The origin of the Universe
On a clear night in the Wicklow Mountains, far from the lights of Dublin, the sky is full of stars. The light that reaches your eyes from each one left it years, centuries or millions of years ago: looking at the sky means looking at the past. Cosmology is the branch of science that uses that light to reconstruct the origin and evolution of the universe.
The Big Bang model
When measuring the light emitted by distant galaxies, astronomers consistently observed that their spectral lines were shifted towards the red: almost all of them seemed to be moving away from us at enormous speeds. (Hubble later showed, in 1929, that the further away a galaxy is, the greater that shift, which is interpreted as a greater speed of recession.) Why do almost all galaxies move away from one another instead of moving randomly? The answer is that space itself is expanding, carrying the galaxies apart. The Big Bang is a scientific model: it is the best explanation we have, and it can be revised if new evidence appears.
Explain what the Big Bang theory consists of and state approximately how long ago the origin of the Universe occurred.
Explain what the Big Bang theory consists of and state approximately how long ago the origin of the Universe occurred.
Name two lines of evidence supporting the Big Bang theory and briefly explain each one.
Name two lines of evidence supporting the Big Bang theory and briefly explain each one.
The expansion of the universe: Hubble's law and redshift
Hubble observed that the light from distant galaxies shifts towards the red. Explain what redshift is and what conclusion about the Universe can be deduced from this…
Hubble observed that the light from distant galaxies shifts towards the red. Explain what redshift is and what conclusion about the Universe can be deduced from this observation.
A classmate claims: "Since from Earth we observe that almost all galaxies are moving away from us in every direction, the Milky Way must occupy the exact physical centre…
A classmate claims: "Since from Earth we observe that almost all galaxies are moving away from us in every direction, the Milky Way must occupy the exact physical centre of the universe where the Big Bang occurred."
- a) Use the analogy of the surface of an inflating balloon with dots drawn on it to refute their claim: deduce what an astronomer located in any of the other galaxies would observe.
- b) Identify an important limitation of this analogy: explain why placing the Big Bang in the "hollow inner centre" of the balloon is a conceptual error about the expansion of space.
Formation of galaxies, stars and chemical elements
How did we get from a cloud of gas to the stars, the Sun and the atoms in your body?
Formation of the Solar System
About 4,600 million years ago, a cloud of gas and dust (a nebula) enriched with elements from older stars began to collapse under gravity.
Describe the main stages of the formation of the Solar System from a nebula: collapse of the cloud of gas and dust, formation of the Sun at the centre, and the…
Describe the main stages of the formation of the Solar System from a nebula: collapse of the cloud of gas and dust, formation of the Sun at the centre, and the protoplanetary disc.
Science communicator Carl Sagan popularized the phrase "we are made of starstuff." Analyze the origin of the atoms in your body based on these data: hydrogen represents…
Science communicator Carl Sagan popularized the phrase "we are made of starstuff." Analyze the origin of the atoms in your body based on these data: hydrogen represents about $62\,\%$ of the total atoms, oxygen $24\,\%$, carbon $12\,\%$, and iron $0.001\,\%$ (essential in hemoglobin to transport oxygen).
- a) Justify why carbon, oxygen, and iron had to form inside or during the death of earlier stars and not during the Big Bang.
- b) Deduce why it was impossible for rocky planets with life like ours to exist around first-generation stars (Population III, formed shortly after the Big Bang consisting solely of hydrogen and helium).
The observational evidence for the Big Bang
What is cosmic microwave background radiation? Explain why its detection is considered key evidence for the Big Bang.
What is cosmic microwave background radiation? Explain why its detection is considered key evidence for the Big Bang.
Cosmic timeline
Look at the timeline of the Universe (not to scale): the letters A, B, C and D mark four events — Big Bang, formation of the Sun, formation of the Earth, and appearance…
Look at the timeline of the Universe (not to scale): the letters A, B, C and D mark four events — Big Bang, formation of the Sun, formation of the Earth, and appearance of life — without indicating which is which. Identify which event corresponds to each letter, order them from oldest to most recent, and state approximately how long ago each one occurred.
In the "cosmic calendar," the $13\,800$ million years of the universe are condensed into a single year of 365 days (January 1st at 00:00 h is the Big Bang and December…
In the "cosmic calendar," the $13\,800$ million years of the universe are condensed into a single year of 365 days (January 1st at 00:00 h is the Big Bang and December 31st at 24:00 h is the present moment).
- a) Calculate approximately on what date (month and day) the Earth formed ($4\,600$ million years ago) and at approximately what date and time the human genus (Homo, about $2.8$ million years ago) appeared. Show your calculations.
- b) In light of these results, refute the anthropocentric argument that the universe exists with the main purpose of harboring the human species, comparing the fraction of time occupied by our existence with that of the cosmos.
Back to the beginning: If looking far away means looking into the past, the most distant and ancient light we can observe is the cosmic microwave background radiation, which shows us what the universe was like only 380,000 years after the Big Bang.