Download Chapter 18 - the Universe Begins

THE COSMIC ENGINE
CHAPTER 18
The Universe begins
The first minutes of the Universe released energy which
changed to matter, forming stars and galaxies
Introduction
Cosmology, the study of the Universe and its origins,
is one of the most fascinating areas of study. Our
knowledge about the Universe is increasing rapidly.
New observatories orbiting Earth high above the
atmosphere can see parts of the electromagnetic
spectrum never properly observed before. The
nature and structure of the Universe is yet to be fully
understood. Indeed it is widely believed that the
matter we can observe is only about one-tenth of
what is ‘out there’. Dark matter and the even more
mysterious dark energy are believed to be exerting
their influence on the galaxies, but we know nothing
about them.
Figure 18.1 An artist’s
impression of the moment
after the Big Bang, known as
inflation
18.1
The Big Bang
■
Outline the discovery of the expansion of the Universe by
Hubble, following its earlier prediction by Friedmann
Describing the origins of the Universe
SECONDARY
SOURCE
INVESTIGATION
■
PFA
P1, 2
PHYSICS SKILLS
11.1 A, E;
12.3 A, B, C, D;
12.4 A, C, E, F;
14.1 A–H
The origins of the Universe have long been the topic of thought and conjecture. Once the
enormous size of galaxies and the distances between them came to be realised in the early
20th century, astronomers had to explain how the Universe had not been contracted back
into itself by the force of gravity long ago.
Identify data sources and gather secondary information to
describe the probable origins of the Universe
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CHAPTER 18 THE UNIVERSE BEGINS
In 1922, a Russian mathematician, Alexander Friedmann (1888–1925), found a
solution to the equations in Einstein’s 1915 general theory of relativity, which
required an expanding universe. Friedmann’s solution was not at first accepted by
Einstein until Einstein had the opportunity to revise the calculations that conflicted
with Friedmann’s. When Einstein discovered his error, he published an apology to
Friedmann. While an expanding universe fitted mathematical requirements, it was yet
to be observed.
NOTE: Alexander Friedmann was an interesting man, having interest in mathematics,
cosmology and meteorology. In 1925, in his role as the director of meteorology in
Leningrad (now St Petersburg), Friedmann set a new hot-air balloon record, ascending
to an altitude of 7400 m to make meteorological observations. During World War II
Friedmann flew bombing raids and became involved in formulating equations to predict
the path of bombs being dropped.
Figure 18.2
Alexander Friedmann
Observations of nebulae—fuzzy balls of light seen through
telescopes—were thought to be ‘island universes’ in space. Their
true identity remained a mystery for many years. Edwin Hubble
(1889–1953) made careful observations of a number of very
luminous yellow giant variable stars called Cepheid variables.
This type of star could be seen within the nebulae known as M31
(the Andromeda galaxy) (see Fig. 18.3). By finding the Cepheids’
period of variation of their light output, Hubble was able to use
the work of Henrietta Leavitt, who 15 years before found that all
Cepheid variable stars have a period–luminosity relationship.
That is, the longer they take to vary their light output, the more
luminous they are. In 1924, Hubble determined the distance to
M31 to be around 900 000 light years.
NOTE: Distances in space are so huge that using normal
units such as metres or kilometres becomes very difficult.
The distance to the next nearest star to the Earth is about
4 ⫻ 1016 metres, or 40 000 000 000 000 km. A light year
is the distance that light will travel in a vacuum in one Earth
year: 9.46 ⫻ 1015 m. Light travels at 3.0 ⫻ 108 m s–1 in
a vacuum. At this speed, it takes a little over one second
to reach the Moon. Light from the Sun takes eight minutes
and 20 seconds to reach the Earth. The light from the next
nearest star, Proxima Centauri, takes 4.3 years to get to
us. Using the Hubble Space Telescope, light that has taken
nearly 13 billion years to travel across the Universe has
been collected and photographed.
Figure 18.3 M31, the
Andromeda galaxy, now
measured as being 2.5 million
light years away
Figure 18.4 Edwin Hubble and the 100-inch
telescope that he used to make his
observations on Mount Wilson in California
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THE COSMIC ENGINE
NOTE: There are other commonly used astronomical distance units, the ‘parsec’
(about 3.26 light years) and the ‘astronomical unit’, or AU, the distance between the
Sun and the Earth.
The expanding Universe
Simulation: the
Doppler effect
Figure 18.5
The
expanding Universe
Figure 18.6
Hubble’s data
appeared similar
to that shown on
this graph of the
speed of recession
of galaxies versus
their distance
Recession velocity v (km s1)
SR
Hubble’s calculation of the enormous distance to the M31 nebula prompted further
observations of the spectra of similar objects. By carefully comparing the wavelengths
of spectral lines of hydrogen from the nebulae to the wavelengths of the same spectral
lines observed in the laboratory, the relative speeds of the nebulae to the Earth could be
found. This was done using the Doppler effect—a shortening of the wavelengths (shift to
the blue end of the spectrum) indicates an approaching nebulae while a lengthening (a
shift to the red end of the spectrum) is due to a receding source. Hubble measured the
relative speed of a small number of galaxies. He quickly realised that they are moving
much faster than any known object within our
own galaxy. He concluded that they were
indeed separate galaxies in their own right,
a long distance away from our own. He
assumed that their apparent brightness was
an indication of their distance. Putting the
data together, he was able to show a simple
relationship: in general, the further away the
galaxy is, the faster it is moving away from us.
