State of the Universe

Transcription

1 State of the Universe Consensus on Big Bang scenario, as evidence is compelling Universe began billions of years ago Then very hot and dense, now expanding and cooling May re collapse, or expand endlessly, depending upon density Conscious life is the wild card

2 Necessity of Big Bang Hints from 19th century science Thermodynamics First Law: Energy is conserved Second Law: Quality or usefulness of energy irreversibly decreases One-way slide of universe to equilibrium But if universe is running down at finite rate, then it can't have existed forever Olbers Paradox Why is the night sky dark? Universe infinite in space & time should be in equilibrium at 6000K If spatially finite, then would gravitationally collapse Hence universe must be of finite age

3 Evidence for Big Bang Red Shifts Elements radiate specific colors Systematic red shifts of spectra first observed by Hubble in 1920s Interpreted stretching of wavelengths to expansion of universe v = H 0 d H 0 ~ 75 (km/s)/mpc t ~ 1/H 0 ~ 15 Gyr Stellar and Expansion Ages If universe expanding, then must have been once together Extrapolating backwards, everything together ~10 10 yr ago This is in rough agreement with ages of oldest stars fusing H to He

4 Cosmological Nucleosynthesis Protons, neutrons, electrons "froze out" ~1 sec Nuclear furnace burned for ~3 minutes ~75% H, ~25% He, trace amounts of D= 2 H (1 part in 10 5 ) agrees with estimates of interstellar gas Relic Background Radiation of Primeval Fireball From opaque plasma to transparency ~10 5 yr Expansion stretches visible radiation to microwaves First observed by Penzias & Wilson in 1960s Thermal radiation at T~2.7K Dipole asymmetry (1 part in 10 3 ) QM fluctuations (11 parts in 10 6 )

5 Meaning of Big Bang Expanding ring-of-beads analogy Expansion of space between galactic clusters not clusters moving through space Every galactic cluster is center of expansion No edge to expansion Extrapolating backward, space & time created in Big Bang Before then, there was no "before"

6 Fire or Ice? Ultimate Collapse or Endless Expansion? Ballistic analogy flat but accelerating open & under dense flat & critically dense now closed & over dense t In simple cosmological models, future of universe related to its history, geometry, and density Can probe history with telescopes, which are also timescopes Can probe geometry by surveying galaxies and summing angles of large triangles Can probe density by counting stars

7 Density Bright Matter Infer sun's mass by its gravitational attraction Milky Way ~10 11 stars Visible universe ~10 10 galaxies solar masses in visible universe lyr across = 1% critical density Dark Matter Galaxies spinning and moving too fast ~10!-100! as massive as brightness suggests MACHOs: MAssive Compact Halo Objects WIMPs: Weakly Interacting Massive Particles Vacuum Energy Energy equivalent to mass via Einstein s E=mc 2 Nonzero energy density in the quantum vacuum keeps empty space under tension While mass, energy, and pressure gravitate, tension antigravitates Hence, vacuum energy accelerates the expansion

8 Inventory of a Flat but Accelerating Universe 4% atoms 23% cold dark matter 73% clear tension (dark energy) 96% of the energy density of the universe has so far not been detected in the laboratory We re not only not the center of the universe, we re not even made of the stuff of the universe!

9 Life in the Universe Millenia Species should flourish if can: Stabilize population without disastrous conflict Minimize ecosphere damage Overcome fossil fuel energy depletion bottleneck Eons Our descendants: Technology will be limited only by the laws of physics Can colonize the galaxy by planet hopping in ~30 Myr, a fraction of a galactic "year" (200 Myr revolution of solar system about Milky Way) Will not be human, perhaps not flesh and blood Whatever their form, will embody our essential humanity (values, culture, artistic & intellectual achievements)

10 Forever? Physics of an Expanding Universe 3 min of nucleosynthesis 10 5 yr of plasma Stars shine by thermonuclear fusion yr M W Degenerate stars devour dark matter yr and disintegrate via proton decay T ~ 1/M Black Holes devour matter yr and QM evaporate e + # e # " e + e " e + e " e + e " $ Electron-positron pairs, $ $ # $ photons & neutrinos # e + e " > yr # $ # e + e " e + e " $ $ # This is just the beginning! e + e " # Forever is a long time...

11 Forever? Life in an Expanding Universe How will our remote descendants deal with the slow degeneration of the universe? Can survive in dilute e " e + sea only by adapting their physical form and technologizing nature Minimum requirement is thought Assume thought possible by a finite state machine Information processing dissipates energy Superbrain/supercomputer could survive by slowing its activity via periodic hibernations as universe winds down Subjective time depends on info processing But brain must grow, else doomed to repeat finite number of brain states...

12 Forever? Physics of a Collapsing Universe 3 min of nucleosynthesis 10 5 yr of plasma Stars shine by thermonuclear fusion >10 6 yr Assume maximum expansion ~ yr, T ~ 1K (depends on density) Planets boil T ~ 300K Stars explode T ~ 6000K Atoms & BH merge Spacetime singularity There is no "after"

13 Forever? Life in an Collapsing Universe Hopeless? Death is not so simple In simplest models of collapsing universe, volume of space is finite but unbounded Our remote descendants, allied with life everywhere, may inhabit and gain control of entire universe Collapsing universe is finite in space, but infinite in momentum and energy space Again, subjective time depends on info processing Plausible that info processing at 1 GK might make imminent end seem years away Might supercomputer/superbrain experience an infinite number of thoughts in the finite time available? Depends on details of the collapse...

14 For What? What to do? Infinite information processing permits and infinite variety of simulated activity & virtual reality After all scientific questions are answered, after all technological problems are solved, then the really hard problems (like those in art and ethics?) can be addressed... Thoughts may know no bounds, even if the universe does

15 References Steven Weinberg The First Three Minutes (New York, Basic Books, 1977) Paul Davies The Last Three Minutes (New York, Basic Books, 1994) Frank Tipler The Physics of Immortality (New York, Doubleday, 1994) Fred Adams & Gregory Laughlin The Five Ages of the Universe (New York, The Free Press, 1999)