ASTRONOMY 9: HISTORY OF COSMOLOGY

Handout #25

J. E. Baker
UC Berkeley, Spring 2000

A Brief History of Everything

• Basic building blocks of particle physics
  • Fermions are divided into three ``families'' including six leptons and six quarks:
    • Leptons: electron, muon, tauon and 3 neutrinos (one associated with each)
    • Quarks: up/down, charmed/strange, top/bottom
    • Protons, neutrons are combinations of 3 up/down quarks
    • Zoo of particles arises from combinations of the above
  • We know of four major forces (mediated by bosons):

    1.

    Strong nuclear force: short range (10^-13 m), binds atomic nuclei, carried by 8 gluons, strength $\sim$ 1

    2.

    Electromagnetism: 1/r², carried by photons, strength $\sim$ 0.01

    3.

    Weak nuclear force: short range, causes fission and neutron decay reactions, carried by W⁺, W^-, Z⁰, strength $\sim 10^{-13}$

    4.

    Gravity: 1/r², carried by graviton, strength $\sim 10^{-41}$ (weakest by far, but wins in cosmology because it is long range and only attractive!)

  • Inner space--outer space connection: physics of very small/high energy determines physics of the whole early universe!
  • When temperature is high enough in early universe, photons have enough energy to create particle/anti-particle pairs spontaneously
  • The more massive the particle, the higher the required temperature
  • Universe evolves from a state of simplicity into one of complexity (analogy: steam to liquid water to snowflake)
• t < 10^-43 sec (Planck time, $t_\mathrm{Pl} = \sqrt{\hbar G/c^5}$)
  • At this time, all 4 forces are united into a single ``Theory of Everything'' (TOE) or super-gravity!
  • Universe was very simple, only a single force, all particles behave the same way
  • But we don't know what the TOE is yet: how to unite quantum mechanics and general relativity?
  • Einstein and many others tried but failed, now ``holy grail'' of physics
  • Best guess: TOE may come out of ``M-theory'' (string theory: particles are not point-like, but might be higher-dimensional entities, like loops!)
  • So, all conjectures about this period are sheer speculation!
  • Spacetime as a chaotic ``foam''
• t = 10^-43 sec
  • Gravity splits off as a separate force
  • Gravitons no longer in equilibrium with other particles
  • G.R. takes over, current theory has a hope of describing the universe
  • Theories of strong + weak + EM are called ``grand unified theories'' (GUTs)
  • GUTs are incomplete and direct testing would require particle accelerators bigger than the solar system! Need indirect evidence from cosmology
• $t \sim 10^{-35}$ sec, $T\sim 10^{28}$ K
  • Inflation (1980, Alan Guth)
    • Universe expands exponentially (dominated by vacuum energy, $\Lambda$) for a brief period, by a factor of 10²⁷!
    • Four main reasons why this needs to have happened (things unexplained in the Big Bang paradigm without inflation):

      1.

      Flatness Problem

      • If $\Omega$ is even slightly different from 1 in the early universe, quickly becomes very different from 1 (like a pencil balanced on its point)
      • Natural prediction is that $\Omega$ should be very close to zero now, or the universe should have recollapsed after a Planck time!
      • Why should $\Omega$ be so incredibly close to 1 early on?
      • Inflation explains this: if you blow the natural curvature up by a factor of 10²⁷, the part of the universe we live in will look flat

      2.

      Horizon Problem

      • CMB is very smooth: points on opposite sides of sky have same temperature to 1 part in 100,000!
      • Without inflation, light could only have traveled across regions about $1^\circ$ in size by the time the universe became transparent
      • No signal could have yet been sent from one side of the sky to the other side, so how did they know to be the same temperature?
      • Inflation explains this: observable universe comes from a much smaller region, signals could have crossed before inflation

      3.

      Relic Problem

      • Many particle physics theories predict weird particles (like magnetic monopoles) to be formed in great abundance in the early universe
      • But no one has ever seen one!
      • Inflation explains this: universe expands so much that density of particles declines to nearly zero

      4.

      Origin of Inhomogeneities

      • Need little ``seeds'': places of slightly higher density which can grow under their own gravity into galaxies, stars, cats, etc.
      • Where do they come from?
      • Uncertainty principle: all systems show tiny fluctuations
      • Inflation allows them to get stretched out big enough to explain where large-scale structure came from

