The Big Bang teaches us that our expanding, cooling universe used puro be younger, denser, and hotter con the past.
Con every direction we care esatto observe, we find stars, galaxies, clouds of miscela gassosa and dust, tenuous plasmas, and radiation spanning the gamut of wavelengths: from radioricevente sicuro infrared esatto visible light onesto varieta rays. No 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 esatto garantisse that it all came from somewhere. If you want puro know the answer onesto the biggest question of all – the question of our cosmic origins – you have puro pose the question esatto the universe itself, and listen preciso what it tells you.
Today, the universe as we see it is expanding, rarifying (getting less dense), and cooling. Although it’s tempting puro simply extrapolate forward mediante 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 puro go as far as possible: all the way back onesto infinitesimal sizes and infinite densities and temperatures, or what we know as per singularity. This timore, of a singular beginning onesto 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 verso 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 preoccupazione sicuro some of the most stringent constraints ever obtained by humanity. Einstein’s first solution was for the weak-field limit around verso scapolo mass, like the Sun; he applied these results sicuro our Solar System with dramatic success. Very quickly, a handful of exact solutions were found thereafter. (Credit: LIGO scientific collaboration, Tau. Pyle, Caltech/MIT)
Where did all this che tipo di from?
Like most stories con science, the origin of the Big Bang has its roots mediante both theoretical and experimental/observational realms. On the theory side, Einstein put forth his general theory of relativity con 1915: verso novel theory of gravity that sought preciso 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.
- Durante 1916, Karl Schwarzschild found the solution for verso pointlike mass, which describes a nonrotating black hole.
- Durante 1917, Willem de Sitter found the solution for an empty universe with a cosmological constant, which describes an exponentially expanding universe.
- From 1916 esatto 1921, the Reissner-Nordstrom solution, found independently by four researchers, described the spacetime for per charged, spherically symmetric mass.
- Con 1921, Edward Kasner found a solution that described per matter-and-radiation-free universe that’s anisotropic: different mediante different directions.
- Durante 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 in 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.