There are many questions about the Big Bang, particularly about what came before it (if anything). But scientists do know some things. Read on for some of the most mind-bending discoveries about the beginning of everything.
The universe is expanding
Hubble made his discovery by measuring something called redshift, which is the shift toward longer, red wavelengths of light seen in very distant galaxies. (The farther away the object, the more pronounced the redshift.) Hubble found that redshift increased linearly with distance in far-off galaxies, indicating that the universe isn't stationary. It's expanding, everywhere, all at once.
Hubble was able to calculate the rate of this expansion, a figure known as the Hubble Constant, according to NASA. It was this discovery that allowed scientists to extrapolate back and theorize that the universe was once packed into a tiny point. They called the first moment of its expansion the Big Bang. [From Big Bang to Present: Snapshots of Our Universe Through Time]
Cosmic microwave background radiation
What they'd detected, it turned out, was the first light of the universe: cosmic microwave background radiation. This radiation dates back to about 380,000 years after the Big Bang, when the universe finally cooled enough for photons (the wave-like particles that make up light) to travel freely. The discovery lent support to the Big Bang theory and to the notion that the universe expanded faster than the speed of light in its first instant. (That's because the cosmic background is quite uniform, suggesting a smooth expansion of everything at once from a small point.)
Direct evidence of inflation
No extra dimensions so far
The results, published in July 2018 in the Journal of Cosmology and Astroparticle Physics, suggest that if there are any other dimensions out there, they're tiny — they would affect areas of the universe less than 1 mile (1.6 kilometers) in size. That means that string theory, which posits that the universe is made of tiny vibrating strings and predicts at least 10 teensy dimensions, could still be true.
Expansion accelerating . . .
The discovery dates back to 1998, when physicists announced the results of several long-running projects that measured particularly heavy supernovas called Type Ia supernovas. The results (which won researchers Saul Perlmutter, Brian P. Schmidt and Adam G. Reiss a Nobel Prize in 2011), revealed weaker-than-expected light from the most distant of these supernovas. This weak light showed that space itself is expanding: Everything in the universe is gradually getting farther away from everything else.
Scientists call the driver of this expansion "dark energy," a mysterious engine that could make up about 68% of the energy in the universe. This dark energy seems to be crucial to making theories of the beginning of the universe fit observations that are being conducted now, such as those made by NASA's Wilkinson Microwave Anisotropy Probe (WMAP), an instrument that has produced the most precise map of the cosmic microwave background yet.
… Even faster than expected
Why does this matter for the origins of the universe? Because physicists must be missing something. According to NASA, there may have been three separate dark energy "bursts" during the Big Bang and shortly thereafter. Those bursts set the stage for what we see today. The first might have started the initial expansion; a second may have happened much faster, acting like a heavy foot pressed on the universe's gas pedal, causing the universe to expand faster than previously believed. A final dark energy burst may explain the accelerating expansion of the universe today.
None of this is proven — yet. But scientists are looking. Researchers at the University of Texas at Austin McDonald Observatory are using a newly upgraded instrument, the Hobby-Eberly Telescope, to look for dark energy directly. The project, the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), is measuring the faint light from galaxies as far away as 11 billion light-years, which will allow researchers to see any changes in the universe's acceleration over time. They'll also be studying the echoes of disturbances in the 400,000-year-old universe, created in the dense soup of particles that made up everything right after the Big Bang. This, too, will reveal the mysteries of expansion and explain the dark energy that drove it.