    • Chaotic Inflation (Linde, 1990s)
      • Little bubbles of vacuum can inflate into whole new ``universes''
      • Could have been happening throughout eternity
      • Each bubble might have very different physical properties
      • Might explain why universe seems ``designed'' for life (change physical constants a tiny bit, and stars, people, etc. could not exist!)
      • We naturally live in one of the (few?) regions which is conducive to life
  • Baryogenesis (creation of matter)
    • After strong force splits off, total number of baryons (particles made of quarks) is conserved
    • Anti-matter counts as a ``negative'' baryon
    • Before split, can create a slight excess of matter over anti-matter
    • Excess is very slight only 1 part in 10<sup>9</sup>!
    • Rest of the matter gets annihilated by equal amounts of anti-matter, leaving 10<sup>9</sup> photons for every baryon
  • Strong force splits off as a separate force
    • Theory of electromagnetism plus weak (electroweak) well understood since 1970s
    • Strong force is less well understood
• $t\approx 10^{-11}$ sec, $T\approx 10^{15}$ K
  • Weak interaction splits from EM, all 4 forces now in current state
• $t\approx 10^{-6}$ sec, $T\approx 10^{12}$ K
  • Universe cools to point where quarks can no longer be free
  • Protons, neutrons are formed
  • Before this time, quark-gluon plasma (state of matter to be created at RHIC!)
• $t\approx 1$ sec, $T\approx 10^{10}$ K
  • Neutrinos (light, weakly interacting particles) stop interacting with other particles
  • Begin to travel freely through the universe
  • Recent result from neutrino experiments: they probably have a (very small) mass!
  • Early idea (1980s) for what the dark matter could be made of
  • But $\Omega_\nu$ probably much less than 1%, too small to affect cosmology
• $t\approx 3$ min, $T\approx 10^9$ K
  • Nucleosynthesis: formation of helium and small amounts of other light elements
• $t\approx 50,000$ years, $T\approx 10^5$ K, $z\approx 20,000$
  • Until this point, radiation (photons, relativistic particles) density was greater than matter density
  • Afterwards, matter becomes dominant
  • Expansion speeds up, from $R\propto t^{1/2}$ to $R\propto t^{2/3}$
• $t\approx 500,000$ years, $T\approx 10,000$ K, $z\approx 1100$
  • Universe becomes transparent, electrons become bound to atoms
  • CMB travels freely through the universe
• $t\approx 10^6$ to 10<sup>8</sup> years, $z\approx 500$ to 30
  • ``Dark Ages'': CMB redshifted out of the visible range
  • Gas begins collapsing
  • Primordial black holes formed?
• $t\approx 10^8$ to 10⁹ years, $z\approx 30$ to 5
  • First stars form out of primordial hydrogen and helium, begin to form heavier elements
  • Galaxies begin to form
  • Giant black holes at the centers of galaxies begin to shine as quasars
• $t\approx 5$ to 10 billion years, $z\approx 1$
  • Great clusters of galaxies formed
  • Curvature or a cosmological constant might begin to affect the expansion of the universe
• $t\approx$ now - 5 billion years
  • Our solar system, Earth, life forms
• t= now ($\approx$ 15 billion years), z=0
  • Humans evolve 100,000 yrs ago
  • Civilization begins 10,000 yrs ago
  • Astro 9 begins 4 months ago
  • Census 2000: what are the contents of the universe today?
    • Recall $\Omega_X$ measures the fraction of the critical density contributed by a certain kind of stuff (X)
    • Stars and luminous gas contribute only a tiny fraction, about 0.5%: $\Omega_\ast\approx$ 0.003-0.007
    • Total for ordinary matter (baryons, known from nucleosynthesis calculation) is 5%: $\Omega_B\approx 0.05$
    • Neutrinos are probably at least 0.3% (1999 discovery of mass) but less than 10% (else structure formation is suppressed too much): $\Omega_\nu\sim$ 0.003-0.10
    • Weird relic particles (non-baryonic ``cold dark matter'' or CDM) between 25% and 45%: $\Omega_\mathrm{CDM} \approx$ 0.25-0.45
      • Known to exist for many reasons:
      • Motions of galaxies in clusters are too fast unless there is extra mass beyond what we can see (Zwicky, 1930s)
      • Stars orbiting in galaxies are also moving too fast
      • Structure formation is suppressed to much unless there is some CDM (1980s)
    • Total for matter (known from studies of galaxy motions and clusters): $\Omega_m\approx 0.3$-0.5
    • ``Dark energy'' (something like a $\Lambda$): $\Omega_\Lambda \approx 0.6$-0.9
      • Evidence from supernovae (1998)
      • More evidence: CMB tells us universe is flat (e.g., BOOMERANG, 2000), but we know total for matter is $<50\%$, so dark energy must make up the difference
    • Total for everything: $\Omega\approx 0.9$-1.1
• $t\approx$ now + 5 billion years
  • Our Sun burns out, expands into a ``planetary nebula'' engulfing the Earth
• $t\approx$ 10 to 100 trillion years
  • Last of the stars (small, faint red dwarfs) use up their fuel
  • More massive stars die much earlier
  • Dark, degenerate stellar remnants remain (white dwarfs)
• $t\sim 10^{20}$ years
  • Galaxies ``evaporate'': stellar remnants are ejected into intergalactic space
• $t\sim 10^{37}$ years
  • Protons probably decay (depends on GUT physics): stars evaporate into photons
• $t\sim 10^{100}$ years
  • Last of the massive black holes (billions of solar masses) evaporate explosively into photons (``Hawking radiation'')
  • Less massive black holes evaporate earlier
  • Gravity has lost the great cosmic battle, entropy has won: ultimate heat death (speculative, of course!)
• Fate of Life in an Ever-Expanding Universe
  • Amount of energy we can hope to collect is probably finite: consciousness must someday come to a halt
  • Particularly bad if there is $\Lambda$; we live in a shrinking event horizon, behind which most other galaxies will disappear in <10<sup>12</sup> years
  • Speculative solution: create a ``baby universe'', and escape (or at least send a signal) to it!

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