Download or view

Teachers Notes Booklet 4: Cosmology
Page 1 of 14
The European Space Agency
The European Space Agency (ESA) was formed on 31 May 1975. It currently has 17
Member States: Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland,
Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland &
United Kingdom.
The ESA Science Programme currently contains the following active missions:
Venus Express – an exploration of our
Cluster – a four spacecraft mission to
sister planet.
investigate
Rosetta – first mission to fly alongside
Sun and the Earth's magnetosphere
and land on a comet
XMM-Newton – an X-ray telescope
Double Star – joint mission with the
helping to solve cosmic mysteries
Chinese to study the effect of the Sun
Cassini-Huygens – a joint ESA/NASA
on the Earth’s environment
mission to investigate Saturn and its
SMART-1 – Europe’s first mission to
moon Titan, with ESA's Huygens probe
the Moon, which will test solar-electric
SOHO
propulsion in flight, a key technology for
atmosphere and interior
future deep-space missions
Hubble Space Telescope – world's
Mars Express - Europe's first mission
most important and successful orbital
to Mars consisting of an orbital platform
observatory
searching for water and life on the
Ulysses
planet
investigate the polar regions around the
INTEGRAL – first space observatory to
Sun
-
interactions
new
–
views
the
first
between
of
the
the
Sun's
spacecraft
to
simultaneously observe celestial objects
in gamma rays, X-rays and visible light
Details on all these missions and others can be found at - http://sci.esa.int.
Prepared by
Anne Brumfitt
Content Advisor
Chris Lawton
Science Editor, Content Advisor, Web Integration & Booklet Design
Karen O'Flaherty
Science Editor & Content Advisor
Jo Turner
Content Writer
© 2005 European Space Agency
Teachers Notes Booklet 4: Cosmology
Page 2 of 14
Booklet 4 – Cosmology
Contents
4.1
Olbers' Paradox ............................................................. 4
4.2
RedShift ....................................................................... 6
4.3
Big Bang....................................................................... 8
4.4
Cosmic Microwave Background ........................................ 9
4.5
Open, Flat, Closed? ...................................................... 10
4.6
Critical Density ............................................................ 11
4.7
Dark Matter ................................................................ 12
4.8
Other Materials............................................................ 13
Figures
4.1
Recreation of Hubble's Original Data ................................. 6
4.2
Fate of the Universe ..................................................... 10
Teachers Notes Booklet 4: Cosmology
Page 3 of 14
4.1 Olbers' Paradox
Isaac Newton believed gravity demands that the Universe be without a centre or an
edge, and of infinite extent in all directions. According to Newton, a finite and bound
Universe would 'fall down into the middle of the whole space, and there compose one
great spherical mass'. In an infinite Universe, he believed, 'the fixed stars, being equally
spread out in all points of the heavens, cancel out their mutual pulls by opposite
attractions.'
Olbers' Paradox
The Universe as Newton saw it gave rise to a paradox, known as Olbers' paradox after
Heinrich Olbers, who raised the issue in 1823. He deduced that in an infinite Universe of
infinite age, there would be an infinite number of stars. If you were, therefore, to look
in any direction in the sky, your line of sight would eventually hit on a star's surface.
Since every direction would lead to a star, and given the absolute luminosity of a star
and the inverse square law for the dimming of light with distance, the night sky would
be infinitely bright. Olbers' paradox asks the question "why is the night sky dark?"
Solutions to Olbers' Paradox
The German astronomer Johannes Kepler had first pondered the problem of the dark
night sky in 1610, and came up with his own solution to it: that the Universe of stars
extends only to a finite distance, beyond which the viewer encountered only empty
space. Of course, this solution prompts the questions: how far away is the boundary
and what lies beyond it?
Olbers himself suggested that starlight is gradually absorbed while travelling through
space, and that this cuts off the light from any stars beyond a certain distance. This
does not, however, solve the problem, because any absorbing gas or dust would simply
heat up until the starlight it had absorbed would be reradiated. Ultimately, the energy
we would detect as light from Earth would be the same.
It was the American poet Edgar Allan Poe who came up with one of the first
scientifically reasonable solutions to the paradox. He suggested that the Universe is not
old enough to fill the night sky with light. He reasoned that while the Universe may be
infinite in size, there has not been enough time since it first came into being for
starlight to reach us from the farthest corners of space.
Teachers Notes Booklet 4: Cosmology
Page 4 of 14
Astronomers now conclude that the Universe began some 12-15 billion years ago. We
are, however, only seeing the part of it that lies within 12-15 billion light years from us,
with the rest of the stars beyond our sight. The number of stars whose light reaches us
is not enough to fill the sky with light. In addition, astronomers now argue that while
the Universe is infinite, there are a finite number of stars filling it, and the expansion of
the Universe explains the lack of absolute starlight in the night sky.