Surprisingly, M31, the Andromeda galaxy that
is the closest major galaxy to our Milky Way, is
moving towards us.
The explanation for the expanding Universe
(see Fig. 18.5) is that it is the space between
the galaxies that is expanding, in the same way
that dots drawn on the surface of a balloon move
further apart when the balloon is inflated (see
Fig. 18.9).
The relationship can be
summarised as v = HoD
where v = velocity at which
the galaxy is moving away
D
from us, Ho = the Hubble
1 000
o
H
v=
constant and D is the
D
Hs
=
distance to the galaxy. The
v
value of the gradient of the
graph shown in Figure 18.6
50
above gives the Hubble
constant. Its unit is s–1. The
Galaxies
inverse value of Ho, or 1/
Ho, gives the age of the
0
Universe. Many corrections
to the value of Ho have been
0
2
1
made over recent years
Distance to galaxy D
and continue to be made.
(Megaparsecs)
(1 Megaparsec = 3.25 million light years)
Each time the value of Ho
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CHAPTER 18 THE UNIVERSE BEGINS
is revised, the estimated age of the Universe is revised. Currently, it is about 13.7 billion
years. Another technique used by astronomers to measure the age of the Universe is to
observe the background microwave radiation, a remnant of the original radiation released
when the Universe formed. The wavelength of this radiation has been gradually increasing
as the Universe expands. A computer-enhanced map of this radiation is shown in
Figure 18.7.
Figure 18.7 The remnant radiation from the Big Bang seen now in the microwave region of the
electromagnetic spectrum
If the Universe is expanding, from where did it originate? The nature of the expansion
means that from wherever the Universe is being observed, it seems that it is expanding
away from that point. The ‘centre’ of the Universe cannot be found. However, if time is wound
back to the instant that everything came together, what was it that caused the Universe to
form? This event was at first mockingly referred to as the ‘Big Bang’, a term that has now
been adopted widely by cosmologists and society in general. The Big Bang is now thought
to have created the Universe. It is considered to be a massive explosion that created energy
and matter, as well as spae and time.
The equivalence of mass and energy
18.2
PFA
■
Identify that Einstein described the equivalence of energy
and mass
In September 1905 Einstein first wrote about the equivalence of energy and mass. His
work on the behaviour of charged particles in magnetic fields led him to believe that
as a particle loses energy, it also loses a tiny amount of mass.
P1
‘Outlines the
historical
development of major
principles, concepts
and ideas in physics’
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THE COSMIC ENGINE
Figure 18.8 A light particle (a photon), not visible here, is annihilated and
becomes two oppositely charged particles that leave behind a vapour trail in
this cloud chamber, a way of seeing the paths left behind by the passage of
gamma rays
Einstein became convinced of the link between mass and
energy when applying his special theory of relativity (studied
in the Space Module in the HSC Course) to perfectly inelastic
collisions. When relativistic speeds become involved, part of
the loss of kinetic energy of the system can only be explained
by a change in mass of the objects involved.
Probably the most well-known scientific equation,
Einstein wrote it originally as m = L/c2, where L was a form
of energy due to relativity and its effects. This form of the
equation was re-written with E as the subject to make it
E = mc2.
It took 40 years after the publication of Einstein’s concept
of the equivalence between mass and energy for the world to witness what such
frightening amounts of energy could do. The atomic bombs dropped on Hiroshima
and Nagasaki in 1945, ending World War II, threw the world’s largest nations into
an arms race that could have ended the world—all using E = mc2. This is ironic
considering Einstein’s often-stated anti-war views.
The most precise confirmation of the change in the mass of a radioactive nucleus
when it emits a gamma ray was made in 2005. It showed Einstein’s E = mc2 equation
to be accurate to within four-hundred-thousandths of one per cent.
WWW>
USEFUL WEBSITES:
Einstein explains the equivalence of energy and matter:
http://www.aip.org/history/einstein/voice1.htm
Einstein light from the UNSW:
http://www.phys.unsw.edu.au/einsteinlight/jw/module5_equations.htm
ANSTO’s information on E = mc2 and more:
http://velocity.ansto.gov.au/velocity/ans0008/article_02.asp
The American Museum of Natural History:
http://www.amnh.org/exhibitions/einstein/energy/emc2.php
18.3
After the Big Bang
■
■
Describe the transformation of radiation into matter which
followed the ‘Big Bang’
Identify that Einstein described the equivalence of energy
and mass
Before the Big Bang occurred, nothing existed, not even time itself. Asking an
astrophysicist what happened before the Big Bang is like asking someone ‘How
is your dog?’ when they do not have a dog! The Big Bang was not like a normal
explosion. It was a sudden expansion and coming into existence of a huge amount
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CHAPTER 18 THE UNIVERSE BEGINS
Figure 18.9 Nuclear power stations harness the energy
released when matter is converted into energy
of energy. The tiny fraction of a second after
the Big Bang is referred to as the period of
‘inflation’ (see Fig. 18.1). This very young
Universe contained extremely hot energy—too
hot for even the most basic building blocks of
matter to exist. After this time, as the Universe
expanded (see Fig. 18.9) and cooled, energy
began to condense into matter (according to
Einstein’s mass–energy relationship), forming
matter and anti-matter in approximately
equal proportions. As the anti-matter collided with the matter, it was annihilated and
converted back into energy (see Fig. 18.10).
