Astronomy Rough Notes – Big Bang – Part 2
DISCLAIMER: These notes do NOT cover everything you need to know. You may need to look up some item or concept online or in a text. Test questions are not exact copies of the OBJECTIVES but if you know the OBJECTIVES thoroughly, you should do well on the exams.
HANDOUTS: None
OBJECTIVES:
What “tweak” was added to the Big Bang model to explain the “Horizon Problem” and the “Flatness Problem”?
In the last decade or so, what have astronomers recently discovered about the expansion of the universe?
What is the ultimate fate of the universe under this scenario?
CLASS:
“Horizon Problem” (not needed for exam)
If the universe was born with some size (even say 10-20 m), it was too big for information to travel from one part of the universe to another quickly. So one spot would not have time to come to a temperature equilibrium with another spot. In other words the early universe would not all be at the same temperature.
BUT the CMB (radiation from the early universe) is very smooth (only very tiny variations in temperature from spot to spot).
That means that the early universe all had the same temperature.
How can it be so uniform in temperature?
How did each part of the early universe “know” the temperature of the other parts?
There just was not enough time for information to get from one spot in the universe to another.
One spot is beyond the “horizon” of another spot.
Discuss “Flatness Problem” (not needed for exam)
Draw a triangle in flat space and the sum of the interior angles of a triangle is 1800.
If you draw a triangle on a sphere like a basketball, the sum of the angles of a triangle is > 1800.
If you draw a triangle on the inside of a bowl, the sum of the angles is <1800.
The universe turns out to be essentially indistinguishable from flat.
Out of all the possibilities for curvature, why did the universe end up with the one choice of flat?
Solution to “Horizon” and “Flatness” problems (know this for exam)
1979 – Alan Guth – Inflation
Just after birth, the universe under goes a huge growth spurt in a tiny fraction of a second.
Universe expands way faster than the speed of light.
Explains “Horizon Problem” because the universe would have started much smaller and would have had time for all spots to come to the same temperature.
Explains “Flatness Problem” because if the universe expands enough it seems almost flat.
Reasons inflation might happen (not needed for exam)
-Forces split
Other possibilities sometimes include
-Broken symmetries/ phase change
-Supercooled universe
- Vacuum energy
Direct Evidence for Inflation (not for exam)
Clem Pryke (University of Minnesota) et.al.
From “Astronomers’ Update”, April 2014, Bell Museum
Why Are We Here?
Human existence, in fact the existence of planets, stars and galaxies, depends on what happened in the earliest moments of creation. In March (2014), a team co-led by Clem Pryke, of the Minnesota Institute for Astrophysics, announced the first look at what the universe was like 0.000 000 000 000 000 000 000 000 000 000 000 001 seconds (a trillionth of a trillionth of a trillionth of a second) after the Big Bang. This astounding announcement was from an experiment called BICEP2 in Antarctica, claiming the detection of twists in the polarization of the Cosmic Microwave Background from the generation of gravitational waves at that very early time. They weren't searching blindly -- a 1980 theory of the early universe, called "inflation", was proposed by Alan Guth, currently a professor at MIT. Assuming this result holds up, it pushes our understanding of our immense and wonderful universe to a whole new level, as the details of those earliest times are now open for our exploration.
[Pictures of early universe signal: ]
For more information try for a more technical article see
For a fun, “minutephysics” video, try
Stretching and Squeezing the Universe
In last month’s issue of Astronomers’ Update, we highlighted the exciting discovery by Clem Pryke, of the Minnesota Institute for Astrophysics, and his colleagues, about signals they detected in the cosmic microwave background, reflecting conditions at the instant after the Big Bang. The key number from their measurements, called the "tensor-to-scalar" ratio, was approximately 0.2, higher than most scientists expected.Pryke quipped, "This has been like looking for a needle in a haystack, but instead we found a crowbar." The "scalar" part of this ratio measures how much the density of matter and energy varies from one position in the universe to another. The "tensor" part of the ratio measures the strength of the gravitational waves at these early times. The waves actually distort space as they move through, alternately stretching in one direction and compressing in the other, as illustrated in the time sequence below. Ever mindful of possible experimental pitfalls, scientists around the world eagerly await confirmation of these exciting results.
Further information
Figure: Credit: Background -- NASA (Hubble Deep Field)
Future of universe (for exam)
Supernova studies suggest the universe is speeding up, not slowing down
Dark energy
Some Dark Energy references:
1. National Geographic
2. Hubble Site on Dark Energy
3. "Introduction to Cosmology" by Barbara Ryden.
4. NASA “Imagine” site on expansion with Dark Energy
Revised model of Big Bang (for exam)
Universe (space and time)
•Springs into existence (tiny) ~ 13.7 Byrs ago
•For tiny fraction of a second, inflating rapidly
•For first 9Byrs,
• Expanding, cooling, rate of expansion slowing
• Due to gravity
•For last 5Byrs,
• Rate of expansion revving up
• Due to dark energy
Composition of universe (for exam)
From Planck data, learn the approximate amounts
~5% “normal” matter (4.9%)
~1/4 dark matter (26.8%)
~2/3 dark energy (68.3%)
What went bang? What was there before the Big Bang?(not needed for exam)
(Lots of speculation)
Search on quantum fluctuations
“The New Cosmology”, Sky and Telescope, October 2003
“Where Did Our Universe Come From?”, Sky and Telescope, December 2006
(A decent, four-page summary of how the universe may have begun and how we are not getting “something” from “nothing”. Mentions virtual energy, negative gravity, etc.You may have to read it several times.)
Multiverses? – no evidence but interesting speculation(not for exam)
(See Greene’s “Fabiric of the Cosmos” at )
Conformal cyclic cosmology (CCC) suggested by Gurzadyan and R. Penrose (not for exam)
Suggest our “aeon” leads to another Big Bang. See “Cycles of Time” by him. As of November 2010, claims some evidence in concentric rings in the CMB around big black holes. See for an article and for a brief interview
Not for the exam but side note on size of universe
Universe is at least 250X bigger than visible universe
Visible universe is ~90 Bly (CMB traveled at least 45 Byrs to us – universe expanding)
An explanation from Jeff R in Mpls
Light from the most distant part of the Universe we can see, the cosmic microwave background radiation (CMB), has been travelling toward us for almost 14 billion years, so it has covered a distance in that time of almost 14 billion light-years. That light was emitted by hot hydrogen gas which was only 40 million light-years away from our current location. That is "million" with an "m".
That hydrogen gas was moving away from this location at very high speed because of the cosmic expansion, which is not limited by the speed-of-light limitation. Those hydrogen atoms, or whatever they may have turned into, are now 45 billion (with a "b") light-years away from our location.
Here's a very compact web page which shows how some of that works:
Revised 6 January 2016