Lecture #25: Plan Cosmology The early Universe (cont d) The fate of our Universe The Great Unanswered Questions
Announcements Course evaluations: CourseEvalUM.umd.edu Review sheet #3 was emailed to you this a.m. Review session: Monday, December 11, 6-8 pm, PHYS 1412, your TAs will be answering your questions
Reminder: Light from the Early Universe Today (after a factor 1000x expansion since last scattering) λ m ~ 1 mm = 10 6 nm T ~ 3 K Cosmic Microwave Background (Penzias, Wilson 1965; Nobel Prize)
Reminder: Clumpiness in the CMB Fluctuations: ΔT / T ~ few 10-6 (Planck Satellite 2009 13 )
The small temperature/density fluctuations in the CMB are the seeds for proto-galaxies, clusters, & superclusters A Timeline of the Universe
A Timeline of the Universe
Birth of the Universe A Timeline of the Universe Early Universe Fate of the Universe!
Radiation, Matter and Antimatter Early Universe was very hot!!! today
Radiation, Matter and Antimatter Early Universe was very hot!!! Blackbody radiation has enough energy to create matter and antimatter (recall: E = Mc 2 ) For a proton-antiproton pair to form, T > 10 13 K (t < 10-6 sec) Matter and antimatter annihilate on contact, releasing energy (= Mc 2 ) Predominance of matter today 10-9 asymmetry between matter & antimatter in the past!
The Origin of Helium At t < 1 minute or so, only protons and neutrons existed At t > 1 minute or so, temperature and density were low enough for deuterium ( 2 H = 1 p + 1 n) to form by fusion At t > 100 seconds or so, temperature was cool enough so that deuterium could fuse into helium At t > 3 minutes or so, fusion into He stops 25% by mass of He in Universe (as observed!)
We ll come back to t << 10-6 sec under the Great Unanswered Questions, but let s first discuss the fate of the Universe
Fate of the Universe Fate of Universe is controlled by total energy: Energy of expansion (positive) Gravitational binding energy (negative)
Fate of the Universe If total energy > 0 Expansion continues forever ( open Universe) If total energy < 0 Expansion will halt, the Universe will contract and eventually collapse ( closed Universe)
Definition: Critical Density The density at which the total energy of the Universe is zero gravitational energy balances expansion ρ C = 3H 2 8πG where H = Hubble constant (~70 km/s/mpc) G = gravitational constant ρ c ~ 1 x 10-26 kg / m 3 ~ 5 hydrogen atoms / m 3!
Fate of the Universe (< late 1990s) Ω M ρ / ρ C, where ρ is the measured density of the Universe If Ω M > 1, the Universe will recollapse ( closed Universe) If Ω M < 1, the Universe will expand forever ( open Universe) If Ω M = 1, the Universe is exactly at the critical density ( flat Universe) Note: if Ω M 1, then the Universe is younger than ~10 billion years, i.e. less than the age of globular clusters! impossible!
Fate of the Universe (< late 1990s) Ω M Ω VM + Ω DM (= visible + dark matter) Observations: Ω VM ~ 0.03 Measurements: Ω DM ~ 10 x Ω VM ~ 0.3 So Ω M ~ 0.3 < 1 Universe expands forever.
Supernovae Findings (> late 1990s) New method: use supernovae as standard candles! Expansion rate at a time when the Universe was half its current size (z = 1) was slower than it is today! The expansion rate is increasing with time! Accelerating expansion Dark energy... very puzzling! (2011 Nobel Prize in Physics: S. Perlmutter, A. Riess, B. P. Schmidt)
Supernovae Findings (> late 1990s) New method: use supernovae as standard candles! Expansion rate at a time when the Universe was half its current size (z = 1) was slower than it is today! The expansion rate is increasing with time! Accelerating expansion Dark energy... very puzzling! (2011 Nobel Prize in Physics: S. Perlmutter, A. Riess, B. P. Schmidt)
Supernovae Findings (> late 1990s) New method: use supernovae as standard candles! Expansion rate at a time when the Universe was half its current size (z = 1) was slower than it is today! The expansion rate is increasing with time! Accelerating expansion Dark energy... very puzzling! (2011 Nobel Prize in Physics: S. Perlmutter, A. Riess, B. P. Schmidt)
Accelerating Expansion (t > 7.5 billion years) The fate of our Universe is eternal expansion!
Great Unanswered Questions 1. Origin of accelerating expansion 2. What makes up the dark matter 3. The birth of our Universe 4. (Life in the Universe? Next lecture )
1. Origin of Accelerating Expansion: What is Dark Energy? Unknown form of energy Very homogeneous Low density ~10-27 kg / m 3 Energy density is constant Provides an outward push to accelerate expansion Dark energy makes up ~70% of all of the energy in the Universe: Ω = Ω VM + Ω DM + Ω DE ~ 1 (flat) Independently confirmed with SNe, CMB, and large-scale structure More work needs to be done to characterize this dark energy
2. What Makes up the Dark Matter? (see also Lecture #23) Big Bang predicts the baryon (= proton + neutron) density: Ω B ρ B / ρ C ~ 0.05 «Ω DM ~ 0.3 Dark matter is predominantly non-baryonic Non-baryonic candidates: Light ( Hot ) dark matter particles: e.g., neutrinos Massive ( Cold ) dark matter particles: e.g., WIMPs (Weakly Interacting Massive Particles)
Cold Dark Matter Simulation: Galaxy cluster formation Millenium simulation, Springel et al. (MPA)
Observed galaxy clustering favors cold dark matter But dark matter particles have not yet been detected! Stay tuned Credit: KICC
3. The Birth of our Universe [NOT ON THE FINAL!]
Fundamental Forces in Nature Gravitational Force Force between massive bodies Infinite in range, but weakens with distance Electromagnetic Force Force between charged bodies Infinite in range, but weakens with distance Strong Nuclear Force Force that holds atomic nuclei together Very short range 10-15 meters! Weak Nuclear Force Force responsible for radioactive decay Very short range 10-18 meters!
Relative strength of four forces depends on temperature Today 10-6 sec 10-35 sec 10-43 sec GUTs Grand Unified Theories
Decay asymmetry due to weak force caused matter / antimatter imbalance Today 10-6 sec 10-35 sec 10-43 sec GUTs Grand Unified Theories
Phase transition (e.g. water freezing) @ 10-35 sec Release of vast amount of energy Today 10-6 sec 10-35 sec 10-43 sec GUTs Grand Unified Theories
The Epoch of Inflation (10-35 10-33 sec) By 10-33 second, the visible universe expanded from the size of a proton to the size of a basketball (x 10 20 ) This rapid expansion is called inflation This coincides with the time when the strong force became distinct from the electroweak force
What happened before 10-35 second? All forces except gravity are indistinguishable: GUT force Today 10-6 sec 10-35 sec 10-43 sec GUTs Grand Unified Theories
Planck time = 5.4 x 10-44 second Gravity becomes indistinguishable from all other forces??? No good theory of quantum gravity Planck Time
What about t < 0??? Very uncertain! Universe created from vacuum fluctuations??? Astrophysics meets religion
Lecture Activity #12 What is the fate of our Universe?