THE UNIVERSE AND COSMOLOGY
• Cosmology is defined as the study of the entire Universe, including
its origins and evolution with time.
• Cosmology has been the most difficult field of Astronomy to study
observationally, because of the vast distances and correspondingly
faint and/or small apparent sizes of the individual objects of study.
• Cosmology involves not only the study of individual galaxies and
other objects, as a function of their apparent distances, but also the
characteristics of diffuse background radiation, over the entire ranges
of distances and of the electromagnetic spectrum.
• Because of the expansion of the Universe, distant objects appear to
be moving away from us, as first realized by Edwin Hubble, at speeds
(to first order) proportional to their apparent distances (as based on
other observational evidence).
• This motion results in “red-shifts” of the apparent wavelengths of
radiation received from these objects, indicating speeds comparable
to the speed of light (and other forms of electromagnetic radiation).
• This recessional red-shift, therefore, provides a means for
determining the distances to these very distant objects.
THE UNIVERSE AND COSMOLOGY
• The age of the Universe, until very recently, was not well
established, but is now believed to be about 13.7 billion years, or
about 3 times the age of our Solar System.
• The Universe is thought to have originated in a point-like
concentration, which began expanding outward in a “big bang” at
the speed of light.
• Over very large distances, the expansion velocity and other
properties of the Universe are such that the theory of general
relativity must be used for quantitative descriptions.
• The Hubble Constant H0 = v/R, indicated that the apparent
recession velocity of an object, v, is proportional to its distance, R,
to the distances accessible until recently with ground-based
telescopes.
• However, with the advent of more powerful space- and groundbased telescopes and instrumentation, there are now indications
that H0 is not constant, but changes with apparent distance
(redshift).
• Accurate determinations of H0 are needed in order to determine
the age and evolution of the Universe.
THE UNIVERSE AND COSMOLOGY
• Establishment of values for H0 requires measuring the recession
velocity of objects for which other means (other than redshift) of
determining the distance, R, are available.
• One such means is to observe Cepheid variable stars, whose
periods of variation are a known function of absolute luminosity.
• Another method, applicable to greater distances, is observations
of Type I supernovas (exploding white dwarfs) which are much
brighter than Cepheid variables (but unpredictable in advance).
• The Hubble Space Telescope has provided the capability to
observe Cepheid variables and other distance indicators to
much greater distances than has been possible previously.
• The self-gravitation of the Universe acts to slow down the
expansion with time since the Big Bang (characterized by the
deceleration parameter q0); hence it is presumed that the most
distant objects would appear to be receding at a faster rate than
in direct proportion to apparent distance.
• However, recent observations with HST and other instruments
has indicated that the expansion of the Universe is increasing
with time!
Globular
Cluster
in M31
Star in Our
Galaxy
This very deep-exposure HST image of the halo of the (relatively) nearby Andromeda Galaxy (M31) also
reveals, in the background, a large number of very distant galaxies.
Great Observatories Origins Deep Survey (GOODS)
THE UNIVERSE AND COSMOLOGY
• Determinations of distance require not only measurements of
redshift, but also indicators of the absolute brightnesses of
distant galaxies (for comparison with observed brightnesses).
• The rate of expansion, and its variation with time, are dependent
on the total mass of the Universe.
• If the total mass is greater than a certain critical mass, the
expansion of the Universe eventually ceases, and then
reverses.
• If the total mass is less than the critical mass, the expansion of
the Universe continues indefinitely, and may proceed at an
increasing rate with time.
• The most recent measurements, using the Hubble Space
Telescope and large ground-based telescopes, appear to
indicate that the expansion of the Universe is accelerating with
time.
• This also appears to indicate the presence of “dark energy” and
“dark matter”, which cannot be detected directly with currently
available instrumentation.
Previous Models of the Expanding Universe
(Most Recent Results indicate negative value of q0!)
RELATIVITY AND COSMOLOGY
• The distances and velocities involved in the study of cosmology
are so great that the laws and relationships involved in
classical (Newtonian) physics must be replaced with the
relationships described by Einstein’s Theory of Relativity, often
broken down into the Special and General theories of relativity.
• An important feature of the theory of relativity is that no
material object or information can travel faster than the speed
of light, which is about 300,000 km/sec (or 186,000 miles/sec).
