I am on record as having said more than 20 years ago that we would know dark matter’s nature long before today. Although that prediction has proved wrong, I have not given up hope. Dark energy entered the picture in 1998, when researchers measuring the distances and speeds of supernovae found that the expansion of the universe was actually accelerating. Gravitational attraction pulling galaxies toward one another seemed to be overwhelmed by a mysterious new force latent in empty space that pushes galaxies apart—a force that came to be known as dark energy. Theorists working on string theory or loop quantum gravity are tackling this challenge, but the phenomenon seems so far from being accessible by any experiment that I am not expecting answers anytime soon. Today we know that the universe is far bigger and stranger than anyone suspected. Not only does it extend beyond the Milky Way to untold numbers of other galaxies—this would come as a surprise to astronomers of the 19th and early 20th century to whom our galaxy was “the universe”—but it is expanding faster every day.
The galaxies are also surprisingly pristine, chemically speaking, lacking in elements heavier than hydrogen and helium. Taken together, the new observations suggest galaxies formed earlier and faster than previously thought.
Aristarchus of Samos and the Heliocentric Model
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Cosmology Is in Crisis Over How to Measure the Universe
Since around 1990, several dramatic advances in observational cosmology have transformed cosmology from a largely speculative science into a predictive science with precise agreement between theory and observation. These advances include observations of the microwave background from the COBE, WMAP and Planck satellites, large new galaxy redshift surveys including 2dfGRS and SDSS, and observations of distant supernovae and gravitational lensing. These observations matched the predictions of the cosmic inflation theory, a modified Big Bang theory, and the specific version known as the Lambda-CDM model.
Cantor argued that the mathematical properties of infinite sets/things are simply radically different to those that are finite, making Craig’s library or Hilbert’s Hotel not absurd. Craig takes it to be obviously absurd for a subset to be both smaller than and equal to its set . For infinite sets it’s not absurd, it’s actually their defining characteristic. When we think of libraries or hotels, we have in mind our ideas about finite sets of things, but Cantor argued such intuitions are not applicable to infinite sets.
The geocentric Ptolemaic system was the prevailing theory until the 16th century when Nicolaus Copernicus, and subsequently Johannes Kepler and Galileo Galilei, proposed a heliocentric system. This is one of the most famous examples of epistemological rupture in physical cosmology. Our scientific origin story is something called the standard model of cosmology.
This is the approach of the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey. Within the standard cosmological model, the equations of motion governing the universe as a whole are derived from general relativity with a small, positive cosmological constant. The solution is an expanding universe; due to this expansion, the radiation and matter in the universe cool down and become diluted. At first, the expansion is slowed down by gravitation attracting the radiation and matter in the universe. However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the universe starts to accelerate rather than decelerate. While Heber Curtis argued for the idea that spiral nebulae were star systems in their own right as island universes, Mount Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the Milky Way star system only.