Astronomy 422. Lecture 23: Early Universe

Transcription

1 Astronomy 422 Lecture 23: Early Universe

2 Exam 3 ch Chapter 28: Active galaxies and Sunyaev-Zeldovich Effect Know formulas (basics about radiation) Skip ch 28.4 about grav. lensing BUT do the last few pages on Lyalpha forest/damping etc (and see lecture notes) Superluminal motion formulas Know what the S-Z effect is and how it is used Chapter 29: Newtonian Cosmology Know formulas 29-3,4,5,6,7,8 e.g. how Hubble parameter relates to scale factor 29.15, 18 and a simple derivation using escape velocity these are definitions of parameters like the density parameter, know them Know 29.10/65/108/110: These are all Friedmann's equation expressed in slightly different ways. What is the difference, what do they mean?

3 Exam 3 continued Chapter 30 Early Universe Know what the successes and problems are with the Hot Big Bang cosmological model. Know what inflation is, and how it solves 4 major problems with the Hot Big Bang model. Know what the Planck time is and the history of the universe after that including major events like: inflation, radiation dominated, matter dominated, CMB decoupling/recombination, epoch of re-ionization Know what the matter-antimatter asymmetry is. Know about the origin of structure.

4 Astro 422 Presentations: Thursday April 28: 9:30 9:50 _Isaiah Santistevan 9:50 10:10 _Cameron Trapp 10:10 10:30 _Jessica Lopez Tuesday May 3: 9:30 9:50 Chris Quintana 9:50 10:10 Austin Vaitkus 10:10 10:30 Kathryn Jackson Thursday May 5: 9:30 9:50 _Montie Avery 9:50 10:10 _Andrea Tallbrother 10:10 10:30 _Veronica Dike 10:30 10:50 _Kirtus Leyba

5 Key concepts: The Early Universe: A summary Structure formation End of the Universe

6 The Early Universe: conditions We know the the conditions & expansion rate of the universe today. If we run the expansion backwards we can predict the temperature & density of the Universe at anytime in its history using basic physics we can study how matter behaves at high temperatures & densities in laboratory experiments Current experimental evidence provides information on conditions as early as s after the Big Bang. Theories take us back to as little as as s.

7 GUT Era (10 43 < t < sec) The universe contained two natural forces: Gravity Grand Unified Theory (GUT) force electromagnetic + strong (nuclear) + weak forces unified Lasted until t~10 38 sec old at this time, the universe had cooled to K the strong force froze out of the GUT force Inflation (10 36 < t < sec) the energy released by this caused a sudden and dramatic inflation of the size of the universe

8 Electroweak Era (10 34 < t < sec) The universe contained three natural forces: gravity, strong, & electroweak Lasted until t~10 10 sec at this time, universe had cooled to K the electromagnetic & weak forces separated Experimentally verified in 1983! discovery of W & Z bosons electroweak particles predicted to exist above K (80-90 GeV)

9 Particle Era (10 11 < t < 10 3 sec) The four natural forces were now distinct Particles were as numerous as photons. When the universe was 10 4 sec old: quarks combined to form protons, neutrons, & their anti-particles At 10 3 sec old, the universe cooled to K protons, antiprotons, neutrons, & antineutrons could no longer be created from two photons (radiation) the remaining particles & antiparticles annihilated each other into radiation slight imbalance in number of protons & neutrons allowed matter to remain Electrons & positrons are still being created from photons

10 Era of Nucleosynthesis (10 3 sec < t < 3 min) During this era, protons & neutrons started 'fusing' but new nuclei were also torn apart by the high temperatures When the universe was 3 min old, it had cooled to 10 9 K at this point, the fusion stopped Afterwards, the baryonic matter leftover in the Universe was: 75% Hydrogen nuclei (i.e. individual protons) 25% Helium nuclei trace amounts of Deuterium & Lithium

11 Era of Nuclei (3 min < t < 3.8 x 10 5 yr) The universe was a hot plasma of H & He nuclei and electrons. photons bounced from electron to electron, not traveling very far the universe was opaque At t~380,000 yrs: universe had cooled to a temperature of 3,000 K electrons combined with nuclei to form stable atoms of H & He photons free to stream across the universe, decoupling

12 Era of Atoms (3.8 x 10 5 < t < 10 9 yr) The universe was filled with neutral atomic gas. Usually referred to as the Cosmic Dark Ages Density enhancements in the gas and gravitational attraction by dark matter: eventually form protogalactic clouds the first star formation lights up the universe which provokes the formation of galaxies

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14 When is the Epoch of Reionization? Sometime between z = 30 and z = 7 Begins at Cosmic Dawn with the formation of first stars & galaxies

