The Big Bang teaches us that our expanding, cooling universe used to be younger, denser, and hotter con the past.
Per every direction we care onesto observe, we find stars, galaxies, clouds of gas and dust, tenuous plasmas, and radiation spanning the gamut of wavelengths: from radio puro infrared puro visible light esatto modo rays. Mai matter where or how we immagine at the universe, it’s full of matter and energy absolutely everywhere and at all times. And yet, it’s only natural to garantit that it all came from somewhere. If you want sicuro know the answer onesto the biggest question of all – the question of our cosmic origins – you have preciso pose the question to the universe itself, and listen puro what it tells you.
Today, the universe as we see it is expanding, rarifying (getting less dense), and cooling. Although it’s tempting sicuro simply extrapolate forward con time, when things will be even larger, less dense, and cooler, the laws of physics allow us esatto extrapolate backward just as easily. Long ago, the universe was smaller, denser, and hotter. How far back can we take this extrapolation? Mathematically, it’s tempting esatto go as far as possible: all the way back esatto infinitesimal sizes and infinite densities and temperatures, or what we know as per singularity. This timore, of a singular beginning preciso space, time, and the universe, was long known as the Big Bang.
The modern cosmic picture of our universe’s history begins not with a singularity that we identify with the Big Bang, but rather with per period of cosmic inflation that stretches the universe esatto enormous scales, with uniform properties and spatial flatness
But physically, when we looked closely enough, we found that the universe told per different story. Here’s how we know the Big Bang isn’t the beginning of the universe anymore.
Countless scientific tests of Einstein’s general theory of relativity have been performed, subjecting the intenzione puro some of the most stringent constraints ever obtained by humanity. Einstein’s first solution was for the weak-field limit around a solo mass, like the Sun; he applied these results preciso our Solar System with dramatic success. Very quickly, per handful of exact solutions were found thereafter. (Credit: LIGO scientific collaboration, T. Pyle, Caltech/MIT)
Where did all this che razza di from?
Like most stories sopra science, the origin of the Big Bang has its roots durante both theoretical and experimental/observational realms. On the theory side, Einstein put forth his general theory of relativity per 1915: a novel theory of gravity that sought sicuro overthrow Newton’s theory of universal gravitation. Although Einstein’s theory was far more intricate and complicated, it wasn’t long before the first exact solutions were found.
- Sopra 1916, Karl Schwarzschild found the solution for a pointlike mass, which describes per nonrotating black hole.
- Durante 1917, Willem de Sitter found the solution for an empty universe with verso cosmological constant, which describes an exponentially expanding universe.
- From 1916 esatto 1921, the Reissner-Nordstrom solution, found independently by four researchers, Codice sconto tastebuds described the spacetime for verso charged, spherically symmetric mass.
- Durante 1921, Edward Kasner found per solution that described verso matter-and-radiation-free universe that’s anisotropic: different con different directions.
- In 1922, Alexander Friedmann discovered the solution for an isotropic (same sopra all directions) and homogeneous (same at all locations) universe, where any and all types of energy, including matter and radiation, were present.
That last one was very compelling for two reasons. One is that it appeared puro describe our universe on the largest scales, where things appear similar, on average, everywhere and con all directions. And two, if you solved the governing equations for this solution – the Friedmann equations – you’d find that the universe it describes cannot be static, but must either expand or contract.