An Introduction to Astronomy Part XIV: Cosmology - PowerPoint PPT Presentation

1 / 37
About This Presentation

An Introduction to Astronomy Part XIV: Cosmology


An Introduction to Astronomy Part XIV: Cosmology Lambert E. Murray, Ph.D. Professor of Physics – PowerPoint PPT presentation

Number of Views:457
Avg rating:3.0/5.0
Slides: 38
Provided by: Lamb192


Transcript and Presenter's Notes

Title: An Introduction to Astronomy Part XIV: Cosmology

An Introduction to AstronomyPart XIV Cosmology
  • Lambert E. Murray, Ph.D.
  • Professor of Physics

  • Cosmology is the study of the structure and
    evolution of the Universe on its grandest scale.
    In the study of cosmology we wish to answer
    question like
  • What do our observations tell us, if anything,
    about the size and geometry of the Universe?
  • What do our observations tell us, if anything,
    about how the Universe came into existence?
  • What do our observations tell us, if anything,
    about the future of the universe?

What Have we Learned about the Nature of the
Universe so far?
  • There seems to be an infinite number of galaxies
    (of various types). No matter the direction you
    look, as we build bigger and bigger telescopes,
    you see more and more galaxies.

They are Everywhere
Galactic Clusters and Superclusters
  • There are enormous distances between galaxies,
    but as we look deeper and deeper into our
    Universe we find that galaxies are grouped
    together by gravitational attraction.
  • If we plot the location of galaxies, we begin to
    see large-scale structure within the Universe.
  • We find that although there are small scale
    fluctuations within the universe we can think
    of the universe as rather homogeneous and
    isotropic on the very largest scale.

Structure Within the Universe
Computer Modelof Universe
What Else Have we Learned about the Nature of the
Universe so far?
  • Hubbles work indicates that the farther away a
    galaxy is, the faster it moves away from us.
    This is true in every direction we look.

The Hubble Law
Are we in a Special Place?The Cosmological
  • Since we see a large number of galaxies in all
    directions, and these are all moving away from us
    are we at the center of the universe (a very
    special place)?
  • A fundamental assumption in the study of
    cosmology is that we are not located in a unique
    region within the universe this is the
    Copernican Principle.
  • In addition, we assume that the universe at the
    largest scale is isotropic and homogeneous, i.e.,
    that the universe looks the same in all
    directions and that this is true no matter where
    you are. This is the cosmological principle.
  • This principle assumes that our location within
    the universe is characteristic of any other
    location within the universe and is the only
    assumption that will allow us to make any
    progress in understanding our Universe as a whole.

An Expanding Universe
  • If we are not located at a special spot in the
    Universe, and
  • If we would expect to obtain the same Hubble plot
    of the recessional speeds of galaxies from any
    arbitrary location within the Universe,
  • This means that the distance between any two
    galaxies (on average) is increasing all over the
  • Thus, the Universe is expanding like a raisin
    cake or an expanding balloon.

Is the Universe Infinite?Olbers Paradox
  • Olbers paradox If our universe is infinite in
    time and space, and if we see galaxies wherever
    we look, why is the sky dark?
  • Although the light intensity drops off as 1/R2,
    the number of stars you can see in an infinite
    universe increases like R2 so these two effects
  • Likewise, if the universe is filled with dark
    material which would absorb the light, that
    material would heat up and eventually glow
    especially if there were an infinite number of
  • Conclusion The universe must not be infinite in
    space and time!

The Big Bang
  • Initially many astronomers believed that the
    Universe was static and infinite. Olbers
    paradox raised serious questions about this
  • Hubbles observations of an expanding Universe
    seems to indicate that the Universe had a
    beginning in time and perhaps in space.

The Hubble Age of The Universe
  • Hubbles Law can be written as velocity of
    recession (constant) x distance
  • the constant is Hubbles Constant, H0
  • We believe that Hubbles constant is constant
    over all the universe at any instant of time, but
    it may actually change over time.
  • Running the expansion backwards
  • Since H0 velocity/distance,
  • 1/H0 distance/velocity time
  • Thus, 1/H0 is a measure of the age of the
  • Using a value for H0 of 50 km/sec/Mpc (kilometers
    per second per megaparsec) leads to an age of
    about 19.7 x 1010 years, or roughly 20 billion
    years, while using a value of 100 km/sec/Mpc
    leads to an age of only about 10 billion years.