15 Current Best Estimates & Limits

16 LWA 1 Science Program: Dark Ages The predicted brightness temperature of the 21cm line from the HI gas is displayed as a function of time, redshift & frequency. Figure 1 from Pritchard & Loeb, 2010 Nature The Dark Ages through Cosmic Dawn encompasses the formation of the 1st galaxies & black holes. LWA1 offers a unique window into this era. LWA1 has two major funded projects in place: LEDA (PI Greenhill): Constrain Dark Ages signal [LG001] Probe thermal history & Lyα output of 1st stars & galaxies by characterizing HI trough only means to detect z >15 New correlator, total power hardware & data reduction pipeline Cosmic Dawn (PI Bowman):Constrain final absorption peak -dual-beam technique to minimize foregrounds [LB002] 16

17 Lyman-α photon production (likely from stars) determines magnitude of decoupling from the dashed curve LEDA: Inference LEDA EDGES LWA1 project [LG001] Production of ionizing photons determines the difference between dashdot and solid curves Case where IGM not reheated prior to reionization. It takes just 10-3 ev per baryon to significantly change this curve.

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19 Era of Galaxies ( t > 10 9 yr) The first galaxies came into existence about 1 billion years after the Big Bang.

20 Accelerating Universe

21 Accelerating Universe

22 Structure formation - the big picture Structure formation is theory of transition from a very homogeneous state in the early universe, as reflected in the CMB, to the structure of galaxies and clusters we observe today. Quantum fluctuations from inflationary era were 'stretched' to cosmic scales Density fluctuations frozen in until matter-radiation equality Density fluctuations grow during matter-dominated era (>10,000 yr ABB) due to gravitational instability Astronomical structures form when fluctuation amplitude becomes large

23 Gravitational instability

24 From CMB, we know that at the time of recombination perturbations were about 10-5 of the average density. Now, it is about 10 5 (in a galaxy). How do we get there?

25 Baryon Acoustic Oscillations Eisenstein et al. 2004

26 Theory of structure formation What is the spectrum of initial density fluctuations? How do density fluctuations grow in an expanding universe? in dark matter? in baryonic matter? How does growth occur for stars and galaxies of primordial gas? Did big or little form first? Let's consider growth of perturbations in a static, and then in an expanding universe.

27 Static spacetime Idealized problem: infinite space constant density ρ 0 everywhere consider only gravity What happens? Nothing. A given particle is attracted equally in all directions. Now suppose that in a sphere of radius R, there is a constant additional density ρ 0 δ <<1. What happens? Dense region contracts further.

28 Gravitational collapse: Consider a particle on edge of the density perturbation. Gravity on it is sum of background (constant density) plus perturbation. Background gives no net force Only perturbation matters Extra mass Acceleration How long does it take to shrink by Density contrast doubles in time

29 Collapse on static background Suppose spacetime is not expanding. We know that at a given redshift, the critical density is Close to this in early universe. Time scale to collapse is then Very short at z=1000. This would form structure easily, within a few million years. 'Static background' valid for molecular clouds. But universe itself is expanding.

30 Collapse on expanding background Now we suppose that there is a slight density enhancement, but that the spacetime is expanding. As long as ρ<<1, the density decreases. The density enhancements are proportional to R(t), =1/1+z (assuming matter domination) As a result, the growth of the perturbations is much slower. From models, the first structures may be expected at around z=10-20.

31 Non-linear collapse Now suppose that we reached a point where ρ~1 The overall expansion becomes unimportant, and the collapse will be in free fall. First, consider dark matter, no non-gravitational interactions. Collapse cannot proceed indefinitely! When random motions become comparable to orbital speeds, a quasi-equilibrium is reached.

32 Next step: gas physics If all matter were non-interacting, elementary particles, nothing would happen. achieve relaxed configuration sit there indefinitely But, normal matter interacts: gas collides, radiates cooling causes sinking in potential This means normal matter can achieve much higher densities once density is high enough, stars can form

33 The first stars How would the first star in a large volume differ from today's stars? no metal enrichment (only H, He, little Li, Be) Little cooling This implies that the Jeans mass remains high. Winds, pulsations are less effective driven by metal line opacities! First stars could be very massive M sun Possible quasi-stars with10 5 M sun

34 From small to large In the standard picture of primordial fluctuations, there is a larger δ on smaller scales The first structures therefore form on small scales BUT: temperature prevents structure to form at a too small scale compromise: 10 5 M sun globular clusters? Small fluctuations are strongest on top of big fluctuations Therefore, hierarchical structure formation small structures form early big structure forms late small halos merge to big

35 Consequences and predictions One prediction relates to the number of 'small' halos to 'big' halos. MW should have hundreds.oops? Why so few dwarf galaxies? Maybe if too small, first star blows away all gas, just BH and dark matter would still have dark matter halos possibly seen with lensing! If BH are formed at centers of all halos, would expect many mergers of BH when halo merge. gravitational waves?

