Differences

This shows you the differences between two versions of the page.

--- courses:ast100:2 [2024/11/04 05:25] – [2. Galactic age] asad
+++ courses:ast100:2 [2024/11/05 11:31] (current) – [5. From Speed to Age] asad
@@ Line 179: / Line 179: @@
 **Hermes:** Let’s return to Earth now. This time, let’s take a boat and travel along the Tsangpo River.
-===== - From Speed to Age =====
+===== - Age from Speeds =====
 **Shashi:** A boat is actually the perfect setting to explain what I want to talk about now. At the start of the 20th century, we didn’t even know there were galaxies outside the Milky Way. Everyone thought the Milky Way was the entire universe. In the 18th century, the French astronomer Charles Messier cataloged many nebulae, which were actually galaxies, but back then, they were thought to be gas clouds within our galaxy. However, not everyone dismissed the idea of other galaxies; for example, the German philosopher Immanuel Kant suggested that these known nebulae could be separate "island universes." But without knowing the distances, it was impossible to prove.
@@ Line 202: / Line 202: @@
 **Socrates:** Yes, the sound gets sharper as it approaches.
-**Shashi:** So, the frequency of the sound increases, and the wavelength decreases. Now the boat is passing us and moving away. Listen—the sound becomes less sharp, meaning the frequency decreases, and the wavelength increases. The diagram illustrates this phenomenon on a cosmic scale. Since the universe is expanding, all galaxies are moving away from each other, just like points drawn on a balloon would move apart as it’s inflated. The galaxies themselves aren’t moving; rather, the space (the balloon) is expanding, increasing the distances between them. If a galaxy is moving away from us, its light frequency will appear lower, similar to the boat’s sound as it moved away. Since red has a lower frequency than blue, this shift in frequency is called **redshift**. We observe that the light from all galaxies is redshifted, moving toward the red end of the spectrum. The degree of this shift indicates the galaxy’s speed of recession. Just as the dungchen on the boat would sound even lower if the boat were moving faster away, the light from a galaxy appears more redshifted the faster it’s moving away. Hubble used this relationship to create his famous diagram, with the speed of recession on the x-axis, distance on the y-axis, and a straight line indicating the relationship.
+**Shashi:** So, the frequency of the sound increases, and the wavelength decreases. Now the boat is passing us and moving away. Listen—the sound becomes less sharp,  the frequency decreases, and the wavelength increases. The diagram illustrates this phenomenon on a cosmic scale. Since the universe is expanding, all galaxies are moving away from each other, just like points drawn on a balloon would move apart as it’s inflated. The galaxies themselves aren’t moving; rather, the space (the balloon) is expanding, increasing the distances between them. If a galaxy is moving away from us, its light frequency will appear lower, similar to the boat’s sound as it moved away. Since red has a lower frequency than blue, this shift in frequency is called **redshift**. We observe that the light from all galaxies is redshifted, moving toward the red end of the spectrum. The degree of this shift indicates the galaxy’s speed of recession. Just as the dungchen on the boat would sound even lower if the boat were moving faster away, the light from a galaxy appears more redshifted the faster it’s moving away. Hubble used this relationship to create his famous diagram, with the speed of recession on the x-axis, distance on the y-axis, and a straight line indicating the relationship.
 **Socrates:** But what does this mean?