By the time you have completed the 2.12. Introduction & Exploration Activities, you should be able to:
Understand how Western Civilization perceived the Universe 1) before Copernicus, 2) between Copernicus and Newton, 3) between Newton and Hubble, and 4) today.
Describe what Hubble’s law is and whether it implies that galaxies are moving through the Universe or being carried by an expanding Universe.
Describe whether the concepts of ‘center’ and ‘edge’ apply to the Universe, or not.
Understanding the Universe
Answer the questions below.
How did western civilization perceive the universe before Copernicus?
Answer
Western Civilization conceived of a relatively small solar-system-sized, unchanging & eternal, Earth-centered Universe. This model is often exemplified by Ptolemy’s model.
How did western civilization perceive the universe between Copernicus and Newton?
Answer
Western Civilization conceived of a much larger, Sun-centered, eternal & unchanging Universe. This model was developed by Nicolaus Copernicus.
How did western civilization perceive the universe between Newton and Hubble?
Answer
Western Civilization conceived of a galaxy-sized, eternal Universe. This model was developed by Isaac Newton and his successors.
How does western civilization perceive the universe today?
Answer
Humanity conceives of the Universe as finite, but unimaginably large, complex, and dynamic. This conception is based on the work of Willem de Sitter, Aleksandr Friedmann, Georges Lemaitre, Edwin Hubble, Albert Einstein, George Gamow, Robert Penrose, and others.
Hubble’s Law
Hubble’s law tells us that distant galaxies' speeds are proportional to their distance. We read about the rising dough analogy and the galaxies on an expanding balloon.
According to Hubble’s Law, are galaxies moving through, or being carried by an expanding universe?
Answer
Galaxies are not moving through the Universe, they are carried by the Universe as it expands. Thus, the ‘speeds’ of galaxies in Hubble’s Law represent how quickly the Universe is stretching, not how quickly galaxies are moving through the Universe.
Based on what you know about the falsification of scientific explanations, what does Hubble’s Law indicate about the Newtonian Universe?
Answer
Hubble’s law falsifies the Newtonian model of the Universe. In other words, it demonstrates that a Newtonian Universe has limited explanatory power and indicates the need for a model of the Universe that better explains nature ‘as it is’. As you’ll discover during the rest of this unit, the more-true explanation that emerged from the falsification of the Newtonian Universe was the Big Bang model of the Universe. It may be that humanity identifies the limits of Big Bang Theory at some point in the future. Or it may be that Big Bang Theory describes nature ‘as it is’. Although we cannot know the future of Big Bang Theory, we can—as disciple scholars—appreciate it for what it is: the most-true explanation of the nature and development of the Universe that humanity has ever known.
Imagine that you live inside one of the raisins (or chocolate chips, if you prefer) in some rising bread dough—illustrated in the image above. From our perspective inside the ‘local’ raisin (our galaxy), we observe the effect of the dough rising on the motions of and distances to all other raisins in the dough.
To do this, we measure the distances between the raisins at 2 o’clock, as the dough begins to rise, and then again at 3 o’clock, once the dough has finished rising.
During that hour, the dough tripled in size, which caused the distances between raisins to triple: the nearest raisin was 1 cm away before rising and 3 cm away after rising, the next raisin moved from 2 to 6 cm, the third from 3 to 9 cm, the fourth from 4 to 12 cm, …. Thus, the nearest raisin moved 2 cm further away (from the local raisin) as the dough rose, the second raisin moved 4 cm further, the third raisin moved 6 cm, the fourth raisin moved 8 cm, …. Now, knowing the distance traveled by each raisin during the hour the dough rose allows us to calculate the speed of each raisin relative to the local raisin. As you know, speed is the distance traveled divided by the duration of the trip (e.g., 60 miles per hour means you traveled 60 miles in 1 hour). So, the raisins traveled at the following speeds as the bread rose: 2 cm/hour for raisin one, 4 cm/hour for raisin two, 6 cm/hour for raisin three, 8 cm/hour for raisin four, …. Okay, with the data in hand, let’s plot the distance to each raisin at the end of rising (on the x-axis) versus the speed traveled by each raisin during rising (on the y-axis). This graph is already plotted above.
Before interpreting our observations, let’s be clear: the raisins in the analogy are galaxies in the Universe and the dough is the fabric of spacetime. To help you think more deeply about how this analogy relates to Hubble’s Law, answer each of the questions below.
From our vantage point inside the ‘local raisin’ (local galaxy), are the rest of the raisins (galaxies) moving towards or away from us?
Answer
All of the raisins (galaxies) are moving away from us; that is, away from the ‘local raisin’ (the Milky Way).
Look at the plot of raisin speed versus raisin distance. Do you recognize this pattern?
