1. Lecture 31: The Age & Fate of the Universe Astronomy 1143 – Spring 2014

2. Key Ideas: Expansion history of the Universe Calculate the time since the Big Bang Hubble time gives an approximate answer Current best estimate 13.798 +/- 0.037 Gyr Test w/Ages of Oldest Stars – Hydrogen burning Fate of an Accelerating Universe: Expands forever at an ever-increasing rate Star formation will end, protons may decay, black holes will evaporate, other quantum effects may become important Ends in a cold, dark, disordered state.

3. Back to the Beginning The Universe is expanding now. In the past: Universe was smaller. Galaxies were closer together in space. If we go back far enough in time: All galaxies (matter) in one place. How far back = “Age of the Universe”

4. Road Trip Analogy You leave Columbus by car for Florida, but leave your watch behind. How long have you been on the road? Your average speed = 100 km/h Your odometer reads: distance = 230 km Time since you left: T = distance  speed T = 230 km  100 km/h = 2.30 hours

5. The Hubble Time: T 0 Hubble’s Law says A galaxy at distance d away has a recession speed, v = H 0 d If locally, v is its average speed, then: T 0 = d / v but since, v = H 0  d, T 0 = d / H 0  d = 1 / H 0 Hubble Time: T 0 = 1 / H 0 Estimate of the Age of the Universe

6. Example H 0 =72 km/s/Mpc What is the Hubble Time? First issue: units!

7. But… Cosmic expansion is not expected to be constant over all times. But galaxies still need to get to their current distances. If faster in the past than 72 km/s/Mpc: Galaxies would take less time to get to their current distances T 0 would overestimate the age of the Universe. If slower in the past than 72 km/s/Mpc: Galaxies would take more time to get to their current distances T 0 would underestimate the age of the Universe.

8. So, How Old is it Really? Need two hard-to-measure numbers: Hubble Parameter, H 0 : How fast the universe is expanding now. Density Parameter,  : How the matter & energy density affected the expansion rate in the past. Can include an   term that enhances the expansion rate Needed to determine the expansion history.

9. Best Estimate of the Age: 13.798 ± 0.037 Gyr To calculate this number, we need to know many other properties of the Universe, such as H 0,  m, and  . We can test whether our measurement makes sense by looking at the ages of objects in the Universe. They should all be younger than 13.798 Gyr..

10. Testing the Finite Age of the Universe Ages of the Oldest Stars We can determine how long stars should be burning H into He in their cores Longer for lower masses When stars run out of hydrogen, their outward appearance changes dramatically as they evolve into red giants Measure masses of stars that are turning into red giants – get their ages Stars cannot be older than the Universe

11. Globular Clusters

12. Results Accurate measurements have been made for ~30 globular clusters Ages from 11-13 Gyr Biggest uncertainties Distance to cluster Amount of dust between cluster and us Composition of stars in cluster Improvements with Gaia! Oldest star clusters -- younger than Universe

13. Epochs of the Universe In the distant past, the Universe was radiation- dominated First 72,000 years Then the Universe is matter-dominated Next 10 billion years Density of matter dominates the budget of the Universe Now & in the future, the Universe is dark-energy- dominated All-pervading dark energy is most important

14. Epochs of the Universe

15. Current State of Universe We are living in a great time in the Universe for life Lots of stars Lots of planets Lots of light Enough stars have died to create metals to form planets and us Not so much time has past that all the gas has been used up and all stars are dead

16. Still have gas to make stars in the Galaxy today

18. Fading Out 10 trillion years 10 million years

19. Sometime in the future…. White dwarfs, neutron stars, black holes

20. Epoch of Star Formation The Present Day (t  14 Gyr): Most stars are metal rich, and make more metals ejected in supernova explosions. Next generation starts with a little less Hydrogen and few more metals. Some fraction of the star’s matter gets locked away in stellar remnants: White dwarfs, neutron stars & black holes.

21. The Once & Future Universe As the Universe expands: Expansion continues forever at a faster rate Space between galaxy clusters widens Universe cools down at a faster rate Details of the future Universe depend upon Stellar Evolution Gravity Quantum Mechanics

22. End of Star Formation After t=10 12 years: Successively more matter is locked up in stellar remnants, depleting the free gas reserves. Cycle of star birth & death is broken: Nuclear fuel is exhausted Red dwarfs burn out as low-mass white dwarfs Remaining matter is locked up in black dwarfs, cold neutron stars, and black holes The last stars fade into a long night…

23. Solar System “Evaporation” After t=10 17 years: Gravitational encounters between stars are rare, but disrupt orbiting systems: Planetary systems get disrupted by stellar encounters and their planets scattered. Wide binary systems are broken apart. Close binary stars coalesce into single remnants.

24. Dissolution of Galaxies After t=10 27 years: Stellar remnants within galaxies interact over many many orbits. Some stars gain energy from the interaction and ~90% get ejected from the galaxy. Others lose energy and sink towards the center. The last 10% coalesce into Supermassive Black Holes.

25. Will Dark Energy Help Tear Galaxies Apart? One of the many things we don’t know about dark energy is whether it has constant energy density for all time (cosmological constant) increasing or decreasing energy density with time (quintessence) If it is a cosmological constant, Universe will continue to accelerate, but gravity will be important. If the energy density increases with time, then dark energy could overwhelm gravity Right now, the dark energy seems fairly constant

26. Dissolution of Matter? After t=10 33 years: Some particle models predict that protons are unstable. Protons decay into electrons, positrons, to neutrinos. All matter not in Black Holes comes apart. Current experimental limits suggest that the proton decay time may  10 32 years.

27. If protons don’t decay 10 1500 years from now – matter quantum tunnels to become iron years from now – iron quantum tunnels to become neutron stars – black holes Regardless – Universe will become very cold, very thin and very uninteresting.

28. Black Holes Aren’t Permanent Virtual particles (particle/anti-particle) pairs are constantly being created and then immediately annihilate. If they are formed near a black hole’s event horizon, however, then one particle can go into the black hole while the other is released Black hole is emitting particles – Hawking radiation Energy must come from somewhere – mass is lost by the black hole

29. Hawking Radiation

30. Evaporation of Black Holes Higher energy is radiated as the black hole mass decreases After t=10 67 years: Remaining stellar-mass black holes evaporate by emitting particles and photons via Hawking Radiation. After t=10 100 years: Supermassive Black Holes evaporate one-by- one in a last final weak flash of gamma rays. End of the epoch of organized matter

31. Positronium Positronium is formed when an electron and positron orbit each other, bound by electric forces Eventually will release energy, get close and annihilate t~10 83 years Size~10 45 Mpc Cannot happen unless the imporance of dark energy decreases…

32. The Big Chill After black holes have all evaporated: Universe continues to cool off towards a Radiation Temperature of absolute zero. If protons decay, only matter is a thin, formless gas of electrons, positrons, neutrinos. Only radiation is a few increasingly redshifted photons. The end is cold, dark, and disordered...

33. Some say the world will end in fire. Some say in ice. From what I’ve tasted of desire I hold with those who favor fire. But if it had to perish twice, I think I know enough of hate To say that for destruction ice Is also great And would suffice. Robert Frost Fire and Ice (1920)