• Special relativity deals with unaccelerated motions (moving in a
straight line at constant velocity) whereas general relativity
deals with accelerated motions.
• The more comprehensive General Theory of Relativity also
includes a theory of gravitation.
• Relativity requires four dimensions (three space dimensions,
and time) to display (called “spacetime”), and so is somewhat
difficult to conceptualize pictorially.
RELATIVITY AND COSMOLOGY
• The history of the theory of relativity dates back to the late 19th
and early 20th centuries, when experiments by Michelson and
Morley were unable to determine a reference position of rest
from which the speed of light could be determined.
• If light waves were analogous to sound waves, their velocity
should be higher when approaching the light source, and lower
when receding from the light source.
• They found that the speed of light they measured was the
same regardless of the direction or motion of the measuring
apparatus (including Earth’s rotation on its axis and revolution
around the Sun).
• Einstein’s theory of relativity states that the speed of light is
the same for all observers, regardless of their relative motions;
however, the observed wavelength of light changes with
motion toward or away from the light source.
RELATIVITY AND COSMOLOGY
• Einstein’s theory also postulated that:
– No material object can travel at or faster than the speed of light
– The energy equivalent of matter is given by the equation E = mc2
– The classical expression for kinetic energy of a material object,
KE = 1/2mv2, is replaced, at speeds comparable to (but still less
than) the speed of light, by a relationship that increases more
rapidly than the square of velocity, and becomes infinite at v=c.
• The energy equivalence of matter is the basis of the nuclear
reactions that produce energy by fission or fusion of atomic
nuclei, which power the Sun and stars, as well as nuclear
reactors (and weapons) utilized on Earth.
• An important factor in this theory is that there is no preferred
point of reference in the Universe (it does not have a “center”
or an “edge”).
• A feature of the general theory, proven by astronomical
observations, is that massive objects (such as our Sun) can
bend the paths of light rays coming from more distant stars
behind them.
SPEED LIMIT – STRICTLY ENFORCED
Captain Kirk, pull over!
Effects of Special Relativity on Views of Fixed and
Moving Observers
Stationary observer at position A sends
flash of light to observer and mirror at
position B, moving at velocity v. Time to
send and receive is 2a/c.
Moving observer at B sees A moving to the
left between A’s sending and receiving the
(reflected) light, and so finds a longer path
length of travel. If the speed of light is c,
the time also has to be longer.
RELATIVITY AND COSMOLOGY
• An important aspect of Einstein’s special theory of relativity, is that
the apparent speed of light is the same, to an observer, regardless of
the differential velocity of the light source toward or away from the
observer.
• There is also no standard reference for determining whether the
observer or the light source is the moving object.
• Although this may appear to be contradictory, if the same light source
is observed simultaneously by another observer at rest relative to the
light source, it is actually compensated by apparent time dilation (as
viewed by the moving observer), and/or apparent contraction of the
length of a meter stick carried by the moving observer, as observed
by the stationary observer (“Lorentz-Fitzgerald contraction”):
and
RELATIVITY AND COSMOLOGY
• According to the equivalence of matter and energy, specified by
Einstein’s equation, E = mc2, the mass term in the classical
relationship for kinetic energy, KE = ½mv2 is replaced by the
relationship
which approaches infinity as v approaches c.
• Likewise, the relativistic Doppler effect varies not directly with line-ofsight velocity,  =0 (1+ v/c) (applicable for v << c), but according to

0
v2
1 2
c
Note, the apparent speed of light is the same, regardless of velocity
of the observer toward or away from the light source, but the
apparent wavelength of the light shifts to shorter or longer
wavelengths per the above equation.
OBSERVED EFFECTS OF SPECIAL RELATIVITY
• One of the well-observed phenomena that provides direct support of
relativity theory, is the lifetime of muons (a type of sub-atomic
particle) that are produced when cosmic rays (mostly very high
energy protons) collide with the nuclei of Earth’s upper atmospheric
atoms.
• The half-lifetimes of these muons, as measured in the laboratory,
are extremely short (about 1.5 x 10-6 second), so even if they were
traveling close to the speed of light they would travel only 450
meters in this half-lifetime.
• However, the muons that are observed travel distances much larger
than this, about 1800 meters, in this lifetime.
• This is a manifestation of “time dilation”- time runs slower for the
high-velocity muon by a factor of 4 or more, vs. the time recorded by
observers on (or at fixed locations above) Earth.
