The Beginning of Time

Review: evidence for dark matter evidence for dark matter comes from  motions of stars and gas in galaxies  motions of galaxies within groups and clusters  temperature of hot gas in clusters  gravitational lensing

nature of dark matter baryonic (normal matter; MACHOs) non-baryonic (exotic matter; WIMPs)  hot (moves close to speed of light)  cold (moves slowly)

observational evidence MACHOs make up only a small fraction of dark matter in our galaxy (microlensing) hot dark matter would not clump to produce observed large scale structure (computer simulations) the dark matter is believed to be mostly non-baryonic (WIMPy) and cold

The fate of the Universe the amount of mass in the Universe (or the average mass-density) determines whether the Universe will go on expanding, or will eventually recollapse.

possible futures: expansion continues forever at about the same rate (coasting) expansion continues forever, but slows down all the time, until eventually the expansion is infinitely slow (critical) at some point, gravity wins, and the Universe starts to contract (recollapsing) expansion accelerates (accelerating)

critical density the dividing line between a Universe that goes on expanding forever and one that recollapses is called the critical density the value of the critical density (for H 0 = 70 km/s/Mpc) is 9.2 x kg/m 3 or 1.36 x M sun /Mpc 3

the density parameter  astronomers like to express the density of the Universe today in terms of the critical density:  =  0 /  c then   > 1   0 >  c  recollapse   = 1   0 =  c  critical   < 1   0 <  c  expand forever

Remember, General Relativity says: mass tells space-time how to curve the curvature of space-time tells mass how to move therefore, the amount of mass in the Universe also determines the geometry of space-time

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so all these things are connected amount of matter in the Universe (  ) fate of the Universe (indefinite expansion or recollapse) geometry of the Universe (global curvature of space-time) Hubble law (relationship between distance and recession velocity)

what this means… this means that if we could measure Hubble’s Law over a large range of time (or distance) we could find out how the expansion rate has changed over time this would allow us to determine 

the discovery of “dark energy”… white dwarf supernovae used as standard candles indicate that the expansion of the Universe is accelerating ! (galaxies are moving away from each other faster now than they were in the past) this means that there must be a repulsive force that pushes matter apart (sort of the opposite of gravity) this force is termed “dark energy”

cosmic mass/energy budget stars 0.01 gas 0.04 dark matter 0.25 dark energy 0.7  mass+energy = 1 (or very close)

The Big Bang the Universe is expanding and was denser in the past the Universe had a finite beginning (in time) at the moment of its creation, the Universe was a “hot singularity”

the Universe cools off as it expands…

A Brief History of the Universe < seconds: Planck era  before the Planck time (all known laws of physics break down and we can’t say anything about what conditions were like) seconds: GUT era  all four forces are “unified” (have the same strength) seconds: electroweak era  the electromagnetic and weak force become distinct.

The Planck Time t P = [Gh/c 5 ] 1/2 = 1.35 x s

the particle era seconds: particle era  plasma of fundamental particles (matter and anti-matter)  for some unknown reason, there must have been slightly more matter than anti-matter, at least in our corner of the Universe  at the end of the particle era, matter and anti-matter annihilate, leaving mostly matter. Photons outnumber protons by a billion to 1.

Big Bang Nucleosynthesis seconds– 3 minutes: nucleosynthesis temperatures of 10 9 K allow hydrogen nuclei to fuse into helium nuclei. proton neutron deuterium 3H3H 4 He

the density of baryons  deuterium is formed in the course of fusing hydrogen to helium, and some is still left over the observed ratio of deuterium to hydrogen and helium tells us the density of baryons (protons and neutrons) during the era of nucleosynthesis observations of deuterium abundance show that the density of baryons is about 5 % of the critical density

prediction: hydrogen 75 % helium 25% at very high temperatures (>10 11 K) protons and neutrons can change into one another as the Universe cools, protons (which are slightly less massive) become favored ratio of protons to neutrons at the time when nucleosynthesis begins is predicted to be 7 to 1.

Recombination 3 minutes – 500,000 years: recombination hydrogen and helium nuclei capture electrons and become neutral atoms. The Universe becomes transparent to photons.

Stars and Galaxies the plasma of neutral atoms gradually cools and protogalactic clouds form. The first stars form out of the hydrogen and helium, make heavy elements, etc. planets life astronomers etc…