lecture27
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The document discusses the fate and evolution of the universe based on its density. It explains that if the density is greater than the critical density, the universe will collapse in a "Big Crunch", while if it is less it will expand forever. Observations find the density is approximately equal to the critical density, implying a flat universe. However, ordinary and dark matter only account for about 30% of the density, so some unknown "dark energy" is proposed to make up the remaining 70% and explain the observed accelerated expansion of the universe.
lecture27
- 1. Introduction to modern astronomy 27 島袋隼⼠(Hayato Shimabukuro)（云南⼤学、 SWIFAR） ©GETTYIMAGES
- 2. The fate of the cosmos ρ > ρcr ρ < ρcr (a)The universe turns to shrink and is collapsed (Big Crunch). We do not know what happened. (b)The universe expands forever. What physics determines the evolution of the universe?
- 3. The geometry of space •If the space is flat, the sum of a triangle’s angles becomes 180∘ •If the space is opened, the sum of a triangle’s angles becomes greater than 180∘ •If the space is closed, the sum of a triangle’s angles becomes less than 180∘
- 4. Rμν − 1 2 gμνR + Λgμν = 8πG c4 Tμν The geometry of space Einstein equation 时空的扭曲 物质（能量） 时空的⼏何学 Open universe Closed universe Flat universe In order to consider the evolution of the universe, we need Einstein’s theory of general relativity.
- 5. The geometry of space What physics determines the evolution of the universe? ρcr = 3H0 8πG = 9 × 10−27 kg/m3 Remember the critical density •If the density of the universe is greater than , the universe becomes a closed universe. ρ ρcr Closed universe •If the density of the universe is smaller than , the universe becomes an opened universe. ρ ρcr Open universe •If the density of the universe is equal to , the universe becomes a flat universe. ρ ρcr Flat universe How large is the density of the universe ? ρ
- 6. The density of the universe •Now we understand the density of the universe is important to determine the future of the universe. •Recent observations suggest the density of the universe is roughly equal to the critical density. ρ ∼ ρcr Or Ω0 = ρ ρcr ∼ 1 This implies that our universe is almost flat.
- 7. But, we face another mystery. The density of the universe We find or . But, the content of the density is very strange. ρ ∼ ρcr Ω0 = ρ ρcr ∼ 1 ρ = ? Ordinal matter
- 8. But, we face another mystery. The density of the universe We find or . But, the content of the density is very strange. ρ ∼ ρcr Ω0 = ρ ρcr ∼ 1 ρ = No! Ordinal matters such as galaxies, and stars make up only a few percent of the density of the universe. ? Ordinal matter
- 9. But, we face another mystery. The density of the universe We find or . But, the content of the density is very strange. ρ ∼ ρcr Ω0 = ρ ρcr ∼ 1 ρ = + ? Ordinal matter Dark matter
- 10. But, we face another mystery. The density of the universe We find or . But, the content of the density is very strange. ρ ∼ ρcr Ω0 = ρ ρcr ∼ 1 ρ = + ? Ordinal matter Dark matter No! Even if we include the density of dark matter, . does not become 1!! Ω0 ∼ 0.25 Ω0
- 11. Supernova as a standard candle Then, how can we achieve ?? Ω0 = 1 •As I told you before, measuring distance is important in astronomy. We can measure the distance of a supernova by observing its brightness. Fainter stars are more far away from us, brighter stars are closer to us. Before I explain the answer, we focus on the Type Ia supernova observation.
- 12. Then, how can we achieve ?? Ω0 = 1 •As I told you before, measuring distance is important in astronomy. Before I explain the answer, we focus on the Type Ia supernova observation. Independent on the distance to the supernova, we can also measure the redshift of the supernova by using its spectrum. Supernova as a standard candle z = 0 0
- 13. Supernova as a standard candle Now we can obtain the supernova's distance and redshift independently. By using the cosmological model (宇宙学的理论), we can associate the distance with redshift. dL ≈ c H0 z ( 1 + 1 − q0 2 z ) acceleration parameter. q0 : •If , the expansion of the universe accelerates（加速） and distance becomes larger for a given redshift q0 < 0 •If , the expansion of the universe decelerates（减速） and distance becomes shorter for a given redshift q0 > 0
- 14. Accelerating cosmic expansion •If , the expansion of the universe accelerates（加速） and distance becomes larger for a given redshift q0 < 0 •If , the expansion of the universe decelerates（减速） and distance becomes shorter for a given redshift q0 > 0 •The observation of supernova indicates . q0 < 0 The expansion of the universe is accelerating!
- 15. Accelerating cosmic expansion •If , the expansion of the universe accelerates（加速） and distance becomes larger for a given redshift q0 < 0 •If , the expansion of the universe decelerates（减速） and distance becomes shorter for a given redshift q0 > 0 •The observation of supernova indicates . q0 < 0 The expansion of the universe is accelerating!
- 16. Dark energy and accelerating expansion Ordinal matter or dark matter Gravitational force attract each other Dark energy To let the expansion of the universe accelerate, dark energy should behave as the anti-gravitational force! Q. Why is the expansion of the universe accelerating? A. We do not know the answer, but we assume dark energy causes it.
- 17. Dark energy and accelerating expansion Q. Why is the expansion of the universe accelerating? A. We do not know the answer, but we assume dark energy causes it.
- 18. The density of the universe ρ = + + Ordinal matter Dark matter Dark energy = 1
- 19. The density of the universe ρ = + + Ordinal matter Dark matter Dark energy = 1 Dark energy: ~70 %, Dark matter: ~25 %, Ordinal matter: ~5 %
- 20. The fate of the universe The expansion of the universe is accelerating now, but we do not know the future!
- 23. Summary • Our universe is homogeneous and isotropic at large scales • Our universe is expanding and the future of the universe depends on the density of the universe. • According to supernova observations, the expansion of the universe is accelerating. We assume this is caused by dark energy.