• Another (as yet untested!) manifestation is that of the “traveling
twin”. If one twin takes a flight at nearly the speed of light to another
solar system, and returns (after the visit) at the same speed, he or
she will have experienced (by their clock) a shorter elapsed time,
and will appear physically younger, than the stay-at-home twin.
RELATIVITY AND COSMOLOGY
•
•
•
•
•
According to the principle of relativity, there is no preferred location in the
Universe, and hence no absolute reference point for a coordinate system
(such as the Sun provides for our solar system, or the center of our Galaxy
provides for the stars within it).
The speed of light, as witnessed by an observer, is the same regardless of
position in space, or velocity relative to the observed object.
The concept of spacetime is a 4-dimensional coordinate system, consisting
of three spatial dimensions and time.
An observer can only detect “events” that occur within his or her “light cone”,
whose boundaries correspond to velocities equal to the speed of light.
Time can change only in one direction (forward, in the light cone diagram).
Distance (3 dimensions)
Velocity = Speed of Light
Time
Observer
THE COSMOLOGICAL PRINCIPLE
• The cosmological principle states that the Universe is
isotropic (homogeneous in its structure, and distribution of
galaxies), on a scale greater than 200 Mpc.
• From this principle, we infer that there is not (to our current
knowledge) a “center” or an “edge” of the Universe.
• Therefore, no matter where we are located in the Universe, it will
appear that we are at the center of the expansion, with generally
uniform distributions (in direction and distance) of galaxies as
seen from our location (there is no “preferred” location in the
Universe).
• The large-scale structure of the Universe, as determined from
surveys of galaxies over wide ranges of distance (out to about
750 Mpc) and in all directions in the sky, appear to show
randomly distributed structures in their distribution, in the forms
of “voids”, “walls”, and “bubbles”.
• However, there appear to be no large-scale structures (greater
than about 200 Mpc) in the distribution of galaxies.
THE COSMOLOGICAL PRINCIPLE
• A two-dimensional analog to the three-dimensional expansion of the
Universe is given by placing adhesive paper dots on the surface of
a partially-inflated balloon.
• As the balloon is further inflated, the dots will appear (to an
observer, such as an ant, on any one of the dots) to move outward,
in all directions (in the 2-dimensional space of the balloon’s
surface), at a velocity proportional to distance.
• Therefore, no matter where on the balloon the observing ant is
located, it will have the impression that it is at the center of the
expansion.
• Likewise, no matter where we are located in the Universe, we will
see uniform expansion away from our apparently central location; in
the (apparently) three-dimensional Universe, time serves as a
fourth dimension for the general expansion.
• As mentioned previously, detailed analysis of this process requires
use of Einstein’s general theory of relativity, which treats time as a
fourth dimension.
THE “BIG BANG” AND THE ORIGIN OF ELEMENTS
• The current hypothesis concerning the origin of the Universe is
that it started out from a point (or very compact) object,
composed of pure energy, which exploded outward in a “big
bang” at the speed of light, and has continued to expand outward
at this speed ever since.
• Currently, the Universe consists of matter, as well as energy in
the form of heat, kinetic energy, and electromagnetic radiation,
and (hypothetically) “dark energy”.
• Energy and matter can be interchanged, according to Einstein’s
equation E = mc2, where E is energy, m is mass, and c is the
speed of light (300,000 km/sec).
• Currently, in our Universe, the matter which it contains far
exceeds (in energy equivalence) the electromagnetic radiation
(in the form of starlight and the cosmic microwave background
radiation) and so is considered to be matter-dominated.
THE “BIG BANG” AND THE ORIGIN OF ELEMENTS
• Stars (including our Sun) generate most of their energy by
converting matter into energy, by means of thermonuclear
reactions (such as fusion of hydrogen to make helium and other,
heavier elements).
• In the early Universe, the opposite process was at work, in
which energy was converted into matter (elementary particles
such as electrons, protons, and neutrons) which in turn were
converted into the light elements, hydrogen and helium (and a
small amount of lithium).
• An example of this process is the interaction of two high-energy
gamma-ray photons to produce an electron and a positron (the
process of “pair production”), the inverse of the reaction which
we observe in the laboratory at present.
• To produce electron-positron pairs requires photons of about
1010 K; however, to produce proton-antiproton pairs requires
energy equivalent temperatures above 1013 K!