Teachers Notes Booklet 4: Cosmology
Page 5 of 14
4.2 RedShift
Early in the 20th century, astronomers noticed that features in spectra from distant
galaxies were shifted towards the red end of the spectrum. The degree of redshift is
related to the velocity of recession, or how fast away from us the galaxy is moving. In
1929 astronomer Edwin Hubble produced a paper that plotted the relationship between
the velocity of recession and the distance.
Figure 4.1: Recreation of Hubble's Original Data
The gradient of the plot can be expressed as shown below:
v = H xd
[4.1]
v = Velocity of Recession ( kms −1 )
H = Hubble Parameter (kms - 1 /Mpc)
d = Distance (Mpc)
The Hubble Parameter has the effective unit of 1/Time, which means that determining
the value of the Hubble Parameter will give the age of the Universe. The current value
is:
H0 = 75 ± 10 kms −1 /Mpc
∴ Age of Universe = 13.5 Giga Years
Hubble's observations suggested that the farther away the galaxy the faster the
recession. This phenomenon is described as the Doppler redshift. It is similar to the
effect that makes a car sound lower-pitched as it travels away from you. A similar effect
Teachers Notes Booklet 4: Cosmology
Page 6 of 14
applies to light as well, so that if an astronomical object is travelling away from the
Earth, its light will be shifted to longer, red wavelengths.
It is this Doppler red shift in the spectra of distant galaxies that leads scientists to
conclude that the Universe is expanding.
Teachers Notes Booklet 4: Cosmology
Page 7 of 14
4.3 Big Bang
In 1948, Russian-born physicist George Gamow came up with the idea that if all
galaxies are travelling away from each other at high speed, there must have been a
point way back in the past when the entire Universe was concentrated in a single point.
The term "the Big Bang" was originally coined in order to disparage this theory, but
inevitably the phrase has entered our vocabulary as a description of the creation event,
when space and time originated.
The Big Bang model proposes that around 12 to 15 billion years ago the mass of the
Universe was contained in an infinitely small volume. Some unknown mechanism
caused this volume to expand rapidly and form the Universe we see today. The very
early Universe (fractions of a second after the initial event) was extremely hot, whereas
the Universe we live in today has cooled to just a few degrees above absolute zero.
One of the hardest concepts to grasp is that the Universe is everything that there is, all
the matter and energy, and dimensions. There is no 'outer edge' to the Universe, and it
is not expanding into a void. This is because the dimensions that we normally use here
on Earth (three spatial and one time) are not valid when we consider the Universe.
Teachers Notes Booklet 4: Cosmology
Page 8 of 14
4.4 Cosmic Microwave Background
According to the Big Bang theory, the early Universe was an extremely hot place, which
has been expanding ever since, as the gas within it cools. Scientists in the late 1940s
and 1950s started to realise that if this were true, the Universe should be filled with
radiation that the remnant heat left over from the Big Bang. This relic radiation is
known as the cosmic microwave background (CMB).
The discovery of the existence of the cosmic microwave background was the result of a
happy accident. Two scientists at the Bell Laboratories in the US, Arno Penzias and
Robert Wilson, were using a 6 metre microwave horn antenna to calibrate radio
sources. The antenna had originally been designed to communicate with satellites and,
by chance, operated in a very narrow band. Through all their observations, and those of
previous operators, a consistent offset in the temperature of the system of 3.3 K was
observed. They had no idea of the cause and spent the best part of a year in
investigation the problem – including the removal of a couple of nesting pigeons!
They discussed the problem with leading scientist Robert Dicke then at Princeton
University. He, along with a number of other scientists, had embarked on a programme
to search for background emission at 10K using highly sophisticated equipment. The
group realised the nature of the emission and that it represented an afterglow of the
Big Bang. Penzias and Wilson received the Nobel Prize for physics in 1978 for their
findings.
A Uniform Background
One of the basic tenets of the Big Bang model is that the Universe is expanding, which
automatically implies that it was smaller, denser and hotter in the past. Thus, if we
know how old the Universe is and its expansion rate, we can also estimate what its
current thermodynamic temperature must be, and consequently its radiation frequency.
Scientists have calculated that the expansion of the Universe has led to a background
radiation of a temperature of 2.73 K, which falls into the microwave region of the
temperature spectrum.
Human eyes cannot see the microwaves from the CMB, although detectors such as
those to be carried by ESA's Planck mission will be able to detect them. CMB is the
farthest and oldest light that any telescope can detect.