A Correction to the Hubble Age of the Universe
  • Most astronomers believe that Hubbles constant
    has changed with time. Gravity should be
    slowing the expansion of the Universe, causing
    a deceleration.
  • Thus, the initial expansion should have been
    faster, leading to a younger universe one with
    an age of about 2/3 of the age determined from
    the present value of Hubbles constant.
  • Astronomers are using several different
    techniques to determine an accurate value of
    Hubbles constant so that we can get a handle of
    the actual age of the Universe.

Measurements of Hubbles Constant
  • Hubbles initial measurements of H0 was 550
    km/sec/Mpc, but by the 1990s the most frequently
    quoted values were 50 70 km/sec/Mpc.
  • Hubbles measurements did not take into account
    several effects now known to astronomers that
    would have influenced his measurements.
  • 2/3 of the Hubble age calculated from these
    values would give 9 13 billion years for the
    approximate age of the Universe.

An Alternate Age Measurement
  • An alternate method of determining the age of the
    Universe is to determine the age of objects
    within the Universe, since the Universe cannot be
    younger than the oldest objects in the Universe.
  • We can get an estimate of the age of globular
    clusters based upon their HR diagrams observing
    their turning points.

HR Diagram for M13
An Age Crisis
  • Before the mid 1990s, the oldest globular
    clusters were thought to be 14 17 billion years
    old, so the Universe must be older than this.
  • If the age of the Universe is only 9 13 billion
    years according to Hubbles expansion constant,
    we clearly have an age crisis.

Measuring the Change in the Expansion Rate of the
  • Since the Hubble constant is probably not
    constant in time, a measure of the Hubble
    constant at any instant of time gives an
    incomplete picture of the situation.
  • The answer to this problem would be to measure
    the Hubble constant for different times in the
    life of the Universe but how can be do that?
  • By looking at the redshifts of galaxies at
    different distances (times) from us, we might be
    able to determine how the Hubble constant changes
    in time.

Measuring the Change in the Expansion Rate of the
Universe II
  • Measuring the redshift of nearby galaxies is
    problematic because gravitational attraction
    between the nearby galaxies induces peculiar
    motions of these galaxies.
  • These problems should be less pronounced as we
    move further away (back in time), but the use of
    Cepheid variables as standard candles is limited
    to the nearer galaxies, where they can be
  • In the 1990s it was discovered that Type Ia
    supernovae all have nearly the same peak
    luminosity, and these can be seen at distances
    thousands of times more distant than Cepheids.
  • Measurement of these supernovae give the present
    preferred value of 65 km/sec/Mpc.

Resolving the Age Crisis
  • After corrections were made to the Hubble Space
    Telescope, measurements of H0 based primarily on
    Cepheid variable stars in 18 galaxies (1999) gave
    a value of 70 to 80 km/sec/Mpc with an
    uncertainty of 7 km/sec/Mpc.
  • Type Ia supernovae measurements indicate a value
    for H0 of about 65 km/sec/Mpc, giving an age of
    the Universe of 15 billion years if there is no
    deceleration, or about 12 billion years with
  • Better measurements of the age of globular
    clusters now gives an age estimate of 11 14
    billion years for the oldest globular clusters.

Additional Evidence for the Big Bang?
  • Cosmic Microwave Background
  • In the 1940s Russian physicist George Gamov
    worked out a theory for the creation of elements
    (a nucleosynthesis) in a big-bang type event.
  • He predicted that copious amounts of radiation
    would be emitted from the hot gas of this Big
    Bang with a black-body spectrum of about 3000K
    when the Universe became transparent to photons.
    (Prior to this the photons were trapped by the
    non-transparent universe.)
  • Later it was realized that this radiation would
    have been red-shifted over time and would look
    like emissions from a very cool gas.
  • In the mid-60s Robert Dicke at Princeton began
    building a receiver to detect this radiation.

