# General Astronomy/An Accelerating Universe

Space is flat. This is shown by the density of matter, and cosmic background radiation.

The radiation left over from the Big Bang is about 3 Kelvin.

The rate of expansion of the universe is greater in proportion to the distance.

## Other Cosmological Models

Throughout history, astronomers have been seeking models of the universe. The three best known are those by Ptolemy, Newton, and Einstein. They all had an explanation why the universe didn't collapse.

Ptolemy's model had the Earth at the center, surrounded by an "8th Sphere" of stars. The sphere held up the stars, so the universe didn't collapse.

Newton's universe was infinitely large. He postulated a universe where the gravitational force was in balance. In an infinitely large area, there is no center of mass; therefore, the universe could not collapse. Like many of his time, Newton also believed in an infinitely old universe. Given an infinite amount of time, if the universe was going to collapse, it would've done it already.

Flaws with Newton's model were apparent even in his time. Heinrich Olbers wondered: If the universe is infinitely large and infinitely old, why isn't the night sky as bright as the sun? This question became known as Olbers' Paradox. Olbers' proposed solution was that the universe was filled with dust, obscuring some of the starlight. However, this only appeared to solve the problem. Dust absorbs and radiates energy, so that the light being obscured would be transmitted anyway.

Einstein's model for the universe was a constant size. He postulated a repulsive ${\displaystyle \lambda }$ (lambda) force that counterbalanced gravity and kept everything from collapsing.

Immanuel Kant, a philosopher, proposed a disc-shaped universe.