Beginnings and Endings

Study of the universe itself

Size - finite or infinite?



Evolution and ultimate fate?

The Redshift and the Hubble Law

Galaxies are receding with redshifts proportional to distances (Hubble law)

Vr = H * D

Vr velocity of recession

D distance

H Hubble constant = 71 km/sec/Mpc

The Expanding Universe

Entire universe is expanding - no obvious center.

All clusters of galaxies receding from each other.

General Relativity: space-time is expanding - galaxy clusters are NOT expanding into empty space

Universe 13.7 x109 yrs old

Evidence for the Big Bang

Recession of galaxies is consistent with Big Bang, but doesn't prove it

Cosmic Microwave Background Radiation (CMBR) suggests universe once hot and dense

CMBR - faint emission at ~ 1 mm, almost uniform intensity across sky

Predicted: Early universe contained hot matter, radiation

Characteristics similar to observed CMBR

As universe expanded, radiation redshifted

Almost uniform distribution

Black body temp. ~ 2.73 K now

Observed universe ~75% H, ~23% He

Source of observed He - nuclear reactions when universe hot, dense

To explain CMBR and observed He abundance, universe must have had a hot, dense phase.

Big Bang - The "Standard Model"

Universe - space, time, energy, matter originated in


Universe started to expand, temp. dropped, photons transform to particles & anti-particles and reverse

As temp. dropped below 1013 K at 10-6 sec, protons and neutrons no longer formed, but particles and anti-particles annihilated each other.

Matter exists because there were not exactly equal numbers of particles and anti-particles.

Few seconds later, electrons & positrons no longer created, most annihilated each other, leaving some electrons

After 100 - 200 sec, fusion produced deuterium, helium, small amounts of lithium, beryllium

Explains amount of He, H

Universe expands, time passes

t = 380,000 years, T ~ 3000 K

Nuclei capture electrons - producing neutral atoms

Space transparent to radiation (Decoupling)

Radiation expands through out universe, redshifted

Stars form 2x108 years later

Galaxies formed by t ~ 109 years

Top-down: superclusters, then clusters, then galaxies

Bottom-up: small galaxies, then large galaxies, then clusters, then superclusters (Hubble Deep Field)

The Future of the Universe

If mean density > critical density, gravity halts expansion, collapse into Big Crunch (closed universe)

If mean density < critical density, universe expands forever (open universe)

If mean density = critical density, universe barely expands forever (flat universe)

Omega (&Omega): ratio of observed mean density to critical density

Density of luminous matter 4.4% critical density, &Omegalum =.044

But there is evidence for dark matter

The Nature of the Dark Matter

We don't know

Baryonic matter: MUST BE < 20% critical density

Elementary particles: e.g. neutrinos, predicted particles

No models match observed universe


With dark matter, omega = 0.27

Why is omega so close to 1? (Flatness problem)

Horizon problem - when CMBR released, parts of universe we see now could not interact

How can all of CMBR have same properties?

Inflationary hypothesis - early universe experienced dramatically rapid expansion

Solves horizon and flatness problems

Hyper expansion flattens curvature until indistinguishable from 1

What caused inflation?

Long-Term Future of the Universe

Recent observations: omega = 1.0

Flat universe - expands forever

1014 years - all stars dead - white dwarfs, neutron stars, black holes

1020 years - galactic black holes & dead stars

Eventually - dead stars disintegrate - electrons positrons, neutrinos, and photons

Recent results of supernova surveys suggest expansion accelerating

Expansion dominated by repulsive force overcoming gravity

73% of universe is 'dark energy'

If repulsive force is strong enough - Big Rip.........

TO BE CONTINUED...............

Prof. Donna Weistrop

University of Nevada, Las Vegas