Physics 101 - Astronomy - Spring 2019
Class notes for day 29, May 2, 2019
Cosmology
Cosmology seeks to explain how the universe began, how it evolves, and
its fate.
The Universe - is all of the matter, energy, and spacetime that will ever
be detectable from Earth or that will ever affect us.
Einstein's theory of general relativity predicts that the entire universe could
expand (or it could contract). To determine which, it was necessary to make
observations. Hubble and others measured the velocity of galaxies using analysis
of the light they emit, and found that all the galaxies are moving away from us,
so we have concluded that the Universe is expanding. Expansion of the universe would cause a type of
redshift of light as the expansion continues. This is evident when we look
at the cosmic microwave background (CMB). This was first seen by a radio
receiver designed and built by Arno Penzias and Robert Wilson of Bell
Laboratories.
The cosmic Background Explorer satellite (COBE) (a satellite designed to study
the CMB) operated from 1989-1994 and earned a Nobel prize for John Mather and
George Smoot (2006). And then the Wilkinson Microwave Anisotropy Probe
(WMAP, another satellite) mapped the details of the CMB and shows red shifts due
to Earth's motion relative to the local universe. The entire Milky Way is moving
at 600 km/s (1.3 million mi/h) toward the Centaurus cluster (relative to the
CMB).
Currently, the Planck spacecraft is studying the CMB in greater detail to find
how there are small fluctuations in the CMB which tell us something about the
early universe.
The main results of these satellite observations are: the part of the universe that we see
displays isotropy (same in all directions to 1 part in 100000) and
homogeneity (uniform over large distances over 1 billion Light Years).
The brief history of everything
The best theory that we have for the evolution of the Universe is called the
Big Bang theory of the Universe.
The origin of the universe occurred about 13.8 billion years ago, in an event
called the Big Bang.
Into what is the universe expanding? Nothing. The Big Bang created space and
time (spacetime), as well as all matter and energy in the universe. Spacetime is
expanding to accommodate the expansion of the universe.
Big Bang Theory
Astronomers believe that the universe began as an exceedingly dense cosmic
singularity (almost a point) that expanded explosively in an event called the Big Bang. The
Hubble law describes the ongoing expansion of the universe and the rate at
which superclusters of galaxies move apart. The observable universe
extends about 13.8 billion light-years in every direction from Earth to what is
called the cosmic light horizon. We cannot see any objects that may exist
beyond the cosmic light horizon because light from these objects has not had
enough time to reach us.
According to the theory of inflation, very early in its existence, the
universe expanded super rapidly for a short period, spreading matter that was
originally near our location throughout a volume of the universe so large that
we cannot yet observe much of it. The observable universe today is thus a
growing volume of space containing matter and radiation that was in close
contact with our matter and radiation during the first instant after the Big
Bang. This explains the isotropic and homogeneous appearance of the universe.
Without this "inflationary universe" model, we have trouble explaining the
isotropy and homogeneity of the visible part of the universe.
Four basic forces—gravity, electromagnetism, the strong nuclear force, and the
weak nuclear force— explain the interactions observed in the universe. According
to current theory, all four forces were identical just after the Big Bang. At
the end of the Planck time (about 10-43 s after the Big Bang),
gravity became a separate force. A short time later, the strong nuclear force
became a distinct force. A final separation created the electromagnetic force
and the weak nuclear force.
Before the Planck time, the universe was so dense that known laws of physics do
not describe the behavior of spacetime, matter, and energy.
In its first 30,000 years, the universe was radiation dominated, during which
time photons prevented matter from forming clumps. Then it was matter-dominated,
during which time superclusters and smaller clumps of matter formed. Today it is
dark-energy-dominated. Dark energy of some sort supplies a repulsive
gravitational force that causes superclusters to accelerate away from each
other. So the Universe is not just expanding, it is expanding at an increasing
rate (accelerating expansion). Astronomers think that during the first 379,000 years of the
universe, matter and energy formed an opaque plasma, called the primordial
fireball. Cosmic microwave background radiation is the greatly redshifted
remnant of the universe as it existed about 379,000 years after the Big Bang.
By 379,000 years after the Big Bang, spacetime expansion caused the temperature
of the universe to fall below 3000 K, enabling protons and electrons to combine
to form hydrogen atoms. This event is called the era of recombination. The
universe became transparent during the era of recombination, meaning that the
microwave background radiation contains the oldest photons in the universe.
Clusters of galaxies and individual galaxies formed from pieces of enormous
hydrogen and helium clouds, each of which became a separate supercluster of
galaxies.
All of the superclusters and some of the clusters of galaxies within each
supercluster are moving away from one another. During the matter-dominated
era, structure formed in the universe. As the universe goes farther into the
dark energy-dominated era, the large-scale structure of superclusters of
galaxies will fade away.
The Fate of the Universe
The average density of matter and dark energy in the universe determines the curvature of space and the ultimate fate of the universe. Observations show that the universe is "flat" (parallel beams of light would remain at the same distance) and that the cosmic microwave background is almost perfectly isotropic, resulting from a brief period of very rapid expansion (the inflationary epoch) in the very early universe. The universe is accelerating outward and it will expand forever.
The Final Exam is on Tuesday, May 7 at 3 p.m. The exam will cover the entire course, including a few questions on galaxies and cosmology.