Question

Scientists often suggest that the expansion of the universe is similar to the expansion of raisin bread as it bakes in an oven. As the loaf increases in size, individual raisins move farther apart in the expanding bread. During a homework assignment, two students suggest the following analogies for the universe, but these are not considered as good as the raisin bread analogy. Why? a) The universe expands similarly to the concentric ripples formed when a rock is thrown into a pond. b) The universe is similar to a Jell-O mold enclosing pieces of fruit (galaxies).

Short answer

The ripple analogy implies a central point of expansion, and the Jell-O does not show galaxies moving apart.

Step-by-step solution

Understanding the Raisin Bread Analogy

The raisin bread analogy suggests that as the bread rises and expands, the distance between the raisins increases uniformly. This is akin to the universe's expansion where all galaxies move away from each other as space expands uniformly.

Analyzing the Ripple Analogy

In the ripple analogy, waves form concentric circles that move outward from a central point. This suggests a central origin or point of expansion in the universe, which contradicts the idea that the universe expands uniformly in all directions with no central point.

Evaluating the Jell-O Mold Analogy

While the Jell-O mold analogy captures the concept of enclosed galaxies within a medium, the pieces of fruit (galaxies) don't move further apart due to the expansion of the Jell-O. This contradicts the expanding universe theory where galaxies move away from each other as the universe expands.

Key concepts

Raisin Bread Analogy

Imagine a loaf of raisin bread baking in the oven. As the bread rises and bakes, it expands in all directions. The raisins scattered throughout the loaf gradually move away from each other. This is similar to how galaxies in the universe behave as the universe expands. Everything gets further apart as space itself grows.
The key feature of this analogy is how each part of the bread expands at the same rate, just like in our universe where space itself is expanding uniformly in every direction. There's no central point of expansion, which means every galaxy moves away from every other galaxy no matter where you are in space.

• Uniform expansion in all directions.
• No center of the expansion.
• Analogous to galaxies moving apart.

The raisin bread analogy is effective because it clearly illustrates these concepts without implying a center of the universe.

Ripple Analogy

The ripple analogy might seem compelling at first, but it suggests a misleading idea about how the universe expands. When you throw a stone into a pond, ripples form and spread out from a central point, moving concentrically in waves.
This implies there is a center from which the universe is expanding, akin to waves moving outward from the point of impact in the pond.

Why It Falls Short

• Suggests a central point of origin.
• Contradicts the idea of uniform expansion.
• Not all regions move apart at the same rate.

Our universe has no known center where the expansion originates from. Because ripples have a clear center and move outward, this analogy does not capture the universe's uniform expansion model.

Jell-O Mold Analogy

The Jell-O mold analogy involves thinking of the universe like a Jell-O mold that contains pieces of fruit, representing galaxies.
The idea is that as the Jell-O sets, it expands, but unfortunately, this analogy doesn't work well for illustrating how our universe expands.
While the Jell-O can contain the galaxies, it doesn't account for them moving apart.

The Issues with This Analogy

• Doesn't demonstrate galaxies moving away from each other.
• Jell-O sets but doesn’t expand dynamically.
• Doesn't illustrate uniform expansion across all regions.

The universe’s actual expansion involves galaxies moving farther from each other, akin to the raisin bread example, without the aid of any expanding containment like Jell-O. This is why the raisin bread analogy works better in explaining cosmic expansion.