2 L dt 2 = - GM L 2

8 Note: no pressure forces because Universe is homogenous Contributions to the gravitating mass come from matter plus energy density from radiation: Matter density ρ m Radiation with energy density u has pressure: P = 1 3 u gravitating mass density is: " = " m + 3P c 2 Mass within sphere, radius L, is: M = "V = 4 3 #L3 "

9 Substitute into acceleration equation: d 2 L dt 2 = - G L 2 " 4 3 #L3 $ Since L = L 0 a(t), with L 0 a constant, can write this as an equation for the evolution of the scale factor a(t): a = - 4"G 3 $ &# m + 3P % c 2 ' ) a ( (also substituting for ρ in the above expression) Matter density ρ m > 0 Pressure of radiation is also positive RHS of the equation is always negative Impossible to have a static Universe

10 Lack of static solutions is not a problem - Universe is expanding. But this was not known in Einstein therefore modified the equations of General Relativity so the equation becomes: a = - 4"G 3 $ &# m + 3P % c 2 ' ) a+ * ( 3 a Λ is the cosmological constant (the factor 3 is just convention). A positive cosmological constant tends to accelerate the expansion - i.e. as if the Universe is filled with material with a negative pressure. Is a static solution stable?

11 Properties of the cosmological constant Cosmological constant is assumed to be a smooth component i.e. it does not cluster or clump together in the same way as ordinary matter. Original cosmological constant was constant in time! This is just an assumption, however - models in which the vacuum energy varies with time are called quintessence. For Λ to be important today, it must have a value comparable to the first term in the equation: " ~ 4#G$ m ~ 10 %36 s -2 (for ρ ~ g cm -3 ) `Fundamental unit of time is the Planck time: t Planck = Gh c 5 =10 "43 s #2 Might guess that " ~ t Planck bad guess by factor Cosmological constant problem

12 Which terms are most important? a = - 4"G 3 $ &# m + 3P % c 2 ' ) a + * ( 3 a Early times - energy density of radiation is large compared to the energy density of matter Later, matter dominates Finally, if Λ is non-zero, eventually it dominates Radiation dominated Matter dominated Cosmological constant dominated Each of these changes in different way as Universe expands - distinct expansion laws

13 Galaxies 626 The cosmic microwave background

14 Cosmic Microwave Background Following recombination, photons that were coupled to the matter have had very little subsequent interaction with matter. Now observed as the cosmic microwave background. Arguably the most important cosmological probe, because it originates at a time when the universe was very nearly uniform: Fluctuations were small - easy to calculate accurately (linear rather than non-linear) Numerous complications associated with galaxy and star formation (cooling, magnetic fields, feedback) that influence other observables not yet important. Basic properties: isotropy, thermal spectrum Anisotropies: pattern of fluctuations

15 Basic Properties of the CMB Excellent first approximation: CMB has a thermal spectrum with a uniform temperature in all directions Thermal spectrum: support for the hot big bang model Isotropy: evidence that the universe is homogenous on the largest observable scales

16 The thermal radiation filling the universe maintains a thermal spectrum as the universe expands Suppose that at recombination the radiation has a thermal spectrum with a temperature T ~ 3000 K. Spectrum is given by the Planck function: B " = 2h" 3 c 2 1 e h" / kt #1 At time t, number of photons in volume V(t) with frequencies between ν and ν + dν is: dn(t) = 8"# 2 c 3 1 e h# / kt $1 V(t)d#

17 Now consider some later time t > t. If there have been no interactions, the number of photons in the volume remains the same: dn(t) = dn(t ' ) However, the volume has increased with the expansion of the universe and each photon has been redshifted: V(t ' ) =V(t) a3 (t ' ) a 3 (t) " ' =" a(t) a(t ' ) d" ' = d" a(t) a(t ' ) Substitute for V(t), ν and dν in formula for dn(t), and use fact that dn(t ) = dn(t)

18 Obtain: dn(t ' ) = 8" # '2 1 V(t ' )d# ' c 3 e (h#' / kt )$(a(t')/ a(t )) %1 which is a thermal spectrum with a new temperature: T ' = T a(t) a(t ' ) Conclude: radiation preserves its blackbody spectrum as the universe expands, but the temperature of the blackbody decreases: T " a #1 "(1+z) Recombination happened when T ~ 3000 K, at a redshift z = 1090.

19 CMB Anisotropies Universe at the time of recombination was not completely uniform - small over (under)-densities were present which eventually grew to form clusters (voids) etc. In the microwave background sky, fluctuations appear as: A dipole pattern, with amplitude: "T T #10$3 Origin: Milky Way s velocity relative the CMB frame. Reflects the presence of local mass concentrations - clusters, superclusters etc. Smaller angular scale anisotropies, with ΔT / T ~ 10-5

20 Experiments detect any cosmic source of microwave radiation - not just cosmic microwave background: Low frequencies - free-free / synchrotron emission High frequencies - dust CMB dominates at around 60 GHz Also different spectra - can be separated given measurements at several different frequencies

21 WMAP results K band - 22 GHz W band - 94 GHz Directly `see the primordial CMB anisotropy at these frequencies

22 Full sky map from WMAP Dipole subtracted (recall dipole is much larger than the smaller scale features) Galactic foreground emission subtracted as far as possible

23 Characterizing the Microwave Background Sky First approximation - actual positions of hot and cold spots in the CMB is `random - does not contain useful information Cosmological information is encoded in the statistical properties of the maps: What is the characteristic size of hot / cold spots? ~ one degree angular scale How much anisotropy is there on different spatial scales? CMB is a map of temperature fluctuations on a sphere - conventionally described in terms of spherical harmonics

