Earth in the Universe · The Universe and the Solar System
The origin of the universe
Next time you look at the stars from a Texas backyard or a campsite in Yellowstone, remember: the light reaching your eyes left each star 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, and that the further away a galaxy was, the greater that shift: all of them seemed to be moving away from us at enormous speeds. Why do almost all galaxies move away from one another instead of moving randomly?
Practice★☆☆Not started
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.
The expansion of the universe: Hubble's law and redshift
The balloon analogy
Picture a balloon with dots drawn on its surface. As you inflate it, every dot moves away from every other dot, and the farther apart two dots start, the faster they separate. Each dot "sees" all the others receding, so no dot is the center. The surface of the balloon is a model of space: the galaxies are the dots.
Practice★★☆Not started
Hubble observed that the light from distant galaxies shifts toward 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 toward 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 center…
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 center 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 center" of the balloon is a conceptual error about the expansion of space.
Formation of galaxies, stars and chemical elements
Formation of the Solar System
About 4.6 billion years ago, a cloud of gas and dust (a nebula) enriched with heavy elements from earlier stars began to collapse under its own gravity, perhaps triggered by a nearby supernova.
Practice★★☆Not started
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 center, 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 center, and the protoplanetary disk.
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).
A quick timeline of the early universe: within the first few minutes after the Big Bang, the universe was hot and dense enough for primordial nucleosynthesis to fuse protons and neutrons into helium nuclei, with traces of lithium. Then it cooled for about 380,000 years before atoms formed and light could travel freely.
The model makes two testable predictions. First, the first minutes of the universe should have produced about 75% hydrogen and 25% helium by mass, with only traces of lithium. Telescopes measure almost exactly these proportions in the oldest gas clouds, which stars have barely changed.
Second, the light released when the universe became transparent has been stretched by expansion for about 13.8 billion years. It should now be a faint glow of microwaves with a temperature of about 2.7 K ($-270\,^\circ\text{C}$), nearly the same in every direction. Satellites such as NASA's COBE and WMAP detected exactly that, matching the prediction.
Practice★★☆Not started
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.
To compress the timeline into one year, use a proportion: the fraction of cosmic history that has passed equals the fraction of the year.
Practice★☆☆Not started
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 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 is the Big Bang and December 31st at 24:00 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.
Summary
Let's practice! 3 medium exercises in this section