<P> The concept of a Universe filled with a luminiferous aether remained in vogue among some scientists until the early 20th century . This form of aether was viewed as the medium through which light could propagate . In 1887, the Michelson--Morley experiment tried to detect the Earth's motion through this medium by looking for changes in the speed of light depending on the direction of the planet's motion . However, the null result indicated something was wrong with the concept . The idea of the luminiferous aether was then abandoned . It was replaced by Albert Einstein's theory of special relativity, which holds that the speed of light in a vacuum is a fixed constant, independent of the observer's motion or frame of reference . </P> <P> The first professional astronomer to support the concept of an infinite Universe was the Englishman Thomas Digges in 1576 . But the scale of the Universe remained unknown until the first successful measurement of the distance to a nearby star in 1838 by the German astronomer Friedrich Bessel . He showed that the star 61 Cygni had a parallax of just 0.31 arcseconds (compared to the modern value of 0.287"). This corresponds to a distance of over 10 light years . The distance to the Andromeda Galaxy was determined in 1923 by American astronomer Edwin Hubble by measuring the brightness of cepheid variables in that galaxy, a new technique discovered by Henrietta Leavitt . This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the Milky Way . </P> <P> The earliest known estimate of the temperature of outer space was by the Swiss physicist Charles É . Guillaume in 1896 . Using the estimated radiation of the background stars, he concluded that space must be heated to a temperature of 5--6 K. British physicist Arthur Eddington made a similar calculation to derive a temperature of 3.18 K in 1926 . German physicist Erich Regener used the total measured energy of cosmic rays to estimate an intergalactic temperature of 2.8 K in 1933 . </P> <P> The modern concept of outer space is based on the "Big Bang" cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître . This theory holds that the universe originated from a very dense form that has since undergone continuous expansion . The background energy released during the initial expansion has steadily decreased in density, leading to a 1948 prediction by American physicists Ralph Alpher and Robert Herman of a temperature of 5 K for the temperature of space . </P>

How many miles to get into outer space