The Big Bang is how astronomers believe the universe began.
The universe began its existence as a single point 13.8 billion years ago, with the Big Bang. After a phase of extremely rapid expansion, the universe has been steadily expanding, but it is increasing in size in an accelerated fashion now (driven by the “dark energy”). An observable leftover from the time of the Big Bang is so-called background radiation – these photons were able to emerge 375 000 years after the Big Bang, have flown around since, and cooled down to 2.7K (-454.765F) which is near the absolute zero degree Kelvin temperature of their surroundings in space. Precise measurements of the distribution of this afterglow radiation across the sky must be reproduced by cosmological models that attempt to mathematically understand the space and time evolution of our expanding universe from 13.8 billion years ago until today.
Illustration of the universe’s expansion from the Big Bang (left) until today (right).
Different epochs and phases are labeled.
Credit: NASA/WMAP Science Team - Original version: NASA; modified by Cherkash
The Planck satellite measured the distribution of the very subtle temperature fluctuations of the photons left over from after the Big Bang (shown in red vs blue patches). The fluctuations reflect tiny density variations from which all structure and galaxies later formed, including the Milky Way.