Teachers Notes Booklet 4: Cosmology
Page 9 of 14
4.5 Open, Flat, Closed?
The shape of the Universe is finely balanced between two forces:
a) The momentum of expansion
b) The pull of gravity
The strength of this gravity depends on the density and pressure of the matter
contained within the Universe. The ultimate fate of the Universe, therefore, depends on
how much mass it contains.
If there is sufficient mass, then the gravity of the material will inevitably stop the
expansion of the Universe, causing it to eventually collapse in on itself. This is referred
to as a 'closed' Universe.
When scientists refer to an open Universe, this refers to the prospect of there not being
sufficient mass to stop its expansion, which will continue forever.
Finally, a 'flat' Universe is one in which there is exactly the right amount of mass to
stop expanding at some point in the future, but not enough to cause a contraction.
Many scientists view this as an aesthetically pleasing solution.
Figure 4.2: The Different Fates of the Universe Depending on Critical Density
Teachers Notes Booklet 4: Cosmology
Page 10 of 14
4.6 Critical Density
The critical density is the minimum density that ensures that the Universe could not
expand forever, but will not collapse back on itself either. The value of critical density is
defined as:
ρc =
3H 2
8πG
[4.2]
The derived value of critical density is, therefore, dependent on the Hubble Constant.
The greater the accuracy of H, as derived in equation [4.1], the more precise the value
for ρc.
Another useful equation relates to the matter density parameter, Ω , the critical value of
which is defined as
Ωc =
ρ
ρC
[4.3]
where ρ is the observed density of the Universe and ρc the critical density. The fate of
the Universe can then be defined in terms of Ω.
Ω < 1 »» open Universe
Ω = 1 »» flat Universe
Ω > 1 »» closed Universe
Teachers Notes Booklet 4: Cosmology
Page 11 of 14
4.7 Dark Matter
It is small wonder that cosmologists are fascinated by the opportunities that exist to
determine the critical density of the Universe. Scientists already have a good measure
of the likely mass of the Universe from their observations of other galaxies. However,
the gravity that can be measured indicates that the likely mass in the Universe is quite
different.
Currently, scientists are seeking to solve this dilemma by looking at other aspects of
our Universe for clues as to its actual mass, and the density of this mass.
One of the aspects that are currently being studied is dark matter. In the last ten years,
scientists have been accumulating evidence that there is a form of matter in the
Universe that cannot be seen, and is not made up of ordinary material in the form of
protons, neutrons and electrons (which is known as 'baryonic matter'). They have come
to this realisation after estimating the mass of distant galaxies by measuring the speed
of their rotation. They found that their estimates of the mass of these galaxies,
including our own, are roughly ten times larger than can be explained purely by the
presence of stars, gas and dust. Consequently, they argue, there must be something
else present, and they have given it the name 'dark matter'.
When trying to understand the nature of this dark matter, scientists have come up with
a number of possible explanations. One of these relates to objects that have been
nicknamed MACHOs (MAssive Compact Halo Objects). These are objects that have a
mass that is less than one twentieth of our Sun, and which consequently shine only
dimly. They are not luminous enough to be directly detectable by our telescopes.
Scientists think they may be responsible for gravitational lensing, the bending of light
predicted by Einstein's theory of general relativity.
Scientists also speculate that new forces or new types of particles could make up much
of the dark matter. They have called these particles WIMPs, or Weakly Interacting
Massive Particles, which could have been produced shortly after the Big Bang.
ESA's forthcoming Planck mission could answer some of the fundamental questions
about the nature of dark matter. Its objective is to analyse, with the highest accuracy
ever achieved, the remnants of radiation that filled the Universe immediately after the
Big Bang.
Teachers Notes Booklet 4: Cosmology
Page 12 of 14
4.8 Other Materials
This is booklet four in a series of six booklets currently available. The full range of titles
is:
Booklet 1
Introduction to the Universe
Booklet 2
Stellar Radiation and Stellar Types
Booklet 3
Stellar Distances
Booklet 4
Cosmology
Booklet 5
Stellar Processes and Evolution
Booklet 6
Galaxies and the Expanding Universe
Each booklet can be used to cover a topic on its own, or as part of a series. Booklets 5
and 6 expand on the material covered in the other booklets and there is, therefore,
some overlap in content.
All the booklets can be accessed via the ESA Science and Technology at:
http://sci.esa.int/teachernotes
For other educational resources visit the ESA Science and Technology Educational
Support website at:
http://sci.esa.int/education
Teachers Notes Booklet 4: Cosmology
Page 13 of 14
Teachers Notes Booklet 4: Cosmology
Page 14 of 14