LSAT 159, Section 2, RC Passage 3, Dark Energy

RC Passage 3 Explanation

PASSAGE TEXT: Since 1929, cosmologists have known that the universe is expanding. Until recently, they …

Paragraph Summaries

1. Astronomers expected the universe’s expansion to be slowing due to gravity, but 1990s observations of supernovas showed they were farther away than predicted. This revealed that the universe’s expansion is actually accelerating.
2. One explanation for acceleration is dark energy, a force that stretches space and may make up 2/3 of the universe. The simplest version treats it as a constant pressure in empty space (proposed and rejected by Einstein).
3. Some scientists argue dark energy comes from vacuum energy, but this idea faces a major issue because the predicted amount of vacuum energy is way larger than what would be needed to explain acceleration.
4. A competing theory rejects dark energy and proposes that gravity behaves differently over large distances. This would have testable consequences and might be easier to test than the dark energy model.

Analysis

There’s a lot of science here. Here’s an overview of the theories:

1. Dark energy as a constant pressure built into empty space (Einstein’s cosmological constant idea)
2. Dark energy as vacuum energy from quantum virtual particles
3. Gravity itself changes over extremely large distances

The first two theories are both grounded in dark energy and just explain the dark energy differently. The third entirely rejects the dark energy theory.

The author doesn’t try to argue for which one is right. The focus is analysis of the merits and issues with each, and how each theory forces scientists to rethink core assumptions.

There’s also mention of testability. The third theory is more radical than dark energy but might be easier to evaluate because it would produce detectable effects within our solar system.