The Expanding Universe
Some of the greatest unanswered questions of modern physics (and particularly of modern astrophysics and cosmology) are those on the topic of the universe. What is the universe? How big is it? Is it finite or infinite? What is beyond it's edges? Does the universe have edges?
These are the sort of questions which have been at the forefront of cosmological research since the beginning of the 20th century.
In 1927, the Belgian astrophysicist Georges Lemaître derived the expansion of the universe from Einstein's equations of general relativity. Two years later, American astronomer (and galaxy enthusiast) Edwin Hubble confirmed this in what has now become known as Hubble's Law. When Hubble plotted the recession velocities of galaxies against their distance from the Earth, he noticed that all of them are moving away from us (bar those in our Local Group) and that, generally, the further a galaxy is away from the Earth, the faster it will be moving away. This was substantial evidence for the fledgling Big Bang theory as the widely believed theory of the universe was the Steady State theory (Einstein even went as far as to add a cosmological constant to his equations of general relativity to adjust for this).
The Doppler effect is a well known principle in physics, whereby the wavelength of a wave emitted by an approaching source is shortened and the wavelength of a wave emitted by a recessing source is extended. This is commonly observed in ambulance sirens which are increased in pitch as the ambulance approaches, reaching a peak as the ambulance reaches the observer, and then reduced in pitch as the ambulance approaches.
Because light acts in a wave-like manner, the same applies for light emitted from galaxies. Light from galaxies which are receding is stretched towards the red end of the spectrum so appears redder. Light from galaxies also has specific black lines (known as absorption line) running through its absorbed spectrum which are also shifted by the Doppler effect. These correspond to specific elements and so have a known rest wavelength. By measuring the amount by which these absorption lines are "red-shifted", the recession velocity of a galaxy can be calculated.
Something in the Background
In 1964, two astronomers, Arno Penzias and Robert Wilson, set to work using the Holmdel Horn radio antenna at Bell Labs. To obtain accurate readings, they needed to calibrate their antenna so pointed it at a seemingly blank region of the sky. Rather than the subtle interference they were expecting to find, there was a constant background noise which couldn't be explained by nearby equipment. In trying to find where the background noise was coming from, they completely reassembled the telescope and cleaned out the bird droppings which had collected inside. To their amazement, the background noise was still there. They contacted a team of researchers who confirmed that what they had found was Cosmic Microwave Background radiation (CMB radiation). It was for this discovery that they were awarded the Nobel Prize in Physics 1978.
This CMB radiation is the signature of the Big Bang, the radiation left after the "explosion". The irregularities in the radiation may indicate the conditions of the early universe and lead us towards the answers to many of our questions about the universe.