Doppler shifting - wavelength emitted by something moving away from us is shifted to a lower frequency
Sound of a fire truck siren - pitch of the siren is higher as the fire truck moves towards you, and lower as it moves away from you
Visible wavelengths emitted by objects moving away from us are shifted towards the red part of the visible spectrum
The faster they move away from us, the more they are redshifted. Thus, redshift is a reasonable way to measure the speed of an object.
When we observe the redshift of galaxies, almost every galaxy appears to be moving away from us – the Universe is expanding.
The expansion of the Universe
Edwin Hubble's 1929 observation that galaxies were generally receding from us provided the first clue that the Big Bang theory might be right.
The abundance of the light elements H, He, Li
The Big Bang theory predicts that these light elements should have been fused from protons and neutrons in the first few minutes after the Big Bang.
The cosmic microwave background (CMB) radiation
The early universe should have been very hot. The cosmic microwave background radiation is the remnant heat leftover from the Big Bang.
Red shift - as light from distant galaxies approach earth there is an increase of space between earth and the galaxy, which leads to wavelengths being stretched
In 1964, Arno Penzias and Robert Wilson, discovered a noise of extraterrestrial origin that came from all directions at once - radiation left over from the Big Bang
In June 1995, scientists detected helium in the far reaches of the universe - consistent with an important aspect of the Big Bang theory that a mixture of hydrogen (75%) and helium (25%) was created at the beginning of the universe
~13.7 billion years ago
How do we know?
Spreading (Red Shift) - know distances, rates of retreat, relative positions
Pervasive background radiation of 2.7°C above absolute zero - afterglow of the Big Bang
Cosmic background radiation temperature on celestial sphere