THE “BIG BANG” AND THE ORIGIN OF ELEMENTS
• For reasons as yet unknown, there was a dominance of protons
and electrons over antiprotons and positrons in the early
Universe; the former (along with neutrons) constitute the entire
mass of the objects in the current Universe.
• No significant amount of new matter is thought to have been
created since the first minute of the “Big Bang”!
• It is thought that the Universe was “radiation dominated” for the
first few thousand years of its existence, following which it was
“matter dominated”.
• The first atoms are thought to have been created (by
combination of electrons with protons and neutrons) about 106
years after the “big bang”.
• The creation of neutral atoms (by combination of free electrons
with positive nuclei) also resulted in de-coupling of the
electromagnetic radiation from matter.
• This made the Universe much more transparent to the
electromagnetic radiation, which previously was strongly
scattered by the free electrons.
THE EARLY EVOLUTION OF THE UNIVERSE
• The first element to be produced (by combination of electrons with
protons) was hydrogen, followed by deuterium (by combination of
neutrons with protons, and addition of an electron).
• The next element, helium, was produced by fusion of deuterium
with hydrogen (1H + 2H  3He, and 3He + neutron  4He).
• A small percentage (about 2 x 10-5 atoms per proton in the
Universe) of lithium was also created at this time.
• This primordial process was essentially the entire source of the
lithium in the current Universe!
• The observed abundance of lithium in the present-day Universe
can be used to estimate the current total density of the Universe
(based on extrapolation, backwards in time, to the period when the
lithium was being created).
• This calculation indicates that the current density of the Universe
is only a few percent of the critical density (above which the
Universe will eventually reach a maximum size and then recollapse).
THE EARLY EVOLUTION OF THE UNIVERSE
• However, studies of the motions of galaxies in clusters and
superclusters indicate that the total mass (based on gravitational
interactions) of the Universe is about 1/3 of the critical density.
• This, in turn, implies the existence of “dark matter” which has
gravity but is invisible (also implied from the rotation velocity vs.
central distance curves of individual galaxies).
• This also implies that this “dark matter” is not made up of
protons and neutrons (and so cannot be in the form of brown or
white dwarfs, neutron stars, or black holes).
• Although the energy equivalence of matter in the current
Universe far exceeds that of the cosmic background radiation,
the latter still exceeds the energy content of starlight due to all of
the currently existing stars and galaxies.
• The recent determination that the expansion of the Universe is
accelerating, not decelerating as previously hypothesized, also
indicates the presence of a currently unknown “dark energy”.
THE UNIVERSE AND COSMOLOGY
• The Hubble Space Telescope (HST) and new large ground-based
telescopes have allowed imaging and redshift measurements of
galaxies at much greater distances from Earth than previously
possible.
• Observations of very distant galaxies are required to determine the
time /distance variation of the expansion rate, and whether the total
mass of the Universe is less than, equal to, or greater than the critical
mass.
• However, recent observations appear to indicate a less than perfectly
linear increase in velocity with distance, which would indicate that the
rate of expansion has increased with time since the “big bang”.
• The planned Next Generation Space Telescope (NGST), recently renamed the James Webb Space Telescope (JWST), will extend our
observations of galaxies to much greater distances than currently
possible, by the use of a much larger aperture than HST, and by
observing farther into the infrared (corresponding to larger redshifts).
THE UNIVERSE AND COSMOLOGY
• The intense radiation field which accompanied the Big Bang is
still evident today, as the cosmic background radiation.
• This background radiation was first detected in ground-based
microwave measurements by A. Penzias and R. Wilson in 1965.
• The Cosmic Background Explorer satellite (COBE), launched in
1989, had as a major objective the detailed measurement of this
microwave background radiation.
• This radiation, initially characteristic of multi-million degree
temperatures, has been red-shifted by the expansion of the
Universe so that it now corresponds to black-body radiation
characteristic of a temperature of 2.73 K.
• Small variations in the intensity of this background radiation over
the sky may indicate the initial fragmentation of the material into
clumps capable of condensing to form galaxies.
• The recently launched Wilkinson Microwave Anisotropy Probe
(WMAP) mission is now obtaining much more detailed mapping
of the cosmic background radiation over large regions of the sky.
