# What "revolutionary" discoveries were made by Hubble Telescope [closed]

Forgive me for the naive question. Besides taking pictures of tons of objects in space, what kinds of revolutionary discoveries were made as a result of data from Hubble? Did Hubble shed light on any open problems or did it lead astronomers to conclude the existence of something that they had not known to have existed before?

• The American Astronomical Society had a press conference about the 25 year history of the Hubble Space Telescope January this year. They use a very very bad media player, you have to click "Seminar for Science Writers: HST @ 25", you cannot download it, it loads very slowly and if you happen to click anywhere it will immediately reset. But the talks are very informative. Commented Jan 27, 2015 at 16:59

• Hubble measured the distance to some Cepheid variables with very good accuracy. Cepheids are variable stars that periodically oscillate between larger and more luminous states and smaller and more dimmer states. They are used as "standard candles" in astronomy because their luminosity and periods can be used to easily calculate how far away they are. In fact, Edwin Hubble used Cepheids to establish Hubble's law, $$v=H_0D$$. It states how fast objects are moving away from us ($$v$$) as a function of distance ($$D$$). $$H_0$$ is Hubble's constant. It was originally written as $$H$$, but it was determined that it isn't actually a constant at all! $$H_0$$ is used as the value in the present era.
Anyway, the measurements from Hubble were used to establish the distance to the Cepheid variables and their recessional velocities. With that, $$H_0$$ could be determined more accurately than ever. Freedman et al. published a paper detailing the results. The team found that $$H_0 = 72 \pm 8 \text{ kms}^{-1}\text{ Mpc}^{-1}$$.