chapter 29.1 the age of the universe 29.2 model of the universe & 29.3 beginning of the universe

expansion of the universe

• is accelerating

• keeps galaxies from colliding

• dark energy: the source of acceleration of the expansion of the universe

• must be homogenous and isotropic as the universe is, thus the expansion rate must be the same everywhere during any epoch of cosmic time

• began everywhere at once

• cosmic expansion causes the universe to undergo a uniform change in scale over time

beginning of the universe

• when all the matter we find today was once concentrated in an infinitesimally small volume

Big Bang

• the explosion of the infinitesimally small volume of all matter at the beginning of time

age of universe

13.8 billion years old, with an uncertainty of about 100 million years

critical density

• the mass per unit volume that will be just enough to slow the expansion to 0 at sometime infinitely far into the future

  • if the actual density of the universe is higher than this critical density, then the expansion will ultimately reverse and the universe will begin to contract

  • if the actual density is lower, then the universe will expand forever

closed universe model

• actual density is higher than the critical density and there is no dark energy

• universe will stop expanding at some time in the future and begin contracting

• universe in imploding on itself

open universe

• density of the universe is less than the critical density

• universe expands forever

• universe is thus infinite, time and space have no end

empty universe and coasting universe

• neither gravity nor dark energy is important enough to affect the expansion rate, which is therefore constant throughout time

average density of matter in space estimate

• 1-2 × 10^-27 kg/m³ (10-20% of critical threshold)

first few minutes of the big bang

• In the first fraction of a second, the universe was unimaginably hot.

• By the time 0.01 second had elapsed, the temperature had dropped to 100 billion (10¹¹) K.

• After about 3 minutes, it had fallen to about 1 billion (10⁹) K, still some 70 times hotter than the interior of the Sun.

• After a few hundred thousand years, the temperature was down to a mere 3000 K, and the universe has continued to cool since that time.

• so hot that it contained mostly radiation and not the matter we see today

  • the photons that filled the universe could collide and produce material particles; energy could turn into matter and vice versa

• the hotter the universe was, the more energetic were the photons available to make matter and antimatter

  • To take a specific example, at a temperature of 6 billion (6 × 10⁹) K, the collision of two typical photons can create an electron and its antimatter counterpart, a positron. If the temperature exceeds 10¹⁴ K, much more massive protons and antiprotons can be created.

evolution of early universe

• by the time universe was 0.01 second old, it consisted of a soup of matter and radiation

  • matter included: protons, neutrons

  • each particle rapidly collided with others

• temperature no longer high enough to allow colliding photos to produce neutrons or protons, but it was sufficient for the production of electrons and positrons

• subatomic particles that would play a role as dark matter

• all particles jiggled about on their own; universe was too hot for protons and neutrons to combine to form nuclei of atoms

formation of atomic nuclei in the early universe

• 3 minutes old, temperature about 900 million K

• protons and neutrons could combine

• deuterium lasted long enough that collisions could convert some of it into helium

  • estimated that 10 times more helium formed in the first 4 minutes of the universe than in all the generations of stars during the succeeding 10-15 billion years

• little bits of lithium could also form

• by 4 minutes, helium was having trouble forming, because the temperature dropped