All-Sky Map of the Cosmic Microwave Background Radiation
Obtained by the Cosmic Background Explorer (COBE)
The color variations are due to our solar system’s (and Galaxy’s)
proper motion relative to the Universe and the background radiation
(which otherwise appears nearly uniform in distribution).
All-Sky Map of the Cosmic Microwave Background Radiation
Obtained by COBE, after Local Background Variation
Subtraction and Contrast Enhancement
Wilkinson Microwave Anisotropy Probe (WMAP)
Wilkinson Microwave Anisotropy Probe (WMAP)
WILKINSON MICROWAVE ANISOTROPY PROBE
(WMAP)
• The WMAP mission, successor to the COBE mission, is the
most recent to observe the cosmic background radiation and the
overall structure of the Universe.
• WMAP is providing much higher resolution imagery and other
information about the cosmic background microwave radiation,
as revealed by very slight, small-scale temperature fluctuations.
• These and other, related data are both verifying and
strengthening theories of the origin and early evolution of the
Universe, such as the Big Bang and Inflation theories.
• The WMAP map of the sky in the 2.73 K background radiation
corresponds to a view of the Universe when it was only about
380,000 years old!
• These (and other) measurements have also refined and more
accurately determined our previous estimates of the age of the
Universe, now indicated to be 13.7 billion years (with 1%
accuracy!).
ALL-SKY MAP OBTAINED BY THE WILKINSON
MICROWAVE ANISOTROPY PROBE (WMAP)
COSMIC EVOLUTION
FUTURE RESEARCH OBJECTIVES IN COSMOLOGY
• The recent WMAP (and other) space missions, in combination with
ground-based observations and theoretical research, have greatly
advanced our knowledge of our Universe and its history, in recent
years.
• However, they have also revealed new and unexpected mysteries,
which will provide much to investigate in the foreseeable future.
• Among the most important of these, are the realization that
previously unknown “dark matter” and “dark energy” are major
components of our Universe, for which the laws of physics, as we
know them, have no immediate explanation!
• The current observations indicate that matter, in the forms we know
(protons, neutrons, and electrons), constitute only a few percent of
the total!
• In addition, we still do not have direct information about the events
and time scales of the origin of the Universe (in the time period 0 to
380,000 years) - much less, about what happened prior to that!
THE MORE WE DISCOVER, THE MORE WE FIND WE DON’T KNOW!
• Perhaps the most important result of our most recent observations
of the distant Universe, is that many of the aspects of cosmology we
set out to prove, have been proven false!
• In particular, observations of the redshifts of very distant galaxies
have shown that the rate of expansion of the Universe is
increasing with time, instead of decreasing with time, as previously
assumed.
• This can be explained only by the presence of previously unknown
dark energy which uniformly permeates the entire Universe.
• In addition, and even prior to this, a previously unknown cold dark
matter was required to explain the rates of revolution of stars
around the center of our Galaxy, and of external galaxies.
• These previously unknown contributions to the total mass and
energy of the Universe constitute more than 95% of the total!
• Clearly, there is still a great deal that needs to be done, by both
observational and theoretical research, to complete our
understanding of our Universe, its origins, and its future.
Inventory of Matter and Energy in the Universe
THE JAMES WEBB SPACE TELESCOPE
• The James Webb Space Telescope (JWST, previously known as
the Next Generation Space Telescope, NGST) is planned as the
follow-on to the Hubble Space Telescope, particularly in the
research areas of distant galaxies and cosmology.
• The JWST is designed to study the earliest galaxies and some
of the first stars formed after the Big Bang.
• The JWST will have a larger collecting mirror than the HST, and
will be optimized for studies in the infrared wavelength range.
• It will also be placed in a very distant orbit from Earth (as was
the Spitzer space telescope) to avoid heating and infrared
radiation interference by Earth and its atmosphere.
• Because of the very large mirror size (6.5 meters in diameter)
and sunshield, the JWST is launched in a “folded up”
configuration, and deployed only after launch into space.
• The JWST is currently planned for launch in August, 2011.
THE JAMES WEBB SPACE TELESCOPE
• JWST science objectives involve finding answers to the following
questions:
–
–
–
–
–
What is the shape of the Universe?
How do galaxies evolve?
How do stars and planetary systems form and interact?
How did the Universe build up its present elemental/chemical composition?
What is dark matter?
Artist’s Concept of JWST in Solar Orbit