7.2.2 – Research – The Big Bang
The Universe is all of time and space (and all of space’s contents). It includes planets, moons, minor planets, stars, galaxies,
comets, the contents of intergalactic space, and all matter and energy. The ancient Greeks thought that Earth was the center of
the Universe. In the 1514, Copernicus popularized the idea that the Sun was at the center of the Universe. Kepler’s
astronomical observations in the early 1600s led scientists to realize that our solar system was within the Milky Way Galaxy and
that the Milky Way was simply one of many many different galaxies. In the early 1900s, observations and data began to suggest
that the Universe was actually expanding! At this time, a few possible causes of this were suggested, however by the 1950s,
data pointed towards the currently accepted explanation for what occurred at the “beginning of the Universe,” which got tagged
by the media as the Big Bang Theory. As we begin to reach the present, we get more and more data thanks to more powerful
telescopes. The Big Bang theory offers a comprehensive explanation for a broad range of observations, including the
abundance of light elements (H and He), the cosmic microwave background, and Hubble’s Law.
Simply, our universe sprang into existence approximately 13.8 billion years ago. It sprang from a singularity, which is a place in
space-time where the gravitational field is infinity (see the equation to the right); meaning as singularity is infinitesimally small,
has infinite mass, is thus infinitely dense, and is infinitely hot. (I know, it’s mind-blowing…).
We have no idea where this singularity came from, why it was there, or what was there
before it. Nothing? Something? Anything? We don’t know. But we do know that the
observable universe that we know started from this singularity.
13.8 billion years ago, something happened to this infinitely dense and hot singularity. Not an explosion, as the name implies,
but an expansion. The matter in the singularity rapidly started to expand. As it expanded, it cooled. The further it expands, the
more it cools, and the less dense that the entire universe is because it continues to expand and take up more space. We
(humanity, Earth, our Solar System, the Milky Way, etc) are inside this ever expanding and cooling universe.
Timeline
The history of the universe is broken down into 4 different parts:
1) Very Early Universe – 0 seconds to 10-32 seconds after the Big Bang started rapidly expanding
a. Little is understood about physics at the temperatures that existed here. It is currently being studied, and
phrases like “String Theory” or the “Grand Unified Theory” are referring to research being done with particle
accelerators that would lead towards a greater understanding of this time period. We are fairly sure that the
4 fundamental forces of physics (electromagnetic, gravitation, weak nuclear interaction, and strong nuclear
interactions) were all combined as one force due to the massive heat and pressure.
2) Early Universe – 10-32 seconds until 150 million years after Big Bang
a. As the universe continued to expand, it cooled. Approximately 3 minutes after expansion started, it cooled
enough that simple atomic nuclei (protons and neutrons). However, stable neutral atoms didn’t form until
377,000 years later, which is supported by cosmic microwave background radiation. During this time, the
majority of all atoms produced were Hydrogen (~75% of all atoms) and Helium (~25% of all atoms), with
some trace Lithium (<1% of all atoms) formed.
3) Period of Structure Formation – 150 million years until the
Distant Future
a. First stars form due to gravitational attractions of
gasses and dust. Stars then interact and pull
together due to gravity and form galaxies, then
groups, then clusters, and superclusters. Everything
started small and was built up bigger as cosmic time
passed.
b. The Milky Way formed 8.8 (1.7) billion years ago. (4.8 billion years after the Big Bang)
c. The Solar System formed 4.6 billion years ago. (9 billion years after the Big Bang)
d. CURRENT DAY (13.7999 billion years after the Big Bang)
4) The Distant Future – 100 billion years after the Big Bang????
a. Stars will eventually stop being born. There are many ideas regarding what will happen here, but the only
certainty now is that if there was a beginning to the universe, there must be an end.
Evidence
1) Hubble’s Law. In 1929, Edwin Hubble (the astronomer that has a famous telescope named after him) discovered that the
universe was expanding, because every deep space object, regardless of the direction that he looked, is moving away from
Earth. But how did he figure that out?
Basically, Hubble looked through special telescopes that would let
him analyze light being emitted by stars or galaxies in a special
way. If we are talking about light, you need a quick refresher in
the electromagnetic spectrum. Recall that most light emitted by a
star is not visible. There are 7 types of light waves in the
electromagnetic spectrum, and from longest wavelength to
shortest wavelength, they are: radio waves, microwaves, infrared
rays, visible light (remember RoyGBiv?), ultraviolet light, x-rays,
and gamma rays. The image above should help out a lot.
