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.
