Today’s post is from Rich, who attends the What Are You Reading Now? Book Club:
The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality by Richard Panek
In 1915 Einstein published his general relativity theory, showing that gravity distorts space to create its attractive force. One of his equations showed that the universe should be expanding, but our knowledge of the universe then was mostly of the stars in our Milky Way galaxy, and Einstein assumed that the expansion was incorrect. Then in 1924, using a new, much stronger telescope, Edwin Hubble announced that there are other galaxies in the universe, much further away than ours. In 1929 Hubble went further, and announced that the galaxies that are farther away are moving away from us proportionately faster. This implies an expanding universe which resulted from a massive ‘explosion.’ When Einstein heard about this, he lamented that not trusting his equation was his life’s greatest blunder.
It is natural to think that there should be some way the universe can last forever, rather than this Big Bang being a one-time event. Fred Hoyle theorized that matter is created in the ‘center’ of the universe, and expansion goes on forever with no Big Bang. His theory was finally disproved when background radiation left over from the Big Bang was discovered in 1965. An alternate theory held that the expansion of galaxies would eventually slow down from gravitational attraction, causing the galaxies to reverse, and eventually move back into an eventual Big Crunch, with a second Big Bag to follow. That theory was also disproved, as this book explains.
Vera Rubin did not become an astronomer until 1965, when she was thirty-seven, with four children. After studying the Andromeda spiral galaxy, she was surprised to discover that the speed of its inner stars is not faster than the speed of outer stars, as we might expect from our knowledge of our own solar system. This situation with Andromeda and other spiral galaxies implied that mass was not centralized, but was spread out, possibly around the outer regions of the galaxies. The problem is that mass is not visible where it should be. This invisible mass has become known as dark matter, although it has not been identified.
Another surprise for astronomers is the velocity of the expansion of the universe as discovered by Hubble. Everyone had assumed that it would be slowing down, but measurements done over many years suggest that the expansion is speeding up! Something is pushing galaxies to accelerate, and that would seem to be an energy force that is unknown. When that dark energy is converted to mass by Einstein’s equation, and added to dark matter estimates, it adds up to 96% of the total mass of the universe. Only 4% is left for the visible parts of galaxies, and the stars (such as our sun) in those galaxies. So the known universe is only 4%.
This book is primarily the story of the astronomers and other scientists who discovered and studied these strange results. There is very little discussion of what dark matter and dark energy consist of, mostly because no one knows.