36 Structure formation summary: The evolution of structure depends on the cosmological parameters (ie, the amount and type of matter and energy and the expansion rate) The evolution of large-scale structure in the Universe is governed by gravity. All structures today formed by gravitational amplification of the small fluctuations that we think were generated by inflation. We think our Universe is dominated by cold dark matter. In Universes dominated by cold dark matter (CDM), structure forms hierarchically, i.e., small things form first, and merge to form larger things. Structure on various scales in CDM is nearly self-similar (substructure in galaxies looks like substructure in clusters)

37 The Five Ages of the Universe 1) The Primordial Era 2) The Stelliferous Era 3) The Degenerate Era 4) The Black Hole Era 5) The Dark Era

38 1. The Primordial Era: y Something triggers the Creation of the Universe at t=0

39 Now = 14 billion years ~ y 2. The Stelliferous Era: y

40 How Long do Stars Live? A star on Main Sequence has fusion of H to He in its core. How fast depends on mass of H available and rate of fusion. Mass of H in core depends on mass of star. Fusion rate is related to luminosity (fusion reactions make the radiation energy). So, lifetime α mass of core fusion rate α mass of star luminosity Because luminosity α (mass) 3, mass lifetime α (mass) 3 or 1 (mass) 2 So if the Sun's lifetime is 10 billion years, the smallest 0.1 M Sun star's lifetime is 1 trillion years.

41 How Long do Galaxies Live? Only as long as they can continue to manufacture stars. To do that the galaxy needs gas. So, lifetime α Mass of gas star formation rate = 10 billion years (for MW) Galaxies with modest star formation rates can shine for perhaps 1 trillion years

42 3. The Degenerate Era: y Most stars leave behind a white dwarf Mass between 0.1 and 1.4 M_sun

43 The Degenerate Era: y Some failed protostars never got hot enough to ignite hydrogen fusion: Brown Dwarfs Mass < 0.08 M_sun Brown dwarf collisions can create occasional warm spots in an increasingly cool universe

44 The Degenerate Era: y

45 The Degenerate Era: y Neutron stars: Cold and no longer pulsating Mass ~ 1.5 M_sun

46 The Degenerate Era: y Black holes Supermassive black holes Stellar mass black holes

47 Galaxy evolution: dynamic relaxation during the Degenerate Era Galaxies continue to merge to form large meta-galaxies (entire local group merges into a single galaxy); spirals merge --> ellipticals Massive remnants sink to the center of the galaxy Less massive remnants get ejected from the galaxy (all the brown dwarfs are gone by y).

48 What happens to Solar systems like ours? Inner planets are fried during end of stelliferous era Planets are gradually stripped away during stellar encounters in the degenerate era y

49 Dark Matter Annihlation of WIMPs (Weakly Interacting Massive Particles)? - In the halo of the galaxy - In the cores of white dwarfs (power ~ Watts, 10-9 L_sun) Surface temperature ~60 K Steady energy source for ~10 20 y

50 Proton Decay Predicted lifetime of protons (and neutrons) is y - In white dwarfs (power ~ 400 Watts, L_sun) Surface temperature ~0.06 K Composition changes to frozen H Star expands Slow decay over ~10 36 y Eventual disintegration into photons Neutron stars, planets, dust, all face the same fate.

51 4. The Black Hole Era: y Black holes inherit the Universe - mostly in the form of stellar mass black holes Some electrons, positrons, neutrinos and other particles remain Planets, Stars and Galaxies are all long gone

52 4. The Black Hole Era: y Black holes eventually start to decay by Hawking Radiation

53 Hawking radiation continued: Effective Temperature ~ 1/mass Universe cools with time, so that after y, the Universe is cooler than a 1 solar mass black hole (10-7 K) After y, even 1 billion solar mass black holes have begun to evaporate. Final stage of black hole radiation is explosive with 10 6 kg of mass converted into energy After y, even the most massive black holes are gone. NB: If acceleration (non-zero cosmological constant) is assumed then Universe expands and cools much faster.

54 5. The Dark Era: > y Only some elementary particles and ultra-long-wavelength photons remain inside a vastly expanded Universe. Density is unimaginably low. Our observable Universe now has a size of cubic meters. In the Dark Era there will be one electron every cubic meters. Heat death - nothing happens, no more sources of energy available Or.

55 1. The Primordial Era: y Something triggers the Creation of a child Universe

56 Child Universe Living on borrowed energy: mc 2 = 1/2 m v 2 esc Energy of expansion is about equal to energy in matter Ω = 1