Astronomy: The Big Bang and Cosmology
Exploring evidence for an expanding universe
Astronomy: The Big Bang and Cosmology
Exploring evidence for an expanding universe
Astronomy - Grade 9-12
- 1
In your own words, describe what the Big Bang model says about the early universe. Include at least two important features of the model.
Focus on expansion, temperature, density, and time.
The Big Bang model says that the universe began in a very hot, dense state and has been expanding and cooling over time. It also explains that space itself is expanding and that the early universe produced light elements such as hydrogen and helium. - 2
A galaxy has a measured redshift, meaning its spectral lines are shifted toward longer wavelengths. Explain what this tells astronomers about the galaxy's motion and how it supports the idea of an expanding universe.
A redshift shows that the galaxy is moving away from us. When many distant galaxies are observed to be moving away, and more distant galaxies generally move away faster, this supports the idea that the universe is expanding. - 3
Use Hubble's law, v = H0d, to find the recessional velocity of a galaxy that is 200 megaparsecs away. Use H0 = 70 kilometers per second per megaparsec.
Multiply the Hubble constant by the distance.
The recessional velocity is v = 70 x 200 = 14,000 kilometers per second. The galaxy is moving away at about 14,000 kilometers per second. - 4
A distant galaxy is moving away at 21,000 kilometers per second. Using H0 = 70 kilometers per second per megaparsec, estimate its distance in megaparsecs.
Using d = v divided by H0, the distance is 21,000 divided by 70 = 300 megaparsecs. The galaxy is about 300 megaparsecs away. - 5
The cosmic microwave background is often described as leftover radiation from the early universe. Explain why this radiation is important evidence for the Big Bang model.
Think about what should remain if the early universe was hot and filled with radiation.
The cosmic microwave background is important because it is the cooled remains of radiation from the hot early universe. Its nearly uniform glow in all directions matches a major prediction of the Big Bang model. - 6
Explain why astronomers say they are looking back in time when they observe very distant galaxies.
Astronomers say they are looking back in time because light travels at a finite speed. Light from a very distant galaxy may have taken millions or billions of years to reach Earth, so we see the galaxy as it was when that light left it. - 7
A spectral line that normally has a wavelength of 500 nanometers is observed from a galaxy at 550 nanometers. Calculate the redshift using z = (observed wavelength - rest wavelength) divided by rest wavelength.
Subtract first, then divide by the rest wavelength.
The redshift is z = (550 - 500) divided by 500 = 50 divided by 500 = 0.10. The galaxy has a redshift of 0.10. - 8
List three major pieces of evidence that support the Big Bang model and briefly explain each one.
Three major pieces of evidence are galaxy redshifts, the cosmic microwave background, and the abundance of light elements. Galaxy redshifts show that space is expanding, the cosmic microwave background is leftover radiation from the hot early universe, and the amounts of hydrogen and helium match predictions from early-universe nuclear reactions. - 9
During the first few minutes after the Big Bang, the universe was hot enough for nuclear reactions to form light elements. Name two elements formed in large amounts during this period and explain why heavier elements were not formed in large amounts then.
The early universe changed temperature very rapidly.
Hydrogen and helium were formed in large amounts during the first few minutes. Heavier elements were not formed in large amounts because the universe expanded and cooled too quickly for many heavier nuclei to build up. - 10
A student says, "The Big Bang was an explosion into empty space." Explain why this statement is not accurate.
The statement is not accurate because the Big Bang was not an explosion into preexisting empty space. The model describes the expansion of space itself from a hot, dense early state. - 11
Cosmologists have found that the expansion of the universe is speeding up. What is the name commonly given to the unknown cause of this accelerated expansion, and what does it appear to do?
This is different from dark matter, which affects gravity in galaxies and clusters.
The unknown cause is commonly called dark energy. It appears to act as a form of energy associated with space that causes the expansion of the universe to accelerate. - 12
Put these events in the correct order from earliest to latest: formation of the first stars, cosmic microwave background released, rapid early expansion, formation of the solar system.
The correct order is rapid early expansion, cosmic microwave background released, formation of the first stars, and formation of the solar system. This order shows that the universe expanded and cooled before stars and planets formed.