This process continued until, about a millionth of a second after coming into
existence, the Universe had expanded and cooled sufficiently for matter to be able to
exist. (Interestingly, slightly more matter than anti-matter was formed, so that most of
the matter we detect now is not anti-matter.)
By the first second after the Big Bang, the
fundamental particles of matter had formed. Protons,
neutrons and electrons existed. By three seconds
old, the Universe contained the basic and simplest
elements—almost all hydrogen and some helium.
However, it took another 380 000 years before
the Universe had cooled sufficiently for photons
(i.e. light) to travel freely through space, making it
transparent for the first time.
Almost 25 years before the Big Bang theory was
first coined, Einstein proposed that matter and energy
were linked, and could be transformed from one
form to the other. A number of scientists were working on this idea around the early
1900s. It was Einstein who, in 1905, published his now famous equation, E = mc2.
This made the relationship between matter and energy quantifiable. The number
‘c ’ in Einstein’s equation is the speed of light in a vacuum, 3.00 ⫻ 108 m s–1. A very
small amount of matter can be transformed into a very large amount of energy.
Einstein based his prediction of the possibility of the atomic bomb and nuclear
energy by realising that when the nuclei of large atoms split, they lose a small
amount of mass. If many atoms could be made to do this simultaneously, the small
change in mass would create a very large amount of energy (see Fig. 18.8). Einstein’s
predictions were, of course, found to be true. Nuclear power stations and atomic
weapons obtain their energy from E = mc2.
Figure 18.10
Another way
of showing the
expansion of the
Universe, with dots
on the surface of
an inflating balloon
representing
galaxies
NOTE: The proposition of the Universe coming into existence in one single event was at
first dismissed by many astronomers, including the well-regarded British astronomer Fred
Hoyle. When asked what he thought about such a theory in 1949, he jokingly referred to it
as the ‘big bang’ in an attempt to show how silly such an idea was. The term has remained
in use to this day. Astronomers (including Fred Hoyle) today widely accept the Big Bang, as
more observations (including the existence of cosmic microwave background radiation) are
made that match and support the once ridiculed theory.
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THE COSMIC ENGINE
Figure 18.11 An artist’s image of the stages of the Universe shortly after the Big Bang
18.4
The formation of stars and galaxies
■
Outline how the accretion of galaxies and stars occurred
through:
– expansion and cooling of the Universe
– subsequent loss of particle kinetic energy
– gravitational attraction between particles
– lumpiness of the gas cloud that then allows gravitational
collapse
Once the Universe was 380 000 years old, it is believed that it was spread throughout
with matter as we know it—atoms with protons and neutrons in the nucleus with
electrons orbiting the nucleus. Most atoms were hydrogen, the simplest of the
elements with one proton and one electron. For some reason that is still not fully
understood, this matter was not distributed evenly. Regions with higher densities
began to become attracted and clumped together under the influence of gravity. This
could only happen once the speed of the particles had slowed sufficiently. With no
direct observations of the Universe at this age, we can only imagine what occurred.
The matter in the Universe was probably spread out like a three-dimensional spider’s
web. With tiny variations in density, gravity caused the denser regions to ‘fall into’
themselves, or coalesce into large lumps. This in turn resulted in the formation of
the first stars, in which the heavier elements were formed. Regions where stars were
more densely spaced collected themselves into huge groups that we now observe as
galaxies. Figure 18.11 summarises this sequence.
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CHAPTER 18 THE UNIVERSE BEGINS
The Big Bang
The very young Universe expanded
and cooled, allowing matter in the
form of simple elements (mostly
hydrogen) to form.
The matter was spread throughout
the Universe but with very small
variations in density.
The Universe continued to expand
and cool, resulting in the slowing
down of the speed of the atoms.
The more dense regions of matter
began to coalesce under the force of
attraction due to gravity.
Density and pressure in the core of
these clumps of matter allowed nuclear
fusion to occur—synthesising the
heavier elements. Stars are formed.
Figure 18.12 The cooling of the Universe and formation of the first stars
CHAPTER REVISION QUESTIONS
1. What did Friedmann predict about the Universe? Why was this prediction made?
2. Describe what observations were made by Hubble that supported Friedmann’s prediction.
3. What is meant by the ‘equivalence of mass and energy’?
4. What was the ‘Big Bang’?
5. Outline the steps from the Big Bang until the formation of the first stars.
6. Why did matter not simply spread out evenly through the Universe?
7. Research and identify a minimum of three other secondary sources that contain
information describing the probable origins of the Universe.
8. When a gas expands, it cools. How does this relate to the early Universe?
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