24 Spherical harmonics Any quantity which varies with position on the surface of a sphere can be written as the sum of spherical harmonics: "T T #,$ measured anisotropy map as function of spherical polar angles θ and φ % ( ) = a lm Y lm (#,$) l,m weight - how much of the signal is accounted for by this particular mode The spherical harmonic functions themselves are just (increasingly complicated) trignometric functions, e.g.: spherical harmonic function Y 22 (",#) = 5 96$ 3sin2 "e 2i#

25 l = 6, m = 0 l = 6, m = 3

26 Having decomposed the observed map into spherical harmonics, result is a large set of coefficients a lm. Next compute the average magnitude of these coefficients as a function of l: C l " a lm 2 Plot of C l as a function of l is described as the angular power spectrum of the microwave background. Each C l measures how much anisotropy there is on a particular angular scale, given by: " ~ 180o l Angular power spectrum is basic measurement to compare with theory

27 Observational determinations of CMB anisotropy Early 2000 amount of anisotropy large scales small scales Red curve is a theoretical model - evidence for a peak but curve is not significantly constrained by the data at high l

28 Compilation of all available data includes WMAP and some ground based / balloon experiments sensitive to smaller angular scales Peak at degree scales Plateau at large scales Decline toward very small scales Want to understand physical origin of each of these features

29 The power spectrum reflects fluctuations in the density at the time of recombination: Photons escape from the overdense region Recombination Consider a slight overdensity collapsing during the radiation dominated phase. Photons escaping at recombination: Escape from a hotter, denser region Are redshifted escaping from a deeper potential well Have a Doppler shift due to relative velocity

30 How this works in detail depends upon the scale of the fluctuations: Largest scales (low l) On the largest scales, perturbations have not had time to collapse significantly prior to recombination. At low l, directly see the fluctuations generated at an earlier epoch. Intermediate scales (~degree) Overdensities start to collapse, but increased pressure causes them to bounce - leading to oscillations.

31 Maxima and minima of these oscillations lead to the strongest signals in the microwave background: Doppler peaks First peak (compression) occurs at degree scales Small scales Start End Recombination is not instantaneous Photons will `leak out of small over / under-densities during the process - damping very small scale fluctuations Exponential suppression of anisotropy at the smallest scales

32 Dependence on Cosmology 1. Is the universe flat, open, or closed? Doppler peaks define a physical scale at recombination Angular scale this corresponds to depends upon the geometry of the universe: Blue curve effect of changing the geometry Open universe position of the peaks is shifted to smaller angular scales (i.e., larger multipole l)

33 Observed position of the first peak is at l = 220 " total =1.02 ± 0.02 i.e., the universe is flat (or very close to being flat) What does this imply about the cosmological constant? Directly: almost nothing - CMB anisotropy is mainly sensitive to the total energy density, not to the individual contributions from matter and cosmological constant Indirectly: estimate (by other means) that the total matter density is perhaps Ω m = 0.3 (mostly dark matter). Need `something else to make up the inferred value of Ω total = 1. A cosmological constant with Ω Λ ~ 0.7 as deduced from SN is consistent with this.

34 2. Baryon content of the universe Increasing the fraction of baryons: Increases the amplitude of the Doppler peaks Changes the relative strength of the peaks - odd peaks (due to compressions) become stronger relative to even peaks (due to rarefactions)

35 Full power spectrum from WMAP and other experiments is consistent with the predictions of ΛCDM (i.e., the family of cosmological models that includes dark matter plus a cosmological constant): Simplest such models have 6 free parameters: Being able to fit the data is a genuine success! Parameters are mostly well constrained by the data

36 Adding in other cosmological information, e.g., from the supernovae measurements, further constrains the model: Further information provided by: Lyman-α forest, galaxy clustering (2dF, Sloan Digital Sky Survey), weak gravitational lensing

37 Today s best guess universe Age: t 0 =13.7 ± 0.2 Gyr Hubble constant: H 0 = 71 km s -1 Mpc -1 Density of ordinary matter: " baryon = 0.04 Best fit CMB model - consistent with ages of oldest stars CMB + HST Key Project measure Cepheid distances CMB Density of all forms of matter: CMB + SNe " matter = 0.27 Cosmological constant: CMB + SNe " # = 0.73

38 Theory of inflation links the power spectrum of fluctuations to the nature of the vacuum energy driving inflation: Power spectrum of inflationary fluctuations is parameterized (k is the wave-number in the Fourier transform from spatial coordinates) P(k) = Ak n n =1+2" # 6$ Original Harrison-Zeldovich expectation was n=1 (a scale invariant spectrum). With inflation, generically one expects n slightly different than 1 (either lower or higher). Inflation predicts very little variation of n with k. Observational evidence not yet definitive but WMAP3 data favor n~0.95.

39 Summary: The universe is flat and will expand forever Ordinary matter (stars, gas, dark baryons) is negligible Cold dark matter and dark energy dominate the evolution of the universe, and currently make roughly equal contributions to the total energy density Suggests that the universe at the time of recombination is well understood - we know the initial conditions that eventually gave rise to galaxies, stars, quasars, etc The big unknown: the nature of dark energy Is it a cosmological constant or something more interesting that varies with redshift? Joint Dark Energy Mission is being planned (SNe at higher z, larger red galaxy survey; read the task force report on the web)