This special telescope, using a prism, would not only show the full visible colors of the rainbow, but it would also leave
black lines on the spectrum depending on which elements were present within the star or galaxy. Each element has a
specific line that it produces. The spectral lines for the first 20 elements are found on the image to the right. Notice that
each element is different, and has a specific pattern and a different set of lines.
Now this is where Edwin Hubble became famous. When he
looked into space, he would see the spectral lines and patterns
for specific elements. They matched up perfectly, except they
were shifted closer to the red side of the color spectrum (look at
the image below to see the difference) than the element would be
if we tested it on Earth. Every direction he looked into the night
sky with his telescope, the stars or galaxies were red shifted. And
Hubble figured out that this meant that the light waves (looking at
the picture above with the full wave on it) actually expanded,
meaning the universe was expanding. Not only could he tell that
everything was traveling away from us, but he could also tell
HOW FAST objects were moving away from us by how red
shifted the object was. Digging a little more, he found a
correlation between how far away an object was and how fast it
was moving away from us.
This is key.
Everything
in deep
space
moving
away from not just Earth, but everything in deep space is
moving away from everything else. The space between ALL
objects continues to increase at a constant rate.
Not only that, but the further away an object was, the faster that
the object was speeding away from Earth. For every 3 light-years
of distance, the object is moving an additional 42 miles per
second faster. For example, an object 3 light years away is
traveling away from us at 42mi/sec, while an object at a distance
of 6 light years is traveling 84mi/sec away from us, and 9 light
years away is traveling 126mi/sec away from us.
Stepping backwards, if we know how fast objects are moving, we
can use those numbers to calculate where they came from and
how long they have been traveling. If you take the speeds of all
of the different stars and galaxies and super-clusters that we have data of, and reverse calculate where they come from,
they condense down to a single point 13.7999 billion years ago.
2) Cosmic Microwave Background Radiation (CMB) is radiation left over from an early stage of the development of the universe.
While microwaves are invisible to the naked eye, they can be picked up by radios and radio-telescopes, and was accidentally
discovered in 1964.
This explanation requires about 2 or 3 steps to get to the point, but this is a really big deal, so…
Recall that the more tightly packed that matter is, the higher the pressure and temperature. As the matter and energy
expanded, it cooled. When subatomic particles (protons, neutrons, and electrons) formed 3 minutes after the Big Bang, it
was still too hot for them to interact together. After 377,000 years, the universe cooled enough that electrons and protons
became bound to form the first neutral hydrogen atoms (this process was called recombination). This is important because
heat and light energy that was caused by the Big Bang could be scattered by protons and electrons individually, but not by
these new stable Hydrogen atoms. Once these new stable Hydrogen atoms formed, heat and light continued traveling in
whatever direction it was already moving and kept moving in that direction without being disturbed by protons or electrons
anymore. Because of the expanding universe, this heat was red-shifted. When heat is red-shifted, it turns into microwaves.
So when astronomers in the 1960s accidentally pointed their radio-telescopes towards the night sky, they saw cosmic
microwave background radiation. These CMBs are simply capturing the red-shifted heat from when protons and neutrons
could finally attach to each other and form the first atoms of Hydrogen close to 13.4 billion years
3) The abundance of light elements, Hydrogen and Helium, occur in ratios throughout the universe that are indicative of an
incredibly hot, dense beginning. The fact that 25% of the mass of the universe is Helium shows how hot the initial temperatures
of the universe were. If it were cooler, we would have a smaller percentage of helium found in the universe. And recall, this
massive temperature comes from massive pressure, where everything is packed together, in an initial singularity that 13.7999
years ago started the Big Bang.
Review Questions
1.
2.
3.
4.
How long ago did the universe form?
What did the universe start as? What properties did that have?
What is the equation for gravity?
What are the 3 major stages for the development of the universe, how long was each stage, and what
important thing happened in each stage?
5. How long ago did the Milky Way form?
6. How long ago did our Solar System form?
7. What are the 7 types of light from longest wavelength to shortest wavelength? (Say RoyGBiv instead of
Visible light)
8. Explain red-shifting to me?
9. What are the 3 major pieces of evidence that support the Big Bang Theory?
10. What are the 2 major parts of Hubble’s Law?
11. What allowed Cosmic Background Radiation to actually start traveling in the direction it is traveling?