3 Observational Cosmology Evolution from the Big Bang Lecture 2

Transcription

1 3 Observational Cosmology Evolution from the Big Bang Lecture 2 Sean McGee smcgee@star.sr.bham.ac.uk

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3 Nucleosynthesis

4 Nucleosynthesis Light elements and isotopes deuterium, helium-3, lithium, helium-4 are not produced inside stars The early universe should only have protons and neutrons separately. What determines how many protons to neutrons? And assuming all neutrons are in Helium-4, what determines the ratio of H to He?

5 Nucleosynthesis Light elements and isotopes deuterium, helium-3, lithium, helium-4 are not produced inside stars The early universe should only have protons and neutrons separately. What determines how many protons to neutrons? And assuming all neutrons are in Helium-4, what determines the ratio of H to He? Three things are important: - protons are lighter than neutrons - Free neutrons decay with a half life of 610 seconds - Stable isotopes exist in which neutron doesn t decay

6 Nucleosynthesis Start our exploration when T >> O(MeV) [the nuclear binding energy] protons and neutrons are free particles in thermal equilibrium (Maxwell-Boltzmann distribution) While T >> MeV, the n/p ratio is ~ 1.

7 Nucleosynthesis Reactions maintain until:

8 Nucleosynthesis Protons and neutrons can form Helium-4 by: p + n D; D + p 3 He D + D 4 He This can proceed until T = 0.06 MeV. (t = 340 seconds) This timescale to cool to this temperature makes the decay of neutrons relevant (T = 610 seconds) Reduces neutron number density by e (-340s ln 2)/610s So Nn/Np = 1/5 * = 1/ 7.3 If the neutron half-life was much shorter, all neutrons would decay and only hydrogen would form. Assume all neutrons go into He-4, then Y = 2Nn / (Nn + Np) = 0.24.

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10 The stages of the Universe/baryons Ionized - The early universe was ionized. And the high scattering cross-section of photons off free elections (σ T = m 2 ) means they are tightly coupled. As the Universe expands and cools, the radiation energy density drops as a -4, whilst the energy density of matter drops only as a -3. Equality of matter and radiation density reached at t~50,000 yr. Recombination The combining of electrons with nuclei, occurs at t~240,000 yr. Thus, the baryons become neutral. Decoupling The radiation (photons) are no longer tightly coupled to baryons. The scattering cross-section of photons off bound elections (away from line transitions) is much lower (t~350,000 yr (z=1100) at T~3000 K). Last-scattering surface - The CMB photons effectively travel freely from this with their blackbody spectrum cooling from 3000 K to 2.7 K as the universe expands by a factor of Reionization Energetic photons from stars and quasars ionize the diffuse baryons again Accreted into halos Form stars/galaxies, are heated by shocks or ejected. Heated outside of dark matter halos.

11 Flatness problem The early universe is radiation dominated.

12 Flatness problem The early universe is radiation dominated.

13 The CMB fluctuations Spherical harmonics with l=0-4 (top-bottom) and m=0-4 (left-right). Dipole subtracted The characteristics of the CMB fluctuations on the celestial sphere can be studied quantitatively by expanding them as a series of spherical harmonics T T (, )= 1 X lx l=0 m=0 a lm Y m l (, ) The spatial frequency of the fluctuations is then characterised by the value of l, and so the variation in fluctuation amplitude with spatial frequency is generally displayed as a sort of power spectrum. Power spectrum of CMB fluctuations fitted with a ΛCDM model

14 The origin of the CMB fluctuations The primary peak in the power spectrum is caused by modes which have just had time to collapse to maximum compression in the time between entering the horizon and decoupling. Perturbations on somewhat smaller scales will have had time to rebound, and so will not be so hot. The second peak corresponds to fluctuations on sizes small enough to allow a full in-out oscillation (generating a cool spot), the third peak to in-out-in, and so on. Power spectrum of CMB fluctuations from WMAP and a variety of ground-based microwave experiments Hinshaw et al 2006

15 The origin of the CMB fluctuations The details of this complex power spectrum provide some of the best constraints available today on key cosmological parameters. For example, the physical scale of the first peak is set by the horizon scale at last scattering, which has to occur at z~1000, since it is determined mostly by the temperature of the Universe. The angular scale at which this peak is seen depends upon the curvature of space-time, so that an open Universe would show a smaller angular scale, and the peak would be shifted to higher spatial frequency. As a result, CMB results constrain Ω to be very close to 1. The observed angular scale (~1 ) of the primary CMB peak constrains the Universe to be very close to a flat geometry.

16 The origin of the CMB fluctuations The amplitude of the oscillations is sensitive to the baryon to photon ratio, and hence to Ω b. More baryons cause stronger compression (since they have mass) but less rebound. Hence they strengthen the odd numbered peaks relative to the even ones. A baryon fraction Ω b =0.046, in excellent agreement with nucleosynthesis constraints, fits the data very well. Effect of varying Ω b on the predicted power spectrum of fluctuations.

17 Flatness problem The early universe is radiation dominated.

18 Flatness problem The early universe is radiation dominated.

19 Flatness problem The early universe is radiation dominated.

20 Flatness problem The early universe is radiation dominated.

21 Flatness problem The early universe is radiation dominated.

22 Flatness problem The early universe is radiation dominated.

23 Flatness problem The early universe is radiation dominated.

24 Flatness problem The early universe is radiation dominated.

25 Growth of structure begins from initial dark matter perturbations Complex evolution: star formation, supernovae, AGN, interactions, mergers etc. Baryons cool within dm potential wells and form stars The basic idea Population of low redshift galaxies, clusters etc.

26 Cosmological model (W m, W L, h); dark matter Primordial fluctuations dr/r(m, t) Dark matter halos (N-body simulations) Well established Well understood Gasdynamic simulations Gas processes (cooling, star formation, feedback) Galaxy formation/evolution Semi-analytics DIFFICULT!