Answer
Yes! The pattern is identical to Hubble’s Law of universal expansion. In both laws, the distance and speed of objects (raisins and galaxies) are strongly correlated; that is, every distance corresponds to just one speed (and vice versa). If we were serious bakers, we could call our law ‘The Law of Raisin Bread Expansion’, and it would apply to all batches of dough that expand. Some dough might expand more, some less, but all plots would produce a straight-line relationship.
So, what is causing the raisins (galaxies) to move apart. In other words, what is the best explanation of ‘The Law of Raisin Bread Expansion’?
Answer
The raisins (galaxies) are moving apart because the dough is expanding (spacetime is stretching).
Are the raisins moving through the dough? If not, describe what is causing the observed motion.
Answer
The raisins are not moving through the dough; instead, the raisins are being carried by the dough as the dough expands (the yeast gives off gas, causing the dough to stretch—which increases the distance between raisins).
In the same way, Hubble’s Law indicates that the observed motion of distant galaxies results from the galaxies being carried by spacetime as it inflates/stretches. That doesn’t mean that galaxies do not move through the Universe. Galaxies are moving through the Universe at relatively slow speeds (due to motion left over from their formation and motion caused by gravitational interactions with nearby galaxies), but this ‘local’ motion is not the cause of the Hubble’s Law velocities. Instead, ‘Hubble’ velocities result from the expansion of the Universe, and local motions have relatively minor effects on observed Hubble motions.
Now, imagine a much larger loaf and consider that you move to another raisin (galaxy)—any other raisin, near or far. In fact, consider making distance and velocity measurements from many different raisins (galaxies). Would you observe the same ‘Hubble’s Law’ relationship from raisins (galaxies) other than the local raisin (Milky Way)? In other words, does it matter which raisin (galaxy) you observe from?
Answer
The short answer is no, it makes no difference which galaxy you observe from. Many who are new to Hubble’s Law incorrectly conclude that the motion of distant galaxies (away from the observer) implies that our galaxy is at the center of the Universe. This is not true. Instead, observations from every raisin are the same. That is, observers in any galaxy would also see all distant galaxies moving away from them. Said another way, observations from any galaxy would establish Hubble’s Law.
Ends of the Universe
The Stretching Balloon Surface Universe. This analogy is based on a partially inflated child’s balloon on which are drawn small galaxy-shaped dots, as shown in the image below. To appropriately understand this analogy, you must consider that nothing exists beyond the surface of the balloon. In other words, imagine that you are a two-dimensional being—a stick figure—that exists entirely within the surface of the balloon. You can move in any direction across the surface, but you cannot leave the surface. Now, locate yourself in one of the galaxies and observe what happens as the balloon is inflated to a larger size, as illustrated below.
From your existence within the surface of the balloon, describe the motion of all other galaxies relative to your own as the balloon inflates.
Answer
You would observe that all other galaxies are moving away from you.
In this analogy, are distant galaxies moving away from you because they are moving across the surface of the balloon? If not, what is causing the distances between galaxies to increase?
Answer
No, the distances between galaxies is not increasing because the galaxies are moving across the surface of the balloon (i.e., through the Universe). Instead, the galaxies are moving away from each other because the surface of the balloon is stretching.
Again, from your existence within the surface of the balloon, describe which galaxies are traveling away from you more slowly and which are moving away more quickly. Also, describe why.
Answer
The nearer galaxies are ‘moving away’ more slowly and the more distant galaxies are ‘moving away’ more quickly. Why? Because—just like the bread dough—the surface of the balloon stretches uniformly during inflation, so larger initial distances between galaxies experience the same degree/percent of stretching as shorter distances but yield a greater amount of stretching.
If this doesn’t make sense to you yet, go back to the pumpkin bread analogy. In this analogy, the rising dough tripled all distances, but that tripling took the distance between the local raisin and raisin one from 1 cm to 3 cm (= 2 cm of movement); whereas the same degree of stretching moved raisin two 4 cm—twice as much as raisin one. Does it make sense now?
In this analogy, if you plotted the distance to galaxies against their speed (as they move away from you), would you recreate Hubble’s Law?
Answer
Yep, you sure would! And, as above, it wouldn’t matter which galaxy was your’s: observers in every galaxy would make the same observations.
Okay, these are the last questions, and then we’ll summarize what these analogies teach us. Once again, answer each of the following questions from your existence within the surface of the balloon: Does the surface of the balloon have a center or an edge? In other words, can you place your finger on the balloon and say, “Here’s the center!” or, “Here’s the edge!”
Answer
Counterintuitive as it may be, the surface of a balloon has no center and no edge! The same is true for Earth’s surface: Although Earth has a center and an edge, Earth’s surface has neither. Walk as far as you like on the surface of the Earth and you never reach an edge, you just go round and round. Similarly, declaring that where you now sit is the center of Earth’s surface makes no sense: your location is no more the center than any other location on Earth’s surface. And the issue isn’t with Earth’s surface, the problem lies with your intuition: most of us just aren’t very familiar with the geometry of continuous surfaces. Continuous surfaces just don’t have edges or a center.
