3. It is expanding: the galaxies are moving apart, accelerating slightly The mystery of Dark Energy

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1 II. Cosmology: How the universe developed Outstanding features of the universe today: 1. It is big, and full of galaxies. 2. It has structure: the galaxies are clumped in filaments and sheets The structure problem 3. It is expanding: the galaxies are moving apart, accelerating slightly The mystery of Dark Energy 4. It contains a uniform background glow of microwaves The smoothness ( horizon ) problem 5. It is almost flat: there is very little curvature. The flatness problem 6. There is more matter than antimatter. The antimatter problem 7. Galaxies are much heavier than the sum of their quarks and electrons. The mystery of Dark Matter Overall picture: the big bang billion years ago (only about 3 times older than the oldest rocks on earth) the universe was very dense and hot, and has been expanding and cooling ever since. 14

2 (1) The universe is big, and full of galaxies Powers of 10: the sizes of things in the universe Large-scale structure (walls, voids, etc in distribution of galaxies) Size of a Cluster of galaxies Distance between galaxies in a cluster 1 billion light-years 50 million light-years 5 million light-years Size of one galaxy m 100,000 light-years Star to star m 1 light-year Solar system m 1 light-day Planet Earth Man 10,000,000 (10 7 ) m 1 m What would a trip out to a billion light years be like? 15

3 Galaxies as far as the eye can see Now we have much more knowledge about the distant parts of the universe: The Hubble deep field image. Direction: just above Ursa Major (the plough or big dipper ) Exposure time: 10 days. Area covered: a dime, 75 feet away. Depth: about 10 billion light years. 16

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5 (2) Structure: How are galaxies arranged in space? Sloane Digital Sky Survey is mapping all the visible galaxies in the universe. We see signs of structure: voids and filaments. Where did they come from? Why are the quarks and electrons arranged that way? 18

6 (3) The expanding universe Edwin Hubble (1929): Other galaxies seem to be moving away from us. This just means the whole universe is uniformly expanding. There is no center to the universe. Are galaxies, stars etc individually getting bigger? No. 19

7 The Hubble Law Hubble law: the speed with which a galaxy recedes from us is proportional to its distance. v }{{} velocity = H }{{} Hubble const d }{{} distance H = 70 ± 3 km/s/megaparsec. 1 Megaparsec 3 million light years. 20

8 How do we know distant galaxies are receding? Doppler shift of the light from them. What is Doppler shift? If something is coming towards you, waves that it emits have higher frequency (blue shift). If it is moving away, waves have lower frequency (red shift). In everyday life, we notice this with sounds of passing vehicles. In cosmology it is actually slightly different: space itself expands and stretches the light waves. Galaxies can (and do) recede from us faster than the speed of light. Misconceptions about the big bang, Lineweaver and Davis, Scientific American, March

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10 (4) The cosmic microwave background An amazingly uniform and perfect blackbody thermal spectrum, with a temperature of 3 K. 23

11 (4) The cosmic microwave background, deviations from thermal spectrum Wilkinson Microwave Anisotropy Probe These tiny deviations help to confirm that the universe is flat. 24

12 (5) The geometry of the universe: space is flat What do we mean by flat? What are the alternatives? The geometry of the universe depends on the overall energy density ρ relative to a critical density ρ c. ρ/ρ c = Ω 0 > 1 ρ/ρ c = Ω 0 < 1 ρ/ρ c = Ω 0 = 1 More energy: closed universe Less energy: open universe Critical energy: flat universe 25

13 Matter and dark energy both contribute, Ω 0 = Ω m + Ω Λ Current observations indicate that they add up to exactly the critical energy density, giving a flat universe: Ω m 0.26 = 0.22 }{{} Ω Λ 0.74 Ω 0 = 1 dark matter }{{} normal matter Because there is some dark energy, the expansion of the universe is accelerating, and will go on forever. How does the Cosmic Microwave Background tell us that Ω m + Ω Λ = 1? 26

14 (6) Matter vs antimatter The laws of physics have symmetry with respect to matter and antimatter. But our universe consists entirely of matter. How did the asymmetry arise? How do we know there isn t any antimatter? Even empty space contains Hydrogen, a few atoms per m 3. If there were antimatter anywhere in the universe it would annihilate with the matter around it, creating distinctive gamma rays that we would see on earth. 27

15 (7) Dark Matter Dark matter is some additional type of matter, not made of quarks and electrons. The evidence for it comes from rotation curves of galaxies and gravitational lensing by galaxy clusters. We know it is not ordinary atoms, i.e. quarks and electrons, from our measurements of deuterium abundance in interstellar clouds. 28

16 Cold Dark Matter and the evolution of large scale structure The best explanation of the observed distribution of galaxies comes from assuming cold dark matter, which is made of some type of heavy particle, something that is not included in the standard model of particle physics. The VIRGO consortium has run huge computer simulations of how a universe with cold dark matter would evolve. How would the evolution of the universe look if you could see dark matter? 29

17 III. Inflation: a solution to cosmological puzzles. Inflation is the hypothesis that the very early universe, between and seconds old, underwent a sudden massive expansion, much faster than it is expanding now. Inflation has the potential to solve the flatness, structure, and smoothness problems. Inflation naturally leaves the universe very close to being flat. Inflation provides the seeds of structure by stretching quantum fluctuations out to galactic sizes. Inflation explains why the universe (specifically, the CMB) looks the same in all directions: before inflation it was a tiny thermally-equilibrated region. 30

18 Inflation and the Structure problem: Inflation stretches out tiny quantum fluctuations in the matter density Inflation Gravitational clumping After inflation they are big enough to lead to inhomogeneities in the cosmic microwave background, and ultimately to large scale structure and galaxies. 31

19 IV. The fate of the universe Current observations indicate that the universe is open, and will expand for ever years (now) stars burn H and He to heavier elements years years years years the bright stars burn out: galaxies consist of dark matter and dim stars, brown dwarfs or white dwarfs. the galaxies collapse into black holes, throwing out some of their dim stars into the intergalactic void. if GUTs are correct, protons decay: atoms fall apart into electrons, positrons, and neutrinos. the black holes evaporate into photons; the universe becomes a cold, rarefying gas of electrons, positrons, photons, and neutrinos. 32