Universe by liwenting

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									             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 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
• However, with the advent of more powerful space- and ground-
  based telescopes and instrumentation, there are now indications
  that H0 is not constant, but changes with apparent distance
• Accurate determinations of H0 are needed in order to determine
  the age and evolution of the Universe.
• 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!
 in M31

                                                                                           Star in Our

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)
• 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
• 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
• 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!)
• 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.

• 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.
• 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.

   Captain Kirk, pull over!
      Effects of Special Relativity on Views of Fixed and
                      Moving Observers

                                            Moving observer at B sees A moving to the
Stationary observer at position A sends
                                            left between A’s sending and receiving the
 flash of light to observer and mirror at
                                            (reflected) light, and so finds a longer path
position B, moving at velocity v. Time to
                                              length of travel. If the speed of light is c,
        send and receive is 2a/c.
                                                   the time also has to be longer.
• 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
• 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”):

• 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

  which approaches infinity as v approaches c.
• Likewise, the relativistic Doppler effect varies not directly with line-of-
  sight velocity,  =0 (1+ v/c) (applicable for v << c), but according to
                                    1 2
    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.
• 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
• 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.
•   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


• 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
• 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.
• 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
• 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 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.
• 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!
• 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 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 re-

• 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 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
• 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
• 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 re-
  named 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 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)
• 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%
• 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
• 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!
• 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
• 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 (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.
• JWST science objectives involve finding answers to the following
   –   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

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