Penzias Wilsons Microwave Experiment
  • 1964-65 Bell Labs scientists Arno Penzias
    Robert Wilson were working on a microwave horn
    antenna for satellite communication.
  • They were troubled by a persistent background
    noise. After trying everything to eliminate the
  • recalibrated antenna
  • cooled their detectors
  • removed nesting pigeons from horn
  • cleaned antenna
  • could not eliminate the static
  • they found that the background persisted.

(No Transcript)
A Cosmic Background?
  • The peak in the background radiation from
    Penzias and Wilsons experiment occurred at 7.35
    cm (4080 MHz)
  • They found that the background was constant
    regardless of time of day, season of year,
    direction in the sky, etc.
  • When they learned of Dickes work, they realized
    that they might be seeing this residual radiation
    remnant from the Big-Bang now called the Cosmic
    Microwave Background Radiation.
  • If a blackbody curve is assumed, the 7.35 cm peak
    corresponds to a background temperature of
    roughly 3.5 Kelvin.
  • 1976 Penzias Wilson received the Nobel Prize
    for their work.

Experimental Measurements of the Cosmic
Background Radiation
  • Very little of the 3 K background radiation can
    penetrate the atmosphere
  • This made it difficult to measure radiation curve
  • Early attempts used balloons, rockets, aircraft,
  • In 1989 NASA launched the Cosmic Background
    Explorer (COBE) which orbited the Earth.
  • Within two months it was determined that this
    background followed (within 1) a blackbody
    radiation curve corresponding to a temperature of
    2.735 Kelvin (within 0.06K).

Blackbody Spectrum of COBE Satellite and fit to a
2.73 K Curve
Variations in the Background Radiation Pattern
  • COBE data is accurate enough to measure the
    motion of the earth relative to the background
    radiation by analyzing variations in the data to
    1 part in 10,000. This is shown in the next
  • A more detailed analysis, with the dipole effect
    of the Earths motion subtracted out, shows small
    scale variations in the background radiation data.

COBE Dipole Speeding Through the Universe
Credit NASA, COBE, DMR, Four-Year Sky Map
Astronomy Picture of the Day, February 5, 1996
(No Transcript)
A map of the brightness of the cosmic microwave
background made by the cosmic background explorer
(COBE) satellite. Notice the patchiness of the
brightness. Each pink patch may represent a
"lump" of matter from which groups of galaxies
ultimately grew. The patches were approximately
one half billion light years across when they
emitted the radiation. (NASA GSFC and the COBE
Science Working Group.)
Confidence in the Big Bang
  • There seems little doubt that the Universe as we
    know it is expanding, and that it was the result
    of a Big Bang.
  • Small fluctuations in the cosmic background
    radiation are consistent with our observations of
    the grouping of galaxies within the Universe.

(No Transcript)
Recent Confirmation
  • The excellent agreement between the current
    theories of cosmology and the most recent data
    from Cosmic Background Radiation (WMAP) indicate
  • The age of the Universe is 13.7 billion years to
    within 1.
  • That the Universe is flat and that the visible
    universe makes up only 4 of the Universe.

WMAP Composition of the Universe
Theoretical Models for the Expansion of the
  • Until recently, precise, unambiguous,
    experimental measurements of H0 with distance did
    not exist.
  • Theory indicates that there are three basic
    models for our Universe
  • Closed Universe (WM gt 1)
  • Flat Universe (WM 1)
  • Open Universe (WM lt 1)

Diagram of Theoretical Models for the Expansion
of the Universe
The Future in Different Models
  • In a closed universe, the recessional speed of
    galaxies would decrease to zero and that universe
    would begin to collapse as gravity overcame the
    initial expansion from the Big Bang. This
    universe would eventually collapse completely
    or perhaps oscillate.
  • In a flat universe, the recessional speed
    continues to decrease and approach zero only as
    time approaches infinity.
  • In an open universe, the recessional speed
    decreases, but never reaches zero.

A Surprise
  • Recent measurements of large-z Type Ia Supernovae
    indicate that the recession rate follows none of
    the previous patterns, but actually are
    consistent with an accelerating universe which
    would require some sort of anti-gravity
    repulsive force which is now associated with
    Einsteins cosmological constant L.
  • Many suggest that the source of this
    anti-gravity is a vacuum energy or dark
    energy due to quantum fluctuation.

End of Part XIV
Write a Comment
User Comments (0)